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pp.714-721
A stoichiometric mixture of evaporating materials for ZnAl2Se4 single-crystal thin films was prepared in a horizontalelectric furnace. These ZnAl2Se4 polycrystals had a defect chalcopyrite structure, and its lattice constants were a0=5.5563Åand c0=10.8897Å.To obtain a single-crystal thin film, mixed ZnAl2Se4 crystal was deposited on the thoroughly etched semi-insulating GaAs(100) substrate by a hot wall epitaxy (HWE) system. The source and the substrate temperatures were 620oCand 400oC, respectively. The crystalline structure of the single-crystal thin film was investigated by using a double crystal X-ray rocking curve and X-ray diffraction ω-2θ scans. The carrier density and mobility of the ZnAl2Se4 single-crystal thin filmwere 8.23×1016cm−3 and 287m2/vs at 293K, respectively. To identify the band gap energy, the optical absorption spectra ofthe ZnAl2Se4 single-crystal thin film was investigated in the temperature region of 10-293K. The temperature dependence ofthe direct optical energy gap is well presented by Varshni's relation: Eg(T)=Eg(0)−(αT2/T+β). The constants of Varshni'sequation had the values of Eg(0)=3.5269eV, α=2.03×10−3eV/K and β=501.9K for the ZnAl2Se4 single-crystal thin film.The crystal field and the spin-orbit splitting energies for the valence band of the ZnAl2Se4 were estimated to be 109.5meVand 124.6meV, respectively, by means of the photocurrent spectra and the Hopfield quasicubic model. These results indicatethat splitting of the ∆so definitely exists in the Γ5 states of the valence band of the ZnAl2Se4/GaAs epilayer. The threephotocurrent peaks observed at 10K are ascribed to the A1-, B1-exciton for n=1 and C21-exciton peaks for n=21.
