Structural and optical properties of submonolayer CdSe/ZnSe superlattices grown with varying thickness of the ZnSe spacer layer are studied. High-resolution electron microscopy images demonstrate that submonolayer CdSe depositions result in two-dimensional nanoscale CdSe islands which are anticorrelated for spacer layer thicknesses exceeding 3 nm, while predominantly vertically correlated growth occurs for thinner spacers, in agreement with most recent theoretical predictions. Vertical ordering of the CdSe islands leads to two lines in photoluminescence ͑PL͒ and optical reflectance spectra originating from excitons localized at vertically coupled and uncoupled CdSe quantum islands, respectively. In edge PL, these lines exhibit different polarizations: predominantly TM and predominantly TE for coupled and uncoupled states, respectively. Stimulated emission in edge geometry and resonant waveguiding effects are observed for both states. The TE and TM components of the stimulated emission of the same state show an energy splitting. At the highest excitation densities we observe saturation of the stimulated emission in the edge geometry, and the development of a peak in surface emission that is strongly increasing with excitation intensity. This peak is attributed to stimulated emission in surface geometry, which is made possible by the ultrahigh material gain in quantum dots and the self-adjustment of the gain spectrum and the cavity mode. ͓S0163-1829͑99͒02024-X͔
I. INTRODUCTIONEMTS based on the GaN/AlGaN materials system are rapidly becoming the semiconductor device of choice for RF and power switching applications. These devices require a semi-insulating buffer to suppress leakage and punch-through. RF devices frequently make use of iron (Fe) doping to render the GaN insulating, but for the higher voltages required for many power switching applications, it has been found that carbon (C) doping delivers higher breakdown voltage and lower off-state leakage [1,2]. Unfortunately it has also been found that using carbon can result in a transitory increase in R ON , also known as current-collapse (CC), when switched from the off to the on-state [2,3]. With field plates now universally used to control surface effects, it is clear that the remaining CC in these devices mostly results from charge storage in deep levels in the buffer. Our previous studies have shown that the difference in CC between Fe and C doping results from their acceptor trap levels pinning the bulk Fermi level in the upper and lower halves of the bandgap respectively [4]. GaN:C is p-type with its low hole density, and hence high resistivity, giving long time constants for charging processes (a hole density of only 10 4 cm -3 was inferred in [5]
We report the investigation of a single quantum dot charge storage device. The device allows selective optical charging of a single dot with electrons, storage of these charges over timescales much longer than microseconds. Reliable readout of the charge occupancy is realized by time gated photoluminescence technique. This device enables us to investigate the tunneling escape of electrons at high electric fields up to several microseconds and, therefore, demonstrates that with more elaborate pulse sequences such structures can be used to investigate charge and spin dynamics in single quantum dots.
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