Electrical conductivity studies on carbazole thin films K. Pisharady Sreejith * , C.S. Menon, C. Sudarsanakumar School of Pure and Applied Physics, Mahatma Gandhi University, P.D.Hills, Kottayam, Kerala 656 560, India article info Article history: Received 23 August 2007 Received in revised form 2 February 2008 Accepted 22 March 2008 Keywords: Thin films: vacuum deposition Organic semiconductors Electrical conductivity abstract Thin films of carbazole have been prepared using vacuum evaporation technique. The electrical con- ductivity studies are carried out in both low and high temperature regions and the activation energies have been determined. In the low temperature region the electrical conduction is due to hopping of charge carriers in a coulomb gap. Carbazole thin films have been used to fabricate capacitors and the variation of capacitance, dielectric constant; conductivity and dielectric loss in the frequency range between 100 Hz and 3.16 MHz are investigated. The effect of annealing on the dielectric properties is also investigated. The surface topography of the deposited films is studied using scanning electron micros- copy. Film morphologies are found to change by annealing. Ó 2008 Elsevier Ltd. All rights reserved. 1. Introduction The mechanism of charge conduction in organic semi- conductors have been the subject of intensive study due to the wide variety of applications of electro active and photo active polymers and their monomers [1]. Out of a number of polymers Poly (Vinyl Carbazole) (PVK) and Poly (N-Vinyl Carbazole) are found to be ef- ficient for use in organic light emitting diodes [2–4]. This work is devoted to carbazole which is a monomer of PVK. Use of this monomer provides comparable electroluminescence and rectifi- cation properties as that of its polymer unit while avoiding short- ening effect of polymer chain length during evaporation [5]. The investigation of new organic based dielectric materials is important for the fabrication gates in Organic Field Effect Transistors [6]. The key interest behind this fact is that a single material can simulta- neously used as a channel and a dielectric material since the elec- trical properties of these materials can alter by doping or by varying preparation conditions [7]. There are several methods to improve the electrical and structural properties of vacuum deposited thin films and thus achieving better performance such as post de- position annealing, substrate temperature and use of different substrates. In this work we investigate the effect of annealing on the electrical and structural properties of vacuum deposited car- bazole thin films. 2. Experimental details Natural carbazole is originally procured from Aldrich chemical company (U.S.A). Highly polished and thoroughly cleaned micro glass slides are used as substrates. The evaporation is carried out in a molybdenum boat using Hind Hivac12A4 vacuum coating ma- chine, under a base pressure of 6.68  10 4 Pa. Thicknesses of the films are counter checked by Tolansky’s multiple beam interference technique [8]. Thermal annealing is carried out in a specially designed furnace equipped with digital temperature controller cum recorder. The electrical conductivity study down to 127 K is done using a programmable Keithley electrometer (Model No. 617). The samples are mounted on the sample holder of the conductivity cell and it is cooled down below the room temperature using liquid nitrogen. The temperature in the conductivity cell is measured using a calibrated chromel–alumel thermocouple. Since carbazole is a photosensitive material, conductivity mea- surements are made in darkness and in a vacuum of the order 0.13 Pa to avoid any possible contamination of the films. A.C. elec- trical conductivity measurements are performed using a Hioki 3532 LCR Hi-tester. 3. Results and discussion 3.1. D.C. conductivity studies According to Davis and Mott [9] electrical conduction in a sem- iconducting material is due to three processes and the total con- ductivity can be expressed as s Total ¼ s Intrinsic þ s Excitation þ s Hopping (1a) The intrinsic conductivity is related to the generation of carriers which when thermally excited from Fermi level (E F ) to the con- duction band or valence band for electrons or holes as the case may be. The expression for intrinsic conductivity can be given as * Corresponding author. Tel.: þ910479 2309784. E-mail address: skpishar@yahoo.co.in (K.P. Sreejith). Contents lists available at ScienceDirect Vacuum journal homepage: www.elsevier.com/locate/vacuum 0042-207X/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2008.03.028 Vacuum 82 (2008) 1291–1295