Multilayer MoTe2 Research Articles

Realization of single MoTe2 crystal in-plane TFET by laser-induced doping technique

Significant recent progress has been achieved in the fabrication of tunnel field-effect transistors (TFETs) utilizing transition metal dichalcogenides (TMDCs) materials, particularly focusing on out-of-plane heterojunction structures. Due to the inherent limitations of doping technology for TMDCs, there have been limited investigations into the development of in-plane TFETs. In this study, we present the realization of an in-plane TFET based on a single crystal of multilayer MoTe2, utilizing a regioselective doping technique through laser irradiation. By constructing a p+/i/n++ homojunction structure, a band-to-band tunneling dominated performance with a minimum subthreshold swing value of 75 mV/dec and an on/off ratio of 105 was obtained at a low temperature. Furthermore, the “OFF” and “ON” state currents of the TFET operation were smaller than the gated diode operation in this structure, which is consistent with the tunneling mechanism.

Thickness-dependent carrier polarity of MoTe2 transistors with NiTe2 semimetal contacts

We demonstrated that the carrier polarity of MoTe2 transistors can be modulated by controlling the channel thickness with NiTe2 semimetal contacts. The multilayer MoTe2 transistors (thickness >7.1 nm) exhibit a symmetric ambipolar conduction, and a transition to unipolar p-type polarity occurs as the channel thickness decreased down to ∼2.3 nm. The position of the semimetal NiTe2 work function was verified to be located at the mid-gap of multilayer MoTe2, and the observed transition was interpreted by a synergistic effect of the channel thickness-dependent band alignment and charge transfer behavior with unique NiTe2 semimetal contacts.

Oxidation of tantalum disulfide (TaS2) films for gate dielectric and process design of two-dimensional field-effect device

Ta-based high-κ dielectrics can be synthesized via the oxidation of TaS2 films. In this study, we investigated the wet and dry oxidation of TaS2 films via thermal annealing and plasma irradiation, respectively. The specific vibration observed via Raman spectroscopy, the bonding states observed via x-ray photoelectron spectroscopy, and capacitance measurements confirmed the oxidation of TaS2 films with a dielectric constant of ∼14.9. Moreover, the electrical transport of the TaS2 films along the in-plane direction indicated a change in conductivity before and after the oxidation. The thickness of the oxidized film was estimated. Accordingly, the layer-by-layer oxidation was limited to approximately 50 nm via plasma irradiation, whereas the TaS2 films within 150 nm were fully oxidized via thermal annealing in ambient air. Therefore, a Ta-oxide/TaS2 structure was fabricated as a stack material of insulator and metal when the thickness of the pristine film was greater than 50 nm. In addition, Ta-oxide films were integrated into bottom-gated two-dimensional (2D) field-effect transistors (FETs) using the dry transfer method. 2D FETs with multilayer MoTe2 and MoS2 films as p-type and n-type channels, respectively, were successfully fabricated. In particular, the Ta-oxide film synthesized via dry oxidation was used as a gate dielectric, and the device process could be simplified because the Ta-oxide/TaS2 heterostructure can function as a stack material for gate insulators and gate electrodes. An anti-ambipolar transistor consisting of an MoTe2/MoS2 heterojunction was also fabricated. For the transfer characteristics, a relatively sharp on-state bias range below 10 V and sufficiently high peak-to-valley ratio of 106 at V DS = 3 V were obtained using the high-κ gate dielectric of Ta-oxide despite the presence of the multilayer channels (∼20 nm).

