Highly enhanced second-order nonlinear susceptibilities in tailored GaN-AlGaN-AlN quantum well structures

A tailoring proposal for design of the strained quantum well structures, optimized with respect to the intersubband resonant second-order nonlinear properties, is presented in this article. A genetic-algorithm-based method is used in order to obtain the optimal potential shape, doping concentration and location in strained GaN-AlGaN-AlN quantum wells, and the structures are analyzed by a numerical solution of the Schrödinger-Poisson self-consistent method. In general form two types of asymmetric structures with remarkable results are obtained with different resonant frequencies, and in both cases results show a considerably high enhancement in the magnitude of the second-order nonlinear susceptibilities in higher resonant frequencies in comparison with a single quantum well structure with the same well width (5.02×10⁻⁸ m/V at to 2.9×10⁻⁵ m/V at and 2.43×10⁻⁵ m/V at ). The optimized structures exhibit considerable absorption coefficient and electroabsorption properties due to high dipole transition matrix element, high dopant concentration and reasonable Fermi level.     

Study of Be δ-doped GaAs/AlAs multiple quantum wells by the surface photovoltage spectroscopy

We report a surface photovoltage and differential surface photovoltage (DSPV) study of Be δ-doped GaAs/AlAs multiple quantum wells (QWs) with widths ranging from 3 to 20 nm and sheet doping densities from 2 × 10¹⁰ to 2.5 × 10¹² cm⁻² per well aiming to characterize their electronic properties and structural quality. From a line shape analysis of room temperature DSPV spectra the interband excitonic transition energies and broadening parameters for a large number of QW-related subbands have been established. A study of well-width and quantum number dependencies of the excitonic linewidths allowed us to evaluate the various broadening contributions to the spectral line shapes in QWs of different design. It was found that an average half monolayer well-width fluctuations are the dominant broadening mechanism of the excitonic line for QWs thinner than 10 nm. In QWs thicker than 10 nm, the spectral line broadening originates mainly from thermal broadening as well as Stark broadening due to random electric fields of ionized impurities and exciton scattering by free holes.     

Hydrogenic impurity ground state in quantum well: the envelope function revisited

We present a new variationnal method for calculating the ground state energy of an electron bound to an impurity located in a quantum well. This method relies on an envelope function which is determined exactly from a formal minimization procedure. The obtained energies are lower by as much as 10% than the ones found by the widely used free electron envelope function. Their large width limits are reached with exponentially small corrections as they should. We also find that, except for narrow wells, the shape of these exact envelope functions strongly depends on the impurity position, being consequently quite different from the usual free electron ones. In order to discuss the improvements brought by our new procedure in the most striking way, we have used a model semiconductor quantum well with infinite barrier height and simplified band structure. Extensions can be made to finite barrier and more realistic band structures, following the same technique. Received 11 December 2000     

Electron-phonon effects on Stark shifts of excitons in parabolic quantum wells: Fractional-dimension variational approach

Electron-phonon effects on Stark shifts of excitons in parabolic quantum wells are studied theoretically by using a fractional dimension method in combination with a Lee-Low-Pines-like transformation and a perturbation theory. The numerical results for the exciton binding energies and electron-phonon contributions to the binding energies as functions of the well width and the electric field in the Al_0.3Ga_0.7As parabolic quantum well structure are obtained. It is shown that both exciton binding energy and electron-phonon contributions have a maximum with increasing the well width. The binding energy and electron-phonon contribution decrease significantly with increasing the electric-field strength, in special in the wide-well case.     

Detecting spatially localized excitons in InGaN quantum well structures with a micro-photoluminescence technique

Spatially localized excitons are observed in InGaN quantum well structures at 4 K by using a micro-photoluminescence (PL) technique. By combining PL and nano-lithographic techniques, we are able to detect PL signals with a 0.2 μm spatial resolution. A sharp PL line (linewidth of <0.4 meV) is clearly obtained, which originates from a single localized exciton induced by a quantum dot like a local potential minimum position. Sharp PL spectra detected in three QWs with different indium compositions confirm that there are exciton localization effects in quantum wells in the blue-green (about 2.60 eV, 477 nm) to purple (about 3.05 eV, 406 nm) regions.     

On the propagation characteristics of metal clad waveguides with a quantum well

The electromagnetic modes of planar metal clad dielectric waveguides containing an n-doped quantum well (QW) are studied theoretically. Special attention is paid on the coupling between metal surface plasmons and intersubband plasmons and the manifestation of this coupling in the propagation characteristics of metal/QW/dielectric and multimode metal/QW/dielectric/metal waveguide structures. The results obtained indicate that the modification of the propagation characteristic induced by the above-mentioned coupling is substantial only in the case of metal/QW/dielectric waveguide structures.     

Barrier D centers in a quantum disc

The bound states of the barrier D⁻ center, which consists of a positive ion located on the z-axis at a distance λ from the two-dimensional quantum disc plane with a confined parabolic potential and two electrons in the disc plane bound by the ion, are studied under a perpendicular homogeneous magnetic field. The binding energies of the three lowest bound states are calculated as a function of the applied magnetic field strength γ. Discontinuous ground state transitions induced by an external magnetic field have been obtained. We have investigated the effect of the impurity position and found that the transition of the ground-state occurs for finite λ with increasing γ.