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The present study reports on the high-temperature low-cycle fatigue (LCF) behavior of a ferritic bimodal 12Cr oxide dispersion strengthened steel. The fully reversed strain-controlled LCF tests were conducted at a constant total strain rate with different axial strain amplitude levels. The measured cyclic stress response showed four distinct stages which include instant initial cyclic softening followed by gradual cyclic hardening thereafter continuous linear cyclic softening and finally crack initiation and growth stage. The rate and amount of these stages depend on both strain amplitude and testing temperature. The cyclic stress-strain and strain-life relationships were obtained through the test results, and related LCF parameters were calculated. Microstructural investigations, using electron microscopy, were carried out to shed light on the deformation mechanisms. At 550 °C, no appreciable microstructural evolution occurred, which is consistent with the manifestation of Masing behavior. A propensity towards cross-slip and hence the formation of three-dimensional dislocation structures increased with increase in loading amplitude and testing temperature, which assisted in accumulating higher inelastic strain and led to slightly higher cyclic hardening at these conditions. Furthermore, W-enriched Laves phase particles precipitated at grain boundaries. The evolution of cell structures with strain amplitude at 650 °C resulted in a small deviation from Masing behavior. The damage studies revealed multiple surface initiated transgranular cracks under higher strain amplitudes and a single surface initiated transgranular crack under lower strain amplitudes. The different stages of the crack path, whose durations depend on the testing conditions, revealed distinct fatigue fracture features.

In the present work, high temperature deformation behavior of oxide dispersion strengthened T91 was investigated and linked to the corresponding microstructure. First, tensile properties are presented and discussed in terms of yield strength, tensile stress and total elongation as a function of temperature. The results are compared to the matrix material and other ODS alloys. Second, transmission electron microscopy was applied to as received and deformed tensile test specimens. It is shown that the Y2O3 particle diameter increases slightly upon deformation at elevated temperatures. Additionally, distinctive coarsening of M23C6 carbides occurs at prior austenite grain boundaries. At temperatures above 500°C, dislocations are straight and pile up at grain boundaries due to thermally activated climbing. Oxide dispersion strengthened T91 provides high strength due to strong particle/dislocation interactions and good toughness properties.

Transmission electron microscopy (TEM) was applied to study the microstructure and composition of a novel austenitic oxide dispersion strengthened (ODS) steel (16 wt.% Cr, 14 wt.% Ni, 1 wt.% W). Special attention was paid to characterize the dispersed nano-oxide particles. It was found, that these embedded particles have a size distribution ranging from 1 to 70 nm. The particles with sizes up to 20 nm, have a density of ∼1023 m−3, are mainly Y2O3 or Y2Ti2O7. The Y2Ti2O7 particles with size up to 3 nm were found to be fully coherent with the γ-Fe matrix (without defects) with relationships 〈001〉 Y2Ti2O7 II 〈001〉 Fe, 〈111〉 Y2Ti2O7 II 〈111〉 Fe and (110) Y2Ti2O7 II (110) Fe. The larger Y2Ti2O7 were found semi-coherent. Due to the misfit of ∼6% between the Y2Ti2O7 crystal lattice and the matrix, which have similar FCC structures, misfit dislocations were observed. The Y2O3 particles, having a smaller misfit of ∼2%, but with BCC structure, reveal coherence with the matrix with relationship 〈001〉 Y2O3 II 〈001〉 Fe, 〈111〉 Y2O3 II 〈111〉 Fe, and {110} Y2O3 II {110} Fe. It was observed that different nanoparticles have a tendency to agglomerate which leads to the loss of their coherency. Independent of the sizes and composition, the nano-oxide particles (which are either fully- or semi-coherent) were able to pin dislocations under repulsive interaction which is known to improve mechanical properties.

