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An ab initio-based approach is used to investigate the facet stability of GaN during tri-halide vapor phase epitaxy (THVPE). First, surface reconstructions are analyzed to create surface phase diagrams as a function of the gaseous pressure and temperature. Next, a triangular wedge model is used to compute absolute surface formation energies with the bulk state as a reference. A Wulff construction is used to predict the crystal growth form composed of energetically preferred facets. The calculated results can be used to control the shape of GaN grown by THVPE.

The initial growth of CdTe having a thickness of 20–500 nm on (2 1 1) Si substrates was investigated to observe the growth of CdTe at the interface for hetero-epitaxial growth. The dependence of surface morphologies at the initial growth on the II/VI and the growth time was evaluated. The (1 3 3) surface orientation at the interface was shown in CdTe films grown on (2 1 1) Si substrates, which was irrespective of the II/VI. For a relatively high II/VI of 36, hillocks having a (2 1 1) orientation were observed on the underlayer having a (1 3 3) orientation. CdTe having a (2 1 1) orientation was dominant during the growth, due to its lateral coalescence on the underlayer. The dependence of the orientation between (1 3 3) and (2 1 1) CdTe films on (2 1 1) Si substrates on the II/VI was explained by the difference between the step-flow growth on the step and the spontaneous nucleation on the terrace.

The dependence of growth temperature for the epitaxial films grown by tri-halide vapor phase epitaxy (THVPE) on the crystal characteristics, such as the surface morphology, the full width at half maximum (FWHM), the threading dislocation density (TDD), the impurity concentration, and the photoluminescence (PL) was investigated. The epitaxial films grown at relatively high growth temperature of 1300-1350 °C showed that the crystal quality, such as FWHM and TDD retained that for the used substrate. The near-band-edge emission for PL for 1300-1350 °C growth showed lager intensities due to low nonradiative recombination center (NRC). Moreover, the epitaxial growth on the supercritical acidic ammonia technology (SCAAT™) substrate was demonstrated. The TDD was as low as 2 × 104 cm-2, which indicated that the epilayer grown by THVPE retained the superior crystal quality of SCAAT™.

In this paper, we investigated the effects of the growth temperature and chlorine gas on the growth of thick GaN on an N-polar GaN substrate via tri-halide vapor phase epitaxy. The results revealed that free Cl2 is necessary for high-speed growth. When the growth temperature increases, the crystal quality improves, and a high growth rate and high crystalline quality can be simultaneously achieved.

Single-crystalline CdTe films were grown in both (133) and (211) surface orientations on (211) Si substrates by vapor-phase epitaxy using metallic Cd source as a group-II precursor. The orientation of epitaxial films depended on the ratio of group-II and -VI precursors, i.e., II/VI. The orientation of epitaxial films was changed from (133) to (211) by increasing the II/VI under the CdTe growth condition. The surface morphology for (133) CdTe was smooth, whereas the surface for (211) CdTe was composed of hillocks with (111), (110), (101), and (100) facets. The full width at half maximum (FWHM) of the epitaxial films with the same thickness showed that the crystalline quality of (133) CdTe was better than that of (211) CdTe. The dependence of the orientation between (133) and (211) CdTe films on (211) Si substrates on the II/VI was explained by the difference between the step-flow growth on the step and the spontaneous nucleation on the terrace.

Thermodynamic analysis of CdTe growth using cost-effective metallic Cd and dialkyl telluride was performed. The major vapor species at source zone in equilibrium were gaseous Cd for the group-II precursor, and Te 2 and H 2 Te for the group-VI precursors. The driving force for the CdTe deposition was still positive even at 650 °C. This indicates that CdTe formation from gaseous Cd can proceed thermodynamically. Furthermore, the calculations showed that CdTe decomposes at higher temperature and increasing the II/VI ratio increases the limit of the growth temperature, which coincides with the experimental results.

We successfully performed epitaxial CdTe growth on a Si (211) substrate with vapor-phase epitaxy using a cost-effective metallic cadmium source as a group-II precursor. The thermodynamic data demonstrate that the combination of metallic Cd and diisopropyl-telluride (DiPTe) with a H2 carrier gas enables the growth of CdTe crystals. A CdTe single crystal with a (422) surface orientation was obtained when a growth temperature between 600°C and 650°C was employed. The surface morphology and crystalline quality were improved with increasing film thickness. The full-width at half-maximum of the x-ray rocking curves with a film thickness of 15.7 μm for the skew-symmetrical (422) and asymmetrical (111) reflection were 528 arcsec and 615 arcsec, respectively.

