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van der Waals Epitaxy of High-Mobility Polymorphic Structure of MoTe Nanoplates/MoTe Atomic Layers with Low Schottky Barrier Height 2

Rochelle S. Lee, Donghwan Kim, Sachin Apparao Pawar, TaeWan Kim, Jae Cheol Shin, and Sang-Woo Kang ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b07720 • Publication Date (Web): 04 Jan 2019 Downloaded from http://pubs.acs.org on January 6, 2019

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van

der

Waals

Polymorphic

Epitaxy

of

Structure

High-Mobility of

Mo6Te6

Nanoplates/MoTe2 Atomic Layers with Low Schottky Barrier Height Rochelle S. Lee1, Donghwan Kim1,2, Sachin A. Pawar1, TaeWan Kim3,*, Jae Cheol Shin1,*, and Sang-Woo Kang2 1Department 2Advanced

of Physics, Yeungnam University, Gyeongsan 38541, Republic of Korea

Instrumentation Institute, Korea Research Institute of Standards and Science

(KRISS) Daejeon 34113, Republic of Korea 3Department

of Electrical Engineering and Smart Grid Research Center, Chonbuk National

University, Jeonju 54896, Republic of Korea

KEYWORDS: MOCVD, transition metal dichalcogenides, MoTe2, Mo6Te6, 1T´-2H polymorphs

ABSTRACT High contact resistance between two-dimensional (2D) transition metal dichalcogenides (TMDs) and metal electrodes is a practical barrier for applications of 2D TMDs to conventional devices. A promising solution to this is polymorphic integration of 1T´-phase semi-metallic and 2H-phase semiconducting TMD crystals, which can lower the Schottky barrier of the

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TMDs. Here, we demonstrate the van der Waals epitaxy of density-controlled single isolated 1T´-Mo6Te6 nanoplates on 2H-MoTe2 atomic layers by using metal-organic chemical vapor deposition. Importantly, in situ grown 1T´-Mo6Te6 nanoplates significantly reduce the contact resistance of the 2H-MoTe2 atomic layers, providing a record high mobility of 1,139 cm2/Vs for Pd/1T´-Mo6Te6/2H-MoTe2 back-gated field-effect transistors (FETs), along with a low Schottky barrier height (qϕb) of 8.7 meV. These results lead to the possibility of ameliorating the high contact resistance faced by other TMDs, and furthermore, offer polymorphic structures for realizing higher mobility TMD devices.

Two-dimensional (2D) thin films of transition metal dichalcogenides (TMDs) are of great interest in various device applications. In particular, 2D molybdenum ditelluride (MoTe2) is attractive for near infrared applications as a result of its narrow band gap of 1.1 eV, as compared to the larger band gap (1.5-1.9 eV) of other 2D semiconductors such as MoS2, MoSe2, WS2, and WSe2.1-3 Compared to other TMDs (i.e., AB2 where A = Mo, W and B = S, Se), MoTe2 provides an easier phase transition, highly ambipolar behavior,4 and a large Seebeck coefficient.5 Furthermore, MoTe2 monolayers offer the advantages of low thermal conductivity,5 unsaturated magnetoresistance, pressure driven superconductivity, and topological insulation.6 MoTe2 commonly exists in three crystalline forms, i.e., the hexagonal, monoclinic, and orthorhombic crystal structure, otherwise known as the 2H semiconducting phase, 1T´ semimetallic phase, and 1T metallic phase, respectively. Recently, polymorphism of MoTe2 was induced under particular synthesis conditions.7-9 Changed polymorphs can exhibit electronic properties that differ by orders of magnitude. Thus, the resulting different phases of MoTe2 suggest design strategies for achieving high device performance. Particularly, an integration of


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the 2H semiconducting phase with a 1T´ semi-metallic phase crystal is intriguing because it can overcome the major barrier (i.e., high contact resistance) now faced by 2H-MoTe2 monolayers.10 Recently, it has been reported that a polymorphic integration of 1T´-phase and 2H-phase MoTe2 crystals lowered the Schottky barrier at metal electrodes.10-15 The polymorphs reported previously, however, have been mostly driven by a thermal-induced process that facilitates the generation of lattice defects and interface disorder in crystalline structures.10 Moreover, an atomically abrupt polymorphic interface over a large area is difficult to obtain via the thermal-induced process. The thermally synthesized MoTe2 polymorphs have, thus far, been formed either in bulk or nano-sized crystals, making device fabrication very difficult.10, 16, 17

