Density Of 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.

Photostimulated Currents in Single Crystals of Lead Magnoniobate and Lead Magnoniobate Titanate

The spectral dependences of photostimulated currents in single crystals of lead magnoniobate and lead magnoniobate titanate are investigated. The results suggest that the distribution of the density of impurity states in the bandgap of the studied crystals is quite wide. These levels could participate in the formation of diffuse phase transitions.

The Effect of Hermanson’s Spatial Dielectric Function on the Density of Impurity States in a Gallium Arsenide Quantum Dot of Rectangular Cross-Section

We have carried out a theoretical study of the effect of Hermanson’s spatial dielectric function on the density of impurity states (DOIS) for a shallow hydrogenic donor impurity located in the center of a Gallium Arsenide (GaAs) Quantum Well Dot (QWD) of rectangular cross-section. The density of impurity states (DOIS) of an unscreened (hydrogenic) donor impurity was calculated and compared with that of the screened (non-hydrogenic) donor impurity for the same system. Our calculations were carried out using a trial wave function within the effective mass approximation. Our calculations have been carried out with finite barriers. In this study, we first calculated the ground state binding energies of both hydrogenic and non-hydrogenic donor impurity for different dot sizes. The donor binding energies in the two regimes are then used to compute the DOIS. The results show that for both hydrogenic and non-hydrogenic donor impurities, the DOIS sharply rises to a peak, and then decreases almost exponentially with increase in binding energy. The results also show that the DOIS obtained for the non-hydrogenic donor impurities is markedly enhanced over that for purely hydrogenic donor impurities in which a dielectric constant is employed in the potential. In fact, the enhanced DOIS is observed throughout the range of values for binding energy considered. To a good extend there is good agreement between our results and those reported in the literature. It is therefore, important that the effect of the spatial dielectric function should be considered when designing optoelectronic devices.

Hydrogenic donor in asymmetric [formula omitted] quantum wells

In this work we study the effects of barrier height ratio VL/VR on asymmetric AlxLGa1−xLAs/GaAs/AlxRGa1−xRAs quantum wells in absence of external influences. We use a variational method within the effective mass approximation to observe the effects of the barrier height ratio. It has been observed that the binding energy of the impurity, the expectation value of the electron–impurity distance along the z-direction, density of impurity states, and edge binding energy difference depend strongly on the barrier height ratio. Furthermore we present new curves that analyze the binding energy of asymmetric quantum wells as functions of barrier height ratio.

Effect of potential shape and height on the density of the impurity states in nanotube with two quantum wells

Recently heterostructures with two GaAs/GaAlAs quantum wells (QWs) in a section of the tube layer has been fabricated and optical properties of the embedded QWs has been studied. The ground state binding energy of an off-axis donor in a cylindrical nanotube, containing two GaAs/GaAlAs quantum wells (QWs) in the presence of a uniform magnetic field is calculated as a function of the donor location as well as the density of states. A trial function for describing the asymmetric electron charge distribution is taken as a product of the combination of 1s and 2px, y subband wave functions and an unknown function that depends only on electron-ion separation. We found that the increasing the magnetic field the increasing the binding energy while the impurity is located in the QW1, whereas the opposite occurs when the impurity is located in the QW2. Two peaks in the curves of the binding energy, as a function of the impurity position, are also found as well as in the density of impurity states.

Effect of hydrostatic compression on the indium-impurity-induced superconducting transition in Pb0.3Sn0.7Te

This paper reports on a study of the low-temperature conductivity and parameters of the superconducting state, namely, the critical temperature Tc and the second critical magnetic field Hc2, in the (Pb0.3Sn0.7)0.95In0.05Te solid solution under hydrostatic pressure P ≤ 9 kbar at T = 4.2 K. The choice of this material has been motivated by the fact that, according to earlier observations, it undergoes a superconducting transition at Tc ∼ 2.3 K, i.e., close to the maximum value Tc ∼ 2.9 K found for the (PbzSn1 − z)0.95In0.05Te solid solutions with a lead content z ∼ 0.15–0.25. It has been demonstrated that an increase in the pressure to P ≤ 9 kbar leads to a bell-shaped dependence Tc(P). The observed dependences are assigned to the effect of hydrostatic compression on the band structure of the solid solution and indicate a shift in the position of the Fermi level EF with increasing pressure within the impurity band of the In quasi-local states. In this case, EF passes through a maximum in the density of impurity states at P = 3–5 kbar.

Donor impurity in nanotube with two GaAs/GaAlAs quantum wells: Magnetic field effects

Micro-tubes containing two GaAs/GaAlAs quantum wells (QWs) in a section of the tube layer has been fabricated and optical properties of the embedded QWs has been studied. The ground state binding energy of an off-axis donor in a cylindrical nanotube, containing two GaAs/GaAlAs quantum wells (QWs) in the presence of a uniform magnetic field is calculated as a function of the donor location as well as the density of states. A trial function for describing the asymmetric electron charge distribution is taken as a product of the combination of 1s and 2px,y subband wave functions and an unknown function that depends only on electron-ion separation. We found that the increasing the magnetic field the increasing the binding energy while the impurity is located in the QW1, whereas the opposite occurs when the impurity is located in the QW2. Two peaks in the curves of the binding energy, as a function of the impurity position, are also found as well as in the density of impurity states.

