Two step synthesis of ultrathin transition metal tellurides

Transition metal dichalcogenides (TMDs) are rich material systems with numerous polytypes, band structures, and properties ranging from semimetallic to semiconducting, superconducting, and insulating.^1–5 Within the larger family of TMDs, the transition metal tellurides (TMTs) exhibit more structural variety than sulfides and selenides, which are typically only stable in the 2H phase, while TMTs are known to crystalize in 2H, 3R, 1T, 1T′, and 1T_d phases.^3,6 This assortment of polytypes results in a range of properties including more conventional semimetallic and semiconducting states as well as exotic states including type-II Weyl semimetals, type-II Dirac semimetals, charge density wave, and superconducting phase.^7–11 

Among TMTs, MoTe₂, WTe₂, and PtTe₂ have generated significant attention, where MoTe₂ is stable in two phases—2H (hexagonal) and 1T′ (monoclinic), WTe₂ only in the T_d (orthorhombic) phase, and PtTe₂ in the 2H hexagonal phase under ambient conditions. In the 2H phase, MoTe₂ is a semiconductor with a 1.1 eV bandgap, while in the 1T′ phase, it is a narrow band semiconductor with a 0.06 eV gap that undergoes a transition to the T_d phase, which is a Weyl semimetal, upon cooling.¹² T_d-WTe₂ is a type-II Weyl semimetal exhibiting a charge density wave phase and superconductivity,⁴ and 2H-PtTe₂ is a type-II Dirac semimetal that shows a thickness-dependent metal semiconductor transition.^9,13

As with most TMDs, the synthesis of high-quality large area TMT films is a major challenge. As such, a range of thin film synthesis techniques are currently being investigated including chemical vapor deposition (CVD), molecular beam epitaxy, sputtering, eutectic solidification, and tellurization of metal films.^6,14–18 Compared to sulfur and selenium, tellurium has a much weaker chemical activity, and the metal-telluride bonding energies are lower presenting obstacles to synthesis.⁶ One simple and versatile method to produce a variety of TMTs with minimal equipment or chemical exchange is by thermally reacting thin transition metal films with Te vapor.^18–20 In addition to its versatility, this two-step process is also highly scalable and fast, making it viable for numerous commercial industries.

In this paper, we present a systematic study on the synthesis of thin MoTe₂, WTe₂, and PtTe₂ films using a simple two-step process that involves first the deposition of continuous thin metal films by kinetically controlled pulsed DC magnetron sputtering followed by thermal tellurization. Using detailed structural and surface analysis, we identify temperatures necessary for producing the best crystal quality for each TMT film. Additionally, we investigate synthesis under various transport gases, demonstrating the necessary importance of H₂ during the tellurization process. From the temperature-dependent resistivity measurements, we observe the MoTe₂ semiconducting 1T′ to semimetallic T_d phase transition at 205 K. In T_d−WTe₂, we observe a semiconducting behavior, while 2H-PtTe₂ shows metallic behavior. Ellipsometry of these TMTs from UV to IR is presented providing the optical constants of these materials out to 12 μm.

Transition metal telluride films were deposited on c-plane sapphire using a two-step synthesis process involving first deposition of thin metal (Mo, W, and Pt) films by pulsed DC magnetron sputtering. Immediately following (<15 min), the ultrathin sputtered metal films were loaded into the CVD reactor and exposed to Te vapor at elevated substrate temperatures in a CVD reactor, forming MoTe₂, WTe₂, or PtTe₂, Fig. 1. Metal films (Mo, W, and Pt) were deposited in a custom-built sputtering system (base pressure of 3 × 10⁻⁹ Torr) from 1.3″ Mass sputter sources (Meivac, Inc.) held 7 cm from the substrate at a 30° angle using a pulsed power supply (Advanced Energy). The power to the magnetron was optimized at 90 W with a pulse frequency of 65 kHz and a positive pulse width of 0.4 μs at a pressure of 10 Torr under 25 SCCM argon flow.²¹ A deposition time of 4 s for Mo and W produced 1.5 and 2.8 nm. In the second step, metal films were converted to TMTs by thermal reaction with Te vapor. Metal films along with a boat filled with Te shot (>99.99%) were loaded into a CVD reactor [see Fig. 1(a) for schematic], evacuated to a pressure of <60 mTorr and then purged with H₂. The reactor pressure was then increased from vacuum to 500 Torr under a flow of 250 SCCM pure H₂, 5% H₂ in N₂ or pure N₂. Films presented below were tellurized under pure H₂ unless specifically stated otherwise. After reaching 500 Torr, the furnace temperature was ramped to the reaction temperature (400–700 °C) at ∼30°/min, held at temperature for 1 h, and rapidly cooled under flowing gas at ∼15°/min. Due to the high vapor pressure of Te, 700 °C was identified as the maximum reaction temperature, beyond which the Te source was depleted in <1 h.

