Valence-band characterization of AgGaSe2 thin films

AgGaSe2 thin films were prepared at post-deposition annealing temperatures from 100 to 350 deg C for 15 min onto glass substrates by the stacked elemental layer deposition technique in vacuum. The structural properties of the films were ascertained by the x-ray diffraction method. The atomic compositions and optical properties of the films were measured by energy dispersive analysis of x-ray and UV-VIS-NIR spectrophotometry, respectively. The lattice parameters are independent and the grain size varies directly with post-deposition annealing temperatures. The films demonstrate compositional uniformity. The nature and extent of the band gap energy of the films on various post-deposition annealing temperatures have been analysed. Two types of electronic transitions are observed. Direct allowed and direct forbidden transitions vary from 1.67 eV to 1.75 eV and from 2.05 eV to 2.08 eV, respectively. The splitting of the valence band is proposed to be due to spin-orbit interaction. The spin-orbit splitting is 0.33 eV at 300 deg C, which is nearest to the single crystal value. The p-d hybridizations vary directly while spin-orbit splitting varies inversely with post-deposition annealing temperature. The admixture of Ag 4d levels with otherwise p-like valence bands is calculated to be 13%.