Characterisation of Polycrystalline Thin Films of Tellurium
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CHARACTERISATION OF POLYCRYSTALLINE THIN FILMS OF TELLURIUM
A K RAY,
C A HOGARTH** AND R SWAN***
*School of Engineering Information Technology, Sheffield Hallam University, Pond Street, Sheffield Si lWB, UK. **Physics Department, Brunel University, Uxbridge, Middlesex UB8 3PH, UK. ***Electrical & Electronic Engineering Department, South Bank University, Borough Road, London SEl OAA, UK. ABSTRACT
Structural, electrical and optical characterisations were performed on tellurium film deposited under high vacuum conditions on ultrasonically cleaned Coming glass 7059 substrates. SEM and TEM studies reveal the polycrystalline nature of deposited tellurium films. At steady state high field conditions, space charge limited conduction is believed to have occurred due to the existence of a fast decaying density of trapping states in the energy gap. The DC conductivity is believed 6to exhibit a TV' 4 law dependence below room temperature, giving a value of 2x10-1 eV/m 3 for conduction states close to the Fermi level. From the power law dependence on frequency of AC conductivity, the barrier for bipolaron hopping is estimated to 0.33eV. Optical absorption in 9nm thick tellurium samples is due to direct transitions both allowed and forbidden producing two edges, the first one at 4eV and the other at about 0.5eV, respectively. INTRODUCTION
Tellurium may be regarded as a useful main channel material in the fabrication of thin film charge-coupled devices for the operation in the UV frequency regime [1]. Polycrystalline tellurium films are known to exhibit some anomalous behaviour with regard to the dependence of carrier-concentration on temperature [2]. According to ordinary semiconductor theory, the concentration of carriers increases while their mobility decreases with the rise of temperature. It has been pointed out that, as the temperature is raised from 80K, the carrier-concentration of polycrystalline tellurium films decreases to a minimum value at 176K and then begins to increase. The activation energy is found to be a temperature-dependent quantity, giving values of 0.0095eV and 0.18eV, respectively for low and high temperature regions. Annealing has also an effect on the activation energy [3]. These observations are explained in terms of the occupancy level of traps, the existence of which is inherent with the polycrystalline nature of Te films. The majority of reported work on electrical properties of tellurium is primarily concerned with thin films evaporated at a moderate vacuum in the order of 10rtorr. We have developed a technique of depositing amorphous tellurium films on insulating substrates under ultra-high vacuum (UHV) conditions. Results of our preliminary work on electrical properties of UHV thin tellurium films have recently been reported in an earlier paper [4]. In order to study intrinsic electrical properties of polycrystalline Te films, further DC and AC measurements have been performed at different temperatures on tellurium films evaporated under high vacuum onto Coming glass 7059 substrates held at
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