Carrier Fermi Research Articles

Coulomb Drag by Injected Ballistic Carriers in Graphene n+−i−n−n+ Structures: Doping and Temperature Effects

The Coulomb carrier drag effect in lateral −i−n− graphene diodes and field‐effect transistors with the injection of ballistic electrons into the n‐region are evaluated. Calculating the drag factor determining the amplification of the injected current, its dependence on the structural parameters, carrier Fermi energy in the n‐region, and temperature is found. As demonstrated, the drag factor exhibits a maximum in certain values of the Fermi energy (i.e., the gate voltage and density of the remote donors) and the temperature. The parameter determining the shape of the current–voltage characteristics (monotonous or S shaped) is also calculated. The obtained results can be used for the optimization of the structures under consideration for different devices, in particular, voltage‐ and current‐driven switches, frequency multipliers, and terahertz emitters.

Plasmon Damping in Electronically Open Systems.

Rapid progress in electrically controlled plasmonics in solids poses a question about possible effects of electronic reservoirs on the properties of plasmons. We find that plasmons in electronically open systems [i.e., in (semi)conductors connected to leads] are prone to an additional damping due to charge carrier penetration into contacts and subsequent thermalization. We develop a theory of such lead-induced damping based on the kinetic equation with microscopic boundary conditions at the interfaces, followed by perturbation theory with respect to transport nonlocality. The lifetime of the plasmon in an electronically open ballistic system appears to be finite, of the order of conductor length divided by carrier Fermi velocity. The reflection loss of the plasmon incident on the contact of the semiconductor and perfectly conducting metal also appears to be finite, of the order of Fermi velocity divided by wave phase velocity. Recent experiments on plasmon-assisted photodetection [Nat. Commun. 9, 5392 (2018)NCAOBW2041-172310.1038/s41467-018-07848-w] are discussed in light of the proposed lead-induced damping phenomenon.

Microscopic quantum description of second-order nonlinearities in two-dimensional hexagonal nanostructures beyond the Dirac cone approximation

Single layers of hexagonal two-dimensional nanostructures such as graphene, silicene, and germanene exhibit large carrier Fermi velocities and, consequently, large light-matter coupling strength making these materials promising elements for nano-opto-electronics. Although these materials are centrosymmetric, the spatial dispersion turns out to be quite large allowing the second-order nonlinear response of such materials to be comparable to the non-centrosymmetric 2D ones. The second-order response of massless Dirac fermions has been extensively studied, however a general approach correct over the full Brillouin zone is lacking so far. To complete this gap, in the current paper we develop a general quantum-mechanical theory of the in-plane second-order nonlinear response beyond the Dirac cone approximation and applicable to the full Brillouin zone of the hexagonal tight-binding nanostructures. We present explicit calculation of the nonlinear susceptibility tensor of 2D hexagonal nanostructures applicable to arbitrary three-wave mixing processes.

Substrate Doping Effect and Unusually Large Angle van Hove Singularity Evolution in Twisted Bi- and Multilayer Graphene.

Graphene has demonstrated great potential in new-generation electronic applications due to its unique electronic properties such as large carrier Fermi velocity, ultrahigh carrier mobility, and high material stability. Interestingly, the electronic structures can be further engineered in multilayer graphene by the introduction of a twist angle between different layers to create van Hove singularities (vHSs) at adjustable binding energy. In this work, using angle-resolved photoemission spectroscopy with sub-micrometer spatial resolution, the band structures and their evolution are systematically studied with twist angle in bilayer and trilayer graphene sheets. A doping effect is directly observed in graphene multilayer system as well as vHSs in bilayer graphene over a wide range of twist angles (from 5° to 31°) with wide tunable energy range over 2 eV. In addition, the formation of multiple vHSs (at different binding energies) is also observed in trilayer graphene. The large tuning range of vHS binding energy in twisted multilayer graphene provides a promising material base for optoelectrical applications with broadband wavelength selectivity from the infrared to the ultraviolet regime, as demonstrated by an example application of wavelength selective photodetector.

Spin polarization dependence of quasiparticle properties in graphene

We address spin polarization dependence of graphene's Fermi liquid properties quantitatively using a microscopic Random Phase Approximation theory in an interacting spin-polarized Dirac electron system. We show an enhancement of the minority-spin many-body velocity renormalization at fully spin polarization due to reduction in the electron density and consequently increase in the interaction between electrons near the Fermi surface. We also show that the spin dependence of the Fermi velocity in the chiral Fermi systems is different than that in a conventional two-dimensional electron liquid. In addition, we show that the ratio of the majority- to minority-spin lifetime is smaller than unity and related directly to the polarization and electron energy. The spin-polarization dependence of the carrier Fermi velocity is of significance in various spintronic applications.

