Post-thermal annealing effects on the optical, structural and morphological properties of hydrogenated silicon (Si:H) thin films prepared by layer-by-layer (LBL) deposition technique

Hydrogenated silicon (Si-H) thin films were deposited from a mixture of pure silane (SiH₄) and hydrogen (H₂) gases in a home-built plasma enhanced chemical vapour deposition (PECVD) system using the layer-by-layer (LBL) deposition technique. The substrate temperature, deposition pressure and radio-frequency (RF) power were maintained at 150degC, 0.8 mbar and 20 W respectively during the deposition process. The films were annealed at temperatures of 100degC to 500degC in steps of 100degC interval. Optical transmission spectroscopy in the ultra-violet to near infrared region was used to determine the film thickness, refractive index and the optical energy gap of the films. The hydrogen content and the microstructure parameter were determined from the Fourier transform infrared (FTIR) spectra of the films. The structure and surface morphology of the films were analyzed using X-ray diffraction (XRD) and atomic force microscopy (AFM) respectively. The results showed that annealing temperatures strongly influenced these properties of the films. The film thickness and the optical energy gap decreased while the refractive index increased with increase in annealing temperature when annealed at temperatures of 300degC and above. The film structure remained amorphous when annealed at these temperatures. The hydrogen content decreased continuously with increase in annealing temperature when annealed up to 300degC, increased slightly when annealed at 400degC and continued to decrease when annealed at 500degC. Structural order decreased significantly with increase in annealing temperature when annealed at temperatures of 300degC and above. High concentration of cone-like structures was observed in the AFM images of the films annealed at 200degC and 400degC.