It has been reported that TMDs can be synthesized using molecular beam epitaxy (MBE),

which offers accurate control of film thickness and ensures high material quality with an abrupt interface. Nevertheless, MBE has inherently poor throughput, resulting from the necessity of ultra-high vacuum conditions which remains a major challenge, whereas powder-based chemical vapor deposition (CVD) method results in a lack of material homogeneity due to the possible incorporation of impurities during the synthesis process.18 Metal-organic chemical vapor deposition (MOCVD) is a reliable, well-defined, and high-yield growth system for conventional semiconductor materials. The vapor phase reaction under pure hydrogen carrier gas at low chamber pressure ensures high material quality and wafer-scale uniformity. As compared to the conventional CVD method, the pyrolysis process of metalorganic sources, which are precisely controlled by a mass-flow controller, not only avoids the incorporation of impurities, but also facilitates the precise optimization of the growth parameters (i.e., growth rate, precursor ratio, reaction temperature, chamber pressure, etc.). Here, we demonstrate van der Waals epitaxy of a wafer-scale polymorphic structure using MOCVD; 1T´-Mo6Te6 nanoplates/2H-MoTe2 atomic layers. van der Waals epitaxy offers a


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great deal of freedom in the combination of materials with different crystal structures,19 resulting in an atomically abrupt interface between the 1T´-Mo6Te6 nanoplate and the 2HMoTe2 atomic layers. More importantly, the polymorphic structure of 1T´-Mo6Te6 nanoplates/2H-MoTe2 atomic layers demonstrated here exhibits superior transport properties as compared to conventional 2H-MoTe2 atomic layers with very low contact resistance, resulting in significantly improved mobility. The polymorphic back-gated field-effect transistor (FET) exhibited a mobility of 1,139 cm2/V·s which is significantly higher than that of the reported contact phase engineered 1T´-2H MoTe2 FETs,10, 20 and showed a low contact resistance (Rc) of approximately 29 MΩ·μm and an ultra-low Schottky barrier height (qϕb) of 8.7 meV.

RESULTS AND DISCUSSION

Figure 1. Polymorphic structure of Mo6Te6 nanoplates/MoTe2 atomic layers grown on eight-inch SiO2/Si Substrate using MOCVD. (a) Schematic of MOCVD system used for


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growth. (b-g) Optical images of the polymorphic structure grown with different H2 flow rates, i.e., 50 sccm (b, c), 200 sccm (d, e), and 250 sccm (f, g). (h) STEM-BF image of the polymorphic structure. Inset shows a low-resolution TEM image of the polymorphic structure on the SiO2/Si substrate. The scale bar in the inset indicates 200 nm. (i, j) Atomic structures of the Mo6Te6 nanoplate (i) and MoTe2 atomic layer (j). (k, l) STEM-HAADF of the nanoplate (k) and the polymorphic structure (l).

A diagram of the MOCVD system employed in this work is presented in Figure 1a. The Mo(CO)6 and (C4H9)2Te precursors are injected into the reactor with H2 carrier gas. The individual precursors are precisely controlled by mass flow controllers (MFCs). The MOCVD has a vertical showerhead reactor that not only disfavors the undesired reaction of the residual gas but also increases the uniformity of the grown materials.21 The additional H2 port in the shower head is for the ambient gas, leading to a uniform chemical reaction on the substrate surface. Note that the heater is located inside the substrate holder, resulting in an onset of the pyrolysis reaction directly on the substrate surface at 400 °C, which is above the pyrolysis temperature of Mo(CO)6 and (C4H9)2Te. Because the precursors are in the gas phase, adsorption of adatoms (i.e., Mo and Te) occurs only on the substrate surface through pyrolysis reactions. The residual gases after pyrolysis reactions are removed by an exhaust system. We have already demonstrated that MOCVD offers optimum nucleation-density conditions for the 2D TMDs with large-scale uniformity and controlled thickness.22 Figure 1b, d, and f show the optical images of the polymorphic structure of 1T´-Mo6Te6 nanoplates/2H-MoTe2 atomic layers grown on eight-inch SiO2/Si substrates. The uniform diffraction color on the substrate surface suggests that the variations in size, shape, and number density of the Mo6Te6 nanoplates are small across the substrate. The optical microscope images