Pressure-dependent thermopower of individual Bi nanowires

The 240–620nm diameter nanowires were freely suspended and thermopower measurements were carried out over the temperature range 4–300K and for stresses as high as 1GPa. The peaks of up to 80μV∕K that are observed around 40K are interpreted in terms of a model of diffusion thermopower under strong electron and hole-boundary scattering. The partial Seebeck coefficients are calculated from the stress-dependent carrier Fermi energies obtained from magnetoresistance measurements. The prospect of Bi nanowire arrays achieving high thermoelectric figure of merit is discussed.

Magnetoquantum oscillations and confinement effects in arrays of 270-nm-diameter bismuth nanowires

We present a study of the electrical transport properties of 270-nm-diameter bismuth nanowire arrays embedded in an alumina matrix which are capped with layers of bulk Bi to produce a very low contact resistance. The resistance of the Bi nanowires has been measured over a wide range of temperatures (1.8--300 K) and magnetic fields (-8--8 T) for the longitudinal and transverse orientation. At low magnetic fields, the longitudinal magnetoresistance exhibits field-periodic modulations whose periods are consistent with theoretical predictions for the Aharonov-Bohm ``whispering gallery'' modes of electrons with long mean free path. At high magnetic fields, as the carrier cyclotron radius becomes smaller than the wire diameter, we observe Shubnikov--de Haas oscillations associated with both holes and electrons. These represent a detailed study of magnetoquantum oscillations in high-density nanowire arrays. Overall, the hole periods are increased by 5% and the carrier density is decreased by 13% from the values for bulk Bi, which is consistent with recent theoretical estimates of the effect of confinement on the carrier's Fermi surface.

Thermoelectric power of bismuth nanowires

We report here measurements of the thermoelectric power and longitudinal magneto-Seebeck coefficient of 200 nm diameter single-crystal bismuth nanowires. Nanowires of pure Bi and of n-type-doped Bi (with Te at about $5\ifmmode\times\else\texttimes\fi{}{10}^{18}{\mathrm{cm}}^{\mathrm{\ensuremath{-}}3})$ were measured. The wires are imbedded in porous anodic alumina. The data are taken on arrays of wires connected in parallel, at temperatures from 8 to 300 K, and, between 10 and 80 K, in magnetic fields from 0 to 5 Tesla. It has been theoretically calculated that bismuth nanowires should have a strongly increased thermoelectric figure of merit over bulk Bi, when the diameter is decreased below about 10 nm. The nanowires in this study were selected because they are easier to prepare and handle. The temperature-dependent thermopower data are consistent with the partial electron and hole thermopower values calculated using the carrier Fermi energies obtained from Shubnikov--de Haas oscillations on the same samples.

An analytic approximation with a wide range of applicability for electron initiated Auger transitions in narrow-gap semiconductors

Theoretical calculations of Auger transition rates are complicated by the interconnection of energy and momentum conservation. The use of a flat valence band where the heavy holes have infinite mass decouples energy and momentum conservation and greatly simplifies the calculation. This flat valence band model has been used to obtain a simple analytic approximation for Auger transition rates. It requires just two parameters to cover a wide range of temperature and carrier Fermi levels (both degenerate and nondegenerate) and their values may be found either by comparison with an accurate calculation or from Auger lifetimes determined experimentally. The results have been applied to InSb and Cd0.2188Hg0.7812Te where good agreement with accurate theoretical values using realistic band structures has been attained. The same model has also been used to provide a simple analytic approximation for impact ionization probability rates, again agreeing well with accurately determined values. Such approximations will prove to be useful in modeling semiconductor transport effects and devices.

Current, potential, electric field, and Fermi carrier distributions around localized elastic scatterers in phase-coherent quantum magnetotransport.

The spatial distributions of the current, Fermi carriers, electric field (due to space charges) and electrostatic potential in a disordered mesoscopic structure are calculated in the presence of arbitrary magnetic fields. These distributions are useful in elucidating and visulaizing many features of quantum magnetotransport, such as the formation of edge states at high magnetic fields and their near-perfect transmittivity, the evolution of the integer quantum Hall effect, the creation of magnetic bound states around an impurity, the magnetic response of current vortices that form as a result of quantum interference between scatterers and the walls of a quantum wire, the dependence of the quantized-conductance steps in a backgated quantum wire on an applied magnetic field, the behavior of residual-resistivity dipoles and the electrostatic space-charge potential in a magnetic field, the dependence of the sign of the magnetoresistance on the impurity configuration, etc. We examine the current, Fermi carrier concentrations, electric field, and both chemical- and electrostatic-potential profiles associated with each of these phenomena, and relate them to the observed terminal characteristics in each case.

