Session R16: Transport in Transition Metal Dichalcogenides

Memristive Phenomena in Polycrystalline Single Layer MoS$_{\mathrm{2}}$

Recently, a new class of layered two-dimensional semiconductors has shown promise for various electronic applications. In particular, single layer transition metal dichalcogenides (e.g. MoS$_{\mathrm{2}})$ present a host of attractive features such as high electrical conductivity, tunable band-gap, and strong light-matter interaction. However, available growth methods produce large-area polycrystalline films with grain-boundaries and point defects that can be detrimental in conventional electronic devices. In contrast, we have developed unconventional device structures that exploit these defects for useful electronic functions.[1] In particular, we observe grain-boundary mediated memristive phenomena in single layer MoS$_{\mathrm{2}}$ transistors. Memristor current-voltage characteristics depend strongly on the topology of grain-boundaries in MoS$_{\mathrm{2}}$. A grain boundary directly connecting metal electrodes produces thermally assisted switching with dynamic negative differential resistance, whereas a grain boundary bisecting the channel shows non-filamentary soft-switching. In addition, devices with intersecting grain boundaries in the channel show bipolar resistive switching with high on/off ratios up to \textasciitilde 10$^{\mathrm{3}}$.[1] Furthermore, the gate electrode in the field-effect geometry can be used to control the absolute resistance of the on and off states. Complementary electrostatic force microscopy, photoluminescence, and Raman microscopy reveal the role of sulfur vacancies in the switching mechanism. \textit{References: 1. Sangwan et al., Nature Nanotechnology, 10 403-406 (2015) }   

Transport measurements on monolayer and few-layer WSe2

The behavior of the electrical contacts often dominates transport measurements in mono and few-layer transition metal dichalcogenide (TMD) devices. Creating good contacts for some TMDs is particularly challenging since the fabrication procedure should prevent the TMD from oxidizing or chemically interacting with the contacts. In this talk, we discuss our progress on creating mono and few-layer WSe2 devices with both good electrical contacts and minimal effects from the substrate, polymer contamination, oxidation and other chemistry. For example, we have developed a technique for encapsulating metallic contacts and WSe2 flakes together in hexagonal boron nitride with multiple gates to separate and control the contributions from the channel and the Schottky barriers at the contacts. Research supported in part by Samsung GRO grant US 040814   

Negative to Positive Crossover of Magnetoresistance in WS2 nanoflakes with Ohmic Contact

We report studies on the transport measurements of WS2 nanoflakes including contact optimization and magnetoresistance measurement. We find that the platinum electrodes deposited by focused ion beam (FIB) technology on WS2 exhibit an ohmic contact, which provides a pathway to solve the dilemma of Shottky barrier for WS2 devices. A temperature-modulated negative-to-positive crossover of magnetoresistance (MR) is also observed, replenishing the existing data which mainly emphasizes field effect transistor (FET) related transport. Our work may stimulate further studies and potential electronic and optoelectronic applications on transition-metal dichalcogenides.   

Shubnikov-de Haas oscillations of high mobility holes in monolayer and bilayer WSe$_{\mathrm{2}}$: spin-valley locking, effective mass, and inter-layer coupling

We study the magnetotransport properties of high mobility holes in monolayer and bilayer WSe$_{\mathrm{2}}$, measured in dual-gated samples with top and bottom hexagonal boron-nitride dielectrics, and using platinum bottom contacts. Thanks to the Pt high work-function combined with the a high hole density induced electrostatically by an applied top gate bias, the contacts remain ohmic down to low (1.5 K) temperatures. The samples display well defined Shubnikov-de Haas (SdH) oscillations, and quantum Hall states (QHS) in high magnetic fields. In both mono and bilayer WSe$_{\mathrm{2}}$, the SdH oscillations and the QHSs occur predominantly at even filling factors, evincing a two-fold Landau level degeneracy consistent with spin-valley locking. The Fourier transform analysis of the SdH oscillations in dual-gated bilayer WSe$_{\mathrm{2}}$ reveal the presence of two subbands, each localized in the top or the bottom layer, as well as negative compressibility. From the temperature dependence of the SdH oscillation amplitude we determine a hole effective mass of 0.45m$_{\mathrm{e}}$ for both mono and bilayer WSe$_{\mathrm{2}}$. The top and bottom layer densities can be independently tuned using the top and bottom gates, respectively, evincing a weak interlayer coupling.   

