Ramamoorthy, Harihara

Preferred name

Alternative Name

Ramamoorthy, H.

Main Affiliation

School of Engineering

harihara.ra@kmitl.ac.th

Now showing 1 - 10 of 12

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Plasmon-mediated energy relaxation in graphene
(2015-12-28)
Ferry, D. K.
;
Somphonsane, R.
;
Ramamoorthy, H.
;
Bird, J. P.
Energy relaxation of hot carriers in graphene is studied at low temperatures, where the loss rate may differ significantly from that predicted for electron-phonon interactions. We show here that plasmons, important in the relaxation of energetic carriers in bulk semiconductors, can also provide a pathway for energy relaxation in transport experiments in graphene. We obtain a total loss rate to plasmons that results in energy relaxation times whose dependence on temperature and density closely matches that found experimentally.
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Probing charge trapping and joule heating in graphene field-effect transistors by transient pulsing
(2017-07-24)
Ramamoorthy, H.
;
Somphonsane, R.
;
Radice, J.
;
He, G.
;
Nathawat, J.
We use pulsed electrical studies to investigate the various processes that limit the current carrying capacity of graphene high frequency transistors. By investigating the transient response of these devices over a time scale that spans some twelve orders of magnitude, we identify the presence of four distinct processes that degrade the current: (1) charge injection into deep traps within the interior of the oxide; (2) Joule heating of the transistor substrate by hot carriers in the graphene channel; (3) equilibration of interfacial-state filling in response to voltage transients, and; (4) leakage of captured charge from the deep traps, once the pulsed voltage is removed. The time scale associated with these processes ranges from nanoseconds to hours, with process (1) being the fastest and process (4) the slowest. By pulsing the transistors on time intervals as short as a few nanoseconds, we therefore demonstrate how it is possible to obtain output characteristics from them that are essentially free from the influence of these different mechanisms. Under such conditions, the hot-carrier drift velocity is shown to saturate at the large values expected for intrinsic graphene. Beyond graphene, this approach of pulsed characterization of transistor performance should be broadly applicable to studies of other two-dimensional semiconductors, including transition-metal dichalcogenides, black phosphorous, silicene, and topological insulators.
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In-situ current annealing of graphene-metal contacts
(2018-12-19)
Ramamoorthy, Harihara
;
Somphonsane, Ratchanok
We study the effect of current-induced annealing on contact resistance between graphene and metal contacts, demonstrating that this technique may be used as an effective in-situ annealing procedure to improve the graphene-metal contact resistance which has long been an issue in the characterization of graphene-based devices. By studying as many as 30 devices with varying sample sizes and geometry, we are able to reduce the overall resistance systematically to around 400 Ω⋅μm, which is competitive with the best values obtained in the literature to treat this problem. We also demonstrate the effectiveness of current annealing in desorbing contaminants from the surface of the graphene layer, simultaneously shifting the charge-neutrality point to zero back-gate voltage, thus allowing the tuning of carrier density on both the electron and hole sides of the Dirac spectrum.
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Energy relaxation of hot carriers in graphene via plasmon interactions
(2016-03-01)
Ferry, D. K.
;
Somphonsane, R.
;
Ramamoorthy, H.
;
Bird, J. P.
Energy relaxation of hot carriers in graphene is studied theoretically and experimentally at low temperatures, where the loss rate may differ significantly from that predicted for electron–phonon interactions. We show here that plasmons, important in the relaxation of energetic carriers in bulk semiconductors, can also provide a pathway for energy relaxation in transport experiments in graphene. Reflecting the linear nature of graphene’s bands, we obtain a total loss rate to plasmons that is independent of carrier density. This results in energy relaxation times whose dependence on temperature and density closely matches that reported experimentally.
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Reversing hot-carrier energy-relaxation in graphene with a magnetic field
(2014-05-12)
Ramamoorthy, H.
;
Somphonsane, R.
;
He, G.
;
Ferry, D. K.
;
Ochiai, Y.
We investigate the influence of a perpendicular magnetic field on hot-carrier energy relaxation in bilayer graphene. Working in the regime of incipient Landau quantization, we find that the magnetic field influences the relaxation in a very different manner, dependent upon the position of the Fermi level relative to the Dirac point. While for carrier densities >10 12 cm-2 relaxation is slowed by the magnetic field, as the density of free carriers approaches zero it instead becomes quicker. We discuss this behavior in terms of the emergence of the zero-energy Landau level, and the role of charge puddling in graphene. © 2014 AIP Publishing LLC.
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Negative differential conductance & hot-carrier avalanching in monolayer WS2 FETs
(2017-12-01)
He, G.
;
Nathawat, J.
;
Kwan, C. P.
;
Ramamoorthy, H.
;
Somphonsane, R.
