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We review on time and spatially resolved two-color pump–probe magneto-optical Kerr effect (MOKE) microscopy studies of electron spins in bulk n-GaAs and GaAs (110) quantum wells (QWs) at low lattice temperatures. The influence of photocarrier heating by above-bandgap optical spin excitation on the spatially resolved magneto-optical spin detection is considered in detail. We demonstrate that a continuous-wave (cw) measurement of the local Kerr rotation at a fixed arbitrary probe wavelength in general does not correctly reveal the local spin polarization when hot electrons are present. For bulk GaAs we determine the true lateral electron spin polarization profile from cw measurements of the spatial dependence of the full excitonic Kerr rotation spectrum. For the (110) QWs, we directly obtain the electron spin diffusion coefficient from picosecond real space imaging of the time evolution of an optically excited electron spin packet, which we observe with a spectrally broad probe pulse.

We studied the contribution of the breaking of antiferromagnetically coupled spin clusters to the total magnetization in thin (CdMn)Te layers as a function of tile layer thickness by reflectivity spectroscopy in magnetic fields up to 45 T. The experimental results show that the contribution of the breaking of antiferromagnetically coupled spin clusters is reduced by decreasing layer thickness.

We use time-resolved photoluminescence (TRPL) spectroscopy to unequivocally clarify the microscopic origin of the nanosecond free exciton photoluminescence rise in GaAs at low temperatures. In crucial distinction from previous work, we examine the TRPL of the GaAs free exciton second LO-phonon replica. This enables us to simultaneously monitor the unambiguous time evolution of the total exciton population and the cooling dynamics of the initially hot free exciton ensemble. We demonstrate by a model based on the Saha equation and the experimentally determined cooling behavior that the long-debated slow photoluminescence rise is caused by time-dependent shifts in the thermodynamic quasiequilibrium between free excitons and the uncorrelated electron-hole plasma.

We report on all optical spatially resolved spin diffusion experiments in an unstrained, unbiased n-GaAs layer. Optical pump and probe intensities are varied over a wide range to study the impact of optical disturbance on spin transport. Both quantities have a considerable influence on the measured spin diffusion length and spin lifetime. Furthermore, an effective spin diffusion coefficient was obtained as a function of temperature.

We summarize experimental studies on the optical polarization properties of MBE-grown shape-asymmetric self-assembled CdSe/ZnSe quantum dots (QDs). The zero magnetic field fine structure splitting of excitonic transitions in structurally anisotropic QDs gives rise to an effective linear-to-circular and circular-to-linear optical polarization conversion upon resonant excitation of the QD ensemble. The effect is qualitatively modelled in terms of a pseudospin formalism. Furthermore, we show how an in-plane magnetic field leads to a complex rotation behavior of the axis of the optical polarization. Our analysis based on the exciton pseudospin Hamiltonian unambiguously displays that these effects are induced by isotropic and anisotropic contributions to the heavy hole Zeeman term. Likewise we show that the latter mechanism can be used to tune the optical respone of shape-anisotropic QDs such that the optical polarization properties are equal to QDs of perfect D2d symmetry. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We review on optical studies of exciton-electron interaction in modulation doped quantum wells (QWs) based on ZnSe and CdTe semiconductor compounds. The optical properties of QWs containing diluted two-dimensional electron gases (2DEGs) are well described in terms of charged excitons (trions). Their parameters such as oscillator strength, binding energy, and fine structure are considered in details. In case of moderate densities of 2DEGs, when the Fermi energy falls between the trion and exciton binding energies, the optical properties can be understood in terms of combined exciton-electron excitations. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

