Recombination and diffusion processes in polar and nonpolar bulk GaN investigated by time-resolved photoluminescence and nonlinear optical techniques

Abstract

Optically-injected carrier dynamics were investigated in bulk polar and nonpolar GaN in 10¹⁵-to-10²⁰ cm^-3 carrier density range, exploring single- and two-photon photoexcitation conditions. The excitation decay and recombination rates were monitored by time-resolved photoluminescence and free-carrier absorption techniques, while diffusivity was investigated by light-diffraction on transient grating technique. Carrier dynamics in c- and m-plane thick freestanding HVPE GaN revealed nearly linear increase of carrier lifetime with temperature in the 80 - 800 K range whereas the bipolar carrier diffusivity decreased with temperature. This feature suggests that the measured long lifetime values of 40-50 ns at RT result from diffusion-governed carrier flow to interface defects at GaN hexagons, which act as centers of nonradiative recombination. The fast PL transients under carrier injection to submicrometer thick layer were fitted by using the determined diffusivity and lifetime values and revealed a strong impact of vertical carrier diffusion, surface recombination, and reabsorption processes. Radiative and nonradiative emission rates were analyzed by various optical techniques to discriminate contribution of excitons and free carriers at various temperatures and injected carrier densities.