Temperature dependence of absorption edge in MOCVD grown GaN

We have studied the temperature dependence of absorption edge of GaN thin films grown on sapphire substrate by metal-organic chemical vapor deposition using optical absorption spectroscopy. A shift in absorption edge of about 55 meV has been observed in temperature range 273–343 K. We have proposed a theoretical model to find the energy gap from absorption coefficient using α = α_max + (α_min − α_max)/[1 + exp 2(E − E_g + KT)/KT]. Temperature dependence of band gap has also been studied by finding an appropriate theoretical fit to our data using E_g(T) = E_g(273 K) − (8.8 × 10⁻⁴T²)/(483 + T) + 0.088 (Varshni empirical formula) and E_g(T) = E_g(273 K)−0.231447/[exp(362/T)−1] + 0.082 relations. It has been found that data can be fitted accurately after adding a factor ∼0.08 in above equations. Debye temperature (483 K) and Einstein temperature (362 K) in the respective equations are found mutually in good agreement.