Low temperature deposition of highly transparent and conducting Al-doped ZnO films by RF magnetron sputtering

Radio frequency (rf) magnetron sputtering process was optimized to deposit highly conducting and transparent polycrystalline Al-doped ZnO (ZnO:Al or AZO) films at low temperature. Deposition was done in pure argon atmosphere on unheated glass substrates. Various properties of AZO films, such as residual stress, crystallite size, Al content, carrier concentration and mobility, resistivity, optical transmittance, band-gap and refractive index were measured and their variation with deposition power and working pressure was analyzed. In the first step deposition power was varied from 200 W to 1200 W at a constant working pressure, and then in the next step working pressure was varied from 0.05 to 0.5 Pa while keeping deposition power constant at the optimized value. Film thickness was maintained constant at 600 ± 50 nm. Obtained results were compared with those of other researchers wherever possible. With increasing deposition power it was observed that film properties improve up to a certain power and then start to deteriorate with further increase of power. Electrical properties were found to depend more strongly on structural properties of films, compared to optical properties. At optimized deposition power film properties showed a weaker dependence on working pressure in the used range. Best quality AZO films with 6.7 × 10−4 Ω-cm electrical resistivity and 90% average visible transmittance were obtained at 600 W deposition power and 0.1 Pa working pressure.