Raman studies in CdS thin films in the evolution from cubic to hexagonal phase

CdS polycrystalline thin films prepared onto glass substrates by the chemical bath deposition method at 80°C showed cubic crystalline structure (β-CdS). Upon thermal annealing in Ar + S₂ flux at different temperatures in the range 200-510°C, the crystalline structure evolution of layers from the cubic modification (as-grown sample) to hexagonal phase (annealed at highest temperature samples) has been observed. At around 300°C the critical point of the phase transition has been experimentally determined to occur. Raman spectroscopy measurements were carried out at room temperature for as-grown and annealed layers. The A₁(LO)-+ E₁(LO) modes at 305 cm^-1 and replicas appear for as-grown and annealed samples. The E₁(TO) and the A₁(TO) modes were also observed. A classic simple approach is proposed in order to explain the presence of the TO modes.