Variational calculations of neutral bound excitons in GaAs quantum-well wires

The binding energy of an exciton bound to a neutral donor (D-0,X) in GaAs quantum-well wires is calculated variationally as a function of the wire width for different positions of the impurity inside the wire by using a two-parameter wavefunction. There is no artificial parameter added in our calculation. The results we have obtained show that the binding energies are closely correlated to the sizes of the wire, the impurity position, and also that their magnitudes are greater than those in the two-dimensional quantum wells compared. In addition, we also calculate the average interparticle distance as a function of the wire width. The results are discussed in detail.

OPTICAL CHARACTERISTICS OF GAAS/ALGAAS RIDGE-QUANTUM-WELL-WIRES GROWN BY MBE ON NONPLANAR SUBSTRATES

With conventional photolithography and wet chemical etching, we have realized GaAs/AlGaAs buried ridge-quantum-well-wires (RQWWs) with vertically stacked wires in lateral arrays promising for device application, which were grown in situ by a single-step molecular beam epitaxy growth and formed at the ridge tops of mesas on nonplanar substrates. Confocal photoluminescence (CPL) and polarization-dependent photoreflectance (PR) are applied to study optical characteristics of RQWWs. Lateral bandgap modulation due to lateral variation of QW layer thickness is demonstrated not only by CPL but also by PR. As one evidence for RQWWs, a large blue shift is observed at the energy level positions for electronic transitions corresponding to quantum wells (QWs) at the ridge tops of mesas compared with those corresponding to QWs on nonpatterned areas of the same sample. The blue shift is in contradiction with the fact that the GaAs QW layers at the tops of the mesas are thicker than those on nonpatterned areas. The other evidence for RQWWs, optical anisotropy is provided by the polarization-dependent PR, which results from lateral quantum size effect existing at the tops of the mesas.

OPTICAL-PHONON MODES AND FROHLICH POTENTIAL IN ONE-DIMENSIONAL QUANTUM-WELL WIRES

By extending our microscopic model on optical-phonon modes in quantum wells to one-dimensional (1D) quantum-well wires (QWW), the optical displacements and associated electrostatic potentials of optical-phonon modes in 1D QWW are calculated. The modes can be clearly divided into confined LO bulklike, TO bulklike modes, and extended interfacelike modes provided the bulk phonon dispersion is ignored. The character of each type of mode is illustrated with special attention to the interfacelike modes, which are hybrids of longitudinal- and transverse-optical waves from the corresponding bulk materials. Based on the numerical results, approximate analytical formulas for bulklike modes are presented. As in 2D wells, both the optical displacements and Frohlich potentials for the bulklike modes vanish at the interfaces. The finite dispersion of bulk phonons has a more pronounced effect on the 1D phonon modes because interfacelike modes show mixed characteristics of 2D interface and bulklike modes.

Binding Energy of Biexcitons in GaAs Quantum-Well Wires

The binding energy of a biexciton in GaAs quantum-well wires is calculated variationally by use of a two-parameter trial wavefunction and a one-dimensional equivalent potential model. There is no artificial parameter added in our calculation. Our results agree fairly well with the previous results. It is found that the binding energies are closely correlative to the size of wire. The binding energy of biexcitons is smaller than that of neutral bound excitons in GaAs quantum-well wires when the dopant is located at the centre of the wires.

Self-assembled quantum dots, wires and quantum-dot lasers

Molecular beam epitaxy-grown self-assembled In(Ga)As/GaAs and InAs/InAlAs/InP quantum dots (QDs) and quantum wires (QWRs) have been studied. By adjusting growth conditions, surprising alignment. preferential elongation, and pronounced sequential coalescence of dots and wires under specific condition are realized. The lateral ordering of QDs and the vertical anti-correlation of QWRs are theoretically discussed. Room-temperature (RT) continuous-wave (CW) lasing at the wavelength of 960 nm with output power of 3.6 W from both uncoated facets is achieved fi-om vertical coupled InAs/GaAs QDs ensemble. The RT threshold current density is 218 A/cm(2). A RT CW output power of 0.6 W/facet ensures at least 3570 h lasing (only drops 0.83 dB). (C) 2001 Elsevier Science B.V, All rights reserved.

Quantum confinement effect in silicon quantum-well layers

The electronic states and optical transition properties of silicon quantum-well layers embedded by SiO2 layers are studied by the empirical pseudopotential homojunction model. The energy bands, wave functions, and the optical transition matrix elements are obtained for layers of thickness from 1 to 6 nm, and three oriented directions (001), (110), and (111). It is found that for Si layers in the (001) direction the energy gap is pseudodirect, for these in the (111) direction the energy gap is indirect, while for those in the (110) direction the energy gap is pseudodirect or indirect for a thickness smaller or larger than 3 nm, respectively. The optical transition matrix elements are smaller than that of diner transition, and increase with decreasing layer thickness. When the thickness of a layer is smaller than 2 nm, the Si QW layers have larger transition matrix elements. It is caused by mixing of bulk X states with the Gamma(1) state. The calculated results are compared with experimental results.