GaN layers of thickness 0.5–1.3 mm were grown at 1280 °C at a growth rate of 95–275 µm/h by tri-halide vapor-phase epitaxy on nonpolar m -plane ( 10 1 0 ) and semipolar ( 10 1 1 ) ammonothermal GaN substrates. For nonpolar m -plane ( 10 1 0 ) with a −5° off-angle, the full widths at half maximum (FWHMs) of X-ray rocking curves (XRCs) and the basal plane stacking fault (BSF) density increased from 50 to 178″ and from 4.8×10 1 to 1.0×10 3 cm −1 , respectively, upon increasing the growth rate from 115 to 245 µm/h. On the other hand, the XRC-FWHM and the BSF density for semipolar ( 10 1 1 ) grown at 275 µm/h were as small as 28″ and 8.3×10 1 cm −1 , respectively.

The growth of thick InGaN layers on free-standing GaN (0001¯) substrates was studied using tri-halide vapor phase epitaxy. It was found that high-indium-content InGaN can be grown under higher InCl 3 input partial pressure at higher growth temperature, which allows the fabrication of a high crystalline quality InGaN layer with a smooth surface morphology. Using the growth conditions of high InCl 3 input partial pressure and high growth temperature, crack- and droplet-free InGaN layers with a thickness of over 10 µm and with an indium fraction of 0.05 were successfully grown. Although the surface showed many hillocks, the number of hillocks was reduced upon growth of thicker InGaN layers. Photoluminescence measurements confirm that thick InGaN layers could be successfully grown without degradation of the crystalline quality.

Tri-halide vapor phase epitaxy (THVPE) of thick GaN using GaCl3 was investigated for fabricating low-cost, high-crystalline-quality GaN substrates instead of the conventional manufacturing method of GaCl-based hydride vapor phase epitaxy (HVPE). The growth rate and upper growth temperature limit of GaN using THVPE were found to be much higher than those obtained using conventional HVPE under the same growth conditions. Drastic reduction in the number of dark spots measured by cathodoluminescence at room temperature was observed for the high-temperature-grown GaN layer on the (000-1) GaN/sapphire template due to the enhancement of precursor migration on the growing surface. It was found that the incorporation of impurities such as O, C, and Cl can be reduced even on the N-polarity GaN by increasing the growth temperature. The possibility of enlargement of the crystal diameter by growing the N-polarity GaN layer using THVPE was also proposed.

Deep-level luminescence at 3.3 eV related to the presence of Al vacancies (V Al ) was observed in room temperature photoluminescence (RT-PL) spectra of homoepitaxial AlN layers grown at 1450 °C by hydride vapor-phase epitaxy (HVPE) and cooled to RT in a mixture of H 2 and N 2 with added NH 3 . However, this luminescence disappeared after removing the near surface layer of AlN by polishing. In addition, the deep-level luminescence was not observed when the post-growth cooling of AlN was conducted without NH 3 . Secondary ion mass spectrometry (SIMS) studies revealed that although the point defect density of the interior of the AlN layers remained low, the near surface layer cooled in the presence of NH 3 was contaminated by Si impurities due to both suppression of the surface decomposition by the added NH 3 and volatilization of Si by decomposition of the quartz reactor walls at high temperatures. The deep-level luminescence reappeared after the polished AlN wafers were heated in presence of NH 3 at temperatures above 1400 °C. The surface contamination by Si is thought to generate V Al near the surface by lowering their formation energy due to the Fermi level effect, resulting in deep-level luminescence at 3.3 eV caused by the shallow donor (Si) to V Al transition.

