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# Nucleation and coalescence of tungsten disulfide layers grown by metalorganic chemical vapor deposition

Haonan Tang<sup>a</sup>, Sergej Pasko<sup>a</sup>, Simonas Krotkus<sup>a</sup>, Thorsten Anders<sup>a</sup>, Cornelia Wockel<sup>a</sup>, Jan Mischke<sup>a</sup>, Xiaochen Wang<sup>b</sup>, Ben Conran<sup>b</sup>, Clifford McAleese<sup>b</sup>, Ken Teo<sup>b</sup>, Sreetama Banerjee<sup>c</sup>, Henry Medina Silva<sup>c</sup>, Pierre Morin<sup>c</sup>, Inge Asselberghs<sup>c</sup>, Amir Ghiami<sup>d</sup>, Annika Grundmann<sup>d</sup>, Songyao Tang<sup>d</sup>, Hleb Fiadziushkin<sup>d</sup>, Holger Kalisch<sup>d</sup>, Andrei Vescan<sup>d</sup>, Salim El Kazzi<sup>e</sup>, Alain Marty<sup>f</sup>, Djordje Dosenovic<sup>f</sup>, Hanako Okuno<sup>f</sup>, Lucie Le Van-Jodin<sup>g</sup>, Michael Heuken<sup>a,d,*</sup>

## ARTICLE INFO

Communicated by Markus Weyers

### Keywords:

A1. Crystal morphology

A1. Low dimensional structures

A1. Nucleation

A3. Metalorganic chemical vapor deposition

B1. Nanomaterials

B1. Sulfides

## ABSTRACT

The nucleation of tungsten disulfide WS₂ crystallites and coalescence behaviour of monolayer WS₂ films grown on C-plane sapphire have been studied in a commercial AIXTRON close coupled showerhead metalorganic chemical vapor deposition (MOCVD) reactor using tungsten hexacarbonyl and di-tert-butyl sulfide as precursors. The influence of sapphire substrates offcut (both orientation and angle), as well as the substrate annealing conditions and growth temperature, on the nucleation and size of the WS₂ domains has been investigated. A higher nucleation temperature combined with increased substrate annealing temperatures was found to promote an increase in the WS₂ domain size. The dependence of surface coverage and coalescence behaviour of the WS₂ domains on the growth duration has also been determined, allowing monolayer WS₂ films with < 10 % bilayer to be obtained. Crystalline quality of the coalesced WS₂ films has been assessed by grazing incidence XRD and misorientation mapping using 4D-scanning transmission electron microscopy, showing a slight in-plane misorientation of the crystallites in the WS₂ monolayer. The electrical performance of the coalesced WS₂ films was assessed using backgated TLM structures demonstrating mobility values of up to 5 cm²/Vs and a high Ion/Ioff ratio greater than 10⁸.

## 1. Introduction

A fundamental understanding of the nucleation and coalescence mechanisms of two-dimensional (2D) layers by metalorganic chemical vapor deposition (MOCVD) is of critical importance for the growth of high crystalline quality 2D materials with acceptable electrical performance suitable for industrial scale applications. Wafer-scale MOCVD growth of transition metal dichalcogenide (TMDC) layers on sapphire substrates has been demonstrated by many research groups in recent

years [1–7], however, the understanding of the growth mechanisms for the optimization of the crystalline quality is still missing. One approach to obtain high crystal quality TMDC layers is unidirectional growth of nucleated crystallites that coalesce to form a layer without antiphase grain boundaries. This has been demonstrated for MOCVD of WS₂ [7] and WSe₂ [8] by optimization of nucleation, ripening and growth conditions, or for CVD growth of MoS₂ using MoO₃ and S as precursors on C-plane sapphire substrates with specific offcut (towards A-plane) [9].

Herein, we report on the preliminary study of nucleation of the

<footnote><sup>a</sup> AIXTRON SE, Dornkaulstraße 2, 52134 Herzogenrath, Germany</footnote>

<footnote><sup>b</sup> AIXTRON Ltd, Anderson Road, Buckinghamway Business Park, Cambridge CB24 4FQ, UK</footnote>

<footnote><sup>c</sup> imec, Kapeldreef 75, 3001 Leuven, Belgium</footnote>

<footnote><sup>d</sup> Compound Semiconductor Technology, RWTH Aachen University, 52074 Aachen, Germany</footnote>

<footnote><sup>e</sup> Applied Materials-NUS Advanced Materials Corporate Lab, 5 Engineering Drive 1, 117608 Singapore, Singapore</footnote>

<footnote><sup>f</sup> University Grenoble Alpes, CEA, CNRS, IRIG-MEM/Spintec, F-38000 Grenoble, France</footnote>

<footnote><sup>g</sup> University Grenoble Alpes, CEA, Leti, F-38000 Grenoble, France</footnote>

<footnote>* Corresponding author at: AIXTRON SE, Dornkaulstraße 2, 52134 Herzogenrath, Germany.
E-mail address: M.Heuken@aixtron.com (M. Heuken).</footnote>
crystallites and coalescence behaviour of WS₂ monolayers grown by MOCVD in an AIXTRON commercial research scale close coupled showerhead (CCS®) MOCVD reactor.