In order to obtain well-controlled size and optimized structural properties, ZnO nanoparticles were grown from ethanolic solutions using acetate, acetylacetonate and dimethylpropylendiamine as stabilizer molecules. The low-temperature syntheses yield considerably large amounts of pure, nanocrystalline powder. Structural properties like particle size, crystal structure and quality were investigated by x-ray diffractometry, photoluminescence, and Raman spectroscopy. Particle sizes down to 4 nm and a size dependent anisotropic shape are observed. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Formation and properties of quantum dots (QDs) and related nanostructures by conventional molecular beam epitaxy (MBE) and two other variants of the technique, have been systematically investigated. While MBE growth of CdSe on ZnSe at 300 °C results in the formation of rough CdSe quantum-well-like structures, by one of the variants, which combines epitaxial growth of a CdSe layer at 230 °C and subsequent in-situ annealing at 310-340 °C, tiny but distinct three-dimensional (3D) QDs are formed. We demonstrate that QD formation by this method, developed originally by Rabe et al. (J. Cryst. Growth 184/185, 248 (1998) [30]), essentially takes place during the annealing step, by a process of “controlled roughening” of the as-grown quasi-two-dimensional (2D) CdSe layer. In comparison to conventional MBE, this process results in reduction of the areal density of the self-organized QDs by up to two orders of magnitude, thus making them distinctly discernible. In the second variant technique, similar to that applied originally for the self-organization of CdTe/ZnTe quantum dots by Tinjod et al. (Appl. Phys. Lett. 82, 4340 (2003) [34], QD-formation takes place as a result of deposition of an amorphous group-VI layer onto a strained 2D CdSe layer at ∼ 50 °C, and its subsequent re-desorption at temperatures of 280-320 °C. While deposition and desorption of amorphous Se leads to the formation of large isolated QDs, we show that use of amorphous Te results in the self-assembly and ordering of a rich variety of nanostructures. Luminescence, morphology, and composition of QDs and the underlying formation mechanism of the three processes are discussed in detail. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We have demonstrated growth of the dilute magnetic II–VI semiconductor (Zn,Mn)Se on As-passivated (100) oriented Si substrates. Low temperature epitaxy has been used to optimize the growth start. The morphology has been assessed by Nomarski and scanning electron microscopy (SEM). SQUID measurements show the full magnetic moment of the Mn ions and optical investigations show a clear Zeeman splitting of up to 30 meV. All observations indicate a high crystalline quality of the epitaxial layers. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Zn 1-x Mn x Te crystals (0 ≤ x ≤ 0.30) have been grown by the high pressure Bridgman technique. The free exciton energy of Zn 1-x Mn x Te was determined from the photoluminescence and reflectance measurements in the temperature range from 1.7 to 270 K. The magneto-photoluminescence and magneto-reflectance were studied at T= 1.7 K and in magnetic fields up to 6 T. The magnetization of Zn 1-x Mn x Te was calculated from the giant Zeeman effect. It was found that there is an efficient energy transfer channel from the photo-excited carriers into the Mn 2+ ions by spin-flip scattering processes. It allows one to change the spin temperature of the Mn 2+ subsystem and consequently to control its magnetization by tuning laser excitation density. The magnetization of the Zn 1-x Mn x Te can be changed by 60% of its maximum value.

Layer-by-layer epitaxial growth of CdSe on ZnSe has been observed at temperatures as low as 170–240 °C. Beyond this range, Reflection high energy electron diffraction (RHEED) specular spot intensity oscillations disappear, signifying a multilayer growth mode. While conventional epitaxial growth of CdSe at ∼300 °C results in formation of a rough layer-like structure, a method combining the epitaxial growth of a quasi-two-dimensional (2D) CdSe layer at 230 °C and subsequent annealing at 310 °C induces the formation of discrete CdSe quantum dots (QDs). Here we show that the formation of QDs by this method relies essentially on the layer-by-layer growth of CdSe at 230 °C, which allows an induced roughening of the as-grown 2D layer, during the subsequent annealing step. The layer-by-layer growth mode results possibly due to the nucleation of small and irregular precursor 2D clusters during growth at low temperatures, at the kink sites of which potential barriers for downward inter-layer transport of adatoms is low/absent.

