Large area synthesis, characterization, and anisotropic etching of two dimensional tungsten disulfide films
Introduction
Graphene-like two-dimensional (2D) semiconducting transition metal dichalcogenides (TMDs), especially tin disulfide (WS2), with sizeable band gaps have showed vast potential for electronics [1] and optoelectronics [2] due to exotic physical and electronic properties, recently. WS2 is a layered semiconductor material with a direct band gap of ∼1.95 eV [3] in monolayer form. For electronics, the presence of a band gap allows field-effect transistors (FETs) to have high on/off ratios [4], and its ultrathin nature allows the channel length to be reduced relative to those fabricated with conventional semiconductors [5]. For optoelectronics, the direct band gap produces high photoconductivity [6], and strong photoluminescence (PL) [3]. To explore new fundamental properties and to further develop their electronic and optoelectronic applications, synthesis of large-area atomically thin WS2 layers with uniform properties by a facile and scalable method is an essential requirement.
Recent top-down approaches including mechanical exfoliation [7] and liquid exfoliation [8] to obtain high crystalline WS2 flakes have attracted considerable attention. However, lateral dimensions of the flakes synthesized by the exfoliation methods are limited to few microns, which limits their applications in large-scale electronics and optoelectronics.
In contrast, chemical vapor deposition (CVD) techniques have great potential in producing large-area WS2 over macroscopic sizes, which are ideal for integration with current CMOS platform. Typically, CVD growth of WS2 includes vapor phase reaction or deposition of gaseous metal and chalcogenide feed stocks [9], [10], and sulfurization or decomposition of the pre-deposited films [11], [12].
Among the CVD techniques, sulfurization of the pre-deposited thin films is emerging as a quick and easy way to obtain large-area atomically thin WS2 films on insulating substrates. Recently, several studies have reported direct sulfurization of tungsten oxide (WO3) films deposited either by thermal evaporation [12] or by atomic layer deposition (ALD) [11]. However, to the best of our knowledge, large-area growth of atomically thin WS2 films on silicon dioxide (SiO2) substrates by sulfurization of the tungsten (W) films deposited by electron-beam (e-beam) deposition has not been reported yet.
In parallel with the recent advances in the synthesis of TMDs, numerous methods have been developed to enable their identification and characterization. PL spectroscopy is a powerful method of probing the electronic structure of TMDs [13]. Recent studies report [14] that molybdenum disulfide (MoS2) shows laser-dependent PL spectra, where the PL intensity and position are affected by varying the duration and intensity of laser irradiation. To the best of our knowledge, there is no systematic study concerning the effect of laser power on PL spectra of WS2, which is essential to probing the PL characteristics of WS2.
Recently, there has been an increasing interest in the nanoscale control of edge structures of TMDs. Nanoscale control of edge structures offers new pathways toward fine tuning the electronic, optical, chemical, magnetic, and catalytic properties of TMDs [15], [16], [17], [18]. One way to engineer edge structures of TMDs is heating them in an oxygen (O2) environment [19], [20]. For example, high density triangular pits with the molybdenum (Mo) or sulfur (S) terminated zigzag edges on the surface of MoS2 sheets have been obtained upon annealing them in air, which might arise from the anisotropic etching of the active MoS2 edge sites [19]. Such edge terminated MoS2 structures find applications in diverse catalytic reactions [21], [22]. As mentioned above, most studies so far have focused on the oxidative etching of MoS2. However, to the best of our knowledge, the etching behavior of WS2 films has yet to be experimentally studied.
In this study, we demonstrate the large-area synthesis of atomically thin WS2 films on SiO2 substrates. Briefly, the W films are deposited on SiO2 substrate by using e-beam deposition method. In the next step, the as-deposited W films are annealed at 500 °C and then sulfurized at 850 °C to obtain WS2 films. The fundamental morphologic, electronic, optical and chemical properties of the pre-deposited W films and WS2 films are investigated using optical microscopy, atomic force microscopy (AFM), scanning electron microscopy (SEM), X-ray photoemission spectroscopy (XPS), Raman spectroscopy and PL spectroscopy. We also systematically investigate the dependence of PL spectra of WS2 films on laser power. Furthermore, we introduce a simple method to etch WS2 films with well-oriented triangular-shaped pits by heating them in air.
Section snippets
Materials synthesis
WS2 films were synthesized via three steps: i) deposition of the W films, ii) annealing of the W films, and iii) sulfurization of the annealed W films. First of all, the W films with a thickness of ∼15 nm were deposited on SiO2 substrates (300 nm thick SiO2 layer deposited thermally on Si wafer) by e-beam deposition method. The deposition of the films was carried out in a load-lock chamber (Temescal BJD 1800) at a base pressure of ∼10−6 Torr and deposition pressure of ∼10−5 Torr, with an
Results and discussion
The morphology and thickness of the W films before and after annealing are determined by AFM analysis. The AFM height image (Fig. 1a) of the as-deposited films reveals island type morphology with a height of about 15 nm. The simultaneously recorded AFM phase contrast image (Fig. 1b) reveals an additional contrast on the surface of islands, which may be attributed to the ‘native oxide layer’.
The as-deposited W films are annealed in open air for 60 min at 500 °C. Annealing is found to induce
Conclusion
In summary, we report a facile method for the large-scale synthesis of atomically thin WS2 films with strong PL properties via sulfurization of the pre-deposited W films. Thermal annealing of the W films before sulfurization is found to substantially improve the quality of WS2 films in terms of thickness and uniformity. We also find that WS2 films show laser power dependent PL characteristics, where the PL peak shifts toward lower photon energy, and its intensity increases with laser power.
Acknowledgments
The authors would like to thank Dr. Darshana Wickramaratne (from UCSB) for valuable feedback. Financial support for this work was provided by the STARnet center C-SPIN (Center for Spintronic Materials, Interfaces, and Novel Architectures) through the Semiconductor Research Corporation sponsored by MARCO and DARPA. XPS data were acquired with equipment funded by the NSF, Grant No. DMR-0958796.
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