The variation of growth conditions such as growth temperature, duration, as well as sapphire substrate offcut and substrate annealing conditions (temperature, carrier gas, duration) on the nucleated WS₂ crystallites size and surface coverage was investigated.

## 2. Materials and methods

MOCVD growth of WS₂ layers was performed in an AIXTRON CCS® 2D R&D system in a 19 × 2'' configuration. Tungsten hexacarbonyl (W (CO)₆, WCO, 94 nmol/min) was used as the tungsten (W) source and di-tert-butyl sulfide (S[C(CH₃)₂], DTBS, 410 µmol/min) was used as the sulfur (S) source. WS₂ was deposited on sapphire substrates in a one-step continuous growth process at temperatures in the range 700–850 °C at 50 hPa reactor pressure with high S/W molar ratio of > 4000 and growth duration varying from 900 s to 3300 s. To investigate the effect of substrate miscut on the nucleation and coalescence of WS₂, C-plane (0001) sapphire substrates having different offcut angles from C-plane (0.2° towards the M-plane (C/M) and 0.25° or 1° towards the A-plane (C/A)) were used. To determine the influence of substrate preparation conditions on the crystallite nucleation and coalescence, sapphire substrates were treated with different annealing conditions: in H₂ at 1200 °C and in N₂ at 1200 °C or 1250 °C.

As-grown WS₂ domains were studied using scanning electron microscopy (SEM, Zeiss Sigma) and the domain size, as well as monolayer/bilayer coverage, were analyzed from the SEM images using a Python-based image segmentation program utilizing Otsu's thresholding concept (Supporting information Section 2). Atomic force microscopy (AFM) measurements were performed using Horiba's OmegaScope. The measurements were performed in AC tapping mode using MikroMasch Si tips with nominal radius of 8 nm and spring constant of 0.5 N/m. The coalesced WS₂ films were characterized by Raman spectroscopy (Renishaw InVia, 532 nm laser; 100x objective lens; 1800 mm⁻¹ grating). WS₂ crystal quality was assessed by grazing incidence X-ray

diffraction (GIXRD) with a SmartLab Rigaku diffractometer equipped with a copper rotating anode beam tube (Kα = 1.54 Å) operating at 45 kV and 200 mA; parallel in-plane collimators of 0.5° of resolution were used both on the source and detector sides. The diffractometer is equipped with multilayer mirrors on the incident beam side and Kβ filter on the diffracted beam side; the incidence angle was set to 0.28°, close to the critical angle of the substrate (Fig. S1). The areal density of W and S of the optimized WS₂ monolayers for the estimation of layer stoichiometry was determined by Rutherford Backscattering Spectroscopy (RBS) using 2 MV tandem 6SDH Pelletron accelerator (NEC) with a He⁺ ion beam with energy ranging from 500 keV to 3.5 MeV; the scattering chamber contains a 5-axis goniometer which uses a silicon surface barrier as ion beam analyzer. 4D-Scanning Transmission Electron Microscopy (4D-STEM) experiments were performed in a Titan Ultimate STEM equipped with Merlin detector, and an orientation map was generated by a custom-made script based on the template matching algorithm discussed in [10]. The electrical performance of the coalesced WS₂ films was assessed using backgated transmission line method (TLM) structures allowing extraction of both field-effect mobility as well as TLM mobility.

