Influence of various Cu contents on the microstructure of Cu(In,Ga)Se₂ thin films

The influence of different Cu contents (Cu/(Ga+In)=0.748~0.982) on the microstructure of Cu(In,Ga)Se₂ (CIGS) thin films was reported. The CIGS thin films were grown via a two-step process including DC sputtering deposition of metallic precursor and following selenization. Presence of a series of chalcopyrite diffraction peaks in the X-Ray diffraction (XRD) patterns confirms the existence of chalcopyrite CIGS (CH-CIGS) phase in these CIGS films. The Raman spectra indicate that as the Cu content increases from low to high, the CIGS film sequentially goes through three phase regimes: coexistence of OVC and CH-CIGS phase, single CH-CIGS phase and coexistence of Cu_xSe and CH-CIGS phase. Moreover, the full width at half maximum of CIGS Raman peaks changes with Cu/(Ga+In) and reaches its minimum near Cu/(Ga+In)=0.9 due to better crystallinity and less disorder. Some empirical FWHM-Cu/(Ga+In) relationships were also observed. These results show that Raman spectroscopy is more sensitive to the microstructure of CIGS film than XRD, and can be used for preliminary estimation of the crystal phases and Cu content of CIGS film in a fast and non-destructive way.