Electronic structure and electron g factors of HgTe quantum dots

The electronic structure and electron g factors of HgTe quantum dots are investigated, in the framework of the eight-band effective-mass approximation. It is found that the electron states of quantum spheres have aspheric properties due to the interaction between the conduction band and valence band. The highest hole states are S (l = 0) states, when the radius is smaller than 9.4 nm. the same as the lowest electron states. Thus strong luminescence from H-Te quantum dots with radius smaller than 9.4 nm has been observed (Rogach et al 2001 Phys. Statits Solidi b 224 153). The bandgap of H-Te quantum spheres is calculated and compared with earlier experimental results (Harrison et al 2000 Pure Appl. Chem. 72 295). Due to the quantum confinement effect, the bandgap of the small HgTe quantum spheres is positive. The electron g factors of HgTe quantum spheres decrease with increasing radius and are nearly 2 when the radius is very small. The electron g factors of HgTe quantum ellipsoids are also investigated. We found that as some of the three dimensions increase, the electron g factors decrease. The more the dimensions increase, the more the g factors decrease. The dimensions perpendicular to the direction of the magnetic field affect the g factors more than the other dimension.

Electronic states in InAs quantum spheres and ellipsoids

The eight-band effective-mass Hamiltonian of the free-standing narrow-gap InAs quantum ellipsoids is developed, and the electron and hole electronic structures as well as optical properties are calculated by using the model. The energies, wave functions and transition probabilities of quantum spheres as functions of the radius of quantum sphere R is presented. It is found that the energy levels do not vary as 1/R-2, which is caused by the coupling between the conduction and valence bands, and by the constant terms correspond to the spin-orbit splitting energy. The blueshifts of hole states depend strongly on the coupling from electron states, so that the order of hole states changes as has been predicted in experiment. The exciton binding energies are calculated, the calculated excitonic gaps as functions of the ground exciton transition energy are in good agreement with the photoluminescence measured spectra in details. Finally, the hole energy levels and the linear polarization factors in InAs quantum ellipsoids as functions of the aspect ratio are presented. The state 1S(Z up arrow)((1/2)) becomes the hole ground state when e is larger than 2.4. The saturation value of the linear polarization factors of the InAs long ellipsoids of diameter 2.0 nm is 0.86, in agreement with the experimental results.

Study of persistent photoconductivity and subband electronic properties of the two-dimensional electron gas in modulation doped GaAs/AlGaAs structure

We obtained the high mobility Of mu(2K) = 1.78 x 10(6) cm(2)/V . s in Si-doped GaAs/AlGaAs two-dimensional electron gas (2DEG) structures. After the sample was illuminated by a light-emitting diode in magnetic fields up to 6 T at T = 2K, we did observe the persistent photoconductivity effect and the electron density increased obviously. The electronic properties of 2DEG have been studied by Quantum-Hall-effect and Shubnikov-de Haas (SdH) oscillation measurements. We found that the electron concentrations of two subbands increase simultaneity with the increasing total electron concentration, and the electron mobility also increases obviously after being illuminated. At the same time, we also found that the electronic quantum lifetime becomes shorter, and a theoretical explunation is given through the widths of integral quantum Hall plateaus.

Spin-polarized transport in one-dimensional waveguide structures with spatially-periodic electronic and magnetic fields

We investigate theoretically the spin-polarized transport in one-dimensional waveguide structure with spatially-periodic electronic and magnetic fields. The interplay of the spin-orbit interaction and in-plane magnetic field significantly modifies the spin-dependent transmission and the spin polarization. The in-plane magnetic fields increase the strength of the Rashba spin-orbit coupling effect for the electric fields along y axis and decrease this effect for reversing the electric fields, even counteract the Rashba spin-orbit coupling effect. It is very interesting to find that we may deduce the strength of the Rashba effect through this phenomenon. (c) 2005 Elsevier Ltd. All rights reserved.