Multilayer MoTe2 Field‐Effect Transistor at High Temperatures

AbstractFunctional 2D material‐based devices are likely subjected to high ambient temperatures when integrated into miniaturized circuits for practical applications, which may induce irreversible structural changes in materials and impact the device performance. However, majority of 2D devices’ studies focus on room temperature or low‐temperature operation conditions. Here, the high‐temperature (up to 673 K) electro‐thermal response of molybdenum ditelluride (MoTe2)‐based field‐effect transistors is investigated. The optimal annealing temperature of around 500–525 K for the multilayer MoTe2 devices with two‐fold enhancement in maximum current level, field‐effect mobility, and current on‐off ratio is identified. Furthermore, MoTe2 devices show the transition of electrical response from gate modulation to the degenerately p‐doped (hole dominant) characteristics when the operation temperature increases to ≈600 K. The gate‐dependent electro‐thermal measurements complemented by surface chemistry analysis confirm the near range hopping transport in the MoTe2 channel at high temperature induced by thermally triggered oxidation of MoTe2. These results not only provide the thermal endurance limits of MoTe2 for practical applications, but also indicate the possible applications of MoTe2 for thermal sensing applications.

Intrinsic effect of interfacial coupling on the high-frequency intralayer modes in twisted multilayer MoTe2.

The interfacial coupling at the interface makes the van der Waals heterostructures (vdWHs) exhibit many unique properties that cannot be realized in its constituents. Such a study usually starts with a twisted stack of two flakes exfoliated from the same layered materials to form twisted multilayers, in which the impact of interfacial coupling on the low-frequency interlayer modes had been well understood. However, it is not clear how interfacial coupling affects the high-frequency intralayer modes of twisted multilayers. Herein, we perform high-resolution resonance Raman spectroscopy of the high-frequency intralayer modes in twisted multilayer MoTe2 (tMLM). All the Davydov entities of the out-of-plane intralayer mode are observed and distinguished at 4 K. It is found that the out-of-plane intralayer modes in tMLM are sensitive to its interfacial layer-breathing coupling so that the out-of-plane intralayer modes in tMLM do not show a direct relationship with those of the two constituents. However, the case is quite different for the in-plane intralayer modes in tMLM, whose spectral profile can be fitted by those of the corresponding modes of its constituents. This indicates that the in-plane intralayer modes are localized within the constituents in tMLM because of its negligible interfacial shear coupling at the interface. All the results can be well understood using the vdW model in which only the nearest neighbor interlayer/interfacial interaction is taken into account. This work directly builds the relationship between the Davydov splitting of the high-frequency intralayer vibrations and the low-frequency interlayer vibrations in tMLM, which can be further extended to other twisted materials and the related vdWHs.

Electron beam lithography induced doping in multilayer MoTe2

Molybdenum ditelluride (MoTe2) is an excellent building block for assembling switching devices owing to its appropriate band gap, tunable p- and n-type conduction. Electron beam lithography (EBL) as a powerful method for electronic device fabrication, generally involves thermal process for resist solidification and electron beam writing. Herein, we find that thermal processing in air and electron beam irradiation result in significant p- and n-doping in multilayer MoTe2, respectively. The p- and n-doping levels can be precisely controlled by the annealing temperature (time) and injection Cullen density, respectively. Both p-doping by annealing and n-doping using irradiation are reversible and can realize degenerate-like conduction behavior. The annealing-induced adsorption of molecules as electron acceptors (e.g. O2, H2O) on the surface MoTe2 accounts for the p-doping, whereas irradiation-induced desorption and interface charging are probably responsible for the n-doping. Polymethyl methacrylate (PMMA) coating does not completely hinder but mitigate the p-doping by annealing. In contrast, PMMA coating is invalid in inhibition of the n-doping by irradiation. These findings would promote understanding of the transport properties of intrinsic MoTe2. Besides, the facile, reversible and controllable p- and n-doping methods are helpful for assembling novel electronics with diverse functionality.

Modulation of Electrical Properties with Controllable Local Doping in Multilayer MoTe2 Transistors

