Effects of the surface Cu2−xSe phase on the growth and properties of CuInSe2 films

Drastic changes in average molecularities $\mathrm{(m=}\mathrm{Cu/In})$ from $\text{m≫1}$ to $\text{m=0.92–0.93}$ and in hole concentrations from ${\text{p≫10}}^{19} {\mathrm{cm}}^{\mathrm{-3}}$ to as low as ${\text{p=7.5×10}}^{16} {\mathrm{cm}}^{\mathrm{-3}}$ have been observed in molecular beam epitaxy grown ${\mathrm{CuInSe}}_2$ after selective etching of the Cu–Se phase by a KCN aqueous solution; high hole concentrations and Cu-excess compositions of the as-grown films were attributed to the Cu–Se phase. On the other hand, well-defined photoluminescence emissions were found characteristic of intrinsic ${\mathrm{CuInSe}}_{\mathrm{2}}.$ The presence of the Cu–Se phase made possible the growth of high-quality ${\mathrm{CuInSe}}_{\mathrm{2}}$ epitaxial films at a temperature well below the melting point of any Cu–Se compound. Surface topology measurements showed that the surface of the as-grown films was not fully covered by Cu–Se grains, leaving holes with depths of 200–300 nm after KCN etching. The enhanced two-dimensional growth and the reduced defect concentration imply that a very thin Cu-excess surface layer controls the growth of ${\mathrm{CuInSe}}_{\mathrm{2}}$ when grown under Cu-excess conditions.