Post-patterning of an electronic homojunction in atomically thin monoclinic MoTe₂

Transition metal dichalcogenides (TMDs) have received great attention beyond graphene because of their diverse structural and electronic properties [1–3], which are promising for applications in two-dimensional (2D) optoelectronic [4, 5], spintronic [6–8], and valleytronic devices [9–12]. Unlike graphite or hexagonal boron nitride (h-BN), the 2D layered TMDs show thickness-dependent electronic changes, especially in their atomically-thin geometry [13–15]. Direct-to-indirect bandgap transitions, semimetal-to-semiconductor transitions and inversion symmetry breaking phenomena have been demonstrated based on thickness control [15–17]. Along with unique strong spin–orbit coupling and low-dimensionality, such transitions are expected to derive synergetic applications for a future 2D-based industry [18–20].

Controlling the thickness of large-area 2D materials has been an important long-standing issue in the 2D community [21, 22]. For graphene and hexagonal TMDs, chemical vapor deposition (CVD) [23–25] and liquid exfoliation [26, 27] methods have been developed to address the issue. However, CVD and other chemical methods have limitations when it comes to producing thickness-controlled monoclinic TMDs (e.g. MoTe₂) [28–30]; inevitable impurities and oxidation occur in the processes [31, 32]. This interferes with the fine thickness-control-based phase transition of monoclinic TMDs, which is essential for electronic and energy device applications of the materials. Moreover, recent studies have demonstrated that to control the phase transition of monoclinic TMDs, thickness control needs to be atomic-scale and widely homogeneous.

While a laser-thinning process has been developed for monolayer graphene and MoS₂ [33, 34], these previous studies did not consider thickness-dependent electronic change or homojunctions. This optical technique has not yet been applied to monoclinic TMDs due to the absence of target single-crystalline materials and a lack of understanding regarding the thickness-dependent electronic change, as well as the thinning mechanism. It has been known that a local electronic change between semimetal and semiconductor can be achieved by a local thickness control of monoclinic TMDs [13, 35]. Moreover, the scalable optical technique (laser-thinning) can be extended to the semiconductor industry-level resolution (~10 nm) that has been also realized by optical lithography technique. However, it remains a challenging undertaking to realize homojunction devices with monoclinic TMDs based on the local thickness control.

Here, we report on a post-patterning technique using laser-irradiation to fabricate an atomically-thin homojunction device from monoclinic (known as a 1T' phase in other literatures) MoTe₂. The term post-patterning means that the laser-irradiation is applied to pre-patterned device channels. This method enables a scalable design of electronic homojunctions based on thickness-dependent electronic change without any structural change, in contrast to the previous exfoliation-driven [13] and lattice-change-driven devices [39].

After fabricating a MoTe₂ device, local laser-irradiation was used to decrease the thickness of the monoclinic MoTe₂ channel down to 3 layers in desired areas, with vertical and lateral space resolutions of 5 nm and 500 nm, respectively; the vertical resolution of 5 nm indicates the resolution of thickness control and the 500 nm is originated from the optical resolution by a laser with a wavelength of 532 nm. We used the laser with a wavelength of 532 nm to verify the crystal structures via reliable Raman spectroscopy during post-patterning process. Therefore, the spatial resolution of our post-patterning is just limited by the long wave length that can be resolved in the future study. Raman and photoemission spectra of the laser-thinned monoclinic MoTe₂ regions showed there were no structural changes or oxidation. By converting the thick metallic channel into a thin semiconducting channel, the trivial semimetal device was transformed into a semiconducting homojunction device. This study found that the laser-patterned thin flake region possessed physical properties similar to exfoliated few-layered single-crystalline monoclinic MoTe₂. Therefore, we propose this method as a practical way to design homojunction devices with 2D materials.

High quality single-crystalline monoclinic MoTe₂ samples were synthesized as described in our previous reports [13]. Two different polymorphs of MoTe₂, a semiconducting hexagonal and a metallic monoclinic MoTe₂, were grown by a temperature quenching method, and the monoclinic MoTe₂ single crystals were used in this study. The mechanical exfoliation, laser irradiation and friction force microscopy were conducted in air. The energy and power of the NTEGRA system (NT-MDT) laser were selected to be 532 nm (excitation wavelength) with 15 mW (power) for the laser thinning of the flakes and post-patterning of the device. Temperature-dependent transport of the fabricated device was investigated using a lakeshore probe station system (Model CRX-VF; cryogen-free micro-manipulated probe station) to compare the performance of the device before and after the post-patterning.

The crystal structure of the layered monoclinic MoTe₂, which has a distorted octahedron structural unit, is shown in figure 1(a). Like other 2D materials, a weak van der Waals interaction exists between layers in the monoclinic MoTe₂ crystals [36]. But lattice distortions, or atomically uneven layers, make exfoliation of the thin monoclinic MoTe₂ more difficult compared to other flat 2D materials such as graphene and hexagonal TMDs [37].