Synthesized films were characterized using a variety of structural, surface, optical, and electrical characterization methods. Raman spectroscopy was used to identify the bonding nature and phase of TMTs using a Renishaw inVia system. An accumulation of 20 scans, each of 5 s duration, was collected using a 2.2 mW 488 nm excitation source, 20-μm slits, and 3000 line/mm grating for each measurement. To minimize degradation, measurements were carried out under a N₂ atmosphere in a Linkam stage. The crystal structure and film thickness were investigated using a PANalytical Empyrean x-ray diffractometer using 2θ/ω and grazing incident x-ray reflectance (XRR) scans. Surface morphology was characterized by atomic force microscopy (AFM) using a Bruker Dimension Icon AFM operating in tapping mode. Root mean squared (RMS) roughness presented in the Results section excluded the obvious tall particles. From raw data, the RMS roughness for Mo, W, and Pt films is 0.26, 0.22, and 0.23 nm. The temperature-dependent transport properties were characterized with a Lakeshore 7507 system using 1 cm² samples in van der Pauw geometry with In dot contacts. Complex optical permittivity ɛ = ɛ′ + iɛ″ was obtained by variable angle spectroscopic ellipsometry in both the visible and IR spectral ranges (JA Woollam RC2 and IR-VASE). Ellipsometry data from each of these instruments were modeled simultaneously using JA Woollam WVASE-32 software that incorporates a real permittivity offset and a series of Gaussian oscillators, which are used to model absorption using a Kramers–Kronig consistent line shape for ɛ′. Optical permittivity is related to the index of refraction by ɛ′ = n²–k², ɛ″ = 2 nk.

Thin Mo, W, and Pt metal layers were deposited on sapphire through kinetically controlled magnetron sputtering, Figs. 1(b) and 1(c). By modulating the applied power, the kinetic energy distributions of ionized species incident upon the substrate can be adjusted to overcome the kinetic limitations of thin film growth. The transfer of kinetic energy to atoms on the surface increases their diffusion length promoting 2D layer-by-layer growth over 3D island formation enabling atomically smooth thin metals films on oxide substrates. AFM height maps from Mo, W, and Pt films [Fig. 1(b)] show smooth surfaces with RMS roughness of 0.26, 0.17, and 0.18 nm. The lack of any pitting confirms the continuity at the nanoscale, which is critical to the formation of continuous TMT films as any inhomogeneity or defects in the metal film will propagate into the TMT film. XRR measurements of the as-deposited and tellurized Mo, W, and Pt films are shown Fig. 1(c). The thicknesses of as-deposited continuous metal films were determined to be 1.5, 2.8, and 2.3 nm for Mo, W, and Pt. Upon tellurization, the thickness increases significantly to 4.1, 7.2, and 8.1 nm for MoTe₂, WTe₂, and PtTe₂ films. This 2.5 to 3.5 times increase in thickness is consistent with similar reports on tellurization of metal films.²² 

Figure 2 shows XRD from MoTe₂, WTe₂, and PtTe₂ films tellurized at temperatures from 400 to 700 °C. From the 2θ/ω scans of these films, we observe only peaks consistent with (001) family from 1T′-MoTe₂, T_d-WTe₂, and 2H-PtTe₂ phases based on powder diffraction files reference 98-001-5431 (MoTe₂), 98-007-3323 (WTe₂), and 98-004-1373 (PtTe₂). The lack of any additional diffraction peaks shows the c-axis alignment of the films layered parallel to the substrate surface. 2θ/ω scan from the as-deposited metals films shows no diffraction peaks, suggesting our pulsed DC sputtered metal films are amorphous. For both the MoTe₂ and WTe₂ films, a significant increase in peak intensity and reduction in peak width are observed with increasing temperatures consistent with improved crystal quality, Figs. 2(a) and 2(b). The (002) peak position of MoTe₂ and WTe₂ increases toward 12.6° with increasing temperature due to a reduction in interlayer spacing and improved layer ordering. 2θ/ω scans from the PtTe₂ films show a slight improvement in crystal quality with increased peak intensity, reduction in peak width, and a shift in the (001) peak position toward 16.9° with increasing temperature up to 600 °C. Increasing the temperature above 600 °C results in films with reduced crystal quality at 650 °C and no observed PtTe₂ peaks at 700 °C setting an upper temperature for the PtTe₂ synthesis. Pt films tellurized at 700 °C only show peaks corresponding to the (110) orientation of Pt, suggesting decomposition and loss of Te. This temperature limit of <700 °C is consistent with findings by Manus et al. who found PtTe₂ would only form up to 650 °C due to Te loss at higher temperatures.²² 

