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The In-concentration in InGaAs quantum dots located within a GaAs matrix was determined with the composition evaluation by lattice fringe analysis (CELFA) technique. However, the results obtained with this method cannot account for the three-dimensional shape of quantum dots and their embedding in GaAs. A correction procedure was developed that takes into consideration the shape of the quantum dots and the TEM sample thickness and quantum-dot size. After correction, In-concentration profiles show an increasing Incontent towards the top of the quantum dots which is consistent with the effect of In-segregation and earlier studies using other experimental techniques. Dedicated to Prof. Wolfgang Neumann on the occasion of his 65 th birthday

We report on the investigation of CdSe/ZnSe heterostructures by transmission electron microscopy (TEM) and photoluminescence spectroscopy (PL). CdSe layers with nominal thicknesses tCdSe between 0.5 and 8 monolayers (ML) were embedded in a ZnSe matrix and grown on a GaAs(001) substrate by molecular-beam epitaxy at 280 °C. The Cd-distribution was obtained from high-resolution TEM lattice fringe images using composition evaluation by lattice fringe analysis technique. The measured minimum, average and maximum Cd-concentrations and the overall CdSe contents in the layers increase with the nominal CdSe layer thickness and reach a constant value at tCdSe = 4 ML. The measured CdSe content in the regions with the maximal Cd-concentration continues to increase for tCdSe ≥ 5 ML. The increasing of measured Cd-concentrations/CdSe contents is correlated with a red shift of PL spectra. In the CdSe layers with tCdSe ≥ 5 ML, formation of defects is observed. The increasing density of defects and decreasing intensity of PL spectra with tCdSe suggest that the critical thickness for defect formation during CdSe growth on the ZnSe(001) is between 4 ML and 5 ML. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The influence of the growth conditions during molecular-beam epitaxy on the properties of InAs/GaAs quantum dot structures were systematically investigated by low temperature photoluminescence spectroscopy and transmission electron microscopy. The circular polarization degree (CPD) of the electroluminescence was compared for two quantum-dot spin-injection light-emitting diodes. The CPD depends on the position of the emission energy in the luminescence band. This correlation is similar for both samples despite the strongly different quantum dot morphologies. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

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. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

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. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Using the well established MBE process, we introduced colloidal nanocrystals (NCs) into monocrystalline semiconductor layers. CdSe nanorods (NRs) and CdSe(ZnS) core-shell nanodots (NDs) with radii R between 2 and 4 nm on a ZnSe surface are capped by thin (d ∼ 2R ) and thick (d > 2R ) MBE-grown ZnSe layers of a thickness d . This new growth technique does not rely on strain like the Stranski-Krastanow growth of CdSe/ZnSe quantum dot layers, and thus offers additional degrees of freedom for choosing the NC composition, shape and size. In- and ex- situ characterisations of the samples are showing a ZnSe cap layer of high crystalline quality. After the growth of the cap layer the dots preserve their shape and emission spectrum. High-resolution transmission electron microscope images show the direct connection of the NCs crystal lattice to the crystal lattices of the ZnSe buffer and the epitaxially grown cap layer. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We investigate cubic Zn1−xMnxSe layers (0 ≤ x ≤ 0.43) grown on GaAs(001) by elemental source molecular-beam epitaxy. To determine the Mn concentration, energy-dispersive X-ray analysis, Rutherford backscattering and particle-induced X-ray emission are applied. In the next step we utilize X-ray diffraction to measure both the strained lattice parameter of the films along and perpendicular to the growth direction. The unstrained values are then derived from elasticity theory using elastic constants for cubic Zn1−xMnxSe calculated from published data for hexagonal material. Finally, an extrapolation to x = 1 yields the lattice parameter of cubic MnSe (a0 = 5.90 A). (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Vertically correlated islands of a quality comparable to the model system InAs/GaAs have been fabricated using modified molecular beam epitaxy. While island widths range from 20-50 nm, island heights are very homogeneous, resulting in narrow photoluminescence (PL) emission line width of 20-40 meV even for stacks of 50 layers. Island centers contain Cd concentrations up to more than 80%. Distinct PL double peaks can be assigned to correlated island stacks with large islands and the uncorrelated region, containing much smaller, laterally interacting islands.

Different techniques of transmission electron microscopy were applied to study the Cd distribution and the formation of defects in CdSe/ZnSe structures with a nominal CdSe layer thickness t of three monolayers (ML) and different ZnSe cap layer thicknesses which were varied from 20 to 60 nm. The samples were grown by standard molecular-beam epitaxy at 350 °C. We report on the configuration of defects which appear in the CdSe/ZnSe structure from a critical thickness t > 2.5 ML. The defects originate in Cd-rich regions close to the CdZnSe layer and consist of stacking fault (SF) pairs on {111} crystal planes. The lengths of the dislocation lines and the sizes of the SFs increase with the cap layer thickness. The quantitative measurement of the Cd distribution indicates segregation with an efficiency of R = 0.74 ± 0.04.

Summary Autocorrelation spectroscopy on the basis of thousands of individual near-field photoluminescence spectra of single ultrathin CdSe layers at low temperatures exhibits a strong positive correlation peak around 18 meV energy with a width of 5 meV. Using simulations and experiments as a function of temperature and laser intensity, we can exclude interpretations along the lines of biexcitons or phonon sidebands. We attribute this feature to the splitting of ground state and an excited state in individual quantum islands. This interpretation implies that the potential minima are rather uniform in size and that the distribution of excitons is nonthermal.

