HIGH-RESOLUTION TRANSMISSION ELECTRON-MICROSCOPY STUDY OF CHEMICALLY DEPOSITED CADMIUM-SULFIDE THIN-FILMS FROM AQUEOUS AMMONIA SOLUTIONS

Cadmium sulphide thin films have been deposited chemically at near roo m temperature in aqueous ammonia solutions using thiourea as the sulph ur source. The composition of the solution was [CdSO4] = 1.4 x 10(-2) M, [Thiourea] = 2.8 x 10(-2) M, [NH3] = 1.74 M. The structure of the l ayers and the growth mechanism are studied by using high resolution tr ansmission electron microscopy (HRTEM). The layers are formed of well defined crystallites (few tens of nm size) with only the hexagonal str ucture, whereas the colloids formed in solution by the parallel homoge neous reaction are composed of much smaller crystallites (3-6 nm) and present a mixed hexagonal/cubic structure. This is related to the poly morphic tendency of the CdS structure, and indicates a strong influenc e of local parameters in the deposition conditions. The results also i ndicate that the growth more likely proceeds via an atomic mechanism, rather than a cluster coagulation mechanism. HRTEM images display well defined atomic projections for crystallites oriented with the c axis perpendicular to the surface, and also crystallites with the c axis pa rallel to the surface, with special projections along the [1120BAR] di rection. The intragrain structure of the crystallites is of good quali ty with few extended defects. An interesting side phenomenon has been evidenced, it concerns the growth of large bidimensional monocrystalli ne hexagonal cadmium hydroxide plates (1 mum), probably at the liquid- air interface during the reaction process.