Journal of Crystal Growth (v.300, #1)

Contents ISGN-1 (v-viii).

Preface by Rositza Yakimova; Bo Monemar (1).

It was quite difficult to grow high quality single crystals of nitrides and impossible to control their electric conduction. These problems prevented the development of nitride-based devices for many years. In 1986, a dramatic improvement in the crystalline quality of GaN was achieved by use of low-temperature-deposited (LT) buffer layer technology in metalorganic vapor phase epitaxy. In 1989, the high quality GaN enabled us to produce p-type conduction in nitrides and to control the conductivity of n-type nitrides. These achievements led to the invention of the world's first GaN p–n junction blue/UV LED in 1989. Room temperature UV stimulated emission, which is indispensable for laser operation, was also achieved in 1990 by use of high quality GaN films grown with the LT-buffer layers. These breakthroughs inspired nitride researchers around the world to greater efforts, and eventually led to the commercialization of high-performance blue LEDs and long-life violet LDs as well as the development of nitride-based devices such as high-speed transistors. Furthermore, unique properties such as a large piezoelectric effect were also clarified due to the marked improvements in crystal quality of nitrides.In this paper, key inventions during the development of nitride-based blue LED and LD are reviewed and a recent advance in UV devices is also described.
Keywords: A1. Conductivity control; A1. High-speed transistor; A1. Low-temperature buffer layer; A1. Piezoelectric effect; A1. P–n junction; A3. MOVPE; B2. Nitride semiconductors; B3. Blue light-emitting diode; B3. Laser diode;

Bulk GaN crystal growth by the high-pressure ammonothermal method by M.P. D’Evelyn; H.C. Hong; D.-S. Park; H. Lu; E. Kaminsky; R.R. Melkote; P. Perlin; M. Lesczynski; S. Porowski; R.J. Molnar (11-16).
The rapidly growing gallium nitride device industry continues to be in dire need of high quality, cost effective native substrates. Considerable progress has been made with pseudo-bulk substrates synthesized by hydride vapor phase epitaxy (HVPE), but true bulk methods promise both superior quality and reduced cost. We give an overview of the high-pressure ammonothermal method developed by GE, based on adaptation of high-pressure apparatus developed for diamond growth, together with appropriate raw materials and methods. We describe recent progress, including characterization of and reductions in impurity concentrations, wafering, and successful fabrication of homoepitaxial laser diodes on GE substrates.
Keywords: A1. Ammonothermal; A1. Crystal growth; A1. High pressure; B1. Gallium nitride; B3. Laser diode;

Crystallization of low dislocation density GaN by high-pressure solution and HVPE methods by I. Grzegory; B. Łucznik; M. Boćkowski; S. Porowski (17-25).
The growth of GaN from solution in gallium under high N2 pressure results in very low-dislocation density (<100 cm−2) crystals usually in the form of hexagonal platelets, but of size limited to 1 cm (lateral) and 100 μm (thickness). Nevertheless, even with such small and thin substrates, the blue–violet lasers with optical power as high as 100–200 mW are reproducibly constructed.Deposition of GaN by HVPE on the pressure grown plate-like or needle-like crystals allows stable crystallization (in terms of flatness of the crystallization front and uniformity of the new grown material) at a rate of about 100 μm/h on both types of seeds. For the needle-like seeds, stable crystallization by HVPE is possible also in multiple growth processes resulting in bulk prismatic crystals with diameter exceeding 5 mm.The crystallization by HVPE on the high-pressure seeds is analyzed with the use of defect selective etching and X-ray data for thick (a few of mm) platelets grown on the plate-like seeds, prismatic bulk crystals grown on needle-like seeds and the cross-section samples sliced from the bulk material in different crystallographic orientations.The results of application of the new grown material for epitaxial growth of quantum structures in both polar and non-polar directions, is shortly reported.
Keywords: A2. High-pressure growth from solution; A2. Single-crystal growth; B1. Gallium nitride;

A single-crystal-growth study of GaN was performed between 2000–2400 °C at 6.5 GPa using GaN powder as a starting material. The average crystal size was about 100 μm and the X-ray rocking curve showed a peak width less than 30 arcsec, suggesting a rather low dislocation density. Polycrystalline Al x Ga1− x N (0⩽x⩽1) alloys were synthesized by a solid-phase reaction at 1800 °C and 6.0 GPa. The lattice constants, a and c, of the hexagonal lattice varied continuously with the nominal composition, indicating that the Al x Ga1− x N alloy was formed over the entire composition range. The phase diagram was determined for InN in a high TP span up to 20 GPa and 2000 °C by in situ diffraction measurements with brilliant X-rays from SPring-8, a third generation synchrotron radiation facility. The wurtzite-rocksalt phase boundary was located around 10 GPa with a steep negative dT/dP. InN decomposed into In and N2 between 700 and 800 °C in the wurtzite phase region. The decomposition temperature increased rapidly in the rocksalt phase region, reaching 1700 °C at 18 GPa. Melting was not observed in the TP span investigated and consequently, the possibility of single-crystal growth from the melt was ruled out for InN.
Keywords: A1. Phase diagrams; B2. Semiconducting III–V materials;

Hydride vapour phase epitaxy growth and characterization of thick GaN using a vertical HVPE reactor by C. Hemmingsson; P.P. Paskov; G. Pozina; M. Heuken; B. Schineller; B. Monemar (32-36).
Growth of 2-inch diameter bulk GaN layers with a thickness up to 2 mm is demonstrated in a vertical hydride vapour phase growth reactor. Morphology, dislocations, optical and electrical properties of the material have been investigated using atomic force microscopy, optical microscopy, decorative etching in hot H3PO4, Hall measurements and low-temperature photoluminescence. Atomic force microscopy reveals a two-dimensional step flow growth mode with step bunching for layers with a thickness of 250 μm. As the growth proceeds, the morphology is changed to a hill and valley structure. The EPD was determined to 5×105  cm−2 for a 2 mm thick layer. The Hall mobility and the carrier concentration were determined. For a 1.7 mm thick layer at 300 K the mobility and the carrier concentration is 520 cm2/V s and about 4×1017  cm−3, respectively. Low-temperature photoluminescence spectra measured on a 350 μm thick freestanding layer show the DBE line at 3.4707 eV with a full-width half-maximum of 1 meV, confirming a stress free GaN layer.
Keywords: A1. Crystal morphology; A1. Impurities; A2. Growth from vapour; A3. Hydride vapour phase epitaxi; B2. Semiconducting III–V materials;

This paper addresses the formation of freestanding GaN substrates by a natural separation mechanism, effectively eliminating the need for post-growth processes such as laser liftoff, chemical etching or mechanical lapping to form freestanding GaN substrates. A number of GaN thick films were grown onto sapphire substrates by the hydride vapor-phase epitaxy (HVPE) method with thickness varying from 200 μm to 3.8 mm using either a low-temperature GaN or an AlN buffer as the nucleation step. We have found that samples grown on a low temperature GaN buffer naturally delaminate from the sapphire substrate post-growth over the entire thickness range studied. Furthermore, we have observed that the thinner films have high crack densities leading to the delamination of several smaller freestanding pieces. As the GaN thickness increases, the area of the delaminated pieces also increases, ultimately leading to a 1-to-1 correlation between initial sapphire substrate area and freestanding GaN area. However, the GaN films grown on AlN buffers did not delaminate. These results were accounted for by calculating the thermal stresses in the GaN film and substrate as a function of film thickness using Stoney's equation and assuming that the GaN buffer undergoes decomposition at the growth temperature.
Keywords: A1. Substrates; A2. Growth from vapor; A3. Hydride vapor-phase epitaxy; B1. Nitrides;