We report on low-temperature spatially resolved photoluminescence (SRPL) experiments to study the diffusion of free excitons in a $1.5\text{\ensuremath{-}}\ensuremath{\mu}\mathrm{m}$-thick layer of high-purity epitaxial GaAs. Extending previous SRPL experiments, we analyze the stationary diffusion profiles detected on the second LO-phonon replica of the free exciton. This allows us to circumvent the inherent interpretation ambiguities of the free-exciton zero-phonon line. Moreover, a spatially resolved line shape analysis of the $(FX)\ensuremath{-}2\ensuremath{\hbar}{\mathrm{\ensuremath{\Omega}}}_{\text{LO}}$ replica provides direct experimental access to the pump-induced exciton temperature profile. We demonstrate that only resonant optical excitation prevents the buildup of a temperature gradient in the carrier system, which otherwise severely distorts the stationary and time-resolved free-exciton diffusion profiles.

We report on the observation of macroscopic free exciton photoluminescence (PL) rings that appear in spatially resolved PL images obtained on a high purity GaAs sample. We demonstrate that a spatial temperature gradient in the photocarrier system, which is due to nonresonant optical excitation, locally modifies the population balance between free excitons and the uncorrelated electron-hole plasma described by the Saha equation and accounts for the experimentally observed nontrivial PL profiles. The exciton ring formation is a particularly instructive manifestation of the spatially dependent thermodynamics of a partially ionized exciton gas in a bulk semiconductor.

Resonant photoluminescence and photoluminescence excitation spectroscopy have been applied to CdSe/ZnSe quantumdots where separation between the quantized levels is close to the optical phonon energy. Astrong relaxation enhancement induced by the optical phonon is observed. The fine structure of the phonon replicas is resolved. It is attributed to the optical phonons with large wave vectors from the symmetrical points of the Brillouin zone. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We present comprehensive studies of in-plane optical anisotropy in CdSe/ZnSe quantum dot ensembles. The anisotropy reveals itself as circular-to-linear polarization conversion and as an alignment occurring at resonant excitation without any applied magnetic fields. The variation of the anisotropy in different dots has been analyzed based on angle scans. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We report on spin memory relaxation time τ s of free electrons in CdTe-based quantum wells which is found to be in the nanosecond range. In these studies two different techniques have been exploited: (i) the Hanle effect under cw excitation and (ii) the time-resolved pump-&-probe Kerr rotation measured under excitation by 1.8-ps optical pulses. These independent techniques give very close results τ s =19 ns and τ s = 14 ns, respectively. To our knowledge this is by two orders of magnitude longer than the electron spin relaxation times reported for CdTe-based quantum wells so far.

has been attracted as early as 1929 [1].The existence of negatively (eeh) and positively (ehh)charged excitons (trions) in semiconductors, being ana-log of the hydrogen ions, was predicted by Lampert in1958 [2]. The investigation of three-body complexes hasa fundamental importance, particularly in semiconduc-tors, where there is a possibility to vary parameters in awide spectrum. However, the experimental observationof trions in bulk semiconductors is rather difficult due totheir small binding energies.The interest to experiment and theory of trions hasgrown due to the progress in the semiconductor het-erostructure fabrication. Theoretical calculations per-formed at the end of the 1980s [3] predicted a consid-erable (up to tenfold) increase of the trion binding en-ergy in quantum well heterostructures compared withbulk semiconductors. The first experimental observationof negatively charged excitons (X

The rapidly expanding research in Spintronics, the electronics utilizing the electron spin instead of its charge, is driven by the very interesting potential applications. The actual task is to develop principles for the spin manipulations in spintronic devices. In this Report we suggest and verify experimentally a concept of heteromagnetic semiconductor structures. It is based on spin diffusion between layers of the nanostructure with different magnetic properties and allows controlling the spin-switching rate for magnetic ions. A ten times increase of spin-lattice relaxation rate of magnetic Mn-ions is achieved in (Zn,Mn)Se/(Be,Mn)Te heteromagnetic structures with an inhomogeneous distribution of Mn-ions. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

