Optimization of the electrical properties of magnetron sputtered aluminum-doped zinc oxide films for opto-electronic applications

Magnetron sputtered ZnO:Al films are promising candidates as front electrode in a variety of opto-electronic devices. Here we report on efforts to obtain highly conductive and transparent ZnO:Al films using different deposition conditions for RF, DC and MF (mid frequency) sputtering. Investigations were made to see the effect of target doping concentration (TDC), film thickness, sputter pressure and deposition temperature. RF sputtering from ceramic targets yields low resistivities between 3 and 5×10 −4 Ω cm for target doping concentrations between 4 and 0.5%. With decreasing TDC to 0.5% carrier mobilities up to 44 cm 2/Vs were obtained, accompanied by the extension of the region of high transmission to the near infrared, due to a reduction in free carrier absorption and corresponding shift in plasma wavelength. DC and MF sputtering from metallic targets yielded similar low resistivities at deposition rates up to 200 nm/min. An analysis of mobility ( μ) data of all films as function of the corresponding carrier densities ( N) showed that the μ– N values obtained in this study are in the vicinity to limits suggested in the literature.