富樫 理恵

Herein, triangular‐lattice nanopillar templates are fabricated on sputter‐deposited AlN/Si (111) substrates. Nanotemplate selective‐area growth via radiofrequency‐plasma‐assisted molecular beam epitaxy is employed to grow GaN nanocolumns on the nanopillars. Well‐ordered uniform GaN nanocolumn arrays are obtained by inserting a migration‐enhanced‐epitaxy grown AlN/AlGaN buffer layer, thereby aligning the polarity of GaN to Ga‐polar. Subsequently, bulk InGaN active layers are grown on top of the GaN nanocolumns with increasing growth time (t_g = 10–20 min). In the initial stage of growth (t_g = 10 min), low‐In‐content InGaN grows on the edges of the six‐sided pyramidal top of the GaN nanocolumns. As the growth progresses, low‐In‐composition InGaN fills the sides between InGaN on the edges, while high‐In‐composition InGaN rapidly grows on the top of the c‐plane nanocolumns. High‐angle annular dark‐field scanning transmission electron microscopy reveals the formation of an InGaN core, covered with a low‐In‐composition InGaN shell, on the top of the nanocolumns. At t_g = 20 min, the photoluminescence spectrum exhibits a peak at 669 nm with a full width at half maximum value of 51.7 nm. Thus, the proposed method is suitable for growing red‐light‐emitting well‐ordered InGaN/GaN nanocolumn arrays on Si.

Abstract In this paper, we report achieving extremely high-density packing in high-voltage vertical gallium nitride (GaN) nanocolumn Schottky barrier diodes (NC-SBDs) through the adoption of a bottom-up process. The NC-SBDs were formed via epitaxial growth using Titanium-mask selective area growth (Ti-SAG) by rf-plasma-assisted MBE (rf-MBE), realizing a packing density equivalent to exceeding 10 million columns/mm². Our fabricated NC-SBDs with a period of 300 nm, a diameter of 250 nm, and a drift length of 1.3 μm demonstrated a breakdown voltage (BV) of 260 V with an on-resistance of 2.0 mΩcm², yielding an excellent figure of merit of 33.8 MW/cm² for nanocolumn-based high-voltage devices. We also discuss dielectric reduced surface field effect and impurities within the nanocolumns as potential factors contributing to the achievement of higher BV devices.

Abstract A red InGaN-based nanocolumn micro μLED with an emission diameter of ϕ2.2 μm was demonstrated to achieve an on-wafer external quantum efficiency (EQE) of 2.1% at the peak wavelength of 615 nm. The LED was fabricated by repeating the electrode process on the same nanocolumn pattern area and reducing the emission diameter from ϕ80 to ϕ2.2 μm. The peak EQE, which was maximized at ∼25 A cm⁻², increased by decreasing the emission diameter from 1.2% to 2.1%. This behavior, which differs from that of InGaN-film LEDs, is characterized as a unit of independent nano-LEDs with passivated sidewalls of nanocolumn LEDs.

Abstract In this study, the growth behavior of Indium gallium nitride (InGaN)-based nanocolumn arrays was investigated, and red emission nanocolumn micro-light emitting diodes (μ-LEDs) were fabricated. The internal structure of the InGaN/GaN superlattice (SL) layer under the multiple-quantum-well (MQW) active layers was evaluated using scanning transmission electron microscopy (STEM) analysis. It was revealed that the InGaN crystal plane at the top of the nanocolumn changed from the c-plane, (1-102) plane, to the (10-11) plane as the number of SL pairs increased. A semipolar (10-11) plane was completely formed on top of the nanocolumn by growing InGaN/GaN SLs over 15–20 pairs, where the InGaN/GaN SL layers were uniformly piled up, maintaining the (10-11) plane. Therefore, when InGaN/AlGaN MQWs were grown on the (10-11) plane InGaN/GaN SL layer, the growth of the (10-11) plane semipolar InGaN active layers was observed in the high-angle annular dark field (HAADF)-STEM image. Moreover, the acute nanocolumn top of the (10-11) plane of the InGaN/GaN SL underlayer did not contribute to the formation of the c-plane InGaN core region. Red nanocolumn μ-LEDs with an φ12 μm emission window were fabricated using the (10-11) plane MQWs to obtain the external quantum efficiency of 1.01% at 51 A cm⁻². The process of nanocolumn μ-LEDs suitable for the smaller emission windows was provided, where the flat p-GaN contact layer contributed to forming a fine emission window of φ5 μm.