Figure 3 shows the AFM analysis of MoTe₂, WTe₂, and PtTe₂ films tellurized at temperatures from 400 to 700 °C. From this, we see an increase in surface roughness from 0.39 to 0.79 nm for MoTe_2, 0.58 to 0.96 nm for WTe₂, and 0.69 to 2.92 nm for PtTe₂ films as the tellurization temperature is increased from 400 to 700 °C. As tellurization temperature is increased, the surface morphology of MoTe₂ and WTe₂ does not change significantly retaining the grainy structure observed in the insets of Figs. 3(a) and 3(b). In PtTe₂ films tellurized at 400 °C, AFM shows a grainy but continuous surface topography. As the temperature is increased, films become very rough and discontinuous, which is consistent with our electrical measurements that showed electrical continuity only in films tellurized at 400 °C.

Figure 4 shows the Raman spectra from MoTe₂, WTe₂, and PtTe₂ films annealed at temperatures from 400 to 700 °C. In MoTe₂ films, we observe eight peaks at 78.5, 93.7, 111, 128, 162.6, 190.8, 251, and 260.3 cm⁻¹ at all temperatures consistent with the 1T′ phase.^23,24 As the temperature is increased from 400 to 600 °C, the Raman spectra depict narrowing in the FWHM of all major peaks observed. Tracking the B_g mode at ∼162.5 cm⁻¹, the peak position blue shifts slightly (∼0.3 cm⁻¹) and FWHM reduces by ∼1.1 cm⁻¹ up to 600 °C, where beyond 600 °C little change is observed. This reduction in FWHM to 4.3 cm⁻¹ is comparable to high-quality bulk 1T′-MoTe₂ and a good indication of crystal quality.²⁵ Spectra from WTe₂ films show peaks at 82.7, 88.4, 110.2, 117.8, 134.8, 163.9, and 213.6 cm⁻¹ consistent with T_d.²⁶ Samples annealed at 400 °C only show two broad peaks near 161 and 215 cm⁻¹, indicating low crystal quality or incomplete tellurization. As the temperature is increased up to 600 °C, a clear improvement in crystal quality is observed with a minimum A₁ FWHM of 5.8 cm⁻¹ achieved at 650 °C, Figs. 4(c) and 4(d). Spectra from PtTe₂ films show peaks at 111.7 and 157 cm⁻¹ consistent with the 2H phase.¹⁷ The FWHM of the A_g mode remains mostly unchanged (4.3 cm⁻¹) with increasing annealing temperature from 400 to 600 °C above which the FWHM begins to increase. At 700 °C, no peaks corresponding to PtTe₂ are observed consistent with the XRD data.

To further explore the tellurization process and investigate the effect of H₂, we fixed the growth temperature at 600 °C and annealed under pure N₂, 5% H₂ in N₂, and pure H₂. Figure 5 shows the Raman and XRD results from MoTe₂, WTe₂, and PtTe₂ tellurized under these conditions. From these data, we clearly see the importance of H₂ in the tellurization process, which is most obvious when forming MoTe₂ and WTe₂, Figs. 5(a)–5(d). Under pure N₂ neither MoTe₂ nor WTe₂ films were formed; while under 5% H₂ in N₂, only low-quality MoTe₂ was formed. On the other hand, Pt films were successfully tellurized under all three ambients. Comparison of the Raman and XRD data shows a strong correlation with PtTe₂ crystal quality and the presence of H₂. This observed effect of H₂ is consistent with previous reports by Hynek et al.¹⁸ and Zhou et al.²⁷ on the formation of MoTe₂ and WTe₂ by similar two-step processes. As discussed by Zhou et al., the high Gibbs energy (compared selenides and sulfide), low eutectic melting points, and high transition metal melting points of WTe₂ and MoTe₂ create a significant challenge to synthesis through direct reaction. By introducing H₂, it reacts with the Te metal forming H₂Te, which is more reactive than Te vapor overcoming these challanges.²⁷ As we observe, the concentration of H₂Te is critical, where higher concentrations (i.e., more H₂) are required to form WTe₂ than MoTe₂ likely due to the higher Gibbs energy of WTe₂.²⁷ Additionally, Tong et al. found that PtTe₂ films could be formed in Ar without the presence of H₂, which may be attributed to Pt reactivity and lower formation energy of PtTe₂.^19,28

