J. Phys. Chem Solids Vol 57, No. 12, pp. 1947-1955, 19% zyxwvut Pergamon PII: SOO22-3697(96)00073-X Copyright 0 1996 Elsevier Science Ltd Printed in Great Britain. All rights resewed CO22-3697/96 $15.00 + 0.00 KINETICS OF ELECTRICAL CONDUCTIVITY ENHANCEMENT IN BISMUTH SULPHIDE THIN FILMS. PART II: OPTOELECTRONIC PROPERTIES (FILM) AND PHASE TRANSFORMATIONS (POWDER) UNDER OXYGEN ANNEALING M. E. RINCbN, RAdL SUAREZ and P. K. NAIR Photovoltaic Systems Group, Lab. de Energia Solar&St. de Investigaciones en Materiales, Universidad National Autbnoma de MCxico, Temixco, Mor. 62580, Mixico (Received I October 1995; accepted 8 February 1996) Abstract-The optoelectronic properties of oxygen annealed bismuth sulphide thin films have been determined and correlated to the physico-chemical transformations of the powder studied by Differential Scanning Calorimetry (DSC). A maximum in photoconductivity of 0.54 R-’ cm-’ was obtained at 170°C with short annealing times. The temperature dependence of the saturation rate indicates the adsorption of oxygen up to 230°C. At higher temperatures oxygen gets chemisorbed and strongly diminishes the overall mobility of the film.The physico-chemical transformations observed by DSC on bismuth sulphide powder show two first order irreversible transitions in the temperature range of 200-300°C. The first has been identified with crystallization of bismuthinite (B&S,) and Bi$. The second transition is quite broad and uncovers the decomposition of BiSz into Bi and S and the incipient formation of bismuth oxides and sulphates. At these temperatures, the conversion of bismuth crystals into sulphates and oxides leaves bismuthinite as the only conductive medium, explaining the decrease on conductivity at higher temperatures and duration of annealing. Oxidation of bismuthinite takes place in two stages between 4OO-5OO”C, a high activation energy followed by a non-activated and fast oxidation process. Keywords: A. chalcogenides, A. semiconductors, A. thin films, D. electrical conductivity. 1. INTRODUCTION In Part I of this work [ 11,we have studied the kinetics and mechanisms of conductivity enhancement of bismuth sulphide thin films annealed in low pressure argon and hydrogen environments. In Part II we will present the optoelectronic properties of oxygen annealed thin films and the oxidation kinetics of bismuth sulphide powder to complement previous studies in oxidizing environments, in which we have shown that air annealing in the lOO-250°C range can increase the photoconductivity of bismuth sulphide films obtained by chemical bath deposition by a factor of lo4 [2]. The study of the structural changes in bismuth sulphide films due to oxygen will allow us to complete the transformation scheme presented in Part I and have a better understanding of the struc- ture-property relationship of annealed bismuth sul- phide thin films. 2. EXPERIMENTAL DETAILS 2.1. Deposition of thin films The usual procedure for the deposition of metal sulphide films by chemical bath has been described elsewhere [3]. We used Bi(NO& * 5H20 as the source of Bi3+ ions, triethanolamine, TEA, as the com- plexing agent, and thioacetamide as the source of S2- with a molar ratio of: Bi3’(0.005M) /S 2-(0.04 M)/TEA (0.6 M). The deposition was done at room temperature for 20-24 h. This resulted in uniform, specularly reflecting (black mirror) films with typical thickness of 0.2pm, determined using the Alpha Step 100 instrument. The powder collected at the bottom of the beaker was filtered and dried at room temperature. Previous X-ray diffraction studies (XRD) indicated the amorphous nature of the as-deposited powder. 2.2 Post-deposition treatments Annealing of the films was done in a tubular oven operating below atmospheric pressure (10-l torr) under oxygen flow. The temperature profile of the oven with respect to the internal position of the sample along its axial axis (x-axis) was obtained under different gases and pressures. Two ways of doing the thermal treatment were chosen: (i) inde- pendent run, in which a single sample was placed in 1947