ZnO thin film templates for GaN-based devices

GaN-based optoelectronic devices are plagued by a tendency to non-radiative transitions linked to defects in the active layers. This problem has its origin in (1) intrinsic factors such as GaN's relatively low exciton binding energy (~24meV) and (2) extrinsic factors including the poor availability of native substrates good enough to significantly suppress the defect density. Indeed, the quality and availability of large-area bulk GaN substrates is currently considered a key problem for the continuing development of improved GaN-based devices. Since development of bulk GaN substrates of suitable quality has proven very difficult, a considerable amount of effort is also being directed towards the development of alternative substrates which offer advantages compared to those in widespread use (c-sapphire and 6H SiC). ZnO is promising as a substrate material for GaN because it has the same wurtzite structure and a relatively small lattice mismatch (~1.8%). In this paper, we discuss use of ZnO thin films as templates for GaN based LED.