AbstractIt has been found that substitutional atoms, surface modification, electrostatic gating, and contact electrode engineering can induce doping effects in 2D materials. However, the doping level, area, position, and pattern shape are not well controlled; thus, it is hard to generate high‐performance, multifunctional devices and logic circuits with these doping approaches. Here, it is found that a damage‐free, highly controllable, local doping in multilayer molybdenum ditelluride (MoTe2) flakes can be realized with highly controllable laser beam scanning irradiation. The doping level, area, position, and pattern shape are well controlled by adjusting parameters of the scanning laser beam. With the controllable doping, the subthreshold swing of MoTe2‐based transistors can be decreased by one order of magnitude, well‐matched p‐type and n‐type transports can be formed, and threshold voltages can be modulated, then a complementary inverter with ultra‐high gain ≈242 is realized, which is the highest in all reported inverters based on 2D materials. It is also found that a stable negative transconductance behavior can be created for the MoTe2 transistor by controlling doping position, then a frequency doubler is successfully demonstrated. Therefore, it is proposed that the position‐controlled local doping in multilayer MoTe2 may offer exciting application potentials in future nanoelectronics.

Two-dimensional series connected photovoltaic cells defined by ferroelectric domains

Recently, a large amount of effort has been devoted to bringing p- and n-type two-dimensional (2D) materials in close contact to promise a p–n junction for photodetectors and photovoltaic devices. However, all solar cells based on 2D materials are single p–n junctions so far, where the open circuit voltage is usually limited by the bandgap of semiconductor materials. Here, by using a scanning-probe domain patterning method to polarize the ferroelectric film, we demonstrate a series connected MoTe2 photovoltaic cell with an additive open circuit voltage and output electrical power. The nonvolatile MoTe2 p–n diodes exhibit a rectification ratio of 100. As a photodetector, the device presents a responsivity of 220 mA/W and an external quantum efficiency of 41% without any gate or bias voltages. The open circuit voltage increases linearly with the number of series connected p–n junctions and can be beyond the bandgap of the multilayer MoTe2.

Tailoring the electrical properties of MoTe2 field effect transistor via chemical doping

Abstract Regulating electrical properties is basic to accomplish a dynamic exhibition of electronic devices such as field effect transistors (FETs). Chemical doping is advantageous in this regard being cost effective, efficient and over-simplistic collated to other methods used in semiconductor electronics for effectual tailoring of electrical properties. Here we report on controlling electrical characteristics of multilayer MoTe2 field effect transistor via chemical doping with ammonia (NH3). Shifting of threshold voltage from positive to negative back-gate voltage (Vbg) indicates n-type doping which is further complied with the results of Raman spectroscopy wherein A1g characteristic peak shifted towards lower wave number from 173.5 cm−1 to 169.4 cm−1. Further, the full-width at half maximum (FWHM) for A1g peak is observed to be 3.2–6.0 cm−1 that is broader relative to E12g characteristic peak. After doping, the current on/off ratio is improved from 105 to 107. The electron mobility and charge carrier density was increased from 43 cm2V−1s−1 to 180 cm2V−1s−1 and from 4.2 × 1012 cm−2 to 6.9 × 1012 cm−2, respectively. Our study reveals exceptional results by illustrating improved performance of the MoTe2 FETs, which could be utilized for potential applications in electronic devices.

Nucleation engineering for atomic layer deposition of uniform sub-10 nm high-K dielectrics on MoTe2

Abstract Continuous, uniform, and sub-10 nm Al2O3 high-K dielectrics upon two-dimensional exfoliated multilayer MoTe2 are realized by atomic layer deposition (ALD) based on a nucleation layer (NL) prepared by the ozone-based process, interfacial AlN, and low-temperature (low-T) physical adsorption. The NLs gives rise to significant reduction of the leakage current in the sub-10 nm Al2O3 high-K dielectrics as shown by conductive atomic force microscopy. For the ozone-based NL, X-ray photoelectron spectroscopy (XPS) reveals the oxidation of MoTe2 which is detrimental to the electrical properties of MoTe2. For the AlN NL, XPS reveals that Mo N bonds were formed without Mo O bonds and no chemical shift appeared in Te-3d XPS spectrum, indicating the AlN NL did not result in the MoTe2 oxidation but instead the formation of an MoN layer. For the low-T NL, the XPS spectra of MoTe2 are the same as those of the as-exfoliated MoTe2 flake, consistent with the absence of chemical reactions during low-T physical adsorption. The result demonstrates that the NLs prepared by the low-T physical adsorption and the interfacial AlN are effective and favorable for nucleating high-quality high-K gate dielectrics on MoTe2 transistors.

