Perturbation of charges in AlGaN∕GaN heterostructures by ultraviolet laser illumination

Abstract

Perturbation of charges in AlGaN∕GaN heterostructures by ultraviolet (UV) laser illumination has been studied by Kelvin probe microscopy. It is observed that the charge dipole formed across the AlGaN barrier decreases under UV laser illumination, and typically takes a few days to a few weeks to revert back to the original equilibrium value. Bare surface barrier height of AlGaN∕GaN heterostructures has been calculated based on the observed recovery transients after the UV illumination is switched off, and found to increase with Al alloy composition as well as thickness of the AlGaN layer. After SiNx passivation, the surface barrier is reduced, and the charges cannot be perturbed to a significant extent. It is further observed that by UV illumination through a quartz mask, surface barrier patterns can be imprinted on AlGaN∕GaN heterostructures, which stay for several hours to several days. It is proposed that the imprinted patterns, with a spatial resolution ∼1–2μm, are caused by surface trapping of photogenerated holes.