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Structural Changes in Amorphous Silicon Annealed at Low Temperatures Branko Pivac13DYR'XEþHN1,2, Ognjen Milat3, and Ivan Zulim4 1 5XGMHU%RãNRYLü,QVWLWXWH32%R[+5-10000 Zagreb, Croatia 2 Sinchrotrone Trieste, SS 14km 163.5, I-34012 Basovizza (TS), Italy 3 Institute of Physics, P.O. Box 1010, HR-10000 Zagreb, Croatia 4 Faculty of Electrical and Mechanical Engineering, and Naval Architecture, University of 6SOLW5%RãNRYLüDEE+5-21000 Split, Croatia ABSTRACT The hydrogen dilution in the course of production of amorphous silicon (a-Si) influences its structural properties, which affect significantly light-induced degradation. We used FTIR, X-ray reflectivity and GISAXS analysis to monitor the structural changes occurring during the low temperature annealing of undoped a-Si:H films. FTIR results show that upon annealing at very low temperatures, hydrogen is moved from its positions (voids) where it was accumulated unbonded to silicon and is subsequently trapped at dangling bonds, enhancing disorder. X-ray reflectivity and GISAXS measurements confirmed the enhancement of the void size.

INTRODUCTION The incorporation of hydrogen atoms in the amorphous silicon (a-Si) samples during the deposition saturates silicon dangling bonds, leading therefore, to a decrease in the density of defect states in the gap. Moreover, it reduces the structural disorder, which results in a narrowing the band tails. Consequently, hydrogen influence is of crucial importance for the improvement of the electronic and optical properties of a-Si. Hydrogenated a-Si and its alloys are therefore, promising materials for semiconductor devices such as large-area and low cost solar cells. However, an important problem remains to be solved to make a-Si:H solar cells acceptable for the power use. That is light-induced degradation of efficiency due to so-called Staebler-Wronski effect [1], which is supposed to be closely related to hydrogen incorporated in the a-Si network [2]. Therefore, much effort has been made over the last 20 years to suppress the light-induced degradation. A possible approach to improve the stabilized efficiency would be to improve the stability of the photovoltaic material themselves. In chemical vapor deposition (CVD) techniques applied for a-Si films deposition, the number of H atoms incorporated is at least two orders of magnitude larger than the number of H atoms needed to passivate the dangling bonds in the Si network [3] . Therefore, various approaches were attempted to resolve this problem, like employing hot-wire CVD or varying the H dilution. Some recent results have demonstrated that a-Si:H with lower hydrogen and particularly silicon dihydride (Si-H2) content is more stable against the light exposure [4,5]. The effect of hydrogen dilution on the properties of solar cells has been extensively studied. For example, Okamoto et al. [6] have reported that good quality wide gap a-Si:H could be deposited with higher hydrogen dilution ratio of up to 20:1. However, further increase of hydrogen dilution is reporte