High-speed epitaxial growth of AlN above 1200 ∘ C by hydride vapor phase epitaxy by Toru Nagashima; Manabu Harada; Hiroyuki Yanagi; Yoshinao Kumagai; Akinori Koukitu; Kazuya Takada (42-44).
Aluminum nitride (AlN) epitaxial layer on sapphire (0 0 0 1) substrate was grown at high temperatures above 1200 ∘ C by hydride vapor phase epitaxy (HVPE). A high-temperature growth system was built by combining a conventional hot-wall type furnace and a heating susceptor with integrated heating element. This system realized growth of AlN by HVPE above 1200 ∘ C even in the quartz reactor. Growth rates of AlN stay constant in the temperature range of 1280 – 1410 ∘ C . This result was consistent with the results expected from thermodynamic analysis, and represents that growth of AlN by HVPE is under mass transportation limited process even at high temperatures. Epitaxial growth of AlN with growth rate of 85 μ m / h was achieved at 1380 ∘ C .
Keywords: A1. Crystal structure; A3. Hydride vapor phase epitaxy; B1. Nitrides; B2. Semiconducting aluminum compounds;

Evaluation of AlN single-crystal grown by sublimation method by Michimasa Miyanaga; Naho Mizuhara; Shinsuke Fujiwara; Mitsuru Shimazu; Hideaki Nakahata; Tomohiro Kawase (45-49).
AlN single crystals with thicknesses from 3 μm to 4 mm were grown on SiC substrates by the sublimation method. Evaluations of crystalline quality were performed by X-ray diffraction (XRD), transmission electron microscope (TEM) and etch pit density (EPD) measurement. The FWHM of the XRD rocking curve for AlN ( 1 0 1 ¯ 0 ) reflection was as small as 26 arcsec for the sample of 4 mm thickness, and the dislocation density was estimated to be less than 106  cm−2 by EPD measurement in spite of the large lattice mismatch of 1% between AlN and SiC. TEM observation was conducted to investigate the mechanism of the improvement of the crystalline quality. We observed the significant reduction of dislocations above the interface, allowing growth of 3 μm–4 mm thick AlN with high crystalline quality. These results show that the commercial production of large-sized, high-quality substrates of AlN single crystal is possible using the sublimation technique.
Keywords: A2. Growth from vapor; A2. Single-crystal growth; B1. Nitrides;

Effect of island coalescence on structural and electrical properties of InN thin films by V. Lebedev; V. Cimalla; F.M. Morales; J.G. Lozano; D. González; Ch. Mauder; O. Ambacher (50-56).
In this work, coalescence aspects of InN epitaxy are addressed. The coalescence phenomena have been studied in thin InN epilayers by means of electron microscopy and X-ray diffraction. Coalescence time and the corresponding diffusion coefficients at elevated temperatures were estimated for InN deposition. The substrate temperature was found to impact drastically the coalescence of the epilayer, and consequently, the electrical and transport properties of hexagonal InN material. Additionally, a simple growth model was suggested to explain the formation of domain boundaries and (0 0 0 1) stacking faults formed during the coalescence. In particular, it is shown that two adjacent and tilted, hexagonal-shaped InN domains may form a non-coherent boundary along a { 1 1 ¯ 0 0 } plane. We also suggest that the interaction between tilted domains induces formation of basal dislocations. This interaction has two consequences: a localized lateral growth of the most epitaxially oriented domain (forming a basal (0 0 0 1) stacking fault) followed by the formation of a surface step, and consequently the termination of a threading dislocation by its dissociation and propagation under the formed (0 0 0 1) stacking fault.
Keywords: A1. Coalescence; A3. Molecular beam epitaxy; B1. InN;

Hydride vapor phase epitaxy of InN by the formation of InCl3 using In metal and Cl2 by Yoshinao Kumagai; Jun Kikuchi; Yuuki Nishizawa; Hisashi Murakami; Akinori Koukitu (57-61).
Hydride vapor phase epitaxy (HVPE) of InN using In metal, Cl2 and NH3 as source materials and N2 carrier gas was investigated on sapphire (0 0 0 1) substrates. A thermodynamic analysis of the reaction between In metal and Cl2 gas in the source zone of the HVPE system revealed that the equilibrium partial pressure of InCl3 increased significantly with decreasing source zone temperature, while that of InCl remained almost constant. At a substrate temperature of 500 °C, appreciable growth of InN occurred by decreasing the source zone temperature to 450 °C. This means that InN growth occurs by the reaction between InCl3 and NH3. With an optimum surface nitridation time of the sapphire substrate prior to InN growth, c-axis oriented single-crystalline layers of InN were obtained. Cathodoluminescence (CL) spectrum measured at room temperature showed a strong peak at 0.75 eV.
Keywords: A1. Characterization; A3. Hydride vapor phase epitaxy; B1. Nitrides; B2. Semiconducting indium compounds;

Theoretical approach to initial growth kinetics of GaN on GaN(0 0 1) by Y. Kangawa; Y. Matsuo; T. Akiyama; T. Ito; K. Shiraishi; K. Kakimoto (62-65).
We carried out theoretical analyses based on ab initio calculations incorporates in which free energy of the vapor phase is incorporated in order to determine the initial growth kinetics of c-GaN on GaN(0 0 1)-(4×1). The feasibility of the theoretical approach had been confirmed by calculations of Ga adsorption–desorption transition temperature and transition beam equivalent pressures on the GaAs(0 0 1)-(4×2)β2 surface in our previous work [Y. Kangawa, T. Ito, A. Taguchi, K. Shiraishi, T. Ohachi, Surf. Sci. 493 (2001) 178]. The results of calculations suggest that no Ga adsorption occurs on the initial surface under typical growth conditions but that a Ga adsorption site appears after N adsorption on GaN(0 0 1)-(4×1). That is, in the initial growth stage of c-GaN on GaN(0 0 1)-(4×1), a N-adsorbed structure is formed and then Ga adsorbs on the N adatom.
Keywords: A1. Adsorption; A3. Molecular beam epitaxy; B1. c-GaN;

Influence of hydrogen coverage on Si(1 1 1) substrate on the growth of GaN buffer layer by Yuriko Matsuo; Yoshihiro Kangawa; Rie Togashi; Koichi Kakimoto; Akinori Koukitu (66-69).
The influence of hydrogen coverage on a Si(1 1 1) substrate on the initial growth of a GaN buffer layer was investigated using Ga and Al adsorption energies obtained by ab initio calculations. It was found that absolute values of the adsorption energies of Ga and Al atoms increased as the hydrogen coverage on the substrate surface decreased. Moreover, it was found that the absolute value of Al adsorption energy was larger than that of Ga in any case. These results suggest that it is important to control the substrate surface condition and carrier gas for the growth of a GaN buffer layer on a Si substrate, though an AlN buffer layer can be grown even under H2 ambient.
Keywords: A1. Adsorption; A1. Computer simulation; A1. Surface processes; A3. Vapor phase epitaxy; B1. Nitrides;

Threading dislocation reduction in (0 0 01) GaN thin films using SiN x interlayers by M.J. Kappers; R. Datta; R.A. Oliver; F.D.G. Rayment; M.E. Vickers; C.J. Humphreys (70-74).
The ability of in situ SiN x interlayers to lower the density of threading dislocations (TDs) has been studied for the growth of c-plane (0 0 0 1) GaN epilayers on sapphire by organometallic vapour-phase epitaxy (OMVPE). The TD density in the films may be reduced by up to a factor of 50 to 9×107  cm−2 and depends on the SiN x coverage and the conditions of the overgrowth. The TD reduction method relies on the formation of facetted islands on the SiN x -treated GaN surface and the formation of dislocation half-loops between bent-over TDs during the lateral overgrowth. Dislocations that are not annihilated at the interfacial region during the interlayer overgrowth may bend over at the islands’ inclined side facets and annihilate at their coalescence boundaries. Thus, the TD density was reduced at the expense of greater film thickness by increasing the SiN x coverage and delaying intentionally the coalescence of the GaN islands.
Keywords: A1. High-resolution X-ray diffraction; A3. Organometallic vapour-phase epitaxy; B1. Nitrides;

