The electronic structure of strained ZnO/MgxZn1-xO superlattices and the influence of polarization

Based on the effective-mass model, the lower energies of the electron and the hole of ZnO/MgxZn1-xO superlattices are calculated. Because of the mismatch of the lattice constant between the ZnO well and the MgxZn1-xO barrier, piezoelectric and spontaneous polarization exist in ZnO/MgxZn1-xO superlattices and a macroscopical internal electric held is found when well width L-w >4 nm and Mg concentration x > 0.2. The parameters of ZnO/MgxZn1-xO superlattices such as lattice constant, band offset, etc. are also proposed. Through calculations, we found the internal electric field can change the lowest energies of the electron and hole to 105.4 and 85.1 meV when well width L-w up to 70 angstrom, which will influence the electronic and optical properties of ZnO/MgxZn1-xO superlattices greatly, while the Rashba effect from the internal electric field is so small that it can be neglected. The ground state exciton energies with different Mg concentration x are also calculated by variational method, our results are very close to the experimental results when Mg concentration x <= 0.3. (C) 2008 Elsevier B.V. All rights reserved.

Electronic structure of self-assembled InAs quantum disks in an axial magnetic field and two-electron quantum-disk qubit

We have studied the single-electron and two-electron vertically assembled quantum disks in an axial magnetic field using the effective mass approximation. The electron interaction is treated accurately by the direct diagonalization of the Hamiltonian matrix. We calculate the six energy levels of the single-electron quantum disks and the two lowest energy levels of the two-electron quantum disks in an axial magnetic field. The change of the magnetic field strongly modifies the electronic structures as an effective potential, leading to the splitting of the levels and the crossings between the levels. The effect of the vertical alignment on the electronic structures is discussed. It is demonstrated that the switching of the ground-state spin exists between S=0 and S=1. The energy difference DeltaE between the lowest S=0 and S=1 states is shown as a function of the axial magnetic field. It is also found that the variation of the energy difference between the lowest S=0 and S=1 states in the strong-B S=0 state is fairly linear. Our results provide a possible realization for a qubit to be fabricated by current growth techniques. (C) 2004 American Institute of Physics.

Electronic transport through self-assembled quantum disks

In this paper we study a single electron tunneling through a vertically stacked self-assembled quantum disks structure using a transfer matrix technique in the framework of effective mass approximation. In the disks, the electron is confined both laterally and vertically; we separate the motion in the vertical and lateral directions within the adiabatic approximation and treat the energy levels of the latter as an effective confining potential. The influence of a constant applied electric field is taken into account using an exact Airy-function formalism and the current density is calculated at zero temperature. By increasing the widths of the barriers, we find the peaks of the current density shift toward lower voltage region; meanwhile, they can become even sharper. (C) 2004 Elsevier Ltd. All rights reserved.

Electronic structure and optical property of semiconductor nanocrystallites

Semiconductor nanostructures show many special physical properties associated with quantum confinement effects, and have many applications in the opto-electronic and microelectronic fields. However, it is difficult to calculate their electronic states by the ordinary plane wave or linear combination of atomic orbital methods. In this paper, we review some of our works in this field, including semiconductor clusters, self-assembled quantum dots, and diluted magnetic semiconductor quantum dots. In semiconductor clusters we introduce energy bands and effective-mass Hamiltonian of wurtzite structure semiconductors, electronic structures and optical properties of spherical clusters, ellipsoidal clusters, and nanowires. In self-assembled quantum dots we introduce electronic structures and transport properties of quantum rings and quantum dots, and resonant tunneling of 3-dimensional quantum dots. In diluted magnetic semiconductor quantum dots we introduce magnetic-optical properties, and magnetic field tuning of the effective g factor in a diluted magnetic semiconductor quantum dot. (C) 2004 Elsevier B.V. All rights reserved.