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

Self-assembled quantum dots, wires and quantum-dot lasers

Molecular beam epitaxy-grown self-assembled In(Ga)As/GaAs and InAs/InAlAs/InP quantum dots (QDs) and quantum wires (QWRs) have been studied. By adjusting growth conditions, surprising alignment. preferential elongation, and pronounced sequential coalescence of dots and wires under specific condition are realized. The lateral ordering of QDs and the vertical anti-correlation of QWRs are theoretically discussed. Room-temperature (RT) continuous-wave (CW) lasing at the wavelength of 960 nm with output power of 3.6 W from both uncoated facets is achieved fi-om vertical coupled InAs/GaAs QDs ensemble. The RT threshold current density is 218 A/cm(2). A RT CW output power of 0.6 W/facet ensures at least 3570 h lasing (only drops 0.83 dB). (C) 2001 Elsevier Science B.V, All rights reserved.

Enhancing Kerr nonlinearity in an asymmetric double quantum well via Fano interference

We investigate the enhancement of Kerr nonlinearity in an asymmetric GaAs double quantum well via Fano interference, which is caused by tunneling from the excited subband to the continuum. In our structure, owing to Fano interference, the Kerr nonlinearity can be enhanced by appropriately choosing the values of the detunings and the intensity of the pump field, while cancel the linear and nonlinear absorptions.

Third harmonic enhancement due to Fano interference in semiconductor quantum well

We investigate the emission spectra of the semiconductor quantum well for few-cycle and sub-cycle pulse exciting. We find that Fano interference may induce third harmonic enhancement. Third harmonic enhancement varies with the magnitude and duration of the incident pulse, and may be enhanced by approximately one order of magnitude for the low intensity region of the sub-cycle incident pulse exciting.

Carrier-envelope phase control of carrier-wave Rabi flopping in asymmetric semiparabolic quantum well

We investigate the carrier-wave Rabi flopping effects in an asymmetric semiparabolic semiconductor quantum well (QW) with few-cycle pulse. It is found that higher spectral components of few-cycle ultrashort pulses in the semiparabolic QW depend crucially on the carrier-envelope phase (CEP) of the few-cycle ultrashort pulses: continuum and distinct peaks can be achieved by controlling the CEP. Our results demonstrate that by adjusting the CEP of few-cycle ultrashort pulses, the intersubband dynamics in the asymmetric semiparabolic QW can be controlled in an ultrashort timescale with moderate laser intensity. (c) 2008 Optical Society of America.

Tunneling-induced large cross-phase modulation in an asymmetric quantum well

We propose an asymmetric double AlGaAs/GaAs quantum well structure with a common continuum to generate a large cross-phase modulation (XPM). It is found, owing to resonant tunneling, that a large XPM can be achieved with vanishing linear and two-photon absorptions. (c) 2007 Optical Society of America.

Spin injection in the multiple quantum-well LED structure with the Fe/AlO (x) injector

A spin-injection/-detection device has been fabricated based on the multiple quantum well light emitting diode (LED) structure. It is found that only a broad electroluminescence (EL) peak of a full width at half maximum of 8.6 nm appears at the wavelength of 801 nm in EL spectra with a circular luminescence polarization degree of 18%, despite PL spectra always show three well resolved peaks. The kinetic energy gained by injected electrons and holes in their drift along opposite directions broadens the EL peak, and makes three EL peaks converge together. The same process also destroys the injected spin polarization of electrons mainly dominated by the Bir-Aronov-Pikus spin relaxing mechanism.

Theoretical gain of strained GeSn0.02/Ge1-x-y ' SixSny ' quantum well laser

Using effective-mass Hamiltonian model of semiconductors quantum well structures, we investigate the electronic structures of the Gamma-conduction and L-conduction subbands of GeSn/GeSiSn strained quantum well structure with an arbitrary composition. Our theoretical model suggests that the band structure could be widely modified to be type I, negative-gap or indirect-gap type II quantum well by changing the mole fraction of alpha-Sn and Si in the well and barrier layers, respectively. The optical gain spectrum in the type I quantum well system is calculated, taking into account the electrons leakage from the Gamma-valley to L-valley of the conduction band. We found that by increasing the mole fraction of alpha-Sn in the barrier layer and not in the well layer, an increase in the tensile strain effect can significantly enhance the transition probability, and a decrease in Si composition in the barrier layer, which lowers the band edge of Gamma-conduction subbands, also comes to a larger optical gain.