Understanding the Pyramidal Growth of GaN

Abstract

ABSTRACTBy a combination of conventional, HREM and CBED TEM experiments we have studied wurtzite GaN layers grown by Metal-Organic Chemical Vapour Deposition (MOCVD) on (0001)Al2O3. We experimentally determine the structure of the macroscopic hexagonal pyramids that are visible at the surface of the layers when no optimised buffer is introduced. These pyramids look like hexagonal volcanoes with one hexagonal microscopic chimney (up to 75nm wide) at their core. The crystal inside the chimney is a pure GaN crystal with a polarity opposed to the one of the neighbouring material : the GaN layers grown on (0001)Al2O3 are everywhere Ga-terminated except in the chimneys where they are N-terminated. Some of the N-terminated chimneys grow faster and form macroscopic hexagonal pyramids. Chimneys bounded by Inversion Domains Boundaries (IDBs) originate from steps at the surface of the substrate and may be suppressed by an adapted buffer layer.