We report on efficient injection of electron spins into InGaAs-based nanostructures. The spin light-emitting diodes incorporate an InGaAs quantum well or quantum dots, respectively, as well as a semimagnetic ZnMnSe spin-aligner layer. We show a circular polarization degree of up to 35% for the electroluminescence from InGaAs quantum wells and up to 21% for InGaAs quantum dots. We can clearly attribute the polarization of the emitted photons to the spin alignment in the semimagnetic layer by comparison to results from reference devices (where the ZnMnSe is replaced by ZnSe) and from all-optical measurements.
We investigate electrical spin injection from a semi-magnetic n-type ZnMnSe spin aligner into III -V p -i -n diodes with InGaAs quantum dots (QDs) in the active layer. Quantitative transmission electron microscopy techniques are applied to characterize the different dot types used. Analysis of the circular polarization degree (CPD) of the device emission indicates the spin polarization of the injected electrons. Values of more than 70% are obtained for the wetting layer and high-energy QD states. However, the CPD shows a strong spectral dependence due to spin relaxation at the stage, before the electrons are finally captured in the dots. This is, e.g., evidenced by an initial increase of the polarization degree with rising temperature, attributed to motional narrowing effects. As a prerequisite for more detailed studies, we also demonstrate electrical spin injection into single InGaAs QDs, which should provide the basis for future single spin manipulation experiments. Finally, we suggest GaInNAs as optically active material for the realization of spin-polarized light-emitting diodes with long-wavelength emission. First results indicate CPD values of up to 80% for λ = 1130 nm, suggesting this approach to be very promising.
We investigate n-type chlorine-doped ZnMnSe epilayers with various Mn contents and doping concentrations. In ZnSe, the maximum dopability was 6×1019cm−3, which reduces to 1.1×1019cm−3 at 13% Mn content. At a constant ZnCl2 doping source temperature, the doping concentration decreases continuously with increasing Mn content in the sample. From our optical measurements, we found a lower electron effective mass in Zn0.87Mn0.13Se samples compared to ZnSe. Additionally, the incorporation of Mn increases the resistivity and decreases the mobility of the free charge carriers in the samples.
Pyramidal GaAs structures on top of GaAs∕AlAs distributed Bragg reflectors are investigated as candidates for true three-dimensional cavities with potentially low mode volume and high quality-factor. Different types of single and coupled resonators with base lengths of a few microns are realized using a combination of molecular-beam epitaxy, electron-beam lithography, and wet chemical etching. Embedded InGaAs quantum dots are utilized as light sources to verify the resonator modes. Furthermore, a spatially localized emission through the pyramid facets indicates the future possibility of coupling cavity modes to optical fibers. This could be interesting within the context of single photon emitters.
High Ge content SiGe alloys or pure Ge are essential components of future microelectronics (high performance p-channels) and optoelectronics/microelectronics (on-chip and chip to chip communication) integration scenarios. The successful integration with Si based circuits requires process compatibility. Here we report specifically on recent results with high n and p doping of Ge and contact formation with Al and silicide/germanide metals. Highly Sb and B doped epitaxial layers (/cm3) are grown at low temperatures (< 400oC) and complete electrical activation of dopants is proven by SIMS and TLM measurements. Metal contacts on p-Ge are easily fabricated as Ohmic ones, whereas n-Ge contacts are more critical. We explain this by the Fermi level pinning near the valence band, and the strong influence of thin interface layers on the strength of Fermi level pinning.
PACS 75.50.Pp, 78.20.Ls, 78.60.Fi, 78.67.Hc, 85.60.Jb We report on the injection of electron spins into InGaAs quantum dots with an efficiency of up to 60 %. This injection is observed in p-i-n light-emitting diode structures using the diluted magnetic semiconductor ZnMnSe as spin aligner (spin-LED). The degree of spin polarization is deduced from the circular polarization degree of the photons emitted when the injected electrons recombine in the quantum dots with unpolarized holes. We observe a strong energy dependence of the polarization degree with a strong increase starting from zero to a high value on the high energy side of the emission spectrum. To study the origin of this dependence, we compare results of two quantum-dot samples with emission peaks at 1.2 eV and 1.33 eV, respectively.1 Introduction Semiconductor spintronics is a rapidly evolving field [1] where one tries to take advantage of the electron spin state for information processing applications rather than charge. Two important requirements for such applications are the initialization and the storage of single spin states. These have to be achieved in a reliable way to make further manipulation of the single spin state and eventually controlled interaction between spin states possible.Initialization of electrons in single spin states can be achieved optically by using optical orientation [2] or electrically by electrical injection through a spin aligner which puts all electrons into the desired spin state. The latter approach is much more favourable as it makes connection to current-based devices possible. Several methods to achieve spin alignment have been investigated lately, e.g. using the ferromagnetic semiconductor GaMnAs (here one typically injects holes [3] but also electron injection is possible in an Esaki-diode configuration [4]), ferromagnetic metal contacts, Heusler alloys or diluted magnetic semiconductors (DMS).Here, we use the DMS ZnMnSe in the spin-LEDs, which shows a huge splitting between spin-up and spin-down states in the conduction band already at moderate magnetic fields due to its giant Zeeman effect. Spin polarization is nearly perfect at low temperatures and thus this spin aligner is not a critical factor for the resulting spin polarization. However, spin relaxation at interface defects is crucial, i.e. the growth of II-VI semiconductor layers on top of III-V semiconductor heterostructures has to be as perfect as possible.To store the electron spins in an accessible manner, localized systems are needed and it has been shown that InGaAs quantum dots (QDs) are the ideal material system for long-lasting spin storage. Experimentally, a quenching of spin-flip processes has been observed for photo-excited
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