1470 G. Pimenta et al.: Thin Pyrite Films Prepared by Sulphurization of Electrodeposited Iron Films Thin Pyrite Films Prepared by Sulphurization of Electrodeposited Iron Films G. Pimenta, V. Schroder, and W. Kautek Laboratory for Chemical Surface Technology, Federal Institute for Materials Research and Testing (BAM), Unter den Eichen 87, W-lo00 Berlin 45, Germany Electrochemistry J Elementary Reactions J Endotaxy J Heteroepitaxy J Semiconductors J Spectroscopy, Photoelectron 1 Spectroscopy, X- ray J Thin Film Growth A simple procedure for the production of pyrite (FeS2) thin films starting from the pure elements is presented. A pure, highly (1 10) oriented polycrystalline iron precursor layer could be successfully electro- deposited. Direct sulphurization of those films on Mo foils resulted in pyrite with (200) oriented grains embedded in a randomly oriented matrix. No other iron sulphide phase like e.g. marcasite was observed. XPS shows the presence of at least 97% FeS2. Depth profiles demonstrate the stoichiometric homogeneity across the films. 1. Introduction Pyrite is the most abundant of all sulphides. This fact makes it an attractive material for the low cost mass pro- duction of solar cells. It needs, however, development of thin film preparation methods and characterization of its phys- ical and chemical properties. Several routes have been used for the synthesis of pyrite, namely Chemical Vapour Trans- port, CVT [l - 31, Metal Organic Vapour Deposition, MOCVD [4], and Spray Pyrolysis, SP [S]. Recently, pyrite films were prepared by the sulphurization of iron oxides, SIO [6], and plasma-assisted sulphurization of vaccum evaporated thin iron films [7]. It is expected that well oriented polycrystalline precursor transition metal films are advantageous in respect to en- dotactic growth of semiconducting solar grade chalcogen- ides [S, 91. There is a high manyfold demand on the prepara- tion techniques of these precursor layers: they have to be low cost (a), applicable to large area production (b), and must exhibit high standards in crystallinity and morphology (c). Electrodeposition has the potential to combine all these criteria. In contrast to vacuum techniques, electrocrystalli- zation enables morphological control not only by concen- tration (resp. pressure), temperature, and mass transfer rate, but also by the unique possibility of direct manipulation of the crystallization step by potential control. There are two principle methods involving electroplating steps: (a) the direct electrodeposition of the semiconductor from an electrolyte that contains all the semiconductor com- ponents, as e.g. In2Se3 or CuInSe2 [lo, 111, or (b) the elec- trodeposition of the transition metal followed by annealing in an appropriate atmosphere as has been shown e.g. for InP or In& [12,13]. Method (a) has been, so far, unsuc- cessful for the production of FeS2with pyrite structure [14J There have been no studies on the electrocrystallization of iron [15,16] since more than four decades, expect some industrial metallurgical application reports. In this work, we studied and developed a well oriented thin polycrystalline iron film electrodeposit, and report on the conversion of such films by sulphurization. 2. Experimental The substrates used were sputtered chromium on polyimide (Renker GmbH & Co. KG, Freiburg, FRG), molybdenum 0.127 mm thick foils (99.97% from Alfa Products), polycrystalline copper, and IT0 on glass. The molybdenum and IT0 substrates were de- greased with aceton. All electrolytes were prepared with bidestilled water. Electrodepositions were run under nitrogen in a glove bag. Iron thin films were deposited from a 0.35 M iron (11) ammonium sulphate solution (Merck p.a.) at temperatures of 80"C, 20 C and 5°C and different current densities. Electrodeposition parameters were varied to optimize the films' crystallinity. Optimized iron films with 300 nm thickness calculated from the coulombic charge passed through the cell were deposited by a galvanostatic two-step pro- gram at 5°C (Fig. 1). The use of the strong and short pulse has turned out necessary in order to improve adherence and to obtain homogeneous coverages. The electrodeposited iron films were kept under nitrogen until they were used in conversion experiments. rl il= 100 mA.cm.1 - L iz=5mA.crn.z t,= 1.5 s tz= 155 s Fig. 1 Galvanostatic double-pulse wave used in the electrodeposition of thin iron layers The sulphurization system (Fig. 2) consisted of a tubular oven (Heraeus) controlled via a temperature controller (Eurotherm) pro- vided with a proportional band control heater. A quartz tube re- actor chamber was equipped with an inlet/outlet for nitrogen and sample insertion. The temperature was monitored both by the nor- mal oven temperature controller, and a thermocouple (Philips cro- mel-alumel type K) inside the oven within ca. 1 cm distance from the sample which was connected to a home built cold junction compensation temperature controller. The system was degassed after the introduction of both the sam- ples and the sulphur boat with nitrogen (5.0 N) for 30 min, and Ber. Bunwnges. Phys. Chem. 95 (1991) No. // 0 VCH Verlagsgesellschaft mbH. W-6940 Weinheim. 1991 O~)OS-9021~9/1/1/1-1470 $ 3.50+.25/0