arXiv subjects

Wei Sun Leong

Publications and source records attributed to Wei Sun Leong.

5 recordsLinked to original sources

arXiv2017

Origin of Contact Resistance at Ferromagnetic Metal-Graphene Interfaces

Edge contact geometries are thought to yield ultralow contact resistances in most non-ferromagnetic metal-graphene interfaces owing to their large metal-graphene coupling strengths. Here, we examine the contact resistance of edge- versus surface-contacted ferromagnetic metal-graphene interfaces (i.e. nickel- and cobalt-graphene interfaces) using both single-layer and few-layer graphene. Good qualitative agreement is obtained between theory and experiment. In particular, in both theory and experiment, we observe that the contact resistance of edge-contacted ferromagnetic metal-graphene interfaces is much lower than that of surface-contacted ones, for all devices studied and especially for the single-layer graphene systems. We show that this difference in resistance is not due to differences in the metal-graphene coupling strength, which we quantify using Hamiltonian matrix elements. Instead, the larger contact resistance in surface contacts results from spin filtering at the interface, in contrast to the edge-contacted case where both spins are transmitted. Temperature-dependent resistance measurements beyond the Curie temperature TC show that the spin degree of freedom is indeed important for the experimentally measured contact resistance. These results show that it is possible to induce a large change in contact resistance by changing the temperature in the vicinity of TC, thus paving the way for temperature-controlled switches based on spin.

cond-mat.mtrl-sci↗

arXiv2015

Tuning the Threshold Voltage of MoS2 Field-Effect Transistors via Surface Treatment

Controlling the threshold voltage (Vth) of a field-effect transistor is important for realizing robust logic circuits. Here, we report a facile approach to achieve bidirectional Vth tuning of molybdenum disulfide (MoS2) field-effect transistors. By increasing and decreasing the amount of sulfur vacancies in the MoS2 surface, the Vth of MoS2 transistors can be left- and right-shifted, respectively. Transistors fabricated on perfect MoS2 flakes are found to exhibit two-fold enhancement in mobility and a very positive Vth. More importantly, our elegant hydrogen treatment is able to tune the large Vth to a small value without any performance degradation simply by reducing the atomic ratio of S:Mo slightly; in other words, creating a certain amount of sulfur vacancies in the MoS2 surface, which generate defect states in the band gap of MoS2 that mediate conduction of a MoS2 transistor in the subthreshold regime. First-principles calculations further indicate that the edge and width of defect band can be tuned according to the vacancy density. This work not only demonstrates for the first time the ease in tuning the Vth of MoS2 transistors, but also offers a process technology solution that is critical for further development of MoS2 as a mainstream electronic material.

cond-mat.mes-hall↗

arXiv2014

Low Resistance Metal Contacts to MoS2 Devices with Nickel-Etched-Graphene Electrodes

We report an approach to achieve low-resistance contacts to MoS2 transistors with the intrinsic performance of the MoS2 channel preserved. Through a dry transfer technique and a metal-catalyzed graphene treatment process, nickel-etched-graphene electrodes were fabricated on MoS2 that yield contact resistance as low as 200 ohm-um. The substantial contact enhancement (~2 orders of magnitude) as compared to pure nickel electrodes, is attributed to the much smaller work function of nickel-graphene electrodes, together with the fact that presence of zigzag edges in the treated graphene surface enhances tunneling between nickel and graphene. To this end, the successful fabrication of a clean graphene-MoS2 interface and a low resistance nickel-graphene interface is critical for the experimentally measured low contact resistance. The potential of using graphene as an electrode interlayer demonstrated in this work paves the way towards achieving high performance next-generation transistors.

cond-mat.mes-hall↗

arXiv2014

Gold on WSe2 Single Crystal Film as a Substrate for Surface Enhanced Raman Scattering (SERS) Sensing

Synthesis and characterization of high-quality single-crystal WSe2 films on highly-insulating substrate is presented. We demonstrate for the first time that the presence of gold nanoparticles in the basal plane of a WSe2 film can enhance its Raman scattering intensity. The experimentally observed enhancement ratio in the Raman signal correlates well with the simulated electric field intensity using a three-dimensional electromagnetic software and theoretical calculation. This work provides guidelines for the use of two-dimensional WSe2 films as a SERS substrate.

cond-mat.mes-hall↗

arXiv2013

Low-Contact-Resistance Graphene Devices with Nickel-Etched-Graphene Contacts

The performance of graphene-based transistors is often limited by the large electrical resistance across the metal-graphene contact. We report an approach to achieve ultra-low resistance metal contacts to graphene transistors. Through a process of metal-catalyzed etching in hydrogen, multiple nano-sized pits with zigzag edges are created in the graphene that form strong chemical bonds with deposited nickel metallization for source-drain contacts without the need for further annealing. This facile contact treatment prior to electrode metallization results in contact resistance as low as 100 ohm-um in single-layer graphene field-effect transistors, and 11 ohm-um in bilayer graphene transistors. The treatment is compatible with complementary metal-oxide-semiconductor fabrication processes, and holds great promise to meet the contact performance required for the integration of graphene in future integrated circuits.

cond-mat.mes-hall↗