An ensemble of InAs quantum dots with ground state transition energies centered at 1.216 eV and density 1011dots/cm2 has been studied by time-resolved photoluminescence (PL). The wavelength of the 100-fs excitation pulse was tuned through the ground (excited) state transitions, resulting in resonant (optical phonon sideband) PL. The decay of the PL was time resolved with a streak camera in the interval 1.5–3 ns to avoid scattered laser light. The intensity of the PL was recorded with its polarization both parallel with and perpendicular to the excitation polarization (along one of the crystal’s cleave axes); the ratio is 2.22 at low temperatures and low excitation. A phenomenological rate equation analysis is made, separating the excitations into two classes, one polarized along the excitation polarization and the other unpolarized (either that way immediately after the excitation pulse or scattered from the first class). Excellent fits to the data lead to the conclusion that both classes decay radiatively with a lifetime of 1 ns, and a transfer from the polarized to the unpolarized species takes place with a distribution time of 12 ns at low temperatures and low excitation, dropping rapidly toward zero for temperatures above 30 K and for intense excitation levels. The polarization of a coherently excited ground state exciton should dephase with a rate equal to the sum of the radiative rate plus the inverse of this distribution time.

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)

We investigated InGaAs layers grown by molecular-beam epitaxy on GaAs (001) with transmission electron microscopy (TEM) and photoluminescence spectroscopy. InGaAs layers with In-concentrations of 16, 25 and 28 % and respective thicknesses of 20, 22 and 23 monolayers were deposited at 535 °C. Island formation is observed for the layer with the highest In-concentration. Inconcentration profiles were obtained from high-resolution TEM images by composition evaluation by lattice fringe analysis. The measured profiles can well be fitted applying the segregation model of Muraki et al. [Appl. Phys. Lett. 61 (1992) 557] and are in excellent quantitative agreement with the photoluminescence peak positions. From our data we conclude that island formation occurs when the amount of Indium in the In-floating layer reaches 1.1±0.2 monolayers indium.

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)

Quantitative high-resolution transmission electron microscopy was applied to determine the indium and nitrogen distributions in a GaInNAs/GaAs heterostructure. Two almost identical samples grown by gas-source molecular beam epitaxy on GaAs (001) substrates were investigated. The samples contained InGaAs and GaInNAs wells with the same thickness and In concentration. According to high-resolution X-ray diffractometry the average In concentration is 27% in both samples, with an N concentration of 1.1% in the GaInNAs quantum well. The evaluation of the photoluminescence peak position of the InGaAs sample and additional photoreflectance measurements yielded an In concentration of 30%. In-concentration profiles were obtained with the composition evaluation by the lattice fringe analysis (CELFA) technique for the InGaAs well using the chemical sensitivity of the (002) reflection. The averaged In concentration in the InGaAs quantum well was determined to be (30±1)% in good agreement with the other measurement techniques. The N-concentration profiles in the GaInNAs wells were measured by the comparison of the CELFA results in the samples with and without nitrogen. The N distribution is characterised by two maxima close to the GaInNAs/GaAs interfaces. The measured N concentration in the central part of the quantum well is (2.5±1.0)%.

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)

We investigated the influence of the temperature of the ZnSe cap layer growth on the Cd distribution in CdSe/ZnSe heterostructures by transmission electron microscopy, high-resolution X-ray diffraction and photoluminescence spectroscopy. ZnSe buffer layers were grown by molecular-beam epitaxy on GaAs (0 0 1). For the CdSe layers and ZnSe cap layers, migration-enhanced epitaxy was used. Increasing the temperature for the cap layer growth reduces the Cd concentration and the total CdSe amount in the (Cd,Zn)Se layers leading to a blue shift of the photoluminescence spectra. This is owing to a stronger Cd desorption during the cap layer growth for a higher substrate temperature.

