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Thermodynamic analyses for the growth of group-III sesquioxides, including α-Al2O3, β-Ga2O3, and c-In2O3, by both ozone and plasma-assisted MBE were performed. In either case, under O-rich conditions, the driving force for III2O3 (III = Al, Ga, In) growth ( Δ P III 2 O 3 ) increased with increasing input partial pressure of the group-III metal ( P III o ), without generation of metal droplets. Conversely, under group-III-metal-rich conditions, Δ P III 2 O 3 decreased with increasing P III o and/or decreasing input partial pressure of O3 or O. This decrease was caused by the formation of Ga2O or In2O during growth of β-Ga2O3 and c-In2O3. The decrease of Δ P Al 2 O 3 was smaller because the equilibrium constant of α-Al2O3 formation reaction was very large. Ga and In droplets formed at low temperatures (2O3 < β-Ga2O3 << α-Al2O3.

The heteroepitaxial growth of In2O3 on a (0001) α-Al2O3 substrate was carried out using the mist chemical vapor deposition method. The concentrations of In2O3 powder in the source precursor solution varied between 0.025 and 0.250 mol l−1. An increase in the growth rate was observed with the increasing In2O3 powder concentration, and the highest growth rate of 4.2 μm h−1 was obtained at a powder concentration of 0.150 mol l−1. An improvement in the electrical properties was eventually observed, i.e. the lowest carrier concentration of 3.1 × 1017 cm−3 and the highest Hall mobility of 241 cm2 V−1 s−1 at room temperature were obtained at a powder concentration of 0.150 mol l−1. The temperature-dependent Hall effect measurement for the sample showed pronounced carrier freeze-out in a temperature range of 300–30 K, indicating that the α-In2O3 film had near-non-degenerate conductivity.

In metalorganic vapor phase epitaxy of β-Ga2O3 using triethylgallium (TEGa) and O2 as precursors and Ar as the carrier gas, the gases directly above the substrate were sampled and analyzed by time-of-flight mass spectrometry. TEGa was found to decompose at 400 °C–600 °C via β-hydrogen elimination reaction to generate gaseous Ga, hydrocarbons (C2H4, C2H2, C2H6), and H2. When β-Ga2O3 was grown at temperatures greater than 1000 °C and with input VI/III ratios greater than 100, the hydrocarbons and H2 were combusted and CO2 and H2O were generated. The C and H impurity concentrations measured by secondary-ion mass spectrometry in the β-Ga2O3(010) homoepitaxial layer grown under these conditions were less than their respective background levels. Thus, to grow β-Ga2O3 without C and H contamination, conditions that favor the complete combustion of hydrocarbons and H2 generated by the decomposition of TEGa should be used.

β-Ga2O3 substrates and homoepitaxial films were characterized using the multiphoton excitation photoluminescence (MPPL) method. MPPL emission peaks at 3.2 and 3.36 eV were obtained with broad shoulders on the low energy side. MPPL images showed the presence of nanopipes in the substrate with a unique contrast. Three-dimensional MPPL imaging revealed the distribution and spatial location of the nanopipes. All the nanopipes were elongated along the [010] direction with diameters of approximately 0.4–1.0 μm and lengths of 10–30 μm. The nanopipes were aligned in the [100] and [001] directions with sub-micron spacing in the (0 1 ¯ 0)-oriented substrate. The nanopipes were located under the hillocks at the homoepitaxial surface. The nanopipes are suggested to trigger the hillock growth by transforming into threading dislocations in the homoepitaxial film.

The electrical properties of beta-gallium oxide (β-Ga2O3) and gallium arsenide semiconductors were characterized using the emerging terahertz time-domain ellipsometry (THz-TDE) technique. The dielectric and conductivity properties were obtained from the complex ratio of the measured p- and s-polarized THz pulses reflected from the samples. The carrier concentration and mobility were then deduced using the Drude model, and the results showed good accuracy. This work demonstrates THz-TDE as a promising tool for characterizing semiconductors, especially those with high carrier concentrations and significant absorption in the THz region.