The suppression of the large magnetoresistance in thin WTe2.

The layered nature of WTe$_{\mathrm{2}}$ suggests the possibility of making a single layer WTe$_{\mathrm{2}}$ memory device that exploits the recently observed large magnetoresistance. Presently, the origin of the magnetoresistance is attributed to the charge balance between the electron and hole carriers, yet the exact underlying physical mechanism is unclear. Here we show a systematic suppression of the large magnetoresistance, as well as turn-on temperature, with decreasing thickness of WTe$_{\mathrm{2}}$. We attribute the thickness-dependent transport properties to undesirable parasitic effects that become dominant in thin films of WTe2. Our results highlight the increasing importance of characterizing the parasitic effects for 2D layered materials in a single- to a few-layer thick limit. Finally, our observations support the hypothesis that the origin of the large magnetoresistance may be due to the charge balance between the electron and the hole carriers.   

Landau levels and Shubnikov-de Haas oscillations in monolayer transition metal dichalcogenide semiconductors

We study the Landau level (LL) spectrum using a multi-band $\mathbf{k}\cdot\mathbf{p}$ theory in monolayer transition metal dichalcogenide semiconductors [1]. We find that in a wide magnetic field range the LL can be characterized by a harmonic oscillator spectrum and a linear-in-magnetic field term which describes the valley degeneracy breaking. The effect of the non-parabolicity of the band-dispersion on the LL spectrum is also discussed. Motivated by recent magnetotransport experiments, we use the self-consistent Born approximation and the Kubo formalism to calculate the Shubnikov-de Haas oscillations of the longitudinal conductivity. We investigate how the doping level, the spin-splitting of the bands and the broken valley degeneracy of the LLs affect the magnetoconductance oscillations. We consider monolayer MoS$_2$ and WSe$_2$ as concrete examples and compare the results of numerical calculations and an analytical formula which is valid in the semiclassical regime. Finally, we briefly analyze the recent experimental results [Cui et al., Nat. Nanotechnol. 10 534 (2015)] using the theoretical approach we have developed. [1] New J. Phys. 17, 103006 (2015).   

Anisotropic Electron transport and device applications of atomically thin ReS$_{\mathrm{2}}$

Semiconducting two-dimensional transition metal dichalcogenides are emerging as top candidates for post-silicon electronics. While most of them exhibit isotropic behavior, lowering the lattice symmetry could induce anisotropic properties, which are both scientifically interesting and potentially useful. In this talk, we will present atomically thin rhenium disulfide (ReS$_{\mathrm{2}})$ flakes with unique distorted 1T structure, which exhibit in-plane anisotropic properties. We first fabricated mono- and few-layer ReS$_{\mathrm{2}}$ field effect transistors, which exhibit competitive performance with large current on/off ratios (\textasciitilde 10$^{\mathrm{7}})$ and low subthreshold swings (100 mV dec$^{\mathrm{-1}})$. The observed anisotropic ratio along two principle axes reaches up to 3.1. Furthermore, we successfully demonstrated an integrated digital inverter with good performance by utilizing two ReS$_{\mathrm{2}}$ anisotropic field effect transistors, suggesting the promising implementation of large-scale two-dimensional logic circuits. Recent results on ultra-high responsivity (as high as 88,600 A W$^{\mathrm{-1}})$ phototransistors based on few-layer ReS$_{\mathrm{2}}$ will also be discussed. Our results underscore the unique properties of two-dimensional semiconducting materials with low crystal symmetry for future electronic and optoelectronic applications.   

