Session G60: 2D Material Devices I

Emergent Phenomena in Ferroelectric/van der Waals Heterostructures

The heterointerfaces between ferroelectrics and 2D van der Waals (vdW) materials can be utilized to achieve novel interfacial coupling, nonvolatile field effect control, and nanoscale programmable functionalities. In this talk, I will discuss a range of emergent phenomena in ferroelectric/vdW heterostructures mediated by interfacial charge, lattice, and polar coupling. Ferroelectric copolymer P(VDF-TrFE) has been utilized to induce nonvolatile conduction modulation in mono- to few-layer MoS₂ and ReS₂, with conductivity switching ratio of >1.5x10⁸ achieved. Combining the ferroelectric field effect with nanoscale domain patterning enables local tuning of carrier density in the vdW channel, which can lead to programmable Schottky junction states in MoS₂ [1]. This approach has also been exploited to create directional conducting paths in an insulating ReS₂ channel, which reveals that the conductivity along and perpendicular to the Re-chain can differ by >5.5×10⁴ in monolayer ReS₂ and further points to the emergence of a flat band in 4-layer ReS₂ [2]. We show that the interface-epitaxy between P(VDF-TrFE) and 1T’-ReS₂ leads to the formation of large scale P(VDF-TrFE) thin films composed of highly ordered, close-packed crystalline nanowires with well-defined width (10 and 35 nm) [3]. We also observe an unconventional filtering effect of second harmonic generation signal, which is attributed to the polar alignment between monolayer MoS₂ and the chiral rotation of surface dipoles at the domain walls in ferroelectric oxide PbZr_0.2Ti_0.8O₃ [4]. Our study showcases the rich research opportunities offered by integrating ferroelectrics with 2D materials.   

Spin-orbit-mediated proximity coupling at a magnetic van der Waals interface

Two-dimensional (2D) quantum materials and their integrated superstructures provide emergent phenomena associated with reduced dimensionality, modified lattice symmetry, and enhanced proximity coupling. A remarkable example is Ising superconductivity emerging in a non-centrosymmetric 2D superconductor with broken in-plane inversion symmetry as represented by 2D NbSe₂, where strong Zeeman-type spin-orbit interaction (SOI) locks the orientation of the spins of Cooper pairs to the out-of-plane direction, providing exotic non-BCS superconducting states. In this presentation, we demonstrate a unique proximity coupling between such out-of-plane spin-polarized electrons in 2D NbSe₂ and isotropic local spins in a newly-developed 2D Heisenberg ferromagnet V₅Se₈ [1] across a van der Waals interface [2, 3]. We will in particular focus on the specific regime, where the number of the V₅Se₈ layer was decreased down to the 2D limit so that the transport properties were dominated by NbSe₂ [3]. Very interestingly, we found that the sign of the anomalous Hall effect (AHE) of those samples were positive at the lowest temperature, which was opposite to those of the V₅Se₈ individual films. We also found that the AHE signal of those samples was enhanced with the in-plane magnetic fields, suggesting an additional contribution to the AHE signal except magnetization. We verify by band structure calculations that those unprecedented behavior could be well understood by accepting the idea that NbSe₂ is in a ferromagnetic/ferrovalley state, where characteristic Zeeman-type SOI plays an essential role.

 

References:

[1] M. Nakano et al., Nano Lett. 19, 8806 (2019).

[2] H. Matsuoka et al., Nano Lett. 21, 1807 (2021).

[3] H. Matsuoka et al., submitted.   

Improved n-type Field-Effect Transistor Performance using WSe2/PdSe2 Heterostructure as a Channel Material

We demonstrate superior performance of PdSe₂/WSe₂ van der Waals heterostructure than PdSe₂ or WSe₂ alone as a channel material for field-effect transistors (FETs), where WSe₂ acts as a buffer between PdSe₂ and Ti metal at the drain/source contacts to alleviate the Fermi-level pinning effect and therefore significantly lower the Schottky barrier and contact resistance. As a result, our PdSe₂/WSe₂ heterostructure-based FETs exhibit a two-terminal electron mobility exceeding 200 cm² V^-1 s^-1 at room temperature and approaching 700 cm² V^-1 s^-1 at 77 K, consistent with phonon-limited electron transport. By contrast, the two-terminal electron mobility of FETs based on a few-layer PdSe₂ or WSe₂ alone is substantially lower, especially at low temperatures suggesting that the electron transport is limited by the contacts. Furthermore, the increase of bandgap with decreasing PdSe₂ thickness leads to a higher ON/OFF ratio without substantial degradation of two-terminal electron mobility in FETs consisting of PdSe₂/WSe₂ heterostructures with thinner PdSe₂. We believe the significantly improved device performance enabled by combining different 2D semiconductors will facilitate real-world electronic applications of 2D semiconductors.