AlGaN alloys are the building blocks for deep UV optoelectronics and high-power devices. It has been demonstrated that the highest crystalline quality AlGaN films with high Al content are obtained on AlN single crystal substrates. The resulting mismatch between AlGaN and AlN causes a compressive strain within the AlGaN layers, which varies with composition. Nevertheless, pseudomorphic AlGaN films with Al content higher than 50%, and dislocation densities lower than 104 cm-2 have been achieved, sustaining compressive stresses with thicknesses exceeding 3 µm. Such results demonstrate the advantages of using AlN substrates for this technology, and at such some have been realized on several deep-UV optoelectronics applications. Kinoshita et al. have demonstrated UV LEDs emitting at 265 nm with output powers exceeding 80 mW. UV LEDs grown on these native substrates have higher reliabilities and higher output powers. In addition, optically pumped lasers emitting at wavelengths between 230 nm and 280 nm that display cavity modes and single polarized-state emission with low lasing thresholds have been developed. Nevertheless, there are several limitations related to the performance and further improvement of the active regions as related to their quantum efficiencies and the polarization of their emission is desired. These limitations will be classified in two main categories: (1) identification and control of point defects, and (2) efficient doping. One of the main limitations based on point defects is the one related to the UV absorption band present at 265 nm in AlN. This relatively broad absorption band limits the use of the substrate within the deep UV range and several complicated fabrication procedures were devised to overcome this limitation. In this work, the origin of this absorption band will be discussed, along with other related optical features and the use of co-doping as a way of removing this undesired absorption. Experimental and theoretical results that explore the optical properties of native and impurity point defects as well as point-defect complexes in AlN with respect to the 265 nm absorption band will be presented. Theoretical results, based on DFT, are compared to photoluminescence and absorption measurements of AlN samples grown by physical vapor transport, metal-organic chemical vapor deposition, and hydride vapor-phase epitaxy . Our results demonstrate that there are multiple sources of a deep-UV absorption between 4.5 and 5.0 eV. The presence of the isolated carbon substitutional CN - produces an absorption band centered at 4.7 eV and emission at 3.9 eV, thus its reduction leads to the reduction of the absorption band. In addition, one mechanism that contributes to the apparent UV transparency when the material is co-doped with Si is discussed. These schemes produced transparent AlN wafers suitable for deep UV LED production. The other limitation relates to obtaining technologically useful n-type conductivities in high Al-content AlGaN within the low and high doping regimes. Besides dislocation density reduction, identification and elimination of compensating defects are necessary to achieve this goal. AlGaN films with high Al mole fractions (0.650.8 was primarily caused by an increase in activation energy. This is due to the formation of a Si DX-center, thus inspiring the search for alternative dopants. Preliminary data on Ge doping of AlGaN will be presented to assess its effectiveness as a donor throughout the whole composition regime. In addition, for samples grown with different Si doping levels it is observed that up to a critical Si concentration, the carrier concentration was always proportional to the Si source flow. Any further increase led to a free carrier concentration reduction. PL spectra exhibited deep defect-related peaks corresponding to transitions between Si and deep acceptor states. For Si concentrations less than an Al-content dependent critical concentration, a higher energy peak dominates the spectra. This peak was related to a (VAl-complex)2- based on DFT calculations. For Si concentrations greater than the critical concentration, a (VAl)3- related lower energy peak dominates the spectra. The energy levels associated with these defects increased with Al composition where the energy increase was the same as the increase in band gap. Acknowledgement: Partial financial support from NSF (DMR-1312582 and ECCS-1508854), DARPA (W911QX-10-C-0027), ARPA-E (DE-AR0000299), ARO (W911NF14C0008) is greatly appreciated. All PVT AlN wafers used for homoepitaxial growth were supplied by HexaTech, Inc., Morrisville, NC.

β-Ga 2 O 3 growth by halide vapor phase epitaxy (HVPE) was investigated by thermodynamic analysis. GaCl and O 2 were determined to be appropriate precursors for the growth of β-Ga 2 O 3 by HVPE. When H 2 is not included in the carrier gas, growth is expected up to 1600 °C. However, with an increase of H 2 in the carrier gas, the driving force of Ga 2 O 3 growth decreases. Stable growth at 1000 °C in an inert carrier gas requires an input VI/III ratio above 1. Experimental results for the homoepitaxial growth of β-Ga 2 O 3 using GaCl and O 2 as precursors and N 2 as a carrier gas show that β-Ga 2 O 3 growth by HVPE can be thermodynamically controlled.

A thick and fully relaxed In0.05Ga0.95N layer was epitaxially grown on a GaN substrate after growing partially relaxed intermediate InGaN layers using tri-halide vapor phase epitaxy. In order to control the relaxation ratio of intermediate InGaN layers, a double intermediate structure with different growth rates (low growth rate and high growth rate modes) was adopted. By changing the first intermediate InGaN layer thickness, the relaxation ratio could be varied in the ranges between 25% and 59%. According to X-ray diffraction reciprocal space mapping measurements, a fully relaxed In0.05Ga0.95N layer was successfully grown on a partially relaxed intermediate In0.10Ga0.90N layer with the relaxation ratio of 32%. Thus, a mirror-like, fully relaxed and thick In0.05Ga0.95N layer could be grown with lattice matched to the intermediate InGaN layers by controlling the relaxation ratio.

GaN with a film thickness of 200–600 μm was grown on the as-grown three-dimensional supercritical acidic ammonia technology (SCAATTM) bulk seed that comprised only semipolar { 10 1 ¯ 1 ¯ } planes at a temperature as high as 1390 °C by tri-halide vapor phase epitaxy; further, the GaN film was also characterized. The FWHM value of the X-ray rocking curves was ∼40″, which was almost similar to the value of the used seed. The curvature radii were as large as 40–64 m. Further, the carrier concentrations were observed to be as small as 5.1 × 1017–9.1 × 1017 cm−3. However, the basal plane stacking fault densities were observed to be 3.4 × 101–5.4 × 101 cm−1 and were observed to increase during the growth process because of the as-grown SCAATTM seed surface condition.

In this report, effects of ammonia nitridation and low temperature InN buffer growth were investigated to improve the crystalline quality of InN(101¯3) grown on GaAs(110) by metalorganic vapor phase epitaxy (MOVPE). InN(101¯3) single crystal including less than 0.1% of differently oriented domains was successfully grown by inserting low temperature InN buffer layer. The full width at half maximum (FWHM) values of InN(101¯3) epitaxial layer were drastically decreased from 89 arcmin to 55 arcmin after processing ammonia nitridation of GaAs(110) substrate surface. Furthermore, the FWHM value was decreased to 38 arcmin by increasing growth time, and the mechanism of dislocation annihilation happened during epitaxial growth was discussed.

