Symmetry breaking and friction in few layer phosphorene

National Defense Science and Engineering Graduate Fellowshi

Gate-controlled reversible rectifying behaviour in tunnel contacted atomically-thin MoS2_{2} transistor

Atomically-thin 2D semiconducting materials integrated into van der Waals heterostructures have enabled architectures that hold great promise for next generation nanoelectronics. However, challenges still remain to enable their full acceptance as compliant materials for integration in logic devices. Two key-components to master are the barriers at metal/semiconductor interfaces and the mobility of the semiconducting channel, which endow the building-blocks of ${pn}$ diode and field effect transistor. Here, we have devised a reverted stacking technique to intercalate a wrinkle-free h-BN tunnel layer between MoS$_{2}$ channel and contacting electrodes. Vertical tunnelling of electrons therefore makes it possible to suppress the Schottky barriers and Fermi level pinning, leading to homogeneous gate-control of the channel chemical potential across the bandgap edges. The observed unprecedented features of ambipolar ${pn}$ to ${np}$ diode, which can be reversibly gate tuned, paves the way for future logic applications and high performance switches based on atomically thin semiconducting channel.Comment: 23 pages, 5 main figures + 9 SI figure

A gate-programmable van der Waals metal-ferroelectric-semiconductor memory

Ferroelecticity, one of the keys to realize nonvolatile memories owing to the remanent electric polarization, has been an emerging phenomenon in the two-dimensional (2D) limit. Yet the demonstrations of van der Waals (vdW) memories using 2D ferroelectric materials as an ingredient are very limited. Especially, gate-tunable ferroelectric vdW memristive device, which holds promises in future neuromorphic applications, remains challenging. Here, we show a prototype gate-programmable memory by vertically assembling graphite, CuInP2S6, and MoS2 layers into a metal-ferroelectric-semiconductor architecture. The resulted devices exhibit two-terminal switchable electro-resistance with on-off ratios exceeding 105 and long-term retention, akin to a conventional memristor but strongly coupled to the ferroelectric characteristics of the CuInP2S6 layer. By controlling the top gate, Fermi level of MoS2 can be set inside (outside) of its band gap to quench (enable) the memristive behaviour, yielding a three-terminal gate programmable nonvolatile vdW memory. Our findings pave the way for the engineering of ferroelectric-mediated memories in future implementations of nanoelectronics

Probing the fractional quantum Hall phases in valley-layer locked bilayer MoS2_{2}

Semiconducting transition-metal dichalcogenides (TMDs) exhibit high mobility, strong spin-orbit coupling, and large effective masses, which simultaneously leads to a rich wealth of Landau quantizations and inherently strong electronic interactions. However, in spite of their extensively explored Landau levels (LL) structure, probing electron correlations in the fractionally filled LL regime has not been possible due to the difficulty of reaching the quantum limit. Here, we report evidence for fractional quantum Hall (FQH) states at filling fractions 4/5 and 2/5 in the lowest LL of bilayer MoS$_{2}$, manifested in fractionally quantized transverse conductance plateaus accompanied by longitudinal resistance minima. We further show that the observed FQH states sensitively depend on the dielectric and gate screening of the Coulomb interactions. Our findings establish a new FQH experimental platform which are a scarce resource: an intrinsic semiconducting high mobility electron gas, whose electronic interactions in the FQH regime are in principle tunable by Coulomb-screening engineering, and as such, could be the missing link between atomically thin graphene and semiconducting quantum wells.Comment: 10 pages, 4 figure

In-plane magnetic domains and N\'eel-like domain walls in thin flakes of the room temperature CrTe2_2 van der Waals ferromagnet