Thermodynamic analyses for the growth of group-III sesquioxides, including alpha-Al2O3, ss-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 (Delta P-III2O3) 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, Delta(PIII2O3) decreased with increasing P-III(o) and/or decreasing input partial pressure of O-3 or O. This decrease was caused by the formation of Ga2O or In2O during growth of ss-Ga2O3 and c-In2O3. The decrease of Delta P-Al2O3D was smaller because the equilibrium constant of alpha-Al2O3 formation reaction was very large. Ga and In droplets formed at low temperatures (<420 degrees C), whereas Al droplets were formed at high temperatures (<820 degrees C), and the order that enabled growth at higher temperatures was c-In2O3 < ss-Ga2O3 << alpha-Al2O3. (c) 2023 The Japan Society of Applied Physics

The emission color of InGaN/GaN densely-packed nanocolumn arrays was controlled by the nanocolumn diameter, by which RGBY micro (μ)-LED pixel units were monolithically integrated to be arranged two dimensionally. Here the crystal growth and process of the devices are described. Red-emitting nanocolumn LEDs with the directional radiation beam were fabricated.

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 C 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 5.1μm/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 ' 1018cm%3, and electron mobilities of 16.2 and 22.7cm2V%1 s%1, respectively.

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.

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 10⁸cm^-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.

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

The increase of InN growth rate by a newly developed two-step precursor generation hydride vapor phase epitaxy (HVPE) was investigated for the preparation of freestanding InN and InGaN substrates. An elevated growth rate was achieved by the complete conversion of InCl generated in the first source zone to InCl&lt inf&gt 3&lt /inf&gt in the second source zone, by the supply of additional Cl&lt inf&gt 2&lt /inf&gt . The growth rate reached 12.4 μm/h at a growth temperature of 600 °C, and the rate was observed to decrease above this temperature. Specular InN layers grown at 650 °C exhibited a sharp room temperature photoluminescence peak at 0.73 eV with a bulk electron concentration of 1.2×10&lt sup&gt 18&lt /sup&gt cm&lt sup&gt -3&lt /sup&gt .

Thick Si-doped AlN layers were homoepitaxially grown by hydride vapor phase epitaxy on AlN(0001) seed substrates. Following the removal of the seed substrate, an n-type AlN substrate with a carrier concentration of 2.4 ' 1014cm%3 was obtained. Vertical Schottky barrier diodes were fabricated by depositing Ni/Au Schottky contacts on the N-polar surface of the substrate. High rectification with a turn-on voltage of approximately 2.2V was observed. The ideality factor of the diode at room temperature was estimated to be >8. The reverse breakdown voltage, defined as the leakage current level of 10%3A/cm2, ranged from 550 to 770V.

Thick high-purity β-Ga2O3 layers of high crystalline quality were grown homoepitaxially by halide vapor phase epitaxy (HVPE) using gaseous GaCland O2 on (001) β-Ga2O3 substrates prepared by edge-defined film-fed growth. The surface morphology and structural quality of the grown layerimproved with increasing growth temperature. X-ray diffraction ω-rocking curves for the (002) and (400) reflections for the layer grown at 1000 °C had small full widths at half maximum. Secondary ion mass spectrometry and electrical characteristics revealed that the growth of high-purity β-Ga2O3 layers with low effective donor concentration (Nd - Na < 10¹³cm^-3) is possible by HVPE.

β-Ga2O3 growth by halide vapor phase epitaxy (HVPE) was investigated by thermodynamic analysis. GaCl and O2 were determined to be appropriate precursors for the growth of β-Ga2O3 by HVPE. When H2 is not included in the carrier gas, growth is expected up to 1600 °C. However, with an increase of H2 in the carrier gas, the driving force of Ga2O3 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-Ga2O3 using GaCl and O2 as precursors and N2 as a carrier gas show that β-Ga2O3 growth by HVPE can be thermodynamically controlled.

The influence of the source gas supply sequence prior to growth and the NH3 input partial pressure (PoNH3) on the nucleation of InN islands during the initial stages of hydride vapor phase epitaxy on a nitrided (0001) sapphire substrate was investigated. The crystalline quality of the InN layer after subsequent lateral growth was also examined. When NH3 was flowed prior to growth, single-crystal hexagonal InN islands formed. When InN was grown with a higher PoNH3, the number of InN islands decreased remarkably while their diameter increased. The crystalline quality of InN grown on the hexagonal islands with a high PoNH3 significantly improved with increasing growth time. A strong PL spectrum was observed only from InN layers grown with a high PoNH3. It was thus revealed that an NH3 preflow and a high PoNH3 are effective for producing InN with high crystalline quality and good optical and electrical properties.

Orientation dependent decomposition of sapphire substrates and resultant AlN formation during heat treatment in an atmospheric-pressure mixedgas flow of H2 and N2 was investigated within the temperature range 980–1480 oC. AlN was formed on sapphire in the temperature range 1030–1430 oC for c-, a-, and m-plane sapphire, and 980–1430 C for the r-plane sapphire. At 1480 C, AlN was not formed, and onlysapphire 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 at1380 oC, 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 - andm-plane sapphire from decomposing.

