Higher lying conduction band in GaN and AlN probed by photoluminescence spectroscopy

Photoluminescence spectroscopy has been employed to study the band edge emissions in GaN and AlN epilayers up to 800 K. Two distinctive activation processes have been observed in both GaN and AlN. The first process occurring below T _t=325 K (T _t=500 K) for GaN (AlN) is due to the activation of free excitons to free carriers, whereas the second occurring above T _t with an activation energy of 0.29 eV (0.3 eV) for GaN (AlN) is believed to be associated with a higher lying conduction band (Γ ₃) at about 0.3 eV above the conduction band minimum (Γ ₁). An emission line at about 0.29 eV above the dominant transition in GaN was also observed at 700 K, corroborating the assignment of Γ ₃. The values of T _t are a direct measure of the onset temperature at which free excitons dissociate into free carriers.