While providing a general overview over the current status of self-organization of quantum islands in the II–VI semiconductor system, with the main focus on CdSe embedded in ZnSe, this paper shall give an introduction to the possibilities opened by a modification of the standard growth technique. In molecular beam epitaxy, we have substituted the generally used Cd-elemental source with a CdS-compound source. The sulfur is usually not included in the growing layer. However, its presence can be surfactant-like while the elevated Cd-temperature of the dissociated CdS leads to changed thermodynamic conditions on the growth front. Using migration enhanced epitaxy, nearly perfect quantum wells with respect to lateral homogeneity can be obtained by suppressing the inherent Cd segregation and clustering. These processes are generally responsible for the formation of small islands (SI) (lateral diameter 3–5 nm) even when not attempting to grow island like structures. The suppression of these SI was a first step to gain control over the island formation. Larger islands with central Cd concentrations above 40% are more of interest for device applications, since a population at room temperature is necessary. In particular, high-density systems are required. Using the modified growth mode, well correlated, stacked island systems were obtained. Their outstanding structural and optical properties will be discussed in detail. The absence of a closed wetting layer in the CdSe/ZnSe system and the appearance of island like structures, even at submonolayer nominal deposition, further corroborate the assumption that island formation does not readily occur in a standard Stranski–Krastanow growth mode, which is assumed for InAs/GaAs.

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.

Using CdS compound and elemental Se, we have grown CdSe/ZnSe quantum structures with improved optical and structural properties exploiting an exchange reaction which leads to the substitution of sulfur by selenium. Typical S contamination is below 2%. A possibly enhanced surface diffusion of adatoms caused by the high CdS oven temperature and a surfactant-like effect of the S–Se exchange lead to a suppression of Cd segregation in the case of migration enhanced epitaxy with long Se exposure times. The new growth method leads to CdSe quantum wells with outstanding optical quality. Their properties are compared to CdSe island structures obtained in a non-migration-enhanced growth mode.

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.

Metalorganic vapor-phase epitaxy of ternary ZnMgSe and ZnCdSe layers, quaternary ZnMgCdSe layers and superlattices on InP (1 0 0) is presented. The dependence of the Cd- and Mg-incorporation into the ternary compounds on the molar ratio in the vapor phase, and the influence of the buffer material and the growth conditions on the structural quality are discussed. A tendency to segregation and increased mosaicity is found in quaternary ZnMgCdSe structures with magnesium fractions exceeding 40%, evaluated by X-ray diffraction, photoluminescence and transmission electron microscopy.

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

The structural properties and the Cd-distribution of CdSe layers with a nominal thickness between 0.5 and 3.6 monolayers (ML) in a ZnSe matrix were investigated by plan-view and cross-section transmission electron microscopy. All layers contain a high density of Cd-rich islands with sizes below 10 nm, which are completely embedded in a 3 nm thick CdZnSe wetting layer. Large islands with a density two orders of magnitude below the small island density are additionally observed at t n ⩾3 ML above the critical wetting layer thickness for the transition from the two-dimensional to the three-dimensional growth mode. Composition analyses on a subnanometer scale reveal the Cd-distribution and Cd-content, which increases in the wetting layer and in the small islands with the nominal layer thickness.

Two different types of quantum islands, formed either by strain-modified island growth or in the Stranski-Krastanow (SK) mode, were identified by optical and structural investigations. Pronounced differences in the size, shape and Cd content of these islands lead to different localisation energies of three-dimensionally confined excitons allowing a clear distinction of both island types. The classification was confirmed by plan-view and cross-sectional transmission electron microscopy.

We demonstrate a new technique to grow high-quality CdSe quantum films and islands with a very small sulfur contamination by using a cadmium sulfide compound source as Cd supply and additional Se flux. By monitoring the lattice constant with reflection high-energy electron diffraction, it is shown that the sulfur is almost completely substituted by Se and CdSe with a contamination below 5% sulfur is formed. The quantum structures obtained by the new method are generally of higher quality than those obtained by more conventional growth methods using elemental sources, even if migration enhanced methods were employed. With a brief growth interruption or post-growth annealing step the initially smooth CdSe layer can be reorganized into islands. The duration of this step as well as the initial amount of deposition allows a rather good control over the island formation. A strongly enhanced growth rate is observed for the first few monolayers of the ZnSe capping layer, which indicates a partial dissolution of the islands in the ZnSe growth front and Cd segregation.

