Synthesis and Characterization of (Sn,Zn)O Alloys

SnO exhibits electrical properties that render it promising for solar energy conversion applications, but it also has a strongly indirect band gap. Recent theoretical calculations predict that this disadvantage can be mitigated by isovalent alloying with other group II oxides, such as ZnO. Here, we have synthesized new metastable isovalent (Sn,Zn)O alloy thin films by combinatorial reactive co-sputtering and characterized their structural, optical, and electrical properties. The alloying of ZnO into SnO leads to a change of the valence state of the tin from Sn⁰ via Sn²⁺ to Sn⁴⁺, which can be counteracted by reducing the oxygen partial pressure during the deposition. The optical characterization of the smooth <10 at. % Sn_1-xZn_xO thin films showed an increase in the absorption coefficient in the range from 1 eV to 2 eV, which is consistent with the theoretical predictions for the isovalent alloying. However, the experimentally observed alloying effect may be convoluted with the effect of local variations of the Sn oxidation state. This effect would have to be minimized to improve the (Sn,Zn)O optical and electrical properties for their use as absorbers in solar energy conversion applications.