Exciton dynamics of thick GaN epilayers deposited by MOVPE on Al2O3

Investigating the time-resolved photoluminescence spectra of three GaN layers with respective thickness 2.5 mu m, 7 mu m and 16 mu m, we have found that the recombination dynamics of free excitons depends upon the layer thickness. For the two thinner layers only single exponential decays are found, with tau similar to 35 - 80 ps. For the thickest layer, the decay becomes more biexponential with tau(1) similar to 80 ps and tau(2) similar to 250 ps. These orders of magnitude preserve up to room temperature. By solving the coupled rate equations of a four-level model, we show that this evolution is consistent with a reduction, versus layer thickness, of the density of shallow impurities and deep traps. This reduction follows roughly a L-1 dependence, where L is the layer thickness.