Country：Japan

Country：Japan

Country：Japan

Country：Japan

Publishing type：Research paper (scientific journal)   International / domestic magazine：International journal  

DOI： 10.35848/1882-0786/accc09

Repository URL： http://hdl.handle.net/10466/0002001376

Authorship：Lead author,　Last author,　Corresponding author   Publishing type：Research paper (scientific journal)   International / domestic magazine：International journal  

DOI： 10.1007/s43673-021-00033-0

Repository URL： http://hdl.handle.net/10466/0002000229

Publishing type：Research paper (scientific journal)   International / domestic magazine：International journal  

Abstract

Thermodynamic analyses of β-Ga₂O₃ growth by both ozone and plasma-assisted molecular beam epitaxy were performed. In either case, the growth mechanism was found to differ depending on whether the input VI/III ratio was above or below 1.5. Under O-rich conditions (VI/III > 1.5), the driving force for β-Ga₂O₃ growth (ΔP_Ga₂O₃) was determined to increase linearly with increasing Ga input partial pressure (P°_Ga) because almost all the supplied Ga was used for growth of the β-Ga₂O₃. In contrast, Ga-rich conditions (VI/III < 1.5) caused ΔP_Ga₂O₃ to decrease. Etching of the β-Ga₂O₃ occurred with increasing P°_Ga due to the formation of volatile Ga₂O. This work also demonstrated that the use of ozone allowed growth at higher temperatures than the use of O radicals. The calculated results were in good agreement with experimental values, indicating that β-Ga₂O₃ growth by molecular beam epitaxy can be explained by thermodynamics.

DOI： 10.35848/1347-4065/ac9bb2

Other URL： https://iopscience.iop.org/article/10.35848/1347-4065/ac9bb2/pdf

Repository URL： http://hdl.handle.net/10466/0002000238

Authorship：Last author   Publishing type：Research paper (scientific journal)   International / domestic magazine：International journal  

DOI： 10.1063/5.0100601

Repository URL： http://hdl.handle.net/10466/0002000230

Publishing type：Research paper (scientific journal)   International / domestic magazine：International journal  

In this work, we fabricated Ga2O3(001)/Si(100) and Ga2O3(010)/Si(100) heterointerfaces by surface activated bonding at room temperature and investigated the effect of Si thickness on the thermal stability of the heterointerfaces by heating the bonding samples at different temperatures. The heterointerface with a thin Si exhibited a good thermal stability at 1000 degrees C. A 4 nm thick intermediate layer with a uniform thickness was formed at the as-bonded Ga2O3(001)/Si(100) heterointerface, but for the as-bonded Ga2O3(010)/Si(100) heterointerface, an intermediate layer with a non-uniform thickness was formed. The thickness of both intermediate layers ranged from 3.6 to 5.4 nm and decreased after annealing at 500 degrees C, followed by an increase after annealing at 1000 degrees C. The component of the intermediate layer includes Ga, O, and Si atoms.

DOI： 10.35848/1347-4065/ac4c6c

Repository URL： http://hdl.handle.net/10466/0002000234

Authorship：Last author   Publishing type：Research paper (scientific journal)   International / domestic magazine：International journal  

This study presents vertical Ga2O3 Schottky barrier diodes (SBDs) with a staircase field plate on a deep trench filled with SiO2. It was clarified from device simulation that at high reverse voltage operation, the staircase field plate and the deep trench can effectively alleviate electric field concentration in the Ga2O3 drift layer and the SiO2 layer, respectively. The Ga2O3 SBDs successfully demonstrated superior device characteristics typified by an on-resistance of 7.6 m omega cm(2) and an off-state breakdown voltage of 1.66 kV. These results offer the availability of the trench staircase field plate as an edge termination structure for the development of Ga2O3 SBDs.

DOI： 10.35848/1882-0786/ac620b

Repository URL： http://hdl.handle.net/10466/0002000236

Publishing type：Research paper (scientific journal)   International / domestic magazine：International journal  

The influence of substrate orientation on homoepitaxial growth of beta-gallium oxide by halide vapor phase epitaxy was investigated. Substrates were cut at various angles & UDelta;(b) from the (001) plane (& UDelta;(b) = 0 & DEG;) to the (010) plane (& UDelta;(b) = 90 & DEG;) of bulk crystals grown by the edge-defined film-fed growth method. The growth rate increased with increasing absolute value of & UDelta;(b) near the (001). However, from the (001) to the (010), as & UDelta;(b) increased, the growth rate decreased sharply, and streaky grooves observed in the grown layer on the (001) substrate became triangular pits. The length of the pits decreased with increasing & UDelta;(b), and a pit-free homoepitaxial layer grew at & UDelta;(b) & AP; 60 & DEG;. The valley line of the pits was parallel to the [010] direction; therefore, the length of the pits decreased with increasing & UDelta;(b). In addition, transmission electron microscopy observations of the deepest part of a pit revealed that the pits originate from dislocations propagating in the substrate at an angle of 60 & DEG; with respect to the (001) plane. Therefore, pits are not formed on the grown layer surface when the & UDelta;(b) of the substrate is & SIM;60 & DEG;, because its surface is substantially parallel to the dislocations. The homoepitaxial growth of a pit-free layer on the (011) substrate (& UDelta;(b) = 61.7 & DEG;) was demonstrated, and void defects and dislocations in the substrate were confirmed by the etch-pit method to not be inherited by the homoepitaxial layer.

DOI： 10.1063/5.0087609

Repository URL： http://hdl.handle.net/10466/0002000233

Authorship：Last author,　Corresponding author   Publishing type：Research paper (scientific journal)   International / domestic magazine：International journal  

We studied electrical properties of n-Si/n-Ga2O3 heterojunctions fabricated by surface-activated bonding. The energy barrier heights (q phi(b)) at the Si/Ga2O3 interface for different reverse voltages (V-rev) were derived from temperature-dependent current density-voltage (J-V-T) characteristics. With shifting V-rev to the negative direction, q phi(b) gradually decreased and reached a constant value due to negatively charged interface states. The conduction band offset at the heterointerface was estimated to be 0.18 eV from the V-rev dependence of q phi(b). The q phi(b) calculated from a capacitance of the heterojunction at thermal equilibrium was larger than those derived from the J-V-T characteristics, attributing to spatially inhomogeneous q phi(b) caused by the non-uniform distribution of the charged interface states. The density of shallow interface states was also extracted from the reverse J-V-T characteristics, which was estimated to be about 6 x 10(12) cm(-2) eV(-1).

DOI： 10.1063/5.0080734

Repository URL： http://hdl.handle.net/10466/0002000235

Authorship：Last author   Publishing type：Research paper (scientific journal)   International / domestic magazine：International journal  

Gallium Oxide has undergone rapid technological maturation over the last decade, pushing it to the forefront of ultra-wide band gap semiconductor technologies. Maximizing the potential for a new semiconductor system requires a concerted effort by the community to address technical barriers which limit performance. Due to the favorable intrinsic material properties of gallium oxide, namely, critical field strength, widely tunable conductivity, mobility, and melt-based bulk growth, the major targeted application space is power electronics where high performance is expected at low cost. This Roadmap presents the current state-of-the-art and future challenges in 15 different topics identified by a large number of people active within the gallium oxide research community. Addressing these challenges will enhance the state-of-the-art device performance and allow us to design efficient, high-power, commercially scalable microelectronic systems using the newest semiconductor platform. (c) 2022 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license(http://creativecommons.org/licenses/by/4.0/).

DOI： 10.1063/5.0060327

Repository URL： http://hdl.handle.net/10466/0002000232

Publishing type：Research paper (scientific journal)   International / domestic magazine：International journal  

The drain current temperature dependence is an efficient way to determine the channel temperature in semiconductor devices; however, it has been challenging to use due to the potential interference of trapping effects. A trapping tolerant method is proposed, illustrated here for Ga2O3 MOSFETs, making in situ temperature measurements possible, allowing a thermal resistance of 59 K.mm/W to be measured in Ga2O3 MOSFETs. However, neglecting the effect of trapping causes an error of similar to 15% in the channel temperature measured using the drain current. 3D simulations show that the measured channel temperature is the average temperature value between source and drain contact.

DOI： 10.1063/5.0069655

Repository URL： http://hdl.handle.net/10466/0002000231

Publishing type：Research paper (international conference proceedings)   International / domestic magazine：International journal  

DOI： 10.1117/12.2607613

Repository URL： http://hdl.handle.net/10466/0002000237

Publishing type：Research paper (scientific journal)   International / domestic magazine：International journal  

Understanding the properties of N-implanted beta-Ga2O3 is fundamental for the optimization of doping and isolation structures based on gallium oxide. This paper reports an extensive analysis of the impact of thermal annealing on the concentration and properties of deep levels in N-implanted beta-Ga2O3 Schottky barrier diodes by means of capacitance isothermal transient spectroscopy. Samples with annealing temperatures from 800 to 1200 & DEG;C were considered. The original results presented in this paper demonstrate the following: (a) The instability of current-voltage characteristics detected for all the samples under test can be attributed to the presence of three electron traps with activation energies of 0.6, 0.7, and 1 eV, consistent with previous reports in beta-Ga2O3. (b) The detected traps are not the nitrogen level but intrinsic defects whose concentration is increased by the implantation process. (c) The concentration of deep levels decreases as the annealing temperature increases, demonstrating that the annealing process can effectively restore the quality of the material while keeping the conductivity decrease related to the presence of the nitrogen. Finally, (d) we demonstrate that the residual leakage and the turn-on voltage shift are correlated with the Arrhenius signature of the detected deep levels. An interpretation is proposed to explain the measurement results.

DOI： 10.1063/5.0065434

Publishing type：Research paper (scientific journal)  

Beta-gallium oxide (beta-Ga2O3) has a history of research and development for over 70 years; however, it has attracted little attention as a semiconductor material for a long time. The situation has drastically changed in the past decade, and research on its material properties and developments of growth and device technologies has become active worldwide, mainly from expectations for applications to next-generation power devices. Most of the specific material properties are attributed to its extremely large bandgap energy of 4.5 eV. The other important material feature is that large-size single-crystal bulks can be synthesized by melt growth methods. Herein, after introducing the material properties of beta-Ga2O3 that are important for electronic devices, the current status of bulk melt growth and epitaxial thin-film growth technologies is given. State-of-the-art beta-Ga2O3 diodes and transistors are also discussed, including future prospects.

DOI： 10.1002/pssr.202100357

Publishing type：Research paper (international conference proceedings)   International / domestic magazine：International journal  

DOI： 10.1109/LTB-3D53950.2021.9598435

Publishing type：Research paper (scientific journal)  

This study investigates the band bending profile of gallium-nitride-based heterostructures with respect to the change in the direction of the induced current in the out-of-plane plane direction, which gives the phase of the terahertz emission peak and can be detected by laser THz emission microscopy. We use a wavelength-tunable laser THz emission spectroscope to observe the transport of charges within the band bending regions, excitation of nearby photocarriers, and carrier scattering. We observe flip-flop peak-to-peak THz emission waveforms for a GaN heterostructure field-effect transistor (FET) compared to a strong THz emission radiated from an Al0.3Ga0.7N/GaN high-electron-mobility-transistor (HEMT). The flip-flop THz emission indicates the emission from various interfaces inside the FET structure, and intense THz emission from the HEMT structure indicates band-edge excitation. Our results provide a valuable perspective for characterizing complex heterostructures that provide insight into possible defects, carrier mobilities, and band bending of multilayer interface electronic devices. Published under an exclusive license by AIP Publishing.

DOI： 10.1063/5.0047402

Publishing type：Research paper (scientific journal)   International / domestic magazine：International journal  

Polarized infrared reflectance spectra from beta-Ga2O3 (001) unintentionally doped (undoped) and Sn-doped substrates were investigated. Spectra from an undoped homoepitaxial film grown on the Sn-doped substrate were also investigated. By setting the electric field vector of the incident light E parallel to the crystallographic b-axis in the s-polarized configuration, the spectra for pure transverse optical phonons of A(u) modes were well reproduced by the Drude-Lorentz model. Subsequently, the free-carrier concentrations and carrier mobilities were determined to be in reasonable agreement with those determined by Hall-effect measurements, and at the same time, the film thickness was determined for the homoepitaxial layer. The results ensure the validity of the simplified optical model analyses for any arbitrary surface orientations where the b-axis is parallel to the surface only if no birefringence effects are present by choosing as E//b in s-polarized configuration.

DOI： 10.1063/5.0059070

Publishing type：Research paper (scientific journal)  

DOI： 10.1063/5.0055292

Publishing type：Research paper (scientific journal)   International / domestic magazine：International journal  

An (AlGa)(2)O-3 back barrier was employed for Ga2O3 metal-oxide-semiconductor field-effect transistors (MOSFETs) with a Si-implanted n-Ga2O3 channel layer. The insertion of the back barrier led to strong confinement of electrons in the channel layer, and a shallower pinch-off characteristic with shifting a threshold gate voltage by +8 V was attained for the MOSFET with the back barrier compared to the conventional one without it. The excellent gate controllability represented by a subthreshold slope of 129 mV/decade was also achieved for the back-barrier MOSFET; whereas it was 337 mV/decade for the non-back-barrier one.

DOI： 10.35848/1347-4065/abe3a4

Publishing type：Research paper (scientific journal)   International / domestic magazine：International journal  

Acceptor impurity doping is recognized as a functional tool to extend the capabilities and applications of beta -Ga2O3. The effect of thermal annealing on photoexcited carriers was characterized by measuring the photocurrent spectra of nitrogen (N)-ion-implanted beta -Ga2O3 crystals, where the N was found to cause less crystal damage to the crystal and much lower thermal diffusivity than Mg. The photocurrent intensity at 4.5-5.5 eV showed an increase with an increase in external bias, and the increase was attributed to the photo-generated non-equilibrium electrons and holes. The spectra under a positive external bias showed a distinct onset at 3.0-3.5 eV owing to optical transitions involving deep donor levels formed by the N-implantation. Spectrally integrated responsivity showed a significant change with the annealing temperature by reflecting the recovery of crystallinity and the thermal activation of N impurities. The results also indicate the formation of additional nonradiative recombination centers due to N-implantation.

DOI： 10.1063/5.0031937

Publishing type：Research paper (scientific journal)   International / domestic magazine：International journal  

An anomalous diode-like turn-on behavior was observed in the drain characteristics of current aperture vertical beta -Ga2O3 transistors. This phenomenon was attributable to an electron barrier created by negative fixed charges in the aperture opening, through which electrons were funneled from the gated channel to the drift layer. Electrostatic analysis for deriving the turn-on voltage yielded effective sheet charge densities on the order of 10(11)-10(12)cm(-2). The charged species was conjectured to be acceptor-like point defects diffusing from nitrogen-implanted current blocking layers with an activation energy consistent with migration of gallium vacancies. These results alluded to a possible role of point-defect diffusion in the performance and reliability of ion-implanted Ga2O3 devices.

DOI： 10.1063/5.0031561

Publishing type：Research paper (scientific journal)  

DOI： 10.1016/bs.semsem.2021.04.001

Publishing type：Research paper (scientific journal)  

DOI： 10.1063/5.0029295

Publishing type：Research paper (scientific journal)  

The advent of acceptor-type doping of beta -Ga2O3 through ion-implantation of nitrogen has opened a new design space for junction-type devices with estimated breakdown voltages in excess of a few kVs. However, the presence of deep states due to intrinsic defects in beta -Ga2O3 and implantation damage could be detrimental to the performance and reliability of such devices. We give a phenomenological description and experimental demonstration of the effects of nitrogen implantation in a buried blocking layer on the performance of transistors. The partial activation of acceptor-like states in the buried implanted region has been revealed and estimated to be similar to 20% through a junction spectroscopic technique involving substrate-bias and sub-bandgap illumination, which remains elusive to standard characterization techniques. The characterization technique, along with a space-charge model of the channel and band model of the buried implanted layer, has revealed the presence of photosensitive mid-bandgap (similar to 2.47eV below the conduction band) and tail states near the valence band edge of nitrogen-implanted beta -Ga2O3.

DOI： 10.1063/5.0031480

Publishing type：Research paper (scientific journal)  

beta -Ga2O3 metal-oxide-semiconductor field-effect transistors (MOSFETs) with gate lengths (L-g) of 50-1000nm were fabricated, employing a thin channel layer formed by a shallow Si-ion implantation doping to maintain a high aspect ratio between an L-g and a gate-to-channel distance. The MOSFETs with L-g=200nm had a maximum drain current density of about 250mA/mm and a peak extrinsic transconductance of 17 mS/mm. The short-channel effect was well suppressed for the devices with L-g >= 200nm, leading to excellent RF device characteristics represented by a record maximum oscillation frequency of 27GHz at L-g=200nm. From simple delay-time analysis on the MOSFETs, the effective electron velocity passing through a region under the gate was estimated to be about 2x10(6)cm/s. Moreover, it was analyzed that the parasitic channel charging delay occupied a substantial proportion of the total delay due to a large sheet resistance of the Ga2O3 channel and thus limited their high-frequency device performance. These results suggest that both suppressing the short channel effect with a reduction in L-g to the sub-0.1-mu m range and minimizing the access resistance are important to further improve RF device characteristics of Ga2O3 MOSFETs.

DOI： 10.1063/5.0029530

Publishing type：Research paper (scientific journal)  

Strong accumulation of Si impurities has been observed at a Ga2O3 epilayer/substrate interface. The highly Si-doped region around the interface typically becomes a current conduction path, causing buffer leakage for lateral Ga2O3 field-effect transistors (FETs). To overcome the drawback, we performed Mg- or Fe-ion implantation doping into a Ga2O3 substrate prior to the subsequent molecular-beam epitaxy growth to compensate the accumulated Si donors at the interface. The Mg implantation doping showed a minimal effect on reduction in the interface leakage, irrespective of its concentration. On the other hand, the Fe doping with a high density of 2x10(19)cm(-3) provided a significant decrease in the leakage and decent FET characteristics.

DOI： 10.1063/5.0029286

Publishing type：Research paper (scientific journal)  

With projected performance advantages over silicon and incumbent wide-bandgap compound semiconductors, gallium oxide (Ga2O3) has garnered worldwide attention as an ultrawide-bandgap semiconductor material suitable for high-voltage, high-temperature, and radiation-hard electronics. Thanks to recent breakthroughs in crystal growth and device processing technologies, the research and development of vertically oriented Ga2O3 power transistors has made rapid strides. In this article, we review the important engineering achievements and performance milestones of the two major types of vertical Ga2O3 transistors-current aperture vertical metal-oxide-semiconductor field-effect transistors (MOSFETs) and vertical fin-channel MOSFETs. Challenges underlying the unique processing approaches to these devices and their implications on device reliability are also discussed.

