Modulating microstructure and optical properties of hydrogenated nanocrystalline silicon photovoltaic materials prepared under hydrogen diluted silane PECVD by various DC bias

Hydrogenated nanocrystalline silicon (nc-Si:H) thin films were fabricated by plasma enhanced chemical vapor deposition under the various negative substrate bias voltages with hydrogen as a diluent of silane. The microstructure and optical properties of nc-Si:H thin films were studied by Raman scattering spectroscopy, X-ray diffraction (XRD), transmission electron microscopy, and optical transmission spectroscopy. Raman spectra and XRD pattern reveal that applying negative bias voltages at the moderate level favors the enhancement of crystalline volume fraction, increase of crystallite sizes and decrease of residual stress. We also demonstrated that the negative direct current bias can be used to modulate the volume fraction of voids, refractive index, absorption coefficient, compactness and ordered degree of nc-Si:H films. It is found that the film deposited at −80 V shows not only high crystallinity, size of crystallite, and static index n0 but also low residual stress and volume fraction of voids. Furthermore, the microstructural evolution mechanism of nc-Si:H thin films prepared at different bias voltages is tentatively explored.

The negative direct current bias can be used to modulate the volume fraction of voids, refractive index, absorption coefficient, compactness and ordered degree of nc-Si:H films.(a) Raman spectra of nc-Si:H films deposited at different substrate bias voltages: spectrum (1) 0 V, (2) −40 V, (3) −80 V, and (4) −120 V. (b) Bias-dependent residual stress and crystallite size of the prepared samples. (c) The experimental (filled circles) and calculated (solid curves) transmission spectra of all four nc-Si:H samples. (d) The bias-dependent volume fraction of voids (fvfv) based on Bruggeman’s effective media approximation.Figure optionsDownload as PowerPoint slide