Donor Impurity States Research Articles

A theoretical study of the effects of uniaxial stress and spatial dielectric functions on the density of states of shallow donor impurities in a GaAs quantum well dot of circular geometry

In the present work, we have carried out a comparative study of the effects of uniaxial stress and spatial dielectric functions on the density of impurity states (DOIS) of shallow donor impurities in a GaAs quantum well dot of circular cross-section. Using a trial wave function in the effective mass approximation, we carried out calculations for a range of binding energies of hydrogenic (dielectric constant) and non-hydrogenic (spatial dielectric functions) donors for various applied uniaxial stress and for different uniaxial lengths of the quantum dot. Our results show that, for a constant axial length of the quantum dot and constant uniaxial stress, the DOIS for the donor impurity is markedly enhanced for the non-hydrogenic donor impurity over that for purely hydrogenic donor impurity. At constant axial length, the applied uniaxial stress enhances the DOIS in both cases. The density of impurity states has also been studied for a constant applied uniaxial stress for different axial lengths of the quantum dot. Here, again, the DOIS increases with increasing axial length of the quantum dot. In fact, the enhanced DOIS is observed throughout the range of binding energies considered. These results show that not only does the DOIS vary with the applied uniaxial stress and spatial dielectric functions they are also different for various axial lengths of the quantum dot. These findings indicate that is important to take into account the effect of applied uniaxial stress and spatial dielectric functions when performing experimental studies of electronic, optical and transport properties of such nanostructures as quantum dots.

Hydrogen-like donor impurity states in strongly prolate ellipsoidal quantum dot

In the current article, the binding energy and impurity-related photoionization cross-section are investigated for the case of an infinite confinement potential well model using a variational approach for hydrogen-like impurity states in strongly prolate ellipsoidal quantum dot. The binding energy is calculated as a function of ellipsoidal small and large semiaxes and as a function of impurity displacement. The behavior of the oscillator strength for the quantum transitions between the impurity ground state and the first two non-impurity states are considered for different values of incident light angles and geometrical parameters. In the work, we illustrate the results of photoionization cross-section computations depending on the incident photon energy for a range of incident angles and different values of impurity shift. It is revealed that the threshold frequency decreases with the increase of impurity displacement.

Hydrogenic donor impurity states and intersubband optical absorption spectra of monolayer transition metal dichalcogenides in dielectric environments

The hydrogenic donor impurity states and intersubband optical absorption spectra in monolayer transition metal dichalcogenides (ML TMDs) under dielectric environments are theoretically investigated based on a two-dimensional (2D) nonorthogonal associated Laguerre basis set. The 2D quantum confinement effect together with the strongly reduced dielectric screening results in the strong attractive Coulomb potential between electron and donor ion, with exceptionally large impurity binding energy and huge intersubband oscillator strength. These lead to the strong interaction of the electron with light in a 2D regime. The intersubband optical absorption spectra exhibit strong absorption lines of the non-hydrogenic Rydberg series in the mid-infrared range of light. The strength of the Coulomb potential can be controlled by changing the dielectric environment. The electron affinity difference leads to charge transfer between ML TMD and the dielectric environment, generating the polarization-electric field in ML TMD accompanied by weakening the Coulomb interaction strength. The larger the dielectric constant of the dielectric environment, the more the charge transfer is, accompanied by the larger polarization-electric field and the stronger dielectric screening. The dielectric environment is shown to provide an efficient tool to tune the wavelength and output of the mid-infrared intersubband devices based on ML TMDs.

Shallow Donor Impurity States with Excitonic Contribution in GaAs/AlGaAs and CdTe/CdSe Truncated Conical Quantum Dots under Applied Magnetic Field.

