Effect of Bi incorporation on defect state density in a-Se 85x Te 15 Bi x thin films Ambika Sharma à , P.B. Barman Department of Physics, Jaypee University of InformationTechnology, Waknaghat, Solan, H.P. 173215, India article info Article history: Received 14 August 2008 Received in revised form 13 January 2009 Accepted 20 January 2009 PACS: 61.43 Dq 81.15 Ef 71.55 Jv Keywords: Chalcogenide glasses Space charge limited conduction Density of states abstract Current voltage (I–V) characteristics have been studied at various temperatures in vacuum evaporated thin films of a-Se 85x Te 15 Bi x (x ¼ 0, 1, 2, 3, 4, 5) glassy alloys. Ohmic behavior is observed at low electric field while at high electric field (E10 4 V/cm) current becomes superohmic. An analysis of the experimental data reveals that in case of samples with 0–2 at% Bi, the experimental data fit well with the theory of space charge limited conduction (SCLC) for uniform distribution of localized states in the mobility gap. Such type of behavior is not observed at higher concentration of Bi in the present glassy system. Due to high conductivity in these samples, joule heating due to large currents may prohibit the SCLC mechanism. Density of localized defect states near Fermi level is determined for these samples by fitting the data in the theory of SCLC. The results indicate that density of defect states increases with Bi addition to binary Se 85 Te 15 alloy. This is explained in terms of electronegativity difference of Bi as compared to host elements. & 2009 Elsevier B.V. All rights reserved. 1. Introduction In contrast to amorphous silicon and other group IV tetra- hedrally bonded semiconductors, the chalcogenide glasses have attracted much attention due to their interesting optical, electrical and physical properties which can be controlled by changing their chemical composition [1,2]. This property of chalcogenide glasses makes these materials useful for technological applications including phase change memories [3]. Amorphous selenium (Se) rich semiconducting alloys are of particular interest due to their current uses as photoconductors in high definition TV pick up tubes and particularly in digital X-ray imaging. When Te is added in a-Se, it enters the structure by an isoelectronic substitution so that its coordination remains two. Addition of Te brings about changes in Vander Waals bonds or interchain secondary bonds, because the Te atom is larger than the Se atom and has more electrons in its orbital. One can therefore argue that increase in secondary bonding between chains is due to Te addition. Te addition will also increase the valence alteration pair (VAP) type defects [4], connecting neighboring chains and limiting molecular mobility. The composition Se 85 Te 15 has been studied by several authors [5–7], because of its ease of fabrication in the form of bulk and thin films. The glassy alloys of the Se–Te system are widely used for various applications in many fields as optical recording media because of their excellent laser writer sensitivity, xero- graphy and electrographic applications such as photoreceptors in photocopying and laser printing, infrared spectroscopy and laser fiber techniques [8–10]. Moreover the technological application of Se–Te glassy alloy enhances due to their greater hardness, higher crystallization temperature and higher photosensitivity [11]. The addition of third element expands the glass forming area and also creates compositional and configurational disorder in the system. Several authors [12–14] have reported the effect of impurities in various chalcogenide glasses. The addition of impurities like Bi has produced a remarkable change in the optical, electrical and thermal properties of chalcogenide glasses especially in Se 85 Te 15 glassy alloy. While the optical band gap of Se 85 Te 15 binary alloy decreases with Bi incorporation, the glass transition temperature is found to increase [15]. Density of the system also found to increase with increase in Bi concentration [16], indicating the rigidity and hence density of defect states (DOS) also increase. The conductivity of chalcogenide glasses changes from p to n type due to Bi addition [17–20]. The effect of this impurity on DOS has always been a subject of curiosity, as the knowledge of this parameter is a key factor in chalcogenide glasses for determining the semiconducting properties of these materials. One of the most useful method for the determination of DOS involves the measurement of space charge limited conduction (SCLC). In chalcogenide glasses, the DOS reported by different authors [21–25] ranges from 10 13 to 10 17 eV 1 cm 3 . The present paper reports the SCLC measurements in Se 85x Te 15 Bi x (x ¼ 0, 1, 2, 3, 4, 5) thin films. ARTICLE IN PRESS Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/physb Physica B 0921-4526/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2009.01.026 à Corresponding author. Tel.: +919817164795; fax: +911792245362. E-mail address: ambikasharma2004@yahoo.co.in (A. Sharma). Physica B 404 (2009) 1591–1594