Atomic structure of the (001) surface of CuGaSe2

Hybrid exchange density functional theory is used to study the wide band gap chalcopyrite CuGaSe2. The formation energies of the experimentally observed (4 × 1) and (1 × 1) atomic scale reconstructions on the CuGaSe2 (001) surface are calculated for different environmental conditions. The results suggest that a Se-rich (1 × 1) reconstruction, and a Cu-poor, Se-rich (4 × 1) reconstruction, are the only stable surfaces under all the studied environmental conditions. Two complementary mechanisms for the stabilisation of CuGaSe2 surfaces are proposed, and it is suggested that the presence of Na stabilises the (4 × 1) reconstructions, making them the stable terminations under Na-rich conditions.