Using the effective mass approximation in a parabolic two-band model, we studied the effects of the geometrical parameters, on the electron and hole states, in two truncated conical quantum dots: (i) GaAs-(Ga,Al)As in the presence of a shallow donor impurity and under an applied magnetic field and (ii) CdSe–CdTe core–shell type-II quantum dot. For the first system, the impurity position and the applied magnetic field direction were chosen to preserve the system’s azimuthal symmetry. The finite element method obtains the solution of the Schrödinger equations for electron or hole with or without impurity with an adaptive discretization of a triangular mesh. The interaction of the electron and hole states is calculated in a first-order perturbative approximation. This study shows that the magnetic field and donor impurities are relevant factors in the optoelectronic properties of conical quantum dots. Additionally, for the CdSe–CdTe quantum dot, where, again, the axial symmetry is preserved, a switch between direct and indirect exciton is possible to be controlled through geometry.

The impurity states in InGaAsP/InP coaxial double quantum well wires with the effects of electric and magnetic fields

The hydrogen donor impurity states are calculated in [Formula: see text] coaxial double quantum well wires by the plane wave method under the theoretical framework of effective mass envelope function approximation. The binding energies of impurity in [Formula: see text] state and [Formula: see text] state are obtained as the functions of impurity position, distance between the inner and outer quantum wires, magnetic and electric field strengths. Transition energies are calculated as the functions of impurity position, distance between the inner and outer quantum wires. The effects of quantum wire thickness and distance of quantum wires on impurity states are analyzed in detail. It is found that the effects of electric field and magnetic field on binding energy of [Formula: see text] state are different for impurity located at different positions.

Electric Field Effect on Excited State Binding Energy and Self-Polarization of a Hydrogenic Impurity in a Spherical Quantum Dot

Effects of the electric field and impurity position on the 2p-excited state self-polarization and binding energy of the electron-impurity in a spherical quantum dot are investigated as a function of the dot radius using variational approach. According to results, turning point of 2p-excited state binding energy changes with the electric field strength and position of impurity in the spherical quantum dot. Self-polarization and binding energy between the 1s-ground state and 2p-excited state of a hydrogenic donor impurity in SQD are presented as a function of the dot radius for different electric field value and impurity positions. Obtained numerical results show a good agreement with the literature.

Effect of tilted electric field on stability of excited states of neutral donor impurity in semiconductor quantum wire

The influence of the tilted electric field on the stability of the excited states of neutral donor impurity (D0) in a cylindrical quantum wire (CQW) is investigated. We consider the finite parabolic confinement potential in the CQW. The energies and wave functions for the on- and off-center D0 states are calculated using the variational principle within the single-band effective mass approximation. Moreover, the dependence of Stark shift ΔE in the ground and excited state energies of D0 on the tilted electric field are obtained. The tilted electric field strength is found to have a certain angle at which the states of D0 turn from bound to unbound. For the off-center donor impurity in the intermediate region of the angle, the binding energy of D0 descends rapidly in both the strong and weak confinement regions, and it increases again in the limit of the total lateral electric field. Moreover, our results show a reordering of the on- and off-center D0 energy states when moving from the strong confinement region towards the weak confinement region. The ground state of the off-center D0 loses its stability, faster in the tilted electric field than both the normal and the total lateral electric fields. The Stark shift in D0 shows a blue shift, and in some states of D0, it increases quadratically with the enhancement in the tilted electric field.

Binding energies and photoionization cross-sections of donor impurities in GaN/AlxGa1−xN spherical quantum dot under hydrostatic pressure

In this paper, the binding energy and photoionization cross-section of donor impurity state in [Formula: see text] quantum dot structure are studied theoretically by using variational method. The variation of binding energy and photoionization cross-section with core and shell sizes at different impurity locations under hydrostatic pressure is calculated numerically. The results show that the binding energy decreases monotonously with the core size at different impurity locations for [Formula: see text] core/shell quantum dot. In contrast, for the inverted core/shell quantum dot, the binding energy exhibits different trends with the increase of core size at different impurity locations. But the binding energy decreases monotonically with the shell size for both of them. Moreover, when the photon energy is approximately equal to the donor binding energy, the peak of the photoionization cross-section appears. There will be different peak shifts under different conditions, and its peak intensity increases with the increase of core and shell sizes. When the hydrostatic pressure is applied, the binding energy and the peak strength of the photoionization cross-section increase with the increase of the pressure.