DOI： 10.1109/TED.2020.3016609

DOI： 10.1109/EDTM47692.2020.9117894

Publishing type：Research paper (scientific journal)  

DOI： 10.7566/jpsj.89.036001

Publishing type：Research paper (scientific journal)  

DOI： 10.1016/j.jcrysgro.2019.125427

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DOI： 10.1016/j.jcrysgro.2019.125337

Publishing type：Research paper (scientific journal)  

Enhancement-mode (E-mode) vertical beta-Ga2O3 metal-oxide-semiconductor (MOS) field-effect transistors featuring a current aperture were developed on a single-crystal beta-Ga2O3 (001) substrate. Nitrogen ions were implanted into a drift layer grown by halide vapor phase epitaxy to form current blocking layers (CBLs) for vertical source-drain isolation, while Si ionswere implanted to form degenerately doped source contact regions and a top-gated lateral channel that was fully depleted at 0-V gate bias. The devices delivered a high output current on/off ratio of 2x10(7) despite a nonideal MOS interface that limited the maximum drain current density to <0.1 kA/cm(2). Pulsed operation without current collapse under off-state voltage stress was demonstrated. Hard breakdown occurred prematurely owing to leakage through the CBLs, but is expected to improve with an optimized nitrogen implantation process. The realization of E-mode vertical Ga2O3 transistors based on a manufacturable all-ion-implanted process represents an important step toward practical applications of Ga2O3 power electronics.

DOI： 10.1109/led.2019.2962657

Publishing type：Research paper (scientific journal)  

Publishing type：Research paper (scientific journal)  

DOI： 10.1007/978-3-030-37153-1_32

Publishing type：Research paper (scientific journal)  

DOI： 10.1007/978-3-030-37153-1_1

Publishing type：Research paper (scientific journal)  

DOI： 10.1007/978-3-030-37153-1_28

Publishing type：Research paper (international conference proceedings)  

Gallium oxide (Ga2O3) is an attractive material for next-generation power device applications due to its large breakdown electric field (>7 MV/cm). This paper highlights current-aperture vertical Ga2O3 metal-oxide-semiconductor field-effect transistors fabricated using silicon- and nitrogen-ion implantation processes.

DOI： 10.1117/12.2544818

Publishing type：Research paper (scientific journal)  

DOI： 10.1063/1.5123985

DOI： 10.1109/IMFEDK48381.2019.8950710

Publishing type：Research paper (scientific journal)  

DOI： 10.1016/j.microrel.2019.113453

Publishing type：Research paper (scientific journal)  

A guard ring (GR) was employed to improve the breakdown voltage (V-br) of vertical Ga(2)O(3 )Schottky barrier diodes (SBDs) with or without a field-plate (FP) by eliminating electric field concentration at the edges of anode and FP electrodes. The GR structure was formed by nitrogen (N)-ion implantation. Four types of vertical SBD structures with: 1) neither a GR nor a FP; 2) a GR; 3) a FP; and 4) both a GR and a FP were fabricated on the same substrate. The SBDs with a GR [structures 2) and 4)] showed larger V-br values than their GR-free counterparts [structures 1) and 3)]. Considering the trade-off relationship between V-br and specific on-resistance (R-on), a V-br/R-on combination of 1.43 kV/4.7 m Omega.cm(2) for the GR/FP-SBD corresponds to one of the best balanced data for Ga2O3 SBDs.

DOI： 10.1109/LED.2019.2927790

Publishing type：Research paper (scientific journal)  

We found that nitrogen (N) and silicon (Si) were unintentionally co-doped in Ga2O3 layers grown by plasma-assisted molecular beam epitaxy (PAMBE), and that the ratio between N and Si could be controlled by O/Ga flux ratio during the growth. Note that N and Si act as a deep acceptor (theoretically predicted) and a shallow donor (experimentally confirmed) in Ga2O3, respectively. Taking advantage of this unique characteristic of PAMBE-grown Ga2O3, normally off operation of Ga2O3 metal-oxide-semiconductor field-effect transistors (MOSFETs) was demonstrated. The unintentionally-doped-Ga2O3 channel layer of the MOSFETs had N and Si concentrations of 1 x 10(18) and 2 x 10(17) cm(-3), respectively, and thus it was considered to behave as a p-type material. The MOSFETs showed a turn-on threshold gate voltage of larger than + 8 V, implying formation of an inversion channel in the N-doped Ga2O3 layer. Although the ON-state drain current (I-d) remained in the subthreshold regime owing to limited gate voltage swing, the I-d ON/OFF ratio exceeded five orders of magnitude.

DOI： 10.1109/LED.2019.2919251

DOI： 10.1109/DRC46940.2019.9046470

DOI： 10.1109/ICIPRM.2019.8819211

Publishing type：Research paper (scientific journal)  

DOI： 10.1142/S0129156419400020

Publishing type：Research paper (scientific journal)  

Depletion-mode vertical Ga2O3 metal-oxidesemiconductor field-effect transistors featuring a current aperture were developed on a halide vapor phase epitaxial drift layer grown on a bulk beta-Ga2O3 (001) substrate. Three ion implantation steps were employed to fabricate the n ++ source regions, lateral n channel, and p current blocking layers, where Si and N were selected as the donor and deep acceptor dopant species, respectively. The transistors delivered a drain current density of 0.42 kA/cm(2), a specific on-resistance of 31.5m Omega.cm(2), and an output current on/off ratio of over 108. High-voltage performance of the present devices was hampered by a large gate oxide field in the off-state causing high gate leakage, a limitation that can be readily overcome through optimized doping schemes and an improved gate dielectric. The demonstration of a planar-gate vertical Ga2O3 transistor based on a highly manufacturable all-ion-implanted process greatly enhances the prospects for Ga2O3-based power electronics.

DOI： 10.1109/LED.2018.2884542

Publishing type：Research paper (scientific journal)  

DOI： 10.1109/LED.2018.2887278

Publishing type：Research paper (scientific journal)  

DOI： 10.1063/1.5091688

Publishing type：Research paper (scientific journal)  

A single-crystal beta-Ga2O3 substrate was directly attached to a polycrystalline SiC (poly-SiC) substrate using a surface-activated-bonding method to enhance heat extraction from beta-Ga2O3 devices. The effective thermal conductivity of the n(+)-Ga2O3/n(+)-poly-SiC bonded substrate and the electrical resistance at the heterointerface were characterized by using periodic heating radiation thermometry and analyzing vertical current-voltage characteristics, respectively. Small thermal and electrical resistances at the bonded interface demonstrated the strong prospects of the bonded substrates for applications to high-power vertical Ga2O3 devices. Published under license by AIP Publishing.

DOI： 10.1063/1.5051720

Publishing type：Research paper (international conference proceedings)  

Electroreflectance (ER) spectra were measured using the Schottky barrier diodes. The ER spectra were well fitted by three components of exciton lineshape functions. The transition energies exhibited moderate changes with the external reverse bias by reflecting the broadening of the absorption edge. The results demonstrate the ER measurement is applicable for Ga2O3 and related alloys as a useful tool to investigate their band structure.

Publishing type：Research paper (international conference proceedings)  

Publishing type：Research paper (international conference proceedings)  

Gallium oxide (Ga2O3) is an emerging ultra-wide-bandgap semiconductor especially suitable for high-power and/or high-voltage switching device applications. In this paper, device fabrication process and characteristics of depletion-mode and enhancement-mode vertical Ga2O3 transistors fabricated by using a multiple ion-implantation doping process are discussed.

Publishing type：Research paper (international conference proceedings)  

Publishing type：Research paper (international conference proceedings)  

Publishing type：Research paper (scientific journal)  

DOI： 10.1002/9781119354987.ch25

Publishing type：Research paper (international conference proceedings)  

Publishing type：Research paper (scientific journal)  

Silicon doped homoepitaxial films were grown on beta-gallium oxide (001) substrates by halide vapor phase epitaxy using gallium monochloride, oxygen and silicon tetrachloride gases as precursors. It was confirmed that the n-type carrier density at room temperature was almost equal to the doped silicon concentration, which was controlled in the range of 10(15) to 10(18) cm(-3). In the doped film with the carrier density of 1x10(16) cm(-3), the activation energy and the mobility at room temperature were 45.6 meV and 145 cm(2)/V.s, respectively. The carrier scattering mechanism in the low carrier density film was dominated by optical phonon scattering with the phonon energy of 33 meV. These results suggest that the doped homoepitaxial film grown by halide vapor phase epitaxy is a high quality film with good crystallinity comparable to bulk crystals.

DOI： 10.1016/j.tsf.2018.09.006

Publishing type：Research paper (scientific journal)  

Comparison between pulsed and CW large signal RF performance of field-plated beta-Ga2O3 MOSFETs has been reported. Reduced self-heating when pulse resulted in a power added efficiency of 12%, drain efficiency of 22.4%, output power density of 0.13 W/mm, and maximum gain up to 4.8 dB at 1 GHz for a 2-mu m gate length device. Increased power dissipation for higher V-DS and I-DS resulted in a degradation in performance, which, thermal simulation showed, could be entirely explained by self-heating. Buffer and surface trapping contributions have been evaluated using gate and drain lag measurements, showing minimal impact on device performance. These results suggest that beta-Ga2O3 is a good candidate for future RF applications.

DOI： 10.1109/LED.2018.2865832

Publishing type：Research paper (scientific journal)  

Deep acceptor doping of beta-Ga2O3 with Mg and N was demonstrated by implantation of the impurity ions into n-type bulk substrates. Systematic physical and electrical characterizations were performed to demonstrate recovery of the implantation-damaged crystals and electrical activation of the dopant atoms by thermal annealing at 1000-1200 degrees C in an N-2 atmosphere. N was found to exhibit much lower thermal diffusivity than Mg, thus enabling the use of higher annealing temperatures to maximize N activation efficiency without significantly altering the impurity profile. Consequently, an n-Ga2O3 /Ga2O3:N/n-Ga2O3 structure was capable of sustaining a much larger voltage across its end terminals than its Mg-doped counterpart. The development of an ion implantation technology for acceptor doping of beta-Ga2O3 creates unique opportunities for designing and engineering a variety of high-voltage beta-Ga(2)O(3 )devices. Published by AIP Publishing.

DOI： 10.1063/1.5050040

Publishing type：Research paper (scientific journal)  

Homoepitaxial growth of β-Ga2O3 layers by halide vapor phase epitaxy (HVPE) using O2 or H2O as an oxygen source was investigated by thermodynamic analysis, and compared with measured properties after growth. The thermodynamic analysis revealed that Ga2O3 growth is expected even at 1000 °C using both oxygen sources due to positive driving forces for Ga2O3 deposition. The experimental results for homoepitaxial growth on (0 0 1) β-Ga2O3 substrates showed that the surfaces of the layers grown with H2O were smoother than those grown with O2, although the growth rate with H2O was approximately half that with O2. However, in the homoepitaxial layer grown using H2O, incorporation of Si impurities with a concentration almost equal to the effective donor concentration (2 × 1016 cm−3) was confirmed, which was caused by decomposition of the quartz glass reactor due to the presence of hydrogen in the system.

DOI： 10.1016/j.jcrysgro.2018.04.009

Publishing type：Research paper (scientific journal)  

We used depth-resolved cathodoluminescence spectroscopy and surface photovoltage spectroscopy to measure the effects of near-surface plasma processing and neutron irradiation on native point defects in β-Ga2O3. The near-surface sensitivity and depth resolution of these optical techniques enabled us to identify spectral changes associated with removing or creating these defects, leading to identification of one oxygen vacancy-related and two gallium vacancy-related energy levels in the β-Ga2O3 bandgap. The combined near-surface detection and processing of Ga2O3 suggests an avenue for identifying the physical nature and reducing the density of native point defects in this and other semiconductors.

DOI： 10.1063/1.5026770

Publishing type：Research paper (scientific journal)  

A vertical β-Ga2O3 metal-oxide-semiconductor field-effect transistor featuring a planar-gate architecture is presented. The device was fabricated by an all-ion-implanted process without requiring trench etching or epitaxial regrowth. A Mg-ion-implanted current blocking layer (CBL) provided electrical isolation between the source and the drain except at an aperture opening through which drain current was conducted. Successful transistor action was realized by gating a Si-ion-implanted channel above the CBL. Thermal diffusion of Mg induced a large source-drain leakage current through the CBL, which resulted in compromised off-state device characteristics as well as a reduced peak extrinsic transconductance compared with the results of simulations.

DOI： 10.7567/APEX.11.064102

Publishing type：Research paper (scientific journal)  

© 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Relation between the electrical and optical properties of p-type NiO films prepared either by RF sputtering or by the molecular precursor method is studied in terms of changes in crystallinity, resistivity ρ and absorption coefficients α. Values of α are correlated with electronic structures near the valence band maximum, which are obtained from the X-ray photoelectron spectroscopy measurements and the density functional theory calculations. The ρ and α values are found to be strongly influenced by the crystallinity as well as the Ni vacancies as acceptor-type intrinsic point defects.

DOI： 10.1002/pssb.201700311

Publishing type：Research paper (scientific journal)  

DOI： 10.1063/1.5017845

Publishing type：Research paper (scientific journal)  

The effects of ionizing radiation on β-Ga2O3 metal-oxide-semiconductor field-effect transistors (MOSFETs) were investigated. A gamma-ray tolerance as high as 1.6 MGy(SiO2) was demonstrated for the bulk Ga2O3 channel by virtue of weak radiation effects on the MOSFETs' output current and threshold voltage. The MOSFETs remained functional with insignificant hysteresis in their transfer characteristics after exposure to the maximum cumulative dose. Despite the intrinsic radiation hardness of Ga2O3, radiation-induced gate leakage and drain current dispersion ascribed respectively to dielectric damage and interface charge trapping were found to limit the overall radiation hardness of these devices.

DOI： 10.1063/1.5017810

Publishing type：Research paper (scientific journal)  

Ultrawide-bandgap (UWBG) semiconductors, with bandgaps significantly wider than the 3.4 eV of GaN, represent an exciting and challenging new area of research in semiconductor materials, physics, devices, and applications. Because many figures-of-merit for device performance scale nonlinearly with bandgap, these semiconductors have long been known to have compelling potential advantages over their narrower-bandgap cousins in high-power and RF electronics, as well as in deep-UV optoelectronics, quantum information, and extreme-environment applications. Only recently, however, have the UWBG semiconductor materials, such as high Al-content AlGaN, diamond and Ga2O3, advanced in maturity to the point where realizing some of their tantalizing advantages is a relatively near-term possibility. In this article, the materials, physics, device and application research opportunities and challenges for advancing their state of the art are surveyed.

DOI： 10.1002/aelm.201600501

Publishing type：Research paper (scientific journal)  

The isotropic average conduction band minimum electron effective mass in Sn-doped monoclinic single crystal β-Ga2O3 is experimentally determined by the mid-infrared optical Hall effect to be (0.284 ± 0.013)m0 combining investigations on (010) and (2-01) surface cuts. This result falls within the broad range of values predicted by theoretical calculations for undoped β-Ga2O3. The result is also comparable to recent density functional calculations using the Gaussian-attenuation-Perdew-Burke-Ernzerhof hybrid density functional, which predict an average effective mass of 0.267m0. Within our uncertainty limits, we detect no anisotropy for the electron effective mass, which is consistent with most previous theoretical calculations. We discuss upper limits for possible anisotropy of the electron effective mass parameter from our experimental uncertainty limits, and we compare our findings with recent theoretical results.

DOI： 10.1063/1.5011192

Publishing type：Research paper (international conference proceedings)  

Gallium oxide (Ga2O3) has emerged as a new competitor to SiC and GaN in the race toward next-generation power switching and harsh environment electronics by virtue of the excellent material properties and the relative ease of mass wafer production. In this proceedings paper, an overview of our recent development progress of Ga2O3 metal-oxide-semiconductor field-effect transistors and Schottky barrier diodes will be reported.

DOI： 10.1117/12.2292666

Publishing type：Research paper (scientific journal)  

DOI： 10.1063/1.5030612

Publishing type：Research paper (international conference proceedings)  

DOI： 10.1002/pssb.201700311

Publishing type：Research paper (scientific journal)  

We employ an eigenpolarization model including the description of direction dependent excitonic effects for rendering critical point structures within the dielectric function tensor of monoclinic beta-Ga2O3 yielding a comprehensive analysis of generalized ellipsometry data obtained from 0.75-9 eV. The eigenpolarization model permits complete description of the dielectric response. We obtain, for single-electron and excitonic band-to-band transitions, anisotropic critical point model parameters including their polarization vectors within the monoclinic lattice. We compare our experimental analysis with results from density functional theory calculations performed using the Gaussian-attenuation-Perdew-Burke-Ernzerhof hybrid density functional. We present and discuss the order of the fundamental direct band-to-band transitions and their polarization selection rules, the electron and hole effective mass parameters for the three lowest band-to-band transitions, and their excitonic contributions. We find that the effective masses for holes are highly anisotropic and correlate with the selection rules for the fundamental band-to-band transitions. The observed transitions are polarized close to the direction of the lowest hole effective mass for the valence band participating in the transition.

DOI： 10.1103/PhysRevB.96.245205

Publishing type：Research paper (scientific journal)  

Gallium oxide (Ga2O3) has gained increased attention for power devices due to its superior material properties and the availability of economical device-quality native substrates. This review illustrates recent advances in Ga2O3 device technologies, beginning with an overview of the social circumstances that motivate the development of new-generation switching devices. Following an introduction to the material properties of Ga2O3 from the viewpoint of power electronics, growth technologies of Ga2O3 bulk single crystals and epitaxial thin films are discussed. The fabrication and performance of state-of-the-art Ga2O3 transistors and diodes are then described. We conclude by identifying the directions and challenges of Ga2O3 power device development in the near future.

DOI： 10.1088/1361-6463/aa7aff

Publishing type：Research paper (scientific journal)  

We developed beta-Ga2O3 trench MOS-type Schottky barrier diodes (MOSSBDs) for the first time. A Si-doped Ga2O3 layer was grown via halide vapor phase epitaxy on a single-crystal Sn-doped beta-Ga2O3 (001) substrate. The trench structure was fabricated using dry etching and photolithography. HfO2 film was used as the dielectric film of the trench MOS structure. The specific on-resistances (R-ON, (SP)) of the normalSBDand trenchMOSSBDwere about 2.3 and 2.9 m Omega cm(2), respectively. The reason the R-ON, (SP) of MOSSBD was a little higher than that of the Schottky barrier diodes (SBD) is that the current path decrease as a result of forming the trench MOS structure. The normal SBD had a large reverse leakage current due to the large electricfield at the anodemetal/semiconductorinterface. On the other hand, the trench MOSSBD had several orders of magnitude smaller leakage current. We, thus, demonstrated that incorporating the trench MOS structure in Ga2O3 is highly effective for decreasing the reverse leakage current.