Theoretical investigation of intersubband transition in AlxGa1-xN/GaN/AlyGa1-yN step quantum well

A modified self-consistent method is introduced for the design of AlxGa1-xN/GaN step quantum well (SQW) with the position and energy-dependent effective mass. The effects of nonparabolicity are included. It is shown that the nonparabolicity effect is minute for the lowest subband energy level and grows in size for the higher subband states. The effects of nonparabolicity have significant influence on the transition energies and the oscillator strengths and should be taken into account in the investigation of the optical transitions. The strong asymmetric property introduced by the step quantum well magnifies the weak intersubband transition from the ground state to the third state (1 -> 3). It is shown that in an appropriate scope, the intersubband transition (1 -> 3) has the comparable oscillator strength with transition from the ground state to the second one (1 -> 2), which suggests the possible application of the two-color photodetectors. The results of this work should provide useful guidance for the design of optically pumped asymmetric quantum well lasers and quantum well infrared photodetectors (QWIPs). (c) 2005 Elsevier B.V. All rights reserved.

Computer simulation of a-Si : H/mu c-Si : H diphasic silicon solar cells

Based on our experimental research on diphasic silicon films, the parameters such as absorption coefficient, mobility lifetime product and bandgap were estimated by means of effective-medium theory. And then computer simulation of a-Si: H/mu c-Si: H diphasic thin film solar cells was performed. It was shown that the more crystalline fraction in the diphasic silicon films, the higher short circuit density, the lower open-circuit voltage and the lower efficiency. From the spectral response, we can see that the response in long wave region was improved significantly with increasing crystalline fraction in the silicon films. Taking Lambertian back refraction into account, the diphasic silicon films with 40%-50% crystalline fraction was considered to be the best intrinsic layer for the bottom solar cell in micromorph tandem.

Excitonic energy of vertically stacked self-assmbled InAs quantum dots

We have studied the exciton states in vertically stacked self-assembled quantum disks within the effective mass approximation. The energy spectrum of the electron and hole is calculated using the transfer matrix formalism in the adiabatic approximation. The Coulomb interaction between the electron and the hole is treated accurately by the direct diagonalization of the Hamiltonian matrix. The effect of the vertical alignment of the disks on the ground energy of heavy- and light-hole exciton is presented and discussed. The binding energy is discussed in terms of the probability of the ground wave function. The ground energy of heavy- and light-hole excitons as a function of the magnetic field is presented and the effect of the disk size (the radius of disks) on the exciton energy is discussed.

Electronic structure of self-assembled InAs quantum discs in a magnetic field with varying orientation and two-electron quantum-disc qubits

We have studied the electronic structure of vertically assembled quantum discs in a magnetic field with varying orientation using the effective mass approximation. We calculate the four energy levels of single-electron quantum discs and the two lowest energy levels of two-electron quantum discs in a magnetic field with varying orientation. The change of the magnetic field as an effective potential strongly modifies the electronic structure, leading to splittings of the levels and anticrossings between the levels. The calculated results also demonstrate the switching between the ground states with the total spin S = 0 and 1. The switching induces a qubit controlled by varying the orientation of the magnetic field.

Effect of spontaneous and piezoelectric polarization on intersubband transition in AlxGa1-xN-GaN quantum well

A self-consistent solution of conduction band profile and subband energies for AlxGa1-xN-GaN quantum well is presented by solving the Schrodinger and Poisson equations. A new method is introduced to deal with the accumulation of the immobile charges at the AlxGa1-xN-GaN interface caused by spontaneous and piezoelectric polarization in the process of solving the Poisson equation. The effect of spontaneous and piezoelectric polarization is taken into account in the calculation. It also includes the effect of exchange-correlation to the one electron potential on the Coulomb interaction. Our analysis is based on the one electron effective-mass approximation and charge conservation condition. Based on this model, the electron wave functions and the conduction band structure are derived. We calculate the intersubband transition wavelength lambda(21) for different Al molar fraction of barrier and thickness of well. The calculated result can fit to the experimental data well. The dependence of the absorption coefficient a on the well width and the doping density is also investigated theoretically. (C) 2004 American Vacuum Society.