In this paper we give a brief overview of our studies on dynamical processes in diluted-magnetic-semiconductor heterostructures based on (Zn,Mn)Se and (Cd,Mn)Te. Presence of free carriers is an important factor which determines the energy- and spin transfer in a coupled systems of magnetic ions, lattice (the phonon system) and carriers. We report also new data on dynamical response of magnetic ions interacting with photogenerated electron-hole plasma. (Zn,Mn)Se/(Zn,Be)Se structures with relatively high Mn content of 11% provide spin-lattice relaxation time of about 20 ns, which is considerably shorter then the characterictic times of nonequilibrium phonons ranging to 1 μs. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Dynamics of spin–lattice relaxation of the magnetic Mn-ion system is investigated for nanostructures with (Cd,Mn)Te and (Cd,Mn)Se diluted-magnetic-semiconductor (DMS) quantum dots. A nonequilibrium phonon technique with optical detection of the induced heating of the magnetic ion system was exploited. A strongly nonexponential decay of the spin dynamics is found to be specific for the DMS quantum dots. It is explained in the frame of a model suggesting that the spin dynamics in quantum dots is strongly affected by a spin diffusion into/from a DMS wetting layer, where the spin–lattice relaxation rate differs from that in the quantum dots. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We present a detailed study of combined exciton-electron processes in modulation doped quantum well structures contain a 2DEG of low density. We observed resonance exciton-electron processes in magnetic fields reveal in a separate narrow spectral lines, as well as nonresonant processes reveal in the exciton line broadening. Combine processes with participate of the excitons have been observed as well as with participate of trions (negatively charged exciton-electron complexes).

We use the combination of nonequilibrium phonon and exciton luminescence techniques to study the spin dynamics in diluted magnetic semiconductor structures with (Cd,Mn)Te and (Cd,Mn)Se quantum dots (QDs). We show that the spin–lattice relaxation (SLR) of Mn ions in these structures differs strongly from the SLR in quantum wells. We explain the results by a model where SLR process in structures with QDs is modified by the spin diffusion on Mn ions from the QD to a wetting layer.

A review of our recent experiments on the studies of spin-lattice relaxation in semimagnetic semiconductor quantum wells (QWs) is given. Three series of magnetic nanostructures are studied: undoped (Cd,Mn)Te/(Cd,Mg)Te QWs; n-doped (Cd,Mn)Te/(Cd,Mg)Te QWs with two-dimensional electron gas; p-doped (Cd,Mn)Te/(Cd,Mg,Zn)Te QWs with two-dimensional hole gas. In the experiments, we use a technique which combines an injection of nonequilibrium phonons and an optical detection of the induced changes via the exciton photoluminescence. We observe that doping induces a significant acceleration of the spin-lattice relaxation process, which is due to spin-flip transitions between Mn ions and free carriers. We demonstrate experimentally that it is possible to tune the value of spin-lattice relaxation time by changing the intensity of the laser excitation in the doped QWs.

Properties of interfaces with no-common atom in type-II ZnSe/BeTe heterostructures are studied with polarization spectroscopy. Structures with four possible configurations of normal and inverted interfaces have been investigated. Radiative recombination and its polarization anisotropy have been found to depend crucially on the interface configuration and excitation power. The comparison of interfaces formed by the growth on anion (Se, Te) and cation (Zn, Be) terminated layers has shown that the latter demonstrate a significantly higher nonradiative recombination rate. In agreement with the quantum well (QW) symmetry, the photoluminescence (PL) of the structures with nonequivalent normal and inverted interfaces is highly linearly polarized both at low and high excitation densities. Unexpectedly, a similarly strong PL polarization has been found for structures with equivalent interfaces up to carrier densities of 1012 cm−2 per QW. The polarization is explained by a built-in electric field, it decreases with...

We studied the diffusion of carriers in dilute magnetic semiconductors for conditions when an additional force caused by a spatially inhomogeneous exchange potential is active. The spatially inhomogeneous exchange potential arises from a creation of photocarriers in the Zn 0.988 Mn 0.012 Se/ Zn 0.94 Be 0.06 Se quantum well. The photocarriers enhance the spin temperature of the Mn ions by a spin-flip scattering with the magnetic ions, which results in a reduction of the giant Zeeman splitting. The gaussian like intensity profile of the exciting laser beam causes a locally varying exchange potential for the carriers. A strong influence of the gradient in the exchange potential on the diffusion of the carriers was found. The diffusion of the carriers was examined by a spatially resolved photoluminescence technique.