(Invited) From Field-Effect Transistors to Tunneling Devices and Resistive Random Access Memories in Two-Dimensional Van Der Waals Structures

Devices from low-dimensional materials are fundamentally different from those in the conventional electronic world. The ultra-thin body of transition metal dichalcogenides (TMDs) and black phosphorus (BP) as examples of semiconducting layered structures enables electrostatics gate control that goes beyond what can be achieved by even the most aggressively scaled three-dimensional silicon FinFET technology. Moreover, the arrangement of bonds in a two-dimensional system defines a natural sub-nanometer channel thickness without loss of carrier mobility as would be typically observed when thinning down a three-dimensional semiconductor to similar dimensions. At the same time the anisotropic transport conditions in layered two-dimensional (2D) systems allow for novel device applications. In my talk, I will present three device implementations that take explicitly advantage of the unique properties of two-dimensional van der Walls (vdW) materials. First, I will discuss a multi-layer channel device from MoS2 that exploits the advantage of stacking 2D vdW materials in a three-dimensional (3D) integration approach. I will show how multi-channel TMD field-effect transistors (FETs) are fabricated, will discuss the associated challenges and finally present the experimental transport results. Next, tunneling field-effect transistors (TFETs) from black phosphorus (BP) will be presented that show record high tunneling current densities and good inverse subthreshold slopes in a reconfigurable device geometry. Last, I will present experimental and simulation results on the topic of resistive random access memories (RRAM) from multi-layer MoTe2 and Mo1-xWxTe2. Excellent switching characteristics of these novel types of two-terminal devices are presented in conjunction with a model to explain the observed switching behavior in terms of an electric field induced structural transition from a 2H semiconducting to a distorted transient structure (2Hd) and orthorhombic Td conducting phase.

Enhanced Photoresponse in MoTe2 Photodetectors with Asymmetric Graphene Contacts

AbstractAtomically thin 2D materials are promising candidates for miniaturized high‐performance optoelectronic devices. This study reports on multilayer MoTe2 photodetectors contacted with asymmetric electrodes based on n‐ and p‐type graphene layers. The asymmetry in the graphene contacts creates a large (Ebi ∼ 100 kV cm−1) built‐in electric field across the short (l = 15 nm) MoTe2 channel, causing a high and broad (λ = 400–1400 nm) photoresponse even without any externally applied voltage. Spatially resolved photovoltage maps reveal an enhanced photoresponse and larger built‐in electric field in regions of the MoTe2 layer between the two graphene contacts. Furthermore, a fast (∼10 µs) photoresponse is achieved in both the photovoltaic and photoconductive operation modes of the junction. The findings can be extended to other 2D materials and offer prospects for the implementation of asymmetric graphene contacts in future low‐power optoelectronic applications.

Low-Frequency Noise Characteristics in Multilayer MoTe2 FETs With Hydrophobic Amorphous Fluoropolymers

This letter investigates the low-frequency noise (LFN) properties of the multilayer MoTe2 field-effect transistors (FETs) before and after hydrophobic amorphous polymer (CYTOP) encapsulation in the subthreshold and linear regimes. The noise spectrum density of drain current (SID) shows that the LFN in the multilayer MoTe2 FETs nicely fits to a 1/ $f^{\gamma }$ power law with $\gamma \sim \textsf {1}$ in the frequency range of 10–200 Hz. From the dependence of SID on the drain current, carrier number fluctuation ( $\Delta n$ ) is considered as a dominant LFN mechanism from all operation regimes in the multilayer MoTe2 FETs. Extracted trap density (Nt) based on the McWhorter model in this letter was reduced at least more than one order level compared with the multilayer MoTe2 FETs without CYTOP passivation.

Control of polarity in multilayer MoTe2 field-effect transistors by channel thickness.