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in Figure 1c, e, and g show the morphological properties of the Mo6Te6 nanoplates grown under different H2 carrier gas flows. Isolated single Mo6Te6 nanoplates are clearly seen at H2 flows of 50 and 200 sccm in Figure 1c and e, respectively. At 250 sccm (Figure 1g), however, the Mo6Te6 nanoplates appear to be overlapping bundles, thus, it becomes very difficult to distinguish a single Mo6Te6 nanoplate. Instead, the plates are linked and form a very dense nanoplate-phase network, almost completely covering the substrate surface. No preferred growth direction is observed; the Mo6Te6 nanoplates are randomly oriented on the MoTe2 atomic layers. The nanoplates are horizontally grown on the substrate without noticeable bending. We have found that the growth of the Mo6Te6 nanoplates strongly depends on the H2 flow ratio. At high H2 flows (i.e., >300 sccm), nanoplates are not grown, but instead, 1T´/2H hybrid phases MoTe2 films grow. Further discussion of the MoxTey growth along with different growth parameters can be seen in Figure S1-4 in the supporting information. Figure 1h shows a scanning transmission electron microscope bright field (STEM-BF) image of the polymorphic structure. The inset in Figure 1h shows a low-resolution STEM image of the polymorphic structure grown on the SiO2/Si substrate, indicating that the thickness of the nanoplate is approximately 20-30 nm. The thickness of the nanoplate is further confirmed by atomic force microscopy (AFM), as seen in Figure S5 in the supporting information. The lattice structures of Mo6Te6 and MoTe2, as obtained using electron microscopy characterization, are illustrated in Figure 1i and j, respectively. Scanning transmission electron microscope highannular dark-field (STEM-HAADF) images are used to study the atomic structure of the polymorphs more clearly. The unit cell of an isolated nanoplate consists of two staggered stacks (indicated by blue triangles in Figure 1i) where each stack is formed by an equilateral triangle with one Te atom at each corner and one Mo atom placed between each Te pair (Mo3Te3).

In

Figure 1k, one plane of staggered triangles is apparent in the STEM image, as shown by the


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schematics. The position of the Te atoms along the z axis is indicated in terms of the lattice constant c, which is 4.7 Å (Figure 1i and k). This lattice constant is similar to that of Mo6Te6 nanowires described by Zhu et al., Cho et al., and Murugan et al.17, 20, 23 In Figure 1l, the 2HMoTe2 atomic layers consist of three layers, a monolayer of 0.707 nm in thickness for a total thickness of approximately 2.1 nm, which corresponds to the 2H-MoTe2 atomic layers.24, 25 In Figure 1l, MoTe2 atomic layers with a thin Mo6Te6 crystalline plate on top can be observed. Note that an atomically sharp interface between the MoTe2 and the Mo6Te6 nanoplate can be seen, although they have different crystal structures. The Mo6Te6 is a rectangular crystal with a lattice constant of 4.7 Å, whereas the 2H-MoTe2 has a hexagonal lattice with a lattice constant of 3.5 Å.26 The randomly oriented nanoplates on the MoTe2 further suggest that the heterointerface is not coherent. van der Waals epitaxy to achieve abrupt heterointerfaces for different crystal systems has been researched for many years.19 TMDs are known to be the most suitable materials for van der Waals epitaxy because they are formed of unit layers consisting of transition metal atoms sandwiched by chalcogen atoms, resulting in no dangling bonds on their surfaces.19 Note that the polymorphic structure demonstrated here is different from those obtained in previous studies, in which the crystal phase transition occurs because of thermally induced processes.10, 16, 17 The high-quality 2H-MoTe2 atomic layers are grown over the entire surface of the eight-inch SiO2/Si wafer at a stable temperature of 400 °C. Then, the 1T´-Mo6Te6 nanoplates are grown in situ on the atomic layers without temperature variation. It is well known that the SiO2 substrate creates a high-nucleation-density MoTe2 monolayer, leading to strong preference for the growth of 2D films versus the growth of 3D formations.22 Therefore, continued growth results in MoTe2 atomic layers that are laterally grown on a SiO2 substrate.22 The 2H-MoTe2 atomic layers, however, have very low surface energy because of the absence of dangling bonds. Thus, after the MoTe2 atomic layers cover the SiO2 substrate


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surface, the van der Waals epitaxy is driven with the formation of isolated islands, resulting from a very low substrate surface energy.27

Figure 2. Spectroscopic characterizations of the polymorphic structure. Raman spectra of (a) 1T´-Mo6Te6 nanoplates (inset shows the enlarged range from 200 to 330 cm-1) and (b) 2HMoTe2 atomic layers.

