arXiv subjects

Vasili Perebeinos

Publications and source records attributed to Vasili Perebeinos.

At least 55 records · Page 3Linked to original sources

arXiv2010

Efficient narrow-band light emission from a single carbon nanotube p-n diode

Electrically-driven light emission from carbon nanotubes could be exploited in nano-scale lasers and single-photon sources, and has therefore been the focus of much research. However, to date, high electric fields and currents have been either required for electroluminescence, or have been an undesired side effect, leading to high power requirements and low efficiencies. In addition, electroluminescent linewidths have been broad enough to obscure the contributions of individual optical transitions. Here, we report electrically-induced light emission from individual carbon nanotube p-n diodes. A new level of control over electrical carrier injection is achieved, reducing power dissipation by a factor of up to 1000, and resulting in zero threshold current, negligible self-heating, and high carrier-to- photon conversion efficiencies. Moreover, the electroluminescent spectra are significantly narrower (ca. 35 meV) than in previous studies, allowing the identification of emission from free and localized excitons.

cond-mat.mes-hall↗

arXiv2010

Thermal infrared emission reveals the Dirac point movement in biased graphene

Graphene is a 2-dimensional material with high carrier mobility and thermal conductivity, suitable for high-speed electronics. Conduction and valence bands touch at the Dirac point. The absorptivity of single-layer graphene is 2.3%, nearly independent of wavelength. Here we investigate the thermal radiation from biased graphene transistors. We find that the emission spectrum of single-layer graphene follows that of a grey body with constant emissivity (1.6 \pm 0.8)%. Most importantly, we can extract the temperature distribution in the ambipolar graphene channel, as confirmed by Stokes/anti-Stokes measurements. The biased graphene exhibits a temperature maximum whose location can be controlled by the gate voltage. We show that this peak in temperature reveals the spatial location of the minimum in carrier density, i.e. the Dirac point.

cond-mat.mes-hall↗

arXiv2010

Inelastic Scattering and Current Saturation in Graphene

We present a study of transport in graphene devices on polar insulating substrates by solving the Bolzmann transport equation in the presence of graphene phonon, surface polar phonon, and Coulomb charged impurity scattering. The value of the saturated velocity shows very weak dependence on the carrier density, the nature of the insulating substrate, and the low-field mobility, varied by the charged impurity concentration. The saturated velocity of 4 - 8 x 10^7 cm/s calculated at room temperature is significantly larger than reported experimental values. The discrepancy is due to the self-heating effect which lowers substantially the value of the saturated velocity. We predict that by reducing the insulator oxide thickness, which limits the thermal conductance, the saturated currents can be significantly enhanced. We also calculate the surface polar phonon contribution to the low-field mobility as a function of carrier density, temperature, and distance from the substrate.

cond-mat.mes-hall↗

arXiv2009

Photocurrent imaging and efficient photon detection in a graphene transistor

We measure the channel potential of a graphene transistor using a scanning photocurrent imaging technique. We show that at a certain gate bias, the impact of the metal on the channel potential profile extends into the channel for more than 1/3 of the total channel length from both source and drain sides, hence most of the channel is affected by the metal. The potential barrier between the metal controlled graphene and bulk graphene channel is also measured at various gate biases. As the gate bias exceeds the Dirac point voltage, VDirac, the original p-type graphene channel turns into a p-n-p channel. When light is focused on the p-n junctions, an impressive external responsivity of 0.001 A/W is achieved, given that only a single layer of atoms are involved in photon detection.

cond-mat.mtrl-sci↗

arXiv2009

Energy dissipation in graphene field-effect transistors

We measure the temperature distribution in a biased single-layer graphene transistor using Raman scattering microscopy of the 2D-phonon band. Peak operating temperatures of 1050 K are reached in the middle of the graphene sheet at 210 KW cm^(-2) of dissipated electric power. The metallic contacts act as heat sinks, but not in a dominant fashion. To explain the observed temperature profile and heating rate, we have to include heat-flow from the graphene to the gate oxide underneath, especially at elevated temperatures, where the graphene thermal conductivity is lowered due to umklapp scattering. Velocity saturation due to phonons with about 50 meV energy is inferred from the measured charge density via shifts in the Raman G-phonon band, suggesting that remote scattering (through field coupling) by substrate polar surface phonons increases the energy transfer to the substrate and at the same time limits the high-bias electronic conduction of graphene.

cond-mat.mtrl-sci↗

arXiv2009

Current Saturation and Surface Polar Phonon Scattering in Graphene

We present a study of transport in graphene devices on polar insulating substrates using a tight-binding model. The mobility is computed using a multiband Boltzmann treatment. We provide the scaling of the surface polar phonon contribution to the low-field mobility with carrier density, temperature, and distance from the substrate. At high bias, we find that graphene self-heating effect is essential to account for the observed saturated current behavior. We predict that by optimizing the device cooling, the high bias currents can be significantly enhanced.