Thermodynamic analyses of β-Ga2O3 growth by both ozone and plasma-assisted molecular beam epitaxy (MBE) 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 ( Δ P Ga 2 O 3 ) was determined to increase linearly with increasing Ga input partial pressure ( P Ga o ) because almost all the supplied Ga was used for the growth of the β-Ga2O3. In contrast, Ga-rich conditions (VI/III Δ P Ga 2 O 3 to decrease. Etching of the β-Ga2O3 occurred with increasing P Ga o 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 MBE can be explained by thermodynamics.

The growth of twin-free single-crystal cubic-indium oxide (c-In2O3) layers was investigated by halide vapor phase epitaxy on c-plane sapphire substrates with various off-axis angles. The growth rate of the c-In2O3 layer increased and twin formation was suppressed as the off-axis angle of the substrate was increased. A single-crystal c-In2O3(111) layer grown on a sapphire substrate with a 5° off-axis angle showed a room temperature carrier density and mobility of 1.4 × 1016 cm−3 and 232 cm2 V−1 s−1, respectively. Temperature-dependent Hall measurements of the layer revealed that the mobility is dominated by optical phonon scattering.

Thermodynamic analysis and experimental demonstration of β-Ga2O3 growth by metalorganic vapor phase epitaxy using triethylgallium (TEG) and oxygen (O2) precursors were performed. Thermodynamic analysis revealed that the O2 supplied is preferentially used for the combustion of hydrocarbons and H2 derived from TEG. Therefore, the use of high growth temperatures and high input VI/III ratios is essential for the complete combustion of hydrocarbons and H2, and β-Ga2O3 growth. The use of an inert gas as the carrier gas was also determined as necessary to grow β-Ga2O3 at high temperatures. Based on these results, a ( 2 ¯ 01) oriented smooth β-Ga2O3 layer could be grown on a c-plane sapphire substrate at 900 °C with a growth rate of 1.4 μm h−1 at an input VI/III ratio of 100. The grown layer showed a clear optical bandgap of 4.84 eV, and impurity concentrations of hydrogen and carbon were below the background levels of the measurement system.

The thermal stability of +/- c- and m-plane GaN substrates and its dependence on the m-plane GaN off-cut angle were investigated at and above 1200 degrees C at atmospheric pressure in flowing N-2 w ...

The homoepitaxial growth of thick β-Ga2O3 layers on 2-in.-diameter (001) wafers was demonstrated by halide vapor phase epitaxy. Growth rates of 3 to 4 µm/h were confirmed for growing intentionally Si-doped n-type layers. A homoepitaxial layer with an average thickness and carrier concentration of 10.9 µm and 2.7 × 1016 cm−3 showed standard deviations of 1.8 µm (16.5%) and 0.5 × 1016 cm−3 (19.7%), respectively. Ni Schottky barrier diodes fabricated directly on a 5.3-µm-thick homoepitaxial layer with a carrier concentration of 3.4 × 1016 cm−3 showed reasonable reverse and forward characteristics, i.e., breakdown voltages above 200 V and on-resistances of 3.8–7.7 mΩ cm2 at room temperature.

The thermal stability and reaction with H2 of the surface of a (1012) R-plane sapphire substrate at high temperatures were investigated using an in situ gravimetric monitoring system. Although the R-plane sapphire surface was stable up to 1400 °C in an inert carrier gas, decomposition started at 1200 °C in H2 carrier gas. In addition, the activation energy for the decomposition of R-plane sapphire in H2 carrier gas changed at approximately 1300 °C, which indicates that the rate-limiting reaction for the decomposition of the R-plane sapphire substrate shifts near 1300 °C. These results agree with those for the stability of the C-plane sapphire substrate, as described previously. However, it was found that decomposition rate of the R-plane sapphire substrate was significantly greater than that for the C-plane substrate. The relationship between the decomposition rate and the surface orientation of sapphire and detailed mechanisms for the decomposition of R- and C-plane sapphire in H2 carrier gas are discussed.

