Structural, optical and electrical characterization of thermally evaporated TlBr thin films
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Structural, optical and electrical characterization of thermally evaporated TlBr thin films Natália Destefano and Marcelo Mulato Department of Physics and Mathematics, School of Science, Philosophy and Letters of Ribeirão Preto, University of São Paulo, Av. Bandeirantes 3900, 14040-901, Ribeirão Preto-SP, Brazil ABSTRACT This paper presents the study related to the production of TlBr thin films. Films produced by thermal evaporation present better structural properties than those produced by spray pyrolysis. The main XRD peak of the evaporated films correspond to the (100) crystalline plane. The structure is columnar as revealed by cross section SEM. The thickness decreases with increasing deposition height. Optical band gap of 3.0 eV and electrical resistivity of about 109 ȍcm were obtained. EDS reveals a reduction in the amount of Br in the final films. One order of magnitude was obtained for the photo-to-dark current ratio when irradiation in the medical diagnosis X-ray mammography energy range was used.
INTRODUCTION The properties of high atomic number and high band gap compound semiconductors for Xray and Ȗ-ray detection have been investigated over many years. Crystalline material obtained as bulk crystals and, more recently, polycrystalline thick semiconductor films have been evaluated for this intent [1-3]. Due to its high atomic number (Tl = 81 and Br = 35), high mass density (7,56 g/cm3) and intrinsic band gap (2.68 eV), thallium bromide (TlBr) is a very promising semiconductor candidate for room temperature radiation detectors [4-6]. In previous papers extensive studies on the purity of the TlBr starting powder and single crystal growth (bulk) have been reported by several authors [8-14]. However, there are few works related to the study of this material in the thin film polycrystalline form for applications in large area medical diagnosis equipments [1516]. In this work, spray pyrolysis and thermal evaporation were used as alternative methods for the deposition of TlBr polycrystalline films. Both methods present relative low cost and can be easily expanded for large areas. The aim of this work is to compare the different techniques and to study the influence of the main growth conditions on the final properties of TlBr films (structural, optical and electrical). EXPERIMENT Mili-Q water was used as solvent for the production of films by spray pyrolysis. The solution (0,10g of TlBr dissolved in 100g of water) was stirred at 70oC. Each deposition run used 1 cm x 1 cm substrates at 100oC, on top of which the solution was sprayed inside a positive pressure chamber. Nitrogen was the carrying gas at a rate of 8 l/min. The solution flow was 1/90 ml/s, for a nozzle-spray to substrate distance of 19cm. Films produced by thermal evaporation were based on 0.35g of the starting powder that sits on a tungsten crucible. The home-made thermal evaporator is illustrated in Figure 1. The same substrate sizes were used, and they were held at
room temperature in this case. The separation between evaporation boa
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