Thermodynamic Model of the Role of Hydrogen Dilution in Plasma Deposition of Microcrystalline Silicon
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THERMODYNAMIC MODEL OF THE ROLE OF HYDROGEN DILUTION IN PLASMA DEPOSITION OF MICROCRYSTALLINE SILICON J Robertson Engineering Dept, Cambridge University, Cambridge CB2 1PZ, UK [email protected]
ABSTRACT Hydrogen dilution is used to promote the nucleation and growth of microcrystalline Si (µc-Si) by plasma enhanced chemical vapour deposition (PECVD). The free energy of µc-Si and hydrogenated amorphous silicon (a-Si:H) is analysed as a function of Si:H composition in order to derive the effect of hydrogen dilution. It is shown that increasing the hydrogen content of the a-SiHx precursor phase increases the relative stability of µc-Si slightly, but strongly increases the driving force for nucleation. The higher stability of µc-Si is the fundamental origin of the higher etch rates of a-Si:H, while surface mobility models do not account for sub-surface nucleation of µc-Si. INTRODUCTION Microcrystalline silicon (µc-Si) is the preferred material to hydrogenated amorphous silicon (a-Si:H) for thin film transistors and solar cells because it has a much higher carrier mobility and better electrical stability. µc-Si can be made by plasma enhanced chemical vapour deposition (PECVD) from a hydrogen-diluted silane plasma. Dilution factors up to 100 can be used. However, dilution lowers the growth rate, which is particularly important in the case of solar cell manufacture, so it is important to understand the role of hydrogen dilution. The use of hydrogen dilution has been attributed to the presence of atomic hydrogen. This is also true when the layer-by-layer method of alternating silane and hydrogen plasma cycles is used to give a fine control of the effect of hydrogen during µc-Si growth. This paper provides a description of the nucleation and growth of µc-Si by PECVD and the role of hydrogen within the standard ideas of phase transformation in two phase systems. There have been various proposals for the role of hydrogen dilution and atomic hydrogen in the deposition of µc-Si. The first idea by Veprek [1] proposed that there is a partial chemical equilibrium during deposition between the µc-Si and a-Si:H. He also suggested that the strain energy of Si micro-crystals increased as they became smaller, so that eventually the energy of µc-Si was higher than for a-Si:H, and a-Si:H became the stable phase. A simpler version of this idea is that µc-Si and a-Si:H are both deposited together, and that atomic hydrogen causes a preferential etching of the a-Si:H [2-4]. The second model by Matsuda et al [5-8] is the surface mobility model, which has been applied to a-Si:H, its alloys and µc-Si. Matsuda [6] argued that hydrogen dilution would provide a higher surface coverage by Si-H groups, thereby increasing the surface diffusion length of the SiH3 growth species. This allows the SiH3 to find more stable growth sites to give better a-Si:H, and then to find the most stable growth site to form µc-Si. It was observed that µc-Si does not grow above ~400C, because the SiH3 mobility is then limited by hydrogen desorption from the surface. The t
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