Aron Pinczuk

Electric field effect tuning of electron-phonon coupling in graphene.

Gate-modulated low-temperature Raman spectra reveal that the electric field effect (EFE), pervasive in contemporary electronics, has marked impacts on long-wavelength optical phonons of graphene. The EFE in this two-dimensional honeycomb lattice of carbon atoms creates large density modulations of carriers with linear dispersion (known as Dirac fermions). Our EFE Raman spectra display the interactions of lattice vibrations with these unusual carriers. The changes of phonon frequency and linewidth demonstrate optically the particle-hole symmetry about the charge-neutral Dirac point. The linear dependence of the phonon frequency on the EFE-modulated Fermi energy is explained as the electron-phonon coupling of massless Dirac fermions.

Visualizing Individual Nitrogen Dopants in Monolayer Graphene

Nitrogen atoms that replace carbon atoms in the graphene lattice strongly modify the local electronic structure. In monolayer graphene, substitutional doping during growth can be used to alter its electronic properties. We used scanning tunneling microscopy, Raman spectroscopy, x-ray spectroscopy, and first principles calculations to characterize individual nitrogen dopants in monolayer graphene grown on a copper substrate. Individual nitrogen atoms were incorporated as graphitic dopants, and a fraction of the extra electron on each nitrogen atom was delocalized into the graphene lattice. The electronic structure of nitrogen-doped graphene was strongly modified only within a few lattice spacings of the site of the nitrogen dopant. These findings show that chemical doping is a promising route to achieving high-quality graphene films with a large carrier concentration.

Nitrogen plasma annealing for low temperature Ta2O5 films

A low temperature oxygen/nitrogen plasma process is reported that substantially reduces leakage currents in chemical vapor deposited (CVD) and physical vapor deposited (PVD) films of tantalum oxide. We show that a combination of nitrogen and oxygen in a remote downstream microwave plasma source reduces leakage currents in CVD films of tantalum oxide and also reduces trap densities as measured by charge pumping. The as deposited CVD films show a high level of photoluminescence that is substantially lowered by the plasma anneal due to a reduction in the density of midgap states. For films deposited by PVD in the thickness range of 100 nm we find low leakage currents with a substantial improvement from the introduction of nitrogen into the plasma. However, PVD films in the thickness range of 20 nm show larger relative leakage currents and less of an improvement from the addition of nitrogen. The role of nitrogen in lowering leakage currents and charge trapping is thought to occur from a reduction in the dens...

Observation of anomalous phonon softening in bilayer graphene.

The interaction of electron-hole pairs with lattice vibrations exhibits a wealth of intriguing physical phenomena such as the renowned Kohn anomaly. Here we report the observation in bilayer graphene of an unusual phonon softening that provides the first experimental proof for another type of phonon anomaly. Similar to the Kohn anomaly, which is a logarithmic singularity in the phonon group velocity [W. Kohn, Phys. Rev. Lett. 2, 393 (1959)], the observed phonon anomaly exhibits a logarithmic singularity in the optical-phonon energy. Arising from a resonant electron-phonon coupling effect, the anomaly was also expected, albeit not observed, in monolayer graphene. We propose an explanation for why it is easier to observe in bilayer samples.

Large Physisorption Strain in Chemical Vapor Deposition of Graphene on Copper Substrates

Graphene single layers grown by chemical vapor deposition on single crystal Cu substrates are subject to nonuniform physisorption strains that depend on the orientation of the Cu surface. The strains are revealed in Raman spectra and quantitatively interpreted by molecular dynamics (MD) simulations. An average compressive strain on the order of 0.5% is determined in graphene on Cu(111). In graphene on Cu (100), MD simulations interpret the observed highly nonuniform strains.

Metamorphosis of a quantum wire into quantum dots

Bound states of electron–hole pairs (excitons) in semiconductors possess desirable properties — such as an enhanced oscillator strength for radiative recombination — that hold promise for the next generation of optical devices. However, at typical device operating conditions (room temperature and moderate charge densities), excitons dissociate to form an electron–hole plasma. Dissociation may be prevented by confining excitons to lower dimensions, where their binding energy is expected to increase significantly. But such confinement may in turn influence the dynamical properties of the excitons. Here we report spatially resolved photoluminescence images of excitons confined to an isolated gallium arsenide quantum wire. As the temperature of the structure is lowered, we observe a striking transition from broad and fairly continuous photoluminescence to an intense set of emission peaks which are both energetically sharp and spatially localized. Such behaviour indicates that, at sufficiently low temperatures, the quantum wire acts like a sparse set of quantum dots. Furthermore, at the site of an isolated quantum dot, we observe an unusual decrease in the relaxation rate of excitons, such that they radiate (via recombination) from higher energy states before relaxing to their ground state. We argue that this is the manifestation of an exciton relaxation ‘bottleneck’, the existence of which could pose problems for the development of optical devices based on quantum dots.

Engineering artificial graphene in a two-dimensional electron gas

At low energy, electrons in doped graphene sheets behave like massless Dirac fermions with a Fermi velocity, which does not depend on carrier density. Here we show that modulating a two-dimensional electron gas with a long-wavelength periodic potential with honeycomb symmetry can lead to the creation of isolated massless Dirac points with tunable Fermi velocity. We provide detailed theoretical estimates to realize such artificial graphenelike system and discuss an experimental realization in a modulation-doped GaAs quantum well. Ultrahigh-mobility electrons with linearly dispersing bands might open new venues for the studies of Dirac-fermion physics in semiconductors.

Graphene growth on h-BN by molecular beam epitaxy

Abstract The growth of single layer graphene nanometer size domains by solid carbon source molecular beam epitaxy on hexagonal boron nitride (h-BN) flakes is demonstrated. Formation of single-layer graphene is clearly apparent in Raman spectra which display sharp optical phonon bands. Atomic-force microscope images and Raman maps reveal that the graphene grown depends on the surface morphology of the h-BN substrates. The growth is governed by the high mobility of the carbon atoms on the h-BN surface, in a manner that is consistent with van der Waals epitaxy. The successful growth of graphene layers depends on the substrate temperature, but is independent of the incident flux of carbon atoms.

Inelastic light scattering by electronic excitations in semiconductor heterostructures

In recent years there has been much research on inelastic light scattering by quasi-2-D electron systems in quantum wells and heterostructures. In this paper we present a general description of resonant inelastic light scattering as a spectroscopic method that reveals single particle and collective behavior of electrons and holes in semiconductor microstructures and review some of the more recent work. We consider high-mobility two-dimensional free carrier systems in modulation doped GaAs-(AlGa)As heterostructures, purely space-charge induced quantum wells of GaAs and shallow impurities in GaAs- (AlGa)As quantum wells.

Resonance Raman scattering in CdTe‐ZnTe superlattices

We present a resonance Raman study of strain and excitons in CdTe‐ZnTe superlattices. We obtain the strain configuration of the samples from the strain‐induced shifts in the Raman frequencies of confined longitudinal optical phonons. We find that CdTe‐ZnTe superlattices are not always in a free‐standing configuration, as previously suggested. For individual layer thicknesses of the order of 20–30 A, our superlattices can be grown lattice matched with their Cd0.1Zn0.9Te substrates. In resonance with the fundamental gap of the superlattice, we observe an enhancement of the Raman cross section not only for CdTe phonons but also (in the samples with the thinnest layers) for phonons confined in ZnTe. This behavior can be understood in terms of a small valence‐band offset.