Study on preparation, growth mechanism, and optoelectronic properties of highly oriented WSe 2 thin films
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S. Fiechter Abteilung Solare Energetik, Hahn-Meitner Institut, 14109 Berlin, Germany
R. Tenne Department of Materials and Interfaces, Weizman Institute, Rehovot 76100, Israel (Received 14 January 2000; accepted 16 August 2000)
Recently, highly oriented WSe2 thin films, with the c axis of the crystallites perpendicular to the substrate, were reproducibly obtained by interposing a Ni/Cr thin layer between the substrate and a WO3 precursor film. In the present work the preparation conditions were varied to elucidate the growth mechanism of such films. A model for the growth mode is proposed. Based upon this analysis, WSe2 thin films with improved crystalline and electronic properties were obtained. The photoresponse spectrum for photoelectrochemical cells with the WSe2 electrode immersed into a selenosulfate solution was measured. Quantum efficiency of 0.1% was calculated from this spectrum.
I. INTRODUCTION
Layered compounds of transition metal dichalcogenides (TMDC) attract steadfast attention due to their numerous applications. These compounds serve as solid lubricants1,2 and are also used as catalysts in petroleum refinement.3 More than two decades ago it was shown4 that TMDC are potentially useful for photovoltaic applications, since (i) they possess high absorption coefficient (␣ ⳱ 105 cm−1) for the visible light; (ii) Mo and W dichalcogenides demonstrate extreme stability against photoinduced corrosion due to the nature of their d–d transitions; and (iii) the layered structure of TMDC provides an atomically smooth inert van der Waals surface, which is free from surface states. Experiments with single crystals of transition metal chalcogenides showed that solar-to-electrical conversion efficiencies in the range of 10% to 13% for n-WSe2 in a polyiodide electrolyte could be achieved.5,6 For practical use as absorber layers in photovoltaic (photoelectrochemical) cells, thin films of these compounds are required. However, TMDC polycrystalline films usually grow with the basal {0001} plane of the crystallites perpendicular to the substrate. This orientation leads to poor optoelectronic properties, since diffusion along the field lines leads to severe charge carrier recombination at the prismatic {1010} plane of the crystallites. One way to avoid or considerably reduce the rate of recombination is to prepare films in which the basal plane of the crystallites is parallel to the substrate and 2636
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J. Mater. Res., Vol. 15, No. 12, Dec 2000 Downloaded: 15 Mar 2015
their in-plane dimensions (coherent lengths) are large. The crystal structure and both types of textures for WSe2 crystallites are represented schematically in Fig. 1. Highly textured films of WS2(MoS2) and WSe2 were prepared by high-temperature sulfurization and selenization, respectively, of tungsten trioxide deposited on a quartz glass with Ni or Ni/Cr intermediate layer.7–10 Interposing of a thin Ni layer film combined with various deposition techniques was successfully used for obtaining highly textured thin TMDC films by
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