Surface step morphologies of GaN films grown on vicinal sapphire (0 0 0 1) substrates are studied in molecular beam epitaxy. Using atomic force microscopy, monolayer and multi-layer steps are clearly observed. It is found that step morphologies greatly depend on the vicinal angle and the inclination directions of the substrate. Step bunching on the surface during the growth occurs when the vicinal angle is larger than 1 . 0 ∘ . Furthermore, straight and zigzag steps are formed on the surface with the inclination direction of the vicinal surface toward a-axis and m-axis of the sapphire (0 0 0 1) substrates, respectively. An atomic-scale model is considered to explain the anisotropic step morphologies.
Keywords: A1. Surface structure; A3. Molecular beam epitaxy; B1. Nitride;

Growth of GaN and GaN/AlN multiple quantum wells on sapphire, Si and GaN template by molecular beam epitaxy by X.Y. Liu; P. Jänes; P. Holmström; T. Aggerstam; S. Lourdudoss; L. Thylén; T.G. Andersson (79-82).
GaN layers of 280 nm thick were grown on sapphire, silicon (1 1 1) and GaN template by plasma assisted molecular beam epitaxy. From atomic force microscopy and high-resolution X-ray diffraction, it was found that GaN grown on sapphire and template gave smooth surface (RMS less then 0.5 nm) and very high crystalline quality (FWHM of (0 0 0 2) scan on sapphire only 48 arcsec). However, GaN growth on Si (1 1 1) provided rough surface and poor crystalline quality. The GaN/AlN multiple quantum well structures were grown on sapphire and template. Intersubband absorption spectra from Fourier transform infrared spectroscopy indicated that layers on GaN templates had better performances than on sapphire substrates.
Keywords: A1. Atomic force microscopy; A1. High-resolution X-ray diffraction; A1. Reflection high-energy electron diffraction; A3. Molecular beam epitaxy; B1. Nitrides;

Growth evolution and pendeo-epitaxy of non-polar AlN and GaN thin films on 4H–SiC (1 1 2¯ 0) by S.M. Bishop; J.-S. Park; J. Gu; B.P. Wagner; Z.J. Reitmeier; D.A. Batchelor; D.N. Zakharov; Z. Liliental-Weber; R.F. Davis (83-89).
The initial and subsequent stages of growth of AlN on 4H–SiC (1 1 2¯ 0) and GaN on AlN (1 1 2¯ 0) have been investigated using atomic force microscopy and X-ray photoelectron spectroscopy. The AlN nucleated and grew via the Stranski–Krastanov mode. Densely packed, [0 0 0 1]-oriented individual islands were observed at 10 nm. Additional deposition resulted in the gradual reorientation of the growth microstructure along the [1 1¯ 0 0]. GaN formed via the Volmer–Weber mode with rapid growth of islands along the [1 1¯ 0 0] to near surface coverage at a thickness of 2 nm. Continued deposition resulted in both faster vertical growth along [1 1 2¯ 0] relative to the lateral growth along [0 0 0 1] and a [1 1¯ 0 0]-oriented microstructure containing rows of GaN. Fully dense GaN films developed between 100 and 250 nm of growth, and the preferred in-plane orientation changed to [0 0 0 1]. Lateral growth of GaN films reduced the dislocation density from ∼4×1010 to ∼2×108  cm−2. The high concentration of stacking faults (∼106  cm−1) was also reduced two orders of magnitude.
Keywords: A1. Atomic force microscopy; A1. Crystal morphology; A1. Surfaces; A3. Metalorganic chemical vapor deposition; A3. Pendeo-epitaxy; B1. Nitrides; B2. Semiconducting III–V materials;

Thermal effects on light-emission properties of GaN LEDs grown by metal-organic vapor phase epitaxy by Tohru Honda; Toshiaki Kobayashi; Shinichi Egawa; Masaru Sawada; Koichi Sugimoto; Taichi Baba (90-93).
Photoluminescence (PL) spectra of GaN layers grown on sapphire substrates by metal-organic vapor phase epitaxy (MOVPE) were observed at temperatures from RT to 500 K. The spectra include the near-band-edge emission (NBE) and yellow luminescence (YL). The peak energy of the NBE is shifted towards lower energy with increasing observed temperature. UV light-emitting diodes (LEDs) utilizing band-gap narrowing due to thermal effects are proposed and their advantages for integration are discussed.
Keywords: A1.Electroluminescence; A1.Photoluminescence; A1.UV; A3.MOVPE; B1.GaN; B3.Schottky;

Nucleation conditions for catalyst-free GaN nanowires by K.A. Bertness; A. Roshko; L.M. Mansfield; T.E. Harvey; N.A. Sanford (94-99).
We have examined the initial steps for catalyst-free growth of GaN nanowires by molecular beam epitaxy (MBE) on Si (1 1 1) substrates using AlN buffer layers. These wires form spontaneously under high N-to-Ga ratios for a growth temperature range of about 810–830 °C. Field emission scanning electron microscopy (FESEM) shows that part of the GaN forms a “matrix layer” that also grows with the [0 0 0 1] direction perpendicular to the substrate surface. This layer contains small, dense hexagonal pits in which the nanowires nucleate. Using both FESEM and atomic force microscopy (AFM), we identify the pit facets as {1 0 1¯ 2} planes. The nucleation studies show that the use of an AlN buffer layer is essential to the regular formation of the nanowires and matrix layers under our growth conditions. Our typical AlN buffer layer is 40–50 nm thick. We conclude that the nucleation mechanism for nanowires includes formation of nanocolumns in the AlN buffer layer. The propagation of the nanowires in GaN growth appears to be driven by differences in growth rates among crystallographic planes under N-rich conditions.
Keywords: A1. Nanostructures; A3. Molecular beam epitaxy; B1. Nitrides; B2. Semiconducting III–V materials;

Uniform hot-wall MOCVD epitaxial growth of 2 inch AlGaN/GaN HEMT structures by A. Kakanakova-Georgieva; U. Forsberg; I.G. Ivanov; E. Janzén (100-103).
The hot-wall metalorganic chemical vapor deposition (MOCVD) concept has been applied to the growth of Al x Ga1− x N/GaN high electron mobility transistor (HEMT) device heterostructures on 2 inch 4H-SiC wafers. Due to the small vertical and horizontal temperature gradients inherent to the hot-wall MOCVD concept the variations of all properties of a typical HEMT heterostructure are very small over the wafer: GaN buffer layer thickness of 1.83 μm±1%, Al content of the Al x Ga1− x N barrier of 27.7±0.1%, Al x Ga1− x N barrier thickness of 25 nm±4%, sheet carrier density of 1.05×1013  cm−2±4%, pinch-off voltage of −5.3 V±3%, and sheet resistance of 449 Ω±1%.
Keywords: A3. Metalorganic chemical vapor deposition; B1. Nitrides; B3. High electron mobility transistors;

