Temperature-dependent absorption measurements of excitons in GaN epilayers

Optical absorption measurements were performed on a series of thin GaN epilayers. Sharp spectral features were observed due to the $\text{1s}$ $A$ and $B$ exciton transitions. Using polarization dependent absorption, the $C$ exciton transition was identified. A broad absorption feature was observed at ∼3.6 eV, which is attributed to indirect exciton-phonon absorption. The excitonic structure was found to persist well above room temperature. A fit to the Varshni formula yielded a temperature dependence of $\text{E(T)=E(T=0)}$ −${\text{11.8×10}}^{\text{−4}}{T}^2\text{(1414+T)}$ eV for the $A$ and $B$ excitons. The exciton absorption linewidth was studied as a function of temperature, indicating that GaN exhibits very large exciton-phonon coupling.