High- resolution TEM investigations reveal that CdSe quantum dot samples are often characterised by the existence of two different types of islands, which form a bimodal size distribution. Analysing the density distribution function of the two dominating size classes of islands, we show that islands of an average size of about 16 nm (type B islands) are clearly correlated with a phase transition via a Stranski–Krastanow growth process. The smaller clusters with a size of less than 10 nm (type A islands) are formed during the growth of the wetting layer, bevor the critical wetting layer thickness is reached. The stability range for the formation of dislocation-free SK islands was experimentally determined. We discuss gain measurements and stimulated emission of vertically stacked CdSe–ZnSe quantum dot structures which were grown on tilted GaAs substrates.

Pairs of bright spots are observed in microphotoluminescence intensity maps of ZnSe/ZnMgSSe quantum-well structures. These pairs are exactly aligned parallel to the [1 1 0] or [ 1 1 0] directions. Atomic-force microscopy and plan-view transmission electron microscopy reveal that the bright emission spots are related to pairs of stacking faults. The enhanced radiative recombination is a result of exciton localization in the region where the stacking faults intersect the quantum wells. Cross-section transmission electron microscopy and microphotoluminescence spectroscopy show that in the case of Frank-type stacking faults (oriented along [1 1 0]) the wells are enlarged by up to 12 bilayers. The exciton localization is much shallower in the case of Shockley-type stacking-fault pairs (oriented along [ 1 1 0] ).

The formation and optical properties of CdSe based, self-assembled quantum dot (QD) like nano-structures embedded in ZnSe have been studied. Self-assembling growth was achieved under both standard molecular beam epitaxy (MBE) and low temperature atomic layer epitaxy (230°C) with a subsequent annealing step. While in the case of standard MBE the competition between the relaxation via misfit dislocations and the desired dot formation leads to a low reproducibility, the latter method allows a more controlled formation of the QDs, which is clearly indicated by reflection high energy electron diffraction. In particular, a capping of the structures with ZnSe usually recovers a 2D surface thus allowing a stacking of several sheets of QDs. Small dots with a lateral diameter of 5–6 nm, which corresponds to the bulk exciton Bohr radius, and a height of 5–6 ML could be obtained as confirmed by transmission electron microscopy. The optical and structural properties of the QDs were studied by means of time resolved, resonant photoluminescence and were compared with a series of quantum wells (QW). Because of the high bandgap difference of ZnSe and CdSe, deep potential fluctuations exist within the QWs. These are caused by local interdiffusion and interface roughness and can act like low dimensional traps. However, because of their nature, they are not necessarily laterally isolated and can interact via tunneling and phonon assisted hopping. This leads to a very typical red shift of the emission peak with time in time-resolved photoluminescence (PL). In the case of self-assembled QDs, the potentials defined by the QDs are spatially well separated, as the typical dot densities observed are in the mid 1010–1011 cm−2 range. The interaction between these potentials is thus strongly suppressed, which clearly shows in the temporal evolution of both maximum position and the half width of the emission peaks. For the first time we were also able to demonstrate that CdSe can be grown with a CdS compound and additional Se flux. Sulfur seems to act as a surfactant that leads to surface smoothing and a reduced inhomogeneous broadening of the PL emission. The results are quite promising as the layers grown can be thermally activated to reorganize to coherently strained islands.

We report on structural and optical investigations of submonolayer-CdSe/ZnSe superlattices with varying thicknesses of the ZnSe spacer layers. High-resolution electron microscopy images demonstrate the formation of two-dimensional nanoscale CdSe islands for submonolayer-CdSe depositions. The vertical island arrangement is anti-correlated for spacer layer thicknesses exceeding 30A, while predominantly vertically correlated growth occurs for thinner spacers. The different vertical ordering of the CdSe islands results in two clearly distinguishable lines in photoluminescence and optical reflectance spectra, which are attributed to excitons localized in quantum dots (QDs) formed by vertically coupled and uncoupled (Cd,Zn)Se islands, respectively. δ-like emission of single QDs is demonstrated and the different carrier localization in uncoupled and coupled QDs is reflected in the polarization of the edge emission. Stimulated emission and resonant waveguiding effects are observed for both states. At the highest excitation densities, we observe saturation of the stimulated emission in edge geometry attributed to saturation of localized states.