We have investigated the surface structures of AlN and InN using ab initio calculations based on the density functional theory within generalized gradient approximation. We studied the surface energies obtained from total energy calculations for various (2×2) geometries of cation-terminated (0001) surfaces and anion-terminated (0001) surfaces of AlN and InN, with hydrogen in a carrier gas. It was found that the structures with N–H bonds were favorable under hydrogen ambient, while in the absence of hydrogen, the structures with metal adatoms and N adatoms tended to be stable under metal-rich conditions and N-rich conditions, respectively.

The direct growth of a GaN epitaxial layer on a Si(111) substrate by metalorganic vapor phase epitaxy (MOVPE) was performed using a low-temperature (LT)-GaN buffer layer with no Al-containing intermediate layer (e.g., AlN or AlGaN). No deterioration in the Si surface caused by the reaction between Si and Ga vapor was observed. However, when there were Ga droplets on the surface, Ga and Si formed a Ga?Si alloy, which caused the generation of numerous holes on the surface by melt-back etching at high temperatures. In addition, it was revealed that the coverage of the LT-GaN buffer layer on Si was strongly affected by the hydrogen (H2) partial pressure in the carrier gas. Using nitrogen (N2) carrier gas, a complete coverage of the LT-GaN buffer layer could be achieved directly over the Si surface. These features can be explained by the facts that the Si surface is partially terminated by hydrogen atoms and the coverage of hydrogen on Si surface depends on H2 partial pressure.

We investigate the desorption of As from GaAs(001) surfaces under an atmosphere of hydrogen using the ab initio pseudopotential method. The GaAs surface is (2×4) reconstructed with arsenic-monohydrogen bonds, arsenic-dihydrogen bonds, and Ga–Ga dimer bonds. We propose a mechanism for As desorption from the (001) surface in the presence of ambient hydrogen. The calculated activation energy is close to the experimental value for the (1×1) to (2×4) transition according to an in situ gravimetric (GM) method.

Thermodynamic analyses were carried out to understand the influence of lattice constraint from InN and GaN substrates on the relationship between solid composition x of InxGa1-xN films and input mole ratio RIn (=PIn0/(PIn0+PGa0), where Pi0 is the input partial pressure of element i) during molecular beam epitaxy. The calculation results suggest that a compositionally unstable region is found at the GaN-rich region for InGaN on InN at higher temperatures while that for InGaN on GaN can be seen at the InN-rich region. This is because the maximum enthalpy of mixing shifts toward x~0.10 for InGaN on InN and toward x~0.80 for InGaN on GaN compared with x~0.50 for stress-free InGaN.

The composition pulling effect in metalorganic vapor-phase InGaN epitaxy was theoretically investigated by thermodynamic analysis. The excess energies of biaxial-strained In x Ga1− x N were numerically calculated using empirical interatomic potentials considering different situations: (i) coherent growth on GaN(0001), (ii) coherent growth on In0.2Ga0.8N(0001), and (iii) bulk growth. Using the excess energies, the excess chemical potentials of InN and GaN alloys were computed. Our results show that compressive strain suppresses In incorporation, whereas tensile strain promotes it. Moreover, assuming chemical equilibrium, the relationship between the solid composition and the growth conditions was predicted. The results successfully reproduced the typical composition pulling effect.

We performed a thermodynamic analysis of GaN metalorganic vapor phase epitaxy considering the (0001) and surface states. Surface reconstruction, which depends on growth conditions such as temperature and partial pressure, affects growth processes. To discuss the effects of surface states on growth processes, we investigated the driving force of precursor deposition to form the surface phase defined stoichiometrically. In both N2 and H2 carrier gas cases, we showed surface phase diagrams, calculated driving forces, and discussed the difference in growth orientation.

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.

