T. King

Energy band bowing parameter in AlxGa1−xN alloys

Molecular-beam epitaxy grown AlxGa1−xN alloys covering the entire range of alloy compositions, 0⩽x⩽1, have been used to determine the alloy band gap dependence on its composition. The Al chemical composition was deduced from secondary ion mass spectroscopy and Rutherford backscattering. The composition was also inferred from x-ray diffraction. The band gap of the alloy was extracted from low temperature optical reflectance measurements which are relatively more accurate than photoluminescence. Fitting of the band gap data resulted in a bowing parameter of b=1.0 eV over the entire composition range. The improved accuracy of the composition and band gap determination and the largest range of the Al composition over which our study has been conducted increase our confidence in this bowing parameter.

Dependence of GaN polarity on the parameters of the buffer layer grown by molecular beam epitaxy

The polarity of GaN films grown using GaN and AlN buffer layers on sapphire substrates by molecular beam epitaxy were investigated by atomic force microscopy, hot wet chemical etching, and reflection high-energy electron diffraction. We found that the GaN films grown on high temperature AlN (>890 °C) and GaN (770–900 °C) buffer layers invariably show Ga and N polarity, respectively. However, the films grown using low temperature (∼500 °C) buffer layers, either GaN or AlN, could have either Ga or N polarity, depending on the growth rate of the buffer layer.

Defect reduction with quantum dots in GaN grown on sapphire substrates by molecular beam epitaxy

The GaN films grown on buffer layers containing quantum dots by molecular beam epitaxy on sapphire substrates were investigated. The density of the dislocations in the films was determined by wet chemical etching and atomic force microscopy. It was found that the insertion of a set of multiple GaN quantum-dot layers in the buffer layer effectively reduces the density of the dislocations in the epitaxial layers. As compared to the dislocation density of ∼1010 cm−2 in the typical GaN films grown on AlN buffer layer, a density of ∼3×107 cm−2 was demonstrated in the GaN films grown with quantum dot layers.

Comparative study of Ga- and N-polar GaN films grown on sapphire substrates by molecular beam epitaxy

We report the surface, structural, and optical properties of typical Ga- and N-polar GaN films grown on sapphire substrates by molecular beam epitaxy. The Ga-polar films were grown on AlN buffer while the N-polar films were grown on GaN buffer layers. Atomic force microscopy imaging shows that the as-grown and chemically etched Ga-polar films have a flat and pitted surface while the N-polar surface is rougher with isolated columns or islands. Transmission electron microscopy demonstrates a low density of inversion domains in the Ga-polar films, while a much higher density of inversion domains was observed in the N-polar films. X-ray diffraction curves show a narrower (002) peak for Ga-polar films than that for N-polar films. On the other hand, both Ga- and N-polar films show a similar width of (104) peak. Despite their rough surfaces, high density of inversion domains, and broader (002) x-ray diffraction peaks, N-polar films with low dislocation density were demonstrated. In addition, higher PL efficiency...

Investigation of Defects and Polarity in GaN Using Hot Wet Etching, Atomic Force and Transmission Electron Microscopy and Convergent Beam Electron Diffraction

Availability of reliable and quick methods to investigate defects and polarity in GaN films is of great interest. We have used photo-electrochemical (PEC) and hot wet etching to determine the defect density. We found the density of whiskers formed by the PEC process to be similar to the density of hexagonal pits formed by wet etching and to the dislocation density obtained by transmission electron microscopy (TEM). Hot wet etching was used also to investigate the polarity of MBE-grown GaN films together with convergent beam electron diffraction (CBED) and atomic force microscopy (AFM). We have found that hot H 3 PO 4 etches N-polarity GaN films very quickly resulting in the complete removal or a drastic change of surface morphology. On the contrary, the acid attacks only the defect sites in Ga-polar films leaving the defect-free GaN intact and the morphology unchanged. The polarity assignments, confirmed by CBED experiments, were related to the as-grown surface morphology and to the growth conditions.

