ABSTRACT
We propose a concept of quantum dot based light emitting diode that produces circularly polarized light without magnetic contacts due to the hyperfine interaction at the crossing of the exciton levels in a weak magnetic field. The electroluminescence circular polarization degree can reach 100%. The concept is compatible with the micropillar cavities, which allows for the generation of single circularly polarized photons. Second order photon correlation function includes information about the nuclear spin dynamics in the quantum dot, and the nuclear spin state can be purified by the quantum measurement backaction.
ABSTRACT
Exciton recombination and spin dynamics in (In,Al)As/AlAs quantum dots (QDs) with indirect band gap and type-I band alignment were studied. The negligible (less than 0.2 µeV) value of the anisotropic exchange interaction in these QDs prevents the mixing of the excitonic basis states and makes the formation of spin-polarized bright excitons possible under quasi-resonant, circularly polarized excitation. The recombination and spin dynamics of excitons are controlled by the hyperfine interaction between the electron and nuclear spins. A QD blockade by dark excitons was observed in the magnetic field, that eliminates the impact of nuclear spin fluctuations. A kinetic model which accounts for the population dynamics of the bright and dark exciton states as well as for the spin dynamics was developed to quantitatively describe the experimental data.
ABSTRACT
New spin-dependent photoemission properties of alkali antimonide semiconductor cathodes are predicted based on the detected optical spin orientation effect and DFT band structure calculations. Using these results, the Na_{2}KSb/Cs_{3}Sb heterostructure is designed as a spin-polarized electron source in combination with the Al_{0.11}Ga_{0.89}As target as a spin detector with spatial resolution. In the Na_{2}KSb/Cs_{3}Sb photocathode, spin-dependent photoemission properties were established through detection of a high degree of photoluminescence polarization and high polarization of the photoemitted electrons. It was found that the multi-alkali photocathode can provide electron beams with emittance very close to the limits imposed by the electron thermal energy. The vacuum tablet-type sources of spin-polarized electrons have been proposed for accelerators, which can exclude the construction of the photocathode growth chambers for photoinjectors.
ABSTRACT
We suggest a new spin orientation mechanism for localized electrons: dynamic electron spin polarization provided by nuclear spin fluctuations. The detrimental effect of nuclear spin fluctuations can be harnessed and employed to provide angular momentum for the electrons via the hyperfine interaction in a weak magnetic field. For this, the sample is illuminated by an unpolarized light, which directly polarizes neither the electrons nor the nuclei. We predict that, for the electrons bound in localized excitons, 100% spin polarization can be reached in longitudinal magnetic fields of a few millitesla. The proof of principle experiment is performed on momentum-indirect excitons in (In,Al)As/AlAs quantum dots, where in a magnetic field of 17 mT the electron spin polarization of 30% is measured.
ABSTRACT
The circularly polarized cathodoluminescence (CL) technique has been used to study the free spin-polarized electron injection in semiconductor heterostructures with quantum wells (QWs). A polarized electron beam was created by the emission of optically oriented electrons from the p-GaAs(Cs,O) negative electron affinity (NEA) photocathode. The prepared beam was injected in a semiconductor QW target, which was activated by cesium and oxygen to reduce the work function. To study the spin-dependent injection, we developed a spin-detector prototype, which consists of a compact proximity focused vacuum tube with the source and target placed parallel to each other on the opposite ends of the vacuum tube (photodiode). The injection of polarized low-energy electrons into the target by varying the kinetic energy in the range of 0.5-5.0 eV and temperature in the range of 90-300 K was studied. The CL was polarized to 2 % by the injection of 20 % spin-polarized electron beam with the energy of 0.5 eV at room temperature. The asymmetry (Sherman function) of spin detection was estimated. It was shown that the dependence of the CL polarization degree on the injected electron energy is satisfactory described by the model that considers the electron spin relaxation in the heterostructure matrix and QWs. The results demonstrate that semiconductor detectors are promising for the spin-polarimetry applications based on the optical detection of free-electron spin polarization.
ABSTRACT
Structures with self-assembled InAs quantum dots (QDs) embedded in an AlAs matrix have been studied by steady-state and transient photoluminescence. It has been shown that in contrast to InAs/GaAs QD systems carriers are mainly captured by quantum dots directly from the AlAs matrix, while transfer of carriers captured by the wetting layer far away from QDs to the QDs is suppressed. At low temperatures the carriers captured by the wetting layer are localized by potential fluctuations at the wetting layer interface, while at high temperatures the carriers are delocalized but captured by nonradiative centers located in the wetting layer.
ABSTRACT
The growth of nanosize islands of iron silicides on Si(100) substrates and epitaxial silicon overgrowth atop them have been studied by low energy electron diffraction and reflectance high energy electron diffraction methods. The near optimal formation conditions of iron silicide islands with high density and minimal sizes have been determined by using of atomic force microscopy. Multilayer (8-10) monolithic structures with buried iron silicide nanocrystallites have been grown after the definition of monocrystalline burying conditions of iron silicides nanocrystallites in silicon lattice. The structure of buried nanocrystallites has been studied in multilayer monolithic heterostructures by high resolution transmission electron microscopy. It was established that in multilayer samples the majority of nanocrystallites have beta-FeSi2 structure, but some of them have gamma-FeSi2 structure. It was observed an influence of additional annealing at 850 degrees C on the morphology and structure of nanocrystallites. By means of deep level transient spectroscopy data one and two trap levels have been observed in multilayer structures (without and with additional annealing, respectively). Photoluminescence spectra have been studied at 4.2 K and the causes of its absence from buried beta-FeSi, NC have been analyzed.
