B. Daniel

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.

Conduction-band electron effective mass in Zn0.87Mn0.13Se measured by terahertz and far-infrared magnetooptic ellipsometry

We determine the electron effective mass parameter m*=0.086±0.004m0 of thin-film n-type low-chlorine-doped Zn0.87Mn0.13Se with free-charge-carrier concentration N=4.5×1017cm−3 and optical mobility μ=300±20cm2∕(Vs) using magneto-optic generalized ellipsometry in the terahertz and far-infrared spectral domain for wave numbers from ω=30–650cm−1. The room-temperature measurements were carried out with magnetic fields up to 3 T. We employ synchrotron and black-body radiation sources for the terahertz and far-infrared spectral regions, respectively. Comparison with previous experimental results from samples with considerably higher free electron density and theoretical calculations suggest that our value is sufficiently unaffected by band nonparabolicity and provides a good approximation of the Γ-point conduction band mass in Zn0.87Mn0.13Se. We further provide optical phonon mode parameters and the high-frequency dielectric constant.

Carrier concentration, mobility, and electron effective mass in chlorine-doped n-type Zn1−xMnxSe epilayers grown by molecular-beam epitaxy

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.

Sphalerite–rock salt phase transition in ZnMnSe heterostructures

We report on the investigation of epitaxial MnSe layers grown on ZnSe by transmission electron microscopy. MnSe∕ZnSe superlattices (SLs) with different nominal MnSe thicknesses tMnSe between 2 and 20 monolayers (MLs) were investigated, which were grown by molecular-beam epitaxy on GaAs(001) substrates. Composition profiles of the SLs were evaluated by the measurement of local (002) lattice parameters in growth direction. A MnSe deposition between 2 and 4MLs on ZnSe leads to the formation of intermixed Zn1−xMnxSe layers with sphalerite structure and a Mn concentration x increasing from 50% to 90%. For MnSe layers with a thickness between 6 and 20ML, we observe 5–10nm small MnSe inclusions with a rock salt structure embedded in sphalerite Zn1−xMnxSe with approximately 90%Mn.

Carrier-density-dependent electron effective mass in Zn1−xMnxSe for 0⩽x⩽0.13

We used room-temperature infrared reflectivity measurements to investigate n-type chlorine-doped Zn1−xMnxSe epilayers (0⩽x⩽0.13). By making Drude-Lorentz-type multioscillator fits to our data, we extracted the optical electron effective mass (m*) in doped Zn(Mn)Se:Cl samples with different Mn content and doping concentrations. Our results indicate that m* in Zn1−xMnxSe is lower than that for ZnSe. In n-type chlorine-doped ZnSe samples with different doping concentrations, m* varied from 0.133m0 to 0.152m0, while in Zn0.87Mn0.13Se:Cl samples, we found a variation from 0.095m0 to 0.115m0 within ±9% experimental accuracy. From theoretical calculations, we estimate that the band-edge electron masses in ZnSe:Cl and Zn0.87Mn0.13Se:Cl should be about 0.132m0 and 0.093m0, respectively.

Defects and phase distribution in epitaxial ZnMnSe layers analyzed by transmission electron microscopy

Our work is concerned with the occurrence and distribution of the sphalerite, wurtzite, and rocksalt phases, which can be present in the ZnMnSe system, and the analysis of structural defects. For this purpose, ZnMnSe layers with thicknesses between 700 and 1000nm and Mn concentrations of 0%, 4%, 11%, 14%, 17%, 29%, 31%, 43%, 50%, 70%, 85%, and 100% were deposited by molecular-beam epitaxy on GaAs (001) substrates. The structure analyses were performed by transmission electron microscopy. A high density of stacking faults exceeding 109cm−2 is already present for a Mn concentration of 14% suggesting that lower Mn concentrations should be used for spin-aligning layers. A significant volume fraction of twinned regions is contained in the Zn0.69Mn0.31Se layer. ZnMnSe layers with Mn concentrations of 43%, 50%, and less than 30% consist exclusively of the sphalerite phase. The sphalerite and a small volume fraction of the wurtzite phase are contained in the sample with 31% of manganese. A mixture of the sphaleri...

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.

Magneto‐optical Studies On Magnetic Semiconductors

From the infrared studies on chlorine‐doped Zn(Mn)Se epilayers, first we show that the electron effective mass in Zn1−xMnxSe is lower than that for ZnSe. We review our results with the discussions of effective mass studies reported on other magnetic semiconductors. Secondly, we show how the magneto‐optical studies are essential for clarifying the electronic structure of diluted magnetic semiconductors (DMSs). We describe the magneto circular dichroism (MCD) technique as an indispensable tool for studying the s, p‐d exchange interaction and magnetooptical properties in DMSs with examples of paramagnetic Cd1−xMnxTe and ferromagnetic (FM) Ga1−xMnxAs epilayers.

Optical and acoustical ridge waveguides based on piezoelectric semiconductors for novel integrated acoustooptic components

The interaction of surface acoustic waves (SAWs) and light is spatially restricted to a region close to the surface approximately given by the acoustical wavelength. Therefore optical waveguides very close to the surface are required for high-frequency i.e. short-wavelength acoustic waves. In contrast to existing collinear integrated acoustooptical devices we are aiming at the regime where the optical and acoustical wavelengths are comparable. The periodically modulated refractive index caused by the SAWs may serve as a tunable and switchable optical add/drop comparable to fiber Bragg gratings, though not static. Another aspect of this regime is the phonon energy, which is non-negligible compared to the energy of the photons. So a significant energy shift i.e. wavelength conversion caused by scattering processes can be exploited. Existing integrated optical waveguides based on silica, SOI, lithiumniobate or III-V semiconductors are not suitable for a realization of such components, due to small piezoelectric coefficients or weak optical confinement. In contrast, heterostructures made of II-VI compounds are promising candidates for the proposed applications. Using Beam Propagation simulations we developed an optimized ridge waveguide structure based on a CdSe/CdS heterostructure, grown by molecular beam epitaxy. The waveguide is defined by wet-chemical etching using a standard photoresist mask. The mode field dimensions are about 1 μm x 2 μm, which requires fiber coupling using lensed fibers. We present measured coupling and propagation losses and discuss the integration with acoustical waveguides.