Thermal instability of electrically active centers in heavily Ga-doped ZnO thin films:: X-ray absorption study of the Ga-site configuration

We report a study of the site configuration of Ga atoms in Ga-doped ZnO thin films by means of x-ray absorption spectroscopy (XAS). Ga-related donors remain electrically active after vacuum annealing at 800 degrees C. On the opposite, annealing the films in air (400 degrees C) leads to a dramatic decrease of the conductivity by four orders of magnitude and disappearance of the Burstein-Moss shift. XAS spectra indicate that air annealing induces partial segregation of Ga atoms to nanocrystallites of the spinel ZnGa(2)O(4) or other intermediate phase. The short Ga-O bondlength measured can be at the origin of the here reported instability. (C) 2007 American Institute of Physics.