We report on measurements of the nitrogen-concentration profile in an InGaNAs heterostructure by high-resolution transmission electron microscopy. Two samples grown by gas-source molecular-beam epitaxy on GaAs(001) substrates were investigated which contain InGaAs and InGaNAs wells with the same thickness and In concentration. The indium concentration was determined by high-resolution x-ray diffractometry. Indium-concentration profiles were obtained with the composition evaluation by lattice fringe analysis (CELFA) technique from the sample with the InGaAs wells exploiting the chemical sensitivity of the diffracted (002) beam. Nitrogen-concentration profiles were measured in the InGaNAs wells by comparison of the CELFA results observed in the samples with and without nitrogen. (C) 2004 American Institute of Physics.

We study GaAs–AlAs short-period superlattices (SPSLs) grown on a GaAs(311)A surface using plan-view transmission electron microscopy (TEM). A strong in-plane compositional modulation with a period of 3.2 nm along the [011] direction is revealed by TEM under chemically sensitive imaging conditions and in high-resolution TEM. Our results confirm the formation of highly ordered vertically aligned arrays of GaAs and AlAs quantum wires formed via self-organized growth. Bright photoluminescence (PL) at room temperature in the green and yellow spectral range is observed.

We report a reduction of inhomogeneous broadening in CdSe-related quantum wells in ZnSe by employing a growth technique that uses a CdS-compound source instead of the standard Cd elemental source for molecular-beam epitaxy. Assisted by the low sticking coefficient of sulfur and possibly an exchange reaction between S and Se, only a small S contamination is observed. A comparison with standard layers reveals an increase in quality and homogeneity by a strong reduction of the photoluminescence (PL) linewidth. Samples obtained by our method show extremely little lateral confinement as indicated by a lack of sharp single dot emission lines in micro PL and the absence of the extensive redshift observed in temperature dependent PL of fluctuating well potentials.

We present time-resolved photoluminescence measurements performed on an ensemble of InAs quantum dots with density of 10 11 dots/cm2 and ground state transition energies centered at 1.216 eV. The wavelength of the 100fs excitation pulse was tuned through the ground (excited) state transitions, resulting in resonant (optical phonon assisted) photoluminescence (PL). The PL was detected with its polarization both parallel with and perpendicular to the excitation polarization (along one of the crystal's cleave axes). The decay of the PL was time-resolved with a streak camera in the interval 1.5 - 3ns to avoid scattered laser light. A strong polarization dependence was observed. Considerable amount of the resonant fluorescence signal and even of the non-resonant PL signatures remained linearly polarized on a nanosecond time scale. A phenomenological rate equation analysis is made.

We have investigated the growth kinetics of the self-assembled formation of coherently strained CdSe islands. We have found that two distinctly different types of islands are formed in succession. Analyzing the density distribution function of the two dominating size classes of islands, we show that islands of an average diameter of about 16 nm (type B islands) are correlated with a phase transition via a Stranski–Krastanow growth process. The other islands with a diameter of less than 10 nm (type A islands) is formed during the growth of the first 2 ML. At a coverage of about 3.1 ML CdSe stacking faults appear, indicating the beginning of the plastic relaxation of the quantum dot structure.

