Analysis of thickness-dependent optical parameters of a-Si:H/nc-Si:H multilayer thin films

Abstract

Abstract Multilayer thin films with alternate hydrogenated amorphous (a-Si:H) and nanocrystalline silicon (nc-Si:H) layers are deposited in hot wire chemical vapor deposition (HWCVD) chamber using hydrogen (H2) dilution of silane (SiH4). Two sets (two samples for each set) have different nc-Si:H layer thickness with the same a-Si:H thickness. The transmittance spectra, refractive index and absorption coefficient are obtained using UV–Vis spectroscopy. An analysis of Tauc plot suggests thickness-dependent optical band gap shift. The films with thick nc-Si:H layers show a decrease in band gap from 1.78 to 1.68 eV, whereas the films with lower nc-Si:H layer thickness have an increased band gap from 1.64 to 1.72 eV. Further, the slope of the Tauc plot (B) and disorder indicator (Urbach energy, E u) show that the films with thick nc-Si:H layers are poorly ordered as hydrogen dilution increases. These results illustrate that band gap shift can be attributed to the crystal size effect and film disorder. This shifting of optical band gap with the nc-Si:H layer thickness is useful in tuning of the absorber layer band gap which finds an application in photovoltaics.

Keywords

• Multilayer
• Hot wire CVD
• Hydrogen dilution
• Optical bandgap