G. C. Aers

Terahertz quantum-cascade lasers based on a three-well active module

The authors report on a design of terahertz quantum-cascade lasers based on three-well active modules. Each module consists of two tunnel-coupled wells for the two lasing states and another well for both resonant-phonon depopulation and carrier injection. This design is the simplest so far among the various published working devices. The test device has a lasing frequency of 3.4THz and maximum operating temperature of 142K.

Coherent control of three-spin states in a triple quantum dot

Manipulating the electrons trapped in quantum-dot pairs is one possible route to quantum computation. Translating this idea to three quantum dots would enable a whole host of extended functionality. Researchers now generate and manipulate coherent superpositions of quantum states using the spins across three electrical-gate-defined dots.

Effect of doping concentration on the performance of terahertz quantum-cascade lasers

We characterized a set of terahertz quantum-cascade lasers with identical device parameters except for the doping concentration. The δ-doping density was varied from 3.2×1010to4.8×1010cm−2. We observed that the threshold current density increased monotonically with doping. Moreover, the measured results on devices with different cavity lengths provided evidence that the free carrier absorption caused waveguide loss also increased monotonically. Interestingly, however, the observed maximum lasing temperature displayed an optimum at a doping density of 3.6×1010cm−2.

Studies and modeling of growth uniformity in molecular beam epitaxy

We have developed a numerical model for the flux distribution in molecular beam epitaxy over stationary and rotating substrates. The existence of a temperature profile along the crucible and accumulation of material at the crucible orifice is taken into account. The influence of melt level tilt on the flux distribution is discussed in detail, and, in contrast to previous reports, is found to be small for the cases considered. Accurate thickness maps of GaAs wafers grown with two different types of effusion cells on stationary substrates were obtained using a scanning reflectance system. Excellent agreement of these experimental results with predictions of the model is demonstrated. We explain some of the deficiencies of the present arrangement and calculate the optimum system conditions for obtaining uniformity across a wafer to better than ±0.3%.

Intersubband Raman Laser

An intersubband Raman laser has been realized in an artificial GaAs/AlGaAs three-level quantum-well structure. A CO2 laser in resonance with the one-to-three level transition is used as the pump, while the lasing emission occurs via the three-to-two level transition. The one-to-two level spacing is designed to be in resonance with the AlAs-like longitudinal optical phonon mode, favoring the Raman process. This work presents an alternative mechanism for realizing intersubband lasers and opens up new possibilities in reaching the far infrared region and achieving room-temperature operation.

Intersubband transitions in InGaNAs/GaAs quantum wells

Intersubband transitions are observed in InGaNAs/GaAs quantum wells at wavelengths around 10 μm. The transition energies are correlated with interband transition energies measured in the near infrared. Clear selection rules are observed: the transition is TM polarized. The amplitude of the absorption is consistent with an increase of the electron effective mass as the N content increases.

Substrate and capping layer effects on the phonon spectrum of ultrathin superlattices

GemSin ultrathin superlattices have been grown on (001) Si substrates and partly capped with Si. Raman scattering from acoustic phonons in the long‐wavelength region shows unexpectedly intense broad peaks that develop into an increased number of sharper intense peaks on capping. We identify these peaks with resonant phonon modes and use a linear chain model to expose the importance of substrate‐superlattice‐capping layer interactions in these multilayer structures.

Simple scaling law for positron stopping in multilayered systems

Using a simple model to take into account the backscattering effects of interfaces we have developed a scheme which removes the necessity for time‐consuming Monte Carlo calculations in the generation of positron stopping profiles in multilayer systems. This scheme uses tabulated mean depth and backscattering fraction data for positrons in the materials constituting the multilayer and represents a computation time saving of several orders of magnitude. This makes detailed multilayer defect profiling with positrons a practical possibility.

Enhanced compositional disordering of quantum wells in GaAs/AlGaAs and InGaAs/GaAs using focused Ga+ ion beams

Spatially selective compositional disordering induced by focused Ga+ ion beam implantation in GaAs/AlGaAs and strained InGaAs/GaAs quantum well structures has been studied using photoluminescence. We find that beyond a certain implantation dosage, the degree of intermixing imparted to a given quantum well saturates and may eventually decline as a result of damage to the semiconductor surface. We overcome this limitation by thermally annealing the sample after implantation to repair the crystalline surface. We show that multiple successive implants interspersed with rapid thermal anneals (RTAs) are successful in locally shifting the optical band gap of quantum wells by many times that attributed to a single implant and RTA.

Thermal quenching mechanism of photoluminescence in 1.55 µm GaInNAsSb/Ga(N)As quantum-well structures

The authors report the temperature dependent photoluminescence characteristics of a series of GaInNAsSb∕Ga(N)As double quantum wells which all emit at 1.5–1.55μm at room temperature and whose design is such that the quantum wells have nominally identical valence band profiles but show different confinement depth in the conduction band. The photoluminescence quenching at high temperature demonstrates a thermal activation energy independent of the conduction band offset and can be most plausibly attributed to the unipolar thermalization of holes from the quantum wells to the barriers. This effect will intrinsically limit the flexibility of heterostructure design using GaInNAs(Sb), as it would for any other material system with small valence band offset.