Sameer Grover

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.

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.

Light matter interaction in WS2 nanotube-graphene hybrid devices

We study the light matter interaction in WS2 nanotube-graphene hybrid devices. Using scanning photocurrent microscopy, we find that by engineering graphene electrodes for WS2 nanotubes we can improve the collection of photogenerated carriers. We observe inhomogeneous spatial photocurrent response with an external quantum efficiency of ∼1% at 0 V bias. We show that defects play an important role and can be utilized to enhance and tune photocarrier generation.

Abrupt p-n junction using ionic gating at zero-bias in bilayer graphene

Graphene is a promising candidate for optoelectronic applications. In this report, a double gated bilayer graphene FET has been made using a combination of electrostatic and electrolytic gating in order to form an abrupt p-n junction. The presence of two Dirac peaks in the gating curve of the fabricated device confirms the formation of a p-n junction. At low temperatures, when the electrolyte is frozen intentionally, the photovoltage exhibits a six-fold pattern indicative of the hot electron induced photothermoelectric effect that has also been seen in graphene p-n junctions made using metallic gates. We have observed that the photovoltage increases with decreasing temperature indicating a dominant role of supercollision scattering. Our technique can also be extended to other 2D materials and to finer features that will lead to p-n junctions which span a large area, like a superlattice, that can generate a larger photoresponse. Our work creating abrupt p-n junctions is distinct from previous works that use a source–drain bias voltage with a single ionic gate creating a spatially graded p-n junction.

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.

Multilayer ReS2 Photodetectors with Gate Tunability for High Responsivity and High Speed Applications

Rhenium disulfide (ReS2) is an attractive candidate for photodetection applications owing to its thickness-independent direct band gap. Despite various photodetection studies using two-dimensional semiconductors, the trade-off between responsivity and response time under varying measurement conditions has not been studied in detail. This report presents a comprehensive study of the architectural, laser power and gate bias dependence of responsivity and speed in supported and suspended ReS2 phototransistors. Photocurrent scans show uniform photogeneration across the entire channel because of enhanced optical absorption and a direct band gap in multilayer ReS2. A high responsivity of 4 A W-1 (at 50 ms response time) and a low response time of 20 μs (at 4 mA W-1 responsivity) make this one of the fastest reported transition-metal dichalcogenide photodetectors. Occupancy of intrinsic (bulk ReS2) and extrinsic (ReS2/SiO2 interface) traps is modulated using gate bias to demonstrate tunability of the response time (responsivity) over 4 orders (15×) of magnitude, highlighting the versatility of these photodetectors. Differences in the trap distributions of suspended and supported channel architectures, and their occupancy under different gate biases enable switching the dominant operating mechanism between either photogating or photoconduction. Further, a new metric that captures intrinsic photodetector performance by including the trade-off between its responsivity and speed, besides normalizing for the applied bias and geometry, is proposed and benchmarked for this work.