Impurity‐related optical properties in rectangular‐transverse section GaAs–Ga1–xAlxAs quantum well wires: Hydrostatic pressure and electric field effects

AbstractUsing a variational procedure within the effective mass approximation, we have calculated the influence of an applied electric field and hydrostatic pressure on the shallow‐impurity‐related optical properties in a rectangular‐transverse section GaAs–Ga1–x Alx As quantum well wire. The electric field is applied in the plane of the transverse section of the wire and different angular directions have been considered. The results presented are for the impurity binding energy, its corresponding density of impurity states, and impurity‐related transition energy and polarizability. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Density of impurity states in doped spherical quantum dots

We analyze the electronic states in spherical quantum dots (SQDs) with randomly distributed neutral, D0 and negatively charged D− donors by using the recently developed fractal dimension method, in which the problems of the D0 and the D− are reduced to similar ones for a hydrogen-like atom and a negative-hydrogen-like ion, respectively, in an isotropic effective space with the fractal dimension which depends on the electron–ion donor separation. In order to solve these problems in the new effective space, we use the numerical trigonometric sweep method and the three-parameter Hylleraas trial function. We present novel curves for the density of D0 impurity states for different potentials with and without the repulsive core. We find that the curves of the density of states with repulsive core have additional peaks, whose position and intensity depends on the width and the height barrier at the center of the dot. Our results are in a good agreement with those obtained previously for donor spectra in SQD with square-well potential shape.

Density of impurity states in doped spherical quantum dots

We analyze the electronic states in spherical quantum dots (SQDs) with randomly distributed neutral, D 0 and negatively charged D − donors by using the recently developed fractal dimension method, in which the problems of the D 0 and the D − are reduced to similar ones for a hydrogen-like atom and a negative-hydrogen-like ion, respectively, in an isotropic effective space with the fractal dimension which depends on the electron–ion donor separation. In order to solve these problems in the new effective space, we use the numerical trigonometric sweep method and the three-parameter Hylleraas trial function. We present novel curves for the density of D 0 impurity states for different potentials with and without the repulsive core. We find that the curves of the density of states with repulsive core have additional peaks, whose position and intensity depends on the width and the height barrier at the center of the dot. Our results are in a good agreement with those obtained previously for donor spectra in SQD with square-well potential shape.

Stress effects on shallow-donor impurity states in symmetricalGaAs/AlxGa1−xAsdouble quantum wells

The effects of the compressive stress on the binding energy and the density of shallow-donor impurity states in symmetrical ${\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}/\mathrm{A}\mathrm{l}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$ double quantum wells are calculated using a variational procedure within the effective-mass approximation. Results are for different well and barrier widths, shallow-donor impurity position, and compressive stress along the growth direction of the structure. We have found that independently of the well and barrier widths, for stress values up to 13.5 kbar (in the direct-gap regime) the binding energy increases linearly with the stress. For stress values greater than 13.5 kbar (indirect gap regime) and for impurities at the center of the wells, the binding energy increases up to a maximum and then decreases. For all impurity positions the binding energy shows a nonlinear behavior in the indirect gap regime due to the $\ensuremath{\Gamma}\ensuremath{-}X$ crossing effect. The density of impurity states is calculated for a homogeneous distribution of donor impurities within the barriers and the wells of the low-dimensional heterostructures. We have found that there are three special structures in the density of impurity states: one associated with on-center-barrier-, the second one associated with on-center-well-, and the third one corresponding to on-external-edge-well-impurity positions. The three structures in the density of impurity states must be observed in valence--to--donor-related absorption and conduction--to--donor-related photoluminescence spectra, and consequently these peaks can be tuned at specific energies and convert the system in a stress detector.

Effects of hydrostatic stress on the density of impurity states and donor‐related optical absorption spectra in GaAs–(Ga,Al)As quantum wells

The effects of hydrostatic stress on the density of donor impurity states and donor-related optical absorption spectra in a GaAs–(Ga,Al)As quantum well are investigated. The shallow-donor binding energy for different well widths and different values of the hydrostatic stress has been calculated. It has been found that for wider well widths the binding energy increases slowly with hydrostatic stress contrary to the behavior of the binding energy for wells with smaller widths. In particular, it has been found that the binding energy does not change appreciably with the impurity position when the width of the well is small and for large values of hydrostatic stress. Two structures in both the density of states and the optical absorption spectra, associated with impurities located close to the center and to the edges of the structure, are obtained. Also, it has been observed that the density of states and the optical absorption spectra depend strongly on the applied hydrostatic stress.