Properties of interfaces with no-common atom in type-II ZnSe/BeTe heterostructures are studied with polarization spectroscopy. Radiative recombination and its polarization anisotropy have been found to depend crucially on the interface configuration and excitation power. Effects of external and internal electric fields on the in-plane polarization are studied both numerically and experimentally at low and high levels of photoexcitation density.

Properties of a two-dimensional electron gas with a density of 5×10 11 cm −2 in a ZnSe/(Zn,Be,Mg)Se quantum well structure have been investigated by means of polarized magneto-luminescence in fields up to 47 T and by an optically detected resonance (ODR) technique. With increasing magnetic field, the optical spectra of the electronic system changes its appearance from Landau-level-like with a linear shift of lines, to excitonic-like with a diamagnetic shift. The transition occurs at filling factor 2. Pronounced oscillatory features at even filling factors are found for the luminescence intensity and polarization degree, the energy of optical transitions, and in ODR signal.

In modulation-doped ZnSe/(Zn,Mg)(S,Se) quantum well structures with electron concentration of about 10 cm - a new resonant optical transition involving four particles (one hole and three electrons) was found in external magnetic fields in photoluminescence excitation and reflectivity spectra. This transition reveals as a narrow line between exciton and trion lines. It is identified with a combined process in which a photocreated exciton binds with an electron forming a trion and promotes for the second electron a transition to higher Landau levels.

We use two-color magneto-optical Kerr (MOKE) microscopy to study low-temperature electron spin diffusion in bulk n-type GaAs (n = 1.4 × 1016 cm−3). We show that simple drift-diffusion models do not describe the spin diffusion for above-bandgap optical spin injection correctly. By variation of the excitation energy and lattice temperature we demonstrate that this discrepancy is caused by the influence of the pump-induced local overheating of the electron system with respect to the lattice, which persists over length scales comparable to the spin diffusion length. Consideration of this hot carrier effect is crucial for a reliable extraction of spin propagation parameters from optical experiments.

We report on a process that enables the removal of II-VI semiconductor epilayers from their GaAs growth substrate and their subsequent transfer to arbitrary host environments. The technique combines mechanical lapping and layer selective chemical wet etching and is generally applicable to any II-VI layer stack. We demonstrate the non-invasiveness of the method by transferring an all-II-VI magnetic resonant tunneling diode. High resolution X-ray diffraction proves that the crystal integrity of the heterostructure is preserved. Transport characterization confirms that the functionality of the device is maintained and even improved, which is ascribed to completely elastic strain relaxation of the tunnel barrier layer.

Singlet and triplet states of negatively charged excitons (trions) in ZnSe/(Zn,Be,Mg)Se quantum wells have been studied by means of photoluminescence in pulsed magnetic fields up to 50 T. Singlet state binding energies, measured for different well widths ranging from 48 to 190 A, show a monotonic increase with growing magnetic fields with a tendency to saturation. This behavior is in qualitative agreement with results of model calculations. Quantitatively, the binding energy of singlet states is underestimated by about 50%. The triplet state assigned to the dark triplet becomes detectable in magnetic fields above 15 T. A crossover of the triplet and singlet states is expected in magnetic fields of about 50 T.

We measure the spin–lattice relaxation time of Mn ions in Cd1−xMnxTe/Cd0.66Mg0.27Zn0.07Te (x=0.0035) semimagnetic quantum wells in the presence of a two-dimensional hole gas (2DHG) with sheet densities up to 3×1010 cm−2. We show that the presence of a 2DHG reduces the spin–lattice relaxation time by more than one order of magnitude. This reduction is much stronger than a similar effect induced by the presence of a two-dimensional electron gas, which is due to the heavier hole mass and stronger p–d interaction for holes with Mn ions. We demonstrate experimentally the possibility to tune the spin–lattice relaxation time by varying the density of the 2DHG through above-barrier photoexcitation.

We report on cw and time-resolved photoluminescence (PL) studies of Cd(Mn)Se/Zn(Mn)Se diluted magnetic semiconductor nanostructures grown by molecular beam epitaxy. Excitonic PL intensity, decay time and Zeeman splitting have been studied systematically as a function of Cd(Mn)Se nominal thickness, Mn concentration and sample design. Wave function mapping has been performed, evidencing the formation of semi-magnetic quantum disk islands in the samples with thick enough Cd(Mn)Se insertions.

