{'fraction_non_alphanumeric': 0.03407105416423995, 'fraction_numerical': 0.02766453115899825, 'mean_word_length': 4.953206239168111, 'pattern_counts': {'":': 0, '<': 0, '<?xml version=': 0, '>': 0, 'https://': 0, 'lorem ipsum': 0, 'www.': 0, 'xml': 0}, 'pii_count': 0, 'substrings_counts': 0, 'word_list_counts': {'cursed_substrings.json': 0, 'profanity_word_list.json': 0, 'sexual_word_list.json': 0, 'zh_pornsignals.json': 0}}
Plasma enhanced atomic layer deposition of two-dimensional WS2 from WF6, H2S and H2 plasma

Single and few layers of semiconducting transition metal dichalcogenides (MX2) hold promise as twodimensional (2D) channel material for future low power logic applications [1]. Semiconducting MX2 have significant band gaps, as opposed to graphene where a band gap is absent. Besides their atomic scale thickness, the MX2 surface is chemically inert and lacks dangling bonds. Therefore, few charge traps and scattering centers are expected to inhibit charge transport. For these 2D materials to be integrated into large-scale integrated circuits, a large-area deposition technique becomes indispensable. The ALD principle ensures growth control at the atomic level, which is essential to control the deposition of single and few layered MX2. The feasibility of growing thin films of MX2 by atomic layer deposition (ALD) has been shown in limited reports [2,3]. Nevertheless, growth control down to a single monolayer still has to be proven. Furthermore, to ensure a layered crystalline structure, high temperature anneal treatments are necessary [2]. Finally, also the surface chemistry is not well understood, in particular the control of the partial reduction or oxidation of the metal in the MX2 ALD process [4]. In this work, we demonstrate a plasma enhanced ALD (PEALD) process for WS2 films, using WF6 and H2S in combination with H2 plasma. Deposition is performed on 300mm Si substrates covered with either 20 nm TiN by physical vapor deposition (PVD) or 30 nm ALD-grown and annealed Al2O3 layers. Deposition was first investigated on TiN substrates because of the known reactivity of WF6 and TiN [5]. The initial WF6 reaction, forming metallic W and volatile TiF4, is confirmed to be self-limiting at temperatures between 250 and 350oC (Rutherford backscattering Spectrometry, RBS). However, even though nucleation is enabled, sequential exposures of WF6 and H2S precursors do not yield a steady-state WS2 growth component at temperatures between 300 and 450°C (RBS, Fig. 1a). The deposition stops after ~20 cycles, resulting in the formation of a tungsten subsulfide layer (WSx, with x=~0.7), with ~1.6E+15 W atoms/cm2. We propose that the reduction of W6+ (in WF6) does not occur through reaction with H2S. The steady-state growth of tungsten disulfide (WS2) is enabled by PEALD, using reaction cycles consisting of WF6, H2 plasma and H2S exposures (RBS, Fig. 1a). After the initial growth enhancement, both W and S content increase linearly with the number of reaction cycles. The S content increases almost two times faster than the W content, indicating WS2 deposition with a better stoichiometry. The deposition occurs in a broad temperature window, between 250 and 450oC. Interestingly, the PEALD sequence also yields nearly stoichiometric WS2 when applied on Al2O3 substrates. On polycrystalline Al2O3, up to 6 monolayers of WS2 have been grown, with a S to W ratio of 1.8-2 and good thickness uniformity over the 300mm substrate. The WS2 signatures are clearly observed in the Raman spectrum (Fig. 1b). A polycrystalline layered 2D structure is indicated by X-ray diffraction and Transmission Electron Microscopy. The WS2 planes are mainly oriented parallel to the substrate surface. The impact of the crystallinity and roughness of the Al2O3 layers on the nucleation, growth and 2D structure of WS2 is being further analyzed.