Band gap energies of bulk, thin-film, and epitaxial layers of CuInSe2 and CuGaSe2

Band gap and excitonic resonance energies of high-quality bulk single crystals, polycrystalline thin films, and epitaxial layers of ${\mathrm{CuInSe}}_{\mathrm{2}}$ and ${\mathrm{CuGaSe}}_{\mathrm{2}}$ were determined as a function of temperature by means of photoreflectance, optical absorption (OA), and photoluminescence measurements. OA spectra were fit including excitonic absorption from low temperature up to room temperature (RT). The band gap energy of 1.032 eV and free exciton (FE) resonance energy of 1.024 eV were obtained at RT for strain-free ${\mathrm{CuInSe}}_{\mathrm{2}}$ giving an exciton binding energy of 7.5 meV. The band gap energy of both ${\mathrm{CuInSe}}_{\mathrm{2}}$ and ${\mathrm{CuGaSe}}_{\mathrm{2}}$ was found to be essentially independent of the molar ratio of Cu to group-III atom (Cu/III) for near-stoichiometric and Cu-rich samples. The disappearance of the FE absorption in the In-rich $(\mathrm{Cu/In}\text{<0.88)}$ ${\mathrm{CuInSe}}_{\mathrm{2}}$ thin films was explained by plasma screening of Coulomb interactions. A slight decrease in the band gap energy of the In-rich films was attributed to a degradation of film quality such as high-density defects, grains, and structural disordering. The fundamental band gap energy in strained ${\mathrm{CuInSe}}_{\mathrm{2}}$ and ${\mathrm{CuGaSe}}_{\mathrm{2}}$ epilayers was shown to decrease due to in-plane biaxial tensile strain.