Absorption coefficient measurements for vacuum-deposited Cu-ternary thin films

Abstract

The absorption coefficients, α, of polycrystalline CuInS2 and CuInSe2 thin films have been determined at room temperature using transmission measurements. For each of these semiconductors, an absorption edge corresponding to the direct band-gapl transition (1.54±0.02 eV for CuInS2 and 1.00±0.02 eV for CuInSe2) is observed in the α vs hν spectrum. In p-type CuInS2 thin films, another edge is evident at lower energies (1.41×1.42 eV) and is attributed to transitions from a copper vacancy band to the conduction band. The effects of annealing on the absorption coefficient spectra of these chalcopyrite materials are presented. An additional absorption region, appears in the CuInSe2 spectra at higher energies (hν∠1.28 eV), and is critically dependent on the Se concentration of the films. This effect is discussed in terms of band structure, film stoichiometry and structural characteristics of the films.