Optical absorption spectra of undoped and Co-doped Cd4GeSe6 single crystals

Abstract

The ternary semiconducting compounds of A4BX 6 type ( A = C d , Hg, B = S i , Ge and X = S , Se) exhibit high photosensitivity and strong luminescence in the visible and near-infrared regions [1-4]. These desirable properties make the compounds promising optoelectronic materials. However, not many studies have been made on these compounds because of considerable difficulties in obtaining homogeneous and stoichiometric crystals. Among the few studies undertaken, some attention has been paid to crystal growth [5], photoconductivity [2, 4] and photoluminescence [3] of CdeSiS 6 and Cd4GeSe6. Quenez and Gorochov [6] have reported on the crystal growth of Cd4GeSe6 by the chemical transport technique using iodine, bromide and chlorine as transporting agents. In earlier work [7], the authors have investigated photoluminescence spectra of Cd4GeSe6 crystals. However, the temperature dependence of the optical energy gap in the CdeGeSe6 compound has not been investigated to date. No study of the optical properties of transition metal impurities in this compound has been made. The purpose of the present letter is to report on the temperature dependence of the optical energy gaps of undoped and Co-doped Cd4GeSe6 single crystals grown by the chemical transport technique. The effects of the doping of the cobalt transition metal impurity on the optical absorption of this compound are also studied. Single crystals of Cd4GeSe 6 and Cd4GeSe6:Co (2 mol %) were grown by the chemical transport technique using iodine as a transporting agent. The starting materials were powders of polycrystalline Cd4GeSe 6 and Cd4GeSe6:Co [7] prepared by reacting stoichiometric quantities of cadmium (99.9999% purity), germanium (99.9999% purity), selenium (99.9999% purity) and cobalt (99.99% purity) in an evacuated quartz ampoule using a rotating horizontal furnace. The amount of iodine was 5 mg per cm 3 ampoule volume. The charged ampoule was evacuated to 2 x 10 -6 mmHg and the sealed ampoule was placed in a two-zone furnace with the source zone temperature 580 °C and the growth zone