In order to confirm the 1T´ and 2H phases of Mo6Te6 and MoTe2, respectively, Raman analysis was performed. The Raman emission was collected using a Leica 100x objective (used to locate individual nanoplates); a laser spot size of 0.75 µm was used to focus the beam on the nanoplate, thus collecting information from the nanoplate section only. The characteristic Raman peaks located at 142 and 233 cm-1 are interpreted to be due to Mo6Te6 as shown in Figure 2a and inset, respectively.16, 28 These peaks were not observed in our MOCVD-grown 1T´-MoTe2 thin film as shown in Figure S6 of the supporting information. Compared to prior reports for Raman spectra of Mo6Te6,16, 28 MOCVD-grown Mo6Te6 shows a red shift for both two dominant peaks. This behavior could be explained by the difference of substrate as well as the impurity effects during the MOCVD growth. Apart from the above peaks, the peaks at 122.5, 163, and 270 cm-1 are the typical signature peaks of Au, Bg, and Ag modes, respectively, for 1T´ phase MoTe2, as


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shown in Figure 2a.29-32 The Raman peaks of the 1T´-MoTe2 thin film grown by MOCVD located at identical positions as shown in Figure S6 of the supporting information. Similar peaks for Au (123.8 cm-1), Bg (161.7 cm-1), and Ag (270.2 cm-1) were found for the samples grown with an H2 flow of 200 sccm (Figure S4 in the supporting information) as well as the sample grown with an H2 flow of 250 sccm (Figure S4 in the supporting information), which confirm 1T´ phase behavior of the Mo6Te6 nanoplates. A similar method was used to extract Raman spectra for the MoTe2 atomic layers. As shown in Figure 2b, the MoTe2 atomic layers exhibited 2H phase behavior, i.e., 2H characteristics dominant peaks could be seen at 112, 236, and 170 cm-1, corresponding to in-plane vibration E1g and E12g and out-of-plane vibration A1g modes, respectively, without indicating Raman peaks corresponding to 1T´ phase.29, 33 We note that Raman peak intensity of MoxTey is sensitive to the excitation wavelength.22, 34 The Raman spectra for 633 nm excitation permit unambiguous identification of MoTe2 atomic layers compared to the spectra for 488 and 532 nm excitation (see Figure S7 in the supporting information). Energy dispersive spectroscopy (EDS) and X-ray photoelectron spectra (XPS) analyses were further performed to confirm the structural properties of 1T´-Mo6Te6 nanoplates and 2H-MoTe2 atomic layers (see Figure S8 in the supporting information).


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Figure 3. Electrical properties of 1T´-Mo6Te6 and 1T´-Mo6Te6/2H-MoTe2 FETs. Schematic diagram of 1T´-Mo6Te6/2H-MoTe2 FETs with (a) the highest and (d) the lowest density Mo6Te6 nanoplates. Ids-Vds characteristics of (b) 1T´-Mo6Te6 nanoplates and (e) 1T´Mo6Te6/2H-MoTe2 FETs with Vbg = 0. Transfer curves (Ids-Vbg) for (c) 1T´-Mo6Te6 nanoplates and (f) 1T´-Mo6Te6/2H-MoTe2 FETs, respectively.

