Kalman Varga

First-principles calculations of electron mobilities in silicon: Phonon and Coulomb scattering

Electron mobilities limited by phonon and ionized impurity scattering have traditionally been modeled by suppressing atomic-scale detail, relying on empirical deformation potentials and either effective-mass theory or bulk energy bands to describe electron velocities. Parameter fitting to experimental data is needed. As modern technologies require modeling of transport at the nanoscale and unprecedented materials are introduced, predictive parameter-free mobility modeling becomes necessary. Here we report the development of first-principles quantum-mechanical methods to calculate scattering rates and electronic mobilities limited by phonon and ionized-impurity scattering. We report results for n-doped silicon that are in good agreement with experiment.

Excited Biexcitons in Transition Metal Dichalcogenides

The Stochastic Variational Method (SVM) is used to show that the effective mass model correctly estimates the binding energies of excitons and trions but fails to predict the experimental binding energy of the biexciton. Using high-accuracy variational calculations, it is demonstrated that the biexciton binding energy in transition metal dichalcogenides is smaller than the trion binding energy, contradicting experimental findings. It is also shown that the biexciton has bound excited states and that the binding energy of the L = 0 excited state is in very good agreement with experimental data. This excited state corresponds to a hole attached to a negative trion and may be a possible resolution of the discrepancy between theory and experiment.

Electron transport properties of carbon nanotube–graphene contacts

The properties of carbon nanotube-graphene junctions are investigated with first-principles electronic structure and electron transport calculations. Contact properties are found to be key factors in determining the performance of nanotube based electronic devices. In a typical single-walled carbon nanotube-metal junction, there is a p-type Schottky barrier of up to ∼0.4 eV which depends on the nanotube diameter. Calculations of the Schottky barrier height in carbon nanotube-graphene contacts indicate that low barriers of 0.09 eV and 0.04 eV are present in nanotube-graphene contacts ((8,0) and (10,0) nanotubes, respectively). Junctions with a finite contact region are investigated with simulations of the current-voltage characteristics. The results suggest the suitability of the junctions for applications and provide insight to explain recent experimental findings.

Semiconductor point defect concentration profiles measured using coherent acoustic phonon waves

Coherent acoustic phonon interferometry is used to quantitatively measure depth-dependent point defect concentrations in semiconductor systems with a depth range of the order of tens of microns. Using time-resolved pump-probe techniques, the optical response of ion-beam irradiated GaAs crystals is analyzed as a function of defect concentration ranging over four orders of magnitude. Varying the ion dose quantitatively relates changes in the optical response to local defect concentrations. Thermal annealing is shown to reduce the effect on the optical response, indicating recovery of the crystal lattice through self-interstitial-vacancy recombination.

The Role of Atomic Displacements in Ion-Induced Dielectric Breakdown

Irradiation of electronic devices with heavy ions causes a range of device degradation and failure modes, many of which are characterized and/or triggered by enhanced leakage current through dielectric layers. These damage modes include single-event dielectric rupture (SEDR), long-term reliability degradation (LTRD), and radiation-induced soft breakdown (RISB), and they play a major role in limiting device lifetime and reliability in space applications. The LET-induced transient carrier plasma that is generated along the incident ion path has traditionally been understood as the physical effect ultimately leading to damage in dielectric layers. However, in a recent study we showed that nontrivial densities of atomic displacements are directly generated by incident heavy ions. Here, we report multiscale calculations of the effects of ion-induced atomic displacements on the current-voltage (I-V) characteristics of SiO2 layers. We use both parameter-free quantum mechanical calculations and 3D percolation theory calculations based on Mott defect-to-defect tunneling. We show that ion-induced atomic displacements produce both transient and static low-resistivity paths through SiO2 layers. The calculated I-V characteristics of damaged SiO2 layers agree quantitatively with experimental data and are shown to depend on both the spatial distribution of displacement-induced defects and the distribution of defect energy levels in the SiO2 energy gap.

Characterization of boron charge traps at the interface of Si/SiO2 using second harmonic generation

We report results from optical second harmonic generation studies of boron charge traps near the interface of Si/SiO2. Our data suggest that a static electric field at the interface is formed during the oxide growth process due to the presence of negative boron ions (B−) in the silicon substrate and positive boron ions (B+) in the oxide. We demonstrated that the B+ state traps could be filled through the creation of neutral boron states created by internal photoelectron emission. By fitting our data, we found that the effective interface susceptibility |χ(2)| depends on doping concentration.

First-principles study of field emission from carbon nanotubes and graphene nanoribbons

A real-space, real-time implementation of time-dependent density functional theory is used to study electron field emission from nanostructures. Carbon nanotubes and graphene nanoribbons are used as model systems. The calculations show that carbon nanotubes with iron adsorbates have spin-polarized emission currents. Graphene nanoribbons are shown to be good field emitters with spatial variation of the emission current influenced by the presence of passivating hydrogen.

Determination of optical damage cross-sections and volumes surrounding ion bombardment tracks in GaAs using coherent acoustic phonon spectroscopy

We report the results of coherent acoustic phonon spectroscopy analysis of band-edge optical modification of GaAs irradiated with 400 keV Ne++ for doses between 1011–1013 cm−2. We relate this optical modification to the structural damage density as predicted by simulation and verified by ion channeling analysis. Crystal damage is observed to cause optical modification that reduces the amplitude of the optoacoustic signal. The depth-dependent nature of the optoacoustic measurement allows us to determine optical damage cross-sections along the ion track, which are found to vary as a function of position along the track. Unexpectedly, we find that this optical modification is primarily dependent on the structural damage density and insensitive to the specific defect configuration along the ion track, suggesting that a simple model of defect density along the track is sufficient to characterize the observed optical changes. The extent of optical modification is strongly probe frequency-dependent as the freque...

Conductance of kinked nanowires

The conductance properties of kinked nanowires are studied by first-principles transport calculations within a recently developed complex potential framework. Using prototypical examples of monoatomic Au chains as well as small diameter single-crystalline silicon nanowires we show that transmission strongly depends on the kink geometry and one can tune the conductance properties by the kink angle and other geometrical factors. In the case of a silicon nanowire the presence of a kink drastically reduces the conductance.

The stability of the ground state for positronic sodium

The existence of a stable ground state for the exotic atom is demonstrated by using a modified version of the stochastic variational method to solve the Schrodinger equation for a positron and valence electron moving under the influence of a model potential. The model potential consists of the direct interaction between the positronium and the Hartree - Fock wavefunction for the alkali ion. The exchange potential between the valence electron and the core electrons is approximated with a local potential. The structure of positronic sodium is best described as a slightly stretched and polarized positronium atom orbiting the core at distances of the order of 10 - . It is found that positronic sodium is stable against decay into with a binding energy of 0.000177 Hartree.