Effects of postdeposition heat-treatment on morphology and microstructure of CdTe grown by electrodeposition

The effects of postdeposition heat-treatment on the morphology and microstructure of thin-film CdTe grown by electrodeposition on CdS/SnO2/glass substrates have been investigated. Scanning electron microscopy analysis indicates that the grains of as-deposited CdTe thin films have a faceted morphology with an average grain size of -0.4 μm After annealing, the facets become less well-defined and the tops of the grains become domed or rounded. Grain growth studies indicate that the grain growth exponent n is smaller than the usual value of 0.5. X-ray diffraction (XRD) measurements confirm that as-grown CdTe thin films have a zinc-blende structure with a strongly preferred orientation such that the cubic <111> directions are perpendicular to the film plane while annealed CdTe thin films show near-random orientations, indicating that recrystallization occurs during the heat-treatment. The dynamics of recrystallization was investigated by using the ratio of integrated intensities of XRD peaks as an indication of the degree of recrystallization. It was observed that the orientation first tends toward randomness upon annealing but for longer time anneals tends to a preferred orientation of the cubic <111> axes again. The recrystallization activation energy was determined to be 2.5 ± 0.3 eV. The presence of CdCl₂ in the postdeposition annealing process was observed to accelerate the recrystallization. The residual stress in CdTe films as a function of annealing temperature and time was determined and the type of residual stress changed from compressive to tensile during the annealing.