Reactive magnetron sputtering of tungsten disulfide (WS2−x) films: Influence of deposition parameters on texture, microstructure, and stoichiometry

Abstract

Tungsten disulfide ͑WS 2−x ͒ films ͑0.07ഛ x ഛ 0.7͒ were prepared by reactive magnetron sputtering from a tungsten target in rare gas/H 2 S atmospheres and at substrate temperatures up to 620°C. The nucleation and growth of the films were investigated by in situ energy dispersive x-ray diffraction ͑EDXRD͒ and by ex situ techniques such as electron microscopy, elastic recoil detection analysis, and x-ray reflectivity. From the EDXRD analysis it was found that the films always nucleate with the ͑001͒ planes, i.e., the van der Waals planes, parallel to the substrate surface. For high deposition rates and/or low substrate temperatures a texture crossover from the ͑001͒ to the ͑100͒ crystallite orientation occurs during the growth. High deposition rates, low substrate temperatures, or low sputtering pressures lead to a significant lattice expansion of the crystallites in the c direction ͑up to 3%͒. This is most probably caused by a disturbed or turbostratic film growth induced by the energetic bombardment during film deposition. Reflected and neutralized energetic ions ͑Ar 0 ,S 0 ͒ from the tungsten target and negative ions ͑S − ͒ accelerated in the cathode dark space constitute the main sources of the energetic bombardment leading to crystallographic defects. The energy of these particles can be tailored by ͑i͒ thermalization between target and substrate in the sputtering gas or ͑ii͒ by a reduction of the discharge or target voltage, respectively, by high frequency excitation of the plasma. Films deposited under favorable conditions with respect to low particle energies and at substrate temperatures higher than 200°C exhibit a significant sulfur deficiency of up to about 5 at. % compared to the stoichiometric composition of WS 2 . This is ascribed to an energetic particle bombardment-induced sulfur desorption from the growing films.

Preparation routes based on magnetron sputtering for tungsten disulfide (WS2) films for thin‐film solar cells

“…However, this can only be done by installing a gas inlet and waste disposal system for the H 2 S (a toxic gas), which requires additional technical efforts. This could also be the reason why, to our knowledge, reactive magnetron sputtering of WS x (and also MoS x ) films has been carried out only in the group of Ellmer et al [26,61]. Some of these results, which were achieved by different plasma excitation frequencies (DC, 13.56 MHz and 27.12 MHz) are summarized in the following.…”

“…a pronounced (001) texture with only a few crystallites with the "dendritic" (100/101) orientation. In a recent article on our experiments on direct reactive magnetron sputtering of WS 2-x the results were summarized as follows [61]:…”

“…Reactive magnetron sputtering from a metallic tungsten target in a sputtering gas mixture of argon/H 2 S was used to prepare WS x films [26,27,61]. The advantages of a metallic target are (i) its better mechanical stability, (ii) the lower costs and (iii) the possibility to adjust the stoichiometry ratio of the grown films by changing the argon-to-H 2 S ratio.…”

PLD Grown Polycrystalline Tungsten Disulphide (WS2) Films

“…In thin-film form, WS 2 has been grown by various techniques like sputtering [17], pulsed laser deposition (PLD) [18], chemical bath deposition (CBD) [19], electrodeposition technique [20], and many others. The growth of polycrystalline films with large grain sizes in inclined orientation at relatively low growth temperature is necessary to make efficient solar cells [21].…”

Donor and acceptor levels in semiconducting transition-metal dichalcogenides

“…The chemical potentials are defined by the experimental growth conditions, which can range from metal-rich to sulfur-rich. Bulk MoS 2 and WS 2 are often sulfur deficient, [21,22] even though sulfur excess has been reported as well [23] Here, we will assume {µ Mo/W ,µ S } are in the metal-rich extreme ie. the system is in equilibrium with a hypothetical reservoir of metallic Mo (or W).…”