Impurity Position Research Articles

Binding energy of hydrogen-like donor impurity and photoionization cross-section in InAs Pöschl–Teller quantum ring under applied magnetic field

Using the variational method and the effective mass approximation, we have studied the binding energy and the photoionization cross-section of a donor impurity in InAs Pöschl–Teller quantum rings under applied magnetic fields. This work reports the binding energy and the photoionization cross-section dependencies on the geometry of the structure, impurity positions, asymmetry of the confining potential, magnetic field, and incident photon energy. The results show that the binding energy behaves as a decreasing function of the height, outer radius, confining potential, and magnetic field. Because of the decrease (increasing) of the impurity binding energy there is a redshift (blueshift) of the photoionization cross-section in the system.

Hydrogenic impurity states in zinc-blende In xGa 1− xN/GaN in cylindrical quantum well wires under hydrostatic pressure

Within the framework of effective-mass approximation, the binding energy of a hydrogenic donor impurity in zinc-blende (ZB) In x Ga 1− x N/GaN cylindrical quantum well wires (CQWWs) is investigated using variational procedures. Numerical results show that the ground-state donor binding energy E b increases when the hydrostatic pressure increases for any impurity position. The donor binding energy for a shallow donor impurity located at the center of the CQWWs is the largest. Also, we have found that In concentration is very important value to tailor the system, since the binding energies close to binding energy maxima are strongly dependent to In content.

Hydrogenic-Donor Impurity States in GaAs/AlxGa1−xAs Quantum Dots in the Presence of an Electric Field

We report a calculation of binding energy of the ground state of a hydrogenic donor in a quantum cylindrical GaAs dot surrounded by Ga1−xAlxAs with finite confinement potentials, in the presence of a uniform electric field applied parallel to the dot axis. The binding energy increases inchmeal as the radius of the dot decreases until a maximum value for a certain value of the quantum dot radii, then begins to drop quickly. Results for the binding energies and electronic wave function density of the hydrogenic-donor as functions of the impurity position, dot thickness and applied electric field are also presented.

Off‐center shallow donors in a spherical Si quantum dot with dielectric border

AbstractWithin the effective mass approximation and using a finite element method, the ground state energy and electron cloud localization of the shallow donors in a Si quantum dot (QD) with dielectric border are calculated. Simultaneous effects of dielectric mismatch (DM) at the core–shell interface, the impurity radial position, and the external electric field on the electronic properties are investigated. We found that (i) for a freestanding QD, the binding energy is strongly enhanced due to the additional interactions of the electron with the polarization charges; (ii) the electron cloud distribution can be easily modulated by varying the impurity position; (iii) the electric field‐induced shift in energy levels increases with the DM. Therefore, the electronic energy levels of the nanocrystal could be tuned by properly tailoring the heterostructure parameters (DM with the surrounding matrix, impurity location) as well as by varying the electric field strength. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2012

Local properties of an inhomogeneous two-component correlated superconductor

The effect of random point-like impurities on correlated superconductor has been studied. We use the two component (also called boson-fermion (BF)) model formulated in a real space supplemented with strong on-site electron-electron repulsion U and solve it by means of the real space Bogoliubov-de Gennes approach. It has been shown that due to electron correlations the gap is larger near the impurity sites, contrary to the results for the same model but without U. This naturally leads to positive correlations between the gap magnitude and the position of impurities, as was observed in various scanning tunneling measurements performed on d-wave symmetry BSCCO-2122 superconductors. Strong correlations are responsible for the protection of the low energy quasiparticles against disorder.

Optical properties of acceptor–exciton complexes in ZnO/SiO 2 quantum dots

The binding energy E b of the acceptor–exciton complex (A −,X) as a function of the radius (or of the impurity position of the acceptor) and the normalized oscillator strength of (A −,X) in spherical ZnO quantum dots (QDs) embedded in a SiO 2 matrix are calculated using the effective-mass approximation under the diagonalzation matrix technique, including a three-dimensional confinement of the carrier in the QD and assuming a finite depth. Numerical results show that the binding energy of the acceptor–exciton complexes is particularly robust when the impurity position of the acceptor is in the center of the ZnO QDs. It has been clearly shown from our calculations that these physical parameters are very sensitive to the quantum dot size and to the impurity position. These results could be particularly helpful, since they are closely related to experiments performed on such nanoparticles. This may allow us to improve the stability and efficiency of the semiconductor quantum dot luminescence which is considered critical.