The in-plane motion of excitons (X) and negatively charged excitons (X−) in a doped CdTe/CdMgTe quantum well has been measured at 4.2 K in magnetic fields up to 12 T, using spatially resolved photoluminescence (PL) spectroscopy. The populations of X and X− have been found to be in local dynamical equilibrium. The measured PL patterns in real space could therefore be described by considering free and mobile excitons in equilibrium with localised charged excitons.

Photoluminescence, photoluminescence excitation and reflectivity spectra of CdTe-based modulation-doped quantum well structures with 2DEG have been studied in magnetic fields. Resonant lines corresponding to combined optical transitions in which the photo-generation of excitons as well as trions is accompanied by the excitation of an additional electron from states below the Fermi level to states above the Fermi level were observed and analyzed.

Recently, efficient spin injection, being the first step towards semiconductor spin electronics, by using BeMnZnSe as a spin filter was accomplished. Such a spin filter made it possible to align the spin orientation of conduction electrons and subsequently inject them into GaAs. However, controlling spin orientation of conduction electrons by an external voltage would be very desirable for semiconductor-based magnetoelectronics. This can be accomplished by using spin switch structures, based on resonant tunneling through magnetic quantum wells, with two separate spin-up and spin-down resonances. Here we summarize both our recent results on spin injection as well as on spin aligner and magnetic resonant tunneling structures. For accomplishing the latter, we have developed magnetic resonant tunneling diodes based on BeTe–ZnMnSe–BeTe structures. Resonant tunneling diode is meant to serve as a spin switch because of the existence of two separate spin-up and spin-down resonances. The tunneling carriers have subsequently been injected into a nonmagnetic GaAs p–i–n light emitting diode. Circular polarization of the emitted light is an indicator of the spin polarization of injected electrons. At constant magnetic field and current, degree of spin polarization could be changed from 81% to 38% by only varying the voltage across the magnetic resonant tunneling device.

Microscopic information on the optical properties of Se(g)-type dislocations in ZnSe layers grown on (001) GaAs substrates by molecular beam epitaxy (MBE) have been obtained by using micro-cathodo- and photoluminescence techniques. Se(g)-type misfit dislocations are found to nucleate first in high quality pseudomorphic films above the critical thickness. The individual Se(g) misfit segments are evidenced as the only local emission centres of the so-called Y luminescence. The latter is found to be strongly polarized in the defect line direction and exhibits a pronounced spectrum fine structure, which is related to the particular structural configuration, as determined by the dissociation of perfect 60° dislocation segments into pairs of 30°/90° Shockley partials bordering a stacking fault ribbon. Studying the dislocation induced Y luminescence with respect to polarization properties and correlation with defect structure yields essential arguments supporting the idea that the dislocation-induced luminescence may be ascribed to 1D excitonic states at the line defect. The local strain field believed to be origin of defect bound electronic states could be examined at isolated Se(g) dislocations by analysing the microscopic dependence of observed polarization of the free exciton emission in the layer matrix. The polarization properties of free exciton emission results from anisotropic relief of layer strain by the Se(g) misfit segment arrangement.

A series of ZnSe/GaAs(001) samples grown with various epilayer thicknesses from 50 to 1800 nm is investigated to study the role of polar misfit dislocations in respect of their nucleation and propagation behavior in the process of layer strain relaxation. Photoluminescence spectroscopy and SEM cathodoluminescence, both applied at low temperatures, are exploited to investigate misfit dislocation configurations and related stress relief in the ZnSe epilayers. Expected effects concerning the asymmetry in the dislocation misfit arrangements and optical anisotropy could be proved and analyzed in the framework of a modified Dodson-Tsao-Model.

