Optical characterisation of a-Si:H and nc-Si:H thin films using the transmission spectrum alone S. Halindintwali Æ D. Knoesen Æ T. F. G. Muller Æ D. Adams Æ N. Tile Æ C. C. Theron Æ R. E. I. Schropp Published online: 20 March 2007 Ó Springer Science+Business Media, LLC 2007 Abstract Using the method proposed by Swanepoel (J Phys E: Sci Instrum 17:896–903, 1984), a-Si:H and nc-Si:H thin films have been successfully characterised using the transmission spectrum alone as taken in the UV–visible range. The studied samples were deposited on both Corning glass and Polyethylene napthalate (PEN) substrates using the Hot Wire Chemical Vapour deposition technique (HWCVD). Results on refractive index, absorption coeffi- cient and energy gap will be presented. Thicknesses calcu- lated by the same method will be compared to those obtained by direct measurement using a Dektak profilometer. 1 Introduction Thin film devices based on silicon have found applications in many areas of modern technology such as solar cells and thin film transistors. Hydrogenated amorphous silicon (a- Si:H) has established itself as a material of choice in this area for many years. However due to the degradation of its initial efficiency during operation, nanocrystalline silicon (nc-Si:H) has attracted increasing attention as a new two phased material that shows good stability and has high values of absorption. This material is tipped to be a suitable candidate for application as the active layer in solar cells. This contribution uses a fast and non destructive tech- nique to determine the optical constants of amorphous and nanocrystalline Si thin films. Its limitations will be briefly discussed as well. The knowledge of the optical constants of the material is essential since they refer to its response to the incident electromagnetic radiation in the visible region. The response of a thin film to light is usually expressed in terms of its dispersion or complex refractive index function as ~ nðkÞ¼ nðkÞ ikðkÞ; ð1Þ where n and k are the real and imaginary parts of the optical function respectively. The real part is known as refractive index and the second as attenuation index or extinction coefficient. The attenuation of the light intensity after travelling a distance l in the material follows an exponential function as I ¼ I 0 expð 2xk c lÞ; ð2Þ where x is the angular frequency and c the speed of the light. The absorption coefficient a (in cm –1 ), defined as the inverse of the ‘‘characteristic penetration depth’’, is linked to the extinction coefficient by the use of Eq. 2 as a ¼ 4pk k : ð3Þ Amorphous silicon behaves as a direct band gap mate- rial; because of its lack of translational symmetry, the law S. Halindintwali (&)  D. Knoesen  T. F. G. Muller  D. Adams  N. Tile Physics Department, University of the Western Cape, Private Bag X17, Bellville 7535, RSA e-mail: shalindintwali@uwc.ac.za C. C. Theron Materials Research Group, iThemba LABS, P.O. Box 722, Somerset West 7129, RSA R. E. I. Schropp SID-Physics of Devices, Utrecht University, P.O. Box 80.000, 3508 TA Utrecht, The Netherlands 123 J Mater Sci: Mater Electron (2007) 18:S225–S229 DOI 10.1007/s10854-007-9194-8