Impurity incorporation and diffusion from regrowth interfaces in N-polar GaN photocathodes and the impact on quantum efficiency

Abstract

We investigate the mechanisms of growth interruption mediated interfacial impurity diffusion in N-polar GaN, and through this understanding, we offer an approach toward achieving high quantum efficiency III-nitride photocathode devices. N-polar photocathode structures consisting of a p-GaN active layer and a thin unintentionally doped GaN (u-GaN) cap layer were grown on an N-polar u-GaN template with varied growth interruptions at interfaces. Unintentionally incorporated oxygen and silicon impurity spikes at regrowth interfaces were measured by secondary ion mass spectrometry. Pipe and bulk diffusion of the oxygen impurity is observed from the regrowth interfaces when the overgrown layer required higher temperature growth, whereas only bulk diffusion is seen otherwise. Furthermore, the proximity of regrowth interfaces to the surface is observed to impact the diffusivity of oxygen. Growth interruption between the p-GaN active layer and cap layer resulted in a low quantum efficiency of 0.27%, while uninterrupted growth of the p-GaN/u-GaN cap photocathode achieved a quantum efficiency of 10.79%. We attribute the low quantum efficiency of the interrupted cap photocathode to the high density of oxygen within the active region of the device. Understanding of impurity incorporation at regrowth interfaces, dominating driving mechanisms behind diffusion of these species, and their impact on material properties are critical elements in designing high performing devices.