The effect of threading dislocations, Mg doping, and etching on the spectral responsivity in GaN-based ultraviolet detectors

Abstract

Presently, GaN-based photodiodes grown on sapphire substrates exhibit broad spectral cutoffs, poor visible rejection, and low quantum efficiencies when illuminated through a p-type contact/window layer. We, therefore, investigate the effect of threading dislocations, Mg doping, and etching on the photoconductivity spectra in GaN. Highly resistive bulk GaN:Mg nearly free of threading dislocations has more than 1000 times better visible rejection ratio compared to highly dislocated and comparably doped p-GaN thin films grown on sapphire substrates. However, the heavy Mg compensation in both the bulk and the epitaxial film causes a broad spectral cutoff. Unintentionally doped semi-insulating GaN with low dislocation density has a similar rejection ratio compared to bulk GaN:Mg, but much sharper spectral cutoff due to the absence of intentional doping. Furthermore, postgrowth processing steps such as etching and polishing significantly increase the surface recombination compared to the as-grown surface.