Stimulated emission threshold in thick GaN epilayers: interplay between charge carrier and photon dynamics

•SE threshold increases with layer thickness despite reduction of defect density.•Photon dynamics is the main factor controlling SE threshold.•Carrier lifetime weakly affects SE threshold.•Layer thickness is a compromise between higher overlap and lower optical losses. Room-temperature luminescence properties of a dense electron-hole plasma were studied in a set of GaN epilayers with thickness varying from 2 to 25 µm. The stimulated emission threshold was measured by photoluminescence and light-induced transient grating techniques under short-pulse excitation. Both techniques revealed the stimulated emission threshold increase with layer thickness despite the reduction of defect density. Numerical modeling of photoexcited carrier dynamics showed the different roles of carrier and photon populations. The stimulated emission threshold is mainly determined by the photon dynamics with weak influence of nonradiative carrier recombination. Increasing layer thickness results in the decreasing overlap of the gain layer with the optical mode, which reduces the transfer of energy from carriers to photons.