Rapid delineation of extended defects in GaN and a novel method for their reduction

Availability of reliable and quick methods to investigate extended defects and polarity in GaN films are of great interest to researchers investigating and exploiting GaN-based structures. The step immediately following the determination of defect density (DD) is to explore ways in which the DD can be reduced. In this paper, we report a systematic investigation of DD determination in GaN which is followed by a novel technique, use of quantum dots, to reduce the DD. We have used photo-electrochemical (PEC) and hot wet etching to determine the DD. We found the density of whiskers formed by the PEC process to be similar to the density of hexagonal pits formed by wet etching and to the dislocation density obtained by transmission electron microscopy (TEM). Hot wet etching was also used to investigate the polarity of MBE-grown GaN films together with convergent beam electron diffraction (CBED) and atomic force microscopy (AFM). We have found that hot H 3 PO 4 etches N-polarity GaN films very quickly resulting in the complete removal or a drastic change of surface morphology. We also report on the improvement in the GaN crystal quality by using multiple layers of quantum dots (QDs) as part of a strain-relieving buffer layer. Samples with QDs generally showed narrower X-ray diffraction peaks and higher photoluminescence efficiency than the control samples without QDs. Insertion of QDs reduced the dislocation density, as determined by a defect revealing etch, from ∼10 10 cm -2 to ∼5 × 10 7 cm -2 . Preliminary TEM investigations show that many of the dislocations terminate at QDs.

Properties of AlxGa1−xN layers grown by plasma-assisted molecular-beam epitaxy under Ga-rich conditions

AlxGa1−xN films were grown by plasma-assisted molecular-beam epitaxy on (0001) sapphire substrates under Ga-rich conditions. To control the AlxGa1−xN composition over the entire range, the Al and Ga arrival rates were fixed while the nitrogen arrival rate was varied. We have found that the Al fraction increased with decreasing N flow due to preferentially favorable bonding of Al and N over Ga and N. Consequently, the growth rate decreased as the Al mole fraction increased. A photoluminescence quantum efficiency at 15 K was markedly higher for the AlxGa1−xN layers grown under Ga-rich conditions (3%–48%) compared to the layers grown under N-rich conditions (1%–10%), indicating much reduced nonradiative recombination in samples grown under Ga-rich conditions.

Near Bandedge Cathodoluminescence Studies of AlN Films: Dependence on MBE Growth Conditions

Aluminum nitride films were grown by molecular beam epitaxy on sapphire (0001) substrates. In some samples, alternating GaN/AlN thin layers were incorporated prior to the deposition of the main AlN layer in an effort to possibly reduce defects. The cathodoluminescence spectra show a sharp bandedge peak, which we tentatively assign to the optical recombination of a donor bound exciton (D 0 ,X), sometimes accompanied by weak one- and two-longitudinal optical phonon replicas. There is also a broad band with maxima in the range 3200-3700 A. A large value for the ratio (D 0 ,X)/(broad band) tends to be associated with a narrow (D 0 ,X) linewidth.

Polarity of GaN Grown on Sapphire by Molecular Beam Epitaxy with Different Buffer Layers

We report on polarity of GaN films grown on sapphire substrates by molecular beam epitaxy using different buffer layers and growth conditions. On high temperature AlN or GaN buffer layers, the GaN films typically show Ga or N-polarity, respectively. When low temperature (either AlN or GaN) buffer layers are employed, GaN films of both polarities can be grown, but these films have high density of inversion domains. Insertion of additional GaN/AlN quantum dot layers between the buffer layers and the GaN films provides strain relief and a significant improvement in the quality of the GaN epilayers.

A Comparative Study of MBE‐Grown GaN Films Having Predominantly Ga‐ or N‐Polarity

Wurtzitic GaN epilayers having both Ga and N-polarity were grown by reactive molecular beam epitaxy (MBE) using a plasma-activated nitrogen source on c-plane sapphire. The polarities were verified by convergent beam electron diffraction (CBED). High-resolution X-ray diffraction, atomic force microscopy (AFM) imaging and transmission electron microscopy (TEM) were employed to characterize the structural defects present in the films. The different topographic features of Ga and N-polarity samples and their appearance after wet etching were correlated to the measured X-ray rocking curve peak widths for both symmetric [0002] and asymmetric [1014] diffraction. For Ga-polarity samples, the [0002] diffraction is narrower than the [1014] diffraction, while for N-polarity ones the [0002] peaks are broader than [1014]. The half width of [1014] peaks for both polarity types were in the range of 5-7 arcmin indicative of, among possibly other defects, a high density of pure edge threading dislocations lying parallel to the c-axis. The 1-2 arcmin [0002] linewidths of Ga-polarity samples suggest a low density of screw dislocations, which corresponds with the TEM observations where the screw dislocation density is less than 10 7 cm -2 . In N-polarity samples, however, the [0002] diffraction peak was typically wider than 5 arcmin, suggesting either a higher density of edge dislocations and inversion domains in N-polarity samples, or the columnar structural features in AFM images, where the effective coherence length for X-ray diffraction is drastically reduced.