Effects of the surface Cu_2-xSe phase on the growth and properties of CuInSe₂ films

Drastic changes in average molecularities (m=Cu/In) from m≫1 to m=0.92-0.93 and in hole concentrations from p≫10¹⁹ cm^-3 to as low as p=7.5×10¹⁶ cm^-3 have been observed in molecular beam epitaxy grown CuInSe₂ 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 CuInSe₂. The presence of the Cu-Se phase made possible the growth of high-quality CuInSe₂ 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 CuInSe₂ when grown under Cu-excess conditions.