In this report, effects of ammonia nitridation and low temperature InN buffer growth were investigated to improve the crystalline quality of InN(10 (1) over bar3) grown on GaAs(110) by metalorganic vapor phase epitaxy (MOVPE). InN(10 (1) over bar3) 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(10 (1) over bar3) 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. (c) 2013 Elsevier B.V. All rights reserved.

AlN layers were grown on (0001) sapphire substrates by hydride vapor phase epitaxy (HVPE) at 1100 degrees 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 AI-polarity on (0001) sapphire substrates by HVPE. (C) 2011 Elsevier B.V. All rights reserved.

Heat treatment of (0001) sapphire substrates in the temperature range 980-1480 degrees C was investigated in an atmospheric-pressure mixed flow of H-2 and N-2 for various molar fractions of H-2 (F degrees=H-2/(H-2+ N-2)). At 1330 degrees C, AIN whiskers formed on the sapphire surfaces only when the heat treatment was performed in the presence of both H-2 and N-2 (0 &lt;F degrees&lt; 1). The amount of AIN that formed was a maximum for F degrees=0.750, whereas no AIN was formed at F degrees=0 (N-2 flow) or 1.000 (H-2 flow). When F degrees=0.750, AIN whiskers formed in the temperature range 1030-1430 degrees C, whereas no AIN was formed outside this temperature range. These experimental results are explained in terms of thermodynamic analysis. (C) 2011 Elsevier B.V. All rights reserved.

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 degrees C. High-temperature heteroepitaxy at 1000 degrees C on (0001) sapphire substrates was realized by two-step growth using the ZnO layer grown at 400 degrees C as a buffer layer. Two-dimensional layer growth at 1000 degrees C was realized on buffer layers thicker than 0.4 mu m. Photoluminescence (PL) measurements performed at room temperature for the ZnO layer grown on the 0.4-mu m-thick buffer layer showed a significant blueshift of near-band-edge emission (NBE). A thick buffer layer of 0.8 mu m 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. (C) 2011 The Japan Society of Applied Physics

The influence of the carrier gas used during the thermal cleaning of r-plane sapphire substrates and the subsequent first AlN layer growth at 1050 degrees C on two-step growth of a-plane AlN layers by hydride vapor phase epitaxy (HVPE) was investigated. When hydrogen (H-2) was used as the carrier gas, the decomposition of r-plane sapphire occurred during the thermal cleaning, and unintentional nitridation of the sapphire surface occurred at the beginning of the growth of the first AlN layer, which resulted in the occurrence of misoriented AlN grains in the second AlN layer grown at 1450 degrees C. When a mixture of H-2 and nitrogen (N-2) was used as the carrier gas, nitridation of the sapphire surface occurred during the thermal cleaning, which also resulted in the occurrence of misoriented AlN grains. A single-crystalline a-plane AlN layer free of misoriented grains could be obtained by using only N-2 as the carrier gas during the thermal cleaning and the growth of the first AlN layer to prevent nitridation of the sapphire surface. (C) 2011 The Japan Society of Applied Physics

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 (11 0) was confirmed by 20 - omega 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) over bar 3) and (1 1 (2) over bar 0) InN have been grown on GaAs (1 1 0) surfaces when the growth temperature was below 550 degrees C, while (1 0 (1) over bar 3) semi-polar InN layers could be grown by increasing the growth temperature above 575 degrees C. It was also found that the in-plane anisotropy of semi-polar 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) over bar 3) plane parallel to GaAs (1 1 0) and InN ((2) over bar 1 1 0) plane parallel to GaAs((1) over bar 1 0). 2010 Elsevier B.V. All rights reserved.