Electronic structures of wurtzite ZnO and ZnO/MgZnO quantum well

The band structures of wurtzite ZnO are calculated using the empirical pseudopotential method (EPM). The 8 parameters of the Zn and O atom pesudopotential form factors with Schluter's formula are obtained. The effective mass parameters are extracted by using k.p Hamiltonian to fit the EPM results. The calculated band edge energies (E-g, E-A, E-B, and E-C) at Gamma point are in good agreement with experimental results. The ordering of ZnO at the top of valence band is found to be A(Gamma(7))-B(Gamma(9))-C(Gamma(7)) due to a negative spin-orbit (SO) splitting. Based on the band parameters obtained, the valence hole subbands of wurzite ZnO/MgxZn1-xO tensile-strained quantum wells (QWs) with different well widths and Mg compositions are calculated using 6-band k.p method. (c) 2005 Elsevier B.V. All rights reserved.

Effects of electric field on the electronic structure and optical properties of quantum rods with wurtzite structure

The Hamiltonian of wurtzite quantum rods with an ellipsoidal boundary under electric field is given after a coordinate transformation. The electronic structure and optical properties are studied in the framework of the effective-mass envelope-function theory. The quantum-confined Stark effect is illustrated by studying the change of the electronic structures under electric field. The transition probabilities between the electron and hole states decrease sharply with the increase of the electric field. The polarization factor increases with the increase of the electric field. Effects of the electric field and the shape of the rods on the exciton effect are also investigated. The exciton binding energy decreases with the increase of both the electric field and the aspect ratio. In the end, considering the exciton binding energy, we calculated the band gap variation of size- and shape-controlled colloidal CdSe quantum rods, which is in good agreement with experimental results.

Electronic structure of coupled vertically stacked self-assembled InAs quantum disks in a vertical electric field

In the framework of the effective-mass and adiabatic approximations, by setting the effective-mass of electron in the quantum disks (QDs) different from that in the potential barrier material, we make some improvements in the calculation of the electronic energy levels of vertically stacked self-assembled InAs QD. Comparing with the results when an empirical value was adopted as the effective-mass of electron of the system, we can see that the higher levels become heightened. Furthermore, the Stark shifts of the system of different methods are compared. The Stark shifts of holes are also studied. The vertical electric field changes the splitting between the symmetric level and the antisymmetric one for the same angular momentum. (C) 2003 Elsevier Ltd. All rights reserved.

Electronic properties of sulfur passivated undoped-n(+) type GaAs surface studied by photoreflectance

Photoreflectance (PR) has been used to study surface electronic properties (electric field, Fermi level pinning, and density of surface states) of undoped-n(+) (UN+) GaAs treated in the solution of ammonium sulfide in isopropanol. Complex Fourier transformation (CFT) of PR spectra from passivated surface shows that the sulfur overlay on GaAs surface makes no contribution to Franz-Keldysh oscillations (FKOs). The barrier height measured by PR is derived from surface states directly, rather than the total barrier height, which includes the potentials derived from Ga-S and As-S dipole layers. Comparing with native oxidated surface, the passivation leads to 80 meV movement of surface Fermi level towards the conduction band minimum, and reduction by more than one order in density of surface states. (C) 2003 Elsevier Science B.V. All rights reserved.

Electronic structure and optical properties of quantum rods with wurtzite structure

The Hamiltonian of the wurtzite quantum rods with an ellipsoidal boundary is given after a coordinate transformation. The energies, wave functions, and transition possibilities are obtained as functions of the aspect ratio e with the same method we used on spherical dots. With an overall consideration of both the transition matrix element and the Boltzmann distribution we explained why the polarization factor increases with increasing e and approaches a saturation value, which tallies quite well with the experimental result. When e increases more and more S-z states are mixed into the ground, second, and third states of J(z)=1/2, resulting in an increase of the emission of z polarization. It is just the linear terms of the momentum operator in the hole Hamiltonian that cause the mixing of S and P states in the hole ground state. The effects of the crystal field splitting energy, temperature, and transverse radius to the polarization are also considered. We also calculated the band gap variation with the size and shape of the quantum rods.

Electronic structure and transport properties of quantum rings in a magnetic field

The electronic structure of quantum rings is studied in the framework of the effective-mass theory and the two dimensional hard wall approximation. In cases of both the absence and presence of a magnetic field the electron momenta of confined states and the Coulomb energies of two electrons are given as functions of the angular momentum, inner radius, and magnetic-field strength. By comparing with experiments it is found that the width of the real confinement potential is 14 nm, much smaller than the phenomenal width. The Coulomb energy of two electrons is calculated as 11.1 meV. The quantum waveguide transport properties of Aharonov-Bohm (AB) rings are studied complementarily, and it is found that the correspondence of the positions of resonant peaks in AB rings and the momentum of confined states in closed rings is good for thin rings, representing a type of resonant tunneling.