The thermal and chemical stabilities of group-III sesquioxides (Al2O3, Ga2O3, and In2O3) were comparatively investigated at an atmospheric pressure at heat treatment temperatures ranging from 250 to 1450 °C in a flow of either N2 or H2. In a flow of N2, the thermal decomposition of α-Al2O3 was not observed at the temperatures investigated, while the decompositions of β-Ga2O3 and c-In2O3 occurred above 1150 and 1000 °C, respectively, with no generation of group-III metal droplets on the surfaces. In contrast, the chemical reactions of α-Al2O3, β-Ga2O3, and c-In2O3 began at low temperatures of 1150, 550, and 300 °C in a flow of H2. Thus, the presence of H2 in the gas flow significantly promotes the decomposition of group-III sesquioxides. The order of thermal and chemical stabilities (α-Al2O3 ≫ β-Ga2O3 > c-In2O3) obtained experimentally was verified by thermodynamic analysis, which also clarified dominant decomposition reactions of group-III sesquioxides.

In this work, the first-ever growth of cubic-In2O3 at 1000 °C by halide vapor phase epitaxy (HVPE) was achieved, using gaseous InCl and O2 as precursors in a N2 flow. The growth rates of In2O3 layers on (001) β-Ga2O3 and (0001) sapphire substrates were 4.1 and 5.1 µm/h, respectively, even at the high growth temperature applied, which are approximately half the growth rate of β-Ga2O3 homoepitaxially grown by HVPE at 1000 °C. Theoretical thermodynamic analyses were also conducted, and results confirmed the growth of In2O3 at temperatures above 1000 °C by HVPE. The as-grown In2O3 layers were light yellow-green in color. The In2O3 layers grown on the (001) β-Ga2O3 and (0001) sapphire substrates exhibited an optical absorption edge at about 370 nm, in addition to n-type conductivity with electron concentrations of 2.7 × 1018 and 1.7 × 1018 cm−3, and electron mobilities of 16.2 and 22.7 cm2 V−1 s−1, respectively.

The decomposition processes of AlN from its surface in a hydrogen atmosphere were studied using the ab initio calculation method based on the density functional theory. The activation energies of Al- and N-atoms desorption from (0001)AlN surface were calculated. It was found that Al-atoms desorb from (0001)AlN surface as AlH molecules, and N-atoms as NH3 molecules. Desorption of AlH molecules is a rate limiting reaction which conforms well with experimental data.

A freestanding (0001)AlN substrate with a thickness of 112 µm was successfully prepared by hydride vapor phase epitaxy (HVPE) of AlN at 1230 °C on a (111)Si starting substrate and subsequent removal of the Si substrate using a chemical etchant. The AlN substrate was transparent and displayed a smooth surface without cracks. Plan-view transmission electron microscopy (TEM) observations at the top surface of the AlN substrate revealed that an average dislocation density of 3 ×109 cm-2 was achieved. The dislocations were found to run inclined from the direction of growth. The AlN substrate exhibited a near-band-edge emission at 209.4 nm in the room-temperature photoluminescence spectrum.

The hydride vapor phase epitaxy (HVPE) of InN with various configurations was studied by thermodynamic analysis. For the high-speed growth of InN, analyses of the source zone and InN growth zone were performed. It was shown that the use of Cl2 instead of HCl as a chlorine source is necessary for the HVPE of InN. An additional supply of Cl2 was found to be effective for the preferential generation of a reactive InCl3 precursor. A new system, the (In–Cl2)–Cl2–NH3 system, was proposed for the HVPE of InN with a high growth rate.

The formation mechanism of AlN whiskers on sapphire substrates during heat treatment in a mixed flow of H2 and N2 was investigated in the temperature range of 980–1380 °C. AlN whiskers grew above 1030 °C after covering the sapphire surface with a thin AlN layer. The existence of pits on the sapphire surface beneath the thin AlN layer was observed. Both AlN whisker and pit densities of samples were on the same order of 108 cm−2. These results suggested the following mechanism. First, the sapphire surface reacts with H2, and the generated Al gas reacts with N2 to form a thin AlN layer on sapphire. Then, the sapphire surface reacts with H2 diffusing to the AlN/sapphire interface. The Al gas escapes through dislocations in the AlN layer to leave pits on the sapphire surface, and finally reacts with N2 to form AlN whiskers on the top surface.

