Precessed electron diffraction study of defects and strain in GaN nanowires fabricated by top-down etching

Abstract

Gallium nitride (GaN) nanowires (NWs) have been fabricated by top-down etching from GaN heteroepitaxial films, which provides an accurate control of their position and dimensions. However, these NWs contain, similar to the initial GaN films, high density of structural defects such as threading dislocations (TDs). In this work, different strategies to reduce the density of defects along the NWs have been compared based on two different wet etching approaches followed by a rapid thermal annealing (RTA) at 750 °C. The addition of a 30 nm SiNx coating is also explored. The defects and strain/stress along the NWs have been studied by high resolution transmission electron microscopy, diffraction contrast imaging in two-beam conditions and 4D STEM, as well as strain maps calculated from scanning precession electron diffraction measurements. RTA reduced the density of TDs at the middle of GaN NWs with bare surfaces by approximately 25%. The reduction increased to approximately 70% by RTA of GaN NWs with surfaces coated by amorphous SiNx, which is attributed to enhancement of dislocation movements by stresses induced from differential thermal expansion of GaN and SiNx. These results suggest a process route that, if optimized and combined with reduction of NW diameter, could establish etching as an efficient fabrication method for high crystal quality GaN NWs.