DOI： 10.1109/LED.2017.2696986

Publishing type：Research paper (scientific journal)  

Enhancement-mode alpha-Ga2O3 metal-oxide-semiconductor field-effect transistors with low series resistance were achieved by Si-ion implantation doping of the source/drain contacts and access regions. An unintentionally doped Ga2O3 channel with low background carrier concentration that was fully depleted at a gate bias of 0 V gave rise to a positive threshold voltage without additional constraints on the channel dimensions or device architecture. Transistors with a channel length of 4 mu m delivered a maximum drain current density (I-DS) of 1.4mA/mm and an IDS on/off ratio near 10(6). Nonidealities associated with the Al2O3 gate dielectric as well as their impact on enhancement-mode device performance are discussed. (C) 2017 The Japan Society of Applied Physics

DOI： 10.7567/APEX.10.041101

Publishing type：Research paper (scientific journal)  

We evaluate and compare the performance and potential of GaAs and of wide and extreme bandgap semiconductors (SiC, GaN, Ga2O3, and diamond), relative to silicon, for power electronics applications. We examine their device structures and associated materials/process technologies and selectively reviewthe recent experimental demonstrations of high voltage power devices and IC structures of these semiconductors. We discuss the technical obstacles that still need to be addressed and overcome before large-scale commercialization commences.

DOI： 10.1109/TED.2017.2653759

Publishing type：Research paper (scientific journal)  

Ga2O3 field-plated Schottky barrier diodes (FP-SBDs) were fabricated on a Si-doped n(-)-Ga2O3 drift layer grown by halide vapor phase epitaxy on a Sn-doped n(+)-Ga2O3 (001) substrate. The specific on-resistance of the Ga2O3 FP-SBD was estimated to be 5.1 m Omega.cm(2). Successful field-plate engineering resulted in a high breakdown voltage of 1076 V. A larger-than-expected effective barrier height of 1.46 eV, which was extracted from the temperature-dependent current-voltage characteristics, could be caused by the effect of fluorine atoms delivered in a hydrofluoric acid solution process.

DOI： 10.1063/1.4977857

Publishing type：Research paper (international conference proceedings)  

Publishing type：Research paper (international conference proceedings)  

DOI： 10.1109/DRC.2017.7999413

Publishing type：Research paper (scientific journal)  

Anisotropic optical properties are investigated on beta-Ga2O3 films and a single crystal by spectroscopic ellipsometry measurements. The (2011) films grown on (0001) alpha-Al2O3 contain threefold in-plane rotational domains, and the refractive index and absorption coefficient a obtained by assuming an isotropic material are found to be smaller than those in the single crystal. By measuring the off-normal transmission ellipsometry spectra of the (010) beta-Ga2O3 substrate, the optical anisotropy in a biaxial crystal as well as the gradual increase in a are recognized as origins of the scattering in optically determined bandgap energies. (C) 2016 The Japan Society of Applied Physics

DOI： 10.7567/JJAP.55.1202B2

Publishing type：Research paper (scientific journal)  

Gallium oxide (Ga2O3) is an emerging wide-bandgap semiconductor for high-power, low-loss transistors and diodes by virtue of its excellent material properties and suitability for mass production. In this paper, we begin by discussing the material properties of Ga2O3 that make it an attractive alternative to not only Si but also other wide-bandgap materials such as SiC and GaN. State-of-the-art Ga2O3-based devices that have been fabricated to date demonstrate the performance potential for power electronics applications. (C) 2016 The Japan Society of Applied Physics

DOI： 10.7567/JJAP.55.1202A1

Publishing type：Research paper (scientific journal)  

DOI： 10.7567/JJAP.55.120201

Publishing type：Research paper (scientific journal)  

Al2O3 films were deposited on beta-Ga2O3(010) and beta-Ga2O3(201) substrates by atomic layer deposition at 250 degrees C, and their interface state densities (D-it) at shallow energies were evaluated using a high-low capacitance-voltage (C-V) method. Al2O3/beta-Ga2O3(010) showed lower D-it values (5.9 x 10(10) to 9.3 x 10(11)cm(-2)eV(-1)) than Al2O3/beta-Ga2O3(201) (2.0 x 10(11) to 2.0 x 10(12)cm(-2)eV(-1)) in an energy range of -0.8 to -0.1 eV. Cross-sectional transmission electron microscopy analysis indicated the formation of a uniform amorphous Al2O3 layer on the beta-Ga2O3(201) substrate. In contrast, a crystalline Al2O3 interlayer with a thickness of 3.2 +/- 0.7nm with an amorphous Al2O3 top layer was formed on the beta-Ga2O3(010) substrate, which effectively decreased D-it. Moreover, thicker interlayers showing lower D-it values at deep state levels were formed at deposition temperatures higher than 100 degrees C, which were evaluated by shifts in the C-V curves. (C) 2016 The Japan Society of Applied Physics

DOI： 10.7567/JJAP.55.1202B5

Publishing type：Research paper (scientific journal)  

Electron paramagnetic resonance was used to study the donor that is responsible for the n-type conductivity in unintentionally doped (UID) beta-Ga2O3 substrates. We show that in as-grown materials, the donor requires high tempeature annealing to be activated. In partly activated materials with the donor concentration in the 10(16) cm(-3) range or lower, the donor is found to behave as a negative-U center (often called a DX center) with the negative charge state DX- lying similar to 16-20 meV below the neutral charge state d(0) (or E-d), which is estimated to be similar to 28-29 meV below the conduction band minimum. This corresponds to a donor activation energy of E-a similar to 44-49 meV. In fully activated materials with the donor spin density close to similar to 1 x 10(18) cm(-3), donor electrons become delocalized, leading to the formation of impurity bands, which reduces the donor activation energy to E-a similar to 15-17 meV. The results clarify the electronic structure of the dominant donor in UID beta-Ga2O3 and explain the large variation in the previously reported donor activation energy. Published by AIP Publishing.

DOI： 10.1063/1.4972040

Publishing type：Research paper (scientific journal)  

The electron mobility in depletion-mode lateral beta-Ga2O3(010) metal-oxide-semiconductor field-effect transistors (MOSFETs) with an n-channel formed by Si-ion (Si+) implantation doping was extracted using low-field electrical measurements on FET structures. An undoped Ga2O3 buffer layer protected the channel against charge compensation by suppressing outdiffusion of deep Fe acceptors from the semi-insulating substrate. The molecular beam epitaxy growth temperature was identified as a key process parameter for eliminating parasitic conduction at the buffer/substrate growth interface. Devices with a resistive buffer showed room temperature channel mobilities of 90-100cm(2)V(-1) s(-1) at carrier concentrations of low-to mid-10(17)cm(-3), with small in-plane mobility anisotropy of 10-15% ascribable to anisotropic carrier scattering. (C) 2016 The Japan Society of Applied Physics

DOI： 10.7567/JJAP.55.1202B9

Publishing type：Research paper (scientific journal)  

The channel temperature (T-ch) and thermal resistance (R-th) of Ga2O3 metal-oxide-semiconductor field-effect transistors were investigated through electrical measurements complemented by electrothermal device simulations that incorporated experimental Ga2O3 thermal parameters. The analysis technique was based on a comparison between DC and pulsed drain currents (I-DS) at known applied biases, where negligible self-heating under pulsed conditions enabled approximation of T-ch to the ambient temperature (T-amb) and hence correlation of I-DS to T-ch. Validation of the device model was achieved through calibration against the DC data. The experimental T-ch was in good agreement with simulations for T-amb between 20 degrees C and 175 degrees C. A large R-th of 48 mm.K/W thus extracted at room temperature highlights the value of thermal analysis for understanding the degradation mechanisms and improving the reliability of Ga2O3 power devices. Published by AIP Publishing.

DOI： 10.1063/1.4966999

Al<inf>2</inf>O<inf>3</inf>films were deposited on β-Ga<inf>2</inf>O<inf>3</inf>(010) and β-Ga<inf>2</inf>O<inf>3</inf>[Formula: see text] substrates by atomic layer deposition at 250 °C, and their interface state densities (D<inf>it</inf>) at shallow energies were evaluated using a high–low capacitance–voltage (C–V) method. Al<inf>2</inf>O<inf>3</inf>/β-Ga<inf>2</inf>O<inf>3</inf>(010) showed lower D<inf>it</inf>values (5.9 × 10¹⁰to 9.3 × 10¹¹cm⁻²eV⁻¹) than Al<inf>2</inf>O<inf>3</inf>/β-Ga<inf>2</inf>O<inf>3</inf>[Formula: see text] (2.0 × 10¹¹to 2.0 × 10¹²cm⁻²eV⁻¹) in an energy range of −0.8 to −0.1 eV. Cross-sectional transmission electron microscopy analysis indicated the formation of a uniform amorphous Al<inf>2</inf>O<inf>3</inf>layer on the β-Ga<inf>2</inf>O<inf>3</inf>[Formula: see text] substrate. In contrast, a crystalline Al<inf>2</inf>O<inf>3</inf>interlayer with a thickness of 3.2 ± 0.7 nm with an amorphous Al<inf>2</inf>O<inf>3</inf>top layer was formed on the β-Ga<inf>2</inf>O<inf>3</inf>(010) substrate, which effectively decreased D<inf>it</inf>. Moreover, thicker interlayers showing lower D<inf>it</inf>values at deep state levels were formed at deposition temperatures higher than 100 °C, which were evaluated by shifts in the C–V curves.

DOI： 10.7567/JJAP.55.1202B5

CiNii Article

Publishing type：Research paper (scientific journal)  

The question of optical bandgap anisotropy in the monoclinic semiconductor beta-Ga2O3 was revisited by combining accurate optical absorption measurements with theoretical analysis, performed using different advanced computation methods. As expected, the bandgap edge of bulk beta-Ga2O3 was found to be a function of light polarization and crystal orientation, with the lowest onset occurring at polarization in the ac crystal plane around 4.5-4.6 eV; polarization along b unambiguously shifts the onset up by 0.2 eV. The theoretical analysis clearly indicates that the shift in the b onset is due to a suppression of the transition matrix elements of the three top valence bands at Gamma point.

DOI： 10.1088/0953-8984/28/22/224005

Publishing type：Research paper (scientific journal)  

We evaluated the band offsets at the SiO2/-Ga2O3 (010) interface by X-ray photoelectron spectroscopy (XPS). Plasma chemical vapor deposition with a liquid tetraethyl orthosilicate source for SiO2 was used to prepare an SiO2(40nm)/Ga2O3 sample and an SiO2(3nm)/Ga2O3 sample for XPS analyses. The bandgap of SiO2 was determined to be 8.7 +/- 0.2eV. A large conduction band offset of 3.1 +/- 0.2eV and a corresponding valence band offset of 1.0 +/- 0.2eV were determined for the SiO2/Ga2O3 interface. These results suggest that an SiO2 gate insulator is favorable for Ga2O3 field effect transistors operating under high temperatures.

DOI： 10.1002/pssb.201552519

Publishing type：Research paper (scientific journal)  

This is a review article on the current status and future prospects of the research and development on gallium oxide (Ga2O3) power devices. Ga2O3 possesses excellent material properties, in particular for power device applications. It is also attractive from an industrial viewpoint since large-size, high-quality wafers can be manufactured from a single-crystal bulk synthesized by melt-growth methods. These two features have drawn much attention to Ga2O3 as a new wide bandgap semiconductor following SiC and GaN. In this review, we describe the recent progress in the research and development on fundamental technologies of Ga2O3 devices, covering single-crystal bulk and wafer production, homoepitaxial thin film growth by molecular beam epitaxy and halide vapor phase epitaxy, as well as device processing and characterization of metal-semiconductor field-effect transistors, metal-oxide-semiconductor field-effect transistors and Schottky barrier diodes.

DOI： 10.1088/0268-1242/31/3/034001

Publishing type：Research paper (scientific journal)  

Temperature-dependent exciton resonance energies E-exciton in beta-Ga2O3 single crystals are studied by using polarized reflectance measurement. The E-exciton values exhibit large energy changes in the range of 179-268 meV from 5 to 300 K. The IR-active A(u) and B-u optical phonon modes are selectively observed in the IR spectroscopic ellipsometry spectra by reflecting the polarization selection rules. The longitudinal optical (LO) phonon energies can be divided into three ranges: (h) over bar omega(LO) = 35-48, 70-73, and 88-99 meV. The broadening parameters, which are obtained from the reflectance measurements, correspond to the lower two ranges of (h) over bar omega(LO) at low temperature and 75 meV above 150 K. The large E-exciton changes with temperature in beta-Ga2O3 are found to be originated from the exciton-LO-phonon interaction. (C) 2016 AIP Publishing LLC.

DOI： 10.1063/1.4943175

Publishing type：Research paper (scientific journal)  

We derive a dielectric function tensor model approach to render the optical response of monoclinic and triclinic symmetry materials with multiple uncoupled infrared and far-infrared active modes. We apply our model approach to monoclinic beta-Ga2O3 single-crystal samples. Surfaces cut under different angles from a bulk crystal, (010) and ((2) over bar 01), are investigated by generalized spectroscopic ellipsometry within infrared and far-infrared spectral regions. We determine the frequency dependence of 4 independent beta-Ga2O3 Cartesian dielectric function tensor elements by matching large sets of experimental data using a point-by-point data inversion approach. From matching our monoclinic model to the obtained 4 dielectric function tensor components, we determine all infrared and far-infrared active transverse optic phonon modes with A(u) and B-u symmetry, and their eigenvectors within the monoclinic lattice. We find excellent agreement between our model results and results of density functional theory calculations. We derive and discuss the frequencies of longitudinal optical phonons in beta-Ga2O3. We derive and report density and anisotropic mobility parameters of the free charge carriers within the tin-doped crystals. We discuss the occurrence of longitudinal phonon plasmon coupled modes in beta-Ga2O3 and provide their frequencies and eigenvectors. We also discuss and present monoclinic dielectric constants for static electric fields and frequencies above the reststrahlen range, and we provide a generalization of the Lyddane-Sachs-Teller relation for monoclinic lattices with infrared and far-infrared active modes. We find that the generalized Lyddane-Sachs-Teller relation is fulfilled excellently for beta-Ga2O3.

DOI： 10.1103/PhysRevB.93.125209

Publishing type：Research paper (scientific journal)  

We investigated the temperature-dependent electrical properties of Pt/Ga2O3 Schottky barrier diodes (SBDs) fabricated on n(-)-Ga2O3 drift layers grown on single-crystal n(+)-Ga2O3 (001) substrates by halide vapor phase epitaxy. In an operating temperature range from 21 degrees C to 200 degrees C, the Pt/Ga2O3 (001) Schottky contact exhibited a zero-bias barrier height of 1.09-1.15 eV with a constant near-unity ideality factor. The current-voltage characteristics of the SBDs were well-modeled by thermionic emission in the forward regime and thermionic field emission in the reverse regime over the entire temperature range. (C) 2016 AIP Publishing LLC.

DOI： 10.1063/1.4945267

Publishing type：Research paper (scientific journal)  

Depletion-mode field-plated Ga2O3 metal-oxide-semiconductor field-effect transistors were demonstrated for the first time. Substantial enhancement in breakdown voltage was achieved with a gate-connected field plate. The device channels, formed by selective-area Si ion implantation doping of an undoped Ga2O3 epilayer, were electrically isolated by the highly resistive epilayer without mesa etching. Effective surface passivation and high Ga2O3 material quality contributed to the absence of drain current collapse. The transistors exhibited an off-state breakdown voltage of 755 V, a high drain current on/off ratio of over 10(9), and stable high temperature operation against 300 degrees C thermal stress.

DOI： 10.1109/LED.2015.2512279

Publishing type：Research paper (international conference proceedings)  

Publishing type：Research paper (scientific journal)  

Publishing type：Research paper (scientific journal)  

The new wide-bandgap oxide semiconductor, gallium oxide (Ga2O3), has gained attraction as a promising candidate for power device applications because of its excellent material properties and suitability for mab production. Fundamental halide vapor phase epitaxy (HVPE) technology for Ga2O3thin film growth has been developed for development of vertical Ga2O3devices. Unintentionally-doped Ga2O3layers grown by HVPE were high purity and high crystalline quality, and an electron density in the Ga2O3film can be precisely controlled by Si doping in the wide range of n=1015- 1018 cm-3. We fabricated vertical Ga2O3Schottky barrier diodes (SBDs) with HVPE-grown n-Ga2O3drift layers on single-crystal β-Ga2O3(001) substrates. The SBDs demonstrated promising device performance as next-generation power devices, such as the specific on-resistances of about 3.0 mωcm2, the ideality factors of almost equal to unity, and the off-state breakdown voltage of about -500 V.

DOI： 10.1541/ieejeiss.136.479

Publishing type：Research paper (scientific journal)  

The polarized transmittance and reflectance spectra of beta-Ga2O3 crystals are investigated, and the data are interpreted in terms of the monoclinic crystal band structure. The energies of the absorption edge can be divided into six ranges, and these ranges can be assigned to the transitions from the valence bands to the conduction band minimum according to the selection rules. The indirect bandgap-energy of 4.43 eV is smaller than the direct bandgap-energy of 4.48 eV at RT; and the energy difference of 0.05 eV nearly matches the theoretically calculated values of 0.03-0.04 eV. (C) 2015 The Japan Society of Applied Physics

DOI： 10.7567/JJAP.54.112601

Publishing type：Research paper (scientific journal)  

The effects of AlOx formation on the emission properties of AlN/GaN heterostructures are comparatively investigated using steady-state and time-resolved photoluminescence (PL) measurements. By analyzing the excitation power and temperature-dependent PL spectra, PL at around 3.45-3.46 eV is attributed to the two-dimensional electron-gas (2DEG)-related emission. Simultaneous changes in the emission energies and lifetimes are observed by forming a crystallized AlOx layer. The results demonstrate a possibility of controlling the surface states by changing the surface oxide structure. (C) 2015 The Japan Society of Applied Physics

DOI： 10.7567/APEX.8.052401

J-GLOBAL

Publishing type：Research paper (scientific journal)  

The thermal conductivities of beta-Ga2O3 single crystals along four different crystal directions were measured in the temperature range of 80-495K using the time domain thermoreflectance method. A large anisotropy was found. At room temperature, the [010] direction has the highest thermal conductivity of 27.0 +/- 2.0 W/mK, while that along the [100] direction has the lowest value of 10.9 +/- 1.0 W/mK. At high temperatures, the thermal conductivity follows a similar to 1/T relationship characteristic of Umklapp phonon scattering, indicating phonon-dominated heat transport in the beta-Ga2O3 crystal. The measured experimental thermal conductivity is supported by first-principles calculations, which suggest that the anisotropy in thermal conductivity is due to the differences of the speed of sound along different crystal directions. (C) 2015 AIP Publishing LLC.

