Removal of 6H-SiC substrate influence when evaluating GaN thin film properties via x-ray
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Removal of 6H-SiC substrate influence when evaluating GaN thin film properties via x-ray Edward A. Preble, Peter Q. Miraglia, Amy M. Roskowski, Sven Einfeldt, and Robert F. Davis Materials Science and Engineering, North Carolina State University, Raleigh, NC 27695 U.S.A. ABSTRACT Non-uniformity in GaN thin films deposited on 6H-SiC can make determining the effects of growth variables difficult. Results presented in this work show the effects of the SiC substrates on the GaN films, and how to correct for these effects to obtain meaningful data about the properties of the thin film rather than the substrate underneath. Rocking curve values of GaN thin films are found to track almost 1:1 with the values of the underlying SiC. Plotting rocking curves with respect to the substrate, as well as a variable of importance can therefore yield more meaningful and reliable comparisons instead of plotting the data for the variable alone. This procedure is used to demonstrate the effects of thickness and AlN and AlGaN buffer layers on GaN thin films. INTRODUCTION A commonly quoted structural property of thin films is the full width half maximum (FWHM) value of its x-ray rocking curve. Broadening of a rocking curve is related to increasing dislocation content and/or the presence of multiple domains of different orientations. Rocking curve data is usually obtained from different crystallographic directions to identify populations of dislocations with different Burgers vectors. This is particularly useful for GaN films on AlN/6H-SiC(0001) substrates, because the primary dislocations are screw- and edge-type, which can be investigated using on-axis and off-axis measurements, respectively. On-axis rocking curves in III-V nitrides are typically taken from the (00.2), (00.4) or (00.6) reflections and their widths are influenced by the screw dislocation content in the film. Off-axis rocking curves can be taken from numerous peaks, commonly including the (10.3), (10.5) or (20.1) reflections and their widths are influenced by both the screw and edge content in the film. The measuring condition for pure edge exists at 90° off-axis, which cannot be measured directly because of wafer geometry. The primary defects of concern in the III-Nitride films are threading dislocations, which are predominantly edge character and oriented normal to the surface of the film[1]. Off-axis measurements are therefore more useful for determining overall dislocation reduction in the films. Methods have been developed that can also enable one to determine the actual dislocation populations from rocking curve information, as well as crystal tilt and twist, provided that there is understanding of the dislocation populations that exist in the crystal as well as understanding of the domain structure of the material being measured.[2,3] Silicon carbide substrates contain numerous domains that are misaligned to each other with an approximate domain size on the order of a millimeter.[4] In GaN/6H-SiC systems the substrate variability causes sufficiently large changes in
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