To highlight the applicability of the 1T´-Mo6Te6 nanoplates/2H-MoTe2 atomic layers polymorphic structure, the electrical properties were investigated. The sample with the highestdensity nanoplates, shown in Figure 1g, was used for the electrical characteristics of the Mo6Te6 nanoplates. As the nanoplates grown at an H2 flow of 250 sccm overlap each other, it allows the nanoplate network to be the channel of the FET. In contrast, the sample shown in Figure 1c was used for electrical characterization of the polymorphs, as it had the lowest density and clearly allowed 2H-MoTe2 to be the channel. Both structures, based on back-gate FETs, were fabricated on a SiO2/p+-Si substrate with Pd (50 nm) for source-drain electrodes, as shown in Figure 3a, Figure 3d and Figure S10a and b in the supporting information. As shown in Figure 3b and c, the I-V characteristics of the 1T´-Mo6Te6 nanoplates reveal semi-


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metallic behavior with weak gate adjustability. Regarding the output I-V characteristics, a linear I-V plot in the voltage range of -2.5 to 2.5 V indicates Ohmic behavior of the Pd/1T´Mo6Te6 nanoplates device (Figure 3b). The transfer curves [i.e., drain-source current (Ids) versus back gate voltage (Vbg)] confirm that the 1T´-Mo6Te6 nanoplate did not show an appreciable response to the back gate (Figure 3c). In contrast, the polymorphic structure (1T´Mo6Te6 nanoplates/2H-MoTe2 atomic layers) device exhibits p-type behavior (Figure 3f). Because the nanoplate is a semimetal, which leads to a high conductivity of 1T´-Mo6Te6 nanoplates/Pd electrode, the channel current readily passes through the 1T´-Mo6Te6 nanoplates via the Pd contact. Non-Ohmic I-V characteristics, observed in the polymorphic structure based device (Figure 3e), may be driven from native oxide formation or contamination from the device process at the interface between the Pd electrode and the polymorphic structure (1T´Mo6Te6/2H-MoTe2). MoxTey is found to be the very reactive with oxygen compared to other TMDs, leading to an increase of material resistivity due to easy absorption of oxygen and water molecules after air exposure.35 In addition, the amount of partial channel current flowing from 2H-MoTe2 atomic layers to Pd electrodes results in further increase of resistance of the device. Although a non-Ohmic contact is formed, the polymorphic structure (1T´-2H phase) based FET exhibited a current on/off ratio of 103 (Figure 3f) and a field-effect mobility of 1,139 cm2/Vs, the fastest result among previously reported hybrid phase 1T´-2H MoTe2 based FET devices.10, 20

We expect that the polymorphic structure provides low Schottky barrier, thus leading to

better transport properties. In addition, we regard that the organic-free fabrication process further enhances the mobility of the polymorphic device. In this process, the sample surface is not treated by photoresist or e-beam resist. The process procedure is seen in figure S9 of the supporting information. The field-effect mobility can be written as (Equation 1):

µ=

(𝐖𝐋 )(𝜺 𝜺𝐝𝑽 )(𝒅𝑽 ) 𝒅𝑰𝒅𝒔

𝟎 𝒓 𝒅𝒔

(1)

𝒃𝒈


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where a channel length (L) of 787µm, width (W) of 590 µm, dielectric thickness (d) of 300 nm, 𝑑𝐼𝑑𝑠

permittivity (𝜀𝑟) of 3.9, and transconductance (𝑑𝑉𝑏𝑔) of 9.83 × 10-6 extracted from a linear fit of the transfer curve (Ids-Vbg at a Vds value of 1 V, as shown in Figure 3f).

Figure 4. Schottky barrier height extraction in the thermionic emission model. (a) Temperature dependent (T = 80 K to 180 K) Ids-Vds characteristics for 1T´-Mo6Te6/2H-MoTe2 polymorphic structure. Inset shows a linear plot of the same Ids-Vds curve. (b) Arrhenius plot ln(Ids/T3/2) versus 1000/T at different values of Vds and (c) extraction of qϕb via the intercept value, where each data point represents the slope obtained from the Arrhenius plot in (b) using a specific value of Vds.

Having identified the Schottky barrier for the metal/1T´-Mo6Te6 nanoplates/2H-MoTe2 atomic layers, we next performed two-probe I-V measurements at various temperatures (from 80 to 180 K) for the 1T´-Mo6Te6/2H-MoTe2 polymorphic structure. In this measurement, transport properties were directly measured by positioning tungsten probe tips on 1T´-Mo6Te6 nanoplates with underlying 2H-MoTe2 (see Figure S10c in the supporting information). The probe tips were slightly rubbed against the nanoplate surface before measurements to remove any possible native oxides. Thus, it provides the electrical path of metal/1T´-Mo6Te6