DOI： 10.1063/1.4916078

Publishing type：Research paper (scientific journal)  

The thermal behavior of Fe as a compensating acceptor impurity in beta-Ga2O3 (010) was studied in view of growing interests in semi-insulating Fe-doped Ga2O3 substrates for the realization of high-performance Ga2O3 field-effect transistors (FETs). An anomalous redistribution of Fe beyond the extent of intrinsic thermal diffusion was revealed by secondary ion mass spectroscopy in device-relevant structures where Ga2O3 grown homoepitaxially on Fe-doped substrates was doped by Si ion (Si+) implantation and annealed at high temperatures. The enhanced Fe diffusion was attributed to an athermal process involving intrinsic defects from the region of implantation damage. An undoped Ga2O3 buffer between the Si+-implanted layer and the Fe-doped substrate effectively suppressed Fe outdiffusion by protecting the substrate against unintentional ion damage or defects from a remote source, thereby preserving the electrical integrity of the Si-doped material. Temperature-dependent current-voltage measurements indicated that the undoped Ga2O3 buffer was highly resistive with inter-device leakage attributable to surface conduction via a variable-range hopping mechanism. The buffer scheme, together with dielectric passivation to eliminate surface leakage, was proposed to constitute an integral process module for future lateral Ga2O3 FET devices. (C) 2015 AIP Publishing LLC.

DOI： 10.1063/1.4906375

J-GLOBAL

Publishing type：Research paper (scientific journal)  

Thick high-purity beta-Ga2O3 layers of high crystalline quality were grown homoepitaxially by halide vapor phase epitaxy (HVPE) using gaseous GaCl and O-2 on (001) beta-Ga2O3 substrates prepared by edge-defined film-fed growth. The surface morphology and structural quality of the grown layer improved with increasing growth temperature. X-ray diffraction omega-rocking curves for the (002) and (400) reflections for the layer grown at 1000 degrees C had small full widths at half maximum. Secondary ion mass spectrometry and electrical characteristics revealed that the growth of high-purity beta-Ga2O3 layers with low effective donor concentration (N-d - N-a &lt; 10(13) cm(-3)) is possible by HVPE. (C) 2015 The Japan Society of Applied Physics

DOI： 10.7567/APEX.8.015503

J-GLOBAL

Publishing type：Research paper (international conference proceedings)  

Gallium oxide (Ga2O3) possesses excellent material properties especially for power device applications. It is also attractive from an industrial viewpoint since large -size, high quality wafers can be manufactured by using simple methods. These two features have drawn much attention to Ga2O3 as a new wide bandgap semiconductor following SiC and GaN. In this report, we describe the recent progress in development on fundamental technologies for Ga2O3 devices, covering wafer production from melt -grown bulk single crystals, homoepitaxial thin-film growth by halide vapor phase epitaxy, as well as device processing and characterization of metal -oxide -semiconductor field-effect transistors and Schottky barrier diodes.

DOI： 10.1109/CSICS.2015.7314495

Publishing type：Research paper (international conference proceedings)  

Publishing type：Research paper (international conference proceedings)  

In order to estimate carrier density on wide bandgap semiconductor beta-Ga2O3, we made a measurement of THz reflectance spectra of beta-Ga2O3 samples by using a THz time-domain spectroscopic method. The tails of reflectance structures were shifted to higher energy side with increasing the carrier density. We treated the structure was caused by plasmon, and calculated the carrier density of each samples. The carrier densities obtained by the THz reflection measurements showed a good agreement with the results by Hall-effect measurements. These results indicated that THz spectroscopy is a useful method to estimate the carrier density of beta-Ga2O3 samples.

Publishing type：Research paper (scientific journal)  

We report successful growth of a crystallized AlOx layer on top of AlN/GaN heterostructures by using RF-plasma molecular-beam epitaxy for exploring a new-type oxide/nitride heterostructure system. The insertion of an AlOx, buffer layer, which was formed by following three steps of (i) an Al metal deposition at 150 degrees C, (ii) an oxidation of the Al metal by oxygen plasma irradiation, and (iii) an annealing of the oxidized layer at 800 degrees C, facilitated the formation of a crystalline AlOx, layer on top of the AlN/GaN structures. Surface morphologies observed by atomic force microscope showed that the AlOx, buffer layer was directly formed on the nitride structure and fully covered the AlN layer. The AlOx, top layer grown on the buffer layer had a flat and smooth surface. A cross-sectional transmission electron microscopy micrograph revealed that the AlOx thin film grown at 800 degrees C on the nitride structure was fully crystallized. (C) 2014 Elsevier B.V. All rights reservecl.

DOI： 10.1016/j.jcrysgro.2014.07.055

J-GLOBAL

Publishing type：Research paper (scientific journal)  

beta-Ga2O3(010) homo-epitaxial growth was performed by plasma-assisted molecular beam epitaxy. Under Ga-rich conditions and for growth temperatures above 650 degrees C, the growth rate was independent of the Ga/O ratio (&gt;1). A high growth rate of 2.2 nm/min for beta-Ga2O3(010) was achieved by optimizing the O flux between 650 and 750 degrees C. Under Ga-rich conditions between growth temperatures of 500-900 degrees C, smooth surfaces with rms roughness below 1 nm were realized. We found that the slightly Ga-rich conditions between 650 and 750 degrees C were optimal for beta-Ga2O3(010) growth with a smooth surface and a high growth rate. (C) 2014 The Japan Society of Applied Physics

DOI： 10.7567/APEX.7.095501

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Polarized Raman spectra were measured from (010) Mg-doped, (100) Si-doped, and (001) unintentionally doped beta-Ga2O3 substrates prepared by either the floating zone growth or edge-defined film-fed growth methods. The A(g) and B-g Raman active modes were perfectly separated in the spectra according to the polarization selection rules. To the best of our knowledge, this is the first experimental observation of a complete set of polarized Raman spectra of beta-Ga2O3. The results are ensured by the high uniformity of crystalline orientation and surface flatness of the present substrates. (C) 2014 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.jcrysgro.2013.12.061

Publishing type：Research paper (scientific journal)  

The band alignment of Al2O3/n-Ga2O3 was investigated by x-ray photoelectron spectroscopy (XPS). With a band gap of 6.8+/-0.2 eV measured for Al2O3, the conduction and valence band offsets at the interface were estimated to be 1.5+/-0.2 eV and 0.7+/-0.2 eV, respectively. The conduction band offset was also obtained from tunneling current in Al2O3/n-Ga2O3 ((2) over bar 01) metal-oxide-semiconductor (MOS) diodes using the Fowler-Nordheim model. The electrically extracted value was in good agreement with the XPS data. Furthermore, the MOS diodes exhibited small capacitance-voltage hysteresis loops, indicating the successful engineering of a high-quality Al2O3/Ga2O3 interface. (C) 2014 AIP Publishing LLC.

DOI： 10.1063/1.4876920

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We investigated the growth temperature dependence of the structural and electrical properties of Sn-doped Ca2O3 homoepitaxial films grown on single crystal beta-Ga2O3 (010) substrates by molecular beam epitaxy. Ca2O3 films with an atomically smooth surface were obtained at growth temperatures of 550-650 degrees C. On the other hand, a delay in the incorporation of Sn atoms in Ca2O3, which was probably due to segregation, occurred in the initial stage of growth at higher than 600 degrees C. To ensure that Sn-doped Ca2O3 films with both high crystal quality and accurately controlled carrier density are obtained, the optimum growth temperature should be set at 540-570 degrees C. (c) 2014 Elsevier EN. All rights reserved

DOI： 10.1016/j.jcrysgro.2014.02.002

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Publishing type：Research paper (scientific journal)  

Gallium oxide (Ga2O3) is a strong contender for power electronic devices. The material possesses excellent properties such as a large bandgap of 4.7-4.9eV for a high breakdown field of 8MVcm(-1). Low cost, high volume production of large single-crystal -Ga2O3 substrates can be realized by melt-growth methods commonly adopted in the industry. High-quality n-type Ga2O3 epitaxial thin films with controllable carrier densities were obtained by ozone molecular beam epitaxy (MBE). We fabricated Ga2O3 metal-semiconductor field-effect transistors (MESFETs) and Schottky barrier diodes (SBDs) from single-crystal Ga2O3 substrates and MBE-grown epitaxial wafers. The MESFETs delivered excellent device performance including an off-state breakdown voltage (V-br) of over 250V, a low leakage current of only few Amm(-1), and a high drain current on/off ratio of about four orders of magnitude. The SBDs also showed good characteristics such as near-unity ideality factors and high reverse V-br. These results indicate that Ga2O3 can potentially meet or even exceed the performance of Si and typical widegap semiconductors such as SiC or GaN for ultrahigh-voltage power switching applications. (C) 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

DOI： 10.1002/pssa.201330197

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Gallium oxide (Ga₂O₃) has excellent material properties for power device applications represented by the extremely large breakdown field due to a large bandgap of 4.7-4.9 eV. Another important feature in industry is that large single-crystal <i>β</i>-Ga₂O₃ bulks and wafers can be fabricated with melt-growth methods. We recently succeeded in fabricating Ga₂O₃ metal-oxide-semiconductor field-effect transistors (MOSFETs) by using newly developed technologies for making single-crystal bulks, growing conductivity-controlled epitaxial films, and fabricating devices. The MOSFETs exhibited excellent device characteristics including an off-state breakdown voltage over 370 V, an extremely low leakage current, and a high on/off drain current ratio of more than ten orders of magnitude. These results indicate that Ga₂O₃ have comparable or even more potential than Si and typical widegap semiconductors SiC and GaN for power device applications.

DOI： 10.1380/jsssj.35.102

CiNii Article

Other URL： https://jlc.jst.go.jp/DN/JALC/10028793754?from=CiNii

Publishing type：Research paper (scientific journal)  

Publishing type：Research paper (scientific journal)  

Single-crystal gallium oxide (Ga2O3) metal-oxide-semiconductor field-effect transistors were fabricated on a semi-insulating β-Ga2O3 (010) substrate. A Sn-doped n-Ga2O3 channel layer was grown by molecular-beam epitaxy. Si-ion implantation doping was performed to source and drain electrode regions for obtaining low-resistance ohmic contacts. An Al2O3 gate dielectric film formed by atomic layer deposition passivated the device surface and significantly reduced gate leakage. The device with a gate length of 2 μm showed effective gate modulation of the drain current with an extremely low off-state drain leakage of less than a few pA/mm, leading to a high drain current on/off ratio of over ten orders of magnitude. A three-terminal off-state breakdown voltage of 370 V was achieved. Stable transistor operation was sustained at temperatures up to 250 ° C. © 2013 AIP Publishing LLC.

DOI： 10.1063/1.4821858

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We developed a donor doping technique for beta-Ga2O3 by using Si-ion (Si+) implantation. For the implanted Ga2O3 substrates with Si+ = 1 x 10(19) -5 x 10(19) cm(-3), a high activation efficiency of above 60% was obtained after annealing in a nitrogen gas atmosphere at a relatively low temperature of 900-1000 degrees C. Annealed Ti/Au electrodes fabricated on the implanted Ga2O3 layers showed ohmic behavior. The Ga2O3 with Si+ = 5 x 10(19) cm(-3) showed the lowest specific contact resistance and resistivity obtained in this work of 4.6 x 10(-6) Omega.cm(2) and 1.4m Omega.cm, respectively. (C) 2013 The Japan Society of Applied Physics

DOI： 10.7567/APEX.6.086502

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Publishing type：Research paper (scientific journal)  

Temperature-dependent cathodoluminescence spectra were measured from (001) unintentionally doped, (100) Si-doped, and (010) Mg-doped beta-Ga2O3 substrates prepared by either the floating zone growth or edge-defined film-fed growth methods. Although beta-Ga2O3 is expected to be an indirect bandgap material, direct Gamma-Gamma transitions were found to be dominant in the optical transmittance spectra. The substrates exhibited no near-band-edge emission and instead exhibited ultraviolet luminescence, blue luminescence (BL), and green luminescence bands. Since the BL intensity strongly depended on the resistivity in the crystals, there was evidence of a correlation between the BL intensity and formation energy of oxygen vacancies. (C) 2013 AIP Publishing LLC.

DOI： 10.1063/1.4816759

Publishing type：Research paper (scientific journal)  

We fabricated gallium oxide (Ga2O3) Schottky barrier diodes using beta-Ga2O3 single-crystal substrates produced by the floating-zone method. The crystal quality of the substrates was excellent; the X-ray diffraction rocking curve peak had a full width at half-maximum of 32 arcsec, and the etch pit density was less than 1 x 10(4) cm(-2). The devices exhibited good characteristics, such as an ideality factor close to unity and a reasonably high reverse breakdown voltage of about 150 V. The Schottky barrier height of the Pt/beta-Ga2O3 interface was estimated to be 1.3-1.5 eV.

DOI： 10.1109/LED.2013.2244057

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Publishing type：Research paper (international conference proceedings)  

New widegap compound semiconductor gallium oxide (Ga2O3) power devices have been attracted as strong candidates for future power electronics. Here, we will introduce material properties, productivity of Ga2O3, and current status of research and development on its transistors and diodes.

Publishing type：Research paper (scientific journal)  

N-type gallium oxide (Ga2O3) homoepitaxial thick films were grown on β-Ga2O3 (010) substrates by molecular beam epitaxy. The epitaxial growth rate was increased by more than 10 times by changing from the (100) plane to the (010) plane. The carrier concentration of the epitaxial layers could be varied within the range of 1016-1019 cm-3 by changing the Sn doping concentration. Schottky barrier diodes (SBDs) and metal-semiconductor field-effect transistors (MESFETs) on β-Ga2O3 homoepitaxial layers were demonstrated for the first time. The SBDs exhibited good device characteristics such as an ideality factor of 1.13, and high breakdown voltage about 125 V. The MESFETs also exhibited excellent characteristics such as a perfect pinch-off of the drain current, off-state breakdown voltage over 250 V, high on/off drain current ratio of around 10 4, and small gate leakage current. These device characteristics clearly indicate the great potential of Ga2O3 as a high-power device material. © 2013 Elsevier B.V All rights reserved.

DOI： 10.1016/j.jcrysgro.2013.02.015

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Publishing type：Research paper (international conference proceedings)  

Depletion-mode Ga2O3 MOSFETs were fabricated on single-crystal beta-Ga2O3 (010) substrates. We applied Si-ion implantation to the ohmic contacts for low contact resistance as well as to the channel layer for reliable doping. An Al2O3 gate dielectric film formed by atomic layer deposition passivated the device surface and significantly reduced gate leakage. The MOSFETs with a simple structure exhibited complete channel pinch-off with a breakdown voltage of 415 V and a drain current on/off ratio of ten orders of magnitude at room temperature. The devices also showed stable operation up to 250 degrees C.

DOI： 10.1109/IEDM.2013.6724713

Publishing type：Research paper (international conference proceedings)  

Publishing type：Research paper (scientific journal)  

We report a demonstration of single-crystal gallium oxide (Ga2O3) metal-semiconductor field-effect transistors (MESFETs). A Sn-doped Ga2O3 layer was grown on a semi-insulating beta-Ga2O3 (010) substrate by molecular-beam epitaxy. We fabricated a circular MESFET with a gate length of 4 mu m and a source-drain spacing of 20 mu m. The device showed an ideal transistor action represented by the drain current modulation due to the gate voltage (V-GS) swing. A complete drain current pinch-off characteristic was also obtained for VGS &lt; -20 V, and the three-terminal off-state breakdown voltage was over 250 V. A low drain leakage current of 3 mu A at the off-state led to a high on/off drain current ratio of about 10000. These device characteristics obtained at the early stage indicate the great potential of Ga2O3-based electrical devices for future power device applications. (C) 2012 American Institute of Physics. [doi:10.1063/1.3674287]

DOI： 10.1063/1.3674287

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Publishing type：Research paper (international conference proceedings)  

β-Ga2O3 has an extremely large bandgap of 4.8-4.9 eV and excellent material properties for power device applications, as summarized in Tab. 1. The breakdown field of β-Ga2O3 is estimated to be 8 MV/cm, from the relation between the band gaps and breakdown fields of other major semiconductors. Another important feature is that large single-crystal β-Ga2O3 substrates can be fabricated with melt-growth methods such as floating-zone (FZ) and edge-defined film-fed growth (EFG). Figure 1 shows a photograph of a 2-inch-diameter single-crystal β-Ga2O3 wafer fabricated by the EFG method. The wafer size depends on the production machine size. Therefore, we consider that Ga2O3 is a very attractive new semiconductor for power device applications because of its material properties and ease of mass production. However, it has not been studied to any extent up to now. © 2012 IEEE.

DOI： 10.1109/DRC.2012.6257021

Publishing type：Research paper (scientific journal)  

A two-stage distributed amplifier monolithic microwave integrated circuit (MMIC) has been designed and fabricated using dual-gate GaN HEMTs. The measured small-signal gain of the MMIC is about 20 dB over the frequency range of 2-18 GHz. Measured peak saturated output power is about 2 W. A low interstage impedance of 25 is chosen for two reasons. It leads to larger size of the HEMTs in the output stage, and hence, increases output power without reducing the bandwidth. It also keeps the inter-stage transmission lines short, and hence, results in a very compact two-stage design with high gain. To enhance the output power further, capacitive division technique is used in the second stage. Dual-gate HEMTs are used, as they are compact and provide superior performance when compared to standard HEMTs.

DOI： 10.1109/TMTT.2011.2144996

CiNii Article

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Publishing type：Research paper (scientific journal)  

Short-gate AlGaN/GaN heterostructure field-effect transistors (HFETs) with extremely thin AlGaN barrier layers were fabricated and characterized from the viewpoint of millimeter-wave applications. The devices showed good direct-current and small-signal characteristics; however, the 30-GHz power characteristics were degraded due to frequency dispersion caused by the SiN(x)/AlGaN interface states. The dispersive behavior of the thin barrier devices measured in pulse I-V curves was different from the commonly observed one for the devices with normal AlGaN barrier thicknesses of 20-30 nm. The first characteristic point was that it happened only for structures with extremely thin barrier layers. As another unique point, the drain current collapse in the pulsed modes was observed only at a positive gate bias. We consider that the unique dispersion of the thin barrier HFETs was caused by the different charging paths related to hot electrons accelerated in a high electric field region of a 2-D electron gas (2DEG) channel. It seems reasonable to suppose that, for the extremely thin barrier structures, the AlGaN surface states can be charged not only by injected electrons from the gate metal through the SiN(x)/AlGaN interface but also by the hot electrons overcoming the barrier from the 2DEG formed at the AlGaN/GaN interface.

