Helical-type surface defects in GaN thin films epitaxially grown on GaN templates at reduced temperatures
Surface pits in the form of v-shaped defects and resulting surface roughening, previously associated solely with InGaN films, were observed and investigated using atomic force microscopy on GaN films grown at 780degreesC via metalorganic vapor phase epitaxy on conventionally and pendeo-epitaxially deposited GaN thin film templates. The density of the v-shaped defects was similar to the density of threading dislocations of similar to 3 x 10(9) cm(-2) (that originate from the heteroepitaxial interface between the GaN template layer and the SiC substrate). Moreover, the V-defect density was diminished with decreases in the dislocation density via increases in the template layer thickness or the use of pendeo-epitaxial seed layers. A concomitant reduction in the full-width half-maxima of the X-ray rocking curves was also observed. A qualitative model is presented that describes the formation of v-shaped defects as a result of interactions between the movement of surface steps, screw-type dislocation cores, and clusters of atoms on the terraces that form under conditions of high surface undercooling. (C) 2003 Elsevier Science B.V. All rights reserved.