Impurity effects on semiconductor quantum bits in coupled quantum dots

We theoretically consider the effects of having unintentional charged impurities in laterally coupled two-dimensional double (GaAs) quantum-dot systems, where each dot contains one or two electrons and a single charged impurity. Using molecular orbital and configuration interaction methods, we calculate the effect of the impurity on the two-electron energy spectrum of each individual dot as well as on the spectrum of the coupled-double-dot two-electron system. We find that the singlet-triplet exchange splitting between the two lowest-energy states, both for the individual dots and the coupled-dot system, depends sensitively on the location of the impurity and its coupling strength (i.e. the effective charge). A strong electron-impurity coupling breaks down the equality of the two doubly occupied singlets in the left and the right dots, leading to a mixing between different spin singlets. As a result, the maximally entangled qubit states are no longer fully obtained in the zero-magnetic-field case. Moreover, a repulsive impurity results in a triplet-singlet transition as the impurity effective charge increases or the impurity position changes. We comment on the impurity effect in spin qubit operations in the double-dot system based on our numerical results.

Electron transport in a two-terminal Aharonov–Bohm ring with impurities

Electron transport in a two-terminal Aharonov–Bohm ring with a few short-range scatterers is investigated. An analytical expression for the conductance as a function of the electron Fermi energy and magnetic flux is obtained using the boundary conditions for describing contacts and impurities. The dependence of the conductance on positions of scatterers is studied. We have found that the conductance exhibits asymmetric Fano resonances at certain energies. The dependence of the Fano resonances on magnetic field and positions of impurities is investigated. It is found that collapse of the Fano resonances occurs and discrete energy levels in the continuous spectrum appear at certain conditions. An explicit form for the wave function corresponding to the discrete level is obtained.

Interplay between size and impurity position of doped quantum dot

We explore the effect of on and off-center repulsive impurities on regular parabolic dots and their impact on the level structures coupled with a variation in the dot size. Information entropy and probability density are invoked to monitor the pattern of evolution of the electronic states. We have considered Gaussian impurity centers of repulsive type. We have found that the dot size and impurity position interplay in a subtle way to modulate the spatial arrangement of impurity doped dot energy levels. The information entropy profile evinces emergence of resonance-like situations and the probability contour delineates the alteration in the spatial extension of the system for different dot sizes and impurity locations.

Band-edge oscillations of the diffraction spectrum of a volume hologram investigated by the air-doping model

Oscillation exists at the high-frequency band edge in the diffraction spectrum of a volume hologram. An air-doping model of a volume hologram is proposed to explain the phenomenon. The numerical results show good agreement with the experimental results, which cannot be explained by the conventional models. The results show that the position of air impurity is the key factor to control the number and width of the oscillations. The present work gives a new approach to generate and control the defect mode in a holographic photonic crystal.

Binding energies of shallow impurities in asymmetric strained wurtzite AlxGa1−xN/GaN/AlyGa1−yN quantum wells

The ground state binding energies of hydrogenic impurities in strained wurtzite AlxGa1−xN/GaN/AlyGa1−yN quantum wells are calculated numerically by a variational method. The dependence of the binding energy on well width, impurity location and Al concentrations of the left and right barriers is discussed, including the effect of the built-in electric field induced by spontaneous and piezoelectric polarizations. The results show that the change in binding energy with well width is more sensitive to the impurity position and barrier heights than the barrier widths, especially in asymmetric well structures where the barrier widths and/or barrier heights differ. The binding energy as a function of the impurity position in symmetric and asymmetric structures behaves like a map of the spatial distribution of the ground state wave function of the electron. It is also found that the influence on the binding energy from the Al concentration of the left barrier is more obvious than that of the right barrier.

Hydrogenic impurity states in zinc-blende In xGa 1−xN/GaN in cylindrical quantum well wires

Within the framework of effective-mass approximation, the binding energy of a hydrogenic donor impurity in zinc-blende (ZB) In x Ga 1 − x N/GaN cylindrical quantum well wires (CQWWs) is investigated using variational procedures. Numerical results show that the ground-state donor binding energy E b is highly dependent on the impurity position and the CQWWs structure parameters. The donor binding energy for a shallow donor impurity located at the center of the CQWWs is the largest. As the impurity position changes from the center of the wire to its edge, the donor binding energy gets smaller. Also, we have found that In concentration is a very important value to tailor the system, since the binding energies close to binding energy maxima are strongly dependent on In content.

Impurity photovoltaic effect in silicon solar cell doped with sulphur: A numerical simulation

The impurity photovoltaic effect (IPV) has mostly been studied in various semiconductors such as silicon, silicon carbide and GaAs in order to increase infrared absorption and hence cell efficiency. In this work, sulphur is used as the IPV effect impurity incorporated in silicon solar cells. For our simulation we use the numerical device simulator (SCAPS). We calculate the solar cell performances (short circuit current density Jsc, open circuit voltage Voc, conversion efficiency η and quantum efficiency QE). We study the influence of light trapping and certain impurity parameters like impurity concentration and position in the gap on the solar cell performances. Simulation results for IPV effect on silicon doped with sulphur show an improvement of the short circuit current and the efficiency for sulphur energy levels located far from the middle of the band gap especially at Ec–Et=0.18eV.

