High-resolution transmission electron microscopy study of chemically deposited cadmium sulphide thin films from aqueous ammonia solutions

Abstract

Cadmium sulphide thin films have been deposited chemically at near room temperature in aqueous ammonia solutions using thiourea as the sulphur source. The composition of the solution was [CdSO₄] = 1.4 × 10⁻²M, [Thiourea] = 2.8 × 10⁻² M, [NH₃] = 1.74 M. The structure of the layers and the growth mechanism are studied by using high resolution transmission electron microscopy (HRTEM). The layers are formed of well defined crystallites (few tens of nm size) with only the hexagonal structure, whereas the colloids formed in solution by the parallel homogeneous reaction are composed of much smaller crystallites (3-6 nm) and present a mixed hexagonal/cubic structure. This is related to the polymorphic tendency of the CdS structure, and indicates a strong influence of local parameters in the deposition conditions. The results also indicate that the growth more likely proceeds via an atomie mechanism, rather than a cluster coagulation mechanism. HRTEM images display well defined atomie projections for crystallites oriented with the c axis perpendicular to the surface, and also crystallites with the c axis parallel to the surface, with special projections along the direction. The intragrain structure of the crystallites is of good quality with few extended defeets. An interesting side phenomenon has been evidenced, it concerns the growth of large bidimensional monocrystalline hexagonal cadmium hydroxide plates (1 μm), probably at the liquid-air interface during the reaction process.