Quantum-confined Stark effect and built-in dipole moment in self-assembled InAs/GaAs quantum dots

Quantum-confined Stark effect and built-in dipole moment in self-assembled InAs/GaAs quantum dots (QDs), which are grown at relative low temperature (460degreesC) and embedded in GaAs p-i-n structure, have been studied by dc-biased electroreflectance. Franz-Keldysh oscillations from the undoped GaAs layer are used to determine the electric field under various bias voltages. Stark shift of -34 meV for the ground-state interband transition of the QDs is observed when the electric field increases from 105 to 308 kV/cm. The separation of the electron and hole states in the growth direction of 0.4 nm, corresponding to the built-in dipole moment of 6.4x10(-29) C m, is determined. It is found that the electron state lies above that of the hole, which is the same as that predicted by theoretical calculations for ideal pyramidal InAs QDs. (C) 2004 American Institute of Physics.

Characterization of InAs quantum dots on lattice-matched InAlGaAs/InP superlattice structures

Self-assembled InAs quantum dots (QDs) in an InAlGaAs matrix, lattice-matched to InP substrate, have been grown by molecular beam epitaxy (MBE). Transmission electron microscopy (TEM), double-crystal X-ray diffraction (DCXRD) and photoluminescence (PL) are used to study their structural and optical properties. In InAs/InAlGaAs/ InP system, we propose that when the thickness of InAs layer deposited is small, the random strain distribution of the matrix layer results in the formation of tadpole-shaped QDs with tails towards random directions, while the QDs begin to turn into dome-shaped and then coalesce to form islands with larger size and lower density to release the increasing misfit strain with the continuous deposition of InAs. XRD rocking curves showing the reduced strain with increasing thickness of InAs layer may also support our notion. The results of PL measurements are in well agreement with that of TEM images. (C) 2004 Elsevier B.V. All rights reserved.

Thermal annealing effect on InAs/InGaAs quantum dots grown by atomic layer molecular beam epitaxy

The effect of rapid thermal annealing on the InAs quantum dots (QDs) grown by atomic layer molecular beam epitaxy and capped with InGaAs layer has been investigated using transmission electron microscopy and photoluminescence (PL). Different from the previously reported results, no obvious blueshift of the PL emission of QDs is observed until the annealing temperature increases up to 800 degreesC. The size and shape of the QDs annealed at 750 degreesC have hardly changed indicating the relatively weak Ga/In interdiffusion, which is characterized by little blueshift of the PL peak of QDs. The QD size increases largely and a few large clusters can be observed after 800 degreesC RTA, implying the fast interdiffusion and the formation of InGaAs QDs. These results indicate that the delay of the blueshift of the PL peak of QDs is correlated with the abnormal interdiffusion process, which can be explained by two possible reasons: the reduction of excess-As-induced defects and the redistribution of In, Ga atoms around the InAs QDs resulted from the sub-monolayer deposition of InGaAs capping layer. (C) 2004 Elsevier B.V. All rights reserved.

THz-field-induced electronic transmission step-structure for a quantum wire

We study theoretically the low-temperature electronic transport property of a straight quantum wire under the irradiation of a finite-range transversely polarized external terahertz (THz) electromagnetic (EM) field. Using the free-electron model and the scattering matrix approach, we show an unusual behaviour of the electronic transmission of this system. A sharp step-structure appears in the electronic transmission probability as the EM field strength increases to a threshold value when a coherent EM field is applied. We demonstrate that this effect physically comes from the inelastic scattering of electrons with lateral photons through intersubband transitions.

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.

