Successful growth of Cu₂Se-free CuGaSe₂ by migration-enhanced epitaxy

CuGaSe₂ films were grown on GaAs (001) substrates by migration enhanced epitaxy, where Cu Ga and Se are alternately deposited. The in situ reflection high energy electron diffraction observation during growth revealed that the segregation of Cu₂Se on CuGaSe₂ can be detected by anomalous diffraction patterns. When the Cu₂Se segregation takes place, a distorted pattern appears in both Cu Ga and Se deposition periods. By optimizing the flux ratios, the anomalous diffraction disappears, and we succeeded in growing high quality CuGaSe₂ single crystal layers free from Cu₂Se segregation on GaAs (001) substrates. The authors also found that, in the CuGaSe₂/GaAs wafers with Cu₂Se segregation, a high density of voids is often observed at the substrate surface. These voids disappeared when the Cu₂Se-free growth conditions were employed. Ga atoms near the GaAs substrate surface were probably drawn out by excess Cu atoms in the growing layer to form CuGaSe₂, leading to the creation of voids with fairly large size.