Pulsed epitaxial lateral overgrowth of GaN by metalorganic vapour phase epitaxy by C. Liu; P.A. Shields; S. Denchitcharoen; S. Stepanov; A. Gott; W.N. Wang (104-109).
A mixed pulsed and normal GaN epitaxial lateral overgrowth (ELO-GaN) by epitaxy metalorganic vapour phase epitaxy (MOVPE) is reported in this study. Monitoring by using an in situ spectroscopic reflectometer has shown that a varying vertical growth rate during the pulsed growth was observed as in the normal ELO-GaN growth process, however, the growth rate was dramatically reduced in pulsed growth. Cross-section scanning electron microscope (SEM) images have shown that a lateral to vertical growth ratio (LTVGR) of 7 was obtained under a set of growth conditions on a template with a GaN trench and SiO 2 mask width of 5 and 15 μ m , respectively, and with the stripes aligned in the GaN 〈 1 ¯ 1 0 0 〉 crystallographic direction. Two types of growth instability associated with pulsed growth were observed under some growth conditions. One is the formation of large steps on the ELO-GaN stripes before coalescence; the other is the formation of hexagonal pyramids on the coalesced surface. The origin of pyramidal formation was found exactly on the coalescence boundaries. A mixed pulsed and normal ELO-GaN growth technique has been established to eliminate the large steps, and formation of pyramids can be avoided by switching to normal growth conditions before ELO-GaN stripes coalesce. The thickness of ELO-GaN has been successfully controlled below 1 μ m before coalescence, and below 3 μ m for a fully coalesced ELO-GaN film by this technique. Atomic force microscope (AFM) has confirmed that ELO-GaN films grown by this technique are of high structural quality.
Keywords: A1. Characterization; A1. Optical reflectance; A3. Metalorganic vapour phase epitaxy; A3. Pulsed growth; B1. Nitrides;

MOVPE growth and cathodoluminescence properties of GAN microcrystal co-doped with Zn and Si by Y. Honda; Y. Yanase; M. Tsuji; M. Yamaguchi; N. Sawaki (110-113).
A (1 1 1) Si micro-facet was fabricated by KOH anisotropic etching on a (0 0 1) Si substrate. On the (1 1 1) Si facet, a GaN:Zn, Si-microcrystal was grown by selective metal-organic vapor phase epitaxy using an AlN intermediate layer and covered with an AlN capping layer. The size of the AlN/GaN/AlN microcrystal was determined by that of the (1 1 1) Si facet on the substrate. The optical properties of sample were evaluated by cathodoluminescence (CL) measurement. The CL intensity of DAP emission band was enhanced by reducing the crystal size. On the other hand, in the case of samples without AlN capping layer, the CL intensity was decreased with the smaller crystals. This result suggests that the optical confinement does enhance the CL intensity in the AlN/GaN/AlN sample.
Keywords: A1. Crystal structure; A3. Metal-organic vapor phase epitaxy; B1. Nitrides;

Aluminum monolayers on Si (1 1 1) for MBE-growth of GaN by X.Y. Liu; H.F. Li; A. Uddin; T.G. Andersson (114-117).
Up to 10 monolayers of Al were deposited on Si (1 1 1) surfaces at low (450 °C) and high (640 °C) temperatures before the molecular beam epitaxy growth of GaN. The influence of the Al monolayers on the overall GaN epitaxial layers was investigated by reflection high-energy electron diffraction, atomic force microscopy, high-resolution X-ray diffraction and transmission electron microscopy. At high-temperature deposition, ∼1.3 monolayer Al gave the smoothest GaN surface and best crystalline quality. At the low temperature, only ∼0.8 ML provided the same GaN quality.
Keywords: A1. Atomic force microscopy; A1. High-resolution X-ray diffraction; A1. Reflection high-energy electron diffraction; A3. Molecular beam epitaxy; B1. Nitrides;

In situ annealing of GaN dot structures grown by droplet epitaxy on (1 1 1) Si substrates by Shigeya Naritsuka; Toshiyuki Kondo; Hiroaki Otsubo; Koji Saitoh; Yo Yamamoto; Takahiro Maruyama (118-122).
The effect of in situ annealing was investigated on GaN dots grown on Si(1 1 1) substrates by droplet epitaxy. In particular, the effect of irradiation by nitrogen plasma during annealing was examined. The density of GaN dots on the sample annealed with nitrogen plasma was approximately 1.0×1011  cm−2, which was much higher than the density of dots on the sample without nitrogen plasma (1.2×108  cm−2). X-ray photoelectron spectroscopy showed that the Ga 2p peaks of the samples annealed both with and without irradiation by nitrogen plasma consisted mainly of Ga–N peaks, while that of the unannealed sample contained an extremely large Ga–O component, which is observed at the higher binding energy side of the Ga–N peak. These results indicate that the degree of nitridation of the dots was much improved by annealing. The high density of GaN dots on the sample annealed with nitrogen plasma, in which case the dot diameters were as small as 20 nm, was presumably enabled through the suppression of both the re-evaporation of Ga atoms and the coarsening of Ga droplets by irradiation by the surface by nitrogen plasma.
Keywords: A3. Molecular beam epitaxy; B1. Nanomaterials; B1. Nitrides; B2. Semiconducting III–V materials;

Cubic-InN was grown at a low temperature of 350 °C using our home-made low-pressure metal-organic chemical vapor deposition (MOCVD). The technology of indium pre-deposition was applied, that is, a layer of indium was deposited on the sapphire surface with a precursor of trimethylindium (TMI) before the growth of InN. Both X-ray diffraction (XRD) and X-ray photoelectron (XPS) spectra show that the pre-deposited indium is able to promote the growth of InN, and meanwhile, suppress the indium aggregation in the grown layer. Atomic force microscopy (AFM) images indicate that the nucleation of InN becomes easier with the pre-deposition of indium. It is proposed that the pre-deposited indium can seed the growth of InN, just like the vapor–liquid–solid (VLS) fabrication of InN whiskers with indium nanoparticles.
Keywords: A2. Vapor–liquid–solid; A3. MOCVD; B1. InN; A1. Indium pre-deposition;

Epitaxial growth of 6H-AlN on M-plane SiC by plasma-assisted molecular beam epitaxy by D.M. Schaadt; O. Brandt; A. Trampert; H.-P. Schönherr; K.H. Ploog (127-129).
A new AlN polytype is grown on M-plane 6H-SiC by plasma-assisted molecular beam epitaxy. A 6H stacking order is deduced from reflection high-energy electron diffraction patterns and the occurrence of an additional [ 1 1 0 ¯ 0 ] 6 H reflection in X-ray diffraction ω−2θ scans. A predominant 6H stacking, intersected by thin lamellae with 2H stacking, is directly observed in high-resolution transmission electron microscopy. Atomic force microscopy (AFM) shows a stripe-like morphology with a roughness of 1.7 nm and a peak-to-valley distance of 12 nm over 25 m2. The AlN films are partially relaxed and of good crystal quality as evidenced from X-ray diffraction ω-scans with a line width of 130 arcsec for both symmetric and asymmetric reflections.
Keywords: A1. Crystal structure; A3. Molecular beam epitaxy; B1. Nitrides; B2. Semiconducting aluminum compounds;

Formation of needle-like and columnar structures of AlN by G.R. Yazdi; M. Syväjärvi; R. Yakimova (130-135).
The present study focused on understanding the formation of needle-like and columnar structures by investigating the initial nucleation of aluminium nitride (AlN) on SiC substrates with SEM, AFM, and XRD. The grown AlN consisted of high concentration (∼8×104  cm−2) hexagonal hillocks (HHs) that originate from threading dislocations in the substrate. The KOH etching technique has been used to examine the origin and formation process of HHs and defect reduction in the grown AlN crystals. A model is introduced to explain the AlN HH formation. The SEM result shows that the AlN columnar structure was formed by merging of needles, which are grown exactly on completed AlN HHs, followed by a lateral growth.
Keywords: A1. Nucleation; A2. Growth from vapor; A2. Single crystal growth; B1. Nitride; B2. Semiconducting III–V material;

