Existence and removal of Cu2Se second phase in coevaporated Cu2ZnSnSe4 thin films

The composition dependence of the electrical properties of Cu2ZnSnSe4 thin films synthesized by coevaporation and the results of phase analyses are reported. We found that the hole concentration depends on the Cu/(Zn + Sn) ratio and is on the order of 1017 cm−3 for the ratio of 0.7 and increases to over 1020 cm−3 when the ratio exceeds 0.9. Raman spectra indicate the coexistence of semimetallic Cu2Se second phase in the thin films with Cu/(Zn + Sn) ratio above 0.9. In order to remove the Cu2Se phase selectively, we attempted a KCN etching. After the KCN etching for 30 min, the Raman peak attributed to the Cu2Se phase disappeared, and the hole concentration decreased to about 1018 cm−3.