Physical Properties of CdS Single Crystal

Abstract

acurate values of many important parameter, such as energy gap, activation energy, carrier concentration, mobility of carriers, are still an open question for study. Also the effect of impurity on the carriers' transport for such materials needs more work. Therefore, the aim of this work was to throw some light on the physical properties of Cds single crystal. It was decided to concentratce on optical, photoconductivity, electrical conductivity, Hall effect and dielectric properties. Measurements were carried out in vacuum cryostate to avoid oxidation in high temperature and to prevent water vapour condensation in the lower temperatures. From the study the following features were present. (1) The sample type was found to bean-type semiconductor, from the Hall effect and the hot probe method. (2) Optical absorption showed an exponential increase with photon energy. The behaviour was found to be temperature dependent. Also,a cross-over in the curves was observed at a certain photon energy value. Optical absorption data showed that the dominant transition is indirect with an optical gap of about ( 2. 03-2.04) eV. On the other hand direct optical energy gap was found in the range (2.19 - 2.24) eV, depending on the measured temperature. (3) The spectral distribution (I -hu) curves showed that the ph intrinsic and impurity transitions are strongly temperature dependent but voltage independent. At higher temperature, the photocurrent was found to be less than the dark component. D.c. photoconductivity measurements showed that the energy gap values were in the range (2.21-2.27) eV and depend on the applied voltage while impurity transitions gave the same gap width value (2.06) eV. Concerning the temperature dependence of the spectral distribution, the curves were divided into two groups. Below about 200°K the curves showed three peaks at 1.8, 2.16 and at 2.44 eV respectively; and above 200"K only one peak at 2.14 eV was observed. • (4) The decay of a.c. photoconductivity with chopping frequency, and so the lifetime, was found to be temperature, voltage and