To examine the effect of hydrogen on the initial growth process of InN, the adsorption processes of group-V sources commonly used in vapor phase epitaxy (VPE) were investigated using first-principle calculations based on the density functional theory (DFT). The ideal and reconstructed surfaces with an adsorbed 0.75 monolayer of hydrogen were used as the initial surface structures to examine the influence of surface hydrogen coverage of InN( 0001) surfaces on the adsorption processes of group-V sources including NH3 and NH2 generated by the decomposition of NH3. From the calculations of the adsorption energies for each case, it was revealed that surface hydrogen coverage did not have an effect on the adsorption of NH3, whereas NH2 was significantly affected by the presence of hydrogen on the surface. In comparison with our previous results, it was also shown that the adsorption energies for NH3 and NH2 on InN(0001) surfaces are smaller than those on GaN and AlN(0001) surfaces. (C) 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Control of the in-plane epitaxial relationship of a c-plane AlN layer grown at 1450 degrees C by hydride vapor phase epitaxy on an a-plane sapphire substrate is achieved. In addition, two in-plane epitaxial relationships are compared. It was possible to grow a c-plane AlN layer on an a-plane sapphire substrate with selectable in-plane matching: AlN [10 (1) over bar 0]//sapphire [0001] could be obtained by growing an intermediate AlN layer at 1065 degrees C, whereas AlN [11 (2) over bar 0]//sapphire [0001] could be grown on the intermediate AlN layer at a low temperature of 700 degrees C. The full-width at half-maximums of X-ray rocking curves for the (0002) and (10 (1) over bar 0) planes were respectively 313.2 and 543.6 arcsec from the AlN layer with the AlN [10 (1) over bar 0]//sapphire [0001] relationship and 428.4 and 788.0 arcsec from the AlN layer with the AlN [11 (2) over bar 0]//sapphire [0001] relationship. (C) 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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 NH3 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 0over(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 NH3 input partial pressure. The surface of In-polar InN became smoother under a high NH3 input partial pressure, whereas the N-polar InN required a low NH3 input partial pressure to achieve a smooth surface. © 2009 Elsevier B.V. All rights reserved.

The activation energies for Al and N desorption from an AlN surface were calculated using the density functional theory (DFT) to obtain a detailed understanding of the decomposition process of AlN(0001) Al and N surfaces under a hydrogen atmosphere. It was found that Al atoms on the AlN(0001) Al surface desorbed as AlH and formed AlH3 by reacting with two H atoms on the surface just after desorption, whereas N atoms on the AlN(0001) N surface desorbed as NH3 molecules from the surface. The desorption energy of Al on the hydrogen terminated surface was more consistent with previous experimental value than that on an ideal surface. This result suggests that the initial surface structure of the AlN(0001) surface is terminated with hydrogen. (C) 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

The temperature dependence of InN growth on (0001) sapphire substrates by atmospheric pressure hydride vapor phase epitaxy (HVPE) was investigated. N-polarity single-crystal InN layers were successfully grown at temperatures ranging from 400 to 500 degrees C. The a and c lattice constants of InN layers grown at 450 degrees C or below were slightly larger than those of InN layers grown above 450 degrees C due to oxygen incorporation that also increased the carrier concentration. The optical absorption edge of the InN layer decreased from above 2.0 to 0.76 eV when the growth temperature was increased from 450 to 500 degrees C. (C) 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

The activation energies for Ga and N desorption from a GaN surface were calculated using the density functional theory to understand the detailed decomposition process of the hydrogen terminated GaN(0 0 0 1) Ga and N surfaces under a hydrogen atmosphere. It was found that the Ga atoms on the hydrogen terminated GaN(0 0 0 1) Ga surface desorbed as GaH molecules from the surface while the N atoms on the hydrogen terminated GaN(0 0 0 1) N surface desorbed as NH3 molecules from the surface. The desorption energies of GaH and NH3 on the hydrogen terminated surface were more consistent with the previous experimental values than those on the ideal surface. These results suggest that the initial surface structure of the GaN(0 0 0 1) surface is terminated with hydrogen.

Self-separation of hydride vapor phase epitaxy (HVPE)grown AlN layer from sapphire substrate was achieved by the formation of voids at the AlN/sapphire interface. Voids could be formed through the decomposition of sapphire beneath a thin (100 nm) protective AlN layer grown at 1065 degrees C by heating at 1450 degrees C in a carrier gas (H-2 + N-2) containing NH3. Additionally, the void size could be controlled by varying the heating time. As the results of controlled formation of voids, after growth of a thick (85 mu m) AlN layer on the thin protective AlN layer heated at 1450 degrees C for 30 min, self-separation of the AlN layer from a sapphire substrate occurred by the compressive stress from difference in thermal expansion coefficients between AlN and sapphire during post-growth cooling. The self-separated thick AlN layer had low concentration of impurities, and showed clear photoluminescence peak at 208.4 nm. (C) 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

A thin protective AlN layer was grown on a (0001) sapphire substrate at 1065° C for main AlN layer growth at high temperature (> 1300° C) by hydride vapor phase epitaxy (HVPE). The formation of surface pits on the surface of the epitaxial AlN layer, grown directly on the sapphire substrate at 1320° C, could be prevented by growing the protective layer. The full‐width at half‐maximum (FWHM) of X‐ray diffraction (XRD) rocking curves of asymmetric (10 ̄1 0) and symmetric (0002) AlN decreased to 19.8 and 9.6 min, respectively. The concentration of oxygen impurities in the layer grown at 1320° C was also reduced from 3× 1019 to 4× 1018 cm− 3 by protecting sapphire substrate at 1065° C.