The Effect of Uniaxial Stress on the Density of States of Shallow Donor Impurities in GaAs Quantum Wells

We have calculated the density of impurity states (DOIS) as a function of applied uniaxial stress of shallow hydrogen donor impurity in a quantum well (QW). The calculations were carried out using a variational procedure within the effective mass approximation. The results show that the density of impurity states is a function of applied stress. It is therefore important to take into consideration the effect of applied stress in a QW when carrying out experimental studies of optical and electronic properties of such heterostructures.

Semiconductor quantum rings: Shallow-donor levels

A theoretical study of shallow-donor states in toroidal GaAs-(Ga,Al)As quantum rings, within the effective-mass approximation and following a variational calculation, is presented. The particular topology of quantum rings is shown to produce interesting confining effects on the impurity states, namely, the effective dimensionality of a quantum ring may increase as it shrinks. The density of impurity states is calculated, and quantum-well wire and quantum-dot regimes are discussed. Also, the effects of an axial magnetic field on free-electron and shallow-donor states are analyzed.

The exact solution for the one-level impurity Green's function in the case of the filled (or empty) substrate energy band

The one-level impurity Green's function is calculated exactly in the case of the substrate with the completely filled (or empty) energy band for the Anderson model. The influence of the electron correlation effects on the impurity density of states is studied in different regions of the parameter space and the nature of the localized states and the continuous spectrum of the elementary excitation is discussed.

EFFECTS OF DIELECTRIC MISMATCH ON DENSITY OF IMPURITY STATES AND OPTICAL ABSORPTION SPECTRA ASSOCIATED WITH IMPURITIES IN QUANTUM WELLS

We calculate the effects of image potential due to dielectric mismatch on the density of impurity states and optical absorption spectra associated with impurities in GaAs/AlAs quantum wells. It is found that the effects of image potential on the density of impurity states and optical absorption spectra are significant, especially when the width of quantum well becomes small. The effects of impurity-ion image potential on the density of impurity states are much larger than those of electron image potential, but the effects of the impurity ion and electron image potentials on the optical absorption spectra are of the same order and opposite. The results also indicate that the effects of image potential on acceptors are comparable with those on donors.

An enhanced perturbational study on spectral properties of the Anderson model

The infinite-U single-impurity Anderson model for rare earth alloys is examined with a new set of self-consistent coupled integral equations, which can be embedded in the large-N expansion scheme (N is the local spin degeneracy). The finite-temperature impurity density of states (DOS) and the spin-fluctuation spectra are calculated exactly up to the order O(1/N2). The presented conserving approximation goes well beyond the 1/N approximation (NCA) and maintains local Fermi-liquid properties down to very low temperatures. The position of the low-lying Abrikosov-Suhl resonance (ASR) in the impurity DOS is in accordance with Friedel's sum rule. For N=2 its shift toward the chemical potential, compared to the NCA, can be traced back to the influence of the vertex corrections. The width and height of the ASR are governed by the universal low-temperature energy scale TK. Temperature and degeneracy N dependence of the static magnetic susceptibility is found to be in excellent agreement with the Bethe ansatz results. Threshold exponents of the local propagators are discussed. The resonant level regime (N=1) and intermediate-valence regime ( mod epsilon f mod < Delta ) of the model are thoroughly investigated as a critical test of the quality of the approximation. Some applications to the Anderson lattice model are pointed out.

Binding energies and density of impurity states in spherical GaAs-(Ga,Al)As quantum dots

The binding energies of hydrogenic donor in both finite and infinite GaAs-(Ga,Al)As spherical quantum dots are calculated as a function of the donor position for different radii within the effective-mass approximation. It is observed an enhancement of the binding energy of donors in quantum dots when compared to results in quantum wells and quantum-well wires, which is an expected consequence of the higher geometrical electronic confinement in these systems. The density of impurity states as a function of the donor binding energy was also calculated. As a general feature it presents structures associated with special impurity positions that may be important in the understanding of the absorption and photoluminescence experiments of doped quantum dots.

Density of States of Hydrogenic Impurities in GaAs/GaAlAs Quantum Wires

AbstractWe investigate the occurrence of impurity density of states of hydrogenic impurities placed on the axis of a cylinder quantum-well wire of GaAs/GaAlAs structure. The effects of disorder are taken into account in the calculation. It is shown that for a specific radius of the wire, the peak energy of the density of states as a function of the impurity concentration increases very fast enlarging the bandwidth. The impurity bands are considered for the observed binding energy and radius as well as for the impurity concentration of experimental interest.

HEAVILY DOPED HARMONIC CONFINED QUANTUM WIRES

Quasi-one-dimensional channels have already been fabricated by holographic lithography on semiconductor heterostructures. We study the formation of an impurity band for shallow donors located inside the channels assuming they have been created by applying a modulated gate voltage in a quantum well of AlxGa1−xAs−GaAs. We calculate the changes in the impurity density of states as a function of the gate voltage. It is shown that the increase of the applied gate voltage leads to higher binding energy and larger impurity bandwidth.