Annihilation mechanism of threading dislocations in AlN grown by growth form modification method using V/III ratio by Masataka Imura; Naoki Fujimoto; Narihito Okada; Krishnan Balakrishnan; Motoaki Iwaya; Satoshi Kamiyama; Hiroshi Amano; Isamu Akasaki; Tadashi Noro; Takashi Takagi; Akira Bandoh (136-140).
High-quality AlN layers with atomically flat surface were grown on a c-plane sapphire substrate by high-temperature metal-organic vapor phase epitaxy (HT-MOVPE). Controlling V/III ratio during growth led to change the growth rate for each facet resulting in the change of the macroscopic form of grain at the transition V/III ratio. The threading dislocations were annihilated with the formation of dislocation loops at the changing of grain form. AlN crystallinity was improved due to the reason that small AlN grains were incorporated by the big AlN grains during growth. These phenomena were confirmed by transmission electron microscopy (TEM) and atomic force microscopy (AFM).
Keywords: A1. V/III ratio; A1. TEM; A1. Threading dislocations; A3. HT-MOVPE; B1. AIN;

Epitaxial lateral overgrowth of a-AlN layer on patterned a-AlN template by HT-MOVPE by N. Okada; N. Kato; S. Sato; T. Sumii; N. Fujimoto; M. Imura; K. Balakrishnan; M. Iwaya; S. Kamiyama; H. Amano; I. Akasaki; T. Takagi; T. Noro; A. Bandoh (141-144).
Epitaxial lateral overgrowth (ELO) of a-plane AlN (a-AlN) has been carried out on patterned a-AlN by high-temperature metal-organic vapor phase epitaxy (HT-MOVPE). The a-AlN templates were grown on +0.5° off r-plane sapphire. The ELO a-AlN layers were found to have coalesced well on the a-AlN templates with trenches formed along 〈 1 0 1 ¯ 0 〉 , while the ELO a-AlN layers were not coalesced on the a-AlN templates with trenches formed along 〈 0 0 0 1 〉 . Both the a-AlN template and the coalesced ELO a-AlN layer were investigated by X-ray diffraction (XRD), atomic force microscopy (AFM), scanning electron microscopy (SEM), cathodoluminescence (CL) and transmission electron microscopy (TEM). Consequently, it was found that the ELO technique is useful for improving the crystalline quality of a-AlN layer by HT-MOVPE.
Keywords: A3. Selective epitaxy; B1. AlN; B1. Nitrides; B2. Metal organic vapor phase epitaxy;

AlN growth on sapphire substrate by ammonia MBE by V.G. Mansurov; A.Yu. Nikitin; Yu.G. Galitsyn; S.N. Svitasheva; K.S. Zhuravlev; Z. Osvath; L. Dobos; Z.E. Horvath; B. Pecz (145-150).
Kinetics of (0 0 0 1) Al2O3 surface nitridation and subsequent growth of AlN films on the sapphire substrate by ammonia molecular beam epitaxy (MBE) are investigated. Surface morphology evolution during AlN growth is studied in situ by reflection high energy electron diffraction and ex situ by atomic force microscopy. It is found that the surfaces of AlN layers thicker than 100 nm have two major features: a quite smooth background and noticeable amount of hillocks. The influence of growth conditions on the AlN surface morphology is studied in order to find a way for reducing of the hillocks density. A modification of nitridated sapphire surface by small amount of Al (1–2 monolayers) with subsequent treatment of the surface under ammonia flux is proposed. An improvement of AlN surface morphology of the layers grown on the modified surfaces is demonstrated.
Keywords: A1. Crystal morphology; A3. Molecular beam epitaxy; B1. Nitrides; B2. Semiconducting III-V materials;

Reactive sputter deposition of AlInN thin films by Q.X. Guo; Y. Okazaki; Y. Kume; T. Tanaka; M. Nishio; H. Ogawa (151-154).
 AlInN films were grown on (0 0 0 1) sapphire and glass substrates by reactive radio-frequency (RF) magnetron sputtering using aluminium and indium targets in an ambience of argon and nitrogen. It was revealed that the Al composition in the AlInN films can be controlled by varying the ratio of the applied RF power of the indium target to that of the aluminium target. The lattice constant for c-axis obtained from the (0 0 0 2) diffraction peak of the AlInN films decreased with the increase of Al composition.
Keywords: A1. Crystal structure; A1. X-ray diffraction; B1. Nitrides; B2. Semiconducting III–V materials;

Growth and characterisation of semi-polar ( 1 l 2 ¯ 2 ) InGaN/GaN MQW structures by M.J. Kappers; J.L. Hollander; C. McAleese; C.F. Johnston; R.F. Broom; J.S. Barnard; M.E. Vickers; C.J. Humphreys (155-159).
Semi-polar ( 1 l 2 ¯ 2 ) GaN films have been grown by organometallic vapour-phase epitaxy (OMVPE) on m-plane ( 1 l ¯ 0 0 ) sapphire. The in-plane orientation of the GaN with respect to the sapphire substrate was determined to be [ 1 l ¯ 0 0 ]GaN∥[ 1 l 2 ¯ 0 ]sapphire and the projections of [0 0 0 1]GaN and [ 1 l 2 ¯ 0 ]GaN∥[0 0 0 1]sapphire. The smooth planar films displayed a distinct in-plane anisotropic mosaicity with the symmetric ( 1 l 2 ¯ 2 ) reflection along the GaN m-direction (XRD ω FWHM of 1080 arcsec) broader than that along the projected c-direction (665 arcsec). TEM analysis indicated the presence of line defects threading through the layer but few stacking faults. Two semi-polar ( 1 l 2 ¯ 2 ) InGaN/GaN 10-period quantum well structures with wells thicknesses (alloy compositions) of 3.4 nm (12%) and 3.9 nm (19%) showed strong photoluminescence peaks at 430 and 500 nm, respectively.
Keywords: A1. High-resolution X-ray diffraction; A1. Photoluminescence; A3. Organometallic vapour-phase epitaxy; A3. Quantum well; B1. Nitride;

Growth of high-In-content InAlN nanocolumns on Si (1 1 1) by RF-plasma-assisted molecular-beam epitaxy by Jumpei Kamimura; Tetsuya Kouno; Shunsuke Ishizawa; Akihiko Kikuchi; Katsumi Kishino (160-163).
In x Al1− x N nanocolumns (x In=0.73–1.00) of c-axis orientation were successfully self-organized on Si (1 1 1) substrates by RF-plasma-assisted molecular-beam epitaxy (RF-MBE). The height, diameter and density of nanocolumns were 0.7–1.1 μm, 40–130 nm, and 0.7–3.0×1010  cm−2, respectively. With increasing In composition from 0.73 to 1.00, the room-temperature photoluminescence (RT-PL) peak wavelengths shifted from 1.06 to 1.87 μm. AlN nanocolumns (when x In=0) were also fabricated, and RT-PL was observed with a peak energy of 5.85 eV.
Keywords: A1. Photoluminescence; B1. AlN; B1. InA1N; B1. InN; B1. Nanocolumn; B1. Nanorod;

Growth of thick Al x Ga1− x N ternary alloy by hydride vapor-phase epitaxy by Takayoshi Yamane; Fumitaka Satoh; Hisashi Murakami; Yoshinao Kumagai; Akinori Koukitu (164-167).
Growth of thick Al x Ga1− x N ternary alloy using AlCl3 and GaCl gases as group III precursors was performed by hydride vapor-phase epitaxy (HVPE). C-axis-oriented AlGaN layers could be grown at 1100 °C. It was found that the growth of Al x Ga1− x N by HVPE was affected by the presence of H2 in the carrier gas. Therefore, the solid composition x in Al x Ga1− x N ternary alloy could be controlled by changing the input Al mole ratio of the group III precursor (R Al) and/or the low range (<10%) of partial pressure of hydrogen (H2) in the carrier gas (F o). The growth rate of approximately 30 μm/h was obtained under inert carrier gas (F o=0.0), while the growth rate decreased rapidly in the low R Al under a low partial pressure of H2 in the carrier gas (F o=0.1). These results are in good agreement with the thermodynamic prospect. Thus, the growth of Al x Ga1− x N using HVPE is thermodynamically controlled.
Keywords: A1. Crystal structure; A3. Hydride vapor-phase epitaxy; B1. Alloys; B1. Nitrides; B2. Semiconducting aluminum compounds; B2. Semiconducting ternary compounds;