DOI： 10.1109/TED.2011.2131653

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Surface donor states with distributed and finite density are implemented in Schrodinger-Poisson simulations of AlGaN/GaN high electron mobility transistors, with the goal of studying their effects on the two-dimensional electron gas. Our recent experimental observations of an increasing surface barrier height with increasing AlGaN thickness are fitted very well by simulations including surface donor levels represented by a constant density of states (DOS) with a density on the order of 10(13) cm(-2) eV(-1). The highest occupied surface states are found to be around 1 eV below the conduction-band minimum, considerably higher in energy than previously reported single surface donor levels. These trends can be explained by the features of oxidized AlGaN surfaces. Furthermore, the surface DOS that fit the experimental results are found to be larger for samples with higher Al concentration.

DOI： 10.1088/0022-3727/43/50/505501

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Publishing type：Research paper (scientific journal)  

The effects of oxidation on the surface structure and chemical bonding states of AlGaN/GaN heterostructures were investigated using x-ray photoelectron spectroscopy (XPS). In comparing Al 2p core-level XPS spectra among as-grown and annealed samples, we found that Al atoms on the surface were highly oxidized after rapid thermal annealing (RTA) at high temperature; not only in an O(2) but also in an N(2) gas atmosphere. The Al oxidation level was almost identical for the samples annealed at 800 degrees C, irrespective of the annealing atmosphere and time; yet there was a strong dependence on the annealing temperature. The dependence of surface barrier height on the annealing condition is associated with Al oxidation behavior. Before the RTA, the barrier height increased together with the AlGaN thickness, indicating an unpinned Fermi level and the existence of low-density and distributed surface donor states. After the high-temperature RTA, however, the height is maintained at a certain value, regardless of the thickness, due to Fermi level pinning by high-density donor states. These results can be explained by the formation of two types of oxide structures providing different types of donor states. (C) 2010 American Institute of Physics. [doi:10.1063/1.3522649]

DOI： 10.1063/1.3522649

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Publishing type：Research paper (scientific journal)  

The dependence of electron density (n(s)) on AlGaN barrier thickness (d(AlGaN)) was studied for AlGaN/GaN single heterostructures whose d(AlGaN) was controlled by low-power Cl-based reactive ion etching (RIE) instead of growth. The samples showed a constant increase not only in n(s) but also in AlGaN surface barrier height (e phi(B)) with d(AlGaN), indicating the existence of low-density and distributed donor states on the AlGaN surface. Such a distribution of donor states differs from the commonly accepted model based on high-density and single-level surface donor states as the source of electrons in the two-dimensional electron gas (2DEG). The presence of a distribution of donor states is confirmed by first-principles calculations for a variety of surface structures for oxidized AlGaN surfaces. Donor states arise from areas of the surface that deviate from the electron-counting rule, leading to occupied surface states in the upper half of the band gap. The oxide formed on the surface after RIE results in a low-density distribution of surface donor states in which the highest occupied levels span the range from 1-2 eV below the AlGaN conduction-band minimum. The density of these states is comparable to the n(s) in the 2DEG and insufficient to pin the Fermi level, leading to a constant increase in e phi(B) with d(AlGaN). (C) 2010 American Institute of Physics. [doi: 10.1063/1.3481412]

DOI： 10.1063/1.3481412

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Transistor operation was demonstrated on non-polar Si delta-doped m-plane AlGaN/GaN hetero-junction field effect transistors (HFETs) grown by metal organic chemical vapor phase deposition. A maximum drain-source current of 340 mA/mm at a gate-source voltage (V(gs)) of +1 V, threshold voltage of -3 V and a maximum transconductance of 95 mS/mm at V(gs) = -1.4 V were exhibited. With increasing Si delta-doping, the sheet carrier concentration increased from 3.1 X 10(12) to 4.8 x 10(12) cm(-2). Good control of sheet carrier concentration by Si delta-doping was observed in non-polar m-plane AlGaN/GaN heterostructures. (C) 2009 The Japan Society of Applied Physics

DOI： 10.1143/APEX.2.061003

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Publishing type：Research paper (scientific journal)  

The effect of SIN. passivation thickness on the power performance of AlGaN/GaN high-electron-mobility transistors (HEMTs) has been studied. A model is proposed to explain the surface-state dispersion and passivation of AlGaN/GaN HEMTs. Based on this model, a multidielectic passivation method has been proposed and demonstrated to both provide lower dielectric capacitance and help remove dc-RIF dispersion.

DOI： 10.1109/LED.2009.2012444

CiNii Article

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Publishing type：Research paper (scientific journal)  

The effects of thin SiNx deposition on AlGaN/GaN heterostructure field-effect transistors were systematically studied by comparing their electrical and device characteristics. Two aspects of the thin SiNx film deposition were investigated: (i) the increase in two-dimensional electron gas (2DEG) density at the heterointerface and (ii) its capability as a gate insulating layer. Three different SiNx deposition methods were studied: catalytic chemical vapor deposition (Cat-CVD), metalorganic chemical vapor deposition (MOCVD), and plasma enhanced chemical vapor deposition (PECVD). A large increase in 2DEG density was obtained after SiNx deposition for all methods. The devices with MOCVD SiNx gate insulator showed a larger gate leakage current than those with the Cat-CVD and PECVD SiNx, implying that a thinning of the AlGaN surface barrier occurred due to Si diffusion into the AlGaN barrier during the high-temperature MOCVD process.

DOI： 10.1063/1.3079798

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Publishing type：Research paper (scientific journal)  

The first demonstration of the m-plane AlGaN/GaN heterojunction field-effect transistor was reported. This transistor exhibited enhancement-mode operation, with the threshold voltage of +2.0 V owing to the nonpolar AlGaN/GaN heterojunction. The maximum drain-source current density and the maximum transconductance were 130 mA/mm at a gate-source voltage (V(gs)) of +9V and 25 mS/mm at V(gs) = +8V, respectively. The on-to-off ratio was over 10(6). (C) 2009 The Japan Society of Applied Physics

DOI： 10.1143/APEX.2.011001

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Publishing type：Research paper (scientific journal)  

This paper describes the device fabrication process and characteristics of AlGaN/GaN heterostructure field-effect transistors (HFETs) aimed for millimeter-wave applications. We developed three novel techniques to suppress short-channel effects and thereby enhance high-frequency device characteristics: high-Al-composition and thin AlGaN barrier layers, SiN passivation by catalytic chemical vapor deposition, and sub-100-nm Ti-based gates. The Al0.4Ga0.6N/GaN HFETs with a gate length of 30 nm had a maximum drain current density of 1.6 A/mm and a maximum transconductance of 402 mS/mm. The use of these techniques led to a current-gain cutoff frequency of 181 GHz and a maximum oscillation frequency of 186 GHz.

DOI： 10.1093/ietele/e91-c.7.984

CiNii Article

J-GLOBAL

Publishing type：Research paper (scientific journal)  

This paper describes the device fabrication process and characteristics of AlGaN/GaN heterostructure field-effect transistors (HFETs) aimed for millimeter-wave applications. We developed three novel techniques to suppress short-channel effects and thereby enhance high-frequency device characteristics: high-Al-composition and thin AlGaN barrier layers, SiN passivation by catalytic chemical vapor deposition, and sub-100-nm Ti-based gates. The Al0.4Ga0.6N/GaN HFETs with a gate length of 30 nm had a maximum drain current density of 1.6 A/mm and a maximum transconductance of 402 mS/mm. The use of these techniques led to a current-gain cutoff frequency of 181 GHz and a maximum oscillation frequency of 186 GHz.

DOI： 10.1093/ietele/e91-c.7.984

Publishing type：Research paper (scientific journal)  

We report on state-of-the-art AlGaN/GaN heterostructure field-effect transistor (HFET) technology in the scope of millimeter-wave applications. 60-nm-long-gate HFETs having 4- and 6-nm-thick Al(0.4)Ga(0.6)N barrier layers and SiN passivation layers formed by catalytic chemical vapor deposition (Cat-CVD) were fabricated on 4H-SiC substrates. Both structures had low sheet resistances of 200-220 Omega/sq that were due to not only high mobilities of 1900 - 2000 cm(2)/(V.s) but also high electron densities of (1.4 - 1.7) x 10(13) cm (- 2), which were provided by the high-Al-composition barrier layers and the Cat-CVD SiN. The devices with the 4- and 6-nm-thick barriers had maximum drain current densities of 1.4 and 1.6 A/mm and peak extrinsic transconductances of 448 and 424 mS/mm, respectively. Maximum f(T) and f(max) reached 190 and 251 GHz, respectively. (c) 2008 The Japan Society of Applied Physics.

DOI： 10.1143/APEX.1.021103

J-GLOBAL

Publishing type：Research paper (scientific journal)  

We have found that SiN passivation by catalytic chemical vapor deposition (Cat-CVD) can significantly increase an electron density of an AlGaN/GaN heterostructure field-effect transistor (HFET). This effect enables thin-barrier HFET structures to have a high-density two-dimensional electron gas and leads to suppression of short-channel effects. We fabricated 30-nm-gate Al0.4Ga0.6N(8 nm)/GaN HFETs using Cat-CVD SiN. The maximum drain current density and extrinsic transconductance were 1.49 A/mm and 402 mS/mm, respectively. Current-gain cutoff frequency and maximum oscillation frequency of the HFETs were 181 and 186 GHz, respectively. These high-frequency device characteristics are sufficiently high enough for millimeter-wave applications. (C) 2007 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.tsf.2007.06.090

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Publishing type：Research paper (international conference proceedings)  

This paper describes device process and characteristics of sub-100-nm-gate AlGaN/GaN heterostructure field-effect transistors (HFETs) for millimeter-wave applications. We developed three techniques to suppress short-channel effects and thereby enhance high-frequency device characteristics: high-Al-composition and thin AlGaN barrier layers, SiN passivation by catalytic chemical vapor deposition, and sub-100-nm Ti-based gates. The Al0.4Ga0.6N(6 nm)/GaN HFETs with a gate length of 60 nm on a 4H-SiC substrate showed a maximum drain current density of 1.6 A/mm and a maximum transconductance of 424 mS/mm. The use of the techniques led to record current-gain cutoff frequency (f(T)) and maximum oscillation frequency (f(max)) of 190 and 241 GHz, respectively. The f(T) and f(max) kept high values over the wide range of drain voltage and current. These results indicate significantly high potential of GaN HFETs for high-power applications in the millimeter-wave frequency range.

DOI： 10.1117/12.767574

Publishing type：Research paper (international conference proceedings)  

DOI： 10.1109/DRC.2008.4800805

Publishing type：Research paper (scientific journal)  

This paper describes approaches that we have used to improve the high-frequency device characteristics of GaN-based heterostructure field-effect transistors (HFETs). We developed three novel techniques to suppress short-channel effects: high-Al-composition and thin barrier layers, SiN passivation by catalytic chemical vapor deposition, and sub-100-nm Ti-based gates. These techniques were used to enhance the high-frequency device characteristics of Al-0.4 Ga-0.6 N/GaN HFETs, which had a record current-gain cutoff frequency of 181 GHz. In addition, high-performance depletion- and enhancement-mode AlN/GaN HFETs, which had DC and RF device characteristics fully comparable with those of state-of-the-art AlGaN/GaN HFETs, were also fabricated with these techniques. (c) 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

DOI： 10.1002/pssa.200674860

Publishing type：Research paper (scientific journal)  

DOI： 10.1143/JJAP.46.L502

Publishing type：Research paper (scientific journal)  

High-performance enhancement-mode (E-mode) AIN (2.5 nm)/GaN heterostructure field-effect transistors (HFETs) were fabricated with a novel method using SiN passivation by catalytic chemical vapor deposition (Cat-CVD). We found that the formation of a 2-D electron gas (2DEG) in the AIN/GaN heterostructure can be controlled by the presence of the Cat-CVD SiN on the barrier layer. Before SiN deposition, the 2DEG at the AIN/GaN heterointerface was completely depleted because of the extremely thin barrier layer. On the other hand, after SiN deposition, the decrease in AIN surface barrier height induced a high-density 2DEG. The E-mode HFETs with gate lengths of 100-180 nm and threshold voltages from +0.14 to +0.55 V showed a maximum drain-current density of 0.70-0.92 A/mm and a maximum extrinsic transconductance of 362-400 mS/mm. A current-gain cutoff frequency of 87 GHz and maximum oscillation frequency of 149 GHz were obtained for the 100-nm-gate devices.

DOI： 10.1109/TED.2007.896607

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Publishing type：Research paper (scientific journal)  

SiN passivation on AlGaN/GaN heterostructures was carried out using catalytic or plasma-enhanced chemical vapor deposition (Cat-CVD or PECVD), which has been found to increase two-dimensional electron gas (2DEG) density. The 2DEG density can be closely related to AlGaN surface properties via polarization effects. AlGaN potential barrier heights of AlGaN/GaN heterostructures with and without SiN passivation were systematically investigated using x-ray photoelectron spectroscopy (XPS) and capacitance-voltage (C-V) measurements. The results for the XPS and C-V measurements were consistent and demonstrated that a reduction in the AlGaN potential barrier height was actually induced by SiN passivation. Furthermore, Cat-CVD SiN passivation lowered the AlGaN potential barrier height more significantly than PECVD SiN passivation did, suggesting that the passivation method can influence the AlGaN potential barrier height.

DOI： 10.1063/1.2472255

Publishing type：Research paper (international conference proceedings)  

AlGaN surface potentials in AlGaN/GaN heterostructures with and without SiN passivation were investigated using x-ray photoelectron spectroscopy (XPS). SiN films were formed on AlGaN surfaces by catalytic chemical vapor deposition (Cat-CVD), which has already been found to increase two-dimensional electron gas (2DEG) density. Based on a simple electrostatic analysis, the 2DEG density is expected to increase as the AlGaN surface potential decreases. This study experimentally demonstrates that a reduction in the AlGaN surface potential is actually induced by Cat-CVD SiN passivation. (C) 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

DOI： 10.1002/pssc.200674757

Publishing type：Research paper (scientific journal)  

DOI： 10.1002/pssa.200674860

CiNii Article

Publishing type：Research paper (scientific journal)  

We fabricated Al0.4Ga0.6N(8 nm)/GaN heterostructure field-effect transistors (HFETs) with a gate length (L-G) of 30 nm on a sapphire substrate. The AlGaN/GaN HFETs, which had a 2-nm-thick SiN passivation and gate-insulating layer formed by catalytic chemical vapor deposition, showed a maximum drain current density of 1.49 A/mm, a peak extrinsic transconductance of 402 mS/mm, a current-gain cutoff frequency (f(T)) of 181 GHz, and a maximum oscillation frequency of 186 GHz. From a delay time analysis of f(T) for the HFETs with L-G = 30-150 nm, the electron velocity overshoot was not observed even when L-G was decreased to 30 nm.

DOI： 10.1143/JJAP.45.L1111

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The authors fabricated SiN/AIN/GaN metalinsulator-semiconductor heterostructure field-effect transistors (MIS-HFETs) using SiN passivation by catalytic chemical vapor deposition (Cat-CVD). Cat-CVD SiN increased the electron density of AIN/GaN HFETs by compensating the surface depletion of the two-dimensional electron gas. The MIS-HFETs had a maximum drain current density of 0.95 A/mm and a peak extrinsic transconductance of 211 mS/mm. A current-gain cutoff frequency of 107 GHz and maximum oscillation frequency of 171 GHz were obtained for the 60- and 70-nm-gate devices, respectively.

DOI： 10.1109/LED.2006.881087

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J-GLOBAL

Publishing type：Research paper (scientific journal)  

We fabricated 60-nm-gate InAlN/GaN heterostructure field-effect transistors (HFETs), which had a 6-nm-thick InAlN barrier with an Al composition of 0.86. The devices had a maximum drain current density of 1.34 A/mm and a maximum extrinsic transconductance of 389 mS/mm. The small-signal performance of the devices shoved a current-gain cutoff frequency of 172 GHz and a maximum oscillation frequency of 206 GHz. These DC and RF device characteristics were superior to the best values reported for AlGaN (8nm)/GaN HFETs with the same gate length.

DOI： 10.1143/JJAP.45.L843

Publishing type：Research paper (scientific journal)  

We investigated the effects of SiN passivation by catalytic chemical vapor deposition (Cat-CVD) on the electrical properties of AlGaN/GaN heterostructure field-effect transistors. The two-dimensional electron density (N-s) greatly increased after the Cat-CVD SiN deposition, and the tendency of the increase was enhanced with decreasing AlGaN barrier thickness. As a result of the large increase in Ns, the sheet resistance (R-sh) significantly decreased after the deposition, and it had low values of 320-460 ohm/square for extremely thin AlGaN barriers of 4 - 10 nm. The increase in Ns showed little dependence on SiN thickness, indicating that the stress applied to the AlGaN barrier by SiN cannot be the origin of the increase. Cat-CVD SiN also improved the in-plane uniformity of mobility for extremely thin-barrier structures, which in turn improved the uniformity of Rsh. Moreover, we found that Cat-CVD was more effective than plasma-enhanced chemical vapor deposition in increasing Ns. A comparison of theoretical calculations and experimental results indicated that these behaviors can be explained by a decrease in the AlGaN surface barrier height due to the SiN deposition. (c) 2006 American Institute of Physics.

DOI： 10.1063/1.2218759

J-GLOBAL

Publishing type：Research paper (scientific journal)  

In this report we present our recent investigation on the co-existence of superconducting and semiconducting properties in InN grown on sapphire (0001) by the use of MBE and MOCVD methods. Magneto-transport measurements were done using InN with a carrier density n. from 4 x 10(17) to 4 x 102 cm 3 in the temperature range from 20 mK to 2 K and under the magnetic field (B) up to 23 T(c) As a result, even InN samples with n, as low as 4 x 10(17) cm(-3) showed superconductivity at T(c) = 0.12 K. The vortex solid of InN was melted by thermal fluctuation and by the external field, similarly to the observation in layered high T(c) superconductors. By the Shubnikov-de Haas oscillation measurements it was found that there were substantial carriers spread in the a-b plane which played an essential role not only for the occurrence of the superconductivity, but also for many other unexpected electronic and optical properties of InN. Based on these results we present a possible mechanism of superconductivity in terms of the interaction between the conduction electrons and the fixed d-electrons of In atoms spread in the a-b plane. (c) 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

DOI： 10.1002/pssb.200565419

Publishing type：Research paper (scientific journal)  

We fabricated sub-0.1 mu m-gate Al0.4Ga0.6N/GaN heterostructure field-effect transistors (HFETs) with AlGaN barrier thicknesses of 4-10 nm. The devices were passivated with 2 nm-thick SiN layers formed by catalytic chemical vapor deposition (Cat-CVD). The Cat-CVD SiN passivation greatly increased electron density, and the effect became more significant with decreasing AlGaN barrier thickness. The HFETs had maximum drain current densities of 1.1-1.5A/mm and peak extrinsic transconductances of 305-438 mS/mm. Peak current-gain cutoff frequency of 163 GHz and maximum oscillation frequency of 192 GHz were obtained for the devices with 8 nm-thick AlGaN barriers. (c) 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

DOI： 10.1002/pssa.200565199

Publishing type：Research paper (scientific journal)  

High sensitivity micro-Hall sensors were fabricated using AlGaN/GaN two-dimensional electron gas heterostructures grown by molecular beam epitaxy for high temperature operation. The room temperature electron mobility and sheet carrier concentration of the heterostructures were 850 cm(2)/V s and 7.7x10(12) cm(-2), respectively. The Hall voltage increased linearly with applied magnetic field up to 0.5 T for all temperatures in the range of temperature from 20 to 600 degrees C. A 100 mu mx100 mu m Hall device with a magnetic sensitivity of 0.35 G/(Hz)(1/2) at 600 degrees C was used to measure the Curie temperature of NdFeB. The AlGaN/GaN micro-Hall sensors will find a wide range of applications including high temperature scanning Hall microscopy, monitoring rotating parts in the engines of automobiles as well as space vehicles which are exposed to extreme changes in temperature and radiation. (C) 2006 American Institute of Physics.