Hidden symmetry and enhanced Rudermann-Kittel-Kasuya-Yosida interaction in P-N junctions of two-dimensional materials

Correlation between magnetic atoms (spins) in non-magnetic two-dimensional (2D) systems and materials is one of the central issues in condensed matter physics. Engineering this correlation relies heavily on the carrier-mediated Rudermann-Kittel- Kasuya-Yosida (RKKY) interaction. However, tailoring and direct detection of spin-spin correlation has been limited to spins separated by a few nanometers due to the rapid $1/R^2$ decay of RKKY interaction with inter-spin distance R. Here we reveal a hidden symmetry -- absent from the Hamiltonian -- in planar P-N junctions, which could qualitatively change the spatial scaling of various response functions in a wide range of 2D systems and materials. In particular, it allows RKKY interaction to attain $1/R$ decay, the slowest decay in extended systems. This dramatically enhances RKKY interaction and enables long-range correlation between distant spins, with applications in nanoscale magnetism, spintronics, and solid-state quantum computation.   

Measurements of Schottky barrier heights formed from metals and 2D transition metal dichalcogedides

Schottky barrier height (SBH) is an important parameter that needs to be considered for designing electronic devices. However, for two dimensional (2D) materials based devices, SBH control is limited by 2D structure induced quantum confinement and 2D surface induced Fermi level pinning. In this work, we explore differences in measuring SBH between 2D and 3D materials. Recently, low temperature I-V measurement has been reported to extract SBH based on thermionic emission equation for Schottky diode. However, 2D devices are not real Schottky diode in that both source and drain metal electrodes make Schottky contact. According to our experimental results, SBH extracted from linear slope of ln (I/T$^{\mathrm{3/2}})$ against 1/T show widely diverse values, dependent on applied voltage bias and tested temperature which affect carrier transport including tunneling or thermionic emission across the metal-2D material interface. In this work, we wish to demonstrate the method to determine SBH and Fermi level pinning which are attributed to 2D transition metal dichalcogedides, differently from conventional 3D materials. .   

Fermi Level Pinning at the Interface of Molybdenum Based Chalcogenides and Metals

MoS$_{2}$ and MoTe$_{2}$ as the layered two dimensional materials have a sizable band gap suitable for future semiconductor application. However, their Schottky/ohmic contact engineering is found difficult to perform when varying contact metals due to Fermi level pinning at their metal interface. In this work, we investigate Schottky barrier heights at the interfaces formed between mono- or bi-layer MoS$_{2}$, MoTe$_{2}$ and Ti, Cr, Au, Pd. By varying temperature in the range from 200 to 500 K, we obtained their current -- voltage and hysteresis characteristics so as to determine accurate Schottky barrier heights. It is found that the Pd contact with MoS$_{2}$ and MoTe$_{2}$ shows the most pronounced Fermi level pinning; -0.8 and -1.2 eV respectively. Furthermore, the pinned energy level is found to be located near the conduction band edge for MoS$_{2}$ whereas it is near the intrinsic level for MoTe$_{2}$. These results are found to be crucial to understand the Fermi level pinning mechanism of two dimensional materials, which can be used for developing future MoS$_{2}$ and MoTe$_{2}$ based transistor devices.   

Weak Fermi Level Pinning Effect in Schottky Junction of $\alpha $-MoTe$_{2}$

Difficulty in hole injection from metal contacts to transition metal dichalcogenide (TMDC) semiconductors has been one of the most serious issues in the application of these 2D materials to future nanoelectronics, which is caused by the strong Fermi level pinning effect in the metal/TMDC Schottky junction. In this work, we found that the holes can be injected efficiently from a large work function metal of Pt to $\alpha $-molybdenum ditelluride ($\alpha $-MoTe$_{2}$; 2H-type), a TMDC semiconductor. The Schottky barrier height for holes at the Pt/$\alpha $-MoTe$_{2}$ interface was extracted to be 40 meV by the temperature dependence of back-gate modulated currents under the flat band condition at the junction, while the Schottky barrier for electrons in the junction with a small work function metal of Ti was found to be 50 meV. Considering the difference in the work functions of Pt and Ti, the Fermi level pinning effect in $\alpha $-MoTe$_{2}$ was found to be much weaker than that in other TMDC semiconductors such as MoS$_{2}$. These results open a way to the realization of complementary type circuits in the 2D materials for future low-power consumption electronics. This work was supported by JSPS KAKENHI Grant Numbers 15K06006, 25107004.   