## 3. Results and discussion

A coalescence study of WS₂ films grown on 0.2° off (C/M) sapphire substrates is presented in Fig. 1 and Fig. S2. Sapphire substrates were annealed at 1200 °C for 10 min in pure H₂ atmosphere and growth was performed at 700 °C temperature. Terminating the growth process after 900 s leads to a partial sapphire coverage (<40 %) with nucleated triangular shaped WS₂ crystallites (Fig. 1a). WS₂ domains are mostly well aligned and have two main antiparallel orientations (0° and 60° domains), similar to results reported in other works on TMDC growth on sapphire [7]. With longer growth times, the domains grow in lateral size (Fig. S3a) resulting in an increased WS₂ coverage (Fig. 1b, 1c) before coalescing into a closed WS₂ film. In addition to lateral growth of WS₂ domains, second layer islands start to appear for the growth times exceeding 2000 s (i.e. at approximately 60–70 % monolayer coverage) as evidenced by additional features appearing in SEM and AFM

<figure><img src="image_3.png" /><figcaption>**Fig. 1.** WS₂ film coalescence study for growth at 700 °C on 0.2° off (C/M) sapphire: a, b) SEM image for 900 s and 3300 s growth time, respectively (Scale bar is 400 nm); c) estimated monolayer and bilayer coverage dependence on growth duration, with dashed lines illustrating the trend; d) Raman spectra of coalesced WS₂ layers at different positions on 2'' wafer; e) typical photoluminescence (PL) spectrum of a coalesced WS₂ monolayer; f) µ-Raman mapping of 2LA(M)/E₁g and A₁g peak difference.</figcaption></figure>
<figure><img src="image_4.png" /><figcaption>Fig. 2. (a) AFM topography micrograph of WS₂ film on sapphire after 2600 s growth; (b) height profiles taken at different positions indicated in (a).</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>Fig. 3. a, b) GIXRD characterization of the WS₂ monolayer: a) radial scans; b) azimuthal scans; c) in-plane misorientation map by 4D-STEM.</figcaption></figure>

topography images (Fig. 2a). We estimate the WS₂ monolayer thickness to be approximately 0.9 nm (Fig. 2b) in line with previous reports on WS₂ on sapphire [7]. Both monolayer and bilayer coverage increases in an approximately linear trend until monolayer coalescence (Fig. 1c). We find a growth time of 3300 s produces a fully closed monolayer film with 7% bi-layer.

Typical Raman spectroscopy characterisation results of coalesced films are shown in Fig. 1d. WS₂ Raman modes related to in-plane (2LA/ $E_{2g}^1$) and out-of-plane ($A_{1g}$) vibrations are clearly distinguished (Fig. S4 a,b,c). As reported in [7], the commonly reported Raman peaks of WS₂ are E'(M), 2LA(M), $E_{2g}^1$(Γ), and $A_{1g}$(Γ). The difference between 2LA/$E_{2g}^1$ and $A_{1g}$ may help determining the number of layers in the WS₂ film on sapphire. The same Raman peak fitting is carried out as shown in Fig. S4a, with a similar distance between 2LA/$E_{2g}^1$(Γ) and $A_{1g}$(Γ) around 65.7 cm⁻¹ (63.3 cm⁻¹ in [7]). In addition, according to the peak position difference in a micro-Raman mapping over a 20 × 20 µm² area (see Fig. 1f), the variation of peak position difference is small ($Δ(2LA/\bar{E}_{2g}^1 - A_{1g}) = 65.7 ± 1.9$ cm⁻¹), indicating uniform WS₂ film formation in the measured area. Raman spectra are similar at different positions across the 2'' wafer indicating uniform WS₂ deposition and slight variations may be related to minor layer coverage variations (e.g. bilayer islands). The absence of Raman peaks in the range 1300–1600 cm⁻¹ (in which D and G modes related to graphitic or amorphous carbon are expected) indicates that the degree of carbon incorporation has been minimized by optimization of the deposition process. Carbon co-deposition in MOCVD of 2D-TMDC layers is a common feature which is attributed to organic chalcogen sources used in the process; however, this may be reduced by an increased H₂ fraction in the carrier gas flow [11]. A typical photoluminescence spectrum of a fully coalesced WS₂ monolayer is presented in Fig. 1e, with the dominant PL band with peak wavelength of 620 nm

(1.995 eV) which corresponds to the intralayer exciton (typical for a WS₂ monolayer).

The areal density of W atoms (1.08·10¹⁵ at/cm²) and S atoms (2.25·10¹⁵ at/cm²) in the coalesced WS₂ monolayer films on 0.2° off (C/ M) sapphire has been determined by RBS, resulting in a calculated layer stoichiometry (S:W ratio) of ~ 2.08, close to the theoretical stoichiometric ratio of 2 (Fig. S5).

