C‐plane bowing in free standing GaN crystals grown by HVPE on GaN‐sapphire substrates with photolithographically patterned Ti masks

Abstract

AbstractIt is suggested that final bowing of {0001} planes in free standing GaN depends mostly on the nucleation mechanism at the beginning of the growth in the presented approach. Typical growth process in this study, includes formation of pyramidal islands through the openings in the mask, growth of the islands on their semi‐polar faces, the coalescence, smoothening of the layer and finally, bulk growth on the c‐plane of the crystal. The initial layer containing large fraction of material grown on semi‐polar faces is strongly n‐type (n > 1019 cm‐3) due to enhanced incorporation of oxygen. In contrast, the growth on c‐plane, after smoothening of the layer, results in much lower electron concentration (n < 1017 cm‐3) thus a corresponding lattice mismatch appears between semi‐polar and c‐plane growth regions. There are three results suggesting that this mismatch is an important source of stress leading to the observed c‐plane bowing: (i) relatively strong dependence of bowing radius on diameter and distance of openings in the mask, (ii) dramatic increase of the bowing radius (R = 10–80 m) if the crystallization front remains rough (entire crystal is strongly n‐type), (iii) agreement of the observed bowing radii (0.9–3 m) with theoretical estimations (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)