Analysis of contacts and V-defects in GaN device structures by transmission electron microscopy

This paper describes the microstructure of ohmic contacts to an AlGaN/GaN heterostructure, of interest for high power transistors, and an analysis of V-defects in an InGaN/GaN multi-quantum well (MQW) light-emitting structure. A combination of different transmission electron microscopy (TEM) techniques has been employed, as they provide complementary information. These include bright field and dark field TEM, high-resolution electron microscopy, X-ray mapping, energy filtered TEM and high angle annular dark field. A full determination of the phase distribution in the ohmic contacts was achieved. The onset of low contact resistance was found to correspond with the formation of an interfacial layer containing both TiN and AlN, and of an intermetallic layer containing Al, Ti and Au in contact with it. The MQW structures were capped with a p-type GaN layer, and TEM and ADF studies of the samples show a number of V-defects 100-200 nm apart along the MQW. Each V-defect incorporates a pure edge (b = 1/3 <11-20>) dislocation, which runs through its apex up to the free surface. The defects contain GaN with no InGaN layers, suggesting the V-pits have been filled in by the capping layer.