To study the electrical properties of the synthesized TMT films, the best quality (as determined by x-ray, AFM and Raman analysis) MoTe₂, WTe₂, and PtTe₂ films were characterized using temperature-dependent resistivity (ρ), Fig. 6. In the 650 °C MoTe₂ sample [Fig. 6(a)], ρ initially increases with decreasing temperature consistent with the semiconducting 1T′ phase identified using Raman and XRD.¹² At ∼205 K, ρ begins to decrease with decreasing temperature showing metallic-like behavior. Bulk 1T′-MoTe₂ undergoes phase transformation to the T_d phase, which is a type-II Weyl semimetal, at ∼240 K.²⁹ Recent measurements on thin film 1T′-MoTe₂ have shown that this transition temperature decreases as film thickness is reduced to the nanometer range due to the confinement of energy bands explaining a transition temperature of 205 K in our 4.1 nm thick films.³⁰ In the 650 °C WTe₂ sample, ρ increases with decreasing temperature over the full temperature range characteristic of a semiconductor or insulator as opposed to a decreasing ρ expected for the semimetallic T_d phase.³¹ We observed semiconducting behavior in WTe₂ films tellurized at 600 and 700 °C. Similar insulating behavior caused by rapid oxidation has been reported for thin T_d-WTe₂ flakes and films.^32–34 Oxidation of these films can be mitigated through postgrowth passivation layers.^24,35 In the 400 °C PtTe₂ sample, we observe a clear decrease in ρ with decreasing temperature consistent with the semimetallic nature of the 2H phase.¹⁷ These electrical results are comparable to other reports on PtTe₂ films synthesized by CVD³⁶ and reaction of Te with Pt metal films.³⁷ 

In Fig. 7, the optical constants n (refractive index) and k (extinction coefficient) for MoTe₂ (650 °C), WTe₂ (650 °C), and PtTe₂ (400 °C) films are plotted over the wavelength range of 0.21–12 μm. From the MoTe₂ film, the index n sharply increases in the visible up to about 0.6 μm followed by a more gradual increase up to about 10 μm. There are small oscillations in n due to absorption features in the extinction coefficient centered around 0.6, 2.2, and 4.9 μm. The observed features in the UV to NIR match with previous reports for bulk 1T′-MoTe₂, although our n and k values are lower.³⁸ In the WTe₂ film, we observe a steady increase in n from 1.8 to 4.4 with a small fluctuation at 0.53 μm corresponding to a small absorption peak in k, which increases steadily up to 2.8. These features are consistent with Buchkov et al. in a study of optical anisotropy of WTe₂.³⁹ In the PtTe₂ films, we observe an increase in n from 1.7 up to a maximum of 3.6 at ∼5 μm followed by a continuous decrease out to 12 μm. Fluctuations can be observed at 0.7, 1.3 and 5 μm over this range corresponding to their counterparts in k. First-principles calculations by Du et al. show similar features at 0.7 and 1.3 μm but do not predict our observed peak at 5 μm as their data do not extend to this spectral region.⁴⁰ The optical data presented in Fig. 6 match well with available literature, which only cover the UV to NIR regions. As such, our measurements, which extend well into the IR, provide important data for application of these TMTs beyond UV, visible, and NIR.

We have reported on the synthesis of few nanometer thick MoTe₂, WTe₂, and PtTe₂ films using a simple yet versatile two-step synthesis method, which involves thermal tellurization of ultrathin metal films. We find that Mo and W are significantly less reactive with Te than Pt requiring higher tellurization temperatures. Additionally, H₂ was identified as a critical part of the process due to its role in the formation of H₂Te, which is more reactive than Te vapor. Based on structural and surface analysis of this collection of films, we provide conditions for producing the best quality for each TMT film. Not only is this simple process suitable for a range of TMTs, but it can also be easily scaled to large areas and adapted for device processing.

This work was funded by the Air Force Office of Scientific Research under Award No. FA9550-19RYCOR050. The authors would like to thank T. Cooper for assistance with transport measurements.

The authors have no conflicts to disclose.

The data that support the findings of this study are available from the corresponding author upon reasonable request.