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nanoplates/2H-MoTe2 atomic layers with a linear I-V behavior (Figure 4a). The drain-source current (Ids) can be calculated using the 2D thermionic equation (Equation 2), which employs 3

the reduced power law 𝑇2 for a 2D transport channel:36 𝑞

(

∗ 𝑆𝑇 3/2exp [ ― 𝑘𝐵𝑇 𝜙𝑏 ― 𝐼𝑑𝑠 = 𝐴2𝐷

𝑉𝑑𝑠 𝑛

)]

(2)

∗ where 𝐴2𝐷 is the 2D equivalent Richardson constant, S is the contact area of the junction, q is

the electric charge, kB is the Botlzmann constant, ϕb is the Schottky barrier height, n is the ideality factor, and Vds is the drain-source bias voltage. Figure 4a shows the temperature 3

variation of the output characteristics. Using the Arrhenius plot and ln (𝐼𝑑𝑠/𝑇2) versus 1000/T for different values of Vds (Figure 4b), the slope S was extracted as a function of Vds, where 𝑆 𝑞

= ―(1000𝑘𝐵)( 𝜙𝑏 ― 𝑞𝜙𝑏

1000𝑘𝐵 ,

𝑉𝑑𝑠 𝑛

). The slope S was plotted against Vds, where the intercept, 𝑆0 = ―

was used to determine qϕb (Figure 4c). The Schottky barrier height for the metal/1T´-

Mo6Te6 nanoplates/2H-MoTe2 atomic layers was calculated to be 8.9 and 8.7 meV for positive and negative Vds values, respectively. The barrier height obtained for the tungsten tips/1T´Mo6Te6/2H-MoTe2 is significantly lower than the previous reported ϕb values of 25.1 meV of Pd/2H-MoTe2,37 and 22 meV of hybrid phase Au/1T´-/2H-MoTe2.10 The ultra-low Schottky barrier value for non-passivated TMD devices clearly proves the advantage of the 1T´-Mo6Te6 nanoplates/2H-MoTe2 atomic layers polymorphic-structured device.


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Figure 5. Resistance measurements of 1T´-Mo6Te6/2H-MoTe2 without any metal electrodes. Ids-Vds characteristics of 1T´-Mo6Te6/2H-MoTe2 using (a) tungsten probe tips at various distances between two point probes. (b) Extracted contact resistance from plotting total resistance (R) against distance between probe tips.

To further investigate the contact resistance of the 1T´-Mo6Te6/2H-MoTe2 polymorphic structures, a transmission line measurement (TLM) method was employed. Without conventional metal contacts, transport properties were also directly measured by positioning tungsten probe tips on 1T´-Mo6Te6 nanoplates with underlying 2H-MoTe2 (see Figure S10c in the supporting information). Contact resistance (Rc) was calculated using a two-point configuration at varying probe tip distances. Rc was extracted from a linear plot, shown in Figure 5b, using the convention TLM equation.38 We note that the extracted contact resistance was approximately 29 MΩ·μm for tungsten probe tips at room temperature. Our findings indicate lower resistances, relative to those previously reported (i.e., 107 kΩ∙µm for 2HMoTe2/Pt,39 and 104 kΩ∙µm for Cr/2H-MoTe2 FETs40). Note that this result verified that the barrier height of the 1T´-Mo6Te6/2H-MoTe2 polymorphic structured device is significantly lower than that of the conventional 2H-MoTe2 ones.


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CONCLUSIONS In conclusion, van der Waals epitaxy of semi-metallic Mo6Te6 nanoplates on 2H-MoTe2 atomic layers was demonstrated using MOCVD with an atomically sharp interface. This growth technique provides a reproducible, versatile, and easy method for the fabrication of high mobility devices. Further, we present the highest recorded mobility (at 1,139 cm2/V·s) for 1T´Mo6Te6/2H-MoTe2 polymorphic structure. The atomically sharp 1T´-Mo6Te6/2H-MoTe2 yields a low contact resistance (Rc) of 29 MΩ·μm and a Schottky barrier (qϕb) of 8.7 meV. The growth presented here can be applied to other TMDs, demonstrating great prospects for high performance device applications.

EXPERIMENTAL PROCEDURES Polymorphic structures of 1T´-Mo6Te6 nanoplates on 2H-MoTe2 atomic layers were grown on a highly doped (

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