Defect structure in GaN pyramids

Abstract

High-quality GaN/AlN layers grown on (111) Si substrates have been used as the seeding layer for lateral epitactic overgrowth of GaN. The selective overgrowth was controlled by depositing a Si₃N₄ mask on the GaN seed layer. Growth of additional GaN resulted in the formation of GaN pyramids above the apertures in the patterned Si₃N₄ mask. Transmission electron microscopy showed that the GaN pyramids, the GaN seed layer, and the AlN buffer layer in the samples have the following epitactic relationship with respect to the silicon substrate: $[11\bar 20]_{{\rm GaN}} ||[11\bar 20]_{{\rm AIN}} ||[\bar 110]_{{\rm Si}}$ and $(0001)_{{\rm GaN}} ||(0001)_{{\rm AIN}} ||(111)_{{\rm Si}}$. The pyramids were found to consist of a defective core region and a nearly defect-free outer region. In the core of the pyramid (at, or above, the aperture in the mask), numerous dislocations thread through the pyramid perpendicular to the interface plane. Some of these threading dislocations, which originated from the GaN/AlN seed layer, bend abruptly through 90° at the edge of this core region. In the outer part of the GaN pyramid, the density of vertically propagating dislocations was much lower. Most of the dislocations in this region are closely parallel to the original (0001) substrate plane. The top few microns of material are found to be essentially defect-free. The growth mechanism of the GaN pyramids is discussed in light of this defect structure.