Impurity Position Research Articles

The effect of hydrostatic pressure on the binding energy and diamagnetic susceptibility of a laser dressed donor impurity in a GaAs/GaAlAs nanowire superlattice

In this paper the effects of hydrostatic pressure and laser radiation on the binding energy and diamagnetic susceptibility of an off-center hydrogenic donor impurity in a nanowire superlattice (NWSL) are studied. The energy eigenvalues and corresponding wave functions of ground and first excited states are numerically computed using finite difference method for a NWSL with circular cross-section which involved an array of spherical quantum dots (QDs). The numerical results show that oscillatory behaviors appear in binding energies and diamagnetic susceptibilities of ground and first excited states as impurity shifts away from center of QDs. Maximum values of ground state binding energy (first excited state binding energy) occur when impurity is located at the center of QDs (at the center of barriers). Additionally, binding energies of ground and first excited states shift towards higher (lower) energies as pressure (laser radiation) increases. An opposite behavior is observed for absolute value of diamagnetic susceptibility as pressure or laser radiation increases. Also, as the QDs’ volume increases the binding energies (I) decrease or (II) increase, reach maximum values and then decrease; which strongly depend on the position of impurity.

Engineering the Effective p-Type Dopant in GaAs/InAs Core–Shell Nanowires with Surface Dangling Bonds

Using first-principles calculation based on density-functional theory, we investigated the effect of surface dangling bond on p-type doping mechanism and the electronic structures in wurtzite (WZ) and zinc blende (ZB) GaAs/InAs core–shell nanowires (NWs) along the [0001] and [111] directions, respectively. The results of the formation energies show that the surface dangling bond of the In atom is a kind of stable defect. Both in WZ and ZB core–shell NWs, we found it is easier and more stable to realize dopant in the GaAs core. Moreover, the position of Cd impurity plays a key role in the formation of p-type nanowires. The farther the distance between the impurity and the surface dangling In atom, the easier it is to form the p-type characteristic of the nanowires. In particular, it shows an intrinsic behavior when doping the Cd impurity near the surface dangling bond. The surface dangling bonds have an ability to capture the holes from the neighbor doping impurity, resulting in the deactivation of dopants. Meanwhile, the transfer of hole moves the valence band down to the lower energy levels and even can lead to a band anticrossing phenomenon in the conduction band. Our results highlight a new physical coupling between the doped state and surface dangling bonds in GaAs/InAs core–shell NWs, and open a new opportunity for the development of tailoring nanoscale electronic properties.

The linear and nonlinear optical properties of an off-center hydrogenic donor impurity in nanowire superlattices: Comparison between arrays of spherical and cylindrical quantum dots

Nanowire superlattices (NWSLs) are objects with a wide range of potential applications in nanoscale electronics and optics. In this paper, we consider an off-center hydrogenic donor impurity in two different NWSL with cylindrical cross-section which involved an array of spherical or cylindrical quantum dots (QDs) along their axis and called Sph and Cyl NWSLs, respectively. The electronic eigenstates and energy eigenvalues of NWSLs are determined by applying finite difference method. Additionally, optical properties such as linear, nonlinear and total intersubband absorption coefficients (ACs) and refractive index (RI) changes are investigated by means of compact density matrix approach. The results show that (I) optical ACs and RI changes of a Sph NWSL are smaller than those of Cyl NWSL. (II) The amplitudes of ACs and RI changes are strongly affected by impurity position and show an oscillatory behavior as the impurity shifts away from the center. (III) Resonance condition in optical spectrum can be adjusted by shape of QDs and impurity position and both blue and red shifts are appeared in ACs as the impurity shifts away from the center of NWSLs. (IV) Total optical absorption saturation intensity can be controlled by the shape of QDs or the impurity position. Moreover, optical incident intensities correspond to the saturation in optical spectrum in Cyl NWSL are smaller than their values for Sph NWSL and have a minimum values as the impurity is located at the center of barriers for both Sph and Cyl NWSLs. (V) By increasing the relaxation time the amplitude of the total AC will increase and for large values of relaxation time a saturation appears in optical spectrum which strongly depends on the Al concentration, impurity position and type of NWSL.

