Microcrystalline Silicon - Relation of Transport Properties and Microstructure
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ABSTRACT Understanding of transport in hydrogenated microcrystalline silicon (gc-Si:H) is difficult due to its complicated microstructure (grains, grain boundaries, amorphous tissue). gc-Si:H layers often exhibit preferential orientation leading to transport anisotropy. Furthermore, specific gc-Si:H growth features lead to the thickness dependence of the structure and properties. gc-Si:H incubation layer was studied by AFM with conductive cantilever measuring simultaneously morphology and local conductivity maps with submicron resolution. Clear identification of Si crystallites (with size of few tens of nanometers) is demonstrated. The crystalline fraction at the surface may be easily evaluated. For the charge collection in solar cells we need to study transport perpendicular to the substrate. Measurement of frequency spectra of A.C. conductivity is introduced as a new tool which can exclude the influence of contact barriers in sandwich geometry and can be used for finding the "true" conductivity perpendicular to the substrate. Using this technique transport anisotropy in some 1 ic-Si:H samples was clearly demonstrated. Finally, it is shown how the transport properties change with growing gc-Si:H thickness and how these changes correlate with the structure observed by AFM.
INTRODUCTION
Recently demonstrated high efficiency and stability of solar cells based on hydrogenated microcrystalline silicon (ýic-Si:H) [1,2] led to a concentrated research effort in many laboratories. As a result we know a lot about the structure and growth of pc-Si:H and its optical properties. Although the improvement of the solar cells proceeds mainly by trial and error, efficiencies as high as 8.5% were reported for single Vac-Si:H junction or 12% for "micromorph" cell, i.e. in tandem of ic-Si:H cell and a cell based on amorphous hydrogenated silicon (a-Si:H) [3,4]. On the other hand, the real understanding of electronic transport in [ic-Si:H remains elusive. One reason is the complicated gc-Si:H microstructure which includes grains, grain boundaries, remaining amorphous tissue and possibly also void fraction. The material is not only inhomogeneous, it also often exhibits preferential orientation and texture and thus anisotropic transport may be expected. Further complication is due to the complex [ic-Si:H growth process which leads to the thickness dependence of the structure. The other reason is that ltc-Si:H represents rather a rich class of the materials and thus widely spread values of important parameters like for example mobility were reported [5-7]. The standard measurement methods often yield only effective values of the transport parameters and thus we need new measurement methods which would be able to select and clarify the underlying phenomena, e.g. differences in local conductivity or transport anisotropy.
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Mat. Res. Soc. Symp. Proc. Vol. 557 © 1999 Materials Research Society
EXPERIMENTAL RESULTS AND THEIR DISCUSSION
Characterization of pc-Si:H initial growth by local transport studies
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