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Figure 1(b) shows a schematic image of the laser thinning process of a monoclinic MoTe₂ flake on SiO₂/Si substrate. When the top MoTe₂ layer is exposed to a laser, local heat accumulates, and sublimation of the layer spreads from the top with a vertical thickness-control resolution of 5 nm. We interpret the vertical thickness resolution to be a function of the thermal conductivity of this material, and therefore layer-by-layer thickness control should be achievable by determining the optimal design of the thermal conductivity in the target 2D layered materials.

During laser-irradiation, the local temperature on the top layer was estimated to be around 400 °C, based on the amount of red-shift in the Raman A_g and B_g peaks (figure 1(c)) [38]. The high temperature heating was conducted in air, and the thickness of the flake gradually decreased and then saturated at 3 layers; at the saturated thickness, the local temperature on the surface cannot be maintained due to rapid heat dissipation into the SiO₂/Si substrate [39].

Figure 1(c) shows Raman spectra of monoclinic MoTe₂ and the temperature-driven Raman shift during the laser-thinning. The red shifted Raman active modes during laser-thinning are restored to their initial positions when the temperature decreases to room temperature [42, 43]. The IR active B_u peak at 194.5 cm⁻¹ shows a blue shift, and a new peak appeared at 138.7 cm⁻¹ after the laser-thinning. The two peaks can be assigned IR active modes B_u with an unusual behavior (blue shift), which could be explained by the role of Te defects [40] in Raman spectroscopy; similar phenomenon, defect-induced symmetry breaking to produce IR active modes, has been reported [41].

In order to address the oxidation issue and observe other chemical changes in the material that may be induced by the laser-thinning process, we investigated changes in the chemical states of Mo and Te atoms in the laser-thinned monoclinic MoTe₂ areas using x-ray photoemission spectroscopy (XPS). Since the laser-thinning involves local thickness engineering, and we intended to conduct post-patterning in the monoclinic MoTe₂, we used synchrotron radiation (hν  =  700 eV) with a 200 nm-sized x-ray spot in our characterization of the laser-thinned area. The scanning image of Mo 3d photoelectrons is well-matched to the optical image of monoclinic MoTe₂ flakes exfoliated on SiO₂/Si substrate in figure 2(a). The pristine and laser-thinned MoTe₂ flakes have Mo and Te atoms as majority components as seen in the wide scan (figure 2(b)). After the laser thinning process, Si 2p and O 1s photoelectrons from SiO₂ substrate are appeared because the sample thickness is similar to the probing depth of photoelectrons (see figure S2 (stacks.iop.org/TDM/4/024004/mmedia)). To examine the surface chemistry, fine scans of Mo 3d and Te 3d photoelectrons were taken from laser-thinned area. In XPS of thin flake on insulating substrate, unavoidable charging effect shifts binding energies of the material. Therefore, instead of binding energy values, we used the binding energy difference between Mo 3d and Te 3d photoelectrons to investigate the surface chemistry before and after laser thinning process in monoclinic MoTe₂. The binding energy differences of laser-thinned monoclinic MoTe₂ flakes are almost similar to the binding energy differences of MoTe₂ single crystals, indicating no surface degradation, deformation or oxidation [39].

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The process of controlling thickness by varying laser exposure time is demonstrated in figure 3. Figure 3(a) shows the large flake that was chosen, with a thickness of 21 nm. Laser patterning was conducted in a square shape with different exposure times, as shown in figure 3(b). Six square areas of 3 µm  ×  3 µm were fabricated with different laser exposure times, 5 s, 10 s, 20 s, 30 s, 40 s and 50 s for each step (the spot size was 200 nm), respectively. The height profiles in figure 3(c), measured by atomic force microscope (AFM), show that thinned regions in the MoTe₂ flake were reduced to 10 nm, 5 nm and 2 nm by exposure times of 5 s, 30 s and 50 s, respectively and the surface roughness of laser-thinned area is estimated to around 1 nm (figure S4). However, the optical contrast in figure 3(b) does not show different levels in the laser-thinned areas unlike the AFM data. This can be explained by light scattering on the laser-treated surface where the roughness is increased by the laser-thinning. In our study, the reliable vertical thickness control was about 5 nm (~7 layers).

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Surface morphology and friction force mapping of the laser-thinned MoTe₂ flake were simultaneously obtained by AFM. This surface-sensitive technique provides information on the atomic height and electronic structure of the engineered area; if an oxidized or insulating area appears after laser-thinning, a local friction force will be measured by the AFM and reveal the material change. Figures 3(d) and (e) show that the patterned areas and their surrounding areas have similar roughness and friction force. A subtle contrast in figure 3(e) can be explained by the increase of roughness in patterned areas during the laser-thinning. But, the overall consistent friction force in figure 3(e) indicates that there were no oxidized or insulating areas near the patterned areas, confirming again the XPS results presented in figure 2.