Dielectric mismatch and shallow donor impurities in GaN/HfO2 quantum wells

In this work we investigate electron–impurity binding energy in GaN/HfO2 quantum wells. The calculation considers simultaneously all energy contributions caused by the dielectric mismatch: (i) image self-energy (i.e., interaction between electron and its image charge), (ii) the direct Coulomb interaction between the electron–impurity and (iii) the interactions among electron and impurity image charges. The theoretical model account for the solution of the time-dependent Schrödinger equation and the results shows how the magnitude of the electron–impurity binding energy depends on the position of impurity in the well-barrier system. The role of the large dielectric constant in the barrier region is exposed with the comparison of the results for GaN/HfO2 with those of a more typical GaN/AlN system, for two different confinement regimes: narrow and wide quantum wells.

Binding energy of the ground and first few excited states of hydrogenic donor impurity in a rectangular GaAs quantum dot in the presence of electric field

Within the quasi-one-dimensional effective potential model and effective mass approximation, we calculate the ground and the first 9 excited-state binding energies of a hydrogenic donor impurity in a rectangular quantum dot (RQD) in the presence of electric field. The analytical form of the quasi-one-dimensional effective potential replacing the three-dimensional Coulomb potential in our model is derived by Fourier transforms. We discuss detailedly dependence of the binding energies on the impurity positions and electric fields. For the ground-state binding energy, our results qualitatively agree with that of Mendoza et al. (2005) in which they only calculated the ground-state binding energies in cubic quantum dots by variational method. However, for first 9 excited-state binding energies, such dependence has complex manner since there are two or three peaks in the electronic probability density distribution curves. The strengths and positions of these peaks in RQD affect the interaction potential between electron and impurity, which appears to be the critical control on the binding energies of impurity. The applied electric field pushes the positions of these peaks downwards, and the strengths of peaks located at the upper half of RQD increase while the strengths of lower peaks firstly decrease, then increase with increasing electric field. The high peak strength can lead to increase of the binding energy while the large distance between the position of peak and impurity center results in reduce of the energy, which is an interesting competition. This competition is more obvious for excited-state binding energies of off-central impurity.

Scanning gate imaging of quantum point contacts and the origin of the 0.7 anomaly

The origin of the anomalous transport feature appearing at a conductance G ≈ 0.7 × (2e2/h) in quasi-1D ballistic devices-the so-called 0.7 anomaly-represents a long standing puzzle. Several mechanisms have been proposed to explain it, but a general consensus has not been achieved. Proposed explanations have been based on quantum interference, the Kondo effect, Wigner crystallization, and other phenomena. A key open issue is whether the point defects that can occur in these low-dimensional devices are the physical cause behind this conductance anomaly. Here we adopt a scanning gate microscopy technique to map individual impurity positions in several quasi-1D constrictions and correlate these with conductance characteristics. Our data demonstrate that the 0.7 anomaly can be observed irrespective of the presence of localized defects, and we conclude that the 0.7 anomaly is a fundamental property of low-dimensional systems.

Impurity position effect on optical properties of various quantum dots

In this work, we have investigated the effect of impurity position on optical properties of a pyramid and a cone like quantum dot. For this goal, we first obtain the energy levels and wave functions using finite element method (FEM) in the presence of impurity. Then, we have studied the influence of impurity location on refractive index changes and absorption coefficients of the two quantum dots. We found that there is a maximum value for total refractive index changes and absorption coefficients at a special impurity position. Also, we have found that the refractive index changes and absorption coefficients of a cone like quantum dot are greater than a pyramid quantum dot in same volume and height. According to the results, it is deduced that the impurity location plays an important and considerable role in the electronic and optical properties of a pyramid and a cone like quantum dot.

Low-frequency spectra of dislocation paths in NaCl crystals in the electron spin resonance scheme

It is shown that the criterion of dislocation mobility resonance in NaCl crystals in the electron spin resonance scheme in the Earth's magnetic field depends on both the sample orientation in this field and the location of impurity atoms in the dislocation core. The measured lowfrequency spectrum of dislocation paths makes it possible to resolve a series of peaks corresponding to different positions of a Ca impurity that correlate with the results from computer simulations of the dislocation core structure.

Hydrostatic pressure and magnetic field effects on donor states in pyramidal quantum dots

The simultaneous effects of the hydrostatic pressure and magnetic field on the ground state of impurity confined by a GaAs pyramid-shaped QD are investigated within the effective mass approximation. By using a variational method, the binding energy and the extent of the electron wavefunction in the structure are examined for different dot sizes and impurity positions. The influence of above mentioned parameters on the diamagnetic shift of the impurity energy is also discussed. We found that the diamagnetic coefficient could be appreciably tuned by a proper tailoring of the heterostructure parameters (dot size, impurity position) as well as by changing the hydrostatic pressure and/or the magnetic field strength.