The effects of temperature ramping rate on GaAs (111)A substrate covered with a thin GaN buffer layer grown at 550°C are investigated in order to determine the ideal parameters for protecting the substrate from thermal decomposition during the growth of a thick GaN layer at 1000°C by metalorganic hydrogen chloride vapor-phase epitaxy (MOHVPE). Crystallization and agglomeration of the GaN buffer layer during heating is found to increase markedly as the temperature ramping rate is decreased, and thus high ramping rates are found to be applicable to the high temperature growth of GaN on GaAs substrate. When a temperature ramping rate of 20.9°C/min is used, a thick GaN layer exhibiting no voids or cracks is obtained due to the protection of the GaAs substrate.

The influence of NH3 input partial pressure on N-polarity InGaN grown by tri-halide vapor phase epitaxy was investigated. It was found that surface morphology, solid composition and optical properties were affected by NH3 input partial pressure. As shown in thermodynamic analyses, the indium content increased due to an increase in the driving force for InN deposition caused by increased NH3 input partial pressure. In addition, the deep level emission around 2.1 eV in photoluminescence measurements drastically decreased at higher NH3 input partial pressures. Ab initio calculations and subsequent secondary ion mass spectrometry measurements suggested the reduction of metal-vacancies and/or carbon impurity incorporation in the InGaN layers.

The thermal stability of β-Ga2O3(010) substrates was investigated at atmospheric pressure between 250 and 1450 °C in a flow of either N2 or a mixture of H2 and N2 using a radio-frequency induction furnace. The β-Ga2O3 surface was found to decompose at and above 1150 °C in N2, while the decomposition of β-Ga2O3 began at only 350 °C in the presence of H2. Heating β-Ga2O3 substrates in gas flows containing different molar fractions of H2 demonstrated that the decomposition was promoted by increasing the H2 molar fractions. Thermodynamic analysis showed that the dominant reactions are in N2 and in a mixed flow of H2 and N2. The second-order reaction with respect to H2 determined for the mixed flows agrees with the experimental results for the dependence of the β-Ga2O3 decomposition rates on the H2 molar fraction.

This is the first study to report InGaN growth by tri-halide vapor phase epitaxy (THVPE) using InCl3 and GaCl3 generated by the reactions between metal sources (i.e., metallic indium and gallium) and gaseous chlorine. The influence of the surface orientation of the initial substrate on InGaN-THVPE growth was investigated using freestanding (0001) and GaN substrates. Only a N-polar InGaN epitaxial layer was obtained by THVPE because of the instability of GaCl3 adsorption toward nitrogen atoms on the Ga-polar surface. In addition, we investigated the influence of the group-III input partial pressure on the growth rate and solid composition of InGaN layers grown on a GaN substrate. The growth rate increased linearly with group-III concentration, and a maximum growth rate of 15.6 µm/h was achieved at P III = 2.0 × 10−3 atm.

Orientation dependent decomposition of sapphire substrates and resultant AlN formation during heat treatment in an atmospheric-pressure mixed gas flow of H2 and N2 (H2/N2= 3/1) was investigated within the temperature range 980–1480 °C. AlN was formed on sapphire in the temperature range 1030–1430 °C for c-, a-, and m-plane sapphire, and 980–1430 °C for the r-plane sapphire. At 1480 °C, AlN was not formed, and only sapphire was decomposed by H2 with the ranking of m- > r- > a- > c-plane. The ranking was contrary to that of the amount of AlN formation at 1380 °C, which occurred by the reaction of gaseous Al generated by the sapphire decomposition and N2. This discrepancy was due to the shape of AlN formed on sapphire; whisker-like AlN does not protect c- and a-plane sapphire from decomposing, while layer-like AlN protects r- and m-plane sapphire from decomposing.

