Growth and Electrical Properties of ZnAl2Se4 Single Crystal Thin Film by Hot Wall Epitaxy

Abstract

A stoichiometric mixture of evaporating materials for <TEX>$ZnAl_2Se_4$</TEX> single-crystal thin films was prepared in a horizontal electric furnace. These <TEX>$ZnAl_2Se_4$</TEX> polycrystals had a defect chalcopyrite structure, and its lattice constants were <TEX>$a_0=5.5563{\AA}$</TEX> and <TEX>$c_0=10.8897{\AA}$</TEX>.To obtain a single-crystal thin film, mixed <TEX>$ZnAl_2Se_4$</TEX> 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 <TEX>$620^{\circ}C$</TEX> and <TEX>$400^{\circ}C$</TEX>, 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 <TEX>${\omega}-2{\theta}$</TEX> scans. The carrier density and mobility of the <TEX>$ZnAl_2Se_4$</TEX> single-crystal thin film were <TEX>$8.23{\times}10^{16}cm^{-3}$</TEX> and <TEX>$287m^2/vs$</TEX> at 293 K, respectively. To identify the band gap energy, the optical absorption spectra of the <TEX>$ZnAl_2Se_4$</TEX> single-crystal thin film was investigated in the temperature region of 10-293 K. The temperature dependence of the direct optical energy gap is well presented by Varshni's relation: <TEX>$E_g(T)=E_g(0)-({\alpha}T^2/T+{\beta})$</TEX>. The constants of Varshni's equation had the values of <TEX>$E_g(0)=3.5269eV$</TEX>, <TEX>${\alpha}=2.03{\times}10^{-3}eV/K$</TEX> and <TEX>${\beta}=501.9K$</TEX> for the <TEX>$ZnAl_2Se_4$</TEX> single-crystal thin film. The crystal field and the spin-orbit splitting energies for the valence band of the <TEX>$ZnAl_2Se_4$</TEX> were estimated to be 109.5 meV and 124.6 meV, respectively, by means of the photocurrent spectra and the Hopfield quasicubic model. These results indicate that splitting of the <TEX>${\Delta}so$</TEX> definitely exists in the <TEX>${\Gamma}_5$</TEX> states of the valence band of the <TEX>$ZnAl_2Se_4/GaAs$</TEX> epilayer. The three photocurrent peaks observed at 10 K are ascribed to the <TEX>$A_1$</TEX>-, <TEX>$B_1$</TEX>-exciton for n = 1 and <TEX>$C_{21}$</TEX>-exciton peaks for n = 21.