    

Charge Inhomogeneity Mediated Low-Frequency noise in Encapsulated Monolayer and Bilayer Graphene Field-Effect Transistors

Among 2D materials family, semimetal graphene is the only material that has promise in designing high mobility field-effect transistors (FETs). One-dimensional (1D) edge contacts to graphene channels in heterostructure GFETs consisting hexagonal boron nitride (hBN) and graphene have recently demonstrated superior device transport properties.¹ Moreover, these devices have also demonstrated low transport noise, necessary for applications such as RF communications.² With low-frequency-noise (LFN) studies on hBN/graphene/hBN heterostructure GFETs showing ultra-low-noise in carrier-rich region, studies comparing LFN in such GFETs consisting single layer (SLG) and bilayer graphene (BLG) near charge neutrality point has not been compared and analyzed thoroughly. In this work, we systematically study temperature-dependent transport and LFN characteristics to model the channel characteristics in SLG and BLG, 1D edge-contacted 2D heterostructure GFETs. Our study shows a substantial difference in electron-hole puddle transport in these devices. As such, these high-performance 1D edge-contacted 2D heterostructure GFETs play a fundamental role to study electron-hole puddle physics.

[1] L. Wang et al., Science 342, 614-617 (2013)

[2] Behera et al., ACS Appl. Electron. Mater. 3, 4126-4134 (2021)   

Electronic Tuning of 2D Materials with Zwitterionic and Functional Polymers

Functional polymers are increasingly recognized for their ability to engineer the electronic properties of 2D materials for device performance enhancement. Although dipole-rich polymer zwitterions have shown significant work function modulation of 2D materials, the contribution of zwitterion structure is not well understood. To this end, a series of zwitterionic sulfobetaine-based random copolymers with varying substituents has been prepared and applied as negative-tone resists on graphene, enabling evaluation of surface potential contrast. The influence of steric footprint on calculated dipole moment and resulting work function measured by ultraviolet photoelectron spectroscopy will be presented. To assess the nature of graphene surface doping by polymer zwitterions, a sample geometry that permits direct access to either the polymer or graphene side is employed using a zwitterionic phosphorylcholine-based polymer coating on graphene. By Kelvin probe and electrostatic force microscopies, a significant upshift in the Fermi level of graphene has been observed from a local field effect rather than pure charge transfer. Overall, understanding how surface dipole orientation impacts the electronics of 2D materials kindles exciting possibilities for precise tuning of device performance.   

Tunneling Spectroscopy of Two-Dimensional Materials based on Via Contacts

We study how electrons transfer between normal metal contacts embedded in hexagonal boron nitride (hBN) and two-dimensional materials with various interface transparency in mesoscopic devices based on ver-tical interconnect access (via) method [1]. We use two target materials in different categories, NbSe2 and monolayer graphene, to demonstrate this planar tunneling method in superconducting and in semiconducting/insulating regimes, respectively. The advantage of precisely controlling the tunnel area size and barrier thickness is key to obtaining high-quality tunneling spectra. This type of via tunneling technique is applicable toa large variety of materials, offering a competitive path to probing materials and interface properties under ultra-low temperature and high magnetic fields otherwise inaccessible with other experimental tools.

[1] E. J. Telford et al, Nano Letters 18, 1416 (2018)   

Characterisation of MoS2 FET Devices Gated Via Novel Ionic Liquids

Ionic liquids (IL), molten salts at room temperature with mobile cations and anions, have demonstrated the ability to electrostatically dope MoS2 and MoSe2 top gated field effect transistors to increase charge density, carrier mobility and even induce superconductivity [1]. Furthermore, ionic liquid gating has been demonstrated to be able to induce changes in the polymorphs of MoS2 and MoTe2 from the semiconducting 2H phase to the semi-metallic 1T’ and the metallic 1T phase [2]. When a gate voltage is applied to the ionic liquid, an electric double layer (EDL) is formed on the surface of the MoS2/MoSe2 sample as a layer of cations or anions accumulate at the sample/IL interface to create a high capacitance, highly doping the surface.

Here, the use of a novel ionic liquid N1114-TFSI as a gating dielectric on MoS2 FET devices will be discussed. N1114-TFSI has a much smaller cation compared to conventionally used DEME-TFSI and EMIM-TFSI, affecting the packing and layering of the anions and cations at the interface which could increase the amount of electrostatic doping of the MoS2 channel. Furthermore, the ability of small cations to intercalate between the layers of the MoS2 channel and the effect it has on the transport characteristics of the device will be explored.

[1] Shi, W., Ye, J., Zhang, Y., Suzuki, R., Yoshida, M., Miyazaki, J., Inoue, N., Saito, Y. and Iwasa, Y., 2015. Superconductivity Series in Transition Metal Dichalcogenides by Ionic Gating. Scientific Reports, 5(1).

[2] Zakhidov, D., Rehn, D., Reed, E. and Salleo, A., 2020. Reversible Electrochemical Phase Change in Monolayer to Bulk-like MoTe2 by Ionic Liquid Gating. ACS Nano, 14(3), pp.2894-2903.   