For an electronic homojunction device by the post-patterning, we applied the laser-thinning process to the device channel of a two-terminal monoclinic MoTe₂ device that had been fabricated by a conventional exfoliation method. The average thickness of the channel following exfoliation was 15 nm (21 layers) with a lateral dimension of 5 µm as shown in figure 4(a). An AFM image of the post-patterned device (using the same device shown in figure 4(a)) is presented in figure 4(b). The thickness of the MoTe₂ channel was decreased to 3 nm (4 layers) by the laser thinning, as shown in the line profile in figure 4(b). We note that it was difficult to obtain a few-layered monoclinic MoTe₂ in which an electronic bandgap is opened by strong spin–orbit coupling, simply by this exfoliation [27]; this could be understood to be a consequence of the strong layer- to-layer interaction in monoclinic MoTe₂, compared to graphite.

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The post-patterning process in figure 4 allowed the channel to have semiconducting regions where atomically clean and thin homojunction could be generated. To demonstrate the electric characteristics of the post-patterned homojunction area, the temperature-dependent electric properties of the device were investigated before and after the laser thinning process, and the results are shown in figures 4(c) and (d). The thick channel device in figure 4(a), composed of 21 layers of monoclinic MoTe₂, exhibits linear current–voltage curves (figure 4(c)), which proves the presence of a semimetallic channel and ohmic contacts in the device. In contrast, the post-patterned device with a 3 nm-thick channel in figure 4(b) exhibits non-linear current-voltage curves with largely increased resistance and semiconducting features; the resistance decreases as the temperature increases. Although the electric properties were completely changed by the post-patterning process, the thinned channel still showed the same Raman features described in figure 1(c), which indicates that monoclinic MoTe₂ is robust to the laser-irradiation process. We further studied the device performance of laser-thinned semiconducting 1T' channel. Unfortunately, above the room temperature, the post-patterned devices exhibit almost zero ON/OFF ratio under the gating bias varying from  −60 V to 60 V (see figure S5 in the supporting materials). This could be understood by large carrier density (~10¹⁴ cm⁻²) [13] and small bandgap (~60 meV) of 1T'-MoTe₂ unlike other semiconducting TMDs which exhibit high ON/OFF ratio mostly due to their low carrier density (~10¹² cm⁻²) and relatively large bandgap (~1 eV).

The thickness-driven electronic change from metallic to semiconducting states is clearly evidenced by the change in Schottky barrier heights of the two devices, which were fabricated using post-patterning on a single device, as shown in figures 4(a) and (b). In the Arrhenius plot of the conductance change as a function of temperature, the few-layered monoclinic MoTe₂ device (red spots) shows a negative slope with an activation energy of ~60 meV, while the initial device (black spots) exhibits metallic characteristics. In order to more precisely compare the activation energy of our laser-thinned device with the activation energy of thin exfoliated flakes, we added three sets of transport data from thin exfoliated MoTe₂ flakes (green, magenta and blue circles in figure 4(e)). The Schottky barrier height is consistent with the value obtained from the exfoliated thin monoclinic MoTe₂ device (open circles), which demonstrates that our laser-thinning method produces a semiconducting MoTe₂ characteristic that has been realized in high quality thin monoclinic MoTe₂.

Recently, it has been reported that the thinner TMDs can have higher degree of disorder that causes a semiconducting transport behavior [44]. In our study, therefore, the exact physical origin of the semiconducting characteristics in thin 1T'-MoTe₂ remains an open question: either bandgap opening owing to spin–orbit coupling like clean 1T'-MoTe₂ single crystal or increased disorder effect in thin layer limit.

We demonstrated a laser-thinning method and its use in the post-patterning of an electronic homojunction in monoclinic MoTe₂. A thickness-dependent electronic change from metallic to semiconducting states in the monoclinic MoTe₂ was enabled by the fabrication of a homojunction between the thick metallic and thin semiconducting regions produced by the laser-thinning process. Despite of the unsatisfactory device performance of the laser-thinned MoTe₂ compared to current transistor, we demonstrated that quantum physics, a new type of metal-insulator transition, can be linked to device application via the homojunction, a conceptually new device structure by our laser-thinning technique.

Investigation of the chemical states of Mo and Te atoms, and friction force, confirmed that the monoclinic MoTe₂ is robust against the laser-irradiation process; oxidation and other degradation were negligible. The Raman spectra obtained during the laser-thinning process showed that the process of thermal detachment induced by the laser-irradiation effectively decreases the thickness of the monoclinic MoTe₂ via local heating effect, at an estimated temperature of 300–400 °C. Vertical thinning and thickness were controlled to a resolution of about 5 nm, and the thinnest sample obtained with this method was 3 layers; realizing layer-by-layer thickness control with this method remains a challenge.

Considering that atomically-thin monoclinic MoTe₂ or other group 6 TMDs have been difficult to obtain by chemical methods or mechanical exfoliation, our study suggests a breakthrough solution of this issue. Our thinned or post-patterned monoclinic MoTe₂ shows physical characteristics that are similar to atomically thin monoclinic MoTe₂. The post- phase-patterning is a practical way to realize few- layered monoclinic MoTe₂ with a 500 nm lateral resolution for device applications, as well as for exploring novel physics such as 2D topological insulator.

Sample preparations

Temperature estimation

Friction force microscopy (FFM)

Scanning photoemission spectroscopy (SPEM)

This work was supported by Samsung Research Funding Center for Future Technology under Project Number SRFC-MA1502-09.