The optical properties of (0001) ZnO layers grown at 1000 °C on (0001) sapphire substrates by halide vapor phase epitaxy (HVPE) were investigated by various photoluminescence (PL) measurements. A layer grown with a H2O/ZnCl2 (VI/II) ratio of 20 on a 0.4-µm-thick buffer layer exhibited a significant near-band-edge (NBE) peak blueshift and degraded internal quantum efficiency (ηint) due to residual compressive stress. Growth with a VI/II ratio of 600 diminished the NBE peak blueshift; however, deep level emission and a reduction of PL decay time (τPL) were caused by point defects generated by excess O source supply. A layer without the NBE peak blueshift and deep level emission was realized by growth with a VI/II ratio of 20 and a buffer layer of 0.8 µm. The ηint and τPL for HVPE-grown layers could be improved to 4.1% and 122.8 ps by using the thick buffer layer and appropriate VI/II ratio.

The heteroepitaxial growth of (0001) ZnO on (0001) sapphire substrates by halide vapor phase epitaxy using a two-step growth procedure was investigated. X-ray diffraction analysis revealed that single-crystal (0001) ZnO layers on (0001) sapphire substrates were grown at 400 °C. High-temperature heteroepitaxy at 1000 °C on (0001) sapphire substrates was realized by two-step growth using the ZnO layer grown at 400 °C as a buffer layer. Two-dimensional layer growth at 1000 °C was realized on buffer layers thicker than 0.4 µm. Photoluminescence (PL) measurements performed at room temperature for the ZnO layer grown on the 0.4-µm-thick buffer layer showed a significant blueshift of near-band-edge emission (NBE). A thick buffer layer of 0.8 µm was found to be necessary for a successful two-step growth without a blueshift of NBE in the PL spectra, which is caused by a large compressive stress.

Using thermodynamic analysis, the solid compositions of Mg in Mg x Zn1-x O grown by halide vapor phase epitaxy (HVPE) are calculated in terms of the various input partial pressures of the gaseous species. It is revealed that the Mg composition shows temperature stability up to 1200 °C, while there is a strong dependence on the input VI/II ratio and the input H2 partial pressure at high temperatures. Based on the thermodynamic calculations, the epitaxial growth of Mg x Zn1-x O layers with different Mg compositions has been demonstrated. The thermodynamic model is found to accurately describe the experimentally observed dependence of Mg compositions on the input VI/II ratio that is used in the growth of Mg x Zn1-x O.

The thermal decomposition and hydrogen etching of a 6H-SiC(0001) Si-face were directly monitored using an in situ gravimetric monitoring system. The monitoring of the weight change of the 6H-SiC Si-face using this system clarified the dependences of the thermal decomposition and hydrogen etching rates on the substrate temperature. Although the thermal decomposition of the 6H-SiC Si-face above 1400 °C generated a graphite layer since only the Si atom directly desorbs from the surface, the etching of the 6H-SiC Si-face by hydrogen did not form this layer, and both Si and C atoms react with hydrogen. Moreover, the surface reaction of the 6H-SiC Si face with H2 and the resultant surface morphology were found to change at approximately 1250 °C.

The indium composition of the InGaN film increases with decreasing growth temperature; however, the crystalline quality of the film is poor when it is grown at low temperatures. To form a high-quality InGaN film with a large indium mole fraction, Nd: YAG pulse laser assisted metalorganic vapor phase epitaxy (MOVPE) was carried out at low temperatures. The results suggest that film quality can be improved by pulse laser irradiation on the surface of the film.

Polarity dependence on GaN decomposition has been investigated by an in situ gravimetric monitoring (GM) method using freestanding GaN (0001). In the H2 carrier gas ambient, the decomposition rate of both GaN (0001) and GaN (0001̄) increased with increasing substrate temperature. The decomposition rate of GaN (0001̄) was faster than that of GaN (0001̄) at temperatures below 820°C, whereas the decomposition rate of GaN (0001) was faster than that of GaN (0001) in the temperature range above 850°C. The decomposition rate as a function of the hydrogen partial pressure (P H2 ) has been observed. The rate-limiting reaction of the GaN decomposition on both surfaces is shifted from N(surface) + 3/2H2(g)→NH3(g) to Ga(surface) + 1/2H2(g) →GaH(g) with increase of temperature.