We have investigated InGaAs layers grown by molecular-beam epitaxy on GaAs(001) by transmission electron microscopy (TEM) and photoluminescence spectroscopy. InGaAs layers with In-concentrations of 16, 25 and 28 % and respective thicknesses of 20, 22 and 23 monolayers were deposited at 535 C. The parameters were chosen to grow layers slightly above and below the transition between the two- and three-dimensional growth mode. In-concentration profiles were obtained from high-resolution TEM images by composition evaluation by lattice fringe analysis. The measured profiles can be well described applying the segregation model of Muraki et al. [Appl. Phys. Lett. 61 (1992) 557]. Calculated photoluminescence peak positions on the basis of the measured concentration profiles are in good agreement with the experimental ones. Evaluating experimental In-concentration profiles it is found that the transition from the two-dimensional to the three-dimensional growth mode occurs if the indium content in the In-floating layer exceeds 1.1+/-0.2 monolayers. The measured exponential decrease of the In-concentration within the cap layer on top of the islands reveals that the In-floating layer is not consumed during island formation. The segregation efficiency above the islands is increased compared to the quantum wells which is explained tentatively by strain-dependent lattice-site selection of In. In addition, In0.25Ga0.75As quantum wells were grown at different temperatures between 500 oC and 550 oC. The evaluation of concentration profiles shows that the segregation efficiency increases from R=0.65 to R=0.83., Comment: 16 pages, 6 figures, 1 table, sbmitted in Phys. Rev. B

Reaching and perusing the quantum limit in conventional devices like memory units is the goal of the current research to allow a further increase in device performance while reducing the device size. While lithographic processes are still widely used for device patterning, these techniques result in corrugated surfaces, inducing undesired surface recombination centers. Thus they are not suitable for high quality, high density quasi-zero dimensional systems with lateral dimensions typically below 30 nm. A self-organization of quantum dots is the alternative. The self-organization of colloidal chalcogenide quantum dots (QD) from liquid solutions or in a glass melt is known since the 1930-ies and has lead to applications like optical absorbers and filters, as the average size of the quantum dot ensemble determines the absorption edge. While a very high density and homogeneity of QD sizes can be achieved, the matrix these QDs are embedded in is generally non-conducting and does not allow to build electrically driven devices. In recent years epitaxial techniques have been developed that allow the embedding of QDs in a semiconducting matrix. These QDs are induced by driving forces that are determined e.g. by the interface energy and the lattice mismatch between the QD and the matrix-material, a perfect crystallinity provided. On the example of CdSe islands embedded in ZnSe we shall discuss the outstanding physical properties of such quasi zero dimensional island structures but also the difficulties in their fabrication.

The morphology and Cd distribution in CdSe/ZnSe quantum dot structures grown by molecular beam epitaxy is compared before and after the ZnSe cap layer deposition. Atomic force microscopy is applied to study the surface topography of uncapped samples without removing the samples from the ultra-high vacuum. The capped structures are analyzed by transmission electron microscopy. It is observed that only a highly specific growth procedure involving a thermal activation step leads to a Stranski-Krastanow morphology with three-dimensional CdSe islands on a thin wetting layer which are preserved after the overgrowth with ZnSe.

GaAs and InGaAs (311) surfaces may be spontaneously corrugated with a height and a period controlled by the film composition, strain and polarity of the substrate. Using image-processed high-resolution transmission electron microscopy we found that both GaAs - AlAs interfaces in short-period superlattices (SPSL) grown on (311)A GaAs substrates are corrugated with a height of 1 nm and a lateral periodicity of 3.2 nm. The same lateral periodicity is also revealed for SPSL grown on (311)B surfaces, but the corrugation height and the degree of order are strongly reduced in this case. A strong optical anisotropy (up to 60%) is found in photoluminescence (PL) spectra for SPSLs grown on (311)A surface and not for (311)B-grown SPSLs. We observed a strong mixing between (Gamma) and X states in the conduction band for the SPSLs grown on (311)A surface which allowed realization of bright PL at room temperature at 650 nm. (311)B and (100)GaAs-AlAs SPSLs grown side by side demonstrated only weak long-wavelength PL due to disorder- induced states. (311)A SPSLs demonstrate also a slow carrier relaxation with characteristic LO-phonon scattering times in excess of 10 ps. Corrugated SLs are particularly advantageous for polarization stabilized surface emitting lasers, bright-red AlGaAs lasers and far infrared emitters and detectors.

