X-Ray Diffraction Analysis of GaN and AlGaN
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X-Ray Diffraction Analysis of GaN and AlGaN H. Kang, N. Spencer, D. Nicol, Z.C. Feng, and I. Ferguson* School of Electrical and Computer Engineering, Georgia Institute of Technology, Atlanta, GA 30332, U.S.A * [email protected] S. P. Guo, M. Pophristic, and B. Peres EMCORE Corporation, 145 Belmont Drive, Somerest, NJ 08873, U.S.A ABSTRACT In this paper, threading dislocation densities in GaN and AlGaN epitaxial layers have been evaluated using two different X-ray analysis techniques; a Williamson Hall (WH) plot and reciprocal space mapping (RSM). GaN and AlGaN have crystalline growth composed of columnar structures that can be estimated by coherence length and angular misorientation measured by X-ray. A WH plot can provide information about coherence length and tilt angle from a linear fit to the linewidth of the triple axis rocking curve (000l) symmetric reflections. RSM is typically used to obtain this data, but it is more involved in technique. The two dominant components of threading dislocation densities (screw and edge types) in the GaN and AlGaN epitaxial layers were found to be similar by both techniques. The treading dislocation density correlates to the size of columnar structure as determined by coherence length, tilt angle, and twist angle. The effect of Al composition in AlGaN alloys on these dislocation densities was investigated and found to depend on strongly on the type of nucleation layer, GaN or AlN. INTRODUCTION GaN-based materials have attracted much attention for optoelectronic device applications where there is a need to operate in the blue-green regime.[1] However, the high threading dislocation density in the III-Nitrides is an issue that encumbers their further development as optoelectronic devices in the ultraviolet.[2] The threading dislocation (TD) density is normally determined using plan view transmission electron microscopy (TEM), however this technique is destructive and typically requires a few days before the this data can be obtained.[3] X-Ray Diffraction (XRD) has been used to obtain functionally equivalent data from some average measurements of crystal microstructure. Since XRD is non-destructive and rapid characterization technique it is the most common technique used to optimize crystalline quality growth parameters. GaN is different to other compound semiconductor materials because it is formed with large areas of crystalline material that are misaligned to each other, a columnar structure (i.e. mosaic structure).[4] Thus an estimate of size of those domains, the average misorientation between them and dislocation density is important in optimizing material growth. Highresolution XRD reciprocal space mapping (RSM) is normally utilized for detailed characterization of such crystalline structures. However, a much quicker and simpler method to obtain similar information utilizes the Williamson-Hall (WH) plot. In this study of GaN and AlGaN, RSM and WH plot techniques were used to characterize the columnar structures and to evaluate the threading dislocation densities
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