DOI： 10.1063/1.2158693

J-GLOBAL

Publishing type：Research paper (scientific journal)  

Al0.4Ga0.6N/GaN heterostructure field-effect transistors (HFETs) with an AlGaN barrier thickness of 8 nm and a gate length (L-G) of 0.06-0.2 mu m were fabricated on a sapphire substrate. We employed two novel techniques, which were thin, high-Al-composition AlGaN barrier layers and SiN gate-insulating, passivation layers formed by catalytic chemical vapor deposition, to enhance high-frequency device characteristics by suppressing the short channel effect. The HFETs with L-G = 0.06-0.2 mu m had a maximum drain current density of 1.17-1.24 A/mm at a gate bias of +1.0 V and a peak extrinsic transconductance of 305-417 mS/mm. The current-gain cutoff frequency (f(T)) was 163 GHz, which is the highest value to have been reported for GaN HFETs. The maximum oscillation frequency (f(max)) was also high, and its value derived from the maximum stable gain or unilateral gain was 192 or 163 GHz, respectively.

DOI： 10.1109/LED.2005.860884

CiNii Article

Publishing type：Research paper (international conference proceedings)  

We report on the superconductivity of n-type InN. There is an optimum carrier density for the occurrence of the superconductivity. The lowest carrier density is limited by the Mott transition of n(c) =2 x 10(17) cm(-3) and the highest density is limited by the superconductivity to metal transition of n(i) similar to 7 x 10(20) cm(-3). We propose a mechanism where the occurrence of the superconductivity is related to the presence of In-In chains of finite lengths in the ab plane. The In-In chains, which originate from the inversion domains of InN grown on sapphire (0001) and elongate along [1120], are coupled to form micro Josephson-junctions.

DOI： 10.1088/1742-6596/51/1/064

Publishing type：Research paper (scientific journal)  

DOI： 10.1002/pssa.200565199

CiNii Article

Publishing type：Research paper (scientific journal)  

We fabricated 120-nm-T-shaped-Mo/Pt/Au-gate AlGaN/GaN high electron mobility transistors (HEMTs) on sapphire substrates. The gate leakage current I-gs of the Mo/Pt/Au-gate HEMT at a gate-source voltage V-gs of -5 V was as much as five orders of magnitude lower than that of the Ni/Pt/Au-gate HEMT under the as-deposited condition. The off-state breakdown voltage, defined as the gate-source voltage when the gate-source current is -1 mA/mm, was about -60 V for the Mo/Pt/Au-gate HEMT. These dc performances are comparable to those of the Ni/Pt/Au-gate HEMTs in which the Schottky contacts were improved with rapid thermal annealing at 500 degrees C. The Mo/Pt/Au-gate HEMTs also exhibited good rf performance without RTA. The cutoff frequency f(T) was more than 50 GHz and the maximum oscillation frequency f(max) was about 100 GHz. (c) 2005 American Vacuum Society.

DOI： 10.1116/1.2013315

Publishing type：Research paper (scientific journal)  

The temperature dependence of the resistivity of InN was investigated as a function of carrier density. The carrier density was changed from ne =1.8× 1018 cm-3 to 1.5× 1019 cm-3 by Si doping. The InN investigated showed metallic conduction above 20 K. At lower temperatures there was a resistivity anomaly originating from carrier localization in the a-b plane, which was confirmed by the magnetoresistance at 0.5 K. The Shubnikov-de Haas oscillation showed that InN had a spherical Fermi surface and its radius increased according to the increase of ne when ne &lt
5× 1018 cm-3. In addition, an oscillation corresponding to the constant carrier density of 4.5× 1012 cm-2 was observed in the field applied perpendicular to the a-b plane. This oscillation showed an anomalous angle dependence on the magnetic field. Taking into account this density, we determined the critical carrier density of the Mott transition to be 2× 1017 cm-3. Anisotropy of localization was observed within the a-b plane, which indicates that the distribution of the electrons was not uniform in the a-b plane. The ne dependence of the magnetoresistance revealed an electronic structure change around 5× 1018 cm-3. From these results, an electronic structure at the fundamental absorption edge of InN grown on sapphire (0001) was presented. © 2005 The American Physical Society.

DOI： 10.1103/PhysRevB.72.085210

Publishing type：Research paper (scientific journal)  

The temperature dependence of the resistivity of InN was investigated as a function of carrier density. The carrier density was changed from n(e)=1.8x10(18) cm(-3) to 1.5x10(19) cm(-3) by Si doping. The InN investigated showed metallic conduction above 20 K. At lower temperatures there was a resistivity anomaly originating from carrier localization in the a-b plane, which was confirmed by the magnetoresistance at 0.5 K. The Shubnikov-de Haas oscillation showed that InN had a spherical Fermi surface and its radius increased according to the increase of n(e) when n(e)&lt; 5x10(18) cm(-3). In addition, an oscillation corresponding to the constant carrier density of 4.5x10(12) cm(-2) was observed in the field applied perpendicular to the a-b plane. This oscillation showed an anomalous angle dependence on the magnetic field. Taking into account this density, we determined the critical carrier density of the Mott transition to be 2x10(17) cm(-3). Anisotropy of localization was observed within the a-b plane, which indicates that the distribution of the electrons was not uniform in the a-b plane. The n(e) dependence of the magnetoresistance revealed an electronic structure change around 5x10(18) cm(-3). From these results, an electronic structure at the fundamental absorption edge of InN grown on sapphire (0001) was presented.

DOI： 10.1103/PhysRevB.72.085210

Publishing type：Research paper (scientific journal)  

We fabricated 120-nm-long-T-shaped-Ti/Pt/Au-gate AlGaN/GaN high electron mobility transistors (HEMTs) on sapphire substrates and annealed them thermally to improve their Schottky contacts (i.e., to reduce interface traps, increase the Schottky barrier height, and get better adhesion), thereby reducing gate leakage current, achieving better gate controllability, and obtaining better dc and rf characteristics compared to the HEMTs with as-deposited Schottky gate metals. Rapid thermal annealing (RTA) was carried out successively at 500, 600, and 650 degrees C. The drain-source current I-ds decreased with RTA temperature up to 600 degrees C and turned to increase at 650 degrees C. The maximum transconductance g(m_max), on the other hand, increased up to 650 degrees C, indicating that RTA improved the dc characteristics of the HEMTs. We confirmed the improvement of Schottky contacts by measuring gate leakage current I-gs and calculating the Schottky barrier height phi(B). After RTA at 600 degrees C, the I-gs at a gate-source voltage V-gs of -10 V was as much as three orders of magnitude lower than it was in the as-deposited condition and the phi(B) was 0.27 eV greater than it was in the as-deposited condition (1.16 eV versus 0.89 eV). After the RTA at 650 degrees C, the phi(B) was slightly less than it was after the RTA at 600 degrees C. RTA at 600 and 650 degrees C also improved the rf characteristics, increasing the cutoff frequency f(T) from 80 to 84 GHz (5% increase) and increasing the maximum oscillation frequency f(max) from 102 to M GHz (20% increase). (c) 2005 American Vacuum Society.

DOI： 10.1116/1.1897706

Publishing type：Research paper (scientific journal)  

The effect of SiN surface passivation by catalytic chemical vapor deposition (Cat-CVD) on Al0.4Ga0.6N-GaN heterostructure field-effect transistors (HFETs) was investigated. The channel sheet resistance was reduced by the passivation due to an increase in electron density, and the device characteristics of the thin-barrier HFETs were significantly improved by the reduction of source and drain resistances. The AlGaN(8 nm)-AlN(1.3 nm)-GaN HFET device with a source/drain distance of 3 mum and a gate length of 1 um had a maximum drain current density of 0.83 Mum at a gate bias of +1.5 V and an extrinsic maximum transconductance of 403 mS/mm. These results indicate the substantial potential of Cat-CVD SiN-passivated AlGaN-GaN HFETs with thin and high Al composition barrier layers.

DOI： 10.1109/LED.2004.842736

CiNii Article

Publishing type：Research paper (scientific journal)  

High-quality InN layers grown on sapphire substrates by plasma-assisted molecular-beam epitaxy were characterized using monoenergetic positron beams. The carrier concentrations of the films were controlled by Si doping (2.1x10(18) to 1.4x10(19) cm(-3)), and the highest obtained Hall mobility was 1300 cm(2) V-1 s(-1). The Doppler broadening spectra of the annihilation radiation and the lifetime spectra of positrons were measured as a function of the incident positron energy for undoped and Si-doped InN films. The line-shape parameter S increased with increasing carrier concentration, suggesting the introduction of vacancy-type defects by a Fermi-level effect. The major defect species were varied with carrier concentration, and its species were identified as In vacancies (V-In) or their related defects. (C) 2005 American Institute of Physics.

DOI： 10.1063/1.1845575

Publishing type：Research paper (international conference proceedings)  

InAlN/GaN heterostructure field-effect transistors (HFETs) were grown on sapphire substrates by plasma-assisted molecular-beam epitaxy. The In mole fraction in InAlN was estimated to be 0.15 from the X-ray diffraction profile. The room-temperature Hall mobility was 654 cm(2) /V(.)s, and the sheet electron density was 2.0 x 10(13) cm(-2). The InAlN/GaN HFET device, which had a source-drain spacing of 3 mu m and a gate length of 1.5 mu m, showed a good DC performance and an excellent pinch-off characteristic. The maximum source-drain current density and extrinsic transconductance were 500 mA/mm and 52 mS/mm, respectively. (c) 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

DOI： 10.1002/pssc.200461389

Publishing type：Research paper (scientific journal)  

AlGaN/GaN heterostructure field-effect transistors (HFETs) with a gate length (L-G) of 60-250nm were fabricated on a sapphire substrate. The HFET structure was grown by plasma-assisted molecular-beam epitaxy, and a 2-nm-thick SiN film was formed on the device surface by catalytic chemical vapor deposition. All of the HFETs showed outstanding DC device performance. They exhibited maximum drain current densities of 1.50-1.55 A/mm and extrinsic transconductances of 340-400 mS/mm. The 60-nm-gate HFET had a current gain cut-off frequency (f(T)) of 152GHz and a maximum oscillation frequency (f(max)) of 173 GHz. To our knowledge, the f(T) and f(max) are the highest ever reported for GaN-based transistors. These superior high-frequency characteristics were achieved with a process using a thin and high-Al-content barrier layer, high-quality catalytic chemical vapor deposition (Cat-CVD) SiN passivation, and sub-0.1-mu m gates defined by electron-beam lithography.

DOI： 10.1143/JJAP.44.L475

Publishing type：Research paper (international conference proceedings)  

DOI： 10.1109/DRC.2005.1553161

Publishing type：Research paper (scientific journal)  

AlGaN/GaN heterostructure field-effect transistors (HFETs) with a gate length (L-G) of 60-250nm were fabricated on a sapphire substrate. The HFET structure was grown by plasma-assisted molecular-beam epitaxy, and a 2-nm-thick SiN film was formed on the device surface by catalytic chemical vapor deposition. All of the HFETs showed outstanding DC device performance. They exhibited maximum drain current densities of 1.50-1.55 A/mm and extrinsic transconductances of 340-400 mS/mm. The 60-nm-gate HFET had a current gain cut-off frequency (f(T)) of 152GHz and a maximum oscillation frequency (f(max)) of 173 GHz. To our knowledge, the f(T) and f(max) are the highest ever reported for GaN-based transistors. These superior high-frequency characteristics were achieved with a process using a thin and high-Al-content barrier layer, high-quality catalytic chemical vapor deposition (Cat-CVD) SiN passivation, and sub-0.1-mu m gates defined by electron-beam lithography.

DOI： 10.1143/jjap.44.L475

CiNii Article

Publishing type：Research paper (scientific journal)  

We report on the barrier thickness dependence of the electrical properties and DC device characteristics of Al0.4Ga0.6N/GaN heterostructure field-effect transistors (HFETs). The HFET structures with 8-25-nm-thick AlGaN barrier layers were grown on sapphire substrates by plasma-assisted molecular-beam epitaxy. All of the fabricated HFET devices with a gate length of 1 mum showed a good DC performance and an excellent pinch-off characteristic. The extrinsic transconductance of the HFET devices increased from 185 to 360 mS/mm with decreasing AlGaN barrier thickness from 25 to 8 nm, while the maximum drain current for a gate bias of + 1.0 V decreased from 1.1 to 0.68 A/mm.

DOI： 10.1143/JJAP.43.L1147

CiNii Article

Publishing type：Research paper (scientific journal)  

InN films with electron densities of 1.6 x 10(18)-1.4 x 10(19) cm(-3) had photoluminescence (PL) properties with clear electron density dependencies. The PL peak shifted to a higher energy with increasing electron density, whereas PL intensity decreased with increasing electron density. These characteristics are typical of degenerate semiconductors with a large density of defects and/or dislocations. We estimated a band-gap energy of 0.6-0.65 eV for intrinsic InN from the shift in the PL peak position and band-gap shrinkage due to the conduction-band renormalization effect. (C) 2004 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.jcrysgro.2004.05.045

Publishing type：Research paper (scientific journal)  

We report on the plasma-assisted molecular-beam epitaxy growth and device characteristics of InAIN/GaN heterostructure field-effect transistors (HFETs). The In mole fraction in InAIN was estimated to be 0.15 from the X-ray diffraction profile. The surface root-mean-square roughness of the InAIN layer was less than 3 nm. The room-temperature Hall mobility was 654 cm(2)/V.s, and the sheet electron density was 1.7 x 10(13) cm(-2). The InAIN/GaN HFET, which had a source-drain spacing of 3 mum and a gate length of 1.5 mum, showed a good pinch-off characteristic. The maximum source-drain current density and extrinsic transconductance were 500 mA/mm and 52 mS/mm, respectively.

DOI： 10.1143/JJAP.43.L768

CiNii Article

Publishing type：Research paper (scientific journal)  

We fabricated non-recessed-gate enhancement-mode (E-mode) AlGaN/GaN high electron mobility transistors (HEMTs) with a gate length L-g of 120nm. As gate metals, Ni/Pt/Au and Mo/Pt/Au were used. The Ni/Pt/Au-gate HEMTs with rapid thermal annealing (RTA) at 500degreesC were normally-off at a gate-source voltage V-gs of 0 V, indicating E-mode operation. Moreover, the Mo/Pt/Au-gate HEMTs also showed E-mode device operation without RTA. The fabricated E-mode HEMTs with both gate metals showed high RF performance. We obtained a cutoff frequency f(T) of more than 50 GHz and a maximum oscillation frequency f(max) of approximately 100 GHz.

DOI： 10.1143/JJAP.43.2255

CiNii Article

Publishing type：Research paper (scientific journal)  

The carrier concentration dependence of the interaction between free carriers and longitudinal optical (LO) phonons of InN is studied by Raman scattering and Fourier transform infrared measurements. InN is grown on a sapphire (0001) surface by plasma-assisted molecular beam epitaxy. The carrier concentration is varied from 1.8 × 1018 to 1.5 × 1019 cm−3 by Si doping. The infrared reflection spectra, to which the vibration in the a − b plane contributes, reveal a linear coupling between the E1(LO) phonon and the plasma oscillation of the free carriers. From the plasma frequency the electron effective mass is estimated to be me⊥* = 0.085m0 for the intrinsic InN. The Raman spectra, to which the vibration along the c axis contributes, reveal that the A1(LO) phonon and free carriers couple nonlinearly, where a Fano interference between the zone-center LO phonon and the quasicontinuum electronic state along the c axis is prominent. With these results, the anisotropic electronic structure of InN is discussed. © 2003 The American Physical Society.

DOI： 10.1103/PhysRevB.68.235204

Publishing type：Research paper (scientific journal)  

The carrier concentration dependence of the interaction between free carriers and longitudinal optical (LO) phonons of InN is studied by Raman scattering and Fourier transform infrared measurements. InN is grown on a sapphire (0001) surface by plasma-assisted molecular beam epitaxy. The carrier concentration is varied from 1.8x10(18) to 1.5x10(19) cm(-3) by Si doping. The infrared reflection spectra, to which the vibration in the a-b plane contributes, reveal a linear coupling between the E-1(LO) phonon and the plasma oscillation of the free carriers. From the plasma frequency the electron effective mass is estimated to be m(eperpendicular to)(*)=0.085m(0) for the intrinsic InN. The Raman spectra, to which the vibration along the c axis contributes, reveal that the A(1)(LO) phonon and free carriers couple nonlinearly, where a Fano interference between the zone-center LO phonon and the quasicontinuum electronic state along the c axis is prominent. With these results, the anisotropic electronic structure of InN is discussed.

DOI： 10.1103/PhysRevB.68.235204

Publishing type：Research paper (scientific journal)  

We have studied Si-doping profiles of InN films grown by plasma-assisted molecular-beam epitaxy and their photoluminescence (PL) properties. We confirmed experimentally that Si acts as a donor in InN. Undoped and Si-doped InN films with electron densities (n) of 1.6 x 10(18) - 1.4 x 10(19) cm(-3) showed clear n dependences of PL properties. The PL peak shifted to the higher energy side with increasing n, and the PL intensity decreased with increasing n. These were characteristics of degenerated semiconductors with a large density of defects and/or dislocations. The band-gap energy of degenerated InN films with n = 1.6 x 10(18) - 4.7 x 10(18) cm(-3) was estimated to be about 0.6 eV by assuming a nonparabolic conduction band and a constant band-renormalization effect. By taking the band-gap shrinkage of about 20 meV due to the conduction-band renormalization into account, we suggest that the band-gap energy of intrinsic InN is 0.6-0.65 eV. (C) 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

DOI： 10.1002/pssb.200303349

Publishing type：Research paper (scientific journal)  

Studying the epitaxial growth of InN films by plasma-assisted molecular-beam epitaxy, we found a buffer layer formed with a low-temperature-grown GaN (LT-GaN) and a low-temperature-grown InN (LT-InN) layers to be effective for improving their structural and electrical properties. Not only the growth temperature of a main part of InN film but growth temperatures and thicknesses of LT-GaN and LT-InN layers strongly influenced InN film properties. The surface of a 260-nm-thick InN film grown at the optimum condition was smooth with the surface root-mean-square roughness less than 4 nm. Its Hall mobility and background electron density at room temperature were 1420 cm(2)/V s and 1.4 x 10(18) cm(-3), respectively. (C) 2003 Elsevier Science B.V. All rights reserved.

