Presence Of Charged Impurities Research Articles

Effects of vertical electric field and charged impurities on the spin-polarized transport ofβ-antimonene armchair nanoribbons

The electronic properties of antimonene, single-layer Sb, are attracting great attention. In this paper, spin transport in armchair antimonene nanoribbon (ASbNR) is investigated. Following the tight-binding model, we calculate both the transmission probability and the conductance by means of the non-equilibrium Green's function (NEGF) method. The effects of an external electric field vertical to the ribbon plane are explored. Our results indicate that the spin-flip rate increases with the vertical electric field. Disorder effects on spin transport are addressed by considering the presence of charged impurities. It is found that charged impurities also enhance the spin-flip rate but to a lesser extent than the out-of-plane electric field.

Reduction of charge impurities in a silicon metal-oxide-semiconductor quantum dot qubit device patterned with nano-imprint lithography

The silicon metal-oxide-semiconductor quantum dot architecture is a leading approach for the physical implementation of semiconductor quantum computing. One major challenge for scalable quantum dots is the presence of charge impurities. Electron-beam lithography (EBL), almost universally used to fabricate quantum dot devices, is known to create such defects at the Si/SiO2 interface. To eliminate the need for EBL, we have transferred the metal gate pattern of a quantum dot onto the silicon substrate using nano-imprint lithography. Critical features with 50 nm scale and separation can be dependably reproduced. By characterizing the bias-dependent charge transport through a quantum point contact barrier, the prevalence of impurities is found to be largely diminished in nano-imprinted devices when compared to similar electron-beam-written counterparts. High-quality charge transport and charge sensing of several quantum dots are obtained. Additionally, gate noise is measured with an average of 1.5 μeV Hz−1/2 equivalent to previous measurements made on devices fabricated with EBL, which suggests that the leading source of impurities produced by EBL are deep, fixed charges. This work offers a path toward reliable quantum dot operation in MOS by improving fabrication techniques to reduce charge impurities.

Phonon- and charged-impurity-assisted indirect free-carrier absorption in Ga2O3

Monoclinic $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$ has a large band gap of 4.8 eV, and can therefore be used as a contact material that is transparent to visible and UV light. However, indirect free-carrier absorption processes, mediated by either phonons or charged impurities, will set a fundamental limit on transparency. We use first-principles calculations to accurately assess the absorption cross section and to elucidate the microscopic origins of these processes. Phonon-assisted absorption is dominated by the emission of phonons, and is therefore always possible. This indirect absorption is inversely proportional to the cube of the wavelength. The presence of charged impurities, whether intentional or unintentional, leads to additional absorption, but for realistic concentrations, phonon-assisted absorption remains the largest contribution. Direct free-carrier absorption also leads to below-gap absorption, with distinct peaks where optical transitions match energy differences to higher conduction bands. In contrast, indirect absorption uniformly reduces transparency for all sub-band-gap wavelengths.

Spin Transport in Armchair Silicene Nanoribbon on the Substrate: The Role of Charged Impurity

In this work, electrical and spin properties of armchair silicene nanoribbon (ASiNR) in the presence of charged impurity are studied. The non‐equilibrium Green's function along with multi‐orbital tight‐binding is applied to obtain the transmission probability. Different types of spin transmission probability in the ASiNR on a substrate are investigated. The charged impurities are located in the underlying substrate. Spin‐flip along the channel is calculated by using the spin transmission probability. The spin diffusion length in ASiNR for differently charged impurities is obtained and compared with the mean free path.

Effects of electrostatic environment on the electrically triggered production of entangled photon pairs from droplet epitaxial quantum dots

Entangled photon pair generation is a crucial task for development of quantum information based technologies, and production of entangled pairs by biexciton cascade decays in semiconductor quantum dots is so far one of the most advanced techniques to achieve it. However, its scalability toward massive implementation requires further understanding and better tuning mechanisms to suppress the fine structure splitting between polarized exciton states, which persists as a major obstacle for entanglement generation from most quantum dot samples. In this work, the influence of electrostatic environment arising from electrically biased electrodes and/or charged impurities on the fine structure splitting of GaAs/AlGaAs droplet epitaxial quantum dots is studied, by means of numerical simulations considering a realistic quantum dot confining potential and electron-hole exchange interaction within a multiband k · p framework. We find that reduction of the fine structure splitting can be substantially optimized by tilting the field and seeding impurities along the droplet elongation axis. Furthermore, our results provide evidence of how the presence of charged impurities and in-plane bias components, may account for different degrees of splitting manipulation in dots with similar shape, size and growth conditions.