cond-mat.mes-hall↗

arXiv2009

Carrier scattering, mobilities and electrostatic potential in mono-, bi- and tri-layer graphenes

The carrier density and temperature dependence of the Hall mobility in mono-, bi- and tri-layer graphene has been systematically studied. We found that as the carrier density increases, the mobility decreases for mono-layer graphene, while it increases for bi-layer/tri-layer graphene. This can be explained by the different density of states in mono-layer and bi-layer/tri-layer graphenes. In mono-layer, the mobility also decreases with increasing temperature primarily due to surface polar substrate phonon scattering. In bi-layer/tri-layer graphene, on the other hand, the mobility increases with temperature because the field of the substrate surface phonons is effectively screened by the additional graphene layer(s) and the mobility is dominated by Coulomb scattering. We also find that the temperature dependence of the Hall coefficient in mono-, bi- and tri-layer graphene can be explained by the formation of electron and hole puddles in graphene. This model also explains the temperature dependence of the minimum conductance of mono-, bi- and tri-layer graphene. The electrostatic potential variations across the different graphene samples are extracted.

cond-mat.mes-hall↗

arXiv2009

Valence Force Model for Phonons in Graphene and Carbon Nanotubes

Many calculations require a simple classical model for the interactions between sp^2-bonded carbon atoms, as in graphene or carbon nanotubes. Here we present a new valence force model to describe these interactions. The calculated phonon spectrum of graphene and the nanotube breathing-mode energy agree well with experimental measurements and with ab initio calculations. The model does not assume an underlying lattice, so it can also be directly applied to distorted structures. The characteristics and limitations of the model are discussed.

cond-mat.mtrl-sci↗

arXiv2009

How does the substrate affect the Raman and excited state spectra of a carbon nanotube?

We study the optical properties of a single, semiconducting single-walled carbon nanotube (CNT) that is partially suspended across a trench and partially supported by a SiO2-substrate. By tuning the laser excitation energy across the E33 excitonic resonance of the suspended CNT segment, the scattering intensities of the principal Raman transitions, the radial breathing mode (RBM), the G-mode and the D-mode show strong resonance enhancement of up to three orders of magnitude. In the supported part of the CNT, despite a loss of Raman scattering intensity of up to two orders of magnitude, we recover the E33 excitonic resonance suffering a substrate-induced red shift of 50 meV. The peak intensity ratio between G-band and D-band is highly sensitive to the presence of the substrate and varies by one order of magnitude, demonstrating the much higher defect density in the supported CNT segments. By comparing the E33 resonance spectra measured by Raman excitation spectroscopy and photoluminescence (PL) excitation spectroscopy in the suspended CNT segment, we observe that the peak energy in the PL excitation spectrum is red-shifted by 40 meV. This shift is associated with the energy difference between the localized exciton dominating the PL excitation spectrum and the free exciton giving rise to the Raman excitation spectrum. High-resolution Raman spectra reveal substrate-induced symmetry breaking, as evidenced by the appearance of additional peaks in the strongly broadened Raman G band. Laser-induced line shifts of RBM and G band measured on the suspended CNT segment are both linear as a function of the laser excitation power. Stokes/anti-Stokes measurements, however, reveal an increase of the G phonon population while the RBM phonon population is rather independent of the laser excitation power.

cond-mat.mtrl-sci↗

arXiv2009

An essential mechanism of heat dissipation in carbon nanotube electronics

Excess heat generated in integrated circuits is one of the major problems of modern electronics. Surface phonon-polariton scattering is shown here to be the dominant mechanism for hot charge carrier energy dissipation in a nanotube device fabricated on a polar substrate, such as $SiO_2$. Using microscopic quantum models the Joule losses were calculated for the various energy dissipation channels as a function of the electric field, doping, and temperature. The polariton mechanism must be taken into account to obtain an accurate estimate of the effective thermal coupling of the non-suspended nanotube to the substrate, which was found to be 0.1-0.2 W/m.K even in the absence of the bare phononic thermal coupling.

cond-mat.mtrl-sci↗

arXiv2008

Chemical Doping and Electron-Hole Conduction Asymmetry in Graphene Devices

We investigate polyethylene imine and diazonium salts as stable, complementary dopants on graphene. Transport in graphene devices doped with these molecules exhibits asymmetry in electron and hole conductance. The conductance of one carrier is preserved, while the conductance of the other carrier decreases. Simulations based on nonequilibrium Green's function formalism suggest that the origin of this asymmetry is imbalanced carrier injection from the graphene electrodes caused by misalignment of the electrode and channel neutrality points.