Anisotropic Zeeman splitting and Stark shift of In1-yMnyAs1-xNx oblate quantum dots

The electronic structure, Zeeman splitting, and Stark shift of In1-yMnyAs1-xNx oblate quantum dots are studied using the ten-band k center dot p model including the sp-d exchange interaction between the carriers and the magnetic ion. The Zeeman splitting of the electron ground states is almost isotropic. The Zeeman splitting of the hole ground states is highly anisotropic, with an anisotropy factor of 918 at B=0.1 T. The Zeeman splittings of some of the electron and hole excited states are also highly anisotropic. It is because of the spin-orbit coupling which couples the spin states with the anisotropic space-wave functions due to the anisotropic shape. It is found that when the magnetic quantum number of total orbital angular momentum is nearly zero, the spin states couple with the space-wave functions very little, and the Zeeman splitting is isotropic. Conversely, if the magnetic quantum number of total orbital angular momentum is not zero, the space-wave functions in the degenerate states are different, and the Zeeman splitting is highly anisotropic. The electron and hole Stark shifts of oblate quantum dots are also highly anisotropic. The decrease of band gap with increasing nitrogen composition is much more obvious in the smaller radius case because the lowest conduction level is increased by the quantum confinement effect and is closer to the nitrogen level. (C) 2007 American Institute of Physics.

Coherent tunnelling in coupled quantum wells under a uniform magnetic fields

Coherent tunnelling is studied in framework of the effective mass approximation for an asymmetric coupled quantum well. The Hartree potential due to the electron-electron interaction is considered in our calculation. The effects of the longitudinal and transverse magnetic field on coherent tunnelling characteristics are discussed. It has been found that the external field plays an important role in modulating the electron states.

The wave function evolution of an exciton in a bilayer system: Turning on a static in-plane electric field

The time evolution of the ground state wave function of an exciton in an ideal bilayer system is investigated within the framework of the effective-mass approximation. All of the moduli squared of the ground state wave functions evolve with time as cosine functions after an in-plane electric field is applied to the bilayer system. The variation amplitude and period of the modulus squared of the ground state wave function increase with the in-plane electric field F-r for a fixed in-plane relative coordinate r and fixed separation d between the electron and hole layers. Moreover, the variation amplitude and period of the modulus squared of the ground state wave function increase with the separation d for a fixed r and fixed in-plane electric field. Additionally, the modulus squared of the ground state wave function decreases as r increases at a given time t for fixed values of d and F-r. (c) 2007 American Institute of Physics.

Depolarization blueshift in intersubband transitions of triangular quantum wires

Major State Basic Research Project 973 program of China 2006CB604907;National Science Foundation of China 60776015 60976008;863 High Technology R&D Program of China 2007AA03Z402

Electronic structure of InSb quantum ellipsoids in an external magnetic field

The shape dependence of electronic structure, electron g factors in the presence of the external magnetic field of InSb quantum ellipsoids are investigated in the framework of eight-band effective-mass approximation. It is found that as the increasing aspect ratio e, the electron states with P character split into three doublets for the different physical interaction and the light-hole states with S character come up to the top of valence bands at e = 2.6 in comparison with the heavy-hole states. In the presence of the external magnetic field, the energy splits of electron states are different for their wave function distribution direction, and the hole ground state remain optical active for a suitable aspect ratio. The electron g factors of InSb spheres decrease with increasing radius, and have the value of about two for the smallest radius, about -47.2 for sufficiently larger radius, similar to the bulk material case. Actually, the electron g factors decrease as any one of the three dimensions increase. The more dimensions increase, the more g factors decrease. The dimensions perpendicular to the direction of the magnetic field affect the g factors more than the other dimensions. (c) 2006 Elsevier B.V. All rights reserved.

Electronic structure and g factors of CdTe quantum ellipsoids

The electronic structure, electron and hole g factors and optical properties of CdTe quantum ellipsoids are investigated, in the framework of eight-band effective-mass approximation. It is found that the light-hole states come down in comparison with the heavy-hole states when the spheres are elongated, and become the lowest states of valence band. When the aspect ratio of the ellipsoid length to diameter (e) changes from smaller than 1 to larger than 1, the linear polarization factors change from negative to positive. The electron g factors of CdTe spheres decrease with increasing radius, and are nearly 2 when the radius is very small. Actually, as some of the three dimensions increase, the electron g factors decrease. More dimensions increase, the g factors decrease. more. The dimensions perpendicular to the direction of the magnetic field affect the g factors more than the other dimension. The light-hole and heavy-hole g factors of quantum spheres are equal, and change from 0.88 to -1.14 with increasing radius. When e < 1 (e > 1) the light-hole g factor is smaller (larger) than the heavy-hole g factor. (c) 2006 Elsevier B.V. All rights reserved.