Collective excitations in the coupled quantum wires

The dielectric response of an electron system composed of an array of parallel quantum wires with weak coupling and strong coupling are studied, and the dispersions of the collective excitations and the single particle excitations (SPE) as functions of wave-vectors are given. It is found that for the nearly isolated quantum wires with several subbands occupation, there are a series of intra-subband collective excitations between corresponding intra-subband SPE spectra. There also exist inter-subband collective excitations when q(x) not equal 0 (q(x) is the wave-vector component in the modulation direction), whose energies are close by the corresponding inter-subband SPE spectra. The energy of the intra-subband mode decreases and that of inter-subband mode increases with q(x) increasing. The collective excitation dispersions show obvious anisotropy in the 1D quantum limit. The calculated results agree with the experiment well. The coupling between quantum wires affects markedly both the collective and single-particle excitations spectra. The system changes to a near-two-dimensional electron system gradually with increasing coupling.

Collective excitations in coupled quantum wells

The dielectric response of a modulated three-dimensional electron system composed of a periodic array of quantum wells with weak coupling and strong coupling are studied, and the dispersions of the collective excitations and the single particle excitations as functions of wave vectors are given. It is found that for the nearly isolated multiple-quantum-well case with several subbands occupation, there is a three-dimensional-like plasmon when q(z)=0 (q(z) is the wave-vector component in the superlattice axis). There also exist intersubband collective excitations in addition to one intra-subband mode when q(z) not equal 0. The intra-subband mode has a linear dispersion relation with q(//) (the wave-vector component perpendicular to the superlattice axis) when q(//) is small. The inter-subband modes cover wider ranges in q(//) with increasing values of q(z). The energies of inter-subband collective excitations are close by the corresponding inter-subband single-particle excitation spectra. The collective excitation dispersions show obvious anisotropy in the 2D quantum limit. The calculated results agree with the experiment. The coupling between quantum wells affects markedly both the collective excitations and the single particle excitations spectra. The system shows gradually a near-three-dimensional electron gas character with increasing coupling. Copyright (C) 1996 Published by Elsevier Science Ltd

Asymmetric stark shifts of exciton in InAs/GaAs pyramidal quantum dots

Within the framework of the single-band effective-mass envelope-function theory, the effect of electric field on the electronic structures of pyramidal quantum dot is investigated. Taking the Coulomb interaction between the heavy holes and electron into account, the quantum confined Stark shift of the exciton as functions of the strength and direction of applied electric field and the size of the quantum dot are obtained. An interesting asymmetry of Stark shifts around the zero field is found. (C) 1997 Elsevier Science Ltd.

Quantum confinement effect in thin quantum wires

The electronic states and optical transition properties of three semiconductor wires Si? GaAs, and ZnSe are studied by the empirical pseudopotential homojunction model. The energy levels, wave functions, optical transition matrix elements, and lifetimes are obtained for wires of square cross section with width from 2 to 5 (root 2a/2), where a is the lattice constant. It is found that these three kinds of wires have different quantum confinement properties. For Si wires, the energy gap is pseudodirect, and the wave function of the electronic ground state consists mainly of four bulk Delta states. The optical transition matrix elements are much smaller than that of a direct transition, and increase with decreasing wire width. Where the width of wire is 7.7 Angstrom, the Si wire changes from an indirect energy gap to a direct energy gap due to mixing of the bulk Gamma(15) state. For GaAs wires. the energy gap is also pseudodirect in the width range considered, but the optical transition matrix elements are larger than those of Si wires by two orders of magnitude for the same width. However, there is no transfer to a direct energy gap as the wire width decreases. For ZnSe wires, the energy gap is always direct, and the optical transition matrix elements are comparable to those of the direct energy gap bulk semiconductors. They decrease with decreasing wire width due to mixing of the bulk Gamma(1) state with other states. All quantum confinement properties are discussed and explained by our theoretical model and the semiconductor energy band structures derived. The calculated lifetimes of the Si wire, and the positions of photoluminescence peaks, are in good agreement with experimental results.

