Sudipta Dubey

Schottky barrier heights for Au and Pd contacts to MoS2

The search of a p-type metal contact on MoS2 has remained inconclusive, with high work function metals such as Au, Ni, and Pt showing n-type behavior and mixed reports of n as well as p-type behavior for Pd. In this work, we report quantitative Schottky barrier heights for Au and Pd contacts to MoS2 obtained by analysing low temperature transistor characteristics and contact resistance data obtained using the transfer length method. Both Au and Pd exhibit n-type behavior on multilayer as well as monolayer MoS2 transistors with Schottky barrier heights of 0.126 eV and 0.4 eV, and contact resistances of 42 Ω.mm and 18 × 104 Ω.mm respectively. Scanning photocurrent spectroscopy data is in agreement with the resulting energy band alignment in Au-MoS2-Pd devices further reinforcing the observation that the Fermi-level is pinned in the upper half of MoS2 bandgap.

A facile process for soak-and-peel delamination of CVD graphene from substrates using water

We demonstrate a simple technique to transfer chemical vapour deposited (CVD) graphene from copper and platinum substrates using a soak-and-peel delamination technique utilizing only hot deionized water. The lack of chemical etchants results in cleaner CVD graphene films minimizing unintentional doping, as confirmed by Raman and electrical measurements. The process allows the reuse of substrates and hence can enable the use of oriented substrates for growth of higher quality graphene, and is an inherently inexpensive and scalable process for large-area production.

Tunable superlattice in graphene to control the number of Dirac points.

Superlattice in graphene generates extra Dirac points in the band structure and their number depends on the superlattice potential strength. Here, we have created a lateral superlattice in a graphene device with a tunable barrier height using a combination of two gates. In this Letter, we demonstrate the use of lateral superlattice to modify the band structure of graphene leading to the emergence of new Dirac cones. This controlled modification of the band structure persists up to 100 K.

Coupling between quantum Hall state and electromechanics in suspended graphene resonator

Using graphene resonator, we perform electromechanical measurements in quantum Hall regime to probe the coupling between a quantum Hall (QH) system and its mechanical motion. Mechanically perturbing the QH state through resonance modifies the DC resistance of the system and results in a Fano-lineshape due to electronic interference. Magnetization of the system modifies the resonator’s equilibrium position and effective stiffness leading to changes in resonant frequency. Our experiments show that there is an intimate coupling between the quantum Hall state and mechanics—electron transport is affected by physical motion and in turn the magnetization modifies the electromechanical response.

Limits on the bolometric response of graphene due to flicker noise

We study the photoresponse of graphene field effect transistors using scanning photocurrent microscopy in near and far field configurations, and we find that the response of graphene under a source-drain bias voltage away from the contacts is dominated by the bolometric effect caused by laser induced heating. We find no significant change in the photocurrent with the optical modulation frequency upto 100 kHz. Although the magnitude of the bolometric current scales with bias voltage, it also results in noise. The frequency dependence of this noise indicates that it has a 1/f character, scales with the bias voltage and limits the detectable bolometric photoresponse at low optical powers.

High Q electromechanics with InAs nanowire quantum dots

In this report, we study electromechanical properties of suspended InAs nanowire resonators. At low temperatures, the nanowire acts as the island of a single electron transistor, and a strong coupling between electron transport and mechanical modes is observed for resonant mechanical driving. Further, as a function of the mechanical drive frequency, the conductance exhibits a Fano lineshape. This arises from the interference between two contributions to potential of the single electron transistor coming from capacitively induced charges and the mechanical oscillation. The quality factor (Q) of these devices is ∼105 at 100 mK.

Evaluating Au and Pd contacts in mono and multilayer MoS 2 transistors

The search of a p-type metal contact on MoS₂ has remained inconclusive, with high work-function metals such as Au, Ni and Pt showing n-type behavior [1] and mixed reports of n as well as p-type behavior for Pd. In this work we report for the first time, quantitative band alignment of Pd and Au-MoS₂ interfaces using low temperature and scanning photocurrent measurements on MoS₂ transistors with varying metal contacts (Au-Au, Pd-Pd and Au-Pd). Our results indicate n-type behavior for Pd contacts on multilayer as well as monolayer MoS₂ transistors and a barrier height (Φ_b) of nearly 0.5 eV, four times that for Au contacts indicating that the MoS₂ Fermi-level is pinned in the upper half of MoS₂ bandgap.