Experimental determination of the massive Dirac-fermion parameters in MoS$_{2}$, MoSe$_{2}$, WS$_{2}$, and WSe$_{2}$

The physics associated with group 6 transition metal dichalcogenides (TMDs) MX$_{2}$ (M $=$ Mo, W; X $=$ S, Se) is one of the most intriguing issues in condensed matter physics. These materials have several interesting aspects, especially the direct to indirect band gap transition and spin-orbit interaction (SOI) induced spin band splitting at the K point. Recently, one reported a minimal band model, called massive Dirac-fermion model, for K point of the monolayer MX$_{2}$ Brillouin zone. There are several parameters in this model obtained by calculations, not by experiment, until now. Here we report the parametric studies on MX$_{2}$ using angle resolved photoemission spectroscopy (ARPES). We factor out the massive Dirac-Fermion parameters from the bulk MX$_{2}$, not monolayer. For confirming the accurate experimental values, we performed the photon energy dependence experiment to find the exact $\Gamma $ point and in-situ potassium-dosing experiment were performed for each MX$_{2}$.   

Electrical transport properties of ReS2 with polymer electrolyte gating in the high-doping limit

Two-dimensional (2D) materials have emerged as promising candidates for future electronic applications. Among them, transition metal dichalcogenides (TMDs) demonstrate not only potential as ultrathin transistor channel material, but also intriguing spin and valley physics, which in principle could allow new types of devices and circuits. Here we report on the first study of two-dimensional anisotropic ReS2 at high doping levels, enabled by polymer electrolyte gating. Significantly increasing the doping level using electrolyte instead of standard solid gate, we measured an unusual modulation of the conductivity at high carrier densities in monolayer ReS2. In the case of thicker flakes, the effect is milder and an insulator-metal-insulator sequence with increasing doping is observed. Transport measurements provide the evidence of major influence of ionic disorder. Furthermore, we discuss possible band structure effects.   

Tempo-spatially resolved dynamics of elec- trons and holes in bilayer MoS2-WS2

We have performed a Density-Matrix Time-Dependent Density-Functional Theory analysis of the response of bilayer MoS2-WS2 to external laser-pulse perturbations. Time-resolved study of the dynamics of electrons and holes, including formation and dissociation of strongly-bound intra- and inter-layer excitonic states, shows that the experimentally observed [1] ultrafast inter-layer MoS2 to WS2 migration of holes may be attributed to unusually large delocalization of the hole state which extends far into the inter-layer region. We also argue that the velocity of the hole transfer may be further enhanced by its interaction with transfer phonon modes. We analyze other possible consequences of the hole delocalization in the system, including reduction of the effects of the electron-electron and hole-hole repulsion in the trions and biexcitons as compared to that in the monolayers. [1] X. Hong et al., Nature Nano 9, 682 (2014).   

Giant Photocurrent Generation at Topological Singularities in Graphene Superlattices

The energy spectrum of graphene away from the Dirac point contains topological critical points where Van Hove singularities (VHSs) appear and are predicted to host fascinating phenomena. However, the required extreme doping has prevented the experimental access to these VHSs. Alternatively, the formation of Moir\'{e} superlattices in twisted graphene bilayers or graphene on hexagonal boron-nitride (hBN) heterostructures generates electronic mini-bands that mimic graphene's energy spectrum but with reduced energy scale, providing a remarkable opportunity to study a variety of physics previously inaccessible. Here we reveal that the formation of saddle point VHSs in the mini-bands of graphene/hBN superlattice enables anomalously enhanced photocurrent generation through a photo-Nernst effect at low magnetic fields. We establish that this enhancement is unambiguously linked to the electronic topological transition at VHSs. The obtained zero-bias photocurrent is giant, with a photoresponsivity as high as about 0.3 ampere per watt, corresponding to an external quantum efficiency exceeding 50{\%}.