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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.

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.

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 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™.

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.

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.

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

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)

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).

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 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.

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.

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.

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.

A new MOVPE-like hydride vapor-phase epitaxy (HVPE) using AlCl 3 solid source is proposed for growing thick and high-quality AlN layers. For the new system, a high-purity AlCl 3 (5 N grade) powder was produced. AlN epitaxial layers were characterized by X-ray diffraction (XRD) and optical absorption measurement. Also, impurities such as oxygen and carbon in the epitaxial layer were analyzed comparing to the conventional HVPE system using Al metal and HCl gas. It is shown that the new HVPE system can be applied to mass production, because the Al-precursor can be transported to the reactor as vapor.

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.

The new wide-bandgap oxide semiconductor, gallium oxide (Ga 2 O 3 ), has gained attraction as a promising candidate for power device applications because of its excellent material properties and suitability for mass production. The Baliga's figure of merit of Ga 2 O 3 is expected to be much larger than those of SiC and GaN due primarily to Ga 2 O 3 's extremely large bandgap of 4.5∼4.9 eV, which will enable Ga 2 O 3 power devices with higher breakdown voltage (V br ) and efficiency than SiC and GaN devices [1]. The other important advantage of Ga 2 O 3 is that large, high-quality bulk single crystals can be grown by using melt growth methods. Recently, we developed a homoepitaxial growth technique for high-purity Ga 2 O 3 thin films on single-crystal Ga 2 O 3 substrates by halide vapor phase epitaxy (HVPE) [2, 3]. This is the first report on Ga 2 O 3 Schottky barrier diodes (SBDs) with epitaxial Si-doped n−-Ga 2 O 3 drift layers grown by HVPE.

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 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)

Three-dimensional momentum of sputtered ions were measured in coincidence with scattered Ar atoms in collisions between Ar q + ( q = 3, 8, 11, and 14) and 6H-SiC and GaN surfaces at grazing-incidence angle ( + , H 2 + , Si + and Ga + ions were observed in time-of-flight-mass spectra. Sputtering yields of H + and H 2 + increased with increasing charge states of incident ions in proportion to q 4 . The momenta of these secondary ions emitted from the surfaces were intensively distributed to the direction of surface normal in spite of the grazing collisions. Almost no significant change among different q was found in the kinetic energy distributions of these secondary ions.

Thick Fe-doped GaN layers were grown by metalorganic hydrogen chloride vapor phase epitaxy (MOHVPE). FeCl2 formed by the reaction between ferrocene (Cp2Fe) and HCl was introduced during GaN growth. Resistivity of the GaN layer increased with increasing Cp2Fe input partial pressure. A semi-insulating (SI)-GaN layer exhibiting room-temperature resistivity of 3.0 × 109 Ω cm was successfully prepared by compensating for the residual donors. The crystalline quality of the GaN layer was found to deteriorate with excess Fe doping. These results indicate that control of background donor impurities is critically important to grow high-quality SI-GaN layers. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

This paper describes the observed magnetic structure and the micromagnetic simulation of cross-shaped, permalloy (Ni 80 Fe 20 ) arrays embedded in silicon wafers. The nano-scale-width, cross-shaped patterns were fabricated using the damascene technique, electron beam lithography, and chemical mechanical polishing. The magnetic poles were observed as two pairs of bright and dark spots at the ends of the crossed-bars using a magnetic force microscope. The force gradient distributions were simulated based on micromagnetic calculations and tip's stray field calculations using the integral equation method. This process of calculation successfully explains the appearance of the poles and complicated spin structure at the crossing region.

GaN growth on GaP(1 1 1)A and (1 1 1)B surfaces are discussed using P desorption energy from the GaP(1 1 1) surfaces calculated by ab initio pseudopotential method and surface morphology of GaN layers on the GaP(1 1 1) substrates obtained by experiments. It was found that the activation energy for P desorption from GaP(1 1 1)A was smaller than that from GaP(1 1 1)B and these values depend on the surface morphology of GaN on GaP(1 1 1) surfaces. These results explain a finding that the GaP(1 1 1)A surface can promise an initial substrate for freestanding GaN substrate like GaAs(1 1 1)A surface.

