Phase transition and related phenomena in chemically deposited polycrystalline cadmium sulfide thin films

Optical and structural properties of chemically deposited CdS films have been characterized by using spectroscopic ellipsometry and electron transmission microscopy. Three representative cases have been studied using grown metastable cubic film and two films annealed at 400 degrees C (30 min) with and without a CdCl2 pretreatment (2 min dip in a CdCl2-saturated methanol solution). Without the pretreatment, the film remains at an intermediate state between the cubic and the stable hexagonal structure. With the pretreatment, a full transition to the hexagonal structure along with a strong recrystallization takes place. Full cubic or hexagonal films present high band gap values (2.44-2.46 eV), whereas band gap narrowing by approximate to 0.1 eV is observed in the second case. This effect is shown to be associated to a spatially modulated structure with a high density of stacking faults. This structure can be identified as a polytype cubic/hexagonal structure, which is a metastable intermediate in the phase transition process. Possible origins of the associated band gap lowering as order/disorder transition or stress effects are discussed.