Cross-sectional Cathodoluminescence Study in Ga-polar and N-polar GaN Epilayers

Abstract

ABSTRACTHighly spatial resolved cross-sectional cathodoluminescence (CL) has been used to study the difference in the defect-formation and growth-mechanism between Ga-polar and N-polar GaN epitaxial layers. These epilayers were grown on sapphire substrates by low pressure MOCVD. Their polarities were controlled by the sapphire nitridation and the trimethylaluminum (TMAl) pre-flow just before the conventional two-step growth, and were confirmed with the coaxial impact collision ion scattering spectroscopy (CAICISS). For the Ga-polar GaN epilayers, cross-sectional CL images show distinct two layers, the upper layer with dark lines along c-direction and the layer close to the film-substrate interface with pyramidal dark regions. These two layers correspond to quasi 2D and 3D growth mode respectively. Whereas, the crosssectional CL image taken on the N-polar epilayers is predominated with small dark spots which were randomly distributed throughout the whole GaN epilayer, illustrating the predominant island growth mode in N-polar films.