Yoshinao Kumagai

Si-doped n-type β -Ga ₂ O ₃ (010) homoepitaxial layers with Si concentrations ([Si]) ranging from 2.1 × 10 ¹⁵ to 1.5 × 10 ¹⁹ cm ^–3 were grown by low-pressure hot-wall metalorganic vapor phase epitaxy using trimethylgallium, O ₂ , and tetramethylsilane (TMSi). The [Si] increased linearly with increasing TMSi supply ratio, and the room-temperature carrier density closely matched [Si], confirming that Si acts as an efficient shallow donor. High crystalline quality was maintained throughout the investigated [Si] range, although surface roughening occurred for [Si] on the order of 10 ¹⁹ cm ^–3 . Temperature-dependent Hall-effect measurements revealed carrier freeze-out upon cooling in samples with [Si] ⩽ 1.8 × 10 ¹⁷ cm ^–3 , whereas the electron mobility showed pronounced maxima at low temperatures. By contrast, the highest-[Si] sample exhibited degenerate conduction above the critical carrier density for the Mott metal–insulator transition, with virtually no carrier freeze-out and almost no temperature dependence of the electron mobility.

We investigated effects of nitrogen radical irradiation on device characteristics of vertical β-Ga2O3(010) fin field-effect transistors (FinFETs). A positive shift in threshold voltage with improved in-plane distribution uniformity occurred for nitridated FinFETs due to fixed negative charges formed in nitridated layers on fin sidewalls, indicating that the nitridation treatment can effectively re-establish the post-etched sidewall surface conditions. The breakdown voltage (Vbr) was also uniformly distributed for the nitridated FinFETs, while the specific on-resistance (Ron) was reduced by adopting multi-fin structure and increasing fin width. The nitridated Ga2O3(010) multi-FinFET with a fin width of 300 nm achieved normally off operation with superior device characteristics of a subthreshold swing of 73 mV/decade, an Ion/Ioff ratio of >1010, an Ron of 9.5 mΩ cm2, a Vbr of 1213 V, and a power figure of merit of 1.55 × 108 W/cm2.

The gaseous species present in a low-pressure hot-wall metalorganic vapor phase epitaxy reactor for β -Ga ₂ O ₃ growth using trimethylgallium (TMGa) as the Ga precursor were analyzed in situ by high-resolution mass spectrometry to clarify the conditions required for high-purity layer growth. Using Ar as the carrier gas, TMGa was found to decompose rapidly above 575 °C, generating hydrocarbons such as C ₂ H ₄ , CH ₄ , C ₂ H ₆ , and C ₂ H ₂ as sources of carbon (C) and hydrogen (H) contamination in the grown β -Ga ₂ O ₃ layers. However, these hydrocarbons were completely combusted by O ₂ , generating CO ₂ and H ₂ O, when β -Ga ₂ O ₃ growth was carried out at high temperatures (≥900 °C) and at high input VI/III ratios of around 1000. The conditions required for complete combustion were verified using an industrial-scale reactor, which also confirmed that high-rate homoepitaxial growth without C and H incorporation can be achieved, thereby providing guidelines for the growth of high-purity β -Ga ₂ O ₃ .

Electrical properties of Si-doped Ga2O3 (010) homoepitaxial thin films grown by low-pressure hot-wall metalorganic chemical vapor deposition (MOCVD) and the effects of high-temperature post-deposition annealing (PDA) on them were studied through device characteristics of Schottky barrier diodes (SBDs) fabricated on the epitaxial substrates. All the SBDs showed ideal n-type Schottky characteristics with excellent in-plane uniformity at room temperature. Temperature-dependent forward current density–voltage (J–V–T) characteristics showed nearly constant ideality factors of 1.02 ± 0.01 from room temperature to 200 °C. The reverse breakdown V of the SBDs with PDA was about 100 V larger than those of the ones without PDA. Furthermore, reverse J–V–T characteristics of the PDA SBDs were well reproduced by the thermionic field emission model for the whole temperature range up to 200 °C. These results indicate that the high-temperature PDA treatment is a useful and effective technique to further improve electrical properties of the MOCVD-grown Ga2O3 epitaxial films.

N-doped β -Ga ₂ O ₃ (010) homoepitaxial layers were grown by halide vapor phase epitaxy using NH ₃ as the doping source. The N concentration, [N], was controlled by varying the input partial pressure of NH ₃ ( P NH 3 0 ) while keeping the partial pressure of O ₂ ( P O 2 0 ) constant. The [N] value was approximately proportional to the control parameter R _N = P NH 3 0 / ( P NH 3 0 + 2 P O 2 0 ) , and increased from about 3.1 × 10 ¹⁸ to 1.3 × 10 ²¹ cm ⁻³ as R _N was raised. At [N] ∼ 10 ²¹ cm ⁻³ , surface roughening, cracking, and degradation of the crystallographic orientation occurred, whereas high crystallinity was maintained for [N] up to the mid–10 ¹⁹ cm ⁻³ range.

