Room temperature photoreflectance of different electron concentration GaN epitaxial layers

Wurtzite-structure GaN epilayers grown by metal-organic vapor phase epitaxy (MOVPE) on sapphire substrates were studied by photoreflectance (PR) spectroscopy performed at room temperature. Several nominally undoped samples with different electron concentrations (5x1015-5x1018 cm-3), deposited under different growth conditions, were investigated. For low electron concentration we have observed three well-resolved excitonic transitions related to A, B, and C excitons. In this case, extremely small broadening of PR lines (few milielectronvolts) has been found. With the increase of carrier concentration in epilayers, we have observed an increase of broadening of the transitions. This has led to the disappearance of excitons in PR spectra and to the observations of one broad non-excitonic feature for highest electron concentration. Additionally, in some cases the Franz-Keldysh oscillations (FKO) have been also observed reflecting the existence of a surface built-in electric field in the epilayers, which do not destroy the excitons