Thermodynamic analyses on hydride vapor phase epitaxy (HVPE) of InGaN using III-chlorides, III-bromides, and III-iodides were performed to search a promising source material having a potential for controlling the solid composition with a high growth rate. In this analysis, various precursors such as InCl3, GaCl3,InBr3, GaBr3, InI3, and GaI3 were selected for comparison. Influence of input mole fraction of hydrogen (H2) in the carrier gas and input V/III ratio on the equilibrium partial pressures at InGaN deposition zone and the driving forces of InGaN deposition are shown. It was found that HVPE growth of InGaN using a combination of InCl3 and GaCl3 under a low input mole fraction of H2 and a high V/III ratio is essential for the control of solid composition and for high growth rate. (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The influence of hydrogen (H2) in the carrier gas on the crystalline quality as well as twin-crystal formation were investigated in the metalorganic vapor phase epitaxy (MOVPE) of InN on GaAs(110). The inclusion of twin crystals of InN(10-13) was suppressed by the introduction of a small amount of H2 during crystal growth. The surface morphology of the InN layer was also significantly influenced by the input partial pressure of H2 in the carrier gas. The suppression of twin-crystal formation and improvements in the surface morphology can be explained by the differences in the stability of the In- and N-polar InN{0001} asymmetric planes in the presence of H2. Both electronic and optical properties were improved by the introduction of H2, which was probably due to the reduction of impurities such as oxygen and carbon in the InN grown layer. (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

AlN layers were grown on (0001) sapphire substrates by hydride vapor phase epitaxy (HVPE) at 1100 °C with a source gas supply sequence of (1) NH3 preflow or (2) AlCl3 preflow. An Al-polarity AlN layer without inclusion of a N-polarity region was grown when AlCl3 was preflown to the sapphire surface prior to AlN growth, while N- and Al-polarity regions were both present in the same AlN layer when NH3 was preflown, since growth was performed on a nitrided sapphire surface. Compared with the AlN layers grown with NH3 preflow, the Al-polarity AlN layers grown with AlCl3 preflow had improved crystalline structural quality, a low concentration of oxygen impurity, and a photoabsorption edge energy of 6.08 eV, which is close to an ideal value. Therefore, the source gas supply sequence has a significant influence on the growth of AlN layers on (0001) sapphire substrates. Thus, preflow of AlCl3 gas to a sapphire surface prior to AlN growth is a key process for high crystalline quality AlN layer growth with uniform Al-polarity on (0001) sapphire substrates by HVPE.

The morphology, structural and optical properties of void-assisted freestanding HVPE-AlN films were investigated by a combination of non-destructive microscopic and spectroscopic techniques. The freestanding approximately 80 μm thick clear film has a wurtzite crystalline structure with remarkable properties around the central film region. The E 2 (high)-phonon frequency coincides with reported stress-free film phonon frequency. The low temperature luminescence study of the growth and interface sides of the film is consistent with the incorporation of a high concentration of oxygen impurities. These results are promising as the growth method amenable to the production of freestanding stress-free large area substrates for epitaxial growth.

A novel hybrid seed substrate for preparing a freestanding (0001) AlN substrate was proposed. The seed substrate consists of thin single-crystalline AlN layer and thick poly-crystalline AlN base, which is expected to remove the differences of thermal expansion coefficient and lattice constant between a subsequently grown thick AlN epitaxial layer and the seed substrate. A freestanding AlN substrate (diameter: 20 mm; thickness: 180 μm) was successfully prepared, although the freestanding AlN substrate included a number of inner cracks. The freestanding substrate had no strong specific absorption in the wavelength range 210–1000 nm and showed an absorption number of 52 cm −1 at 265 nm.

Heat treatment of (0001) sapphire substrates in the temperature range 980–1480 °C was investigated in an atmospheric-pressure mixed flow of H2 and N2 for various molar fractions of H2 (F°=H2/(H2+N2)). At 1330 °C, AlN whiskers formed on the sapphire surfaces only when the heat treatment was performed in the presence of both H2 and N2 (0

In this paper, we use thermodynamic analysis to determine how the nitrogen (N) ratio in the source gases affects the solid composition of coherently grown GaAs 1− x N x ( x ∼0.03). The source gases for Ga, As, and N are trimethylgallium ((CH 3 ) 3 Ga), arsine (AsH 3 ), and ammonia (NH 3 ), respectively. The growth occurs on a Ge substrate, and the analysis includes the stress from the substrate–crystal lattice mismatch. Calculation results indicate that to have just a few percent N incorporation into the grown solid, the V/III ratio in the source gases should be several thousands and the input-gas partial-pressure ratio NH 3 /(NH 3 +AsH 3 ) should exceed 0.99. We also find that the lattice mismatch stress from the Ge substrate increases the V/III source–gas ratio required for stable growth, whereas an increase in input Ga partial pressure ratio has the opposite effect.

