Binding energy and density of shallow impurity states in GaAs–(Ga, Al)As quantum wells : effects of an applied hydrostatic stress

ABSTRACT: The effects of hydrostatic stress on the binding energy and the density of shallow-donor and shallow-acceptor impurity states in a GaAs–(Ga, Al)As quantum well are calculated using a variational procedure within the effective-mass approximation. Results are for different well widths and hydrostatic stresses, as a function of the impurity position along the growth direction of the structure. We have found that in the low-pressure regime the binding energy changes linearly for both donor and acceptor impurities, independently of the sizes of the well. However, for high pressures (greater than 13.5 kbar) this is valid for acceptors but not for donors due to the -X crossover. We have shown that there are two special structures in the density of impurity states, one associated with on-centre and the other with on-edge impurities. Also, we have observed that the density of impurity states depends strongly on the applied hydrostatic stress

Binding energy of the ground and first few excited states of a shallow-donor impurity in rectangular-cross-sectional area GaAs quantum-well wires under applied electric field

ABSTRACT: Using a variational approach within the effective mass approximation we calculate the binding energy of the ground and some excited donor impurity states in quantum-well wires with rectangular and cylindrical transversal sections under the action of applied electric fields. We study the binding energy as a function of the geometry of the system, the applied electric field as well as the impurity position inside the structure. We found that the presence of the electric field breaks down the degeneracy of states for impurities symmetrically positioned within the structure, and that the geometric confinement and the electric field are determinant for the existence of bound excited states in these structures

Estudio de cristales fotónicos basados en ferrofluidos con nanopartículas magnéticas de ferritas de cobalto-zinc

Resultados experimentales previos muestran diversos fen´omenos como la formaci´on de patrones estructurados cuando el ferrofluido se encuentra contenido en una pel´ıcula delgada, bajo la acci´on de un campo magn´etico externo aplicado; si el campo magn´etico es paralelo al plano de la pel´ıcula, una porci´on de las part´ıculas magn´eticas en el fluido se aglomeran para formar cadenas 1D, pero si es perpendicular se forman columnas cil´ındricas de conglomerados de nanopart´ıculas cuyo comportamiento puede ser visto como un cristal fot´onico 2D. En este trabajo se presenta el estudio de cristales fot´onicos basados en ferrofluidos con nanopart´ıculas de ferritas de cobaltozinc (Co0,8Zn0,2Fe2O4), material que no ha sido reportado hasta el momento en aplicaciones fot´onicas y que bajo campo magn´etico se ordena formando cristales fot´onicos. Esto hace que estos cristales sean de gran inter´es en el desarrollo de nuevas tecnolog´ıas. En la primera parte de este trabajo, usando la t´ecnica de la matriz transferencia, se estudian los efectos de la concentraci´on de Zn, el tama˜no y la forma de las nanopart´ıculas sobre la estructura de bandas de un cristal fot´onico 1D hecho de capas alternadas de aire y ferrofluido. Para el ferrofluido, nosotros tomamos nopart´ıculas de ferritas de cobalto-zinc (Co0,8Zn0,2Fe2O4), recubiertas con ´acido oleico e inmersas en etanol. Adicionalmente, usando los resultados de los efectos de la concentraci´on de Zn sobre la PBS, se proponen photonic quantum wells (QWs) que dependiendo de la geometria y de los par´ametros dielectricos, pueden presentar modificaciones notables en los espectros de transmisi´on. En la segunda parte de este trabajo, se utiliza el m´etodo de expansi´on de ondas planas para estudiar la estructura de bandas de un cristal fot´onico 2D constituido por rods de nanopart´ıculas de las ferritas utilizadas en el caso 1D a diferentes campos magn´eticos aplicados, que depende de la permitividad I efectiva y la raz´on de ´area de la fase l´ıquida

Photonic band structure evolution of a honeycomb lattice in the presence of an external magnetic field

ABSTRACT: A standard plane-wave expansion technique is used to investigate the evolution of the photonic band structure of a two-dimensional honeycomb lattice composed by cylindrical shell rods with dielectric permittivities 1 and 2, and embedded in a background with permittivity 3. We have considered the effect of dispersive dielectric responses as well as the influence of an externally applied magnetic field aiming to obtain efficient tunable bandgaps. Present results suggest that a combination of a doped semiconductor constituent with an anisotropic geometry, which breaks symmetry and unfolds degeneracies, provides an efficient realization of photonic systems with tunable bandgaps

