Estimation of density of localized states of a-Se 1002x Sb x ®lms using electrical properties M.A. Majeed Khan, M. Zulfequar, M. Husain * Department of Physics, Jamia Millia Islamia, Jamia Nagar, New Delhi 110 025, India Received 18 September 2000; accepted 7 November 2000 Abstract The density of states (DOS) near the Fermi level is calculated using the dc conductivity (Mott parameters) and SCLC measurements data. The dc conductivity measurements on thin ®lms of a-Se 1002x Sb x (x  0,0.5,2.5, 5 and 10) are reported in the temperature range (219±375 K). At high temperature (314±375 K), the conduction occurs in the extended states while at lower temperature (219±314 K) the conduction due to variable range-hopping. The I±V measurements have also been done in a- Se 1002x Sb x at different electric ®eld. Space charge limited conduction (SCLC) has also been observed in the present system. q 2001 Elsevier Science Ltd. All rights reserved. Keywords: Density of states; Space charge limited conduction; dc conductivity; Amorphous semiconductor 1. Introduction Since the advent of electrophotography, amorphous sele- nium has become a material of commercial importance. Its various device applications, such as recti®ers, photocells, vidicons, and more recently, switching and memory devices [1,2], have found selenium-type materials offer attractive advantages. The effect of impurities on the transport and structural properties has been an important issue since the discovery of these glasses. The density of localized states in the mobility gap controls many physical properties of amor- phous semiconductors. The determination of density of states near the Fermi level N(E F ) has therefore been an important issue in these materials. The transport mechanics of charge carriers in amorphous semiconductors has been the subject of intensive theoretical and experimental investigation for the last few years. These studies have been stimulated by the attractive possibilities of using the structure disorder in amorphous semiconductors for the development of better, cheaper and more reliable solid state devices [1±3]. Due to their low conductivity, amorphous semiconductors are most suitable for high-®eld conduction studies, as the joule heating is negligibly small in these materials at moderate temperatures. Some such studies have been reported in chalcogenide glassy semicon- ductors [4±9] and the results have been interpreted in terms of the space-charge limited conduction. Several workers [10±13] have reported electrical and structural properties of chalcogenide glasses with different composition. The ®eld dependence of the conductivity is measured at different temperatures on vacuum-evaporated thin ®lms having different electrode separation. The dependence of DC conductivity on the electrode separation con®rms the presence of space charge limited conduction (SCLC) in the present samples. One of the most direct method for the determination of the density of localized states N(E F ) in the mobility gap involves the measurement of SCLC which can easily be observed at high ®elds in amorphous materials because of their low conductivity. Such a technique has already been applied to hydrogenated amorphous silicon (a-Si: H) [14±17] and it is generally believed that surface states do not come into the picture in this method. The ®eld effect technique as SCLC technique is not in¯uenced by surface states, unlike ®eld effect experiment where surface states may come into play. To ®nd the change in DOS, samples of low concentration of antimony have been used. The aim of the present work is to do SCLC measurements in an important glassy system a-Se 1002x Sb x , where properties have been found to be highly composition-dependent. These Journal of Physics and Chemistry of Solids 62 (2001) 1093±1101 0022-3697/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved. PII: S0022-3697(00)00288-2 www.elsevier.nl/locate/jpcs * Corresponding author. Tel.: 191-11-631-4631; fax: 191-11- 683-0337. E-mail address: mush.ph@jmi.ernet.in (M. Husain), zulfe_ph@ jmi.ernet.in (M. Zulfequar).