Temperature dependence of the photoreflectance lineshape for GaN films grown by molecular beam epitaxy

The photoreflectance (PR) lineshape of GaN films grown by molecular beam ep itaxy was investigated in the temperature range of 10 to 400 K. At low temp eratures, the lineshape is dominated by excitonic transitions and can be re asonably well fitted using the first derivative of a Gaussian-broadened osc illator. At higher temperatures, the lineshape is more Franz-Keldysh like, but still influenced by excitonic effects. Typical Franz-Keldysh oscillatio ns appear on the high-energy side of the PR spectra, from which the surface electric field can be estimated. At room temperature, we find a remarkable sensitivity of the PR lineshape to the ambient medium. A small change in s urface electric field and a large decrease of the PR amplitude are observed , when the ambient medium is changed from air to vacuum. We address this ef fect to an UV-light induced desorption of oxygen from the sample surface.