Effect of Cu/Al ratio on the properties of CuAlSe2 thin films prepared by co-evaporation

Cu-rich, near-stoichiometric and Al-rich CuAlSe2 thin films were deposited by elemental co-evaporation technique on glass substrates held at 673 K. Powder X-ray diffraction studies revealed that the Cu-rich CuAlSe2 films contain Cu2Se as second phase while near-stoichiometric and Al-rich CuAlSe2 films are single phase. The Cu-rich and near-stoichiometric films are found to have chalcopyrite structure and the Al-rich films have defect chalcopyrite structure due to formation of ordered vacancy compound CuAl2Se3.5. The optical band gaps of Cu-rich, near-stoichiometric and Al-rich films determined from optical absorption studies are found to be 2.48 eV, 2.62 eV and 2.87 eV respectively. The electrical conductivity studies revealed that the Cu-rich and stoichiometric films are p-type and Al-rich films are n-type conducting. The resistivity of the films is found to increase with decrease in Cu/Al ratio.