Twin formation of {103} semi-polar InN layers on GaAs(110) surface has been investigated by metalorganic vapor phase epitaxy (MOVPE). It was found that the growth temperature of InN epitaxial layer strongly affected the twin formation due to the difference in the durability of In-polar and N-polar InN crystal at each growth temperature. From the results of polarity determination and pole figure measurements by high resolution X-ray diffraction (HR-XRD), InN layers grown at high temperatures showed a single domain (no twinning) and the direction of 〈0001〉 InN paralleled to the direction of 〈11〉B GaAs, implying the epitaxial relationship to be (10)//GaAs(110). On the contrary, InN layers grown at relatively low temperatures contained the twin crystal and the [0001] InN of each domain paralleled to either 〈111〉A or 〈11〉B of GaAs. (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The adsorption processes of group-V sources used in metalorganic vapor phase epitaxy (MOVPE) and hydride vapor phase epitaxy (HVPE) were investigated using first-principle calculations based on the density functional theory (DFT) to understand the initial growth process of AlN(0001). To examine the influence of surface hydrogen coverage on the initial growth process, the ideal surface and a reconstructed surface with an adsorbed 0.75 monolayer of hydrogen were used as the initial surface structures. NH3 and NH2 generated by the decomposition of NH3 were considered as group-V sources. From the calculations, the influence of surface hydrogen coverage on the initial growth process of AlN(0001) surfaces in vapor phase epitaxy was clarified. It was revealed that surface hydrogen coverage does not have an effect on adsorption of NH3, whereas NH2 was affected by hydrogen on the surface. It was also demonstrated that the adsorption energies for NH3 and NH2 on the ideal and reconstructed GaN(0001) surfaces are smaller than those on the ideal and reconstructed AlN(0001) surfaces (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Growth of InGaN alloy using hydride vapor phase epitaxy (HVPE) was investigated. Thermodynamic analysis was performed, taking account of the source zones. It was found that InCl 3 and GaCl 3 , which are known to be essential for appreciable InGaN growth by HVPE, could be generated preferentially at the source zone by using a group-III metal and Cl 2 gas. The analysis for the growth zone revealed that a significantly large driving force for both InN and GaN deposition is possible. The calculated vapor–solid distribution was close to a linear relationship by using a high V/III ratio, inert carrier gas, and high temperature. These facts suggest that an InGaN thick layer can be grown with a high growth rate and enough controllability of solid composition by employing InCl 3 and GaCl 3 precursors.

Investigation of the hetero-epitaxial growth of InN on a GaAs (1 1 0) substrate was performed by metalorganic vapor phase epitaxy in order to realize the formation of semi-polar InN layer on it. The orientation relationship between InN and GaAs (1 1 0) was confirmed by 2θ―ω and pole figure of X-ray diffraction measurements. It was found that the crystalline orientation strongly depended on the growth temperature; mixed domains of (1 0 1 3) and (1 1 2 0) InN have been grown on GaAs (1 1 0) surfaces when the growth temperature was below 550 °C, while (1 0 1 3) semi-polar InN layers could be grown by increasing the growth temperature above 575 °C. It was also found that the in-plane anisotropy of semipolar InN layer was suppressed by increasing the growth temperature. Epitaxial relationship of the InN with the GaAs (1 1 0) substrate was InN (1 0 1 3) plane parallel to GaAs (1 1 0) and InN (2 1 1 0) plane parallel to GaAs(1 1 0).

Thermodynamic analyses of halide vapor phase epitaxy (HVPE) for the growth of ZnO were conducted to investigate the effects of growth conditions against growth rates. The partial pressures of gaseous species in equilibrium with ZnO and the resultant driving force for ZnO deposition are calculated with respect to temperatures, input H 2 pressures and input VI/II ratios. It has been revealed that the driving force is weakly dependent on the temperature and is still positive even at 1200 °C, while it greatly decreases at higher temperatures in the presence of H 2 . The driving force is also significantly influenced when the VI/II ratio is below 1000. The experimental growth rates substantially agree with the dispositions of the driving force expected from the thermodynamic analyses.

