Analysis of the carbon-related “blue” luminescence in GaN

Abstract

The properties of a broad 2.86-eV photoluminescence band in carbon-doped GaN were studied as a function of C-doping level, temperature, and excitation density. For GaN:C grown by molecular-beam epitaxy (MBE) the 2.86-eV band is observed in Si codoped layers exhibiting high n-type conductivity as well as in semi-insulating material. The peak position of the “blue” luminescence is constant with temperature in MBE GaN, but in semi-insulating GaN:C grown by metal-organic vapor-phase epitaxy it shifts from 3.0to2.86eV with increasing temperature in the range of 12–150K. The 2.86-eV band undergoes thermal quenching from 200to400K with an activation energy of ∼150meV. The characteristics of the 2.86-eV band are consistent with deep donor-deep acceptor recombination originating from carbon defects, under the assumption that the concentrations of these defects are low compared to the total carbon concentration in heavily C-doped samples. For low excitation density (4W∕cm2) the 2.86-eV band intensity decreases as a function of HeCd laser exposure time over a period of many minutes. However, no transient effects are observed for 20W∕cm2 excitation density. The transient behavior can be best explained using a model based on charge-trapping-induced Coulomb barriers which impede the diffusion of carriers to the 2.86-eV luminescence centers.