A study of cadmium mercury telluride (CMT) thin film structures: some physical observations and characteristics

Polycrystalline thin film structures of the form Cd_1?x Hg _x Te (0 ≤ x ≤ 0.5) were obtained on ultrasonically cleaned glass substrates in an alkaline medium (pH≈ 10 ± 0.2) at 80^°C. The forerunner solutions were obtained from the equimolar solutions of CdCl₂, HgCl₂ and 0.33 M-refluxed Na₂TeSO₃. The preparation parameters such as temperature, pH, deposition time and speed of mechanical stirring were optimized. The as-prepared samples were tightly adherent to the substrate support, less smooth, diffusely reflecting and were analysed for composition. It appeared that incorporation of Hg²⁺ in the lattice of CdTe replaced divalent Cd²⁺ predominantly. However, a small amount of Te^2? replacement by Hg²⁺ ions has also been observed at higher levels of Hg in CdTe (> 0.1 mol. %). The samples were then characterized optically to determine the absorption coefficient, mode of optical transition and the optical bandgaps. The estimated band-gap decreased, typically from 1.51 to 0.83 eV as Hg content in CdTe was increased from 0 to 0.5. The dc electrical conductivity measurements were made in the temperature range from 300 to 550 K and it was observed that the conductivity increased rapidly up to x = 0.1 and thereafter remained more or less the same. The thermopower measurements showed n-type conduction of these films. Both carrier concentration (n) and mobility (μ) were computed from these studies and found to be dependent on the working temperature and the film composition. The surface morphologies of these as-grown layers were then observed via a scanning electron microscope. CdTe crystals are more or less crystalline spherical and hexagonal prismatic in shape whereas HgCdTe samples appear glassy and show hexagonal prismatic whiskers, some less frequent forms of needles.