E. E. Mendez

Shot noise enhancement in resonant-tunneling structures in a magnetic field

We have observed that the shot noise of the tunnel current, {ital I}, in a GaSb-AlSb-InAs-AlSb-GaSb double-barrier structure under a magnetic field can exceed 2qI. The measurements were done at T=4 K in fields up to 5 T parallel to the current. The noise enhancement occurred at each of the several negative-differential conductance regions induced by the tunneling of holes through Landau levels in the InAs quantum well. The amount of the enhancement increased with the strength of the negative conductance and reached values up to 8qI. These results are explained qualitatively by fluctuations of the density of states in the well, but point out the need for a detailed theory of shot noise enhancement in resonant-tunneling devices. {copyright} {ital 1998} {ital The American Physical Society}

Raman spectroscopy of wurtzite InN films grown on Si

Abstract We have used Raman spectroscopy to study indium nitride thin films grown by molecular beam epitaxy on (111) silicon substrates at temperatures between 450 and 550°C. The Raman spectra show well defined peaks at 443, 475, 491 and 591xa0cm −1 , which correspond to the A 1 (TO), E 1 (TO), E 2 high , and A 1 (LO) phonons of the wurtzite structure, respectively. In backscattering normal to the surface the A 1 (TO) and E 1 (TO) peaks are very weak, indicating that the films grow along the hexagonal c -axis. For a growth temperature of 500°C, the E 2 high peak has a full width at half maximum of 7xa0cm −1 , which is comparable to the value reported for wurtzite InN films grown on sapphire.

Drastic Reduction of Shot Noise in Semiconductor Superlattices

We have found experimentally that the shot noise of the tunneling current I through an undoped semiconductor superlattice is reduced with respect to the Poissonian noise value 2eI, and that the noise approaches 1/3 of that value in superlattices whose quantum wells are strongly coupled. On the other hand, when the coupling is weak or when a strong electric field is applied to the superlattice, the noise becomes Poissonian. Although our results are qualitatively consistent with existing theories for one-dimensional multibarrier structures, the theories cannot account for the dependence of the noise on superlattice parameters that we have observed.

Shot noise in negative-differential-conductance devices

We have compared the shot-noise properties at T=4.2u2009K of a double-barrier resonant-tunneling diode and a superlattice tunnel diode, both of which exhibit negative differential-conductance (NDC) in their current–voltage characteristics. While the noise spectral density of the former device was greatly enhanced over the Poissonian value of 2eI in the NDC region, that of the latter device remained 2eI. This result implies that charge accumulation, not system instability, is responsible for shot-noise enhancement in NDC devices.

Spin dynamics of cavity polaritons

Abstract We have studied polariton spin dynamics in a GaAs/AlGaAs microcavity by means of polarization- and time-resolved photoluminescence spectroscopy as a function of excitation density and normal mode splitting. The experiments reveal a novel behavior of the degree of polarization of the emission, namely the existence of a finite delay to reach its maximum value. We have also found that the stimulated emission of the lower polariton branch has a strong influence on spin dynamics: in an interval of ∼150xa0ps the polarization changes from +100% to negative values as high as −60%. This strong modulation of the polarization and its high speed may open new possibilities for spin-based devices.

Shot-noise characteristics of triple-barrier resonant-tunneling diodes

We have found experimentally that the shot noise in InAlAs-InGaAs-InAlAs Triple-Barrier Resonant-Tunneling Diodes (TBRTD) is reduced over the 2eI Poissonian value whenever their differential conductance is positive, and is enhanced over 2eI when the differential conductance is negative. This behavior, although qualitatively similar to that found in double-barrier diodes, differs from it in important details. In TBRTDs the noise reduction is considerably larger than predicted by a semi-classical model, and the enhancement does not correlate with the strength of the negative differential conductance. These results suggest an incomplete understanding of the noise properties of multiple-barrier heterostructures.

Optical gain of type II inter-sub-band quantum cascade lasers

Abstract We have calculated the optical gain of inter-sub-band quantum cascade lasers based on type II heterostructures such as InAs–GaSb–AlSb, using a model that includes the whole band diagram of the structure as well as the dynamics of all the processes involved. A comparison with similar calculations for conventional type I quantum cascade lasers based on the InGaAs–InAlAs heterostructure shows the theoretical superiority of type II lasers, with an optical gain more than 50% higher than that of type I lasers and a much more efficient energy conversion.

Enhancement of Rabi splitting in a microcavity with an embedded superlattice

We have observed a large coupling between the excitonic and photonic modes of an AlAs/AlGaAs microcavity filled with an 84-({rm {AA}})/20({rm {AA}}) GaAs/AlGaAs superlattice. Reflectivity measurements on the coupled cavity-superlattice system in the presence of a moderate electric field yielded a Rabi splitting of 9.5 meV at T = 238 K. This splitting is almost 50% larger than that found in comparable microcavities with quantum wells placed at the antinodes only. We explain the enhancement by the larger density of optical absorbers in the superlattice, combined with the quasi-two-dimensional binding energy of field-localized excitons.

Electric-Field Tuning of Spin-Dependent Exciton-Exciton Interactions in Coupled Quantum Wells

We have shown experimentally that an electric field decreases the energy separation between the two components of a dense spin-polarized exciton gas in a coupled double quantum well, from a maximum splitting of 4 meV to zero, at a field of 35 kVcm. This decrease, due to the fieldinduced deformation of the exciton wave function, is explained by an existing calculation of the change in the spin-dependent exciton-exciton interaction with the electron-hole separation. However, a new theory that considers the modification of screening with that separation is needed to account for the observed dependence on excitation power of the individual energies of the two exciton components.

Control by an electric field of electron–hole separation in type-II heterostructures

Abstract We report the realization of multi-period devices in which each period consists of two-dimensional electron and hole layers whose effective distance is controlled by an external bias. The current–voltage characteristics of devices based on InAs/AlxGa1−xAsSb/Al0.1Ga0.9AsSb type-II quantum well structures show a behavior that is consistent with a predicted enhancement of the radiative recombination in this type of devices at T=77 K . We have found that increasing the number of periods and decreasing the operating temperature considerably improves the electrical performance of the devices.