Dislocations and their reduction in GaN

Abstract

Gallium nitride (GaN) is a semiconductor used to make light emitting diodes, a technology that could decrease global energy demands significantly if used worldwide. Yet there are barriers to making high efficiency GaN based devices: defects, including threading dislocations (TDs), hamper the quality of the GaN crystalline film. The hypotheses proposed to explain the origin of TDs are critically reviewed. It has been suggested that TDs form upon GaN island coalescence during initial stages of crystalline film growth, yet some transmission electron microscopy and atomic force microscopy studies have shown few TDs at coalescence boundaries. Although harmful, TDs have a lesser effect on nitride based devices than on other compound semiconductors. Thus, GaN based devices are able to produce light despite high dislocation densities. This phenomenon has led to debate over the role of TDs in charge carrier recombination, which is reviewed. Some suggest that charge carriers arrive at TDs and recombine in a non-radiative manner, whereas others claim that they are repelled from the dislocations because the dislocation cores are electrically charged. The reduction of TDs in GaN films furthers the drive towards high efficiency devices. The final sections of this review address ways to effect reductions in TD density. Methods include changing growth conditions (including temperature and pressure), dosing the substrate with silane, and the exploitation of interlayers deposited during growth.