Conventional and high-resolution transmission electron microscopy (TEM) was applied to a study of the Cd distribution and structure of defects in single and multiple CdSe layers in a ZnSe matrix. The samples containing nominal CdSe layer thicknesses tn between 1.7 and 3.5 monolayers (ML) were grown by molecular beam and atomic layer epitaxy under different conditions. In all samples ternary CdZnSe wetting layers with Cd-rich regions with sizes of less than 10 nm (small islands: SIs) and a density of ∼1011 cm—2 are observed. The Cd concentration in the wetting layer and the SIs increase with tn. In addition, regions with sizes of 20–30 nm (large islands: LIs) and Cd concentrations >40% occur in specimens with tn > 2.5 ML. In the vicinity of the LIs stacking faults are preferably generated leading to a “coffee-bean” contrast in plan-view TEM images. The multilayer structures with tn ≈ 2.5 ML display a predominant vertical correlation of the SIs at a ZnSe spacer thickness of 12 ML.

Finite-temperature thermodynamic theory is developed for equilibrium arrays of two-dimensional monolayer-high islands in heteroepitaxial systems at submonolayer coverage. It is shown that the entropy contribution to the Helmholtz free energy of the system favors formation of smaller islands at higher temperatures which results in a decrease of the average number of atoms in the islands (the island volume) with temperature. The characteristic temperature T char , at which the island volume is significantly decreased compared to its value at T = 0 K, is found to be far below the characteristic energy of the island formation and to lie in a region of several hundreds of K. Such a temperature dependence can be the basis for decisive experiments aimed at distinguishing between thermodynamic and kinetic effects in the formation of arrays of 2D islands. Results of high resolution electron microscopy and photoluminescence spectroscopy of a submonolayer InAs/GaAs(001) system are in agreement with the theory. It confirms that the formation of a sub-monolayer array of InAs/GaAs(001) islands is predominantly influenced by thermodynamics.

The optical gain of CdSe/ZnSe structures containing stacks of islands grown in the Stranski-Krastanow mode is studied. The islands have a lateral size of ~16 nm and a small height-to-width ratio. Cd contents of ~70 % were estimated from high-resolution transmission electron microscopy investigations. The radiative recombination of excitons localised in the islands lead to a broad emission band around 2.4 eV. At low temperatures resonant gain up to 300 cm was found generated by localised transitions. Introduction. Wide bandgap II-VI quantum dot structures based on self-organised island growth were achieved by several growth modes [1] resulting either in the formation of small 2D-like islands [2] or in islands formed in a three-dimensional (3D) growth process [3,4,5]. Zero-dimensional excitons localised in such islands enable resonant gain. Therefore, efficient excitonic waveguiding [2] can be applied to reduce the threshold density of lasers devices. For structures containing small islands, labelled type A, zero-phonon gain [e.g.,6,7] and optical pumped lasing at room temperature [8,9,10] were achieved. Nevertheless, the limited thermal stability of carrier localisation in type A islands prevents room temperature (RT) operation. Recent investigations on CdSe/ZnSe structures [11] have demonstrated that at room temperature excitons are evaporated from type A islands whereas thermal stability was found for larger islands, grown in the Stranski-Krastanow (SK) mode (labelled type B). In order to take advantage of the better thermal stability in type B islands, we have studied the resonant gain in structures containing such islands. Experimental. The investigated structures were grown by molecular beam epitaxy (MBE) and consist of a single layer or a 10 fold stacks of nominally 2.8 ML thick CdSe coverage deposited at 340 oC. The CdSe layers in the stacks were separated by 12 nm ZnSe spacers. All structures were cladded by 45 nm thick ZnSe bufferand cap layers. To study the 1 on leave from Ioffe-Institute, Polytekhnicheskaya 26, St. Petersburg, 194021, Russia physica status solidi –2– M. Strasburg et al. conditions for the formation of type A islands which have a Cd content below 40 % and those of type B islands which are described in detail in previous publications [5,11], samples were grown both, on exactly (001)-oriented GaAs (exactly oriented) substrates and on substrates which were tilted by an angle of 6 o towards the direction (tilted). The growth on tilted surfaces is believed to enable the step-flow growth mode, and thus to suppress the formation of type A islands. The formation of 3D islands was confirmed by in-situ monitoring of the RHEED pattern. More details of the growth are given in Ref. 12. The samples were studied by transmission electron microscopy (TEM) in plan-view and cross-section geometry using a Philips CM 200 FEG/ST electron microscope with an electron energy of 200 keV and a Scherzer resolution of 0.24 nm. For the optical investigations the samples were mounted in a He-flow cryostat providing temperatures between 4 K and 300 K. The photoluminescence (PL) was excited by the 325 nm line of a cw He-Cd laser. Gain studies were performed in edge geometry using the variable-stripe-length method [13]. These measurements used a pulsed dye laser with ∼20 ns pulse duration pumped by an excimer laser at an excitation energy of 2.85 eV. Luminescence was detected by a photomultiplier attached to a 0.85 m double monochromator. Gain spectra were recorded at 8 K. Results and Discussion. A typical cross-section TEM image of the sample with stacked SK islands (type B) grown on tilted substrate is depicted in Fig. 1. A relative large number of type B islands is revealed by regions with a bright contrast inside the islands. This contrast indicates a high Cd concentration which was estimated to ~70 % [12]. Type B islands having an average lateral size of 16 nm are laterally surrounded by an ~3 nm thick wetting layer (WL) with a significantly lower Cd concentration. The height of the islands exceeds that of the WL only slightly. In some places, the island arrangement is laterally shifted in adjacent

Owing to a large diamagnetism, graphene flakes can respond and be aligned to magnetic field like a ferromagnetic material. Aligned graphene flakes exhibit emergent properties approaching single-layer graphene. Anisotropic optical properties also give rise to a magnetic writing board using graphene suspension and a bar magnet as a pen. This simple alignment technique opens up enormous applications of graphene.