The role of oxygen diffusion in photoinduced changes of the electronic and optical properties in amorphous indium oxide

A stable increase by as much as 10(8) in the conductivity of amorphous indium oxide to sigma greater than or equal to 10(3) Omega(-1) cm(-1) can be achieved by ultraviolet photoreduction. This treatment also increases the absorption coefficient, alpha(h upsilon), by up to a factor of 10(3) for h upsilon < 1.5 eV due to free carrier absorption and causes a 0.1 eV shift of the absorption edge to the blue. These changes are controlled by the Fermi level, E(F), which is presumably determined by doping due to oxygen vacancies. A diffusion constant D > 3 x 10(-12) cm(2)/s for oxygen at 300 K is determined from a constant flow experiment. Oxygen diffusion is verified by secondary ion mass spectrometry with O-18. The functions alpha(h upsilon) and sigma(T) are simulated as E(F) is varied using a simple density of states model appropriate for amorphous semiconductors. These simulations qualitatively agree with the experimental data if transitions from the conduction band tail to the conduction band are assumed to be forbidden.