Atomic Layer Deposition of CdS Films

Pure, polycrystalline CdS deposited by atomic layer deposition (ALD) on Si(100) or glass using dimethyl cadmium and in situ generated H2S is investigated in detail. This ALD system follows saturation behavior typical of ALD systems, and the growth rate monotonically decreases with temperature from 100 degrees C-300 degrees C; by 400 degrees C linear growth rate behavior is no longer seen. The crystal structure as determined by X-ray diffraction and transmission electron microscopy gradually transitions from zincblende to wurtzite with increasing temperature until the film is primarily wurtzite by 400 C. Further, the average grain size increases with temperature. Transmission electron microscopy images and selected area diffraction patterns confirm the presence of zincblende and wurtzite crystals because of stacking faults and demonstrate that {111} crystal planes are more oriented parallel to the substrate at lower temperatures. Ultraviolet-visible spectroscopy shows that the bandgap is 2.3-2.42 eV in the 100 degrees C-400 degrees C range with a slight increase occurring with temperature. The roughness of the films is found to increase both with temperature and cycle number as observed with atomic force microscopy and scanning electron microscopy. Density functional theory calculations were used to understand observations concerning the growth rate and the bandgap of the films deposited at different temperatures.