The low-electrical resistivity and high-optical transmittance in the visible range of Sn-doped In 2 O 3 (ITO) films means that they are widely used for transparent conducting oxide (TCO) electrodes in solar cells, liquid crystal displays, light emitting diodes, and optoelectronic devices . However, the indium required for the production of ITO films is rare, expensive, and toxic. Recently, impurity-doped zinc oxide (ZnO) films have received a lot of attention as a possible remedy for the deficiencies of ITO films. They are low in cost, abundant in nature, non-toxic, and more thermally and chemically stable . Another reason that impuritydoped ZnO films are emerging as a potential alternative for ITO films is due to their optical and electrical properties. Impuritydoped ZnO films have high-optical transmittance in the visible range, and the energy band gap can be controlled by the doping level. The electrical resistivity of impurity-doped ZnO films can be further lowered by the addition of impurity dopants such as Al, Ga, In, Ti, and so on .
ZnO films can be deposited by several methods [16] such as chemical vapor deposition, spray pyrolysis, and DC/RF magnetron sputtering. Many investigators have used the RF magnetron sputtering method. These films are typically prepared in ambient Ar or Ar + O 2 plasma. Although the addition of various dopants to ZnO films (as a way to decrease the electrical resistivity) has been extensively studied, investigators have encountered a bottleneck preventing further improvement. Van de Walle [17, showed that theoretically, hydrogen can act as a shallow donor to become a source of electrical conductivity. Chen et al. reported that the electrical resistivity of ZnO:H films can be significantly reduced by the addition of hydrogen to Ar during RF sputtering. The results show that the presence of hydrogen donors in the ZnO causes an increase in the carrier concentration. Sun et al. [20] reported on the preparation of ZnO:Al films by RF magnetron sputtering with a 2 wt.% Al 2 O 3 target. They used the same Ar + H 2 ambient gas but varied the substrate temperature ranging from 100 to 300 8C. The minimum electrical resistivity of ZnO:Al films occurred when the substrate temperature was 200 8C: its value was 7.9 Â 10 À4 V cm. Liu et al. [21] reported that the hydrogen flow that occurred during the sputtering process had a strong influence on the properties of the resultant ZnO:Al films.
In a previous study , we noted that the electrical properties of TiO 2 -doped ZnO films deposited in pure Ar gas via RF magnetron sputtering could be improved by decreasing the working pressure and increasing the substrate temperature. The lowest electrical resistivity achieved for TiO 2 -doped ZnO films was 2.5 Â 10 À3 V cm. This value is lower than that obtained previously, but it is still higher than that of the trivalent cation-doped ZnO films. Since ZnO films deposited by reactive sputtering have proven to have better conductivity when deposited in an H 2 ambient, it is worth