Bikowski, A.; Ellmer, K.: Electrical transport in hydrogen-aluminium Co-doped ZnO and Zn1-xMgxO films: Relation to film structure and composition. Journal of Applied Physics 113 (2013), p. 053710/1-6
10.1063/1.4790314

Abstract:
ZnO:Al and Zn1-xMgxO:Al films have been deposited in Ar/H2 atmospheres by magnetron sputtering from oxidic targets at two substrate temperatures: room temperature and 300 °C. The electrical transport parameters—carrier concentration, resistivity, and Hall mobility—have been measured and related to the structural properties and the chemical composition. The resistivity q both of ZnO:Al as well as Zn1-xMgxO:Al films decreases with increasing hydrogen flow for the films deposited at room temperature. The decrease is up to 2 orders of magnitude and in both cases due to an increase of the electron concentration and the Hall mobility and occurred despite the fact, that the films became almost X-ray amorphous with increasing hydrogen content. In contrast to these results, for depositions at 300 °C, the resistivity increases with increasing hydrogen portion in the sputtering atmosphere, more strongly for the Zn1-xMgxO:Al films (3 orders of magnitude). Based on literature data, it is concluded that the built-in atomic hydrogen acts as a shallow donor at low deposition temperatures, while it becomes deactivated at Tsub = 300 °C by the formation of molecular H2.