Structural, electrical and optical properties of Cd_x Zn_1−xSe thin films

Abstract

X-ray diffraction and electron diffraction techniques indicate that Cd_x Zn_1−xSe thin films on glass substrates have a polycrystalline nature, with sphalerite structure for x≤0.5 and wurtzite structure for x≥0.6. The crystalline size in each composition increases with increasing the film thickness. The room temperature dark resistivity π varies from one composition to another showing a transition at x=0.55

The temperature dependence of π of the deposited films revealed two conduction mechanisms, one below 352 K due to shallow levels, surface states, and defects introduced during the film growth, and over 352 K due to deep-level ionization following the ordinary semiconducting behaviour.

The thermal activation energy of the free charge carriers decreases linearly with increasing the molar fraction x of the CdSe content up to x=0.55, above which it increases with increasing x.

The optical constants of Cd_x Zn_1−xSe thin films of different compositions were determined in the spectral range 400–2000 nm. The analysis of the absorption coefticient at and near the absorption edge indicates the existence of allowed direct transition energy gaps decreasing with increasing x.