Effects of deposition profiles on RF-sputtered Cu(In,Ga)Se2 films at low substrate temperature

Cu(In,Ga)Se2 (CIGS) thin films were successfully prepared under a hybrid deposition process using radio frequency (RF) sputtering. Different deposition profiles were utilized to give a graded deposition by incorporating a Cu-excess growth step, with Cu/(In + Ga) > 1. The compositional, structural, morphological, optical and electrical properties of the CIGS films prepared under different deposition profiles were investigated. The characterization of energy-dispersive X-ray (EDX) spectrometer shows that the films prepared under the hybrid RF sputtering process represents near stoichiometry of Cu(In,Ga)Se2, with Cu/(In + Ga) < 1. XRD patterns and Cross-sectional SEM images reveal that the crystallinity of CIGS films significantly improves and grain boundaries predominately decrease as the duration of Cu-excess growth step prolonged. This result is attributable to the existence of Cu2−xSe secondary phase during the Cu-excess growth step. The increasing of the duration of Cu-excess growth step yields a CIGS film with engineered optical band gap ranging from 1.30 down to 1.0 eV. Hall measurements show the carrier concentration (N) increases 3 orders of magnitude and resistivity (ρ) gradually decreases with the increasing of the duration of Cu-excess growth step.