Substrate temperature dependence of electrical properties of ZnO:Al epitaxial films on sapphire (12̄10)

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Abstract
ZnO:Al films were deposited on (12̄10) oriented sapphire substrates heated up to 400 °C by rf magnetron sputtering from a ZnO target mixed with Al2O3 of 2 wt%. Films deposited on a substrate heated to a temperature in the range 50–350 °C were (0001) oriented single crystals but those grown at 400 °C consisted of crystallites with the (0001) and (11̄01) orientation. The former films had relatively smooth surfaces whereas the latter exhibited very rough surfaces. Electrical properties such as resistivity, carrier concentration, and the Hall mobility were measured as a function of substrate temperature. The carrier concentration decreased as the substrate temperature was increased up to 300 °C, although the Al content remained unchanged in this temperature range. From these measurements, it was found that the native donors were important as a source of carriers, even in ZnO:Al films. However, it was found that the Hall mobilities for films with a thickness of more than 200 nm experienced minor changes over a growth temperature range from 50 to 350 °C. The minimum resistivity obtained was about 1.5×10−4Ω cm, a value comparable to that for indium tin oxide film.