X‐ray characterization of high quality AlN epitaxial layers: effect of growth condition on layer structural properties

Abstract

.72. Dd, 68.55.Jk AlN layers were deposited directly on (0001) sapphire by a MOCVD two-step pressure process. The best (0002) and (10 12) rocking curves had FWHM values of ~360 arcsec and ~580 arcsec respectively. A detailed X-ray diffraction study was carried out to investigate the effect of growth condition on the layer structural properties. In-plane twist was estimated from the FWHM values of both ω-scans and φ-scans as functions of inclination angle, using the quasi-symmetrical reflection from a group of lattice planes with inclination angle ranging from 0 to 75°. The range of twist spread was found to be sensitive to the substrate nitridation condition. The lowest spread angle obtained with optimized nitridation was ~770 arcsec. As expected, the edge-type threading dislocation density estimated from twist spreading was found to correlate well with the dislocation density measured by TEM. The tilt spreading was found to be sensitive to the growth pressure. By reducing growth pressure to 40 Torr, we obtained layers with tilt spreading less than 120 arcsec. 1 Introduction There is increasing interest in the growth of high quality AlN because of its high temperature stability, high electrical resistivity, high thermal conductivity, UV transparency, and good lattice match with GaN. These properties make AlN a promising substrate for various high power, high temperature electronic and opto-electronic applications. Indeed, recent reports have shown that InGaN based LEDs [1] and GaN HEMTs [2] grown on AlN/sapphire template have substantially improved device performance. Since AlN substrates are not available, heteroepitaxial AlN films are usually deposited on foreign substrates. These AlN films often contain high densities of structural defects. Recently, we reported the development of a two-step pressure process for AlN growth on sapphire substrates [3]. With an optimum nitridation condition, we showed that the new process was effective for the improvement of crystalline quality of AlN films.To understand the effect of growth conditions on the layer structural properties, we undertook a detailed X-ray diffraction study. AlN films usually exhibit a high degree of mosaicity; i.e. the films consist of sub-grains slightly misoriented with respect to each other and the underlying substrate. Such structure is usually described by a spread of tilt angles with respect to substrate normal and a range of twist angles in the basal plane. This spreading contributes to the broadening of the width of the X-ray diffraction peaks. The methods reported in the literature to characterize such twist, however, vary from group to group: some use rocking curves of a quasi-symmetric or a symmetric reflection [4,5], while others

Effect of initial process conditions on the structural properties of AlN films

“…The FWHM of the (0 0 0 2) reflection was used to estimate the tilt distribution. The ''in-plane'' crystalline quality was evaluated using off-axis ð1 0 % 1 2Þ plane rocking curves measured under quasi-symmetric configuration [6].…”

“…Because of the significant differences in growth rate, AlN epilayers with such ''mixed polarity'' domains exhibit rough surface morphology. We also found that the in-plane crystalline quality of these AlN layers was very responsive to the extent of substrate nitridation [6]. While the process optimization can be very reactor dependent, it is possible to identify some of the growth-related characteristics of MOCVD-grown AlN layers that appear to be controlling the crystal quality.…”

An X-ray diffraction technique for analyzing structural defects including microstrain in nitride materials

“…inclination angle w plane is used to generate a conventional plot, similar to those shown in previous reports [5][6][7]. The solid lines in Fig.…”

“…However, in most of the previous nitride analyses [4][5][6][7], microstrain is neglected. The justification for ignoring microstrain effects is based on the fact that lattice tilt and twist are often the most dominant broadening terms in typical heterogeneous epitaxial nitride materials.…”

“…To analyze structural defects in III-nitrides, several approaches based on rocking curve (o-scan) peak width have been reported for measuring the lattice tilt, twist, and limited coherence length [4][5][6][7][8][9] By measuring diffraction profiles for a series of symmetric and asymmetric crystal planes, the broadening terms can be separated using extrapolation methods [4,5,7] based on their geometric dependence. Using only the full-width at half maxima (FWHMs) of the diffraction profiles, it has been found [4][5][6][7] that for those mosiac films, even the rough estimates of threading dislocation density from these X-ray FWHM approaches correlates very well with TEM results. Therefore, the technique may well support the efforts to improve growth processes and allow assessments of the impact of these defects on device performance.…”