Graphene films decorated with TiO2 grown by atomic layer deposition: Characterization and photocatalytic activity study under UV–visible light

The great potential of graphene can be enhanced thanks to the functionalization of its surface. For this aim, different thicknesses of TiO2 were grown on graphene films by atomic layer deposition (ALD) at 200 °C using H2O and TiCl4 as precursors. The changes in electronic structure of graphene after the deposition of TiO2 and the influence of graphene in TiO2 photocatalytic activity under UV–Visible irradiation were studied. Results indicated the presence of inhomogeneity and intrinsic strain effects within the same sample. Undecorated graphene showed pre-existent strain due to the mismatch between graphene film and the underlying substrate, while non-intentional self-doping is caused by the presence of charged impurities. The deposition of TiO2 films with thickness ≤10 nm led graphene to be p-doped, while strain became the dominant effect increasing film thickness. Oxygen vacancies in the film decreased exponentially by increasing the film thickness leading to a stoichiometric O/Ti atomic ratio of TiO2 above 10 nm thickness. The combination of TiO2 and graphene enhanced the efficiency of electron-hole separation of TiO2 under UV–Visible light, leading to a higher photocatalytic activity tested for methyl red molecule degradation, making TiO2/Graphene hybrid material a promising candidate for the photodegradation of pollutants and water purification.

Electronic transport and dynamical polarization in bilayer silicene-like systems

Abstract We investigate the semiclassical electronic transport properties of the bilayer silicene-like systems in the presence of charged impurity. The trigonal warping due to the interlayer hopping, and its effect to the band structure of bilayer silicene is discussed. Besides the trigonal warping, the external field also gives rise to the anisotropic effect of the mobility (at finite temperature) which can be explored by the Boltzmann theory. The dynamical polarization as well as the scattering wave vector-dependent screening within random phase approximation are very important in determining the scattering behavior as well as the self-consistent transport. We detailedly discuss the transport behavior under the short- or long-range potential. The phonon scattering with the acoustic phonon mode which dominants at high temperature is also studied within the density functional theory (DFT). Our results are also valid for the bilayer graphene or bilayer MoS2, and other bilayer systems which with strong interlayer coupling.

Reversed Hall effect and plasma conductivity in the presence of charged impurities

The Hall conductivity of magnetized plasma can be strongly suppressed by the contribution of negatively charged particulates (referred further as “dust”). Once the charge density accumulated by the dust exceeds a certain threshold, the Hall component becomes negative, providing a reversal in the Hall current. Such an effect is unique for dust-loaded plasmas, and it can hardly be achieved in electronegative plasmas. Further growth of the dust density leads to an increase in both the absolute value of the Hall and Pedersen conductivities, while the field-aligned component is decreased. These modifications enhance the role of transverse electric currents and reduce the anisotropy of a magnetized plasma when loaded with charged impurities. The findings provide an important basis for studying the generation of electric currents and transport phenomena in magnetized plasma systems containing small charged particulates. They can be relevant for a wide range of applications from naturally occurring space plasmas in planetary magnetospheres and astrophysical objects to laboratory dusty plasmas (Magnetized Dusty Plasma Experiment) and to technological and fusion plasmas.

Global Phase Diagram of a Three-Dimensional Dirty Topological Superconductor

We investigate the phase diagram of a three-dimensional, time-reversal symmetric topological superconductor in the presence of charge impurities and random s-wave pairing. Combining complimentary field theoretic and numerical methods, we show that the quantum phase transition between two topologically distinct paired states (or thermal insulators), described by thermal Dirac semimetal, remains unaffected in the presence of sufficiently weak generic randomness. At stronger disorder, however, these two phases are separated by an intervening thermal metallic phase of diffusive Majorana fermions. We show that across the insulator-insulator and metal-insulator transitions, normalized thermal conductance displays single parameter scaling, allowing us to numerically extract the critical exponents across them. The pertinence of our study in strong spin-orbit coupled, three-dimensional doped narrow gap semiconductors, such as Cu_{x}Bi_{2}Se_{3}, is discussed.

