Hydrogen Diffusion in Boron-Doped Hydrogenated Amorphous Silicon Films: Crystallization and Induced Structural Changes
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Hydrogen Diffusion in Boron-Doped Hydrogenated Amorphous Silicon Films: Crystallization and Induced Structural Changes F. Kail1,2, A. Hadjadj2, and P. Roca i Cabarrocas1 1 Laboratoire de Physique des Interfaces et des Couches Minces (UMR 7647 CNRS), Ecole Polytechnique, 91128 Palaiseau Cedex, France. 2 Laboratoire d'Analyse des Solides, Surfaces et Interfaces, Unité de Thermique et Analyse Physique (UTAP EA 3802),Université de Reims, 51687 Reims Cedex 2, France. ABSTRACT We have studied the evolution of the structure of boron-doped hydrogenated amorphous silicon films exposed to a hydrogen plasma. From the early stages of exposure, hydrogen diffuses and forms a thick H-rich subsurface. At longer times, hydrogen plasma leads to the formation of a microcrystalline layer via chemical transport without crystallization of the initial layer. We observe that the hydrogen content increases in the films during a plasma exposure and once the microcrystalline layer is formed hydrogen diffuses out of the sample accompanied with a decrease in the boron content. This effect can be attributed to the electric field developed within the heterojunction a-Si:H/µc-Si:H that drives the positively charged hydrogen atoms in the boron-doped layer towards the µc-Si:H layer. INTRODUCTION Hydrogen plays an important role in the deposition, structure and electronic properties of silicon thin films. Therefore, hydrogen diffusion in intrinsic a-Si:H has been extensively studied [1, 2]. However, the effects associated to diffusion of hydrogen during the deposition of films remain poorly understood, especially in the case of doped films, and in the deposition of solar cells where during the growth of the intrinsic layer hydrogen can diffuse and affect the properties of the p-layer layer and the overall cell efficiency[3]. In previous studies we have shown that when a-Si:H is exposed to a hydrogen plasma, two processes can occur. When the walls of the reactor are uncoated with a-Si:H , the hydrogen plasma leads to the etching of the film on the heated substrate [4, 2]. On contrary, if the walls are coated with a-Si:H, the H2 plasma leads to deposition of µc-Si:H by chemical transport [5, 6]. In this work we combine in-situ spectroscopic ellipsometry, and hydrogen effusion measurements to study the kinetics of hydrogen diffusion in boron doped a-Si:H thin films exposed to hydrogen or deuterium plasmas. We observe that hydrogen content increases in the early stages of plasma exposure, but once a microcrystalline layer is formed, hydrogen diffuses out of the sample accompanied with decreasing of boron content. EXPERIMENTS Hydrogenated amorphous silicon films were deposited at 230°C in a conventional radio frequency glow discharge system on 1737 Corning glass and crystalline silicon substrates coated with a 500 Å-thick chromium layer. The films were obtained by decomposition of pure silane under a RF power of 3 W and a total pressure of 71 mTorr. Doping was performed by adding 5 mTorr of trimethylboron (diluted at 2% in hydrogen) to the silane
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