A detailed theory for enhanced optical absorption in thin silicon with a distribution of nanovoids has been worked out in this paper. It is demonstrated that significant enhancement of the effective optical absorption coefficient (by a factor of about two to more than four) in a thin Si layer can be achieved by optimizing the dimensions and distribution of nanovoids. In this work, the absorption in a thin Si layer has been modelled taking into account the diffraction of light by the nanocrystallites between the voids as well as the scattering of light by the voids. This modelling is supposed to be applicable to any semiconductor film having a distribution of nanovoids since the modelling incorporates scattering phenomena due to Rayleigh scattering for small voids and the gradual transition from Rayleigh scattering to diffraction phenomena in the case of large voids including multiple-and back-scattering effects. The consideration of the diffraction of light instead of Mie scattering greatly simplifies the calculation and still predicts the correct behaviour of absorption phenomena in such films. The simulated results obtained using this modelling agree excellently with Brendel's recently reported experimental results. This enhancement of the optical absorptance in a thin Si film with nanovoids has potential application in different devices, e.g. thin Si solar cells. The realization of nanovoids can be achieved by high temperature annealing of double-layer porous silicon, i.e. a quasi-monocrystalline porous silicon (QMPS) layer.
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