The recent discovery of magnetic van der Waals materials has triggered a wealth of investigations in materials science, and now offers genuinely new prospects for both fundamental and applied research. Although the catalogue of van der Waals ferromagnets is rapidly expanding, most of them have a Curie temperature below 300 K, a notable disadvantage for potential applications. Combining element-selective x-ray magnetic imaging and magnetic force microscopy, we resolve at room temperature the magnetic domains and domains walls in micron-sized flakes of the CrTe$_2$ van der Waals ferromagnet. Flux-closure magnetic patterns suggesting in-plane six-fold symmetry are observed. Upon annealing the material above its Curie point (315 K), the magnetic domains disappear. By cooling back down the sample, a different magnetic domain distribution is obtained, indicating material stability and lack of magnetic memory upon thermal cycling. The domain walls presumably have N\'eel texture, are preferentially oriented along directions separated by 120 degrees, and have a width of several tens of nanometers. Besides microscopic mapping of magnetic domains and domain walls, the coercivity of the material is found to be of a few mT only, showing that the CrTe$_2$ compound is magnetically soft. The coercivity is found to increase as the volume of the material decreases

Ru passivated and Ru doped epsilon-TaN surfaces as a combined barrier and liner material for copper interconnects: a first principles study

The reduction of critical dimensions in transistor scaling means that a severe bottleneck arises from the lowest levels of device interconnects. Copper is currently used as an interconnect metal, but requires separate barrier, to prevent Cu diffusion, and liner, to promote Cu deposition, materials. Advanced interconnect technology will require coating of very high aspect ratio trench structures which means that the copper barrier + liner stack should take up only a very small volume of the trench to maintain the low copper resistivity. The current industry standard for Cu diffusion barriers is TaN and Ru is a widely used liner. In this paper we use first principles density functional theory (DFT) computations to explore in detail the interaction of Cu atoms at models of TaN, Ru and combined TaN/Ru barrier/liner materials. This model allows us to explore the role of these materials in Cu adsorption and diffusion (over the surface and into the bulk) at the very early stage of Cu film growth. As a benchmark we studied the behaviour of Cu and Ru adatoms at the low index surfaces of ε-TaN, and the interaction of Cu adatoms with the (0 0 1) surface of hexagonal Ru. These results confirm the barrier and liner properties of TaN and Ru, respectively, while also highlighting the weaknesses of both materials. We then investigate the adsorption and diffusion of Cu adatoms at Ru-passivated and Ru-doped ε-TaN(1 1 0) surfaces. Ru passivated TaN enhances the binding of Cu adatoms compared to the bare TaN and Ru surfaces. On the other hand, the activation energy for Cu diffusion at the Ru passivated TaN surface is lower than that on the bare TaN surface which may promote Cu agglomeration. For Ru-doped TaN we find a decrease in Cu binding energy. In addition, we find favourable migration of the Cu adatoms toward the doped Ru atom, compared with unfavourable migration of Cu away from the Ru site, into the bulk. This suggests that Ru doping sites on the TaN surface can act as nucleation points for Cu growth with high activation energies for agglomeration and can promote electroplating of Cu. Therefore we propose Ru-doped TaN as a candidate for a combined barrier/liner material which will have reduced thickness compared to individual barrier/liner material stacks

Synergistic interplay between Dirac fermions and long-wavelength order parameters in graphene-insulator heterostructures

In this work, we theoretically study the electronic structure, topological properties, and interaction effects of the low-energy Dirac electrons in band-aligned heterostructures consisted of graphene and some correlated insulating substrates. By virtue of the band alignment, charge carriers can be transferred from graphene to the insulating substrate under the control of gate voltages. This may yield a long-wavelength charge order at the surface of the substrate through the Wigner-crystallization mechanism. The long-wavelength charge order in turn exerts a superlattice Coulomb potential to the Dirac electrons in graphene, which reduces the non-interacting Fermi velocity, such that $e$-$e$ Coulomb interactions would play an important role. Consequently, the Dirac points are spontaneously gapped out by interactions leading to a sublattice polarized insulator state. Meanwhile, the Fermi velocities around the Dirac points are drastically enhanced to more than twice of the bare value by interaction effects, which can give rise to large Landau-level spacing with robust quantization plateaus of Hall resistivity under weak magnetic fields and at high temperatures. We have further performed high-throughput first principles calculations, and suggested a number of promising insulating materials as candidate substrates for graphene, which could realize the gapped Dirac state concomitant with low-field, high-temperature quantum Hall effects.Comment: Main text: 7 pages with 3 figures and 2 tables; Supplement info: 16 pages with 8 figures and 4 table

Large scale integration of CVD-graphene based NEMS with narrow distribution of resonance parameters

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