Quantum dot superlattices and their conductance

The one-dimensional energy bands and corresponding conductivities of a T-shaped quantum-dot superlattice are studied in various cases: different periods, with potential barriers between dots, and in transverse electric fields. It is found that the conductivity of the superlattices has a similar energy relation to the conductance of a single quantum dot, but vanishes in the energy gap region. The energy band of a superlattice with periodically modulated conducting width in the perpendicular magnetic field is calculated for comparison with magneto-transport experiments. It is found that due to the edge state effect the electron has strong quantum transport features. The energy gaps change with the width of the channel, corresponding to the deep peaks in the conductance curve. This method of calculating the energy bands of quantum-dot superlattices is applicable to complex geometric structures without substantial difficulty. (C) 1997 American Institute of Physics.

Photoluminescence studies on the interaction of near-surface GaAs/AlxGa1-xAs quantum wells with chemical adsorbates

The chemical adsorption of sodium sulphide, ferrocene, hydroquinone and p-methyl-nitrobenzene onto the surface of a GaAs/AlxGa1-xAs multiquantum well semiconductor was characterized by steady state and time-resolved photoluminescence (PL) spectroscopy. The changes in the PL response, including the red shift of the emission peak of the exciton in the quantum well and the enhancement of the PL intensity, are discussed in terms of the interactions of the adsorbed molecules with surface states.

Subband separation energy dependence of intersubband relaxation time in wide quantum wells

Subband separation energy dependence of intersubband relaxation time in a wide quantum well (250 Angstrom) was studied by steady-state and time-resolved photoluminescence. By applying a perpendicular electrical field, the subband separation energy in the quantum well is continuously tuned from 21 to 40 meV. As a result, it is found that the intersubband relaxation time undergoes a drastic change from several hundred picoseconds to subpicoseconds. It is also found that the intersubband relaxation has already become very fast before the energy separation really reaches one optical phonon energy. (C) 1997 American Institute of Physics.

Exciton states in isolated quantum wires

The exciton states in isolated and semi-isolated quantum wires are studied. It is found that the image charges have a large effect on the effective Coulomb potential in wires. For the isolated wire the effective potential approaches the Coulomb potential in vacuum at large z distance. For the semi-isolated wire the effective potential is intermediate between the Coulomb potential in vacuum and the screened Coulomb potential at large distance. The exciton binding energy in the isolated wire is about ten times larger than that in the quantum well, and that in the semi-isolated wire is also intermediate between those in the isolated wire and in the quantum well. When the lateral width increases the binding energy decreases further, and approaches that in the quantum well. The real valence-band structure is taken into account, the exciton wave functions of the ground state in the zero-order approximation are given, and the reduced mass is calculated. The effect of the coupling between the ground and excited states are considered by the degenerate perturbation method, and it is found the coupling effect is small compared to the binding energy.

High-pressure study of optical transitions in strained In0.2Ga0.8As/GaAs multiple quantum wells

We have measured low-temperature photoluminescence (PL) and absorption spectra of In0.2Ga0.8As/GaAs multiple quantum wells (MQW's) under hydrostatic pressures up to 8 GPa. In PL, only a single peak is observed below 4.9 GPa corresponding to the n = 1 heavy-hole (HH) exciton in the InxGa1-xAs wells. Above 4.9 GPa, new PL lines related to X-like conduction band states appear. They are assigned to the type-II transition from the X(Z) states in GaAs to the HH subband of the InxGa1-xAs wells and to the zero-phonon line and LO-phonon replica of the type-I transition involving the X(XY) valleys of the wells. In addition to absorption peaks corresponding to direct exciton transitions in the wells, a new strong absorption feature is apparent in spectra for pressures between 4.5 and 5.5 GPa. This absorption is attributed to the pseudodirect transition between the HH subband and the X, state of the wells. This gives clear evidence for an enhanced strength of indirect optical transitions due to the breakdown of translational invariance in MQW structures. From experimental level splittings we determine the valence band offset and the shear deformation potential for X states in the In0.2Ga0.8As layer.

