Time-resolved photoluminescence of sub-monolayer InGaAs/GaAs quantum-dot-quantum-well heterostructures

Time-resolved photoluminescence (PL) of sub-monolayer (SML) InGaAs/GaAs quantum-dot-quantum-well heterostructures was measured at 5 K for the first time. The radiative lifetime of SML quantum dots (QDs) increases from 500 ps to 800 ps with the increase of the size of QDs, which is related to the small confinement energy of the excitons inside SML QDs and the exciton transfer from smaller QDs to larger ones through tunneling. The rise time of quantum-dot state PL signal strongly depends on the excitation power density. At low excitation power density, the rise time is about 35 ps, the mechanism of carrier capture is dominated by the emission of longitudinal-optical phonons. At high excitation power density, the rise time decreases as the excitation density increases, and Auger process plays an important role in the carrier capture. These results are very useful for understanding the working properties of sub-monolayer quantum-dot devices.

Quantum-dot growth simulation on periodic stress of substrate

InAs quantum dots (QDs) are grown on the cleaved edge of an InxGa1-xAs/GaAs supperlattice experimentally and a good linear alignment of these QDs on the surface of an InxGa1-xAs layer has been realized. The modulation effects of periodic strain on the substrate are investigated theoretically using a kinetic Monte Carlo method. Our results show that a good alignment of QDs can be achieved when the strain energy reaches 2% of the atomic binding energy. The simulation results are in excellent qualitative agreement with our experiments. (C) 2005 American Institute of Physics.

Interference and non-interference of sidebands in a quantum dot with oscillating levels

When a quantum dot is suffering an AC gate voltage, the sidebands turn up beside the static levels of the dot. We formularized the conductance and current when the effective coupling between levels in the quantum dot induced by the hybrid terms is included using a bi-unitary transform method, and we investigated the interference of the photon sidebands of deferent levels. The interference occurs if the same sidebands of deferent levels overlap, which is possible only when the static levels lie close to and overlap with each other. The overlap of different photon sidebands leads to a simple non-coherent superposition. (C) 2005 Elsevier Ltd. All rights reserved.

Spin and charge currents through a quantum dot connected to ferromagnetic leads

We investigate the spin polarized current through a quantum dot connected to ferromagnetic leads in the presence of a finite spin-dependent chemical potential. The effects of the spin polarization of the leads p and the external magnetic field B are studied. It is found that both the magnitude and the symmetry of the current are dependent on the spin polarization of the leads. When the two ferromagnetic leads are in parallel configuration, the spin polarization p has an insignificant effect on the spin current, and an accompanying charge current appears with the increase of p. When the leads are in antiparallel configuration, however, the effect of p is distinct. The charge current is always zero regardless of the variation of p in the absence of B. The peaks appearing in the pure spin current are greatly suppressed and become asymmetric as p is increased. The applied magnetic field B results in an accompanying charge current in both the parallel and antiparallel configurations of the leads. The characteristics of the currents are explained in terms of the density of states of the quantum dot.

Intraband relaxation and its influences on quantum dot lasers

A comprehensive two-level numerical model is developed to describe carrier distribution in a quantum-dot laser. Light-emission spectra with different intraband relaxation rates (2ps, 7.5ps and 20ps) are calculated and analysed to investigate the influence of relaxation rates on performance of the quantum-dot laser. The results indicate that fast intraband relaxation favours not only the ground state single mode operation but also the higher injection efficiency.

Design and analysis of 1.3 mu m GaAs-based quantum dot vertical-cavity surface-emitting lasers

A theoretical study on 1.3 mu m GaAs-based quantum dot vertical-cavity surface-emitting lasers (VCSELs) was made. Investigation of the influence of VCSELs on the optical confinement factors and the optical loss and the calculation of the material gain of the assembled InGaAs/GaAs quantum dots. Analysis of the threshold characteristic was made and the multi-wavelength cavity and multilayer quantum-dot stack structure is found to be more suitable for quantum dot VCSELs.

Tuning of detection wavelength in a resonant-cavity-enhanced quantum-dot-embedded photodiode by changing detection angle

We have fabricated a resonant-cavity-enhanced photodiode (RCE-PD) with InGaAs quantum dots (QDs) as an active medium. This sort of QD-embedded RCE-PD is capable of a peak external quantum efficiency of 32% and responsivity of 0.27A/W at 1.058 mu m with a full width at half maximum (FWHM) of 5 nm. Angle-resolved photocurrent response eventually proves that with the detection angle changing from 0 degrees to 60 degrees, the peak-current wavelength shifts towards the short wavelength side by 37 nm, while the quantum efficiency remains larger than 15%.

Growth of Ge quantum dot mediated by boron on Ge wetting layer

We, report on the influence of boron on the formation of Ge quantum dots. The investigated structure consists of a Ge wetting layer, on which a sub-monolayer boron is deposited and subsequently a Ge top layer. For sufficiently thin Ge top layers, the strain field induced by boron on Ge wetting layer destabilizes the Ge top layer and causes the formation of small Ge quantum dots. However, for thicker Ge top layers, boron on the Ge wetting layer diffuses into Ge layers, compensates partly the strain and delays the evolution of Ge quantum dots. By this method, small Ge quantum dots with high density as well as size uniformity can be formed by optimizing the growth condition. (c) 2005 Elsevier B.V. All rights reserved.

