The phonon bottleneck in self-assembled InAs/GaAs quantum dots ͑SAD's͒ is observed directly in continuous-wave photoluminescence experiments when exciting one GaAs longitudinal optical ͑LO͒-phonon energy above the ground level of the smallest dot. To overcome the phonon bottleneck, selective photoluminescence ͑PL͒ experiments are performed and multiple phonon-assisted radiative bands are observed. We found that no real crystal states are involved in the experimentally observed phonon emission. Under nonresonant excitation at 5 K, the SAD's photoluminescence band is centered at 1.315 eV. As proven by our photoluminescence experiments at high excitation densities, there are no excited states in such small dots. We interpret the phonon-assisted PL as being due to enhanced Fröhlich interaction between strain-induced polarized excitons in the SAD's and LO phonons. Further experimental support for this model is found from the cleaved-side PL measurements. A light-hole ground state is observed, instead of the theoretically predicted heavy-hole one.

We have experimentally determined the band offsets at a highly strained InAs/GaAs interface by means of coupling between two ultrathin InAs layers embedded in a GaAs matrix. When both InAs layers are separated by a 32-ML barrier, the confined electron and light-hole ͑lh͒ states are split into symmetric and antisymmetric states, whereas the heavy-hole ͑hh͒ level is not split yet. Consequently, the splitting between the hh exciton transitions, which is measured by photoluminescence excitation spectroscopy, is solely determined by the conduction-band offset ⌬E c . Knowing ⌬E c , the hh and lh band offsets ⌬E hh and ⌬E lh were subsequently determined from the coupling-induced shift and splitting in samples with 16-, 8-, and 4-ML barriers. We find a conduction-band offset of 535 meV, a conduction-band offset ratio of Q c ϭ0.58, and a strain-induced splitting between the hh and lh levels of 160 meV. This method for the direct determination of band offsets is explicitly sensitive to the band-offset ratio, and its application is not restricted to particular type-I semiconductor heterostructures as long as the effective-mass-band-offset product for the conduction and valence bands differs by at least a factor of 2. ͓S0163-1829͑99͒01715-4͔

We demonstrate the operation of a novel tunable wavelength surface emitting device. The device is based on a p-GaAs and n-Ga 1−x Al x As heterojunction containing an inversion layer on the p-side, and GaAs quantum wells on the n-side, and is referred to as HELLISH-II (hot-electron light emitting and lasing in semiconductor heterojunction). The device utilizes hot-electron longitudinal transport and, therefore, light emission is independent of the polarity of the applied voltage. Because of this symmetric property, the device can perform light logic functions. The wavelength of the emitted light can be tuned with the applied bias from GaAs band-to-band transition in the inversion layer to e1-hh1 transition in the quantum wells. The operation of the device requires only two diffused in point contacts. Therefore, a two-dimensional array of surface emitters can be fabricated very cheaply and easily. Theoretical modelling of the device operation is carried out and compared with the experimental results. An optimized structure for high-efficiency device operation, as based on our model calculations, is also proposed.

No abstract

The carrier capture into ultrathin InAs layers embedded in a GaAs matrix has been investigated by timeresolved two-wavelength pump-probe phototransmission at 4.2 K. Using an InAs thickness of 1.2 monolayers, we observe switching of the carrier relaxation from optical to acoustic phonon emission. At the light-hole ͑lh͒ exciton transition we find a constant capture time of 20 ps. In contrast, the capture time decreases abruptly from 50 ps to 22 ps within the heavy-hole ͑hh͒ exciton transition as the energy separation between lh and hh states exceeds the threshold for GaAs LO phonon emission. The combination of both characteristics provides strong evidence for a two-step capture process of the holes. First the holes are captured by the weakly confined lh state and then they cool down to the hh state. We calculated the transient bleaching of the excitonic absorption considering both phase-space filling and exciton screening. The calculations show in agreement with the measurements that the phototransmission transients directly reflect the population of the confined InAs states only at excitation densities below 3ϫ10 8 cm Ϫ2 . At larger excitation densities, the phototransmission rise time becomes significantly smaller than the capture times whereas its decay time appears longer than the carrier lifetime.