Numerical Simulation of Microcrystalline Silicon Growth on Structured Substrate
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0910-A13-02
Numerical Simulation of Microcrystalline Silicon Growth on Structured Substrate Martin Python, Evelyne Vallat-Sauvain, Julien Bailat, Christophe Ballif, and Arvind Shah Institute of Microtechnology, University of Neuchâtel, Rue A.-L. Breguet 2, Neuchâtel, Neuchâtel, 2000, Switzerland
ABSTRACT The growth of thin-film silicon close to the amorphous/microcrystalline transition is qualitatively described by a 3D-discrete dynamical growth model on a cubic lattice. The result of this simulation is a representation of the microstructure of the layer as a function of time, i.e. computer-generated animations of growing microcrystalline silicon layers. It permits to follow the evolution of the nucleation and of the growth of the crystalline phase, the surface roughness, the average crystalline volume fraction and the void volume fraction. In these computer simulations, the effects of the substrate surface morphology and of the distribution of particles incidence angle have been studied. Comparison between simulated normal and isotropic incidence on structured substrates indicates that, under microcrystalline growth conditions, shadowing effects lead to the occurrence of cracks in the simulated microstructure. These effects are also evidenced experimentally in the case of µc-Si:H silicon layers deposited by very high frequency plasmaenhanced chemical vapour deposition (VHF-PECVD) on periodic and random nanostructured substrates. INTRODUCTION Thin film µc-Si:H silicon is a material of choice for large-area thin film solar cells. However, because of its indirect bandgap and because of the low thicknesses used (usually 1-2 µm), efficient light trapping is necessary to increase the photogenerated current within the solar cell. For this purpose, one method is to use rough substrates, which can be either random or periodic. Characteristic roughness used for thin film solar cells application are in the order of a few tens to a few hundreds of nanometers and lead to a relative increase of short-circuit current by a few percents to up to 15-20% [1]. However, it is not well documented yet if growth of µc-Si:H on such substrates is conformal (the surface of the layer follows the surface of the substrate) or not and how light trapping is influenced by this effect. In addition, the other electrical parameters (such as open-circuit voltage) of the solar cell are influenced by the substrate’s morphology [2]. This observation has been related with the effect of the substrate on the layer microstructure, resulting from one characteristic of µc-Si:H growth: indeed, microcrystalline grains grow perpendicular to the local plane of the substrate. The collision between grains can lead to cracks, whose possible effects on the electrical characteristics of solar cells are not clearly elucidated yet. Here, the importance of shadowing effects of the nanostructured substrate on the growth of µc-Si:H layers will be studied numerically and experimentally. The 3D model used in this study is based on the approached proposed by Bailat et al. [3
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