CVD of nanophasic (Zn,Cd)S thin films: From multi-layers to solid solutions

Nanostructured ZnxCd1-xS (0 less than or equal to x less than or equal to 1) thin films were synthesized by CVD on SiO2 substrates using M(O-(i)PrXan)(2) (M = Zn,Cd; O-(i)PrXan = S2COCH(CH3)(2)) as precursor compounds. The method of preparation used consisted in the sequential deposition of single-phase MS coatings (M = Zn,Cd) at 300 degreesC in a N-2 flow, followed by ex-situ annealing in an inert atmosphere at various temperatures in the range 400-700 degreesC. The obtained specimens were thoroughly characterized before and after thermal treatments, paying particular attention to their structure, composition, morphology, and optical properties. Annealing in N-2 at 600 degreesC for 3 h resulted in the formation of a homogeneous ZnxCd1-xS solid solution. Lower temperatures did not produce a uniform sample composition, while more severe annealing conditions induced US sublimation. Moreover, a compositional control of the ZnxCd1-xS ternary phase could be achieved by varying the ZnS overlayer deposition time. Irrespective of the annealing conditions, all the obtained films were contaminant-free and nanophasic (circle divide approximate to 25 nm), and possessed a hexagonal crystal structure the lattice parameters of which decreased linearly with increasing zinc molar fraction. A parallel evolution of optical absorption spectra was also observed. In the present paper, the interrelations between film properties and processing conditions are investigated, focusing particularly on the (Zn,Cd)S evolution to ZnxCd1-xS solid solutions.