Optical Properties of Nanocrystalline Silicon Thin Films in Wider Regions of Wavelength
Nanocrystalline silicon thin films were prepared on corning (7059) glass substrates by means of a 150 MHz very high frequency plasma-enhanced chemical vapour deposition. An analysis about the effect of deposition times and substrate temperatures on the optical properties of nanocrystalline silicon is reported. Analysis by UV-Vis-Nir spectrophotometer showed that nc-Si films were almost transparent throughout the visible and infrared (IR) region mainly at 500 nm, while strongly absorbed incident light occurred in the ultraviolet (UV) region. A blue-shift changes at energy around 2.3 eV is observed for absoprtion coefficient if compared to those of bulk silicon crystal due to quantum confinement of nc-Si. The effect of film thickness showed a strong relation in the absorption coefficient value. Thicker films at longer deposition times and lower substrate temperatures reduced absorption coefficient of the films. However, grain size and surface roughness also plays a major role. Optical energy band gap, Eg opt deduced from Tauc’s plot were found to be higher than 1.1.eV within the range of 1.7 - 2.3eV. These results were consistence with those obtained using PL measurements. The existence of nc-Si was previously confirmed using EDX and Raman analysis.
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