The crystal quality and the layer orientation of monolayer WS₂ grown under these conditions were assessed by in-plane GIXRD measurements (Fig. S1). Both radial XRD scans (Fig. 3a) and azimuthal XRD scans (Fig. 3b) have been recorded. The full width at half maximum (FWHM) values of the WS₂ (110) and (010) peaks in the azimuthal scans were in the range of 3.8°-3.9°, indicating reasonable crystal quality of heteroepitaxial WS₂ layers grown on sapphire. In-plane misorientation of the crystallites in the WS₂ monolayer has been investigated by 4D-STEM mapping (Fig. S6) on a representative sample area of 4 µm² (Fig. 3c). A stripe-like pattern with characteristic length of about 45 nm (equivalent to a miscut of 0.27°, which is within the specifications of nominal sapphire miscut value of 0.2° ± 0.1°) is visible in the orientation map of WS₂ suggesting a step-edge-guided nucleation of WS₂ on sapphire terraces. Moreover, a slight misorientation of the crystallites (FWHM 4.2°, inset of Fig. 3c) has been detected that correlates well with the width of the azimuthal XRD peaks performed on the wafer scale sample.

The electrical properties of the coalesced WS₂ layers have been assessed from backgated TLM device structures displayed in an optical microscope image in Fig. 4a. These structures allow to estimate both field-effect and TLM mobility. For electrical evaluation the WS₂ layers are transferred from the sapphire growth template to a 50 nm SiO₂/Si target coupon employing a PMMA/TRT based manual transfer
<figure><img src="image_6.png" /><figcaption>Fig. 4. Electrical characterization of monolayer WS₂ films: a) micrograph of TLM structure; b) estimated field-effect mobility for different channel lengths; c) drain current for different gate voltages Vg.</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>Fig. 5. WS₂ domains at higher growth temperature 850 °C on varying offcut sapphire annealed at 1250 °C/N₂: a) 0.2° off (C/M); b) 0.25° off (C/A); c) 1° off (C/A).</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>Fig. 6. a) WS₂ edge length on sapphire substrates with various offcuts and b) Raman spectra of WS₂ layers on 0.2° (C/M) sapphire (carbon signatures marked).</figcaption></figure>

technique. Basic 2PP- electrical devices, with only minimal process impact, are obtained with a single litho print by e-beam lithography. Contact metals are deposited by evaporation, using a Ni/Pd stack for contact metal, and the device is finalized by lift-off. To avoid impact from a channel patterning process, mechanical channel isolation is performed [12]. TLM test structures are obtained with channel lengths (L<sub>ch</sub>) ranging from 0.5 µm to 20 µm, while the channel width (W) is kept constant at 100 µm. In Fig. 4b, 2PP-field-effect mobility extraction is done for selected channel lengths. For long channels, these values of ~ 5 cm²/Vs overlap with TLM mobility extracted, identifying the contact resistance not to be a dominant factor in the device performance. Moreover, high I<sub>on</sub>/I<sub>off</sub> ratios > 10⁸ (Fig. 4c) were obtained for the WS₂ monolayers with only small variations over the coupon.

For further improvement of crystal quality, different sapphire sub-
strate offcuts and orientations were considered, as the C/A offcut of
sapphire substrates has previously been reported to promote unidirec-
tional growth of MoS₂ crystallites [9]. Also, it has been published that a
certain offcut of the sapphire is needed to obtain uniform electrical re-
sults across the wafer [12]. By using similar process conditions (700 °C
and 900 s growth duration) as for C/M substrates, no unidirectional WS₂

domain orientation was observed on C/A sapphire (Fig. S7), however, in
both cases, two preferential antiparallel (180° rotated) orientations of
the crystallites were present. Significantly higher crystallite sizes were
found (up to 190 nm edge length) when growing on 0.25° (C/A) sap-
phire, as compared to 140 nm edge length on C/M sapphire with similar
offcut angle (Fig. 5 and Fig. 6a, blue boxplots). This indicates that
substrate offcut orientation is indeed an important parameter affecting
the WS₂ nucleation processes. A low value of C/A offcut promotes the
formation of slightly wider sapphire terraces (as visible in Fig. S7),
which may promote surface diffusion thus resulting in larger crystal
domains. However, increasing the offcut value to 1° reduces the terraces
width, again resulting in smaller domain sizes.

We speculate that higher growth temperatures are needed to further
promote adatom surface diffusion in order to achieve an expected step-
edge-guided growth mode with preferential WS₂ orientation (e.g.
observed for MoS₂ in [13]). An increase of growth temperature to 850 °C
leads to more pronounced triangular shapes of WS₂ crystallites (Fig. 5)
and to larger domain sizes (Fig. 6a, red boxplots). The triangular WS₂
domains on the 0.2° (C/M) offcut sapphire are generally oriented
perpendicular to the sapphire terraces formed by the substrate annealing
step, whereas the domains on the 0.25° (C/A) offcut sapphire are ori-
ented parallel to the terraces. This means the substrate offcut orientation
may be an instrument to control the terraces and domain growth.
However, in both cases, antiphase triangular domains (rotated mutually
by 180°) are present in approximately equal ratio, in disagreement with
[13] for MoS₂ grown on 0.25° off (C-A) offcut sapphire.