DOI： 10.1016/S0022-0248(03)00918-7

CiNii Article

Publishing type：Research paper (scientific journal)  

We have studied the growth of InN thin films and InAlN/InN heterostructures on a low-temperature grown GaN (LT-GaN) layer on a sapphire substrate by plasma-assisted molecular-beam epitaxy (PAMBE). Cross-sectional transmission electron microscopy revealed that the InN film stood on the LT-GaN layer like a bridge, indicating that the InN film was almost free standing. PL and optical absorption measurements of the InN film showed that the intrinsic band gap of hexagonal InN is less than 0.7 eV. We successfully grew coherent InAlN layers with an A] content up to 0.15 on the InN layer. This demonstrated that single-crystal InAlN/InN heterostructures can be grown by PAMBE. (C) 2002 Elsevier Science B.V. All rights reserved.

DOI： 10.1016/S0022-0248(02)02362-X

Publishing type：Research paper (scientific journal)  

DOI： 10.1103/PhysRevB.68.235204

CiNii Article

Publishing type：Research paper (international conference proceedings)  

In this paper, we review our recent results obtained for InP-based HEMTs. We have developed several fabrication techniques, and obtained an ultrahigh f&lt
inf&gt
T&lt
/inf&gt
of 562 GHz for a 25 nm long gate pseudomorphic InAlAs/InGaAs HEMT. We also discuss possible applications of our HEMTs and the future of InP-based HEMTs.

DOI： 10.1109/DRC.2003.1226847

Publishing type：Research paper (international conference proceedings)  

We investigated the electronic structure of degenerated InN by Shubnikov-de Haas (SdH) measurements for the first time. The SdH oscillation depends on the angle between the crystal c-axis and the applied field and has two components
one originates from the spherical Fermi surface with effective mass of about 0.085 m0, the other from the layered structure spread in the a-b plane. In the case of Sidoped InN with the carrier density of 2.5×1018 cm-3, about half electron density exists on the a-b plane. © 2003 WILEY-VCH Verlag GmbH &amp
Co. KGaA.

DOI： 10.1002/pssc.200303461

Publishing type：Research paper (international conference proceedings)  

We fabricated sub-50-nm-gate i-AlGaN/GaN high electron mobility transistors (HEMTs) on sapphire and measured their DC and RF characteristics at room temperature. The fabricated HEMTs exhibited true device operation and good pinch-off characteristics down to a gate length Lg of 25 nm. For the HEMTs with a source-drain spacing Lsd of 2 μm, we obtained the Lg dependence of the cutoff frequency fT under a drain-source voltage Vds of 3 V. The peak fT was measured to be 102 GHz at Lg = 35 nm. At Lg = 25 nm, fT started to decrease due to the short-channel effect. The highest fT of 110 GHz was obtained by reducing Lsd from 2 to 1.5 μm and by increasing Vds from 3 to 4 V. © 2003 WILEY-VCH Verlag GmbH &amp
Co. KGaA.

DOI： 10.1002/pssc.200303335

Publishing type：Research paper (international conference proceedings)  

We fabricated sub-50-nm-gate i-AlGaN/GaN high electron mobility transistors (HEMTs) on sapphire and measured their DC and RF characteristics at room temperature. The fabricated HEMTs exhibited true device operation and good pinch-off characteristics down to a gate length L-g of 25 nm. For the HEMTs with a source-drain spacing L-sd of 2 mum, we obtained the Lg dependence of the cutoff frequency f(T) under a drain-source voltage V-ds of 3 V. The peak f(T) was measured to be 102 GHz at L-g = 35 nm. At L-g = 25 nm, f(T) started to decrease due to the short-channel effect. The highest fT of 110 GHz was obtained by reducing L-sd from 2 to 1.5 mum and by increasing V-ds from 3 to 4 V. (C) 2003 WILEY-VCH Vertag GmbH & Co. KGaA, Weinheim.

DOI： 10.1002/pssc.200303335

Publishing type：Research paper (international conference proceedings)  

We investigated the electronic structure of degenerated InN by Shubnikov-de Haas (SdH) measurements for the first time. The SdH oscillation depends on the angle between the crystal c-axis and the applied field and has two components; one originates from the spherical Fermi surface with effective mass of about 0.085 m(0), the other from the layered structure spread in the a-b plane. In the case of Si-doped InN with the carrier density of 2.5 x 10(18) cm(-3), about half electron density exists on the a-b plane. (C) 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

DOI： 10.1002/pssc.200303461

Publishing type：Research paper (scientific journal)  

High-quality indium nitride (InN) films were grown on sapphire substrates by plasma-assisted molecular-beam epitaxy. The structural and electrical properties of the InN films were greatly improved by employing a buffer layer formed with a low-temperature-grown GaN intermediate layer and a low-temperature-grown InN layer. The surface morphology of the InN film was quite smooth - the root-mean-square roughness was less than 5 run. The room-temperature Hall mobility was 1180 cm(2)/V.s, and the residual electron concentration was 1.6 x 10(18) cm(-3). These results indicate that the InN film almost meets the requirements for application to practical devices.

DOI： 10.1143/JJAP.41.L540

CiNii Article

Publishing type：Research paper (scientific journal)  

We fabricated sub-50-nm-gate InAlAs/InGaAs high electron mobility transistors (HEMTs) lattice-matched to InP substrates. The two-step-recessed gate technology and low-temperature process, with all steps taking place below 300degreesC, allowed us to fabricate sub-50-nm-gate HEMTs that had high levels of performance. We succeeded in fabricating ultrashort 25-nm-long T-shaped gates. DC measurements showed that the 25-nm-gate HEMT had good pinchoff behavior, and that its maximum transconductance g(m) was about 770 mS/mm. A cutoff frequency f(T) of 396 GHz was obtained for the 25-nm-gate HEMT. This f(T) is the highest value yet reported for a transistor of any type, and the gate length of 25 nm is the shortest value ever reported for any compound semiconductor transistor that exhibits true device operation.

DOI： 10.1143/JJAP.41.1094

CiNii Article

Publishing type：Research paper (international conference proceedings)  

InN has not been studied so much among III-nitride compound semiconductors, even though it is an attractive material from the viewpoint of practical device application. Theoretical studies have predicted that its saturation and overshoot electron velocities in InN film would be the highest among nitride compounds and that InN-based transistors would be extremely fast and have a cutoff frequency of over 1 THz. However, InN epitaxial film and InN-based heterostructures with quality sufficient for practical device application have not been obtained due to difficulties with InN epitaxial growth, such as a low dissociation temperature and the lack of a suitable lattice-matched substrate. We have reported that a buffer layer formed with a low-temperature-grown GaN (LT-GaN) intermediate layer and a LT-InN layer on a sapphire substrate improves electrical and structural properties of InN epitaxial film. The highest Hall mobility and lowest carrier density of our plasma-assisted MBE-grown InN film at room temperature were 1420 cm2/V·s and 1.4×1018 cm-3, respectively. In this summary, we report on plasma-assisted MBE growth of InN thin film and of an InAlN/InN heterostructure on a sapphire substrate.

DOI： 10.1109/MBE.2002.1037846

Publishing type：Research paper (international conference proceedings)  

We found that inserting a low-temperature-grown layer of GaN between a low-temperature-grown InN buffer layer and a sapphire substrate was an effective way of improving structural and electrical properties of InN films grown by plasma-assisted molecular beam epitaxy. The surface of a 260 nm thick InN film grown under the optimum condition was smooth, with a surface root-mean-square roughness of less than 4 nm. The Hall mobility of the InN film at room temperature was greater than 1500 cm(2)/Vs with an electron density of less than 2.0 x 10(18) cm(-3).

DOI： 10.1002/pssc.200390063

Publishing type：Research paper (scientific journal)  

We fabricated 50-nm-gate InAlAs/InGaAs high electron mobility transistors (HEMTs) lattice-matched to InP substrates by using a conventional process under low temperatures, below 300 degreesC, to prevent fluorine contamination and suppress possible diffusion of the Si-delta -doped sheet in the electron-supply layer, and measured the DC and RF performance of the transistors. The DC measurement showed that the maximum transconductance g(m) of a 50-nm-gate HEMT is about 0.91 S/mm. The cutoff frequency f(T) of our 50-nm-gate HEMT is 362 GHz, which is much higher than the values reported for previous 50-nm-gate lattice-matched HEMTs. The excellent RF performance of our HEMTs results from a shortening of the lateral extended range of charge control by the drain field, and this may have been achieved because the low-temperature fabrication process suppressed degradation of epitaxial structure.

Publishing type：Research paper (scientific journal)  

We have succeeded in fabricating ultra-short 25-nm-gate InAlAs/InGaAs high electron mobility transistors (HEMTs) lattice-matched to InP substrates, The two-step-recessed gate technology and low temperature processing at below 300 degreesC allowed the fabrication of such ultra-short gates. DC measurements showed that the 25-nm-gate HEMT had good pinchoff behavior. We obtained a cutoff frequency f(T) of 396 GHz, within the range of 400 GHz f(T), for the 25-nm-gate HEMT. This f(T) is the highest value yet reported for any type of transistor, and the gate length of 25 mm is the shortest value ever reported for any compound semiconductor transistor that exhibits device operation.

DOI： 10.1109/55.936345

CiNii Article

Publishing type：Research paper (scientific journal)  

Publishing type：Research paper (international conference proceedings)  

Sub-50-nm-gate InAlAs/InGaAs high electron mobility transistors (HEMTs) lattice-matched to InP substrates were fabricated. Our method of fabrication includes the two-step-recess gate technology and a low temperature process, applied at below 300 degreesC. We succeeded in fabricating ultra-short 25-nm-long T-shaped-gates. RF measurements showed that the cutoff frequency f(T), of a 25-nm-gate HEMT is 396 GHz, and this is the highest value yet reported for any type of transistor.

DOI： 10.1109/ICIPRM.2001.929171

Publishing type：Research paper (scientific journal)  

In0.1Ga0.9As/(GaAs)6(AlAs)1 quantum wires (QWRs) were naturally formed in a 3.0-nm- or 4.8-nm-thick In0.1Ga0.9As/(GaAs)6(AlAs)1 quantum well grown on a (775)B-oriented GaAs substrate by molecular beam epitaxy which has a corrugated AlAs-on-InGaAs upper interface (a period of about 40 nm and a vertical amplitude of about 2 nm) and a flat InGaAs-on-AlAs lower interface. Strong polarization dependence [P≡(I∥-I⊥)/(I∥+I ⊥) = 0.15] of photoluminescence spectrum from the (775)B InGaAs QWR structures (Lw = 3.0nm) was observed at 11 K, indicating their good one dimensionality. Graded index separate confinement heterostructure-type self-organized (775)B In0.1Ga0.9As/(GaAs)6(AlAs)1QWR lasers were fabricated, and they showed laser oscillation with threshold current densities of 1.7-3.1 kA/cm2 and lasing wavelengths of 833-868 nm at room temperature (27 °C) under pulsed current condition. © 2000 American Vacuum Society.

DOI： 10.1116/1.591449

Publishing type：Research paper (scientific journal)  

We fabricated 50-nm-gate InAlAs/InGaAs high electron mobility transistors (HEMTs) lattice-matched to InP substrates using a conventional process under low temperatures below 300 degrees C, and measured DC and RF performance. nle measured cutoff frequency f(T) of our 50-nm-gate HEMT is 362 GHz, which is the highest value ever reported for any transistor, and is much higher than the values reported for previous 50-nm-gate lattice-matched HEMTs. The excellent RF performance of our HEMTs might result from the low-temperature fabrication process.

DOI： 10.1143/jjap.39.L838

CiNii Article

Publishing type：Research paper (scientific journal)  

In this paper, we report on the material and device characteristics of pseudomorphic In0.7Ga0.3As/In0.52Al0.48As high electron mobility transistors (HEMTs) grown on a(411)A-oriented InP substrate by molecular-beam epitaxy. The electron Hall mobility in the (411)A HEMT was 2.5 times higher at 15K and 1.3 times higher even at room temperature than that in a HEMT grown on a conventional (100) InP substrate. The (411)A HEMTs with 50 nm gates provided excellent DC anti RF device characteristics. The maximum transconductance was as high as 1.1 S/mm, and the cutoff frequency reached 355 GHz.

DOI： 10.1143/jjap.39.L720

CiNii Article

Publishing type：Research paper (international conference proceedings)  

In this paper, we report the device fabrication and characteristics of 50-nm-gate pseudomorphic In0.7Ga0.3As/In0.52Al0.48As high electron mobility transistors (HEMTs) grown on a (411) A-oriented InP substrate by molecular beam epitaxy, which is the first report, to our knowledge, of HEMTs grown on any InP substrate except a conventional (100)-oriented InP substrate. The pseudomorphic HEMTs on the (411) A InP substrate showed a higher electron mobility than those on the (100) InP substrate at not only low temperatures but also room temperature. The (411) A HEMTs provided excellent DC and RF characteristics, showing a very high transconductance (1.1 S/mm) and one of the best cutoff frequencies (355 GHz) ever reported.

DOI： 10.1109/ICIPRM.2000.850362

Publishing type：Research paper (scientific journal)  

DOI： 10.1116/1.591449

CiNii Article

Publishing type：Research paper (international conference proceedings)  

We fabricated 50-nm-gate lattice-matched InAlAs/InGaAs High Electron Mobility Transistors (HEMTs) using a conventional process under low temperatures below 300 degrees C, and measured DC and RF performance. The measured cutoff frequency f(T) of our 50-nm-gate HEMT is 362 GHz, which is much higher than those in previous works, and the highest value ever reported for any transistor. The excellent RF performance might result from the low-temperature fabrication process.

DOI： 10.1109/ICIPRM.2000.850238

Publishing type：Research paper (scientific journal)  

Self-organized In0.15Ga0.85As quantum wires (QWRs) were formed in an In0.15Ga0.85As quantum well (QW) with GaAs barrier lavers grown on a (5 5 3)B-oriented GaAs substrate by molecular beam epitaxy. The QW has a regularly corrugated interface to the upper GaAs barrier (a lateral period of about 20 nm and a vertical amplitude of about 1.5 nm) and a rather flat interface to the lower GaAs barrier. A very narrow photoluminescence peak from the QWRs was observed at a wavelength of 864 nm at 15 K. The PL peak showed strong polarization dependence [the polarization degree P = (I-parallel to - I-perpendicular to)/(I-parallel to + I-perpendicular to) = 0.25] at 15 K, indicating high one dimensionality of the (5 5 3)B QWRs. The full-width at half-maximum (FWHM) of the PL peak was as small as 8.4 meV (15 K) which is, to our knowledge, the smallest PL-FWHM reported so far for self-organized QWRs. (C) 1999 Elsevier Science B.V. All rights reserved.

DOI： 10.1016/S0022-0248(98)01449-3

Publishing type：Research paper (scientific journal)  

Stripe-geometry lasers with high-density GaAs/(GaAs)(4) (AlAs)(2) quantum wires (QWRs) as an active region were grown on (7 7 5)B-oriented GaAs substrates by molecular beam epitaxy. The (7 7 5)B QWRs were naturally formed at thick parts in a GaAs/(GaAs)(4) (AlAs)(2) quantum well with a corrugated A1As-on-GaAs upper interface and a Aat GaAs-on-AlAs lower interface. The (7 7 5)B QWR lasers with a high one-dimensionality exhibited threshold current densities of 1.9 kA/cm(2) at 20 degrees C for a cavity length of 1 mm, which were about 35% lower than those of conventional QW lasers simultaneously grown on (1 0 0) GaAs substrates. Moreover, they showed a 30% higher characteristic temperature in the range of 20-80 degrees C than the (1 0 0) QW lasers. (C) 1999 Elsevier Science B.V. All rights reserved.

DOI： 10.1016/S0022-0248(98)01492-4

CiNii Article

Publishing type：Research paper (scientific journal)  

Self-organized GaAs/(GaAs)(4)(AlAs)(2) quantum-wire (QWR) lasers were grown on (775) B-oriented GaAs substrates by molecular-beam epitaxy. The QWRs were naturally formed at thick parts in the GaAs/(GaAs)(4)(AlAs)(2) quantum well with a corrugated AlAs-on-GaAs upper interface and a flat GaAs-on-AlAs lower interface. The density of the QWRs was as high as 8 X 10(6) cm(-1). Stripe-geometry lasers with the self-organized (775) B GaAs/(GaAs)(4)(AlAs)(2) QWRs as an active region oscillated at 20 degrees C with threshold current densities of about 3 kA/cm(2) for uncoated mirrors. (C) 1999 American Institute of Physics. [S0003-6951(99)01806-9].

DOI： 10.1063/1.123365

CiNii Article

A high-quality pseudomorphic In0.7Ga0.3As/In0.52Al0.48As quantum-well (QW)-HEMT structure grown on (411)A InP substrates by molecular beam epitaxy (MBE) shows the highest mobility (39,700 cm2/Vs at 77 K) of two-dimensional electron gas (2DEG). The pseudomorphic In0.7Ga0.3As/In0.52Al0.48As QW-HEMT structures consisted of, from the top, a Si-doped InGaAs contact layer (40 nm thick), an In0.52Al0.48As layer (20 nm), a Si-sheet-doped In0.52Al0.48As layer (20 nm) with a nominal doping concentration of 1×1013 cm-2, an In0.52Al0.48As spacer layer (3 nm), a pseudomorphic In0.7Ga0.3As QW channel layer (12 nm), an In0.52Al0.48As layer (300 nm) and an In0.53Ga0.47As buffer layer (25 nm). The high-mobility (411)A sample can be expected to improve high-speed performance of InGaAs/InAlAs HEMTs.