Ordering growth of InAs quantum dots on ultra-thin InGaAs strained layer

InAs quantum dots (QDs) were grown On Ultra-thin In0.15Ga0.85As strained layers by molecular beam epitaxy on GaAs (00 1) substrates. Combining reflection high-energy electron diffraction, atomic force microscopy and transmission electron microscopy, we analyzed the stress field of dislocations in the strained layer/substrate interface. Specially, we revealed the relative position of QDs and dislocations. We found that the difference of the stress field around dislocations is prominent when the strained layer is ultra-thin and the stress field will directly affect the following growth. On the strained layer surface, In0.15Ga0.85As ridges will form at the inclined upside of dislocations. Then, InAs QDs will prefer nucleating on the ridges, there is relatively small stress between InAs and In0.15Ga0.85As. By selecting ultra-thin In0.15Ga0.85As layer (50 nm) and controlling the QD layer at just form QDs, we obtained ordered InAs QDs. (C) 2004 Elsevier B.V. All rights reserved.

Effect of critical thickness on structural and optical properties of InxGa1-xN/GaN multiple quantum wells

InGaN/GaN multiquantum-well (MQW) structures grown by metalorganic chemical-vapor deposition on n-type GaN and capped by p-type GaN were investigated by cross-sectional transmission electron microscopy, double crystal x-ray diffraction, and temperature-dependent photoluminescence. For the sample with strained-layer thicknesses greater than the critical thicknesses, a high density of pure edge type threading dislocations generated from MQW layers and extended to the cap layer was observed. These dislocations result from a relaxation of the strained layers when their thicknesses are beyond the critical thicknesses. Because of indium outdiffusion from the well layers due to the anneal effect of Mg-doped cap layer growth and defects generated from strain relaxation, the PL emission peak was almost depressed by the broad yellow band with an intensity maximum at 2.28 eV. But for the sample with strained-layer thicknesses less than the critical thicknesses, it has no such phenomenon. The measured critical thicknesses are consistent with the calculated values using the model proposed by Fischer, Kuhne, and Richter. (C) 2004 American Institute of Physics.

Photon-assisted Fano resonance and corresponding shot noise in a quantum dot

We have studied the Fano resonance in photon-assisted transport through a quantum dot. Both the coherent current and the spectral density of shot noise have been calculated. It is predicted that the shape of the Fano profile will also appear in satellite peaks. It is found that the variations of Fano profiles with the strengths of nonresonant transmissions are not synchronous in absorption and emission sidebands. The effect of interference on photon-assisted pumped current has also been investigated. We further predict the current and spectral density of shot noise as a periodic function of the phase, which exhibits an intrinsic property of resonant and nonresonant channels in the structures.

Photoluminescence from self-assembled long-wavelength InAs/GaAs quantum dots under pressure

The photoluminescence from self-assembled long-wavelength InAs/GaAs quantum dots was investigated at 15 K under hydrostatic pressure up to 9 GPa. Photoemission from both the ground and the first excited states in large InAs dots was observed. The pressure coefficients of the two emissions were 69 and 72 meV/GPa, respectively. A nonlinear elasticity theory was used to interpret the significantly small pressure coefficients of the large dots. The sequential quenching of the ground and the excited state emissions with increasing pressure suggests that the excited state emissions originate from the optical transitions between the first excited electron states and the first excited hole states. (C) 2004 American Institute of Physics.

Suppression of exciton recombination in symmetric GaAs0.7Sb0.3/GaAs/GaAs0.7P0.3 coupled quantum wells induced by an in-plane magnetic field

Suppression of the exciton recombination in GaAs0.7Sb0.3/GaAs/GaAs0.7P0.3 coupled quantum well (CQW) induced by an external magnetic field is investigated theoretically. Unlike the usual electro-Stark effect, the exciton energy dispersion of an exciton is modified by an external in-plane magnetic field, the ground state of the magnetoexciton shifts from a zero in-plane center of mass (CM) momentum to a finite CM momentum, and the Lorentz force induces the spatial separation of electron and hole. Consequently, this effect renders the ground state of magnetoexciton stable against radiative recombination due to momentum conservation. This effect depends sensitively on the thickness and height of GaAs0.7Sb0.3 layer, therefore it could provide us useful infometion about the band alignment of CQW. (C) 2004 American Institute of Physics.

