Conclusions

The study of hydrogenated nanocrystalline Si (nc-Si:H) is advancing day by day, since nc-Si:H films are regarded as promising materials for commercially accepted large-area solar cells and thin-film transistors. The nc-Si:H films take very complex structures consisting of disordered (amorphous) and ordered (crystalline) forms. The quantitative explanation of the electronic and optical properties of nc-Si:H, therefore, is very hard.

A current understanding of nc-Si:H films has been briefly reviewed in the present chapter, including preparation methods and spectroscopic characterization. It was shown that an effective medium approximation was useful to explain the electronic transport and optical properties in nano- or microcrystalline Si films, which strongly depend on the volume fraction of crystallite. The present chapter also made clear the following unknown properties of these complicated materials: (i) The Hall mobility is sublinearly proportional to the crystallite size, which can be attributed to a fractal nature of the microcrystalline system. (ii) The nonactivated temperature variation of dc conductivity at relatively high temperatures is well explained in terms of the band transport across the potential barriers (percolation-limited thermionic emission process). Hopping transport may dominate only at lower temperatures.

All results predicted here may help further understanding and development of nano-, micro-, and polycrystalline materials.

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