Postdeposition reduction of noble metal doped ZnO films

Insulating ZnO (wurtzite phase) films containing 1% to 2% Pt, Au, Pd, Ga, or Al dopants have been deposited onto silica, silicon, and aluminum substrates under oxidizing conditions. Substantial enhancement in film conductivity is promoted by postdeposition reduction in hydrogen above a critical temperature or by room temperature cathodic reduction in an electrochemical cell. Film reduction requires the presence of atomic hydrogen, formed at the film surface either by dissociative adsorption of gaseous Hz or its electrochemical generation from a buffered aqueous solution. As deposited and reduced films have been characterized using x-ray photoemission spectroscopy, Raman spectroscopy, optical transmission measurements, spectroscopic ellipsometry, voltammetry, chronopotentiometry, and four-point conductivity measurements. Results indicate that film deposition parameters and postdeposition reduction alter the oxidation states of both the zinc and the resident dopant cations. The zinc reduction reaction appears to be quasireversible while the reduced noble metal dopants in the films are not prone to reoxidation. These results may have implications for enhancing stability of the optical response and electrical conductivity exhibited by these films. (C) 1998 American Vacuum Society.