By using tetramethylsilane (TMSi) as a Si dopant source, thick intentionally Si-doped β-Ga₂O₃ homoepitaxial layers were grown by low-pressure hot-wall metalorganic vapor phase epitaxy. The Si concentration was linearly controlled by varying the TMSi supply, and a RT electron density nearly equal to the Si concentration was achieved in the range from 1.8 × 10¹⁶ to 1.3 × 10¹⁹ cm^–3. For the layer with a Si concentration and RT electron density of 1.8 × 10¹⁶ cm^–3, the RT electron mobility of 136 cm²​​​​​ V^–1 s^–1 was found to be limited by polar optical phonon scattering.

Roles of Al-vacancy (VAl) complexes on the cathodoluminescence (CL) spectra of Si-doped AlN grown by halide vapor phase epitaxy (HVPE) on a physical-vapor-transported (0001) AlN substrate are described, making a connection with the results of positron annihilation measurements. A combination of HVPE and AlN substrate enabled decreasing deleterious carbon concentration and dislocation density, respectively, thus accentuating the influences of VAl-complexes on the luminescence processes. A low-temperature CL spectrum of unintentionally doped AlN exhibited predominant excitonic emissions at around 6 eV and a marginal deep-state emission band at around 3.7 eV that originates from residual carbon (<1016 cm−3) on nitrogen sites (CN). However, the sample was revealed to contain a considerable amount (∼1017 cm−3) of vacancy clusters, most likely comprising a VAl and nitrogen-vacancies (VN), namely, VAlVN1−2, which act as nonradiative recombination centers that decrease overall CL intensity at elevated temperatures. With increasing Si-doping concentration ([Si]), major vacancy species progressively changed from VAlVN1−2 to VAlON1−2, where ON is oxygen on N sites, which exhibit other deep-state emission bands ranging from 3.2 to 3.5 eV. Further increase in [Si] gave rise to the formation of donor-compensating defects comprising VAl and Si on the second-nearest-neighbor Al sites (SiAl), abbreviated by VAl−SiAln, which exhibit emission shoulders at around 2.9–3.0 eV. When [Si] exceeded 5 × 1018 cm−3, an emission band at around 4.5 eV emerged, which had been ascribed to originate from the nearest-neighbor SiAlCN complexes. Because VAl-complexes, including those containing impurities, are thermally stable, incorporation of vacancies should be blocked at the growth stage.

Ga₂O₃ fin field-effect transistors (FinFETs) were fabricated on β-Ga₂O₃ (010) substrates, which had on-axis (100)-plane gate sidewalls treated by nitrogen radical irradiation. The typical FinFET with a fin width (W _fin) of 400 nm demonstrated decent on-state device characteristics such as a low specific on-resistance of 6.9 mΩcm², a high drain current on/off ratio of over 10⁹, and a subthreshold slope of 82 mV/decade. The threshold voltage (V _th) of the FinFETs increased with decreasing W _fin, and enhancement-mode operation with V _th > 0 V was achieved for W _fin < 800 nm.

We investigated the electrical properties of unintentionally doped (UID) Ga₂O₃ (010) layers grown by low-pressure hot-wall metalorganic chemical vapor depositions from device characteristics of Schottky barrier diodes (SBDs) fabricated on them. Highly resistive properties of the UID Ga₂O₃ layers were confirmed from current–voltage characteristics. The specific on-resistance of the SBD with the most resistive UID Ga₂O₃ layer was 2.2 × 10⁷ Ωcm². Capacitance–voltage characteristics revealed that most of the SBDs had complete depletion of the UID layers at thermal equilibrium, indicating that their residual effective donor densities were less than 3.0 × 10¹³ cm⁻³.

High-speed growth of thick, high-purity β-Ga₂O₃ homoepitaxial layers on (010) β-Ga₂O₃ substrates by low-pressure hot-wall metalorganic vapor phase epitaxy was investigated using trimethylgallium (TMGa) as the Ga precursor. When the reactor pressure was 2.4–3.4 kPa, the growth temperature was 1000 °C, and a high input VI/III (O/Ga) ratio was used, the growth rate of β-Ga₂O₃ could be increased linearly by increasing the TMGa supply rate. A thick layer was grown at a growth rate of 16.2 μm h⁻¹ without twinning. Incorporated impurities were not detected, irrespective of the growth rate, demonstrating the promising nature of β-Ga₂O₃ growth using TMGa.