Effect of electric and magnetic fields on impurity binding energy in zinc-blend symmetric InGaN/GaN multiple quantum dots.

The binding energy of ground state for hydrogenic impurity in multiple quantum dots is calculated in the framework of effective-mass approximation and using a variational method. It is shown that the binding energy is a function of the size of dots, impurity position and external fields strength. The binding energy has a maximum value when the impurity is located on the center of dots and decreases for other impurity positions. The external electric and magnetic fields change the magnitude and the position of peaks. PACS Codes 73.20.D; 71.21.La; 71.55.Eq

Weak localization in low-symmetry quantum wells

Theory of weak localization is developed for electrons in semiconductor quantum wells grown along [110] and [111] crystallographic axes. Anomalous conductivity correction caused by weak localization is calculated for symmetrically doped quantum wells. The theory is valid for both ballistic and diffusion regimes of weak localization in the whole range of classically weak magnetic fields. We demonstrate that in the presence of bulk inversion asymmetry the magnetoresistance is negative: The linear in the electron momentum spin-orbit interaction has no effect on the conductivity while the cubic in momentum coupling suppresses weak localization without a change of the correction sign. Random positions of impurities in the doping layers in symmetrically doped quantum wells produce electric fields which result in position-dependent Rashba coupling. This random spin-orbit interaction leads to spin relaxation which changes the sign of the anomalous magnetoconductivity. The obtained expressions allow determination of electron spin relaxation times in (110) and (111) quantum wells from transport measurements.

Study of impurity position effect in pyramid and cone like quantum dots

In the present work, we have studied the effect of impurity position on energy levels, the donor binding energy and third harmonic generation of a pyramid and a cone like quantum dot. For this goal, we have calculated the energy levels, wave functions and binding energy using finite element method for various impurity positions. Our results show that the impurity location plays an important and considerable role in the electronic and optical properties in a pyramid and a cone like quantum dot. We found that the third harmonic generation and binding energy of both quantum dots have a maximum value at a special impurity position. The special impurity location is different for the two quantum dots. Also, we have deduced that the third harmonic generation of a cone like quantum dot is larger than a pyramid quantum dot for same volumes and heights.

Shallow impurity states in AlxGa1–xAs cylindrical quantum wire

Polarons bound to a shallow Coulomb impurity center in cylindrical quantum wire is studied by a variational approach. The binding energies of the shallow impurity states in AlxGa1–xAs cylindrical quantum wire are calculated as functions of the composition x and the impurity position. It is confirmed that the binding energies are reduced obviously by the influence of the electron—phonon interaction and the binding energies are increased with increasing the composition x.

Patterns of microsatellite evolution inferred from the Helianthus annuus (Asteraceae) transcriptome.

The distribution of microsatellites in exons, and their association with gene ontology (GO) terms is explored to elucidate patterns of microsatellite evolution in the common sunflower, Helianthus annuus. The relative position, motif, size and level of impurity were estimated for each microsatellite in the unigene database available from the Compositae Genome Project (CGP), and statistical analyses were performed to determine if differences in microsatellite distributions and enrichment within certain GO terms were significant. There are more translated than untranslated microsatellites, implying that many bring about structural changes in proteins. However, the greatest density is observed within the UTRs, particularly 5'UTRs. Further, UTR microsatellites are purer and longer than coding region microsatellites. This suggests that UTR microsatellites are either younger and under more relaxed constraints, or that purifying selection limits impurities, and directional selection favours their expansion. GOs associated with response to various environmental stimuli including water deprivation and salt stress were significantly enriched with microsatellites. This may suggest that these GOs are more labile in plant genomes, or that selection has favoured the maintenance of microsatellites in these genes over others. This study shows that the distribution of transcribed microsatellites in H. annuus is nonrandom, the coding region microsatellites are under greater constraint compared to the UTR microsatellites, and that these sequences are enriched within genes that regulate plant responses to environmental stress and stimuli.

Uniaxial strain-dependent magnetic and electronic properties of (Ga,Mn)As nanowires

Variations in magnetic and electronic properties as a function of uniaxial strain in wurtzite (Ga,Mn)As nanowires (NWs) grown along the [0001] direction were investigated based on density functional theory (DFT). We found that (Ga,Mn)As NWs are half-metal, and the ferromagnetic state is their stable ground state. The magnetism of the NWs is significantly affected by the strain and by the substituent position of Mn impurities. By examining charge densities near the Fermi level, we found that strain can regulate the conductive region of the NWs. More interestingly, the size of spin-down band gap of the NWs is tunable by adjusting uniaxial stress, and the NWs can be converted from indirect to direct band gap under tension.