Spin-polarized ballistic transport in diluted magnetic semiconductor quantum wire systems

The energy dispersion of an electron in a double quantum wire with a diluted magnetic semiconductor barrier in between is calculated. An external magnetic field modifies significantly the energy dispersion of the electron which is different for the two spin states. The conductance exhibits many interesting peaks and dips which are directly related to the energy dispersions of the different electron spin states. These phenomena are attributed to the interwell coupling which can be tuned by the magnetic field due to the s-d exchange interaction.

Structural and optical properties of InAs/In0.52Al0.48As self-assembled quantum wires on InP(001)

Six-stacked InAs/In0.52Al0.48As self-assembled quantum wires (QWRs) on InP(001) by molecular-beam epitaxy (MBE) have been studied by high-resolution transmission electron microscopy (HRTEM) and polarized PL measurements. We obtained the chemical lattice fringe (CLF) image of InAs self-assembled QWRs embedded in the In0.52Al0.48As matrix by the interference between the (002)-diffracted beam and the transmitted beam in the image plane of the objective lens. The results show that the InAs QWRs were bounded by (113), (001) and (114) facets. Both the size and strain distribution in QWRs were determined. It was found that with the growth of successive periods, the height and height fluctuation of InAs QWRs decreased from the bottom period to the upper one. Some suggestions are put forward for further improving the uniformity of the stacked InAs QWRs. (c) 2005 Elsevier B.V. All rights reserved.

High structural and optical quality 1.3 mu m GaInNAs/GaAs quantum wells with higher indium content grown by molecular-beam expitaxy

High structural and optical quality 1.3 mu m GaInNAs/GaAs quantum well (QW) samples with higher (42.5%) indium content were successfully grown by molecular-beam epitaxy. The cross-sectional transmission electron microscopy measurements reveal that there are no structural defects in such high indium content QWs. The room-temperature photoluminescence peak intensity of the GaIn0.425NAs/GaAs (6 nm/20 nm) 3QW is higher than, and the full width at half maximum is comparable to, that of In0.425GaAs/GaAs 3QW, indicating improved optical quality caused by strain compensation effect of introducing N to the high indium content InGaAs epilayer. (C) 2005 American Institute of Physics.

Singlet-triplet transition in quantum dots confined by triangular and bowl-like potentials: the effect of electric fields

We theoretically investigate the energy spectra of two-electron two-dimensional (2e 2D) quantum dots (QDs) confined by triangular potentials and bowl-like potentials in a magnetic field by exact diagonalization in the framework of effective mass theory. An in-plane electric field is,found to contribute to the singlet-triplet transition of the ground state of the 2e 2D QDs confined by triangular or bowl-like potentials in a perpendicular magnetic field. The stronger the in-plane electric field, the smaller the magnetic field for the total spin of the ground states in the dot systems to change from S = 0 to S = 1. However, the influence of an in-plane electric field on the singlet-triplet transition of the ground state of two electrons in a triangular QD modulated by a perpendicular magnetic field is quite small because the triangular potential just deviates from the harmonic potential well slightly. We End that the strength of the perpendicular magnetic field needed for the spin singlet-triplet transition of the ground state of the QD confined by a bowl-like potential is reduced drastically by applying an in-plane electric field.

Influence of dislocations on photoluminescence of InGaN/GaN multiple quantum wells

The influence of dislocations on photoluminescence (PL) of InGaN/GaN multiple quantum wells (MQWs) is investigated by triple-axis x-ray diffraction (TAXRD), transmission electron microscopy (TEM), and PL spectra. The omega scan of every satellite peak by TAXRD is adopted to evaluate the mean screw and edge dislocation densities in MQWs. The results show that dislocations can lead to a reduction of the PL-integrated intensity of InGaN/GaN MQWs under certain conditions, with edge dislocations playing a decisive role. Additionally, the dislocations can broaden the PL peak, but the effect becomes evident only under the condition when the interface roughness is relatively low. (C) 2005 American Institute of Physics.