Transport properties of amorphous hydrogenated silicon±carbon alloys J.A. Schmidt * ,1 , M. Hundhausen, L. Ley Institut f ur Technische Physik, Universit at Erlangen-N urnberg, Erwin-Rommel-Street 1, 91058 Erlangen, Germany Abstract We measure the electron and hole drift mobility and the recombination lifetime in hydrogenated amorphous silicon± carbon alloys by using the moving photocarrier grating (MPG) technique. Samples have been prepared by plasma enhanced chemical vapor deposition of methane/silane mixtures under 90% hydrogen dilution. The optical gap of the samples varies between 1.72 and 2.09 eV. We ®nd a decrease in the electron drift mobility when small amounts of carbon (leading to optical gaps <1.95 eV) are incorporated into the silicon matrix, and a saturation when the carbon content is such that the optical gap of the samples is larger than 1.95 eV. Both electron and hole drift mobilities are correlated with the Urbach energy. We ®nd an increase in the recombination lifetime when the optical gap increases from 1.72 to 1.85 eV, which we assign to a widening of the band tails. A further increase in the gap decreases the recombination lifetime, due to an increase in the density of defects acting as recombination centers. Ó 2000 Elsevier Science B.V. All rights reserved. 1. Introduction In recent years, amorphous hydrogenated sili- con±carbon (a-Si 1x C x :H) alloys have received at- tention because of their potential application in electronic devices [1]. Carbon incorporation into a-Si:H increases the optical gap and decreases the photoconductivity [1,2]. Attempts have been made to ®nd deposition conditions which improve the electrical properties [3]. It has been found that glow-discharge a-Si 1x C x :H deposited under high hydrogen dilution ( P 90%) has a smaller defect density and a larger photoconductivity gain [4,5]. To optimize the material, a detailed knowledge of the transport properties is needed. Up to the mo- ment most of the studies of transport properties have dealt with the steady-state photoconductivity [1±5]. A few groups have measured the electron drift mobility [6±9], and, to our knowledge, only two groups have measured the hole drift mobility [8,9] as a function of the carbon content. With the moving photocarrier grating (MPG) technique [10,11] the drift mobilities of electrons (l n ) and holes (l p ) and the common recombina- tion lifetime (s R ) can be measured independently. In this work we use that method to study the transport properties of a-Si 1x C x :H samples over a range of carbon content. 2. Experimental The a-Si 1x C x :H samples have been deposited by plasma enhanced chemical vapor deposition Journal of Non-Crystalline Solids 266±269 (2000) 694±698 www.elsevier.com/locate/jnoncrysol * Corresponding author. Tel.: +49-9131 85 27259; fax: +49- 9131 85 27889. E-mail address: javier.schmidt@physik.uni-erlangen.de (J.A. Schmidt). 1 Present address: INTEC (UNL-CONICET), G uemes 3450, 3000 Santa Fe, Argentina. 0022-3093/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 9 9 ) 0 0 7 8 7 - 5