We used positron annihilation spectroscopy to study bulk AlN single crystals grown by physical vapor transport (PVT) and hydride vapor phase epitaxy (HVPE). Unlike in previous measurements in AlN, we see a clear Alvacancy related lifetime component already at room temperature in HVPE-AlN. Overall, our results show the more UV-transparent HVPE-AlN to have higher concentration of Al-vacancy related defects than PVT-AlN, suggesting that Al-vacancies are not the origin of UV absorption in AlN. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Void formation at the interface between thick AlN layers and (0 0 0 1) sapphire substrates was investigated to form a predefined separation point of the thick AlN layers for the preparation of freestanding AlN substrates by hydride vapor phase epitaxy (HVPE). By heating 50–200 nm thick intermediate AlN layers above 1400 °C in a gas flow containing H 2 and NH 3 , voids were formed beneath the AlN layers by the decomposition reaction of sapphire with hydrogen diffusing to the interface. The volume of the sapphire decomposed at the interface increased as the temperature and time of the heat treatment was increased and as the thickness of the AlN layer decreased. Thick AlN layers subsequently grown at 1450 °C after the formation of voids beneath the intermediate AlN layer with a thickness of 100 nm or above self-separated from the sapphire substrates during post-growth cooling with the aid of voids. The 79 μm thick freestanding AlN substrate obtained using a 200 nm thick intermediate AlN layer had a flat surface with no pits, high optical transparency at wavelengths above 208.1 nm, and a dislocation density of 1.5×10 8 cm −2 .

The surface morphology of homoepitaxial (0 0 0 1)ZnO thin layers grown by halide vapor phase epitaxy (HVPE) using ZnCl2 and H2O source gases was investigated. Atomic force microscopy (AFM) observations showed that high temperature growth at 1000 °C with a low input H2O/ZnCl2 (VI/II) partial pressure ratio of 20 strongly promoted step-flow growth. X-ray diffraction (XRD) analyses revealed the crystalline quality of the homoepitaxial ZnO layer grown at 1000 °C with input VI/II ratio of 20 is comparable to that of bulk ZnO substrates. Under a constant input partial pressure of ZnCl2, the growth rate of ZnO is constant within the temperature range 700–900 °C, and the growth rate decreases above 900 °C due to the shift of thermodynamic equilibrium of the growth reaction.

InN layers were grown on both-sides-polished (0 0 0 1) freestanding GaN substrates by hydride vapor-phase epitaxy (HVPE) in the growth temperature range from 450 to 650 °C with an input partial pressure of NH 3 ranging from 3.0×10 −2 to 3.8×10 −1 atm. An In-polar InN layer was grown on the (0 0 0 1) Ga-polar surface, while a N-polar InN layer was grown on the (0 0 0 1 ¯ ) N-polar surface of the freestanding GaN substrate. The InN layers of both polarities grown at 550 °C had smooth surfaces, ideal lattice constants of the wurtzite InN structure, and a minimum optical absorption edge energy of about 0.75 eV. Surface morphologies of the InN layers were also dependent on the NH 3 input partial pressure. The surface of In-polar InN became smoother under a high NH 3 input partial pressure, whereas the N-polar InN required a low NH 3 input partial pressure to achieve a smooth surface.

The heterostructures of GaN/SiC/Si were prepared without using AlN or AlGaN buffer layers (AlN buffers) in the metalorganic vapor phase epitaxy of GaN on SiC. GaN (0 0 0 1) with specular surface was obtained. The AlN buffers are usually used in the conventional growth of GaN on SiC due to the poor nucleation of GaN on SiC. Instead, the nucleation of GaN was controlled by varying the partial pressure of H 2 in the carrier gas, the mixture of H 2 and N 2 , during the low-temperature (600 °C) growth of GaN (LT-GaN). After the LT-GaN, the high-temperature (1000 °C) growth of GaN was performed using pure H 2 as the carrier gas. The epitaxial film of cubic SiC (1 1 1) on a Si (1 1 1) substrate was used as the SiC template. Increasing the partial pressure of H 2 in the carrier gas decreased the coverage of SiC surface by LT-GaN. It is suggested that the hydrogen atoms adsorbed on the surface of SiC is preventing the nucleation of GaN.

Surface reactions between a (0 0 0 1) C -plane sapphire and NH 3 , with He as an inert carrier gas, were investigated at high temperatures over 1200 °C using the in situ gravimetric monitoring method. Although the sapphire substrate was stable up to 1400 °C under a He atmosphere, decomposition started to occur at 1300 °C under a 0.1 atm NH 3 +He and the decomposition rates were found to be lower than those in 0.1 atm H 2 +He at each temperature. These results imply that sapphire can be decomposed by NH 3 and/or hydrogen generated by the decomposition of NH 3 over 1300 °C. The decomposition rate in NH 3 +He was decreased with increase in NH 3 flow time, and the decomposition rate became constant after 60 min of NH 3 flow. Moreover, the activation energy for sapphire decomposition before 60 min of NH 3 flow was different from that after 60 min of NH 3 flow time, which indicates that the surface reaction between sapphire and NH 3 and/or hydrogen generated from NH 3 changes depending on the time of NH 3 flow. The dependence of the surface reactions and rate-limiting reactions between sapphire and NH 3 on the time of NH 3 flow is discussed.

