Effect of Silane flow rate on microstructure of Silicon films deposited by HWCVD Purabi Gogoi, Himanshu S. Jha, Pratima Agarwal ⁎ Department of Physics, Indian Institute of Technology Guwahati, Guwahati 781039, India abstract article info Article history: Received 20 August 2011 Received in revised form 23 December 2011 Available online 16 January 2012 Keywords: HWCVD; Microstructure; Nanocrystalline silicon Hydrogenated silicon films ranging from pure amorphous to those containing small crystallites in large crys- talline fraction are prepared using the HWCVD technique without using any hydrogen dilution which is sup- posed to be necessary for the deposition of nanocrystalline Si films. The only parameter that is varied is Silane flow rate. The deposition rate ranges from 6–27 Å/s. The band gap of the films (1.8–2.0 eV) is high compared to the regular films, which is attributed to the improved short and medium range order as well as the pres- ence of low density amorphous tissues in the grain boundary regions. The films show improved stability under long term light exposure due to more ordered structure and presence of hydrogen mostly as strong Si―H bonds. © 2012 Elsevier B.V. All rights reserved. 1. Introduction Hot wire Chemical vapor deposition (HWCVD) technique is con- tinuously drawing the attention of researchers to deposit hydroge- nated amorphous and microcrystalline silicon films and devices due to certain advantages such as i) the lack of ion bombardment induced defects on growing films and ii) higher decomposition rate thus bet- ter utilization of reactant gasses over the conventional plasma en- hanced CVD (PECVD) technique [1]. The deposition rate in the case of HWCVD is also found to be higher especially for nano and micro- crystalline silicon films, where PECVD requires heavy hydrogen dilu- tion causing the deposition rates to drop down significantly. Interestingly, in HWCVD, the microstructure of the films can be easily changed from amorphous to microcrystalline by just varying any of the process parameter i.e., substrate or filament temperature, process pressure [2,3]. In this paper, we present our results on the influence of pure silane gas flow rate on the microstructure and electrical conduc- tivity of silicon films prepared using HWCVD. We observed that lower silane flow rates (SFR) results in microcrystalline silicon films, with- out using any hydrogen dilution. 2. Experimental details Hydrogenated silicon films were deposited using semiconductor grade undiluted silane (Matheson Inc.) on different substrates viz. Corning 1737, Indium Tin Oxide (ITO) coated glass, c-Si wafer and carbon coated Cu grids for different studies. Thin tungsten wire (0.5 mm diameter) is used in a horizontal filament assembly in a load lock based HWCVD system. Prior to deposition, the chamber was evacuated to a pressure less than 10 -4 Pa using turbo molecular pump. The films were prepared by varying the SFR in the range of 2–10 standard cubic centimeters per minute (SCCM), while keep- ing the other parameters such as substrate temperature (T s ), fila- ment temperature (T F ) and process pressure (PP) fixed at 250 °C, 1800 °C, 5 Pa respectively. To avoid the heating of the substrate due to hot filament, the filament to substrate distance was kept at 6 cm. The microstructure of the films was determined using X-Ray dif- fraction (XRD), Raman scattering studies (λ EX = 514.1 nm) and Trans- mission electron microscopy (TEM). Fourier Transform Infrared (FTIR) transmission spectra were recorded in the range of 400–4000 cm -1 for the estimation of total hydrogen content and its bonding configu- ration in the films. Ultraviolet–visible near-infrared (UV–vis-NIR) transmission spectra were recorded in the range of 3000–350 nm for the estimation of thickness, optical constants and optical band gap (E G ). The transport studies were done by temperature dependent dark and photoconductivity measurements on films deposited on Corning 1737 substrates in coplanar geometry using silver paint as electrodes and stability studies by light soaking. For photo conductiv- ity measurements a tungsten halogen white light source with incident intensity of 20 mW/cm 2 on films was used, while for light soaking the light intensity was 100 mW/cm 2 . 3. Results Figs. 1, 2 and 3a–d show the XRD pattern, Raman scattering spec- tra and low resolution TEM micrographs with selected area diffrac- tion (SAD) in the insets of Fig. 3. It is evident from these figures that when SFR is high (≥5 SCCM), the films are amorphous in nature characterized by broad hump at around 2θ ~ 28° in XRD, a broad peak Journal of Non-Crystalline Solids 358 (2012) 1990–1994 ⁎ Corresponding author. Tel.: + 91 361 2582702; fax: + 91 361 2690762. E-mail address: pratima@iitg.ernet.in (P. Agarwal). 0022-3093/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2011.12.095 Contents lists available at SciVerse ScienceDirect Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/ locate/ jnoncrysol