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

Nanostructured ZnxCd1-xS (0<x<1) thin films were synthesized by CVD on SiO2 substrates using M(O-iPrXan)2 (M=Zn,Cd; O-iPrXan=S2COCH(CH3)2) as precursor compounds. The adopted preparative strategy consisted in the sequential deposition of single-phase MS coatings (M=Zn,Cd) at 300°C in N2 flow, followed by ex-situ annealing in an inert atmosphere at different temperatures in the range 400-700°C. The obtained specimens were thoroughly characterized before and after thermal treatments, with particular attention to their structure, composition, morphology and optical properties. Annealing in N2 at 600°C for 3 h resulted in the formation of a homogeneous ZnxCd1-xS solid solution. Lower temperatures did not allow to obtain an uniform sample composition, while more severe annealing conditions induced CdS 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 resulted contaminant-free and nanophasic (≈25 nm), and possessed an hexagonal crystal structure whose lattice parameters 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 in particular on the (Zn,Cd)S evolution to ZnxCd1-xS solid solutions.