Abstract
Photoluminescence spectroscopy has been employed to study the band edge emissions in GaN and AlN epilayers up to 800K. Two distinctive activation processes have been observed in both GaN and AlN. The first process occurring below Tt=325K (Tt=500K) for GaN (AlN) is due to the activation of free excitons to free carriers, whereas the second occurring above Tt with an activation energy of 0.29eV (0.3eV) for GaN (AlN) is believed to be associated with a higher lying conduction band (Γ3) at about 0.3eV above the conduction band minimum (Γ1). An emission line at about 0.29eV above the dominant transition in GaN was also observed at 700K, corroborating the assignment of Γ3. The values of Tt are a direct measure of the onset temperature at which free excitons dissociate into free carriers.
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