Hydrogen induced electric conduction in undoped ZnO and Ga-doped ZnO thin films: Creating native donors via reduction, hydrogen donors, and reactivating extrinsic donors

The manner in which hydrogen atoms contribute to the electric conduction of undoped ZnO and Ga-doped ZnO (GZO) films was investigated. Hydrogen atoms were permeated into these films through annealing in an atmospheric H2 ambient. Because the creation of hydrogen donors competes with the thermal annihilation of native donors at elevated temperatures, improvements to electric conduction from the initial state can be observed when insulating ZnO films are used as samples. While the resistivity of conductive ZnO films increases when annealing them in a vacuum, the degree of increase is mitigated when they are annealed in H2. Hydrogenation of ZnO crystals was evidenced by the appearance of OH absorption signals around a wavelength of 2700 nm in the optical transmittance spectra. The lowest resistivity that was achieved by H2 annealing was limited to 1–2 × 10−2 Ω cm, which is one order of magnitude higher than that by native donors (2–3 × 10−3 Ω cm). Hence, all native donors are converted to hydrogen donors. In contrast, GZO films that have resistivities yet to be improved become more conductive after annealing in H2 ambient, which is in the opposite direction of GZO films that become more resistive after vacuum annealing. Hydrogen atoms incorporated into GZO crystals should assist in reactivating Ga3+ donors