Publishing type：Research paper (scientific journal)  

Publishing type：Research paper (scientific journal)  

In0.14Ga0.86As/(GaAs)(5)(AlAs)(5) Quantum Wires (QWRs) were naturally formed in a thin In0.14Ga0.86As/(GaAs)(5)(AlAs)(5) Quantum Well (QW) with a regularly corrugated AlAs/In0.14Ga0.86As upper interface (a period of about 40 nm) and a flat In0.14Ga0.86As/AlAs lower interface grown on (775)B-oriented GaAs substrates by molecular beam epitaxy. The QWRs were formed side by side with a high density of 2.5 x 10(5) QWRs cm(-1). A photoluminescence from the QWRs formed in the QW with an average well width of 2.2 nm, which have a cross section of about 40 x 4 nm(2), showed a strong polarization dependence [the polarization degree P = (I-parallel to - I-perpendicular to)/(I-parallel to + I-perpendicular to) = 0.19], indicating good one-dimensionality of those QWRs. Full width at half maximum of the PL peak from the In0.14Ga0.86As/(GaAs)(5)(AlAs)5 QWRs was as small as 17 meV at 14 K, which is smaller than any of other naturally synthesized QWRs reported so far. (C) 1998 Elsevier Science B.V. All rights reserved.

DOI： 10.1016/S0167-9317(98)00182-8

Publishing type：Research paper (scientific journal)  

Photoluminescence (PL) decay time and its temperature dependence were studied in high-density GaAs/(GaAs)(4)(AlAs)(2) quantum wires (QWRs) grown on a (775)B GaAs substrate by molecular beam epitaxy, which are naturally formed in a 2.1 nm-wide GaAs/(GaAs)(4)(AlAs)(2) quantum well (QW) with a corrugated upper AlAs/GaAs interface (a lateral period of 12 nm and a vertical amplitude of 1.2 nm) and a flat lower GaAs/AlAs interface. The PL decay rime of the (775)B QWRs (tau(QWR)) was 430 ps at 18 K, which is about 20% longer than that (tau(QW =) 360 ps) of a GaAs/(GaAs)(4)(AlAs)(2) QW simultaneously grown on a (100) GaAs substrate. tau(QWR) increased slowly with increasing temperature (T) as tau(QWR) = 30T(1/2) + 290 (PS), while tau(QW) varied as tau(QW) = 6T + 240 (ps). As a result, tau(QWR) became shorter than tau(QW) at 50 K. The PL decay time of the (775)B QWRs becomes longer again than that of the (100) QW in the rage of 70-80 K. These results suggest good one-dimensional characteristics of the (775)B QWRs. (C) 1998 Elsevier Science Ltd. All rights reserved.

DOI： 10.1016/S0038-1101(98)00075-6

CiNii Article

Publishing type：Research paper (scientific journal)  

Temperature dependence of photoluminescence (PL) was investigated for high-density and highly uniform GaAs/(GaAs)(4) (AlAs)(2) quantum wires (QWRs) grown on a (7 7 5)B GaAs substrate by molecular beam epitaxy, which are naturally formed in an average 2.1 nm width GaAs/(GaAs)(4)(AlAs)(2) quantum well (QW) with a corrugated upper interface (a lateral period of 12 nm and a vertical amplitude of 1.2 nm) and a flat lower interface. As temperature increases in the range of T greater than or equal to 60 K, PL intensity of the (7 7 5)B GaAs/(GaAs)(4)(AlAs)(2) QWRs decreases more slowly and is several times larger compared with that of a GaAs/(GaAs)4(A1As)z QW simultaneously grown on a (1 0 0) GaAs substrate, indicating reduction of the nonradiative decay process of exicitons in the (7 7 5)B QWRs. The full width at half maximum of the PL peak from the (7 7 5)B QWRs is almost independent of temperature above 60 K, while that of the (1 0 0) QW increases in proportion to about k(B)T above 50 K. These results strongly indicate the one-dimensional characteristics of the (7 7 5)B QWRs as theoretically predicted. (C) 1998 Elsevier Science B.V. All rights reserved.

DOI： 10.1016/S1386-9477(98)00197-0

CiNii Article

Publishing type：Research paper (scientific journal)  

Surface morphologies of GaAs layers grown on (775)B-oriented GaAs substrates by molecular beam epitaxy were studied using atomic force microscopy. The surface of a GaAs layer grown on a (775)B GaAs substrate is flat when it is grown at a substrate temperature (T-s) below 580 degrees C, but corrugates regularly for T-s' greater than or equal to 640 degrees C. Similar periodic corrugation of the (775)B GaAs surface formed during thermal annealing at T-s = 640 degrees C under As-4 atmosphere (10(-6) Torr), which is similar in shape to that of a GaAs layer in the early stage of growth (a GaAs layer thinner than 5 nm). As the thickness of a GaAs layer increases from 5 nm, the lateral period and step height of the corrugation increase and tend to saturate for thick GaAs layers (200 nm thick or more). The precise structure of the surface corrugation of a 3-nm-thick GaAs layer (the lateral period of 12 nm and step height of 1.2 nm) was observed as a corrugated AlAs-on-GaAs interface of a GaAs/(GaAs)(5)(AlAs)(5) quantum well structure with an average well width of 3 nm by cross-sectional transmission electron microscopy observation.

DOI： 10.1143/JJAP.36.6285

CiNii Article

Publishing type：Research paper (scientific journal)  

Extremely uniform and high-density GaAs/(GaAs)(4)(AlAs)(2) quantum wires (QWRs) were self-organized in a thin GaAs/(GaAs)(4)(AlAs)(2) quantum well grown on a (775)B GaAs substrate with a regularly corrugated AlAs-on-GaAs interface and a flat GaAs-on-AlAs interface by molecular beam epitaxy. A strong photoluminescence (PL) peak at lambda=692 nm from the GaAs/ (GaAs)(4)(AlAs)(2) QWRs showed large polarization anisotropy [P = (I-parallel to - I-perpendicular to)/(I-parallel to + I-perpendicular to) = 0.19]. A full width at half-maximum of the PL peak from the QWRs was as small as 15 meV at 14 K, which is smaller than those of any self-organized GaAs/AlGaAs QWRs reported so far. Density of the QWRs (8 x 10(5) QWRs/cm) is the same as the highest ever reported. These results indicate that the QWRs grown on the (775)B GaAs. substrate meet requirements for applications to QWR lasers (high uniformity, high density, high optical quality, and simple fabrication process). (C) 1997 American Institute of Physics.

DOI： 10.1063/1.119769

CiNii Article

Publishing type：Research paper (scientific journal)  

GaAs/(GaAs)(2)(AlAs)(2) quantum wires (QWRs) were naturally firmed in a thin GaAs/(GaAs)(2)(AlAs)(2) quantum well (QW) with a regularly corrugated AlAs/GaAs upper interface (a period of 12 nm) and a flat GaAs/AlAs lower interface grown on (7 7 5)B-orienred GNAs substrates by molecular beam epitaxy. The QWRs were formed side by side with an extremely high density elf 8 x 10(5) QWRs/cm, which is the same to that of the previous GaAs/AlAs QWRs grown on the (7 7 5)B substrate (the highest density of QWRs ever reported [8]). A photoluminescence from the QWRs formed in the QW with an average well width (L-w) of 2.1 nm, which have a cross section of about 12 x 2 nm(2), showed a strong polarization dependence (the polarization degree P drop [(I parallel to - I perpendicular to)/(I parallel to + I perpendicular to)] = 0.21). The polarization degree is about twice as large as that of the precious GaAs/AlAs QWRs with an average L-w of 3.3 nm grown on the (7 7 5)B substrate. This improvement of the polarization degree is mainly due to the further reduced QW width, which results in an improved one-dimensional confinement of carriers in the present QWRs.

DOI： 10.1016/S0022-0248(96)00861-5

Publishing type：Research paper (scientific journal)  

GaAs/AlAs quantum wires (QWRs) were found to be naturally formed by a regularly corrugated AlAs/GaAs interface and a flat GaAs/AlAs interface in an AlAs/GaAs/AlAs quantum well with a well width of 3.3 nm grown on a (775)B GaAs substrate by molecular beam epitaxy. The lateral period and vertical amplitude of the AlAs/GaAs interface corrugation were 12 nm and 1.2 nm, respectively. The QWRs were formed side by side with an extremely high density of 8x10(5) QWRs/cm. A photoluminescence peak at lambda = 715 nm from the QWRs with a cross section of about 12x3 nm(2) showed a polarization degree of (I-parallel to - I-perpendicular to)/(I-parallel to + I-perpendicular to) = 0.11 and a very small full width at half maximum of 15 meV at 4.2 K.

DOI： 10.1143/jjap.35.L606

CiNii Article

Publishing type：Research paper (scientific journal)  

Metalorganic molecular beam etching of a GaAs surface is observed at temperatures as low as 370 degrees C, for the first time, when the GaAs surface is exposed to TDMAAs (trisdimethylaminoarsenic) only. At this low temperature, not only the oxide layer is removed, but a smoothening of the surface also occurs. At higher temperatures (greater than or equal to 480 degrees C), however, this reaction of GaAs with TDMAAs is responsible for a roughening of the (100) GaAs surface, forming (411)A micro-facets. At low V/III beam pressure ratios (less than or equal to 0.5), smooth surfaces as well as good optical and electrical properties are obtained for GaAs layers grown even at high temperatures (550 less than or equal to T(sub)less than or equal to 650 degrees C). TDMAAs proves to be an extremely efficient arsenic source which can supply enough As even for a small V/III ratio of 0.25, and provides high GaAs growth-rates up to 2 mu m/h for a TDMAAs flux of 0.15 SCCM and TEG of 0.4 SCCM.

DOI： 10.1016/0022-0248(94)00895-7

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Total pages：279  

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ASIN

DOI： 10.1007/978-3-030-37153-1_28

Scopus

DOI： 10.1007/978-3-030-37153-1_1

Scopus

DOI： 10.1007/978-3-030-37153-1_32

Scopus

Total pages：xxviii, 764 p.   Book type：Scholarly book

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Total pages：512  

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ASIN

DOI： 10.1142/9789811216480_0002

Scopus

DOI： 10.1002/9781119354987.ch25

Scopus

Total pages：冊  

CiNii Books

Total pages：vii, 251p  

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Total pages：vii, 266p  

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Authorship：Lead author,　Last author,　Corresponding author   Publishing type：Article, review, commentary, editorial, etc. (scientific journal)   International / domestic magazine：Domestic journal  

Repository URL： http://hdl.handle.net/10466/0002001380

Authorship：Lead author,　Last author,　Corresponding author   Publishing type：Article, review, commentary, editorial, etc. (scientific journal)   International / domestic magazine：Domestic journal  

Authorship：Lead author,　Last author,　Corresponding author   Publishing type：Article, review, commentary, editorial, etc. (trade magazine, newspaper, online media)   International / domestic magazine：Domestic journal  

Authorship：Lead author,　Last author,　Corresponding author   Publishing type：Article, review, commentary, editorial, etc. (scientific journal)   International / domestic magazine：Domestic journal  

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J-GLOBAL

J-GLOBAL

J-GLOBAL

Gallium Oxide is attracting attention as a new material for optical and electrical applications. It has a large band gap of 4.8 eV. The electron concentration can be controlled in a range between 10^&lt;16&gt; cm^&lt;-3&gt; and 10^&lt;19&gt; cm^&lt;-3&gt;. Large-size bulk crystals can be obtained easily because melt growth is possible under the atmospheric pressure. From these material features, we expect high performance and low production cost of LEDs and high power devices. In this report, we introduce the properties of β-Ga_2O_3 single crystal substrates and the present status of device development.

CiNii Article

The band alignment of Al_2O_3/n-Ga_2O_3 (010) was investigated by x-ray photoelectron spectroscopy (XPS). With a band gap of 6.8±0.2 eV measured for Al_2O_3, the conduction and valance band offsets at the interface were estimated to be 1.5±0.2 eV and 0.7±0.2 eV, respectively. The conduction band offset was also obtained from tunneling current in Au/Al_2O_3/n-Ga_2O_3 (2^^-01) metal-oxide-semiconductor (MOS) diodes using the Fowler-Nordheim model. The electrically extracted value was in good agreement with the XPS data. Furthermore, the MOS diodes exhibited small capacitance-voltage hysteresis loops, indicating the successful engineering of a high-quality Al_2O_3/Ga_2O_3 interface.

CiNii Article

Depletion-mode gallium oxide (Ga_2O_3) MOSFETs were fabricated on single-crystal β-Ga_2O_3 (010) substrates. We applied Si-ion implantation to the ohmic contacts for low contact resistance as well as to the channel layer for reliable doping. An Al_2O_3 gate dielectric film formed by atomic layer deposition passivated the device surface and significantly reduced gate leakage. The MOSFETs with a simple structure exhibited complete channel pinch-off with a breakdown voltage of 415 V and a drain current on/off ratio of ten orders of magnitude at room temperature. The devices also showed stable operation up to 250℃.

CiNii Article

We have analyzed the structural and electrical properties of Al_2O_3/Ga_2O_3 MOS diodes fabricated on a Ga_2O_3 (010) substrate by using transmission electron microscopy and capacitance-voltage measurements. The formation of a characteristic polycrystalline γ-Al_2O_3 film of about 3-nm thick at the initial stage of deposition was observed. The capacitance-voltage characteristic showed hysteresis, and an interface state density of 1.0×10^<12> cm^<-2> at the Al_2O_3/Ga_2O_3 interface was extracted by the Terman method. These results suggest that the polycrystalline film contributes to the formation of interface states in the Al_2O_3/Ga_2O_3 system.

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We fabricated gallium oxide (Ga_2O_3) Schottky barrier diodes using β-Ga_2O_3 (010) single crystal substrates produced by the floating zone. The crystal quality of the substrates was excellent; the X-ray diffraction rocking curve peak had a full width at half maximum of 32 arcsec, and the etch pit density was less than 1×10^4cm^<-2>. The devices exhibited good characteristics, such as an ideality factor close to the unity and a reasonably high reverse breakdown voltage of about 150V. The Schottky barrier height of the Pt/β-Ga_2O_3 interface was estimated to be 1.3-1.5eV.

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Other URL： http://bizboard.nikkeibp.co.jp/houjin/cgi-bin/nsearch/md_pdf.pl/0000319595.pdf?NEWS_ID=0000319595&CONTENTS=1&bt=NE&SYSTEM_ID=HO

Short-gate AlGaN/GaN heterostructure field-effect transistors (HFETs) with extremely thin AlGaN barrier layers were fabricated and characterized from the viewpoint of millimeter-wave applications. The devices showed good DC and RF small-signal characteristics; however, the 30 GHz power characteristics were degraded. The dispersive behavior of the thin-barrier devices measured in pulse I V curves was different from the commonly observed one. We consider from these results that the unique dispersion of the thin-barrier HFETs was caused by the different charging path related to hot electrons accelerated in a high electric field region of a two-dimensional electron gas channel.

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We investigated effects of surface oxidation by annealing during device process on AlGaN surface barrier height of AlGaN/GaN heterostructures by Hall measurement and x-ray photoelectron spectroscopy (XPS). From AlGaN thickness dependence of two-dimensional electron gas density and XPS analysis, a constant surface barrier height regardless of AlGaN thickness was obtained for the samples annealed at 800℃. On the other hand, the samples annealed at 400℃ showed a proportional increase in the height with the thickness. Moreover, XPS peak intensity related to Al-O bonding significantly increased after annealing at 800℃ regardless of atmosphere and saturated in a short time, even though it changed little after annealing at 400℃ compared with before annealing. We consider from the experimental results and their correlation that Al oxide structure formed on the AlGaN surface significantly affects energy distribution and density of surface donors.

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GaN heterostructure field-effect transistors (HFETs) are promising not only as high-power and high-breakdown-voltage devices but also as high-frequency ones, because the saturation electron velocity in GaN is relatively high. By using three characteristic techniques to improve high-frequency characteristics of GaN HFETs, which are (1) high-Al-composition and extremely thin barrier layers, (2) Cat-CVD SiN passivation, and (3) T-shaped short gates defined by electron-beam lithography, we have succeeded in suppressing short-channel effects in short-gate HFETs with a gate length of less than 0.1μm. Here, we report on device characteristics of short-gate AlGaN/GaN and InAlN/GaN HFETs, which are being developed for applications in the frequency range above 50GHz.

CiNii Article

We have succeeded in suppressing short-channel effects by using high-Al-composition and extremely thin AlGaN barrier layers to improve high-frequency characteristics of sub-0.1-μm AlGaN/GaN heterostructure field-effect transistors (HFETs). The devices had maximum drain current densities of 1.1-1.2A/mm and peak extrinsic transconductances of 305-441mS/mm. The current-gain cutoff frequency (f_T) of 163GHz and maximum oscillation frequency (f_<max>) of 192GHz were obtained for the 8-nm-AlGaN-barrier devices with 0.06- and 0.08-μm-long gates, respectively.

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We fabricated ultra-short-gate AlGaN/GaN high electron mobility transistors (HEMTs) and improved Schottky gate contacts by thermal annealing. Using a 27-nm i-Al_<0.22>Ga_<0.78>N layer, we obtained the gate length L_g dependence of the cutoff frequency f_T for the HEMTs with a source-drain spacing L_<sd> of 2μm under a drain-source voltage V_<ds> of 3 V. The peak f_T was measured to be 102 GHz at L_g=35 nm. The highest f_T of 110 GHz was obtained by reducing L_<sd> from 2 to 1.5 μm and by increasing V_<ds> from 3 to 4V. Using a 10-nm i-Al_<0.25>Ga_<0.75>N layer, we fabricated 120-nm Ni/Pt/Au-gate and Mo/Pt/Au-gate HEMTs. Enhancement-mode operation was achieved by using Ni/Pt/Au-gate HEMTs with thermal annealing at 500℃ and Mo/Pt/Au-gate HEMTs without thermal annealing.

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Studying InN epitaxial growth on a sapphire substrate by RF-MBE, we found that a buffer layer formed with a low-temperature-grown GaN intermediate layer and a low-temperature-grown InN layer on a sapphire substrate was very effective to improve the crystal quality and adhesiveness of InN film. The surface of a 260-nm-thick InN film was quite smooth with the RMS roughness of 3.8 nm. The room-temperature Hall mobility and carrier density of the InN film were 1420 cm^2/V・s and 1.4×10^18 cm^-3, respectively.

CiNii Article

InP-based InGaAs/InAlAs high electron mobility transistors (HEMTs) are considered to be one of the most promising devices for millimeter-wave radio communications because of their superior high frequency and low-noise performances. This is due to high electron mobility, high saturation velocity, and high sheet carrier density obtained in this system. A shorter gate is also essential for ultra-high RF performances. In our previous work, RF performances have been significantly improved by reducing gate length (L_g) down to 50 nm using a simple lift-off process and a low temperature process. In this paper, we have developed a sub-50-nm T-shaped-gate fabrication technique using a conventional tri-layer resist system by optimizing electron beam (EB) lithography and reactive ion etching (RIE) conditions, demonstrating a cutoff frequency (f_T)as high as 318 GHz in a 35-nm-gate HEMT.

CiNii Article

CiNii Article

CiNii Article

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