Transition metal tellurides (TMTs) are an exciting group of two-dimensional materials with a wide variety of polytypes and properties. Here, we demonstrate a simple and versatile two-step method for producing MoTe2, WTe2, and PtTe2 films via tellurization of thin metals at temperatures between 400 and 700 °C. Across this temperature range, monoclinic 1T′ phase of MoTe2, orthorhombic Td phase of WTe2, and hexagonal 2H phase of PtTe2 were formed. Based on x-ray diffraction and Raman analysis, temperatures greater than 600 °C were found to produce the best quality MoTe2 and WTe2. In contrast, lower temperatures (400 °C) were preferred for PtTe2, which becomes discontinuous and eventually decomposes above 650 °C. The presence of H2 in the tellurization process was critical to facilitate the formation of H2Te, which is known to be more reactive than Te vapor. In the absence of H2, neither MoTe2 nor WTe2 formed, and although PtTe2 was formed under pure N2, the crystal quality was significantly reduced. Temperature-dependent resistivity (ρ) measurements were performed on the best quality TMT films revealing all films to be highly conductive. MoTe2 showed metallic behavior up to 205 K where it underwent a phase transition from the semimetallic Td to semiconducting 1T′ phase. WTe2 exhibited a consistent semiconducting behavior with a small positive increase in ρ with decreasing temperature, and PtTe2 showed a metallic dependence from 10 K up to room temperature. Spectroscopic ellipsometry for TMT films provides complex optical constants n and k from ultraviolet to infrared.
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 1Td 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 Td (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 Td phase, which is a Weyl semimetal, upon cooling.12 Td-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 Td phase transition at 205 K. In Td−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.
III. RESULTS AND DISCUSSION
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, Td-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 MoTe2, 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 Bg 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 Td.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 Ag 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 Td 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 Td 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 Td-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.
Conflict of Interest
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.