Spin dynamics of ZnSe-ZnTe nanostructures grown by migration enhanced molecular beam epitaxy

We study the spin dynamics of ZnSe layers with embedded type-II ZnTe quantum dots using time resolved Kerr rotation (TRKR). Three samples were grown with an increasing amount of Te, which correlates with increased quantum dot (QD) density. Samples with a higher quantum dot density exhibit longer electron spin lifetimes, up to 1 ns at low temperatures. Tellurium isoelectronic centers, which form in the ZnSe spacer regions as a result of the growth conditions, were probed via spectrally dependent TRKR. Temperature dependent TRKR results show that samples with high QD density are not affected by an electron-hole exchange dephasing mechanism

Decoherence in semiconductor nanostructures with type-II band alignment: All-optical measurements using Aharonov-Bohm excitons

We examine the temperature dependence of the visibility of the excitonic Aharonov-Bohm peak in type-II quantum dots. We obtain a functional temperature dependence that is similar to that determined by transport experiments, namely, with the T − 1 term due to electron-electron collisions and the T − 3 term due to electron-phonon interactions. However, the magnitude of the latter term is much smaller than that for the transport electrons and similar to the interaction strength of the exciton-phonon coupling. Such suppressed electron-phonon interaction ushers a way for all-optical studies of decoherence processes in semiconductor nanostructures as other dephasing mechanisms become more pronounced

Long spin-flip time and large Zeeman splitting of holes in type-II ZnTe/ZnSe submonolayer quantum dots

The Zeeman splitting and degree of circular polarization (DCP) of photoluminescence (PL) from type-II submonolayer ZnTe/ZnSe quantum dots (QDs) have been investigated in magnetic fields up to 18 T. To explain the observed relative intensities and energy positions of the σ+and the σ-PL, a non-Boltzmann distribution for holes with ultra-long spin-flip time, confined to submonolayer QDs, is proposed. The g-factor of electrons, located in the ZnSe barriers, was obtained from fitting the temperature dependence of the DCP, and its value is in excellent agreement with that of bulk ZnSe. The g-factor of type-II excitons was extracted by analyzing the Zeeman splitting, from which the g-factor of holes confined within submonolayer ZnTe QDs was found to be ∼2.65 ± 0.40. This value is considerably larger than that in bulk ZnTe. Tight-binding calculations using an sp3s∗ model were employed to understand the origin of such an increase. The results of the simulation match the experiment and show that the enhancement of the hole g-factor is mostly caused by a reduced orbital contribution to Zeeman splitting arising from the submonolayer thickness of these QDs

Optical anisotropy in type-II ZnTe/ZnSe submonolayer quantum dots

Linearly polarized photoluminescence is observed for type-II ZnTe/ZnSe submonolayer quantum dots (QDs). The comparison of spectral dependence of the degree of linear polarization (DLP) among four samples indicates that the optical anisotropy is mostly related to the elongation of ZnTe QDs. Numerical calculations based on the occupation probabilities of holes in px and py orbitals are performed to estimate the lateral aspect ratio of the QDs, and it is shown that it varies between 1.1 and 1.4. The value of anisotropic exchange splitting for bright excitonic states is found to be ∼200 μeV from the measurement of the degree of circular polarization as a function of the magnetic field. The results also show that heavy-light hole mixing ratio is about 0.16

Increased Short-Term Variability of the QT Interval in Professional Soccer Players: Possible Implications for Arrhythmia Prediction

BACKGROUND: Sudden cardiac death in competitive athletes is rare but it is significantly more frequent than in the normal population. The exact cause is seldom established and is mostly attributed to ventricular fibrillation. Myocardial hypertrophy and slow heart rate, both characteristic changes in top athletes in response to physical conditioning, could be associated with increased propensity for ventricular arrhythmias. We investigated conventional ECG parameters and temporal short-term beat-to-beat variability of repolarization (STV(QT)), a presumptive novel parameter for arrhythmia prediction, in professional soccer players. METHODS: Five-minute 12-lead electrocardiograms were recorded from professional soccer players (n = 76, all males, age 22.0±0.61 years) and age-matched healthy volunteers who do not participate in competitive sports (n = 76, all males, age 22.0±0.54 years). The ECGs were digitized and evaluated off-line. The temporal instability of beat-to-beat heart rate and repolarization were characterized by the calculation of short-term variability of the RR and QT intervals. RESULTS: Heart rate was significantly lower in professional soccer players at rest (61±1.2 vs. 72±1.5/min in controls). The QT interval was prolonged in players at rest (419±3.1 vs. 390±3.6 in controls, p<0.001). QTc was significantly longer in players compared to controls calculated with Fridericia and Hodges correction formulas. Importantly, STV(QT) was significantly higher in players both at rest and immediately after the game compared to controls (4.8±0.14 and 4.3±0.14 vs. 3.5±0.10 ms, both p<0.001, respectively). CONCLUSIONS: STV(QT) is significantly higher in professional soccer players compared to age-matched controls, however, further studies are needed to relate this finding to increased arrhythmia propensity in this population

AS CAPTURE AND THE GROWTH OF ULTRATHIN INAS LAYERS ON INP

Capture of As by (001) InP surfaces exposed to As fluxes under chemical beam epitaxy conditions is investigated by virtual-interface analysis of real-time kinetic ellipsometric data. Intentional growth of ultrathin InAs layers is readily followed. Arsenic accumulated in the absence of growth can be completely removed by exposure to P, showing that As-P exchange occurs only in the outermost layer.64243279328

Interface modification in type-II ZnCdSe/Zn(Cd)Te QDs for high efficiency intermediate band solar cells

A new growth process for type-II ZnCdSe/ZnCdTe quantum dots (QDs) is developed to avoid formation of a deleterious strain-inducing ZnSe interfacial layer (IL) that forms during the migration enhanced epitaxy growth process used to form the QDs. This new growth sequence allows for improved control of the interfacial composition and simplifies the fabrication of the intermediate band solar cell device structure based on these QDs, since additional strain balancing schemes are no longer required to grow stress-free structures. In contrast to previous results, lattice-matched QD superlattices were obtained using near lattice-matched ZnCdSe barrier layers. X-ray diffraction and excitation intensity dependent photoluminescence studies were used to support such a conclusion. Our findings have applications for the growth of other heterointerfaces in which an undesirable IL may form due to lack of a common anion, and to desorption and incorporation of competing elements

II–VI quantum cascade emitters in the 6–8 μm range

We present the growth and characterization of ZnCdSe/ZnCdMgSe quantum cascade (QC) heterostructures grown by molecular beam epitaxy (MBE) and designed to operate at 6–8 μm. These structures utilize the better-understood ZnCdMgSe with InP lattice matched compositions yielding a bandgap of 2.80 eV as compared to previous work which used ZnCdMgSe compositions with bandgaps at 3.00 eV. Grown structures posses good structural and optical properties evidenced in X-ray diffraction and photoluminescence studies. Fabricated mesa devices show temperature dependent I–V measurements with differential resistance of 3.6 Ω, and a turn on voltage of 11 V consistent with design specifications. Electroluminescence was observed in these devices up to room temperature with emission centered at 7.1 μm and line-widths of ∼16% (ΔE/E) at 80 K. The results show that these are well-behaved electroluminescent structures. Addition of waveguide layers and further improvements in well barrier interfaces are being pursued in efforts to demonstrate lasing