Level-oscillation-induced pump effect in a quantum dot with asymmetric constrictions

We have investigated the pump effect induced by the level oscillation in a quantum dot with asymmetric constrictions. The curve of pumped current versus the frequency of level oscillation undulates at zero temperature. The oscillation of the pumped current can be smeared by increasing the temperature and the coupling strength between the quantum dot and the leads. Either the temperature increase or the coupling strength enhancement can lead to a positive or negative effect on the pumped current, depending on the parameters of the quantum dot system. A larger level-oscillation magnitude results in a larger pumped current, especially in the low-frequency case. An analytical expression of the pumped current is obtained in the regime far from adiabatic. A convenient physical picture based on our analytic result is proposed, with which we can explain all the features of the pumped current curves.

Spin-polarized tunneling through a diluted magnetic semiconductor quantum dot

Spin-polarized tunneling through a diluted magnetic semiconductor quantum dot embedded in a tunneling barrier is investigated using the Bardeen transfer Hamiltonian. The tunneling current oscillates with an increasing magnetic field for a fixed bias. Many peaks are observed with an increasing external bias under a fixed magnetic field. Spin polarization of the tunneling current is tuned by changing the external bias under a weak magnetic field.

High-temperature electron-hole liquid and dynamics of Fermi excitons in a In0.65Al0.35As/Al0.4Ga0.6As quantum dot array

State-filling effects of the exciton in a In0.65Al0.35As/Al0.4Ga0.6As quantum dot array are observed by quantum dot array photolumineseence at a sample temperature of 77 K. The exciton emission at low excitation density is dominated by the radiative recombination of the states in the s shell and at high excitation density the emission mainly results from the radiative recombination of the exciton state in the p shell. The spectral interval between the states in the s and p shells is about 30-40 mcV. The time resolved photoluminescence shows that the decay time of exciton states in the p shell is longer than that of exciton states in the s shell, and the emission intensity of the exciton state in the p shell is superlinearly dependent on excitation density. Furthermore, electron-hole liquid in the quantum dot array is observed at 77 K, which is a much higher temperature than that in bulk. The emission peak of the. recombination, of electron-hole liquid has an about 200 meV redshift from the exciton fluorescence. Two excitation density-dependent emission peaks at 1.56 and 1.59 eV are observed, respectively, which result from quantum confinement effects in QDs. The emission intensity of electron-hole liquid is directly proportional to the cubic of excitation densities and its decay time decreases significantly at the high excitation density.

Role of interactions in electronic structure of a two-electron quantum dot molecule

We have studied a two-electron quantum dot molecule in a magnetic field. The electron interaction is treated accurately by the direct diagonalization of the Hamiltonian matrix. We calculate two lowest energy levels of the two-electron quantum dot molecule in a magnetic field. Our results show that the electron interactions are significant, as they can change the total spin of the two-electron ground state of the system by adjusting the magnetic field 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. We found that the energy difference between the lowest S = 0 and S = 1 states in the strong-B S = 0 state varies linearly. Our results provide a possible realization for a qubit to be fabricated by current growth techniques.

Photo-capacitance response of internal tunnelling coupling in quantum-dot-imbedded heterostructures under selective photo-excitation

Under selective photo-excitation, the capacitance response of internal tunnelling coupling in quantum-dots-imbedded heterostructures is studied to clarify the electronic states and the number densities of electrons filling in the quantum dots (QDs). The random nature for both optical transitions and the filling in a QD assembly makes highly resolved capacitance peaks appear in the C-V characteristic after turning off the photo-excitation.

Dependence of elastic strain field on the self-organized ordering of quantum dot superlattices

A systematic investigation of the strain distribution of self-organized, lens-shaped quantum dot in the case of growth direction on (001) substrate was presented. The three-dimensional finite element analysis for an array of dots was used for the strain calculation. The dependence of the strain energy density distribution on the thickness of the capping layer was investigated in detail when the elastic characteristics of the matrix material were anisotropic. It is shown that the elastic anisotropic greatly influences the stress, strain, and strain energy density in the quantum dot structures. The anisotropic ratio of the matrix material and the combination with different thicknesses of the capping layer, may lead to different strain energy density minimum locations on the capping layer surface, which can result in various vertical ordering phenomena for the next layer of quantum dots, i.e. partial alignment, random alignment, and complete alignment.

Gain measurement and anomalous decrease of peak gain at long wavelength for InAs/GaAs quantum-dot lasers

Mode gain spectrum is measured by the Fourier series expansion method for InAs/GaAs quantum-dot (QD) lasers with seven stacks of QDs at different injection currents. Gain spectra with distinctive peaks are observed at the short and long wavelengths of about 1210 nm and 1300 nm. For a QD laser with the cavity length of 1060 mu m, the peak gain of the long wavelength first increases slowly or even decreases with the injection current as the peak gain of the short wavelength increases quickly, and finally increases quickly before approaching the saturated values as the injection current further increases.