Effects of deposition temperature on the effectiveness of hydrogen doping in Ga-doped ZnO thin films

Gallium-doped zinc oxide thin films were prepared on glass substrates by dc magnetron sputtering under various hydrogen contents in sputtering ambient. The carrier concentration of the films deposited at low-temperatures (80 and 160 deg. C) was increased due to the incorporation of hydrogen atoms, acting as shallow donors. A low resistivity of 4.0x10-4 Ω cm was obtained for the film grown at 160 deg. C with H2 10%, which has a carrier concentration of 8.2x1020/cm3. The beneficial effect of hydrogen doping was not observed for the films deposited at 270 deg. C. Both carrier concentration and mobility were decreased by the addition of hydrogen gas in the sputtering ambient. Variations in the electrical transport properties upon vacuum annealing showed that the difference is attributed to the thermal stability of interstitial hydrogen atoms in the films. The hydrogen incorporation was found to induce the lattice expansion and the free carrier absorption in near infrared range. The investigation of the structural and optical properties of the films upon annealing also revealed that the incorporated hydrogen atoms are unstable at high temperature, which is consistent with the results obtained in the electrical properties.