Microstructure of heteroepitaxial GaN grown on mesa-patterned 4H-SiC substrates

Cross-sectional transmission electron microscopy and atomic force microscopy have been used to study the microstructure of a thin heteroeptiaxial GaN film grown on (0001) 4H-SiC mesa surfaces with and without atomic scale steps. Analysis of a mesa that was completely free of atomic-scale surface steps prior to III–N film deposition showed that these GaN layers had a wide variation in island height (1–3μm) and included the presence of pit-like defects on the film surface. This sample had a low dislocation density (5×108∕cm2) as compared to conventionally grown samples on unpatterned (0001) on-axis 4H-SiC (2×109∕cm2), coupled with a 3–5 times increase in grain size. A comparison of a GaN film on the step-free 4H-SiC mesa region with a GaN film on a stepped 4H-SiC mesa region on the same substrate showed that the presence of surface steps reduced the overall grain size of the film from 7–10μm to a grain size of about 2–3μm. Since the GaN films grow via a Volmer–Weber mechanism, a decrease in the number of heterogeneous nucleation sites may allow the growth of large GaN islands before coalescence, thus reducing the number of threading dislocations. These results are promising for the further development of unique, low-dislocation density active regions for GaN device structures on 4H-SiC.