Rapid Crystallization of (001)-Textured TungstenDisulphide (WS2) Thin Films on Metallic Back Contacts:

An in situ Real-Time Diffraction Study

S. Brunken, K. Ellmer and R. Mientus1

Helmholtz-Zentrum für Materialien und Energie, Glienicker Straße 100, 14109 Berlin, Germany1Optotransmitter-Umweltschutz-Technologie e.V., Köpenicker Straße 325b, 12555 Berlin, Germany

We have shown recently by time-resolved, in situ energy-dispersive X-ray diffraction (EDXRD)analysis, that photoactive tungsten disulfide (WS2) films can be crystallized from X-ray amorphoussulfur-rich WS3+x films [1,2]. Such films exhibit a high structural as well as electronic quality,which allows to apply these highly absorbing semiconducting films as absorbers in thin film solarcells [3]. For obtaining highly (001)-textured WS2 films at crystallization temperatures below about700 °C, the addition of a thin metal film (nickel, cobalt or palladium) is essential, since, as we couldshow, the WS2 crystallization occurs via a solid-liquid-solid (SLS) process, which is similar to thewell known formation of whiskers or nanorods by the vapour-liquid-solid (VLS) process,discovered already in the 1960ies by Wagner and Ellis [4]. These investigations have beenperformed with insulating substrates, for instance oxidized silicon or quartz. However, in order touse the WS2 films as absorbers in thin film solar cells, a metallic back contact is needed to extractthe photocurrent. Up to now, high quality WS2 films could not be prepared on metallic layers ormetal sheets, at least not at low temperatures below about 800 °C [5].

In the following we show, that WS2 films can be crystallized or prepared by sulfidation of tungstenfilms also on metals. In a first experiment titanium nitride (TiN), which is a heat-resistant metalliccompound, was used as the back contact. The first crystallizations were not successful since thenickel film, used as crystallization promoter, diffused into the TiN, when the layer stack was heatedup to temperatures of 800 °C. Therefore, the essential NiSx-eutectic liquid could not form and theresulting WS2 films were nanocrystalline without a pronounced texture and hence of poor electronicquality. By using TiN:O or TiN:Ni films, where the grain boundaries were stuffed by oxygen ornickel, the diffusion of the nickel into the TiN was prevented, and the SLS process could take place,leading to the required highly (001)-textured WS2 films on a metallic underlayer.

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Figure 1: Time-dependence of the EDXRD spectra measured during the rapid sulfidation of a tungsten layeron oxidized silicon. The curve on the right-hand side shows the temperature characteristics. The X-ray

intensity increases from red, yellow, green to blue. Parameters: Sulfidation of 100 nm tungsten with 30 nmnickel on top in a 10 Pa H2S atmosphere.

Another possibility, to prepare metal/ WS2 layer stacks is the sulfidation of polycrystalline tungstenfilms, which is explained in the following. The tungsten films (100 nm) were deposited ontooxidized silicon by magnetron sputtering at an argon sputtering pressure, where the W films exhibitalmost no intrinsic stress. A 30 nm thick nickel film was evaporated on top of the W film and thisstack was annealed in a hydrogen sulfide atmosphere (10 Pa). The time-dependences of thedifferent diffraction and fluorescence peaks are shown in Fig.1 (coulor-coded). It can be seen, thatthe tungsten film is completely transformed into tungsten disulfide, when a temperature of morethan 600 °C is reached. It has to be mentioned, that this sulfidation does not occur without thenickel film, pointing to the decisive role of the NiSx eutectics, which has a formation temperature of637 °C. Fig. 2 shows the time evolution of the characteristic parameters of the diffraction peakstogether with the optical reflectance at a wavelength of 680 nm and the substrate temperature. It canbe seen, that the required (001)-texture (c-axis perpendicular to the surface) of the WS2 film occursonly after an intermediate formation of a thin (100)-oriented (c-axis perpendicular to the surface)layer (c-axis parallel to the surface). The halfwidth (FWHM) of the (002)- WS2 diffraction peak is ameasure of the grain size dg perpendicular to the surface. It is clearly visible, that the grain size (dg~ 1/FWHM) increases significantly after the onset of the growth of the (001)-textured WS2 film.The reflectance decreases suddenly when the surface of the tungsten film is sulfidized, which is dueto the rough (100)-textured WS2 layer on top of the tungsten and afterwards the completetransformation of the metallic film (high R) to a semiconducting film (low R).

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Figure 2: Time evolution of the characteristic parameters during the crystallization experiment, displayed inFig.1. The diffraction peak intensities of W (110), WS2 (100) and WS2 (002) are shown together with the

WS2 (002) peak position and half width. Also, the specular reflectance of the W/WSx film at a wavelength of680 nm (R) and the substrate temperature are depicted.

References[1] S. Brunken, R. Mientus, S. Seeger and K. Ellmer, J. Appl. Phys. 103 (2008) 063501.[2] S. Brunken, R. Mientus and K. Ellmer, Thin Solid Films 517 (2009) 3148.[3] H. Tributsch, Solar Ener. Mat. 1 (1979) 257.[4] R. S. Wagner and W. C. Ellis, Trans. Met. Soc. AIME 233 (1965) 1053.[5] A. Matthäus, A. Ennaoui, S. Fiechter, T. Kiesewetter, K. Diesner, I. Sieber, W. Jaegermann,

T. Tsirlina and R. Tenne, J. Electrochem. Soc. 144 (1996) 1013.