The oscillator strength of negatively charged excitons (trions) in ZnSe/(Zn,Mg)(S,Se) quantum-well structures with n-type modulation doping is studied by means of reflectivity as a function of electron concentration, temperature, and external magnetic fields. The trion oscillator strength is found to increase linearly with increasing electron concentration up to $6\ifmmode\times\else\texttimes\fi{}{10}^{10}{\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}.$ The trion nonradiative damping shows no dependence on the electron concentration or external magnetic field. An optical method, based on the analysis of the polarization of the trion line, is proposed to investigate two-dimensional electron gases of low density from ${10}^{9}$ up to ${10}^{11}{\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}.$

We review results of magnetooptical studies related to various methods of spin splitting engineering possible in quantum well (QW) structures made of II-VI diluted magnetic semiconductors (DMS). The experiments made use of unique structures, based on Cd 1-x-y Mn x Mg y Te, with a precise in-plane profiling of either QW width or n-type doping intensity and of structures with digital profiling of the composition of constituent materials in the growth direction. The most typical version of the spin splitting engineering, available also in bulk DMSs, namely the one via an adjustment of (homogeneous) Mn composition, or via the lattice temperature or/and a magnetic field, was used to study magnetooptical properties of QW containing electrons as a function of their g-factor (tuned from g * eff = - 1.46 to + 55!). We also demonstrate three further methods of modifying the spin splitting in low-dimensional structures made of DMSs. They involve: (i) atomically precise control over Mn ion spatial distribution in the growth direction, (ii) reduction of the s-d exchange constant with increasing electron confinement, (iii) modification of Mn spin temperature by the electron gas heated by photo-excited carriers.

We report magneto-optical studies of ZnSe/(Zn,Mg)(S,Se) quantum wells with n-type modulation doping. Polarized photoluminescence and spin-flip Raman scattering spectroscopy is used to investigate the energy and spin structure of neutral and negatively charged excitons. The binding energy of charged excitons changes very little in magnetic fields up to 15 T. An inverted population of neutral exciton spin sublevels is found in magnetic fields; it is associated with the spin-dependent formation of charged excitons.

The in-plane optical anisotropy of ZnSe/BeTe multi-layered heterostructures has been studied by analyzing the linear polarization of the spatially indirect photoluminescence. A pronounced quantum-confined Pockels effect is reported in ZnSe/BeTe double-barrier structures. A tight-binding model is proposed which consistently interprets the experimental findings and suggests a new mechanism of the Pockels effect realized in type-II heterostructures. The model takes into account that the ZnSe/BeTe heterosystem exhibits a type-II band alignment with large band offsets and that in the zinc-blende lattice the chemical bonds are oriented in 〈1 1 1〉-like directions and, when shifted along the [0 0 1] principal axis, alternatively change their orientation in the (0 0 1) plane from [1 1 0] to [1 1 0] and vice versa.

An alternative technological approach for the realization of (Cd,Mn)Te nanostructures is presented. Focused ion beam lithography and subsequent thermal annealing are used for a lateral modulation of the bandgap, allowing both, the realization of quantum wires and quantum dots with adjustable size and a well-defined lateral potential. Due to the high quantum efficiency of the buried nanostructures, photoluminescence spectroscopy even on single quantum dots becomes accessible, allowing an experimental insight on optical transitions in three-dimensionally confined semimagnetic structures.

Spin injection into semiconductors has been a field of growing interest during recent years, because of the large possibilities in basic physics and for device applications that a controlled manipulation of the electrons spin would enable. However, it has proven very difficult to realize such a spin injector experimentally. Here we demonstrate electrical spin injection and detection in a GaAs/AlGaAs p-i-n diode using a semimagnetic II–VI semiconductor (Zn1 − x − yBexMnySe) as a spin aligner. The degree of circular polarization of the electroluminescence from the diode is related to the spin polarization of the conduction electrons. Thus, it may be used as a detector for injected spin-polarized carriers. Our experimental results indicate a spin polarization of the injected electrons of up to 90% and are reproduced for several samples. The degree of optical polarization depends strongly on the Mn concentration and the thickness of the spin aligner. Electroluminescence from a reference sample without spin aligner as well as photoluminescence after unpolarized excitation in the spin aligner sample show only the intrinsic polarization in an external magnetic field due to the GaAs bandstructure. We can thus exclude side effects from Faraday effect or magnetic circular dichroism in the semimagnetic layer as the origin of the observed circularly polarized electroluminescence.