Improvements of surface morphology and sheet resistance of AlGaN/GaN HEMT structures using quasi AlGaN barrier layers by Y. Kawakami; X.Q. Shen; G. Piao; M. Shimizu; H. Nakanishi; H. Okumura (168-171).
We report the growth and characterization results of AlGaN/GaN heterostructures using quasi AlGaN as a barrier layer, which is formed by AlN/GaN super-lattice. It is found that the surface morphology of the heterostructure is greatly improved, where monolayer steps on the surface are clearly observed. Simultaneously, electric properties in such structures are superior to those using the conventional alloy AlGaN caplayers. Low sheet resistance (less than 200 Ω/□) is obtained from our samples with high Al composition (>40%) in average, which shows the great merit of our growth method to the conventional technique. It is expected that the technique can be applied to the high power and high frequency device applications.
Keywords: A1. Surface structure; A3. Molecular beam epitaxy; A3. Superlattice; B1. Nitride;

Growth of Si/III–V-N/Si structure with two-chamber molecular beam epitaxy system for optoelectronic integrated circuits by Y. Furukawa; H. Yonezu; A. Wakahara; S. Ishiji; S.Y. Moon; Y. Morisaki (172-176).
A Si/III–V-N/Si structure for optoelectronic integrated circuits (OEICs) was grown using a two-chamber molecular beam epitaxy (MBE) system to decrease a carrier concentration of Si epilayer for metal oxide field effect transistors (MOSFETs). At first, a GaP layer was grown by migration-enhanced epitaxy on a Si substrate. Two-dimensional growth mode was maintained, and self-annihilation of anti-phase domain was confirmed. The growth process was also identified from reflection high-energy electron diffraction patterns during the growth. Subsequently, an InGaPN/GaPN double-hetero light-emitting diode (LED) and a topmost Si layer were grown by the dislocation-free growth process. It was found that a carrier concentration of the topmost Si epilayer was decreased to 4.0–6.6×1017  cm−3 from 3.0–6.7×1018  cm−3 by using the two-chamber MBE system instead of a single-chamber MBE system. The carrier concentration could be adapted to the fabrication of MOSFETs. Finally, we have fabricated elemental devices on the Si/III–V-N/Si structure and obtained characteristics of pMOSFETs and LEDs successfully. It was confirmed that the two-chamber MBE system could be available to the realization of OEICs.
Keywords: A1. Growth models; A3. Migration-enhanced epitaxy; A3. Molecular beam epitaxy; B1. Nitrides; B2. Semiconducting gallium compounds; B2. Semiconducting silicon;

Effect of low-temperature InGaN interlayers on structural and optical properties of In-rich InGaN by Hyunseok Na; Shinya Takado; Shinya Sawada; Masahito Kurouchi; Takanobu Akagi; Hiroyuki Naoi; Tsutomu Araki; Yasushi Nanishi (177-181).
We have grown In-rich InGaN epilayers on InN by radio-frequency plasma-assisted molecular beam epitaxy (RF-MBE), and the role of thin InGaN interlayers inserted between InN and InGaN were investigated in structural and optical properties of In-rich InGaN. InGaN interlayers (InGaN-ILs) were grown at the same growth condition of upper InGaN except for growth temperature. InGaN layer directly grown on InN was not flat owing to suppressed adatom mobility at N-rich V/III ratio, even though a part of surface was well developed to flat (0 0 0 1¯) plane. Insertion of 1 nm InGaN-IL did not give any change to the surface morphology of InGaN. On the other hand, 3, 10, and 30 nm InGaN-ILs apparently changed InGaN's morphologies to be more continuous, and the luminescence property was also largely improved. However, 10 and 30 nm InGaN-ILs slightly deteriorated the crystal quality of InGaN probably caused by thick interlayer containing a lot of defects. Therefore, we found that 3 nm LT-InGaN was very effective to improve the morphology of InGaN not deteriorating the crystal quality of InGaN. And it was also noticeable that the strain of InGaN was reversed from tensile to compressive by InGaN-IL, which is one of the positive approaches to suppress surface cracks.
Keywords: A1. Crystal morphology; A1. X-ray diffraction; A2. RF-MBE; B1. InGaN; B1. InN;

Growth and characterization of GaPN by OMVPE by Akihiro Wakahara; Yuzo Furukawa; Shinya Itoh; Susumu Hatakenaka; Hiroo Yonezu (182-185).
Growth properties of GaPN layer by organometallic vapor phase epitaxy (OMVPE) have been investigated. The effect of growth temperature on the nitrogen contents reveals that the nitrogen content is limited by thermal desorption of nitrogen related species. The desorption energy of nitrogen related species is not dependent on the group-V vapor composition. From the photoluminescence (PL) measurement, post growth annealing in pure nitrogen is effective to improve the PL intensity, but the effects of annealing is slightly different from the samples grown by molecular beam epitaxy.
Keywords: A3. Metalorganic vapor phase epitaxy; B1. GaNP; B2. Metastable III–V semiconductor;

a-plane GaN grown on r-plane sapphire substrates by hydride vapor phase epitaxy by T. Zhu; D. Martin; R. Butté; J. Napierala; N. Grandjean (186-189).
Thin a-plane ( 1 1 2 ¯ 0 ) GaN layers have been grown on r-plane ( 1 1 ¯ 0 2 ) sapphire substrates by hydride vapor phase epitaxy (HVPE), using either a single-step high-temperature (HT) growth or a two-step growth method similar to that of metal organic vapor phase epitaxy (MOVPE). For the single-step growth procedure, layers were grown under various pressures, ranging from near atmospheric pressure down to 75 mbar. For growth pressures lower than 200 mbar, an improvement in surface morphology is observed. The full-widths at half-maximum (FWHM) of in-plane X-ray diffraction peak anisotropy features are reversed when the growth pressure is decreased from 400 to 100 mbar. These layers are then compared to those obtained by the two-step growth procedure.
Keywords: A1. HRXRD; A1. Morphology; A1. Structure; A3. HVPE; B1. GaN;

Effect of middle temperature intermediate layer on crystal quality of AlGaN grown on sapphire substrates by metalorganic chemical vapor deposition by M. Tsukihara; K. Sumiyoshi; T. Okimoto; K. Kataoka; S. Kawamichi; K. Nishino; Y. Naoi; S. Sakai (190-193).
We investigated the effect of a middle temperature intermediate layer (MTIL) of AlGaN and AlN/AlGaN super lattice structure (SLS) on the crystal quality of AlGaN epilayers grown by low-pressure metalorganic chemical vapor deposition (LP-MOCVD). MTIL-Al0.17Ga0.83N layers could enhance the lateral growth area at 950 °C, and the dislocations made dislocation loop. The full-width at half-maximum from ( 1 0 1 ¯ 2 ) X-ray rocking curve peak of HT-Al0.17Ga0.83N epilayers grown on MTIL decreased from 1220 to 525 arcsec. The dislocation density evaluated by atomic force microscopy density decreased from 1.0×1010 to 1.3×109  cm−2, respectively. These results reveal that MTIL technique is very useful to further improve the quality of AlGaN-based films, and to fabricate the high-efficiency ultra-violet light emitting diode (UV-LED).
Keywords: B1. AlGaN; A1. Dislocation reduction; A3. MOCVD;