Tilted electric-field and hydrostatic pressure effects on donor impurity states in cylindrical GaAs quantum disks

ABSTARCT: In this work we are devoted to the study of donor impurity states in a cylindrical GaAs quantum disk under the effects of tilted applied electric field and hydrostatic pressure. A variational procedure has been performed within the effective mass and parabolic-band approximations. For the hydrostatic pressure effects we consider the Γ-X mixing via a phenomenological procedure

Absorción óptica relativa a impurezas donadoras en pozos cuánticos de GaAs-(Ga,Al)As bajo efectos de presión hidrostática

RESUMEN: En este trabajo se presentan resultados teóricos sobre los efectos de presión hidrostática en el espectro de absorción óptica relativo a impurezas donadoras en pozos cuánticos de GaAs-(Ga,Al)As. Hemos calculado el espectro de absorción óptica relativo a impurezas donadoras considerando diferentes anchos del pozo cuántico y valores de la presión hidrostática. Nuestros resultados muestran dos estructuras bien definidas en el espectro de absorción: una de ellas asociadas con impurezas localizadas cerca al centro del pozo y la otra para impurezas cerca de las paredes de la estructura. Adicionalmente, asociado con la variación con la presión del gap (brecha) en el GaAs, se muestra un importante corrimiento al azul del espectro de absorción

Plasmon polaritons in 1D Cantor-like fractal photonic superlattices containing a left-handed material

ABSTRACT: The propagation of light incident upon a 1D photonic superlattice consisting of successive stacking of alternate layers of a right-handed nondispersive material and a metamaterial, arranged to form a Cantor-like fractal, is considered. Plasmon-polariton excitations are thoroughly investigated within the transfer-matrix approach and shown to strongly depend on the Cantor step number N. More specifically, the number of plasmon-polariton bands corresponds to the number 2N −1 of metamaterial layers within the unit cell

Stress effects on shallow-donor impurity states in symmetrical GaAs/AlxGa1_xAs double quantum wells

ABSTRACT: The effects of the compressive stress on the binding energy and the density of shallow-donor impurity states in symmetrical GaAs/AlxGa12xAs double quantum wells are calculated using a variational procedure within the effective-mass approximation. Results are for different well and barrier widths, shallow-donor impurity position, and compressive stress along the growth direction of the structure. We have found that independently of the well and barrier widths, for stress values up to 13.5 kbar ~in the direct-gap regime! the binding energy increases linearly with the stress. For stress values greater than 13.5 kbar ~indirect gap regime! and for impurities at the center of the wells, the binding energy increases up to a maximum and then decreases. For all impurity positions the binding energy shows a nonlinear behavior in the indirect gap regime due to the G-X crossing effect. The density of impurity states is calculated for a homogeneous distribution of donor impurities within the barriers and the wells of the low-dimensional heterostructures. We have found that there are three special structures in the density of impurity states: one associated with on-center-barrier-, the second one associated with on-center-well-, and the third one corresponding to on-external-edge-well-impurity positions. The three structures in the density of impurity states must be observed in valence–to–donor-related absorption and conduction–to–donor-related photoluminescence spectra, and consequently these peaks can be tuned at specific energies and convert the system in a stress detector

Temperature dependence of band gap ratio and Q-factor defect mode in a semiconductor quaternary alloy hexagonal photonic-crystal hole slab

We present numerical predictions for the photonic TE-like band gap ratio and the quality factors of symmetric localized defect as a function of the thickness slab and temperature by the use of plane wave expansion and the finite-difference time-domain methods. The photonic-crystal hole slab is composed of a 2D hexagonal array with identical air holes and a circular cross section, embedded in a non-dispersive III–V semiconductor quaternary alloy slab, which has a high value of dielectric function in the near-infrared region, and the symmetric defect is formed by increasing the radius of a single hole in the 2D hexagonal lattice. We show that the band gap ratio depends linearly on the temperature in the range 150–400 K. Our results show a strong temperature dependence of the quality factor Q, the maximum ( Q=7000 ) is reached at T=350K, but if the temperature continues to increase, the efficiency drops sharply. Furthermore, we present numerical predictions for the electromagnetic field distribution at T=350