Investigating interface effects on the single MoS2 flake placed on SiO2 and BN surfaces

Due to the atomic thickness, electrical properties of two-dimensional (2D) materials such as MoS₂ are very sensitive to the interface and the underneath substrate. In previous reports, it was discovered that the MoS₂ placed on or sheathed with BN exhibited prominent electrical behaviors. However, the exact comparison of electrical properties of atomic thin MoS₂ placed on either BN or SiO₂ has not been explored yet. Here we placed the BN flake on the Si wafer capped with 300-nm thick SiO₂ and attached a few-layer MoS₂ flake with one part on BN and the other part on SiO₂ surfaces. The as-attached MoS₂ on both BN and SiO₂ was processed to make field-effect transistors and properties of electron transport and device performances were measured. Through a systematic study, it is found that the MoS₂/BN reveals a much lower Schottky barrier and specific contact resistivity as compared with MoS2/SiO₂. For all samples, the MoS₂/BN always shows a higher mobility while the temperature behavior presents either ascending or descending mobilities with increasing temperature. The MoS₂/BN presents a low interface trap density as well. In summary, the using of BN certainly prohibits some interface disorders coming from SiO₂ while there are other intrinsic disorders in MoS₂ that could severely degrade electrical properties of the 2D MoS₂.   

Supression of charge impurities in haffnium oxide capped MoS2

Atomically thin two-dimensional transition metal dichalcogenides offers wide range of applications thanks to its optical and electrical tunability. In this, the gate dielectrics play an important role in tuning the carrier concentration which impacts the optical and electrical properties. Here, we study the influence of gate dielectric on electrical properties of MoS2. We measure and compare the field effect mobility as a function of temperature in MoS2 and hafnium capped MoS2 devices. These measurements give insight into the dominant scattering mechanism in both type devices. We observe suppression of both, impurity scattering as well as phonon scattering in hafnium oxide encapsulated MoS2 device compared to uncapped MoS2 device. When the hafnium oxide encapsulated MoS2 device is further cooled down to lower temperatures, we observe conductance oscillations. We attribute these conductance oscillations to quantum interference due to phase coherence electron transport. Such oscillations are absent in uncapped MoS2 device. Suppressed impurity scattering and the observed quantum interference in encapsulated MoS2 devices indicates that these effects are induced by presence of hafnium oxide. Our work opens new possibilities of exploring quantum phenomenon in encapsulated 2D TMDC devices.   

Large Effective Work-function Increase in Al/SiO2/Si Junction with Graphene Interlayer at Al/SiO2 Interface

We report the significant change of the effective work-function from ~3.31 to ~4.35 eV of Al electrode when the graphene interlayer is inserted at the Al/SiO₂ interface in Al/SiO₂/n-Si junction, confirmed by capacitance-voltage (C-V) measurements. This effective work-function increase is considered to originate from the formation of electric dipole layer reflecting the electron orbital overlapping between Al and graphene (interaction dipole layer). According to the electrostatic analysis of solving Poisson equation for the flat-band conditions, the interaction dipole has its negative side toward the graphene layer, inducing a potential increase across the Al/graphene interface. A similar electrostatic effect of interaction dipole layer is also observed when using a Pt electrode with relatively high work-function. Differently from the Al electrode, the effective work-function of Pt electrode is measured to decrease from ~4.85 to ~4.68 eV, implying that the polarity of interaction dipole layer has its positive side toward the graphene layer. Based on such large effective work-function tuning (~1.04 eV) comparable to the Si band gap (~1.12 eV), the Al electrode accompanied by the partially-transferred graphene interlayer can be used for controlling both n-MOS and p-MOS systems.   

Understanding Weakly Interacting van der Waals Fermi Systems via Renormalization Group Theory

Weak-coupling phenomena of the two-dimensional Hubbard model is gaining momentum as a new interesting research field due to its extraordinarily rich behavior as a function of the carrier density and model parameters. Salmhofer [Commun. Math. Phys. 194, 249 (1998);Phys. Rev. Lett. 87, 187004 (2001)] developed a new renormalization-group method for interacting Fermi systems and Metzner [Phys. Rev. B 61, 7364 (2000);Phys. Rev. Lett. 85, 5162 (2000)] implemented this renormalization group analysis of the two-dimensional Hubbard model. In this work, we demonstrate the spin-wave dependent susceptibility behavior of model graphene-phosphorene van der Waals heterostructure in the framework of renormalization group approach. We implement signlet vertex response function for the weakly interacting van der Waals Fermi system with nearest-neighbor hopping amplitudes [J. Phys.: Condens. Matter 33, 335604 (2021)]. This analytical approach is further correlated with ab initio simulation results and extended for spin-wave dependent susceptibility behavior with possible experimental protocols. We present the resulting compressibility and phase diagram in the vicinity of half-filling, and also results for the density dependence of the critical energy scale.