Characterization of Graphene Gate Electrodes for Metal-Oxide-Semiconductor Devices

ABSTRACT We fabricate and characterize metal-oxide-semiconductor (MOS) devices with graphene as the gate electrode, 5 or 10 nm thick silicon dioxide as the insulator, and silicon as the semiconductor substrate. We find that Fowler-Nordheim tunneling dominates the gate current for the 10 nm oxide device. We also study the temperature dependence of the tunneling current in these devices in the range 77 to 300 K and extract the effective tunneling barrier height as a function of temperature for the 10 nm oxide device. Furthermore, by performing high frequency capacitance-voltage measurements, we observe a local capacitance minimum under accumulation, particularly for the 5 nm oxide device. By fitting the data using numerical simulations based on the modified density of states of graphene in the presence of charged impurities, we show that this local minimum results from the quantum capacitance of graphene. These results provide important insights for the heterogeneous integration of graphene into conventional silicon technology.

Gate tunneling current and quantum capacitance in metal-oxide-semiconductor devices with graphene gate electrodes

Metal-oxide-semiconductor (MOS) devices with graphene as the metal gate electrode, silicon dioxide with thicknesses ranging from 5 to 20 nm as the dielectric, and p-type silicon as the semiconductor are fabricated and characterized. It is found that Fowler-Nordheim (F-N) tunneling dominates the gate tunneling current in these devices for oxide thicknesses of 10 nm and larger, whereas for devices with 5 nm oxide, direct tunneling starts to play a role in determining the total gate current. Furthermore, the temperature dependences of the F-N tunneling current for the 10 nm devices are characterized in the temperature range 77–300 K. The F-N coefficients and the effective tunneling barrier height are extracted as a function of temperature. It is found that the effective barrier height decreases with increasing temperature, which is in agreement with the results previously reported for conventional MOS devices with polysilicon or metal gate electrodes. In addition, high frequency capacitance-voltage measurements of these MOS devices are performed, which depict a local capacitance minimum under accumulation for thin oxides. By analyzing the data using numerical calculations based on the modified density of states of graphene in the presence of charged impurities, it is shown that this local minimum is due to the contribution of the quantum capacitance of graphene. Finally, the workfunction of the graphene gate electrode is extracted by determining the flat-band voltage as a function of oxide thickness. These results show that graphene is a promising candidate as the gate electrode in metal-oxide-semiconductor devices.

Sigmoid-growth curve of lattice-constant against annealing-temperature of Cd1−xZnxTe thin films and its structural, optical and morphological characteristics

Stacked CdTe/Zn/CdTe layers were deposited on glass substrates. The vacuum-evaporated thin films were subsequently annealed in vacuum ambience at various temperatures. Change in lattice-constant of major Cd1−xZnxTe planes against temperature was plotted from the XRD results. The graphs followed sigmoid-growth model and were regressed well by standard Boltzmann and Logistic functions. Lattice-constant varied maximum in between 375–400°C and 425–450°C, giving two separate growth trends. Optical studies suggested that presence of charge impurities and defects reduced the transmittance and band-gap values of the samples. Such reduction occurred, despite of greater formation of Cd1−xZnxTe. Decreasing granularity was however associated with increasing band-gap for samples annealed at 425 and 450°C. SEM micrographs showed that granularity decreased significantly for samples annealed at higher temperatures. EDX results were further used to co-relate the compositional characteristics with structural and optical features.

Quantum capacitance of graphene in contact with metal

We report a versatile computation method to quantitatively determine the quantum capacitance of graphene when it is in contact with metal. Our results bridge the longstanding gap between the theoretically predicted and experimentally measured quantum capacitance of graphene. Contrary to popular assumptions, the presence of charged impurities or structural distortions of graphene are not the only sources of the asymmetric capacitance with respect to the polarity of the bias potential and the higher-than-expected capacitance at the Dirac point. They also originate from the field-induced electronic interactions between graphene and metal. We also provide an improved model representation of a metal–graphene junction.