A freestanding GaN substrate over 2 inches in size was successfully prepared for the first time by hydride vapor phase epitaxy (HVPE) using GaAs as a starting substrate. In the experiment, a GaAs (111)A substrate with a SiO2 mask pattern on its surface was used. A thick GaN layer was grown on the GaAs substrate at 1030°C through the openings in the SiO2 mask. By dissolving the GaAs substrate in aqua regia, a freestanding GaN substrate about 500 µm thick was obtained. The full-width at half maximum (FWHM) in the ω-mode X-ray diffraction (XRD) profile of GaN (0002) plane was 106 arcsec. The dislocation density of the GaN substrate obtained was determined to be as low as 2×105 cm-2 by plan-view transmission electron microscopy (TEM). Hall measurements revealed the n-type conductivity of the GaN substrate with typical carrier concentration and carrier mobility of 5×1018 cm-3 and 170 cm2·V-1·s-1, respectively.

Decomposition of GaN on its surface was investigated under atmospheric pressure using the in situ gravimetric monitoring (GM) method. Weight change of the GaN substrate both in the H2 carrier gas and in the He carrier gas ambient was monitored at temperatures ranging from 600°C to 950°C with and without the presence of NH3 flow. It was found that the GaN decomposition did not occur in the presence of NH3 flow both in the H2 carrier gas and in the He carrier gas. However, without NH3 flow, the decomposition rate of GaN drastically increased as the temperature increased in the H2 carrier gas, whereas the decomposition of GaN was negligible in the He carrier gas. Dependence of the decomposition rate on the H2 partial pressure in the carrier gas (P H 2 ) was also investigated, and it was found that the decomposition rate is proportional to the P H 2 3/2. These results indicate that the decomposition is governed by the reaction of GaN(surface)+3/2H2(g)→Ga(surface)+NH3(g).

Thick hexagonal GaN was grown on GaAs (111)A surfaces by metalorganic hydrogen chloride vapor phase epitaxy (MOHVPE) in the temperature range from 920°C to 1000°C. Both the surface morphology and the photoluminescence (PL) property of the grown layer were greatly improved with increase of the growth temperature up to 1000°C. However, the full-width at half maximum (FWHM) in the ω mode X-ray diffraction (XRD) of the GaN (0002) plane increased with increasing growth temperature above 960°C, due to the bending of the grown layer. The bending could be suppressed by growing a thicker layer, even at 1000°C. A mirror-like GaN layer with the FWHM value of 4.7 min was obtained by growing a 100-µm-thick layer at 1000°C, which indicates that the growth of a thick GaN layer on the GaAs (111)A surface is a promising method for the preparation of freestanding GaN substrates.

It was found that a 50-nm-thick GaN buffer layer grown by metalorganic hydrogen chloride vapor-phase epitaxy (MOHVPE) at 550°C on a GaAs (111)A surface did not deteriorate even after subsequent heating in an NH3 ambient up to 1000°C, whereas numerous pinholes were formed when the buffer layer was grown on a GaAs (111)B surface. In addition, serious arsenic (As) desorption from the GaAs substrate occurred through the pinholes. A single crystal hexagonal GaN layer with a mirror-like surface can be grown at 1000°C on the 50-nm-thick GaN buffer layer grown at 550°C on the GaAs (111)A surface, as confirmed by scanning electron microscopy (SEM) and X-ray diffraction (XRD) analyses. The full-width at half maximum (FWHM) of the ω mode scan for the GaN (0002) plane was 1450 arcsec. Photoluminescence (PL) measurement showed a sharp band-edge emission at 363.1 nm with a FWHM of 96 meV at room temperature.

In situ gravimetric monitoring (GM) of atomic-layer epitaxy (ALE) of cubic GaN on a GaN buffer layer/GaAs (001) is investigated using a halogen-transport system with GaCl and NH3 sources. The cubic GaN growth rate of one monolayer/cycle is obtained at a temperature ranging from 350 to 400°C. It is found that pure cubic GaN can be grown by halogen transport ALE. The growth rate decreases with increasing growth temperature, and a constant growth rate of about 0.45 is observed from 410 to 550°C. In this paper, it is shown that the in situ GM method is a powerful tool for understanding growth mechanism of the group III nitrides, as well as that of GaAs.