Growth conditions for MnGeP2 thin films have been investigated by using molecular beam epitaxy (MBE) method. Mn and Ge were evaporated by K-cells, and P2 was supplied by decomposing tertialybutylphosphine (TBP). GaAs (001) and InP (001) single crystals were used as substrates. An X-ray diffraction peak, which can be assigned to (008) peak of MnGeP2, was observed at nearly the same position as the (004) peak of GaAs. The lattice constant of the MnGeP2 thin film was determined to be 1.13 nm assuming its crystal structure is a c-axis oriented chalcopyrite type structure. Secondary phases such as GeP, MnGe x and MnP were observed for beam fluxes of Mn and Ge as high as 1×10−8 Torr.

We studied experimentally the crystallization of a low-temperature (LT)-GaN buffer layer and its effect on the subsequent crystalline quality of the GaN epitaxial layer. The degree of crystallization was sensitive to the thickness and the temperature ramp rate of the GaN buffer layer. When the LT-GaN buffer layer was more than 100 nm thick, there was relatively little crystallization of the buffer layer and subsequently poor crystalline quality of the GaN epitaxial layer. The optimum crystallization condition for the GaN buffer layer just prior to high-temperature growth was found when the single-crystal regions in the GaN buffer layer occupied a volume 40% as large as the randomly aligned matrix. We could reduce the threading dislocation density in the GaN epitaxial layer by inserting the LT-GaN buffer layers in periodically. To explain this result, we propose a mechanism by which the periodic layer reduces the density of threading dislocations.

In 0.53 Ga 0.47 N film was grown at 600 °C by Nd:YAG pulse laser assisted MOVPE. The optical and structural properties of the film were compared with that grown without laser assistance at the same condition. The results of XRD measurements showed that the crystallinity of the film grown with laser was better than that of the one grown without laser. The surface morphology and cross-sectional SEM image of the film grown with laser revealed that there were no In droplets on the film. The band-edge emission of the film grown with laser at room temperature and 77 K was observed at 840 nm. The results of micro-Raman measurement showed that the film grown with laser had better crystalline structure than that of the film grown without laser and the radiative recombination which contributed to photoluminescence mainly occurred at In 0.53 Ga 0.47 N region. Those results imply that pulse laser enhances the surface migration and reaction of elements in spite of low-growth temperature. We suggest that pulse laser assisted technique is effective for low-temperature growth of InGaN with high indium content.

Electrochemical behavior of polyaniline (PAn) film containing Pd nanoparticles (Pd-NPs) was investigated in propylene carbonate solution. The PAn-Pd particle systems were characterized by using X-ray photoelectron spectroscopy (XPS), UV–visible absorption spectroscopy, and FT-IR spectroscopy. Cyclic voltammetry (CV) and XPS analysis indicated that PAn interacts strongly with Pd-NPs. The electrochemical behavior of PAn film electrode containing Pd-NPs was sensitive to the amount of Pd in the film. From the XPS results, we confirmed that Pd-NPs are stable as metallic state Pd(0) in the PAn-Pd composite film, whereas it exists as ionic state Pd(II) by electrochemical reaction with PAn. Our results indicate that Pd-NPs in the PAn matrix are coordinated to the nitrogen atom of the PAn chain and this leads to the complex formation.

The electrochemical properties of 2,5-dimercapto-1,3,4-thiadiazole (DMcT)—polyaniline (PAn) composite films containing Pd nanoparticles (average diameter: 28 nm) were investigated. Compared to DMcT–PAn composite film, the DMcT–PAn composite film containing Pd nanoparticles showed enhanced redox current and discharge capacity. The enhanced activity is attributed to the nanosize dispersion of the Pd catalyst particles within the DMcT–PAn matrix. UV-Vis spectroscopy and X-ray photoelectron spectroscopy (XPS) results confirmed the interactions between DMcT and Pd nanoparticles. We have obtained the positive effect of Pd nanoparticles addition on the redox activity of the DMcT–PAn composite.