The high field phenomena of inter-valley transfer and avalanching breakdown have long been exploited in devices based on conventional semiconductors. In this Article, we demonstrate the manifestation of these effects in atomically-thin WS<inf>2</inf> field-effect transistors. The negative differential conductance exhibits all of the features familiar from discussions of this phenomenon in bulk semiconductors, including hysteresis in the transistor characteristics and increased noise that is indicative of travelling high-field domains. It is also found to be sensitive to thermal annealing, a result that we attribute to the influence of strain on the energy separation of the different valleys involved in hot-electron transfer. This idea is supported by the results of ensemble Monte Carlo simulations, which highlight the sensitivity of the negative differential conductance to the equilibrium populations of the different valleys. At high drain currents (>10 μA/μm) avalanching breakdown is also observed, and is attributed to trap-assisted inverse Auger scattering. This mechanism is not normally relevant in conventional semiconductors, but is possible in WS<inf>2</inf> due to the narrow width of its energy bands. The various results presented here suggest that WS<inf>2</inf> exhibits strong potential for use in hot-electron devices, including compact high-frequency sources and photonic detectors.
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Conductance fluctuations in graphene in the presence of long-range disorder
(2016-03-04)
Liu, Bobo
;
Akis, Richard
;
Ferry, David K.
;
Bohra, Girish
;
Somphonsane, Ratchanok
The fluctuations in the conductance of graphene that arise from a long-range disorder potential induced by random impurities are investigated with an atomic tight-binding lattice. The screened impurities lead to a slow variation of the background potential and this varies the overall potential landscape as the Fermi energy or an applied magnetic field is varied. As a result, the phase interference varies randomly and leads to fluctuations in the conductance. Recently, experiments have shown that an applied magnetic field produces a remarkable reduction in the amplitude of these conductance fluctuations. We find qualitative agreement with these experiments, and it appears that the reduction in magnetic field of the fluctuations arises from a field induced smoothing of the conductance landscape.
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"freeing" Graphene from Its Substrate: Observing Intrinsic Velocity Saturation with Rapid Electrical Pulsing
(2016-01-13)
Ramamoorthy, H.
;
Somphonsane, R.
;
Radice, J.
;
He, G.
;
Kwan, C. P.
Rapid (nanosecond-scale) electrical pulsing is used to study drift-velocity saturation in graphene field-effect devices. In these experiments, high-field pulses are utilized to drive graphene"s carriers on time scales much faster than that on which energy loss to the underlying substrate can occur, thereby allowing the observation of the highest saturation velocities reported to date. In a dramatic departure from the behavior exhibited by conventional metals and semiconductors, as the electron or hole density is reduced toward the charge-neutrality point, the drift velocity is found to reach values comparable to the Fermi velocity itself. Corresponding current densities are as large as 109 A/cm2, similar to the values reported for carbon nanotubes and for graphene-on-diamond transistors. In essence, our approach of rapid pulsing allows us to "free" graphene from the deleterious influence of its substrate, revealing a pathway to achieve the superior electrical performance promised by this material. The usefulness of this approach is not merely limited to graphene but should extend also to a broad variety of two-dimensional semiconductors.
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Conduction Mechanisms in CVD-Grown Monolayer MoS2 Transistors: From Variable-Range Hopping to Velocity Saturation
(2015-08-12)
He, G.
;
Ghosh, K.
;
Singisetti, U.
;
Ramamoorthy, H.
;
Somphonsane, R.
We fabricate transistors from chemical vapor deposition-grown monolayer MoS<inf>2</inf> crystals and demonstrate excellent current saturation at large drain voltages (V<inf>d</inf>). The low-field characteristics of these devices indicate that the electron mobility is likely limited by scattering from charged impurities. The current-voltage characteristics exhibit variable range hopping at low V<inf>d</inf> and evidence of velocity saturation at higher V<inf>d</inf>. This work confirms the excellent potential of MoS<inf>2</inf> as a possible channel-replacement material and highlights the role of multiple transport phenomena in governing its transistor action.
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Evaluating the sources of graphene's resistivity using differential conductance
(2017-12-01)
Somphonsane, R.
;
Ramamoorthy, H.
;
He, G.
;
Nathawat, J.
;
Kwan, C. P.
We explore the contributions to the electrical resistance of monolayer and bilayer graphene, revealing transitions between different regimes of charge carrier scattering. In monolayer graphene at low densities, a nonmonotonic variation of the resistance is observed as a function of temperature. Such behaviour is consistent with the influence of scattering from screened Coulomb impurities. At higher densities, the resistance instead varies in a manner consistent with the influence of scattering from acoustic and optical phonons. The crossover from phonon-, to charged-impurity, limited conduction occurs once the concentration of gate-induced carriers is reduced below that of the residual carriers. In bilayer graphene, the resistance exhibits a monotonic decrease with increasing temperature for all densities, with the importance of short-range impurity scattering resulting in a "universal" density-independent (scaled) conductivity at high densities. At lower densities, the conductivity deviates from this universal curve, pointing to the importance of thermal activation of carriers out of charge puddles. These various assignments, in both systems, are made possible by an approach of "differential-conductance mapping", which allows us to suppress quantum corrections to reveal the underlying mechanisms governing the resistivity.

Ramamoorthy, Harihara

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