The effects of growth conditions for ZnO layers grown by halide vapor phase epitaxy (HVPE) on (0 0 0 1) ZnO/sapphire templates are investigated. Micron-sized pyramidal ZnO islands nucleate on the template at the initial growth stage and each island grows differently with the process conditions. The high temperature of 1000 °C promotes a lateral growth rate and coalescence between the islands. The full-width at half-maximums (FWHMs) of X-ray rocking curves for the (0 0 0 2) and (1 0 1¯ 1) planes from a fully coalesced ZnO layer are quite narrow values below 160 arcsec. Transmission electron microscopy (TEM) reveals that screw character dislocations in the template do not propagate into the HVPE-grown layer.

Facet control of InN was performed for the improvement of the crystalline quality of InN layers on GaAs (1 1 1)B by metalorganic vapor phase epitaxy (MOVPE). Growth mode of InN could be controlled by changing the input mole ratio of hydrogen in the carrier gas and growth temperature. When the input mole ratio of hydrogen in the carrier gas was 0 (N 2 ambient) and growth temperature was low, InN with a flat surface could be obtained. On the contrary, when a small amount of hydrogen was contained in the carrier gas (0.4% of H 2 ), InN islands having a (1 0 1¯ 2) facet were grown. It was also found that the crystalline quality of InN could be improved by employing a growth mode change during epitaxial growth from three dimensional (3D) to two dimensional (2D), indicating a reduction in dislocation density.

MnP nanowires have been grown by molecular beam epitaxy (MBE) without any preliminary deposited metal catalyst on InP(100) and GaAs(111)B substrates. Mn was supplied using a conventional Knudsen cell, whereas P2 was supplied as a gas by the decomposition of tertiary butylphosphine (TBP) using a cracking cell. The self-assembled growth of nanowires was observed when the substrate temperature exceeded 430 °C. Nanowires obtained on the InP(100) surface had diameters close to 150 nm, lengths of up to 2 µm, whereas nanowires grown on the GaAs(111)B surface had widths of up to 600 nm, lengths of up to 30 µm, and clear atomic facets. Magnetic and optical characterizations were also carried out.

High-temperature growth of thick AlN layers was performed by solid source halide vapor-phase epitaxy (HVPE) with local heating system. It was found that crystalline quality of AlN layers grown on sapphire (0 0 0 1) substrates improved with increase of growth temperature until 1500 °C. Degradation of crystalline quality occurred over 1550 °C, which is thought to be due to the decomposition of AlN crystal or sapphire substrate during the epitaxial growth. The full-width at half-maximum (FWHM) values of (0 0 0 2) and ( 1 0 1 ¯ 0 ) diffraction from the AlN layer grown at 1500 °C were 103 and 828 arcsec, respectively. Secondary ion mass spectrometry (SIMS) measurement showed a high concentration of oxygen incorporated into the AlN layer through the interface between sapphire and AlN at 1550 °C. The near band-edge emission peaking at 208.2 nm was also observed by cathodoluminescence measurement 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.

N-polarity InN layers grown on (0001) sapphire substrates by hydride vapor phase epitaxy (HVPE) were annealed in atmospheric N2, H2, H2 + NH3, or N2 + NH3 ambient at various temperatures. In the N2 ambient, InN decomposition became significant above 600 °C, whereas in the H2 ambient, InN layers reacted with H2 and decomposed at very low temperatures above 350 °C. The process of InN decomposition was also investigated using a first-principles calculation. In a system free from H2, the rate-limiting process was found to be desorption of N atoms from the surface as atomic N (N*), whereas in the H2 ambient, the desorption of In atoms as In, InH or InH3 is the rate-limiting process. Thus, the decomposition reactions of InN layers in the ambient in the presence and absence of H2 are different. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Influences of hydrogen input partial pressure in the carrier gas ( F o = P H 2 o / ( P H 2 o + P N 2 o ) ) on the crystalline quality and polarities of InN on GaAs (1 1 1)A surfaces were investigated by metalorganic vapor phase epitaxy (MOVPE). It was found that the polarity of the InN was affected by the hydrogen gas in the system regardless of the polarity of GaAs starting substrate. The polarity of InN layer grown with the hydrogen partial pressure of F o =0.004 was a mixture of In-polarity and N-polarity, while that grown with F o =0 was In-polarity. Degradation of the crystalline quality of InN grown with F o =0.004 occurred due to the polarity inversion during the growth. The reason why the polarity of InN was influenced by the hydrogen carrier gas could be explained by the preferential growth of N-polarity InN in the H 2 contained ambient and/or the limiting reaction of InN decomposition.

