How much hydrogen and voids are energetically stable in silicon thin films?
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How much hydrogen and voids are energetically stable in silicon thin films? Anna Fontcuberta i Morral, Holger Vach, Pere Roca i Cabarrocas LPICM (UMR 7647), Ecole Polytechnique, 91128 Palaiseau, Cedex France Abstract We have developed a model to account for the effects of hydrogen and voids on the structural stability of silicon thin films. The model is based on both experiments and theory. First, hydrogenated amorphous silicon films (a-Si:H) with various hydrogen contents were obtained by Plasma Enhanced Chemical Vapor Deposition. A linear correlation between hydrogen content and void fraction was observed. By tuning the deposition conditions, polymorphous silicon films with hydrogen contents up to 15%, very small void fractions (0.5%) and excellent electronic properties were also obtained. Density Functional Theory (DFT) calculations were performed to determine the formation energy for four types of silicon tetrahedra of the form Si-SinH4-n (n=1, 2, 3, 4). In our model, these tetrahedral units are considered as the building blocks of the silicon thin films. Considering a homogeneous distribution of hydrogen in the solid, the proportion of the different SiSinH4-n tetrahedra as a function of the hydrogen concentration was calculated. Then, the formation energy of hydrogenated amorphous silicon (a-Si:H) was calculated as a function of the hydrogen content and for various porosities. The model predicts that hydrogen incorporation does render the a-Si:H structure unstable for different hydrogen contents depending on the void fraction. Our results show that polymorphous silicon films with hydrogen concentrations up to 15% can be as stable as standard amorphous silicon with 2% hydrogen content, provided that the presence of hydrogen is not associated with the incorporation of porosity in the film. INTRODUCTION Amorphous silicon was deposited for the first time in 1879 by Ogier1. 90 years later, this material was re-discovered by Chittick et al. who showed that in spite its disordered structure, amorphous silicon did behave like a semiconductor 2,3 . This material became especially important after the acceptation of the fundamental role of hydrogen in the passivation of dangling bonds 4 . Since then, a lot of work has been devoted to the understanding of the different roles of hydrogen in silicon. In particular the StaeblerWronksi effect, which consists in the increase of the density of dangling bonds upon illumination of a-Si:H5 and in which it is believed that hydrogen plays a significant role. It is also generally accepted that a large hydrogen content in a-Si:H leads to poor electronic properties. Large efforts have been devoted to the obtaining of a dense a-Si:H with very low hydrogen content6. However, we have recently shown that polymorphous silicon thin films with high hydrogen content and good electronic properties can be obtained7, 8, which shows that large hydrogen contents are not incompatible with good electronic properties of silicon thin films. The solubility of hydrogen in silicon has been calculat
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