We have investigated influence of substrate polarity on the growth of InN films by RF-MBE. It was found that the temperature required for successful growth of InN strongly depended on the substrate polarity. InN could be grown on C-face SiC at 550 °C, whereas no InN growth occurred on Si-face SiC at 550 °C. InN growth on Si-face SiC was realized at lower temperature of 450 °C. In the case of the growth on free-standing GaN substrates, InN could be grown on N-face GaN at 550 °C, whereas InN growth on Ga-face GaN was realized at 450 °C. It was found that InN with N polarity can be grown at higher temperature than that with In polarity. These results indicate that control of polarity is essential for successful growth of InN.

Thermodynamic analyses incorporating the contribution of lattice constraint from the substrates were carried out to understand the relationship between input In mole ratio and solid composition of stress-free InGaN, InGaN on GaN and InGaN on InN during metalorganic vapour phase epitaxy. The calculated results suggest that a compositionally unstable region is found at the InN-rich region for InGaN on GaN at high temperatures, while that for InGaN on InN can be seen at the GaN-rich region due to the lattice constraint from the substrate. These results imply that incorporating the contribution of lattice constraint is indispensable to predict the thermodynamic properties of such materials.

Thick and high quality GaN layer growth using periodic insertion of low-temperature (LT)-grown GaN buffer layers was investigated by metalorganic hydrogen chloride vapor phase epitaxy (MOHVPE). Both morphological and optical properties of GaN epitaxial layers were drastically improved by inserting a second LT-GaN buffer layer. Also, the thickness of the second buffer layer was found to affect the quality of subsequently grown epitaxial layers. The full-width at half maximum (FWHM) value of X-ray diffraction for () plane (φ scan) of the GaN layer with double buffer layer structure decreased to 608 arcsec whereas that with single buffer structure was 3600 arcsec. These results indicate that the free-standing GaN substrate with low dislocation density can be possible by reiterating the growth sequence of buffer layer and epitaxial layer. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Superlattice stacking structures in the Ga-rich branch of wurtzite InxGa1−xN/GaN were found using semigrand canonical Monte Carlo simulations, which were performed aiming at the construction of the alloy phase diagram [Mori et al., Mater. Phys. Mech. 6, 49 (2003)]. Simulated systems were InxGa1−xN thin films pseudomorphic to the GaN (0001) substrate. Continuing the simulations, we revealed that In-rich layers parallel to the substrate were formed in Ga-rich regions. At 800 K, traversing along the c direction GGGIGGGI… superlattice-type stackings were more frequently observed, where G and I indicate the Ga-rich and In-rich layers, respectively. As the temperature increased along the two-phase boundary the superlattice structure changes so that many In-rich layers were included. The In content of the Ga-rich phase eventually exceeded 0.5 at a higher temperature such as 875 K. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

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.

Magnetic nanostructures embedded in silicon wafers were successfully fabricated by the damascene technique using electron beam lithography. The magnetic properties of embedded square dots (1μm in width and 100nm in height), rectangular dots (300nm×100nm in area and 100nm in height), and circular dots (100nm in diameter and 100nm in height) were investigated. Magnetic force microscopy (MFM) and vibrating sample magnetometer (VSM) measurements showed that the perpendicular anisotropy increased with the aspect ratio, due to a decrease in the areal size. Fine magnetic structures of the 300nm×100nm pattern were clearly resolved by using a special MFM with a low-moment tip.

High temperature growth of thick GaN layers on GaAs (111)A substrates was performed using a two-step growth process by metalorganic hydrogen chloride vapor phase epitaxy (MOHVPE). Temperature ramping rate of the low-temperature (LT)-grown GaN buffer layer was found to be a key factor controlling the crystalline quality of subsequently grown GaN layer at high temperature. When a 50 nm thick LT-GaN buffer layer grown at 550 °C is heated to 1000 °C, it is essential to apply a temperature ramping rate of 15 °C/min or more in order to grow a high-quality GaN layer with a specular surface on a GaAs (111)A substrate.