COHERENT MODEL FOR RESONANT-TUNNELING OF ELECTRONS IN DOUBLE-QUANTUM WELLS UNDER ELECTRIC-FIELDS

Taking the inhomogenous broadening of the electron energy levels into account, a coherent model of the resonant tunneling (RT) of electrons in double quantum wells is presented. The validity of the model is confirmed with the experiments [M. Nido et al., Proc. SPIE 1268, 177 (1990)], and shows that the tunneling process can be explained by the simple coherent theory even in the presence of the carrier scattering. We have discussed the dependence of resonant tunneling on the barrier thickness L(B) by introducing the contrast ratio LAMBDA and the full width at half depth of the RT valley, and found that LAMBDA first increases with increasing barrier thickness, reaches a maximum, and then decreases with a further increase of L(B), in striking contrast to the Fabry-Perot model where a monotonic increase of the peak-to-valley ratio is predicted. We attribute the reduction of LAMBDA with large L(B) to the energy broadening resulting from the carrier scattering. A monotonic decrease of the full width at half depth of the RT valley with an increase of L(R) is also found.

ELECTRIC-FIELD-INDUCED EXCITON-LINEWIDTH BROADENING IN SHORT-PERIOD GAAS/GAXAL1-XAS SUPERLATTICES

We have investigated the Wannier-Stark effect in GaAs/GaAl1-xAs superlattices under electric fields by photocurrent spectroscopy measurements in the range of temperatures 10-300 K. The linewidth of the Oh Stark-ladder exciton was found to increase significantly along with an increase in peak intensity when the electric field increases. We present a mechanism based on an enhanced interface roughness scattering of electronic states due to Wannier-Stark localization in order to explain this increased broadening with electric field. This electric-field-related scattering mechanism will weaken the negative differential conductance effects in superlattices predicted by Esaki and Tsu.

WANNIER LOCALIZATION IN GAAS/GAALAS SUPERLATTICES UNDER ELECTRIC-FIELD

We have studied the Wannier-Stark effect in GaAs/GaAlAs short-period superlattices under applied electric field perpendicular to the layers by room- and low-temperature photocurrent measurements. The changes in the transition intensities with biasing are well fitted to a theoretical calculation based on the finite Kronig-Penney model on which the potential of an applied electric field is superposed. With increasing electric field, the 0h peak grows to a maximum while the -1h and +1h peaks monotonousely decrease. By a comparison of the spectra measured at different temperatures, the two peaks in the room temperature photocurrent spectra at relatively low electric field (1.0 X 10(4) V/cm) are identified to be caused by the Wannier localization effect instead of saddle-point excitons.

WANNIER QUANTIZATION OF A SUPERLATTICE SUBBAND UNDER AN ELECTRIC-FIELD

Wavefunctions of electronic Wannier-Stark states in a superlattice are calculated with a finite Kronig-Penney model. Overlap integrals between electron and heavy-hole wavefunctions centred in the same well layer, and in first- and second-neighbour wells are calculated as functions of the applied field. The results show good agreement with experimental results on photoluminescence. The problem is also treated by a one-band approximation method, which gives a closed expression for the wavefunction of the Wannier-Stark states; this is compared with the results of accurate calculations with the Kronig-Penney model.

Current oscillations and stable electric field domains in doped GaAs/AlAs superlattices

The crossover between two regimes has been observed in the vertical electric transport of weakly coupled GaAs/AlAs superlattices (SLs). At fixed d.c. bias, the SLs can be triggered by illumination to switch from a regime of temporal current oscillation to the formation of a stable electric field domain. The conversion can be reversed by raising the sample temperature to about 200 K. An effective carrier injection model is proposed to explain the conversion processes, taking into account the contact resistance originating from DX centres in the n(+)-Al0.5Ga0.5As contact layers which is sensitive to light illumination and temperature. In addition, quasiperiodic oscillations have been observed at a particular d.c. bias voltage.

Suppression of ballistic electron transmission through a semiconductor II-structure by an external transverse electric field

We have conducted numerical studies of ballistic electron transport in a semiconductor II-structure when an external transverse electric field is applied. The device conductance as a function of electron energy and the strength of the transverse electric field is calculated on the basis of tight-binding Green's function formalism. The calculations show that a relatively weak electric field can induce very large decrease in the electron transmission across the structure. When the transverse electric field is sufficiently strong, electrons can hardly be transported through the device. Thus the performance of the device can be greatly improved for it is much easier to control electron transport through the device with an external transverse electric field.

Perpendicular-electric-field dependence of exciton binding energy studied by continuous-wave photoluminescence

The reduction of exciton binding energy induced by a perpendicular electric field in a stepped quantum well is studied. From continuous-wave photoluminescence spectra at 77 K we have observed an obvious blueshift of the exciton peak due to a spatially direct-to-indirect transition of excitons. A simple method is used to calculate the exciton binding energy while the inhomogeneous broadening is taken into account in a simple manner. The calculated result reproduces remarkably well the experimental observation.

An investigation of the formation mechanisms of electric field domains in GaAs/AlAs superlattices

We have studied the vertical transport and formation mechanisms of electric field domains in doped weakly-coupled GaAs/AlAs superlattices. Under hydrostatic pressure two kinds of sequential resonant tunneling are observed within the pressure range from 0 to 4.5 kbar. A transition from Gamma-Gamma to Gamma-X sequential resonant tunneling occurs at P-t approximate to 1.6 kbar. For P < P-t, the high electric field domain is formed by the Gamma-Gamma process, while for P > P-t it is preferentially formed by the Gamma-X process.

Influence of the lateral confining potential on the ballistic electron transport in a quantum wire

A theoretical investigation of ballistic electron transport in a quantum wire with soft wall confinement is presented. A general method of the electron transmission calculation is proposed for structures with complicated geometries. The effects of the lateral guiding potential on ballistic transport are investigated using three soft wall confinement models and the results are compared with those obtained from the hard wall confinement approximation. It is shown that the calculated transmission coefficients are notably dependent on the lateral confining potential especially when the incident electron energy is larger than the energy of the second transverse mode. It is found that the transmission profile obtained from soft wall confinement models exhibits simpler resonance structures than that obtained from the hard wall confinement approximation. Our results suggest that only in the single-channel regime the hard wall confinement approximation can give reasonable results.

HIGH-EFFICIENCY TOP SURFACE-EMITTING LASERS FABRICATED BY 4 IMPLANTATION USING TUNGSTEN WIRE AS MASK

We report the results of a high efficiency room temperature continuous wave (cw) vertical-cavity surface-emitting laser. The structure is obtained by four deep H+ implantation using tungsten wires as the mask. The fabrication process is the simplest ever reported in vertical-cavity surface-emitting laser fabrication. The largest differential quantum efficiency of 65% and maximum cw light output power over 4 mW have been achieved for the 15X15 mu m(2) device. (C) 1995 American Institute of Physics.