In addition to higher growth temperature, sapphire pre-anneal con-
ditions were also somewhat different (1250 °C in N₂), therefore the
differences observed in the present study cannot be fully attributed to
the higher growth temperature. The increase in temperature, however,
had no clear effect on crystallite orientation (again no unidirectional
crystallites were obtained, however clear two antiparallel orientations
are observed (antiphase domains)). Further increase in temperature was
not considered in this work, as with current conditions, significant
carbon incorporation into WS₂ films grown at 850 °C was already
observed by Raman spectroscopy (Fig. 6b).

4. Conclusions

An AIXTRON research scale MOCVD reactor has been successfully applied for the heteroepitaxial growth of WS₂ layers. The coalescence point of nucleated WS₂ domains was found by an almost linear increase of surface coverage with growth duration. The obtained monolayer WS₂ had preferential orientation according to the in-plane GIXRD measurements: the films (FWHM values of the WS₂ (1 1 0) and (0 1 0) peaks in the azimuthal scans were 3.8°-3.9°), indicating reasonable crystal quality of heteroepitaxial WS₂ layers. A slight in-plane local misorientation of the crystallites in a continuous WS₂ layer has been detected by 4D-STEM with an angle distribution of 4.2° (FWHM). RBS measurements show the grown WS₂ layers to be stoichiometric, with S:W ratio ~ 2.08, and reasonable electrical properties (field-effect and TLM mobility ~ 5 cm²/Vs with high I<sub>on</sub>/I<sub>off</sub> ratios > 10⁸) were obtained for the WS₂/sapphire layers grown under optimized conditions. Finally, sapphire substrate offcut angle and orientation as well as growth temperature were shown to significantly impact initial stages of WS₂ growth. Thus, understanding and controlling crystallite nucleation and coalescence by substrate surface preparation as well as different growth conditions are essential in further improving crystalline quality of TMDC films grown by MOCVD.

CRediT authorship contribution statement

**Haonan Tang:** Software, Investigation, Writing – original draft, Writing – review & editing. **Sergej Pasko:** Investigation, Writing – original draft, Writing – review & editing. **Simonas Krotkus:** Investigation, Writing – original draft, Writing – review & editing. **Thorsten Anders:** Project administration. **Cornelia Wockel:** Investigation. **Jan Mischke:** Investigation, Writing – review & editing. **Xiaochen Wang:** Investigation. **Ben Conran:** Investigation. **Clifford McAleese:** Methodology. **Ken Teo:** Funding acquisition. **Streetama Banerjee:** Investigation. **Henry Medina Silva:** Investigation. **Pierre Morin:** Investigation. **Inge Asselberghs:** Resources. **Amir Ghiami:**

Investigation. **Annika Grundmann:** Methodology. **Songyao Tang:** Investigation. **Hleb Fiadziushkin:** Investigation. **Holger Kalisch:** Investigation, Methodology. **Andrei Vescan:** Resources. **Salim El Kazzi:** Conceptualization. **Alain Marty:** Investigation. **Djordje Dosenovic:** Investigation. **Hanako Okuno:** Investigation. **Lucie Le Van-Jodin:** Project administration. **Michael Heuken:** Supervision, Conceptualization, Resources, Funding acquisition, Writing – review & editing.

**Declaration of Competing Interest**

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

**Data availability**

Data will be made available on request.

**Acknowledgements**

AIXTRON acknowledges funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No. 952792 (2D-EPL) and No. 824123 (SKYTOP). AIXTRON and RWTH (CST) acknowledge BMBF (Neurotec II (16ME0403 and 16ME0399) and NeuroSys (03ZU1106AD) projects). imec contribution was done in the frame of imec IIA core CMOS programs and received funding from European Union's Graphene Flagship grant agreement No. 952792. CEA-IRIG acknowledges, for 4D-STEM analysis, funding from the European Research Council under the European Union's H2020 Research and Innovation program via the e-See project (Grant No. 758385), and LANEF framework (No. ANR-10-LABX-0051) for its support with mutualized infrastructure.

**Appendix A. Supplementary material**

Supplementary data to this article can be found online at https://doi.org/10.1016/j.jcrysgro.2023.127111.

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