Peculiarities of photoluminescence efficiency dependence on excitation intensity in GaN/Al₂O₃ epilayers

Excitation-intensity dependent photoluminescence (PL) as well as effective recombination time of a series of c-plane GaN layers grown on sapphire substrates were measured at room temperature. It was shown that at low excitation level a high background free-electron concentration causes an increase of radiative recombination rate in the near band edge region of GaN that results in an increase of the PL efficiency. In the investigated series, the GaN layers with lower crystal quality but having higher free-electron concentration showed higher PL efficiency at excitation levels under ∼50 kW/cm². At elevated excitation levels the impact of free-carrier concentration decreases and the PL efficiency is mainly determined by nonradiative recombination time. From simulations of excitation-intensity dependent PL efficiency, the Auger recombination coefficient in the investigated GaN samples was estimated to be much less then 10^-30 cm⁶s^-1. A drop of PL efficiency was observed at excitation levels higher than 2500 kW/cm² when PL was registered from sample's surface. The reason of the drop was stimulated emission propagating along the in-plane direction and causing the saturation of photogenerated carrier density. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)