cond-mat.mes-hall↗

arXiv2008

The Effects of Substrate Phonon Mode Scattering on Transport in Carbon Nanotubes

Carbon nanotubes (CNTs) have large intrinsic carrier mobility due to weak acoustic phonon scattering. However, unlike two-dimensional metal-oxide-semiconductor field effect transistors (MOSFETs), substrate surface polar phonon (SPP) scattering has a dramatic effect on the CNTFET mobility, due to the reduced vertical dimensions of the latter. We find that for the Van der Waals distance between CNT and an SiO$_2$ substrate, the low-field mobility at room temperature is reduced by almost an order of magnitude depending on the tube diameter. We predict additional experimental signatures of the SPP mechanism in dependence of the mobility on density, temperature, tube diameter, and CNT - substrate separation.

cond-mat.mes-hall↗

arXiv2008

Computational study of exciton generation in suspended carbon nanotube transistors

Optical emission from carbon nanotube transistors (CNTFETs) has recently attracted significant attention due to its potential applications. In this paper, we use a self-consistent numerical solution of the Boltzmann transport equation in the presence of both phonon and exciton scattering to present a detailed study of the operation of a partially suspended CNTFET light emitter, which has been discussed in a recent experiment. We determine the energy distribution of hot carriers in the CNTFET, and, as reported in the experiment, observe localized generation of excitons near the trench-substrate junction and an exponential increase in emission intensity with a linear increase in current versus gate voltage. We further provide detailed insight into device operation, and propose optimization schemes for efficient exciton generation; a deeper trench increases the generation efficiency, and use of high-k substrate oxides could lead to even larger enhancements.

cond-mat.mes-hall↗

arXiv2008

Conductance Quantization in Graphene Nanoribbons

We report the experimental observation of conductance quantization in graphene nanoribbons, where 1D transport subbands are formed due to the lateral quantum confinement. We show that this quantization in graphene nanoribbons can be observed at temperatures as high as 80 K and channel lengths as long as 1.7 $μ$m. The observed quantization is in agreement with that predicted by theoretical calculations.

cond-mat.mes-hall↗

arXiv2008

Phonon and Electronic Non-radiative Decay of Excitons in Carbon Nanotubes

We investigate theoretically the rates of non-radiative decay of excited semiconducting nanotubes by a variety of decay mechanisms and compare with experimental findings. We find that the multi-phonon decay (MPD) of free excitons is too slow to be responsible for the experimentally observed lifetimes. However, MPD lifetimes of localized excitons could be 2-3 orders of magnitude shorter. We also propose a new decay mechanism that relies on a finite doping of nanotubes and involves exciton decay into an optical phonon and an intraband electron-hole pair. The resulting lifetime is in the range of 5 to 100 ps, even for a moderate doping level.

cond-mat.str-el↗

arXiv2007

Intersubband decay of 1-D exciton resonances in carbon nanotubes

We have studied intersubband decay of E22 excitons in semiconducting carbon nanotubes experimentally and theoretically. Photoluminescence excitation line widths of semiconducting nanotubes with chiral indicess (n, m) can be mapped onto a connectivity grid with curves of constant (n-m) and (2n+m). Moreover, the global behavior of E22 linewidths is best characterized by a strong increase with energy irrespective of their (n-m) mod(3)= \pm 1 family affiliation. Solution of the Bethe-Salpeter equations shows that the E22 linewidths are dominated by phonon assisted coupling to higher momentum states of the E11 and E12 exciton bands. The calculations also suggest that the branching ratio for decay into exciton bands vs free carrier bands, respectively is about 10:1.

cond-mat.mtrl-sci↗

arXiv2007

Exciton Ionization, Franz-Keldysh and Stark Effects in Carbon Nanotubes

We calculate the optical properties of carbon nanotubes in an external static electric field directed along the tube axis. We predict strong Franz-Keldysh oscillations in the first and second band-to-band absorption peaks, quadratic Stark effect of the first two excitons, and the field dependence of the bound exciton ionization rate for a wide range of tube chiralities. We find that the phonon assisted mechanism dominates the dissociation rate in electro-optical devices due to the hot optical phonons. We predict a quadratic dependence of the Sommerfeld factor on the electric field and its increase up to 2000% at the critical field of the full exciton dissociation.

cond-mat.mes-hall↗

arXiv2006

Impact Excitation by Hot Carriers in Carbon Nanotubes

We investigate theoretically the efficiency of intra-molecular hot carrier induced impact ionization and excitation processes in carbon nanotubes. The electron confinement and reduced screening lead to drastically enhanced excitation efficiencies over those in bulk materials. Strong excitonic coupling favors neutral excitations over ionization, while the impact mechanism populates a different set of states than that produced by photoexcitation. The excitation rate is strongly affected by optical phonon excitation and a simple scaling of the rate with the field strength and optical phonon temperature is obtained.

cond-mat.str-el↗