Oscillator strength and carrier dynamics in type I and inverted type I spherical core/ shell nanostructures under external laser field

Abstract For the first time, the effect of laser on the donor impurity states in spherical nano core/shell systems is examined theoretically by variation technique. In this work, a step like laser dressed confinement potential is employed to evolve the donor dynamics in two different spherical core/shell quantum structures namely (a) Type I GaAs/Al0.3Ga0.7As core shell nanostructure (CSN) and (b) Inverted Type I Al0.3Ga0.7As/GaAs CSN. Donor binding energy (BE) is calculated for various laser dressing parameter ( α 0) for various impurity locations (Ri) by varying the dimensions of the systems. Effect of laser on sub-level energy (E0), oscillator strength (P10), transition energy (E10) and the magnitude of dipole matrix element ( | M 10 | ) of the systems are also determined. The diverse behavior of type I and inverted type I CSNs are analyzed. It is witnessed that in type I the carrier is highly confined whereas inverted type I favors tunneling. This study will help in understanding the influence of laser field on donor impurity in CSNs and its application towards optical devices.

Spin preservation during THz orbital pumping of shallow donors in silicon

We investigate the spin relaxation under conditions of optical excitation between the Rydberg orbital states of phosphorus donor impurities in silicon. Here we show that the spin relaxation is less than a few percent, even after multiple excitation/relaxation cycles. The observed high level of spin preservation may be useful for readout cycling or in quantum information schemes where coupling of neighbor qubits is via orbital excitation.

Heat capacity and entropy of Gaussian spherical quantum dot in the presence of donor impurity

The energy states of shallow donor impurity in GaAs/AlGaAs quantum dot heterostructure with Gaussian potential have been calculated by the shifted 1/N expansion method. The effects of the impurity on the ground state energy and average statistical energy have been investigated. The present calculations show that the donor impurity modifies significantly the electron energy levels of spherical quantum dot and thermal properties. In addition, we have also displayed the variations of the heat capacity and entropy of donor impurity in quantum dot with the radius, confining potential depth, dimension and temperature. The comparison shows that our results are in very good agreement with the theoretical reported work.

Simultaneous Effects of Hydrostatic Pressure and Temperature on Donor Impurity States in Wurtzite Inx Ga1 – x N/GaN Concentric Double Quantum Rings

Simultaneous Effects of Hydrostatic Pressure and Temperature on Donor Impurity States in Wurtzite In<i>_x</i> Ga₁ –<i> x</i> N/GaN Concentric Double Quantum Rings

Absorption Spectra of Hydrogen-Like Donor Impurity in GaAs Quantum Well with Modified Pöschl-Teller Potential

Hydrogen-like donor impurity states in a quantum well with modified Poschl-Teller confinement potential have been considered. The dependence of binding energy on hydrogen-like donor impurity position and confinement potential parameters has been revealed. It has been shown that the dependence of binding energy on hydrogen-like donor impurity position has the bell-shaped character. The existence of binding energy maximum is explained by the concurrency of the size-quantization effect and the Coulomb interaction between electron and hydrogen-like donor impurity. The coefficient of an optical absorption in the presence of hydrogen-like donor impurity center has been computed. The absorption spectra for the quantum transitions between the hydrogen-like donor impurity ground state and conduction band ground state have been obtained.

Binding energy and optical absorption of donor impurity states in “12-6” tuned GaAs/GaAlAs double quantum well under the external fields

We have investigated the binding energies of the s-symmetric ground and first excited shallow donor impurity states and the total 1s → 2s absorption coefficient, including the first and third order corrections, in “12-6” tuned GaAs/GaAlAs double quantum well as a function of the impurity position, size of the structure, and the electric and magnetic field intensities. The obtained results show that the electronic and optical properties of tuned GaAs/GaAlAs double quantum well can be adjustable by an appropriate choice of the sample geometry, material parameters and applied external fields which will lead to new potential applications in optoelectronics.

