Unpinned behavior of the surface Fermi level of GaN detected by photoreflectance spectroscopy

The photoreflectance signal from GaN films is sensitive to the ambient medi um. A large decrease in the photoreflectance amplitude is observed, when th e ambient medium is changed from air to vacuum. This effect is attributed t o ultraviolet-light-induced desorption of oxygen from the sample surface le ading to a reduction of the surface barrier height. The effect is absent, w hen a thin Ti layer is deposited on top of the GaN film. A simple model is used to demonstrate that the surface photovoltage can be strongly reduced w ith a decrease of the surface barrier height. (C) 2000 American Institute o f Physics. [S0021-8979(00)05009-X].