Epitaxial growth of CuGaSe2 thin-films by MBE-Influence of the Cu/Ga ratio

By molecular beam epitaxy (MBE) CuGaSe2 (CGS) thin-films with varying Cu/Ga ratios were grown epitaxial on GaAs (100) and stepped GaAs (111)A substrates. Cu/Ga ratios from Cu-poor to Cu-rich were obtained. In this work the appearance of Cu crystallites on the surface of epitaxial CGS (001) layers are observed and strategies to avoid these precipitations are presented. High quality thin CGS films of around 100 nm thickness are obtained, enabling a detailed analysis of the electronic and chemical properties as well as of the crystal structure of the CGS surfaces. The electronic structure with respect to the Cu/Ga ratio was characterized in-situ by XPS and UPS. By LEED a (4 × 1) (Cu-poor and near stoichiometric) and a (4 × 2) (Cu-rich) reconstruction of a zinc blende structure were obtained. For CuGaSe2 (112) the LEED pattern showed a (3 × 1) chalcopyrite reconstruction for Cu/Ga ratios < 1. A (1 × 1) reconstruction of the chalcopyrite structure was observed for Cu-rich (112) samples. The observed dependence of the surface reconstruction on the stoichiometry for CGS grown on GaAs has not been reported in literature so far. Additionally, for Cu-rich stoichiometries a binary phase of Cu2-xSe appeared independently of orientation. The film morphology was investigated ex-situ by SEM.