Semiconductor to Metal to Half-Metal Transition in Pt-Embedded Zigzag Graphene Nanoribbons

The electronic and magnetic properties of Pt-embedded zigzag graphene nanoribbons (Pt–ZGNRs) are investigated using density-functional theory calculations. It is found that Pt–ZGNRs exhibit a semiconductor–metal–half-metal transition as the position of Pt substitutional impurities in the ribbon changes from the center to edge sites. This behavior can be attributed to the interaction between Pt impurities and edge states of ZGNRs, which governs the electron occupation of the edge states. The transition always occurs independent of ribbon width. However, Pt impurity concentration is important for obtaining this transition. Our results demonstrate that Pt–ZGNRs can be used as versatile electronic devices.

The effects of optical phonon on the binding energy of bound polaron in a wurtzite ZnO/MgxZn1−xO quantum well

An improved Lee-Low-Pines intermediate coupling method is used to study the energies and binding energies of bound polarons in a wurtzite ZnO/MgxZn1−xO quantum well. The contributions from different branches of long-wave optical phonons, i.e., confined optical phonons, interface optical phonons, and half-space optical phonons are considered. In addition to electron-phonon interaction, the impurity-phonon interaction, and the anisotropy of material parameters, such as phonon frequency, electron effective mass, and dielectric constant, are also included in our computation. Ground-state energies, binding energies and detailed phonon contributions from various phonons as functions of well width, impurity position and composition are presented. Our result suggests that total phonon contribution to ground state and binding energies in the studied wurtzite ZnO/Mg0.3Zn0.7O quantum wells varies between 28–23 meV and 62–45 meV, respectively, which are much larger than the corresponding values (about 3.2–1.8 meV and 1.6–0.3 meV) in GaAs/Al0.3Ga0.7As quantum wells. For a narrower quantum well, the phonon contribution mainly comes from interface and half-space phonons, for a wider quantum well, most of phonon contribution originates from confined phonons. The contribution from all the phonon modes to binding energies increases slowly either when impurity moves far away from the well center in the z direction or with the increase in magnesium composition (x). It is found that different phonons have different influences on the binding energies of bound polarons. Furthermore, the phonon contributions to binding energies as functions of well width, impurity position, and composition are very different from one another. In general, the electron-optical phonon interaction and the impurity center-optical phonon interaction play an important role in electronic states of ZnO-based quantum wells and cannot be neglected.

Ground state energy and wave function of an off-centre donor in spherical core/shell nanostructures: Dielectric mismatch and impurity position effects

Ground state energy and wave function of a hydrogen-like off-centre donor impurity, confined anywhere in a ZnS/CdSe spherical core/shell nanostructure are determined in the framework of the envelope function approximation. Conduction band-edge alignment between core and shell of nanostructure is described by a finite height barrier. Dielectric constant mismatch at the surface where core and shell materials meet is taken into account. Electron effective mass mismatch at the inner surface between core and shell is considered. A trial wave function where coulomb attraction between electron and off-centre ionized donor is used to calculate ground state energy via the Ritz variational principle. The numerical approach developed enables access to the dependence of binding energy, coulomb correlation parameter, spatial extension and radial probability density with respect to core radius, shell radius and impurity position inside ZnS/CdSe core/shell nanostructure.

Donor-impurity related photoionization cross section in [formula omitted] concentric double quantum rings: Effects of geometry and hydrostatic pressure

The donor-impurity related photoionization cross section in GaAs/Ga1−xAlxAs three-dimensional concentric double quantum rings is investigated. The photoionization cross section dependence on the incident photon energy is studied considering the effects of hydrostatic pressure, variations of aluminum concentration, geometries of the structure, and impurity position. The interpretation of the dipole matrix element, which reflects the photoionization probability, is also given. We have found that these parameters can lead to both redshift and blueshift of the photoionization spectrum and also influence the cross section peak value.

An x-ray fluorescence holographic study on a Bi2Te3Mn0.1 topological insulator

To search the atomic sites of Mn impurities in Bi2Te3Mn0.1 topological insulator, a Mn Kα fluorescence x-ray holography experiment was performed. The reconstructed atomic image around the central Mn atom reveals a hexagonal configuration, where the local lattice constant is slightly larger than the original Bi2Te3 crystal. Thus, the most plausible atomic position of Mn impurities is the substitution with Bi or Te atom. The further-distant atomic images are hardly observed unlike usual crystals, indicating a formation of flat clusters around Mn in this functional crystal.