An absorption study of microcrystalline silicon deposited by hot-wire CVD

The effect of variation of the preparation parameters filament temperature T-fil, gas pressure p and hydrogen dilution (H-2/SiH4-flow ratio) on the absorption spectra of microcrystalline silicon deposited by the hot-wire technique (hw-mu c-Si:K) has been studied by means of Photothermal Deflection Spectroscopy (PDS). We find an enhanced absorption of the mu c-Si:H compared to crystalline silicon in the band gap (defect absorption) as well as in the interband transition region. An increase of absorption has already been reported for mu c-Si:H films prepared by different techniques. In the case of hw-mu c-Si:H we observe a relation between the absorption enhancement and the crystallite size. Increasing the gas pressure from 35 to 400 mTorr (T-fil=1850 degrees C) or the filament temperature from 1750 degrees C to 1950 degrees C (p=100mTorr) the crystallite sizes, deduced from X-ray diffraction measuements, range from 10 to 60 nm. An alteration of the hydrogen dilution by varying the flow ratio between 2.5 and 25 does not affect the crystallite size and the optical absorption remains constant. In our opinion the enhancement cannot be described by a simple superposition of an amorphous and a crystalline absorption coefficient weighted by the volume fractions of the amorphous and crystalline phase, respectively. The possible reasons for the enhanced absorption will be discussed. The variation of the crystallite size with deposition conditions offers the possibility to control the optical absorption of mu c-Si:H which is important for incorporating the material either as window layers or intrinsic layers in solar cells.