Preparation of Cu(In,Ga)2Se3.5 thin films by radio frequency sputtering from stoichiometric Cu(In,Ga)Se2 and Na2Se mixture target

Abstract

Defect chalcopyrite thin films of Cu(In,Ga)2Se3.5 were prepared by rf sputtering from stoichiometric CuInxGa1−xSe2 (x=0.6) and Na mixture target. The composition of the thin films fabricated in the ratio of [Na]/[Cu(In,Ga)Se2] above 5% was changed from the stoichiometric composition of Cu(In,Ga)Se2 to Cu-poor one, and identified as Cu:(In+Ga):Se=1:2:3.5. From the results of x-ray diffraction, the lattice parameters of these thin films were slightly smaller than that of Cu(In,Ga)Se2 and, besides the peaks appearing for chalcopyrite structure Cu(In,Ga)Se2, the additional peak was observed. The optical band gap is increased from 1.24 to 1.36 eV with increasing the [Na]/[Cu(In,Ga)Se2] ratio from 0% to 10% in the target. These films showed n- or p-type conduction.