In this study, electronic properties of field-effect transistors (FETs) fabricated from exfoliated MoTe2 single crystals are investigated as a function of channel thickness. The conductivity type in FETs gradually changes from n-type for thick MoTe2 layers (above ≈ 65 nm) to ambipolar behavior for intermediate MoTe2 thickness (between ≈ 60 and 15 nm) to p- type for thin layers (below ≈ 10 nm). The n-type behavior in quasi-bulk MoTe2 is attributed to doping with chlorine atoms from the TeCl4 transport agent used for the chemical vapor transport (CVT) growth of MoTe2. The change in polarity sign with decreasing channel thickness may be associated with increasing role of surface states in ultra-thin layers, which in turn influence carrier concentration and dynamics in the channel due to modulation of Schottky barrier height and band-bending at the metal/semiconductor interface.

Thickness-dependent carrier mobility of ambipolar MoTe2: Interplay between interface trap and Coulomb scattering

The correlation between the channel thickness and the carrier mobility is investigated by conducting static and low frequency (LF) noise characterization for ambipolar carriers in multilayer MoTe2 transistors. For channel thicknesses in the range of 5–15 nm, both the low-field carrier mobility and the Coulomb-scattering-limited carrier mobility (μC) are maximal at a thickness of ∼10 nm. For LF noise, the interplay of interface trap density (NST), which was minimal at ∼10 nm, and the interfacial Coulomb scattering parameter (αSC), which decreased up to 10 nm and saturated above 10 nm, explained the mobility (μC) peaked near 10 nm by the carrier fluctuation and charge distribution.

Physical origin of Davydov splitting and resonant Raman spectroscopy of Davydov components in multilayerMoTe2

We systematically study the high-resolution and polarized Raman spectra of multilayer (ML) MoTe2. The layer-breathing (LB) and shear (C) modes are observed in the ultralow-frequency region, which are used to quantitatively evaluate the interlayer coupling in ML MoTe2 based on the linear chain model, in which only the nearest interlayer coupling is considered. The Raman spectra on three different substrates verify the negligible substrate effect on the phonon frequencies of ML MoTe2. Ten excitation energies are used to measure the high-frequency modes of N-layer MoTe2 (NL MoTe2; N is an integer). Under the resonant excitation condition, we observe N-dependent Davydov components in ML MoTe2 , originating from the Raman-active A'1(A21g) modes at ~172 cm-1. More than two Davydov components are observed in NL MoTe2 for N larger than 4 by Raman spectroscopy. The N-dependent Davydov components are further investigated based on the symmetry analysis. A van der Waals model only considering the nearest interlayer coupling has been proposed to well understand the Davydov splitting of high-frequency A'1(A21g) modes. The different resonant profiles for the two Davydov components in 3L MoTe2 indicate that proper excitation energy of ~1.8-2.2 eV must be chosen to observe the Davydov splitting in ML MoTe2 . Our work presents a simple way to identify layer number of ultrathin MoTe2 flakes by the corresponding number and peak position of Davydov components. Our work also provides a direct evidence from Raman spectroscopy of how the nearest van der Waals interactions significantly affect the frequency of the high-frequency intralayer phonon modes in multilayer MoTe2 and expands the understanding on the lattice vibrations and interlayer coupling of transition metal dichalcogenides and other two-dimensional materials.

Exfoliated multilayer MoTe2 field-effect transistors

The properties of multilayer exfoliated MoTe2 field-effect transistors (FETs) on SiO2 were investigated for channel thicknesses from 6 to 44 monolayers (MLs). All transistors showed p-type conductivity at zero back-gate bias. For channel thicknesses of 8 ML or less, the transistors exhibited ambipolar characteristics. ON/OFF current ratio was greatest, 1 × 105, for the transistor with the thinnest channel, 6 ML. Devices showed a clear photoresponse to wavelengths between 510 and 1080 nm at room temperature. Temperature-dependent current-voltage measurements were performed on a FET with 30 layers of MoTe2. When the channel is turned-on and p-type, the temperature dependence is barrier-limited by the Au/Ti/MoTe2 contact with a hole activation energy of 0.13 eV. A long channel transistor model with Schottky barrier contacts is shown to be consistent with the common-source characteristics.