The hydride vapor phase epitaxy (HVPE) of {0 0 0 1} AlN films on {1 1 1} Si substrates covered with epitaxial {1 1 1} cubic SiC (3C-SiC intermediate layers) was carried out. 3C-SiC intermediate layers are essential to obtain high-quality AlN films on Si substrates, because specular AlN films are obtained with 3C-SiC intermediate layers, whereas rough AlN films are obtained without 3C-SiC intermediate layers. We determined the polarities of AlN films and the underlying 3C-SiC intermediate layers by convergent beam electron diffraction (CBED) using transmission electron microscopy. For the first time, the polarities of the AlN films and the 3C-SiC intermediate layers were determined as Al and Si polarities, respectively. The AlN films were hardly etched by aqueous KOH solution, thereby indicating Al polarity. This supports the results obtained by CBED. The result is also consistent with electrostatic arguments. An interfacial structure was proposed. The 3C-SiC intermediate layers are promising for the HVPE of AlN films on Si substrates.

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. Funding Agencies|Linkoping Linnaeus Initiative for Novel Functional Materials (LiLi-NFM); Council for Science, Technology and Innovation (CSTI); Cross-ministerial Strategic Innovation Promotion Program (SIP); NEDO, Japan

Secondary-ion mass spectroscopic studies for wurtzite GaN (0 0 0 1) and ( 0 0 0 1 ¯ ) surfaces grown by hydride-vapor phase epitaxy were carried out using highly-charged ions. The secondary ions of impurities adsorbed on the surfaces, such as proton, H 2 + , and C m H n + ( m = 1–6 and n = 0–13), were preferentially desorbed by the grazing incidence of Ar q + ( q = 6–11). A spectrum peak of Ga + was observed only in the (0 0 0 1) surface, whereas an enhancement of the peak intensity of N + was observed in the ( 0 0 0 1 ¯ ) surfaces. The time difference between the two peaks was also observed in the time-of-flight spectra of protons emitted from the (0 0 0 1) and ( 0 0 0 1 ¯ ) surfaces. It was concluded that these differences were due to the surface polarity of GaN.

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.

We report the growth of N- and Al-polar AlN layers on c-plane sapphire by flow-modulation MOCVD (FM-MOCVD) with some flow sequence modifications. Surface polarities were decided by pre-treatment for sapphire substrates prior to AlN seeding layer growth. Low-temperature nitridation (620 °C) was done for the N-polar samples, and no-nitridation was done for the Al-polar. To avoid strong vapor-phase reaction between TMA and NH 3 and to enhance the surface migration of Al atoms, FM-MOCVD technique was used. The flow sequence was modified for optimization of each N- and Al-polar growth. The surface features were completely different between the N- and Al-polar AlN layers in atomic force microscope images. The former had domed structures and the latter atomic steps. The dislocation densities were counted from plane-view transmission microscope images to be about 3 x 10 10 cm -2 for the N-polar layer and about 1×10 10 cm -2 for the Al-polar. Secondary-ion mass spectroscopy measurement revealed oxygen- and carbon-incorporation into the layers. Band-edge and impurity-related emissions were observed from only the Al-polar layer by room-temperature cathodoluminescence, whereas the N-polar one did not emit.

Thick freestanding GaN films were grown by hydride vapor-phase epitaxial method on both pattern-masked and unmasked GaAs substrates. Both approaches resulted on films characterized by a large excess of free carriers at room temperature. The use of a GaAs back surface NiTi substrate protective layer increased the concentration of incorporated iron impurities and yield semi-insulating films. The morphology and crystalline quality of these films show a strong dependence on nucleation layer growth approach and Fe doping concentration. Near infrared photoluminescence and Raman scattering verified the incorporation and activation of the Fe impurities, and its effect on the free carrier concentration.

The thermal stabilities of Al- and N-polarity AlN layers grown on (0 0 0 1) sapphire substrates were investigated at temperatures ranging from 1100 to 1400 °C in various gas flows (He, H 2 and H 2 +NH 3 ). Decomposition of AlN occurred in flowing H 2 , while it did not occur in He or H 2 +NH 3 flow. The decomposition rate in H 2 increased with an increase in the temperature over 1200 °C, and the decomposition rate of the Al-polarity AlN layer was found to be lower than that of the N-polarity AlN layer at each temperature. The decomposition reaction of AlN and the relationship between the polarities of the AlN layers and their different decomposition rates are discussed.

We have demonstrated that high-quality aluminium nintride (AlN) single crystal can be grown at high growth rates by hydride vapor phase epitaxy (HVPE) with a help of using AlN templates and step growth technique. A colorless and transparent AlN layer with 83 μm thick was grown at a growth rate of 57 μm/h at 1450 °C. Its full-width at half-maximum for {0 0 0 2} plane was 295 arcsec and that for {1 0 1¯ 1} plane was 432 arcsec. Grown AlN layer was transparent in the visible and ultraviolet region and a sharp absorption edge with band gap energy of 5.96 eV was confined.

HVPE growth of AlGaN ternary alloy using AlCl 3 -GaCl-NH 3 system is reported. AlGaN alloy was grown directly on sapphire without any buffer layers. It was found that the solid composition could be controlled in all ranges from GaN to A1N by changing the input ratio of the group III precursors under the low partial pressure of H 2 (