W. Löffler

Molecular beam epitaxy of phase pure cubic InN

Cubic InN layers were grown by plasma assisted molecular beam epitaxy on 3C-SiC (001) substrates at growth temperatures from 419to490°C. X-ray diffraction investigations show that the layers have zinc blende structure with only a small fraction of wurtzite phase inclusions on the (111) facets of the cubic layer. The full width at half maximum of the c-InN (002) x-ray rocking curve is less than 50arcmin. The lattice constant is 5.01±0.01A. Low temperature photoluminescence measurements yield a c-InN band gap of 0.61eV. At room temperature the band gap is about 0.56eV and the free electron concentration is about n∼1.7×1019cm−3.

Electrical spin injection from ZnMnSe into InGaAs quantum wells and quantum dots

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.

Optical modes in pyramidal GaAs microcavities

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.

Localized and delocalized modes in coupled optical micropillar cavities.

Optical micropillar Bragg cavities of different diameters and coupled by a small bridge have been realized experimentally by means of a focused ion beam system. The resonator modes in these coupled microcavities are either localized in one pillar or delocalized over the whole photonic structure, a fact that could be exploited to control the coupling between two spatially separated quantum dots, i.e. placed in different pillars, via the enhanced electromagnetic field in such a coupled microcavity. A simplified two dimensional simulation has been used to predict the resonant wavelengths and design the optical modes in these coupled Bragg cavities.

Parallel preparation of highly spin-polarized electrons in single InAs∕GaAs quantum dots

Initialization of electron spins in semiconductor quantum dots (QDs) is a major prerequisite for a successful implementation of such QDs in quantum information applications. It is essential that the initialization is achieved for many individually separable dots in parallel. Here the authors show that exactly this can be accomplished with near-unity fidelity by electrical spin injection from the diluted magnetic semiconductor ZnMnSe into InAs∕GaAs quantum dots. The deviation from unity is smaller than 0.13, more precise determination is limited by the signal-to-noise ratio of their setup. They demonstrate the robust concurrent initialization of several quantum dots with the same high fidelity.

Doping and optimal electron spin polarization in n-ZnMnSe for quantum-dot spin-injection light-emitting diodes

Utilizing the diluted magnetic semiconductor ZnMnSe for electron spin alignment near-perfect spin state preparation in semiconductor quantum dots has been demonstrated. We show that the electron spin polarization depends strongly on the electron concentration in ZnMnSe:Cl. Using a model which takes accurately the Zeeman sublevel occupation into account, we can explain well the experimentally observed results. We find that the electron concentration must be below the effective density of states to obtain full electron spin polarization and best device operation. This is especially important in II-VI spin-aligner materials with a low density of states.

Electrical Spin Injection into Single InGaAs Quantum Dots

In the context of a potential future quantum information processing we investigate the concurrent initialization of electronic spin states in InGaAs quantum dots (QDs) via electrical injection∈dex{spin!injection} from ZnMn(S)Se spin aligners. Single dots can be read out optically through metallic apertures on top of our spin-injection light-emitting diodes (spin-LEDs). A reproducible spin polarization degree close to 100% is observed for a subset of the QD ensemble. However, the average polarization degree is lower and drops with increasing QD emission wavelength. Our measurements suggest that ∈dex{spin!relaxation}spin relaxation processes outside the QDs, related to the energetic position of the electron quasi-Fermi level, as well as defect-related spin scattering at the III–V/II–VI interface should be responsible for this effect, leading us to an improved device design. Finally, we present first time-resolved ∈dex{electroluminescence measurement}electroluminescence measurements of the polarization dynamics using ns-pulsed electrical excitation. The latter should not only enable us to gain a more detailed understanding of the spin and carrier relaxation processes in our devices. They are also the first step towards future time-resolved optical and electrical spin manipulation experiments.

Spin-polarization dynamics in InGaAs quantum dots during pulsed electrical spin-injection

We investigate the fidelity of electron spin initialization in quantum dots utilizing nanosecond-pulsed electrical spin injection through a semimagnetic spin aligner in a spin light-emitting diode. At the onset of the electroluminescence signal, the circular polarization degree of the emitted light, corresponding to the spin polarization degree, is distinctively higher than under constant-current excitation. The observed spin-polarization dynamics are attributed to state filling effects. Additional contributions due to spin-flip mechanisms within the optically active region are identified.

Spin and carrier relaxation dynamics in InAs/GaAs quantum-dot spin-LEDs

We investigate the dynamics of electrons injected into InAs/GaAs quantum dots by initializing and further observing the spin state of the electrons. For this purpose, we use spin polarized light-emitting diodes where the electron spin is set in a semimagnetic ZnMnSe layer. We find that the degree of optical polarization depends strongly on the ground state energy of the quantum dot. A dependence of polarization on dopant concentration in the spin aligner suggests an influence of residual electrons in the quantum dots.

Electrical Spin Injection into InGaAs Quantum Dot Ensembles and Single Quantum Dots

Electrical spin injection from an n‐type ZnMnSe spin aligner into III‐V p‐i‐n diode structures with InGaAs quantum dots (QDs) in the active layer is investigated. Analysis of the circular polarization degree (CPD) of the device emission indicates the spin polarization of the injected electrons. Values > 70% are obtained for the electroluminescence (EL) from the wetting layer and QDs with high ground‐state energy. Towards the low‐energy end of the emission spectrum, the CPD drops strongly. Temperature‐dependent measurements suggest, that this is due to spin relaxation taking place at a stage, when the electrons are not yet finally captured in the dots, i.e. in the GaAs spacer or the wetting layer. Furthermore, we demonstrate electrical spin injection into single InGaAs QDs, a prerequisite for future single spin manipulation experiments within the context of quantum information processing.