Experimental evidence for extended hydrogen diffusion in silicon thin films during light-soaking F. Kail a,b, * , S. Fellah a , A. Abramov a , A. Hadjadj b , P. Roca i Cabarrocas a a Laboratoire de Physique des Interfaces et des Couches Minces (UMR 7647 CNRS), Ecole Polytechnique, Route de saclay, 91128 Palaiseau cedex, France b Laboratoire d’Analyse des Solides, Surfaces et Interfaces, Unite ´ de Thermique et Analyse Physique (UTAP EA 3802), Universite ´ de Reims, 51687 Reims cedex 2, France Available online 20 March 2006 Abstract We have used mass spectrometry to detect hydrogen effusing from silicon thin films exposed to light. Our results indicate a long range diffusion of hydrogen through the whole film, which ends with its release into the vacuum system. The changes in the film properties are characterized by dark and photoconductivity and hydrogen exodiffusion measurements. From the evolution of dark conductivity mea- surements after turning off the light, we show that this long range motion of hydrogen is not due to the heating of the sample. A com- parison of hydrogen exodiffusion spectra of as-deposited and light-soaked samples shows that the weakly-bonded hydrogen content decreases by 30% for a-Si:H films and that the tightly-bonded hydrogen migrates to grain boundaries of crystalline regions in the case of pm-Si:H films. These results clearly demonstrate the long range motion of hydrogen during light soaking. Ó 2006 Elsevier B.V. All rights reserved. PACS: 73.50.Pz Keywords: Amorphous semiconductors; Silicon; Solar cells; Conductivity; Photoconductivity 1. Introduction It is well known that light induces metastable defects in hydrogenated amorphous silicon (a-Si:H), the so-called Staebler–Wronski effect [1], which limits the efficiency of solar cells. Over the last 25 years, large efforts have been made to improve a-Si:H stability, as well as to understand the mechanisms underlying the structural and electronic changes induced by light. Various attempts have been made to find a suitable model to explain this effect [2–4], and most of them associate the light-induced changes with an active role of hydrogen. Using secondary-ion mass spec- trometry, Santos et al. have provided a direct observation of light-enhanced hydrogen motion in a-Si:H/a-Si:D multi- layers [5]. Cheong et al. have also given evidence for long range hydrogen motion in a-Si:H samples light-soaked at room temperature by monitoring the changes in the Raman spectra of an a-WO 3 overlayer [6]. In this paper we give experimental evidence for extended hydrogen diffusion in a-Si:H and polymorphous silicon films during light-soaking. Using mass spectrometry, we detect the hydrogen effusing from the films exposed to light. Photoconductivity, dark-conductivity, and hydrogen exodiffusion measurements were also used to study the changes in the film structure and hydrogen bonding configurations. 2. Experiments Hydrogenated amorphous silicon films were deposited on 1737 corning glass substrates at 230 °C in a conven- tional radio frequency (RF) glow discharge system by the 0022-3093/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2005.10.033 * Corresponding author. Address: Laboratoire de Physique des Inter- faces et des Couches Minces (UMR 7647 CNRS), Ecole Polytechnique, Route de saclay, 91128 Palaiseau cedex, France. Tel.: +33 1 69 33 32 03; fax: +33 1 69 33 30 06. E-mail address: fatiha.kail@polytechnique.edu (F. Kail). www.elsevier.com/locate/jnoncrysol Journal of Non-Crystalline Solids 352 (2006) 1083–1086