Carrier dynamics in microcrystalline silicon studied by time-resolved terahertz spectroscopy L. Fekete, H. Ne ˇmec, F. Kadlec, P. Kuz ˇel * , J. Stuchlı ´ k, A. Fejfar, J. Koc ˇka Institute of Physics, Academy of Sciences of the Czech Republic, Na Slovance 2, 182 21 Prague 8, Czech Republic Received 30 August 2005; received in revised form 27 January 2006 Abstract We present results of optical pump – terahertz probe experiments applied to a set of thin film silicon samples on sapphire substrates. Structure of the films varied from amorphous to fully microcrystalline. Picosecond time scale evolution of electrical transport properties of photoexcited samples is investigated and discussed. Three different mechanisms are found to contribute to the dynamical conductivity at terahertz frequencies. Ó 2006 Elsevier B.V. All rights reserved. PACS: 73.50.Gr; 73.61.Jc; 78.66.Jg; 78.47.+p Keywords: Silicon; Solar cells; Dielectric properties, relaxation, electric modulus; Chemical vapor deposition; Microcrystallinity 1. Introduction Microcrystalline silicon (lc-Si:H) has attracted consider- able attention during the last years for use in photovoltaic applications, as well as active matrix displays and other opto-electronic elements [1]. This is due to the fact that it combines the opto-electronic properties of crystalline sili- con with those of low-cost thin-film technology. Indeed, sil- icon can be grown on cheap substrates, like glass or plastic materials, by various deposition methods including PECVD and hot-wire deposition. The improvement of effi- ciency of the layers suited for applications is critically dependent on the detailed knowledge of charge carrier dynamics. The deposition conditions of the thin films allow for the tuning of such film characteristics as grain size, their arrangement and degree of crystallinity, which are expected to influence strongly the lifetime and mobility of carriers. The time-domain terahertz (THz) spectroscopy is a con- tact-free method highly sensitive to mobile charges, and, consequently well suited for the dynamical investigation of photo-excited semiconductors [2,3]. In this contribution we report on optical pump – THz probe (OPTP) experi- ments for a series of 1 lm-thick lc-Si:H layers with a var- iable crystallinity deposited on sapphire substrates. The experiment allows one to obtain broadband transient THz spectra of the films as a function of the pump–probe delay and, subsequently, to identify different contributing mechanisms. 2. Experimental All samples were prepared by plasma-enhanced chemi- cal vapor deposition (PECVD) at 250 °C and 70 Pa. The deposition parameters were adjusted to obtain samples with different degree of crystallinity from amorphous sample to a highly microcrystalline one. The sample thick- nesses were determined by surface profile measuring. Crys- tallinity of the samples was determined from the integrated 0022-3093/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2006.02.099 * Corresponding author. E-mail addresses: fekete@fzu.cz (L. Fekete), kuzelp@fzu.cz (P. Kuz ˇel). www.elsevier.com/locate/jnoncrysol Journal of Non-Crystalline Solids 352 (2006) 2846–2849