Influence of image charge effect on the binding energy of hydrogen-like donor impurity in a near-surface quantum well under transverse electric field

We present a systematic theoretical study of the influence of image charges on the spectrum of a shallow donor impurity located anywhere in a near-surface quantum well under the electric field applied along the growth direction of the system. We calculate the binding energies of the ground and excited donor impurity states in the effective-mass approximation using the variational method for various values of the quantum well width, electric field strength, and donor positions within the well. Theoretical results indicate that the role of image charges is especially important for small widths of a quantum well. In contrast to the ground state, the influence of image charges on the binding energy of excited states is significant only at small values of the electric field. Therefore such fields can serve as a means to control the transition energies between ground and first excited states.

Binding energy of donor impurity states and optical absorption in the Tietz-Hua quantum well under an applied electric field

For a quantum well which has the Tietz-Hua potential, the ground and some excited donor impurity binding energies and the total absorption coefficients, including linear and third order nonlinear terms for the transitions between the related impurity states with respect to the structure parameters and the impurity position as well as the electric field strength are investigated. The binding energies were obtained using the effective-mass approximation within a variational scheme and the optical transitions between any two impurity states were calculated by using the density matrix formalism and the perturbation expansion method. Our results show that the effects of the electric field and the structure parameters on the optical transitions are more pronounced. So we can adjust the red or blue shift in the peak position of the absorption coefficient by changing the strength of the electric field as well as the structure parameters.

Impurity states in InSe monolayers doped with group II and IV elements

We have used first-principles calculations to investigate the electronic structures of two-dimensional (2D) InSe monolayers doped with group II and IV elements. The results show that substituting Mg, Ca, Ge, and Sn for indium clearly modifies the electronic and magnetic properties of InSe monolayers and that these substitutions should be easily achieved in a Se-rich environment. Mg- and Ca-doped InSe monolayers develop deep acceptor states and magnetism, whereas substituting Ge and Sn for In induces shallow donor impurity states in these monolayers. Furthermore, the results show that substituting Sn for In provides effective n-type carriers in the InSe monolayer because of the low formation energy and transition level (∼61.10 meV). These results should be helpful for related experimental studies.

The effect of stimulated interband emission on the impurity-assisted far-infrared photoluminescence in GaAs/AlGaAs quantum wells

Emission of far- and near-infrared radiations in the n-GaAs/AlGaAs quantum well nanostructures under interband photoexcitation of electron-hole pairs is studied at low lattice temperatures. Optical transitions of nonequilibrium electrons involving donor impurity states in quantum wells are revealed in far- and near-infrared emission spectra. Intensive optical pumping allows to observe near-infrared stimulated emission related to the radiative recombination of electrons from the ground donor state and holes from the valence subband in quantum wells. The possibility of the intensity increase of impurity-assisted far-infrared radiation due to effective depopulation of donor states with interband stimulated emission in quantum wells is demonstrated.

Binding energy and photoionization cross-section of hydrogen-like donor impurity in strongly oblate ellipsoidal quantum dot

Hydrogen-like donor impurity states in strongly oblate ellipsoidal quantum dot have been studied. The hydrogen-like donor impurity states are investigated within the framework of variational method. The trial wave function constructed on the base of wave functions of the system without impurity. The dependence of the energy and binding energy for the ground and first excited states on the geometrical parameters of the ellipsoidal quantum dot and on the impurity position have been calculated. The behavior of the oscillator strength for different angles of incident light and geometrical parameters have been revealed. Photoionization cross-section of the electron transitions from the impurity ground state to the size-quantized ground and first excited states have been studied. The effects of impurity position and the geometrical parameters of the ellipsoidal quantum dot on the photoionization cross section dependence on the photon energy have been considered.

LASER DRIVEN IMPURITY STATES IN TWO DIMENSIONAL CONCENTRIC DOUBLE QUANTUM RINGS

Hydrogenic donor impurity states in the 2D GaAs=Ga0.7Al0.3As concentric double quantum rings have been investigated under the action of intense laser field. An analytical expression of the effective lateral confinement potential induced by the laser field is obtained. The laser dressed effect has been considered both on electron confining and electron-impurity Coulomb interaction potentials. The single electron energy spectrum and wave functions have been found using the effective mass approximation and exact diagonalization technique. The accidental degeneracy of the impurity states has been observed.