Evaluation of sapphire substrate heating behaviour using GaN band-gap thermometry by J.J. Harris; R. Thomson; C. Taylor; D. Barlett; R.P. Campion; V.A. Grant; C.T. Foxon; M.J. Kappers (194-198).
The recent development of a commercial band-gap thermometry system for wide band-gap materials such as GaN (the k-Space Associates’ “ BandiT TM ”) has allowed a systematic study of the relationship between pyrometric or thermocouple temperature-monitoring methods and the directly determined layer temperature. The wide band gap of GaN gives a weak and sample-dependent coupling to radiant heat sources, and it is common in molecular beam epitaxy (MBE) to use a sputtered Mo coating on the rear of the sapphire substrate to improve the efficiency and consistency of heat transfer. We have investigated the role of this backing layer and the use of a PBN diffuser in an MBE chamber, but similar results are expected to be obtained from band-gap thermometry in a metal-organic vapour phase epitaxy (MOVPE) system. The wide range spectrometer used for the band-edge determination can also be employed in a pyrometric mode, at wavelengths both within the band gap of GaN and above it. The latter is insensitive to thickness oscillations, and is less affected by the presence or absence of a Mo backing layer. Results are presented comparing all these measurement techniques, and analysed to show that the Mo backing does not significantly increase the power coupling into the epitaxial layer, although it does improve the accuracy of pyrometric measurements.
Keywords: A1. Heat transfer; A1. Optical absorption; A1. Radiation; A3. Molecular beam epitaxy; B1. Nitrides; B2. Semiconducting gallium compounds;

Electronic structure of nitride surfaces by David Segev; Chris G. Van de Walle (199-203).
We report a systematic and comprehensive computational study of reconstructed GaN and InN surfaces in various orientations, including the polar c plane as well as the nonpolar a and m planes. Two distinct microscopic origins for Fermi-level pinning are identified, depending on surface stoichiometry and surface polarity. We also explain the source of the electron accumulation that has been universally observed at InN surfaces. We predict that such a charge accumulation will be absent on the nonpolar surfaces of InN, when prepared under specific conditions.
Keywords: A1. Computer simulation; A1. Surface structure; B1. Nitrides; B2. Semiconducting III–V materials;

Interfacial chemistry and energy band line-up of pentacene with the GaN (0 0 0 1) surface by J. Uhlrich; M. Garcia; S. Wolter; A.S. Brown; T.F. Kuech (204-211).
The chemical nature of the GaN surface combined with the bulk and surface electronic structure opens new potential application areas for this material. The nature of specific organic-GaN was developed for two cases in which the surface electronic structure of such interfaces was developed and utilized. The band alignment between n-GaN and pentacene was determined for surfaces prepared through reaction with HCl. The energy band offset was estimated through combined X-ray photoelectron spectroscopy and ultraviolet photoemission measurements. XPS measurements indicated that there was no interfacial chemical reaction. The measured valence band offset between the n-GaN and the vapor-deposited pentacene was estimated to be greater than 2 eV providing a favorable band-offset for hole-injection from the GaN layer into pentacene. The surface of a AlGaN/GaN heterojunction field effect transistor (HFET) structures was also functionalized by an adsorbed hemin layer and was shown to be sensitive to the presence of reactive species such as NO. The HFET structure provides enhanced sensitivity to changes in the surface electric field, altered through the adsorption of chemically active species either directly on the surface or through the interactions with surface functionalization.
Keywords: A1. Interfaces; A3. Metalorganic vapor phase epitaxy; B1. Nitrides;

The results of atomistic calculations based on an ab initio tight-binding method are reported in this work for four configurations of the perfect a ⇒ -screw basal dislocation in wurtzite GaN: pure shuffle, pure glide and two mixed shuffle–glide. Configurations with pure character (shuffle or glide) are found to be stable, whereas those with mixed shuffle–glide character are found to be transition-like configurations. Further, the calculations predict that the pure glide configuration, containing threefold coordinated atoms with an sp2 hybridization, to be the most energetically favourable.
Keywords: A1. Line defects; B1. Nitrides; B2. Semiconducting III-V materials;

Evolution of threading dislocations in MOCVD-grown GaN films on (1 1 1) Si substrates by X. Weng; S. Raghavan; J.D. Acord; A. Jain; E.C. Dickey; J.M. Redwing (217-222).
We have quantitatively compared the evolution of threading dislocations (TDs) in GaN films grown on (1 1 1) Si substrates using different buffer/interlayer structures: a compositionally graded Al x Ga1− x N (0⩽x⩽1) buffer layer, a thin high-temperature (HT) AlN interlayer (IL), and an AlN/GaN/Al x Ga1− x N multilayer. Plan-view transmission electron microscopy (TEM) shows a reduction in TD density in GaN films grown on graded Al x Ga1− x N buffer layers, and an increase in TD density in GaN films grown on HT AlN ILs, in comparison with those grown directly on an AlN buffer layer. Cross-sectional TEM reveals bending and annihilation of TDs within the graded Al x Ga1− x N buffer layer, which lead to a decrease of TD density in the overgrown GaN films. On the other hand, a high density of TDs forms at the GaN/AlN IL interface, resulting in an increase in TD density in the GaN film. In addition, growing a thin AlN+GaN bilayer before growing the compositionally graded Al x Ga1− x N buffer layer significantly reduces the TD density in the Al x Ga1− x N buffer layer, which subsequently further reduces the TD density in the overgrown GaN film.
Keywords: A3. Metalorganic chemical vapor deposition; B1. Nitrides; B1. Silicon;

All-optical characterization of carrier lifetimes and diffusion lengths in MOCVD-, ELO-, and HVPE- grown GaN by T. Malinauskas; R. Aleksiejūnas; K. Jarašiūnas; B. Beaumont; P. Gibart; A. Kakanakova-Georgieva; E. Janzen; D. Gogova; B. Monemar; M. Heuken (223-227).
The metrological capability of the picosecond four-wave mixing (FWM) technique for evaluation of the photoelectrical properties of GaN heterostructures grown on sapphire, silicon carbide, and silicon substrates as well as of free-standing GaN films is demonstrated. Carrier recombination and transport features have been studied in a wide excitation, temperature, and dislocation density (from ∼1010 to 106  cm−2) range, exploring non-resonant refractive index modulation by a free carrier plasma. The studies allowed to establish the correlations between the dislocation density and the carrier lifetime, diffusion length, and stimulated emission threshold, to reveal a competition between the bimolecular and nonradiative recombination, and to verify the temperature dependence of bimolecular recombination coefficient in the 10–300 K temperature range. It was shown that the FWM technique is more advantageous than the time-resolved photoluminescence technique for determination of carrier lifetimes in high quality thick III–nitride layers.
Keywords: A1. Characterization; A1. Defects; A3. Hydride vapor phase epitaxy; A3. Metalorganic chemical vapor deposition; B1. Nitrides;

Photoelectric properties of highly excited GaN:Fe epilayers, grown by modulation- and continuous-doping techniques by Z. Bougrioua; M. Azize; B. Beaumont; P. Gibart; T. Malinauskas; K. Neimontas; A. Mekys; J. Storasta; K. Jaras˘iūnas (228-232).
Investigation of photoelectrical properties of iron modulation-doped (MD) and continuously doped (CD) GaN layers has been carried out by transient photo-Hall, photo-conductivity, and time-resolved picosecond four-wave-mixing (FWM) techniques. The MD semi-insulating (SI) layers exhibited prolonged photocurrent relaxation time and the presence of deep defects with thermal activation energy of 217 meV. Low electrical activity of threading dislocations (TD) in the upper part of the MD layers as well as high carrier mobility at low temperature was confirmed by FWM measurements and pointed out to vanishing dislocation-related heterogeneous barriers due to Fe doping. In contrast, shorter carrier lifetimes and low mobility in CD-layers were attributed to Fe-related defects, more “detrimental” centers than dislocations.
Keywords: A1. characterization; A1. Defects; A3. Metalorganic chemical vapor deposition; B1. Nitrides;

