Influence of argon dilution on the growth of amorphous to ultra nanocrystalline silicon films using VHF PECVD process Jhuma Gope a,b , Sushil Kumar a,⇑ , S. Sudhakar a , Kalpana Lodhi a , C.M.S. Rauthan a , P.C. Srivastava b a Physics of Energy Harvesting Division, CSIR-Network of Institutes for Solar Energy, National Physical Laboratory, Dr. K.S. Krishnan Road, New Delhi 110 012, India b Department of Physics, Banaras Hindu University, Varanasi 221 005, India article info Article history: Received 25 September 2012 Received in revised form 17 May 2013 Accepted 17 May 2013 Available online 5 July 2013 Keywords: Amorphous silicon Mixed phase silicon FTIR PECVD abstract Hydrogenated Silicon (Si:H) thin films were deposited by very high frequency plasma enhanced chemical vapor deposition (VHF PECVD) technique from silane (SiH 4 ) diluted in various argon (Ar) concentration (f ar ). The effect of argon concentration (53–95%) on the growth and properties of Si:H films was investi- gated. Device quality amorphous silicon films were grown at f ar = 53% and 63% showed photosensitivity of five orders of magnitude. Nucleation of crystallites were started at f ar = 82% whereas mixed-phase of amorphous (a-Si:H) and ultra nanocrystalline silicon thin film (ultra nc-Si:H) was grown at 87% of f ar . The nucleation and growth of crystallites were clearly observed in the AFM images. Raman spectra showed the splitting and blue shift of TO mode for the films grown at f ar = 82% and 87%, respectively. The variation in the hydrogen bonding configuration of these Si:H films were observed by FTIR measure- ment. The difference in the temperature dependent dark conductivity during heating and cooling runs decreases as the fraction of clustered Si–H bond increases in the films. Ó 2013 Elsevier B.V. All rights reserved. 1. Introduction During the last two decades hydrogenated amorphous silicon (a-Si:H) thin film has received much interest as a candidate for low cost solar cells [1]. Due to its inherent disorder nature, it ab- sorbs sunlight very efficiently (mainly photon energy, E g > 1.8 eV) and thus it require very thin layer (<500 nm) for solar cell. The main drawback of this material is its light induced degradation called Staebler Wronski (SW) effect [2]. Intrinsic nano/microcrys- talline silicon (nc/lc-Si:H) thin film is being emerged as a promis- ing material due to its stabilized efficiency [3,4] without the SW effect as present in a-Si:H material based solar cells. The higher values of absorption coefficient of nc/lc-Si:H films in the near infrared region offer the possibility of achieving higher efficiencies by making solar cells incorporating nc/lc-Si:H. The material grown near the a-Si:H/lc-Si:H transition is ideally known to be the best for active layer of thin-film solar cell [5]. These transition region materials show good photoelectrical property having minimum degradation under light compared to pure amorphous silicon. The structure of Si:H films can be defined as different forms, for instance amorphous, micro/nanocrystalline and polymorphous [6–10] based on the plasma chemistry and process parameters of PECVD. The various gas phase reactions and plasma behaviors mainly depend on the deposition parameters like RF/VHF power, pressure, silane flow rate and dilution of the source gases by hydrogen, argon and helium. Deposition of microcrystalline and polymorphous silicon thin films using pure silane gas was reported by Li et al. [10]. In the present investigation, we report the growth of amorphous and mixed phase material by varying argon dilution with silane gas. Conventionally, high hydrogen dilution is used for the growth of nc/lc-Si:H films by PECVD technique [11]. High hydrogen dilution enhances the density of atomic hydrogen which provides the surface diffusion of film precursors to find energeti- cally favorable sites for the formation of crystalline phase [12]. Alternatively, argon dilution plays an important role in the growth of a-Si:H and nc/lc-Si:H films. In addition to the quality of Si:H material, the deposition rate of device quality a-Si:H and nc/lc- Si:H film is a big issue which found very low in conventional rf- PECVD process. Therefore, in order to obtain high quality Si:H films with high deposition rate, a novel deposition technique VHF-PEC- VD with varying argon dilution at low pressure was investigated in this study. The advantage of VHF PECVD is that device quality film can be deposited due to the reduction in ion energy bombard- ment [13] at high rate. Particularly, emphasis is made on deposi- tion of small crystallites embedded in an amorphous silicon matrix as this type of materials shows better photoelectrical prop- erties and stability. In this work, we studied the effect of argon concentration on the structural, electrical and morphological properties of films. Finally, films were subjected to heating–cooling cycles to test their stability. 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.05.142 ⇑ Corresponding author. Tel.: +91 11 4560 8650; fax: +91 11 4560 9310. E-mail address: skumar@nplindia.org (S. Kumar). Journal of Alloys and Compounds 577 (2013) 710–716 Contents lists available at SciVerse ScienceDirect Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom