PolycrystaUine thin films of CuInSe2 and CuGaSe2 deposited by thermal evaporation onto heated substrates of 7059 glass and A1203 were studied by electron microprobe analysis, X-ray diffractometry, Hall effect and van der Pauw measurements, and spectrophotometry in the near-infrared and visible range, 2000 -400 nm. The resulting films covered a wide range of compositions, 19-29 at.%Cu ([Se]/[metal] ~-1.0). The substrate temperatures ranged from 350 Β° to 500 Β°C. The films were subjected to postdeposition chemical treatments and anneals.
The data for CuInSe2 indicates an absorption coefficient as high as 1.5 Γ l0 s cm -I, though predominantly (2.0 -8.0) Γ 104 cm -I, and band-gap values ranging from 0.94 to 1.02 eV, with band-gap narrowing observed for films deposited at 350 Β°C. The effect of high temperatures and lower copper concentrations in CulnSe: and CuGaSe2 is to shift the onset of absorption to higher energies. The data for CuGaSe2 thin films indicate absorption coefficients up to (1-2)Γ 10 s cm -~ at 500 nm and band gaps ranging from 1.66 to 1.72 eV. The effect of higher substrate temperatures on CuGaSe2 is to eliminate secondary phases and minimize residual absorption below the band gap.
The electrical measurements on CulnSe2 indicate activated conductivity and show the films to be highly compensated by donors and acceptors whose origins are the native defects arising from non-stoichiometry. Slight copper-rich material is characterized as degenerate p-type with very low mobilities. For at.% Cu < 25, carrier concentrations and mobility change drastically with changes in stoichiometry.