Effect of anisotropic strain on phonons in a-plane and c-plane GaN layers by V. Darakchieva; T. Paskova; M. Schubert; P.P. Paskov; H. Arwin; B. Monemar; D. Hommel; M. Heuken; J. Off; B.A. Haskell; P.T. Fini; J.S. Speck; S. Nakamura (233-238).
We have studied phonons in two types of anisotropically strained GaN films: c-plane GaN films grown on a-plane sapphire and a-plane GaN films grown on r-plane sapphire. The anisotropic strain in the films is determined by high-resolution X-ray diffraction (HRXRD) in different measuring geometries and the phonon parameters have been assessed by generalized infrared spectroscopic ellipsometry (GIRSE). The effect of strain anisotropy on GaN phonon frequencies is presented and the phonon deformation potentials a A 1 ( TO ) , b A 1 ( TO ) , c E 1 ( TO ) and c E 1 ( LO ) are determined.
Keywords: A1. High-resolution X-ray diffraction; A1. Infrared spectroscopic ellipsometry; A1. Phonons; B1. GaN;

Investigation of n-GaN/p-SiC/n-SiC heterostructures by A.A. Lebedev; O.Yu. Ledyaev; A.M. Strel’chuk; A.N. Kuznetsov; A.E. Nikolaev; A.S. Zubrilov; A.A. Volkova (239-241).
With the use of sublimation epitaxy in vacuum and hydride vapor phase epitaxy (HVPE) a heterostructure n-GaN/p-6H-SiC/n-SiC on base top of n+6H-SiC substrate was developed. The electrical properties of the n-GaN/p-6H-SiC heterojunction was investigated and prototype of hetero bipolar transistor (HBT) was fabricated. It was shown that this device can work up to ∼350 °C. It was concluded that device parameters can be improved by further optimization.
Keywords: A3. Hydride vapor phase epitaxy. A3. Solid phase epitaxy. B2. Semiconducting materials. B3. Heterojunction semiconductor devices;

A surface study of wet etched AlGaN epilayers grown by hot-wall MOCVD by M. Syväjärvi; A. Kakanakova-Georgieva; G.R. Yazdi; A. Karar; U. Forsberg; E. Janzén (242-245).
Epitaxial layers of AlGaN were grown by hot-wall MOCVD and their surfaces wet chemically etched with phosphorous acid. The as-grown surfaces and the development of the etched surfaces after 10 and 20 min of etching were studied with atomic force microscopy (AFM) and CL. In the as-grown layers growth features may be resolved while the RMS is as low as 1.4 Å in a scan area of 2×2 μm. Surfaces etched for 10 min had developed etch pits and a low RMS roughness of 7 Å indicating a uniform quality of the layers. Micrometer scale hexagonal features were observed after 20 min of etching. In some cases a deep hexagonal etch pit is observed in the centre of the hexagonal feature with a 30° rotation to each other, suggesting that the origin is substrate-induced defects.
Keywords: A1. Characterization; A2. Growth from vapour; A2. Single-crystal growth;

We report on high-resolution X-ray diffraction studies of crystalline perfection and relaxation of elastic strain in GaN and AlN layers grown on SiC and sapphire substrates. Thin (300–500 nm) GaN layers are grown with a threading screw dislocation density of about 1–2×107  cm−2. A density of 1.75–8.5×105  cm−2, the lowest value ever reported for III-nitride epitaxial layers, is observed in thin (200–300 nm) AlN layers. In both cases, low density is observed in a surface layer formed over the defective nucleation layer. A model of spatial distribution of crystalline defects, based on generation of point defects on the growth surface, diffusion and further structural transformation under the action of volumetric elastic strain accounts for these observations. Perfect thin layers may be used as templates for epitaxial growth of perfect layers of other compositions, for example, GaN or InGaN layers for photovoltaic applications.
Keywords: A1. Characterization; A1. Defects; A1. High-resolution X-ray diffraction; B1. Nitrides;

Vacancy defect distribution in heteroepitaxial a -plane GaN grown by hydride vapor phase epitaxy by F. Tuomisto; T. Paskova; S. Figge; D. Hommel; B. Monemar (251-253).
We have used positron annihilation spectroscopy to study the native vacancy distribution in a -plane heteroepitaxial GaN. We show that the Ga vacancy concentration is independent of the layer thickness in the range from 5 to 25 μ m . This is strikingly different from the behavior in c -plane GaN, where the Ga vacancy concentration decreases dramatically with the distance from the GaN/sapphire interface. This difference in the native vacancy profiles is tentatively correlated with the differences in the O impurity and dislocation density profiles in the polar and non-polar materials.
Keywords: A1. Characterization; A1. Point defects; A3. Hydride vapor phase epitaxy; A3. Metalorganic chemical vapor deposition; B1. Nitrides; B2. Semiconducting gallium compounds;

Selective wet etching of lattice-matched AlInN–GaN heterostructures by F. Rizzi; K. Bejtka; P.R. Edwards; R.W. Martin; I.M. Watson (254-258).
Wet etching of AlInN–GaN epitaxial heterostructures, containing AlInN layers with InN mole fractions close to 0.17 has been studied. One molar aqueous solution of the chelating amine 1,2-diaminoethane (DAE) proved to selectively etch the AlInN layers, without the need for heating above room temperature, or photo-assistance. In experiments with a (0 0 0 1)-oriented AlInN-on-GaN bilayer, the mode of removal of the AlInN layer was predominantly lateral etching, initiated from the sidewalls of pit defects in the AlInN layer. The lateral etch rate was estimated at ∼60 nm/h. The GaN buffer layer surface was roughened concurrently with etching of the AlInN, although the DAE solution has no effect on as-grown GaN (0 0 0 1) surfaces. The roughening of the GaN surface is tentatively attributed to the charge accumulation layer expected at the AlInN–GaN heterointerface. The DAE etchant also proved effective at removing buried AlInN layers from trilayer and more complex multilayer structures, leading to the prospect of epitaxial lift-off processes, and the fabrication of three-dimensional engineered microstructures. These capabilities were demonstrated by the production of suspended microdisk structures from a GaN–AlInN–GaN trilayer, using a combination of dry and wet etching.
Keywords: A1. Etching; A3. Metalorganic vapor phase epitaxy; B1. Nitrides; B2. Semiconducting III–V materials;

GaN nanocolumns were grown with AlN buffer layers on (0 0 0 1) sapphire substrates by rf-plasma-assisted molecular-beam epitaxy. The AlN buffer layers underneath the nanocolumns were used to nucleate the nanocrystals. The thickness of the AlN buffer layer affected the column configuration (size, shape), the density and the optical properties of the nanocolumns; when the thickness increased from 1.8 to 8.2 nm, the average column diameter gradually decreased from 150 to 52 nm with a small kink, but the column density peaked at a thickness of 3.2 nm at 5×109  cm−2 and finally decreased to 2×108  cm−2. Based on TEM observations, it is suggested that GaN nanocolumns were not grown just on AlN grain but on the edge of AlN grain. Further, the growth behavior of a nanocolumn as a function of AlN buffer layer thickness is suggested. The room-temperature photoluminescence intensity of the nanocolumns was maximized at a buffer thickness of 4.6 nm, where the intensity was 4 times stronger than that of high-quality bulk GaN crystals grown by HVPE with a threading dislocation density of ∼8×106  cm−2.
Keywords: A1. Nanostructures; A3. Molecular-beam epitaxy; B1. Gallium compounds; B1. Nitrides; B2. Semiconducting III–V materials;