Novel sputtering method to obtain wide band gap and low resistivity in as-deposited magnesium doped zinc oxide films

It has been recently demonstrated that magnesium doped zinc oxide (MZO) can be used to replace the traditional CdS window layer in thin-film solar cells. This is due to its low light absorption and non-toxic nature. In this work, a novel method to deposit high-quality MZO thin films by reactive RF-Sputtering is proposed, avoiding high deposition temperatures and post-deposition thermal treatments. The method consists in covering the Zn target with square Mg shavings distributed in concentric circles. The covering Mg/Zn area ratio was varied to control the final composition of the as-deposited MZO film. The optical, structural, chemical, morphological, and electrical properties were studied according to the covering area ratio. The band gap value of the MZO samples was tuned from 3.30 to 3.71 eV and the electrical resistivity was reduced by two orders of magnitude (from 105 to 103 Ω cm) compared to pure ZnO film. These results are remarkable as the deposition temperature is relatively low (400 °C) and there was no post-deposition thermal treatment. This method produced high-quality transparent, compact, uniform, and pinhole-free MZO films, which can be used as window layer in thin-film solar cells.