Irradiation effects of 65Â MeV Kr-ions on structure and optical band-gap of nc-Si:H films

In this paper, hydrogenated nano-crystalline silicon (nc-Si:H) films with thickness of 1.0 μm were irradiated with 65 MeV Kr-ions, and the irradiation induced modification on structural and optical properties of the films are investigated. Characterization of the samples with X-ray diffraction shows that the average crystallite size decreases slightly from 10.9 to 9.4 nm, and the integrated intensity ratios of (2 2 0) to (1 1 1) peak and (3 1 1) to (1 1 1) peak remain nearly constant after irradiation. The Raman spectra results reveal that the crystalline fraction of the films decreases considerably from 65.6% to 10.3% and at same time the bond angle deviation of amorphous phase increases obviously from 7.7° to 11.5° with increasing the ion fluence. Moreover, the UV-Vis transmittance spectra results show that the optical band-gap decreases from 2.14 to 1.55 eV after irradiation. The irradiation induced atomic displacements leads to the amorphization of crystalline phase and the reduction in structural order of amorphous phase in the nc-Si:H films. The improvement of light absorption and the broadening of valence and conduction band-tails, which related to the decrease of crystalline fraction and short range order, respectively, are responsible for the reduction of optical band-gap.