H. Cruz

TWO-DIMENSIONAL TIGHT-BINDING MODEL OF AC CONDUCTIVITY IN POROUS SILICON

A time-dependent tight-binding model has been developed to study the ac conductivity in porous silicon. Assuming that carriers are allowed to hop between isolated pairs of Si nanocrystals embedded in a SiO2 matrix, the tunneling times have been calculated according to different geometries. The geometrical structure of porous silicon has been modeled with simple percolationlike clusters. By using the tunneling times, the ac conductivity behavior in the high-frequency regime has been calculated in the pair approximation. The conductivity increases with the frequency according to a power law with an exponent lower than unity. It is found that there is a strong dependence of the ac conductivity on the thickness of the SiO2 interconnecting layer.

Resonant tunnelling of electrons through parabolic quantum wells: an analytical calculation of the transmission coefficient

The transmission coefficient for tunnelling through a GaAs/GaAlAs parabolic quantum well has been calculated using a transfer matrix model. In this work the one-dimensional effective-mass equation has been first solved analytically by means of confluent hypergeometric functions and airy functions as envelope functions. It is found that barrier width affects significantly the quantum tunnelling current through a parabolic quantum well.

Dynamics of a two-dimensional electron-hole system in a coupled quantum well

In this work, we have numerically integrated in space and time the effective-mass Schrodinger equation for a two-dimensional electron-hole system in a coupled quantum well system. Considering a time-dependent Hartree potential and an external electric field, we derive the nonlinear dynamical evolution of the carrier wave function. It is found that charge dynamically trapped in both wells produces a reaction field that modifies the system resonant condition. At different electron-hole sheet densities we show the possibility of multiple resonant tunneling peaks in a bilayer system.

Coherent and escape tunneling processes in asymmetric coupled quantum wells

The dynamics of electrons in a Ga1−xAlxAs‐GaAs asymmetric double quantum well heterostructure under an external applied electric field is studied. In particular, we examine the coupling between two different tunneling processes: coherent tunneling between the two quantum wells and escape tunneling to the GaAs bulk. Simple hydrodynamical models are proposed to account for the observed features. The effect of a longitudinal magnetic field on both tunneling processes is also analyzed. In this coupled quantum well system, the possibility exists of a direct observation of an oscillating luminescence signal with a quantum tunneling period modulated by means of an applied longitudinal magnetic field.

Wave-packet oscillations in a strongly driven InAs quantum well

In this work, we have numerically integrated in space and time the effective-mass nonlinear Schrodinger equation for an electron wave packet in a strongly driven InAs quantum well. Considering a time-dependent Hartree potential, we have calculated the tunneling dynamics between the two quantum well barriers when an external ac bias is applied. At certain ac oscillation periods, we have obtained a time-varying wave-function penetration in the barriers with an amplitude which is also oscillating with time. In this way, the possible time-dependent effects of electronic spins in a strongly driven InAs quantum well could be also investigated.

Possibility of multiple tunnelling current peaks in a coupled quantum well system

In this work, we have numerically integrated in space and time the effective-mass nonlinear Schrodinger equation for an electron wave packet in a bilayer electron system. Considering both Hartree and exchange-correlation potentials, we have calculated the tunnelling rates between the two quantum wells when an external bias is applied in the double quantum well system. Due to the nonlinear effective-mass equation, it is found that the charge dynamically trapped in both wells produces a reaction field which modifies the system resonant condition. At different electronic sheet densities, we have shown the possibility of having multiple resonant tunnelling peaks in a bilayer electron system.

Stark effect and excitonic tunneling escape process in semiconductor quantum wells

In this work, we have numerically integrated in space and time the effective mass Schrodinger equation for an exciton in a semiconductor quantum‐well structure. Considering a Coulomb interaction between the electron‐hole pair and an external electric field, we have studied the excitonic tunneling escape process from semiconductor quantum wells. Our method of calculation has been applied to types‐I, ‐II, and ‐III quantum‐well superlattices. In addition, we present the calculated excitonic lifetimes for the GaAs/GaAlAs, InAs/GaSb, and HgTe/HgCdTe systems under an external electric field. In the HgTe/CdTe system, the possibility of having similar electron and hole lifetime values is also found if the applied electric field is large enough.

Possibility of spin device in a triple quantum well system

We have numerically integrated in space and time the effective mass nonlinear Schrodinger equation for an electron wave packet in an InAs triple quantum well system. Considering the local spin-density approximation, we have calculated the tunneling dynamics in the triple quantum well system when an external bias is applied in the center quantum well. In such a device, the injected electronic current that is initially unpolarized could be divided into two spin-up and spin-down polarized currents at the same time and voltage obtaining a double efficiency.

Nonlinear dynamics of excitons in asymmetric coupled quantum wells

In this work, we have numerically integrated in space and time the effective-mass Schrodinger equation for an excitonic wave packet in a coupled quantum well system. Considering an electronic many-body interaction and an external electric field, we have obtained a nonlinear dynamical evolution for the excitonic wave function. At large electronic sheet densities, it is found that the charge dynamically trapped in both wells produces a reaction field which modifies the system resonant condition.

Characteristic times for resonant tunneling through double barrier heterostructures

Abstract Time dependent resonant tunneling of electron wave packets scattered through symmetric and asymmetric double barrier heterostructures under an external electric field is studied by systematically varying the barrier lengths. The different times involved, the charging time, the dwell time, the life time and passage time by transmission, are examined and grouped according to their qualitative behaviour against the barrier widths. It is found that the life time and dwell time are not always proportional.