X-ray photoelectron spectroscopy study of the nucleation processes and chemistry of CdS thin films deposited by sublimation on different solar cell substrate materials

Abstract

Cadmium sulfide has been deposited by evaporation on five different substrates: CdTe, ZnO, Ag, TiO2, and partially reduced titanium oxide (i.e., TiO1.73). The deposition rate and the evolution of the Cd∕S ratio on the different substrates have been determined by x-ray photoelectron spectroscopy. The growth mode of the films has been also studied by analyzing the shape of the backgrounds behind the photoemission peaks (peak shape analysis). It has been found that, under completely equivalent conditions, the deposition efficiency (i.e., sticking coefficient) is large on CdTe and TiO1.73, but very small on ZnO and TiO2. Silver constitutes an intermediate situation characterized by a long induction period where the deposition rate is small and a later increase in deposition efficiency comparable to that on CdTe. For the initial stages of deposition, below an equivalent monolayer, it has been also found that the Cd∕S ratio is smaller than unity on TiO1.73 and ZnO but larger than unity on CdTe and Ag substrates. For sufficiently long deposition times the Cd∕S ratio on the surface reaches unity. Except for silver substrate, cadmium appears as Cd2+ and sulfur as S−2 species at the initial stages of deposition. On the silver surface, cadmium adsorbs as Cd0 at low coverage. Peak shape analysis has shown that cadmium sulfide grows according to layer-by-layer mechanism (Frank–van de Merwe model) when the substrates are CdTe and TiO1.73, but large particles are formed that do not cover the surface for ZnO and Ag substrates (Volmer-Weber growth model). These results are consistent with the different chemical affinities of the substrate towards the atoms of cadmium and sulfur produced during the evaporation of the cadmium sulfide.