Electrical properties of natural and synthetic pyrite (FeS 2 ) crystals
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We have prepared a series of synthetic n-type pyrite (FeS2) crystals by chemical vapor transport and high temperature solution growth. These and natural crystals were characterized with respect to Hall mobility, carrier concentration, and conductivity, and chemically analyzed by mass spectroscopy and atomic emission spectroscopy. The results are compared in detail to previous work on natural crystals. Conductivity and carrier concentration in our measurements varied between 0.05 and 3.5 (fl cm)"1 and between 6.9-1015 and 5.4-1017 cm"3, respectively. The peak mobilities have values ranging from 100 to 2000 cm2/Vs. We find that sulfur deficiency and a number of impurity elements, such as Si, Cu, and Al, can qualitatively account for the trends in the electronic properties.
I. INTRODUCTION
FeS2 crystallizing in the pyrite structure is well known and, in nature, a frequently occurring mineral. The pyrite structure comprises a family of numerous compounds, the general formula of which is MX 2 , where M can represent various elements of the groups la, Ib, Ha, lib, IVa, VIb, Vllb, VHIb, and X can be elements of groups Va or Via.1'2 Examples are SiP2, NiP2, PtAs2, AuSb2, CdO2, CuS2, MnSe2, and RuTe2. A total of approximately 50 different materials are known to crystallize in the pyrite structure. FeS2 attracts interest as a novel semiconducting material for photovoltaic energy conversion since it consists of nontoxic and abundant elements and can be prepared in the form of single crystals and thin films using a variety of growth methods. It has been shown to have a high absorption coefficient (6 • 105 cm"1 at 800 nm), is suitable for thin film applications, has high quantum efficiency of —90%, and an adequate band gap of 0.95 eV.3'4 The energy gap lies between the iron 3d levels. These split in the octahedral sulfur ligand field into a filled t2g and an empty eg band. Since these bands have nonbonding character, it is thought that occupancy changes within the bands do not greatly affect the chemical bond strength. This character may explain the high stability of pyrite observed in photoelectrochemical experiments.3 Polycrystalline thin films of pyrite have previously been prepared by MOCVD (Metal Organic Chemical Vapor Deposition) employing iron pentacarbonyl and sulfur as starting materials,5 or by a simple spray a)
Address correspondence to this author. J. Mater. Res., Vol. 5, No. 7, Jul 1990
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pyrolysis method using an aqueous iron(m) choloride/ thiourea solution.6 The photoconductivity of these films at present is poor because of inhomogeneities, surface states, grain boundaries, and growth defects. The present work focuses on the growth of single crystals of high purity and defined composition in order to study solid state properties such as electric conductivity, carrier concentration, and Hall mobility. The results of a study of these synthetic single crystals in comparison to natural samples are presented in this paper and a discussion is made as to the inte
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