Effect of illumination on hydrogenated amorphous silicon thin films doped with chalcogens S.K. Sharma a,c , Krishnankutty-Nair P. Kumar b, * , K.J. Kang c , R.M. Mehra a a Department of Electronic Science, University of Delhi South Campus, New Delhi 110 021, India b Department of Materials Science and Engineering, University of Texas at Dallas, Richardson, TX 75080, USA c National Research Laboratory, School of Mechanical systems Engineering, Chonnam National University, Kwangju 500 757, South Korea article info Article history: Received 28 July 2008 Available online 29 June 2009 PACS: 68.43. J 68.55.Nq 71.23.Cq 79.60.Dp 81.05.Gc Keywords: Silicon Solar cells Photovoltaics Sensors Conductivity Photonic bandgap abstract Hydrogenated amorphous silicon thin films doped with chalcogens (Se or S) were prepared by the decomposition of silane (SiH 4 ) and H 2 Se/H 2 S gas mixtures in an RF plasma glow discharge on 7059 corn- ing glass at a substrate temperature 230 °C. The illumination measurements were performed on these samples as a function of doping concentration, temperature and optical density. The activation energy varied with doping concentration and is higher in Se-doped than S-doped a-Si:H thin films due to a low defect density. From intensity versus photoconductivity data, it is observed that the addition of Se and S changes the recombination mechanism from monomolecular at low doping concentration films to bimolecular at higher doping levels. The photosensitivity (r ph /r d ) of a-Si, Se:H thin films decreases as the gas ratio H 2 Se/SiH 4 increased from 10 4 to 10 1 , while the photosensitivity of a-Si, S:H thin films increases as the gas ratio H 2 S/SiH 4 increased from 6.8  10 7 to 1.010 4 . Ó 2009 Elsevier B.V. All rights reserved. 1. Introduction Hydrogenated amorphous silicon (a-Si:H) thin films have been studied during the last three decades with growing interest because of their potential applications in electronic and optoelec- tronic devices. Current interests in hydrogenated amorphous silicon thin films and its alloys as promising candidates for the realization of advanced sensing system such as gas sensors [1,2], pixel detector for high energy particles [3,4], optical imaging [5], active matrix displays [6,7] and solar cells [8–10] is clearly seen from these publications. Hydrogenated amorphous silicon thin films have high optical absorption, low temperature deposition (<300 °C) is possible, high uniformity over large area, few constraints on substrate size, material, or topology. Most of the studies were made on preparation and investiga- tion of the properties of a-Si:H alloy with tetrahedrally co-ordi- nated elements such as C and Ge [11]. The effect of alloying halogen such as F or Cl [12] to a-Si:H was also investigated and has shown to have potential in certain applications [13]. So far a-Si, C:H has emerged as leading contender in large band- gap alloys in amorphous silicon based photovoltaic devices by serving as photon energy absorbing window layers in multi-junc- tion cells [9]. Despite this success, fundamental problems of the efficiency and the long-term stability of the material have not been solved due to the photoinduced degradation in a-Si:H thin films, which is associated with light-induced metastable Si dan- gling-bond defect [14,15]. This effect hampers the utilization of amorphous silicon and its alloy in solar cells on a large scale and since then, intensive research has been carried out to eluci- date the degradation mechanism [16–18] and to improve the stability. Recently the selection of Se and S as the doping element, as grown by capacitively coupled RF glow discharge decomposition of silane (SiH 4 ) and hydrogen sulfide (H 2 S) and hydrogen selenide (H 2 Se) diluted in helium (He), terminate the silicon dangling bonds in the same way as hydrogen as shown in Fig. 1 [15]. The temper- ature dependant conduction mechanism of Se and S-doped a-Si:H thin films have been studied by Sharma et al. [19]. Two types of conduction mechanism were observed. The conduction was found 0022-3093/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2009.05.052 * Corresponding author. Tel.: +1 972 833 6264; fax: +82 62 530 0384. E-mail address: padmakumar.nair@gmail.com (K.-N.P. Kumar). Journal of Non-Crystalline Solids 355 (2009) 1638–1643 Contents lists available at ScienceDirect Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/locate/jnoncrysol