Resonant Faraday and Kerr effects due to in-gap states on the surface of a topological insulator

When Dirac electrons on the surface of a topological insulator are gapped, the resulting quantum anomalous Hall effect leads to universal magneto-optical Faraday and Kerr effects in the low frequency limit. However, at higher frequencies different excitations can leave their own fingerprints on the magneto-optics and can therefore be probed. In particular, we investigate the role of localized in-gap states---which inevitably appear in the presence of charged impurities---on these higher frequency magneto-optical effects. We have shown that these states resonantly contribute to the Hall conductivity and are magneto-optically active. These in-gap states lead to peculiar resonant signatures in the frequency dependence of the Faraday and Kerr angles, distinct in character to the contribution of in-gap excitonic states, and can be probed in ellipsometry measurements.

Ferroelectric distortions in doped ferroelectrics: BaTiO3:M(M=V−Fe)

A major challenge in the search for multiferroic materials among transition metal compounds has been that ferroelectricity is primarily found in ${d}^{0}$ materials while magnetism is found in ${d}^{n}$ systems. Considering a well-known ferroelectric oxide, namely BaTiO${}_{3}$, the question we asked within a theoretical study was whether ferroelectric distortions disappeared for the slightest amount of doping. Surprisingly, in the case of V-doped BaTiO${}_{3}$, ferroelectricity was found to be stronger than in the undoped limit. Another surprise was that the presence of charged impurities rather than free carriers was found to be most detrimental to the presence of ferroelectric distortions. These ideas of the low doping limit were used to design alternative multiferroics.

Thermoelectric properties of the interacting two dimensional electron gas in the diffusion regime?

We demonstrate that kinetic coefficients related to thermoelectric properties of the two dimensional electron gas in the diffusive regime are strongly influenced by electron-electron interaction. As an example we consider the thermoelectric coefficients of the diluted two-dimensional electron gas in Si(100) MOSFET's in the presence of charged-impurity scattering. We find that the screening anomaly at $q=2k_F$, also responsible for Friedel oscillations, leads at low electron densities to a large change in the thermoelectric coefficient for the thermopower.

Spectral and optical properties of doped graphene with charged impurities in the self-consistent Born approximation

Spectral and transport properties of doped (or gated) graphene with long-range charged impurities are discussed within the self-consistent Born approximation. It is shown how, for impurity concentrations ${n}_{imp}\ensuremath{\gtrsim}n$ a finite density of states appears at the Dirac point, the one-particle lifetime no longer scales linearly with the Fermi momentum, and the line shapes in the spectral function become non-Lorentzian. These behaviors are different from the results calculated within the Born approximation. We also calculate the optical conductivity from the Kubo formula by using the self-consistently calculated spectral function in the presence of charged impurities.

Reflection of finite-width edge channels.

We have studied experimentally the edge channels in an externally controlled inhomogeneous two-dimensional electron gas (2DEG) under quantum Hall conditions. The 2DEG is inhomogeneous in the sense that the electron concentration changes linearly across the width of the sample. In an impurity-free (or ideal) electron gas, these edge channels are wide compressible strips alternated with incompressible strips. However, when the presence of charged impurities (or disorder) is considered, localized compressible strips appear on both sides of the incompressible strips. We show that the width of the incompressible plus the localized compressible strips can be determined from resistance measurements. We further demonstrate that the macroscopically wide compressible strips in an inhomogeneous, disordered 2DEG act as if they are edge channels; they can be reflected at an interior potential and the values of the quantized resistances can be determined from the Landauer-B\"uttiker model. \textcopyright{} 1996 The American Physical Society.

Density of states in the quasi-one-dimensional electron gas. Multiple-scattering approach

The density of states of cylindrical quantum wires is calculated in the presence of charged impurities. A multiple-scattering approach is used in order to describe impurity bands for small impurity densities and band-tails for large impurity densities. Excited impurity bands are obtained within our approach. An analytical expression is given for the disorder-induced renormalized band-edge in the band-tail regime.

Plasmons localized at point charges in semiconductor quantum wells.

The plasmon excitations of inhomogeneous carrier gases in semiconductor quantum wells in the presence of charged impurities are investigated. The random phase approximation for the carrier gas yields an integral equation which expresses the condition that plasmons may be self-consistently localized in the vicinity of the impurity. The localized plasmon is a density wave trapped at the impurity site and exists in the electron (hole) gas only for negative (positive) impurity charge. Bound states of the intersubband plasmon are found for all densities of the carrier gas in a quantum well, a result which differs qualitatively from the bulk case. Numerical results for the binding energies are given for a range of parameters.