Shubnikov de Haas oscillations of a two-dimensional electron gas in a modulation-doped Zn0.80Cd0.20Se/ZnS0.06Se0.94 single quantum well

The magnetotransport properties of the two-dimensional (2D) electron gas confined in a modulation-doped Zn0.80Cd0.20Se/ZnS0.06Se0.94 single quantum well structure were studied at temperatures down to 0.35 K in magnetic fields up to 7.5 T. Well resolved 2D Shubnikovde Haas (SdH) oscillations were observed, although the conductivity of the sample in the as grown state was dominated by a bulk parallel conduction layer. After removing most of the parallel conduction layer by wet chemical etching the amplitude and number of SdH oscillations increased. From the temperature dependence of the amplitude the effective mass of the electrons was estimated as 0.17 m(0). Copyright (C) 1996 Published by Elsevier Science Ltd

Electronic structures of the zinc-blende GaN/Ga1-xAlxN compressively strained superlattices and quantum wells

The electronic structures of the zinc-blende GaN/Ga0.85Al0.15N compressively strained superlattices and quantum wells are investigated using a 6 x 6 Hamiltonian model (including the heavy hole, light hole and spin-orbit splitting band). The energy bands, wavefunctions and optical transition matrix elements are calculated. It is found that the light hole couples with the spin-orbit splitting state even at the k=0 point, resulting in the hybrid states. The heavy hole remains a pure heavy hole state at k=0. The optical transitions from the hybrid valence states to the conduction states are determined by the transitions of the light hole and spin-orbit splitting states to the conduction states. The transitions from the heavy hole, light hole and spin-orbit splitting states to the conduction states obey the selection rule Delta n=0. The band structures obtained in this work will be valuable in designing GaN/GaAlN based optoelectronic devices. (C) 1996 Academic Press Limited

EER studies of the single quantum well GaAs/AlxGa1-xAs electrode/nonaqueous solution interface

The interfacial behavior of the single quantum well (SQW) GaAs/AlxGa1-xAs electrode in HQ/BQ and Fc/Fc(+) electrolytes was characterized respectively by studying the quantum confined Stark effect and Franz-Keldysh oscillation with electrolyte electroreflectance spectroscopy. The interaction of the surface state of the SQW electrode with redox species and its effects on the distribution of external bias at the interface of the SQW electrode are discussed.

Photoluminescence studies of GaAs/AlGaAs single quantum well intermixed by Ga+ ion implantation

Ga(+)ion implantation followed by rapid thermal annealing (RTA) was used to enhance the interdiffusion in GaAs/AlGaAs single Quantum Wells(SQWs). The extent of intermixing was found to be dependent on the well depth, number of implanted ions and annealing time. A very fast interdiffusion process occurs at the initial annealing stage. After that, the enhanced diffusion coefficient goes back to the umimplanted value. We propose a two-step model to explain the diffusion process as a function of the annealing time : a fast diffusion process and a saturated diffusion process. The interdiffusion coefficient of the fast diffusion was found to be of well depth dependence and estimated to be in the range of 5.4x10(-16) similar to 1.5x10(-15)cm(2)s(-1). Copyright (C) 1996 Published by Elsevier Science Ltd

Optical gain in zinc-blende GaN/Ga1-xAlxN strained quantum well laser

Based on the valence subbands of the zinc-blende GaN/Ga0.85Al0.15N strained quantum wells obtained by a 6x6 Hamiltonian (including heavy hole, light hole and spin-orbit splitting band), optical gain and radiative current density are calculated for the strained quantum well laser structures. The compressive strain in the GaN well region strongly depresses the TM mode optical gain and enhances the TE mode optical gain.