뜨거운 곁쌓기법에 의해 성장된 CaAl2Se4 단결정 박막의 광전기적 특성

Abstract

ABSTRACT

Opto-electrical Properties for CaAl2Se4 Single Crystal Thin Film Grown by Hot Wall Epitaxy

Lee Ki-Jung Advisor : Prof. Hong, Kwang-Joon, Ph.D. Department of Physics, Graduate School of Chosun University

Abstract A stoichiometric mixture of evaporating materials for CaAl2Se4 single crystal thin films was prepared from horizontal electric furnace. To obtain the single crystal thin films, CaAl2Se4 mixed crystal was deposited on thoroughly etched semi-insulating GaAs(100) substrate by the Hot Wall Epitaxy (HWE) system. The source and substrate temperatures were 600 ℃ and 440 ℃, respectively. The crystalline structure of the single crystal thin films was investigated by the photoluminescence and double crystal X-ray diffraction (DCXD). The carrier density and mobility of CaAl2Se4 single crystal thin films measured from Hall effect by van der Pauw method are 9.98 × 1016 cm-3 and 296 cm2/v․s at 293 K, respectively. The temperature dependence of the energy band gap of the CaAl2Se4 obtained from the absorption spectra was well described by the Varshni's relation, Eg(T) = 3.8239 eV - (4.9823 × 10-3 eV/K)T2/(T + 559 K). The crystal field and the spin-orbit splitting energies for the valence band of the CaAl2Se4 have been estimated to be 5.14 meV and 1.27 meV, respectively, by means of the photocurrent spectra and the Hopfield quasicubic model.

These results indicate that the splitting of the Δso definitely exists in the Γ3 + Γ4(C) states of the valence band of the CaAl2Se4/GaAs epilayer. The three photocurrent peaks observed at 10 K are described to the A1-, B1-exciton for n = 1 and C15-exciton peaks for n =15. From the photoluminescence measurement of CaAl2Se4 single crystal thin film , we observed free excition (EX) observable only in high quality crystal and neutral donor-bound exciton (Do,X) having very strong peak intensity. And, the full width at half maximum and binding energy of neutral donor bound excition were 27 meV and 12.6 meV, respectively. By Haynes rule, an activation energy of impurity was 63 meV. The open-circuit voltage, short current density, fill factor ,and conversion efficiency of p-Si/ n-CaAl2Se4 heterojunction solar cells under 80 mW/cm2 illumination were found to be 0.43 V, 24.2 mA/cm2, 0.74 and 9.63 %, respectively.