Theoretical study of thin and thick emitter silicon solar cells

We describe a complete theoretical model to calculate the open circuit voltage, the light generated current density, and the conversion efficiency of thin and thick emitter silicon solar cells. We show the importance of the Fermi carrier degeneracy, in addition to the heavy doping effects such as band-gap narrowing and Auger recombination. For example, in contradiction to recent calculations, the conversion efficiency of thin passivated emitter solar cells is shown to be independent of the surface dopant concentration. Also, in agreement with experimental results, but contrary to old conventional cell designs, it is shown that the conversion efficiencies of thick emitter solar cells can be as high as those of thin emitter cells. The last result will occur whenever the emitter is well passivated and the surface impurity concentration is relatively low (Ns=1×1019 cm−3). For non-passivated solar cells it is better to have thin emitters and high surface dopant concentrations (Ns≳2×1020 cm−3) as usual in conventional solar cells.

Interface States at Au/Gaas Schottky Contacts

ABSTRACTWe present a new method for the characterization of traps at the interfacial layer of metal/semiconductor contacts. The method is based on measurements of the ac-admittance of Schottky contacts over a wide frequency range. The frequency dependence is analyzed within a new Trap Transistor Model which explains the ac-behavior as well as the dc-characteristics. In particular we propose that the ac-current across the interface consists of capacitive as well as of conductive parts. We are able to deduce the density of trap states at the majority carrier Fermi level as well as the capture cross section of the traps. The model is applied to Au/GaAs-Schottky contacts. We find a weak energy dependence for the density of interface states as well as for their capture cross section within the energy range of 0.45eV to 0.57eV below the conduction band edge.

Determination of charge carrier Fermi levels in pure platinum by the two zone approximation method

Determination of charge carrier Fermi levels in pure platinum by the two zone approximation method

Simple phenomenological model for the optical conductivity and dielectric constant of quasi one dimensional metallic conductors

One dimensional metallic behaviour of a mixed valence square planar (MVSP) complex compound has been convincingly demonstrated by the work of Kuse and Zeller. A simple phenomenological theory for the frequency dependent conductivity sigma ( omega ) and dielectric constant epsilon ( omega ) of a quasi one dimensional metallic conductor of this type is developed in terms of a model of linear metallic strands interrupted by insulating lattice defects. Since the insulating nature of the defects gives rise to zero DC conductivity in the absence of interstrand carrier transitions, epsilon ( omega ) and sigma ( omega ) are found to deviate significantly from their respective Drude type counterparts for omega < nu F/l0, where nu F denotes the carrier Fermi velocity and l0 the mean distance between defects in a given strand. If l0 is small by comparison with the 'intrinsic' mean free path for carrier motion along the defect free strand, a large and real static dielectric constant of order (l0/a)2 is obtained, where a denotes the spacing of the metallic atoms comprising the strands. For the particular MVSP compound studied by Kuse and Zeller it is estimated that l0 approximately 100AA, which together with the small effective carrier mass deduced for the same compound, implies the existence of large spacings of the 'boxed' carrier energy levels.

Lattice Thermal Conductivity, Nernst-Ettinghausen Effect, and Specific Heat in Antimony at Low Temperature

The lattice thermal conductivity, the high-field Nernst-Ettinghausen thermoelectric coefficient, and the specific heat of antimony have been determined in the temperature range 0.4-2.4\ifmmode^\circ\else\textdegree\fi{}K. Thermal-conductivity results confirm the predominance of phonon-electron normal scattering in the lowest range of temperatures with the expected ${T}^{2}$ law. The dramatic increase in the lattice thermal conductivity above 1.5\ifmmode^\circ\else\textdegree\fi{}K is thought to be due to the inability of the electrons to scatter phonons with wave numbers $q&gt;2{k}_{F}$, where $2{k}_{F}$ is the diameter of a charge carrier's Fermi pocket. An effective scattering Debye temperature of ${\ensuremath{\Theta}}^{*}=(\frac{2{k}_{F}}{{q}_{D}})\ensuremath{\Theta}\ensuremath{\approx}25\ifmmode^\circ\else\textdegree\fi{}$K is in good agreement with experimental results. Nernst-Ettinghausen results give the total electronic density of states $Z=(1.10\ifmmode\pm\else\textpm\fi{}0.07)\ifmmode\times\else\texttimes\fi{}{10}^{33}$ ${\mathrm{erg}}^{\ensuremath{-}1}$ ${\mathrm{cm}}^{\ensuremath{-}3}$; the presence of a phonon-drag contribution is confirmed and discussed. The specific-heat results, $C=(116.5\ifmmode\pm\else\textpm\fi{}6.4)T+(211.0\ifmmode\pm\else\textpm\fi{}5.3){T}^{3}+1.97\ifmmode\pm\else\textpm\fi{}0.23){T}^{\ensuremath{-}2}$ in \ensuremath{\mu}J ${(\mathrm{m}\mathrm{o}\mathrm{l}\mathrm{e}\phantom{\rule{0ex}{0ex}}\mathrm{\ifmmode^\circ\else\textdegree\fi{}}\mathrm{K})}^{\ensuremath{-}1}$, are compared with the results of transport measurements and with recent specific-heat determinations.