Electronic structure of diluted magnetic semiconductor superlattices: In-plane magnetic field effect

The electronic structure of diluted magnetic semiconductor (DMS) superlattices under an in-plane magnetic field is studied within the framework of the effective-mass theory; the strain effect is also included in the calculation. The numerical results show that an increase of the in-plane magnetic field renders the DMS superlattice from the direct band-gap system to the indirect band-gap system, and spatially separates the electron and the hole by changing the type-I band alignment to a type-II band alignment. The optical transition probability changes from type I to type II and back to type I like at large magnetic field. This phenomenon arises from the interplay among the superlattice potential profile, the external magnetic field, and the sp-d exchange interaction between the carriers and the magnetic ions. The shear strain induces a strong coupling of the light- and heavy-hole states and a transition of the hole ground states from "light"-hole to "heavy"-hole-like states.

Effects of piezoelectricity and spontaneous polarization on electronic and optical properties of wurtzite III-V nitride quantum wells

A theoretical model accounting for the macropolarization effects in wurtzite III-V nitrides quantum wells (QWs) is presented. Energy dispersions and exciton binding energies are calculated within the framework of effective-mass theory and variational approach, respectively. Exciton-associated transitions (EATs) are studied in detail. An energy redshift as high as 450 meV is obtained in Al0.25GaN0.75/GaN QWs. Also, the abrupt reduction of optical momentum matrix elements is derived as a consequence of quantum-confined Stark effects. EAT energies are compared with recent photoluminescence (PL) experiments and numerical coherence is achieved. We propose that it is the EAT energy, instead of the conduction-valence-interband transition energy that is comparable with the PL energy. To restore the reduced transition rate, we apply an external electric field. Theoretical calculations show that with the presence of the external electric field the optical matrix elements for EAT increase 20 times. (C) 2001 American Institute of Physics.

Scattering matrix approach to electronic dephasing in long-range electron transfer

Based on the Buttiker dephasing model, we propose an analytical scattering matrix approach to the long-range electron transfer phenomena. The present efficient scheme smoothly interpolates between the superexchange and the sequential hopping mechanisms. Various properties such as the drastic dephasing-assisted enhancement and turnover behaviors are demonstrated in good agreement with those obtained via the dynamical reduced density-matrix methods. These properties are further elucidated as results of the interplay among the dephasing strength, the tunneling parameter, and the bridge length of the electron transfer system. (C) 2001 American Institute of Physics.

Electronic states of InAs/GaAs quantum ring

In the framework of effective mass envelope function theory, the electronic states of the InAs/GaAs quantum ring are studied. Our model can be used to calculate the electronic states of quantum wells, quantum wires, and quantum dots. In calculations, the effects due to the different effective masses of electrons in rings and out rings are included. The energy levels of the electron are calculated in the different shapes of rings. The results indicate that the inner radius of rings sensitively changes the electronic states. The energy levels of the electron are not sensitively dependent on the outer radius for large rings. If decreasing the inner and outer radii simultaneously, one may increase the energy spacing between energy levels and keep the ground state energy level unchanged. If changing one of two radii (inner or outer radius), the ground state energy level and the energy spacing will change simultaneously. These results are useful for designing and fabricating the double colors detector by intraband and interband translations. The single electron states are useful for studying the electron correlations and the effects of magnetic fields in quantum rings. Our calculated results are consistent with the recent experimental data of nanoscopic semiconductor rings. (C) 2001 American Institute of Physics.

Electronic characteristics of InAs self-assembled quantum dots

Deep-level transient spectroscopy and photoluminescence studies have been carried out on structures containing self-assembled InAs quantum dots formed in GaAs matrices. The use of n- and p-type GaAs matrices allows us to study separately electron and hole levels in the quantum dots by the deep-level transient spectroscopy technique. From analysis of deep-level transient spectroscopy measurements it follows that the quantum dots have electron levels 130 meV below the bottom of the GaAs conduction band and heavy-hole levels at 90 meV above the top of the GaAs valence band. Combining with the photoluminescence results, the band structures of InAs and GaAs have been determined. (C) 2000 Elsevier Science B.V. All rights reserved.