Persistent conductivity in post-growth doped ZnO films following pulsed UV laser irradiation

Solution and rf sputter deposited doped ZnO films were subjected to cumulative 4-ns pulses of 355 nm light from a pulsed Nd:YAG laser at fluences between 5 and 150 mJ/cm2. Film densification, change in refractive index, and an increase in conductivity were observed following room temperature irradiation in air, a carbon monoxide reducing environment, or under vacuum. At fluences between 20 and 80 mJ/cm2, the films did not damage catastrophically under irradiation and high visible transparency persisted. The increase in conductivity is attributed to creation of oxygen vacancies and subsequent promotion of free carriers into the conduction band. Effects were most pronounced for films treated in vacuum. All treated films became insulating again upon equilibration in air at room temperature after several days. Films were characterized by means of UV–VIS–NIR transmission spectroscopy, Raman spectroscopy, and Hall measurements. Analysis of interference fringes in measured transmission spectra allowed evaluation of optical properties. Raman measurements showed an increase of LO mode intensity with respect to TO mode intensity as the films became more conducting in accord with previous work. Results of this study are not only important for continued development of transparent conducting oxides, but also provide compelling evidence for the role of free carriers as initiators of laser damage in wide bandgap metal oxide films.