Effect of a localized impurity on soliton dynamics in the Bose—Einstein condensates

By using a multiple-scale method, we analytically study the effect of a localized impurity on the soliton dynamics in the Bose-Einstein condensates. It is shown that a dark soliton can be transmitted through a repulsive (or attractive) impurity, while at the position of the localized impurity the soliton can be quasitrapped by the impurity. Additionally, we find that the strength of the localized impurity has an important effect on the dark soliton dynamics. With increasing strength of the localized impurity, the amplitude of the dark soliton becomes bigger, while its width is narrower, and the soliton propagates slower.

Simultaneous effects of hydrostatic pressure and applied electric field on the impurity-related self-polarization in GaAs/Ga1−xAlxAs multiple quantum wells

A detailed theoretical study of the combined effects of hydrostatic pressure and in-growth direction applied electric field on the binding energy and self-polarization of a donor impurity in a system of GaAs-(Ga,Al)As coupled square quantum wells is presented. The study is performed in the framework of the effective mass and parabolic band approximations and using a variational procedure. The electron effective mass, the dielectric constant, the barrier height, the well sizes, all them varying with the hydrostatic pressure are taken into account within the study. The results obtained show that the impurity binding energy and its self-polarization bear strong dependencies with the hydrostatic pressure, the strength of the applied electric field, the width of the confining potential barriers, and the impurity position.

RESONANT DONOR STATES IN QUANTUM WELL

A method of calculation of donor impurity states in a quantum well is developed. The used techniques have made it possible to find the binding energy both of ground and excited impurity states attached to each QW subband. The positions of the resonant states in 2D continuum are determined as poles of corresponding wave functions. As a result of such an approach the identification of resonant states in 2D continuum is avoided, introducing special criterions. The calculated dependences of binding energies versus impurity position are presented for various widths of Si / Si 1-x Ge x quantum wells.

Simultaneous effects of hydrostatic pressure and electric field on impurity binding energy and polarizability in coupled InAs/GaAs quantum wires

This work is concerned with the theoretical study of the combined effects of applied electric field and hydrostatic pressure on the binding energy and impurity polarizability of a donor impurity in laterally coupled double InAs/GaAs quantum-well wires. Calculations have been made in the effective mass and parabolic band approximations and using a variational method. The results are reported for different configurations of wire and barriers widths, impurity position, and electric field and hydrostatic pressure strengths. Our results show that for symmetrical structures the binding energy is an even function of the impurity position along the growth direction of the structure. Also, we found that for hydrostatic pressure strength up to 38 kbar, the binding energy increases linearly with hydrostatic pressure, while for larger values of hydrostatic pressure the binding energy has a non-linear behavior. Finally, we found that the hydrostatic pressure can increase the coupling between the two parallel quantum-well wires.

Polaron effects on the binding energy of a hydrogenic impurity in parabolic quantum wires in perpendicular electric and magnetic fields

Using a variational approach, the binding energy of shallow hydrogenic impurities in a parabolic quantum wire is calculated within the effective mass approximation. The polaron effects on the ground-state binding energy in electric and magnetic fields are investigated by means of the Pekar–Landau variation technique. The results for the binding energy as well as a polaronic correction are obtained as a function of the applied fields and the impurity positions.

Hydrostatic pressure effect on the donor impurity states in asymmetric multiple quantum wells

Based on the effective-mass approximation, hydrostatic pressure effect on the donor binding energy in zinc blende (ZB) InGaN/GaN asymmetric multiple quantum wells (AMQWs) is investigated variationally. Numerical results show that the hydrostatic pressure increases the donor binding energy for any impurity position. Moreover, the hydrostatic pressure effect is more noticeable if the impurity is localized inside the wide well of the AMQWs. For any hydrostatic pressure, the donor binding energy is distributed asymmetrically with respect to the center of the AMQWs. In particular, the donor binding energy of impurity located at the center of the wide well of the AMQWs is insensitive to the increment of the inter-well barrier width if the inter-well barrier width is large.

HYDROGENIC IMPURITY STATES IN WURTZITE ZnO/MgZnO QUANTUM DOT

Based on the effective-mass approximation, considering the built-in electric field effect due to the spontaneous and piezoelectric polarizations, the ground-state donor binding energy of a hygrogenic impurity in a cylindrical wurzite (WZ) ZnO / MgZnO strained quantum dot (QD) is investigated variationally. Numerical results show that the ground-state donor binding energy is highly dependent on the Mg composition, the impurity positions and the QD size. The built-in electric field also induces an asymmetric distribution of the ground-state donor binding energy with respect to the center of the QD. In particular, it is found that the ground-state donor binding energy is insensible to the dot height when the impurity is located at the right boundary of the WZ ZnO / MgZnO strained QD if the dot height is large.