Formation of thallium antimony sulfide and thallium bismuth sulfide thin films by heating chemically deposited multi-lay
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Formation of thallium antimony sulfide and thallium bismuth sulfide thin films by heating chemically deposited multi-layer thin films Verónica Estrella, M. T. S. Nair, P. K Nair Centro de Investigación en Energía, Universidad Nacional Autónoma de México Temixco, Morelos-62580, México; [email protected] ABSTRACT Antimony sulfide (Sb2S3) and bismuth sulfide (Bi2S3) thin films of 200 nm thickness each were deposited from aqueous baths on glass substrates. Subsequently, thin films of thallium sulfide (Tl2S) with thickness around 120 nm were deposited on to these films from a bath containing thallium nitrate, sodium citrate, sodium hydroxide and thiourea. The multilayer films of Sb2S3-Tl2S, Bi2S3-Tl2S and Bi2S3-Sb2S3-Tl2S, thus produced, were heated in a nitrogen atmosphere around 300ºC. XRD studies confirmed the formation of TlSbS2, TlBiS2, Tl4Bi2S5 and TlSb3S5 compounds. Optical band gaps of these materials are 1.85 eV (TlSbS2), 0.15 eV (TlBiS2), and about 1 eV for the composite film (Tl4Bi2S5 + TlSb3S5). In the visible spectral region, the optical absorption coefficients of these materials are about 105 cm-1. Values of dark conductivity are 10-7 Ω-1cm-1 (TlSbS2), 10-4 Ω-1cm-1 for TlBiS2 and 10-6 Ω-1cm-1 for the composite film. All the films are photoconductive. 1.INTRODUCTION Ternary compounds of thallium include TlSbS2 and TlBiS2, with reported optical bandgaps (Eg) of 1.7 eV and 0.4 eV respectively [1]. In this paper we report the formation of these materials in thin film form by heating chemically deposited Sb2S3 + Tl2S and Bi2S3 + Tl2S thin films in nitrogen. We report here the deposition method for the binary semiconductor thin films and the structural, optical and electrical properties of the ternary films produced, which include a composite film: Tl4Bi2S5 + TlSb3S5. Since chemical bath deposition is a technique capable of depositing large-area thin films, application of these coatings in large area devices could be a possibility. 2. EXPERIMENTAL The basic concepts underlying the chemical bath deposition of semiconductor thin film have been discussed in previous papers [2-5]. The films were deposited on corning microscope glass slides (75 mm x 25 mm x 1 mm) on which a very fine thin film of ZnS of thickness < 20 nm was coated in a deposition bath prepared as in [6]. The chemical baths and the durations and temperatures of depositions employed in the present work for the different thin films along with the resulting thickness are given below: ZnS substrate film: 1.25 ml of 1 M zinc acetate solution, 1.1 ml of NH3/NH4OH (pH 10) solution, 1.4 ml of 50% triethanolamine (TEA), 0.5 ml 1 M thioacetamide (TA) solution and the rest deionized water to complete 200 ml of bath mixture; Duration, 22 h at 25ºC. Sb2S3 thin films [4]: 0.6 g SbCl3 dissolved in 2.5 ml acetone, 25 ml of 1 M sodium thiosulfate, and the rest de-ionized water to make up the volume to 100 ml; Duration of deposition, 4 h at 10oC; film thickness, 200 nm. Bi2S3 thin films [4]: 10 ml of ≈ 0.5M BiNO3.5H2O solution prepared in triethanolamine-water mixture, 8
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