Transmission electron microscopy (TEM) was applied to study In segregation during molecular beam epitaxy (MBE) growth of InAs-based quantum-dot (QD) structures with a focus on achieving QD luminescence at the technologically relevant wavelength of 1.3 μm. Quantitative composition analyses were carried out with the composition evaluation by lattice fringe analysis (CELFA) technique and In-segregation efficiencies were determined on the basis of atomic-scale composition profiles. The effect of the InAs-deposition rate on the In distribution was studied in detail which needs to be low, in the 0.005 ML/s regime to achieve long-wavelength emission. Further minimization of In-segregation induced intermixing can be achieved by lowering the substrate temperature during GaAs cap layer growth. Finally, a procedure is presented to extract the real composition of QDs taking into account their three-dimensional morphology and embedding in a GaAs cap layer.

InAs quantum dot (QD) layers grown by molecular-beam epitaxy were investigated by transmission electron microscopy (TEM) and photoluminescence (PL) spectroscopy. To achieve the highest possible In concentration in the QDs, InGaAs (instead of GaAs) cap layers with different In concentrations were deposited after the growth of the InAs QD layer. We combine different TEM techniques to determine the shape, size, and composition of the QDs. By applying a post-processing procedure, we are able to reconstruct the In concentration in the QDs which is measured too low in TEM due to the embedding of the QDs in material with lower In concentration and averaging along the finite TEM sample thickness. The determination of the composition of the layers on an atomic scale shows that the In concentration in the QDs increases in growth direction and reaches values up to 90%. Redistribution of indium during the InGaAs cap layer growth leads to a decrease of the In concentration in the cap layer with respect to the nominal In concentration. The observed redshift of the PL peak with increasing In concentration in the cap layer is attributed to the enlargement of island size and the change of the strain in the QD layers.

The influence of the conditions during growth of InAs/GaAs quantum-dot structures on GaAs(001) by molecular-beam epitaxy was investigated systematically with respect to achieving quantum-dot photoluminescence in the 1 eV range. The growth temperature, As flux, growth rate, InAs deposit, and growth interruption time before cap layer growth were varied. Photoluminescence spectroscopy and transmission electron microscopy were used to study the optical and structural properties. Large InAs quantum dots with photoluminescence in the 1 eV range were obtained at a low growth rate of 0.0056 ML/s. Analyzing in particular the low-growth-rate regime, we found that an InAs deposition of at least 2.4 ML and a growth temperature of 500-510 °C were crucial to obtain large quantum dots with a high size uniformity. Composition analyses by transmission electron microscopy revealed a significantly higher In concentration in the quantum dots grown at low growth rate compared to high-growth -rate samples.

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...

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.

Two well distinguishable classes of nanoscale islands were identified in CdSe/ZnSe quantum dot structures by optical spectroscopy and transmission electron microscopy. For 2.1 to 3.1 monolayer CdSe deposition, coherent three-dimensional (3D) islands, formed in the Stranski–Krastanow (SK) mode, are found with typical diameters of ∼16 nm and a coverage-dependent density of up to 3×1010 cm−2. Simultaneously, small islands with lateral extensions below 10 nm and a density of ∼5×1011 cm−2 are formed by strain-modified island growth. Whereas the 3D SK islands dominate the emission properties at room temperature, the latter smaller islands determine the optical properties at temperatures below 120 K.

Stacking-fault pairs in ZnSe/ZnMgSSe quantum-well structures are found to induce enhanced radiative recombination visible as pairs of bright spots in microphotoluminescence intensity maps. Structural investigation by atomic-force microscopy and transmission electron microscopy (plan view as well as cross section) reveal that a widening and bending of quantum wells occurs when they are intersected by Frank-type stacking faults. The enlargement of the well width by up to 12 bilayers evokes an efficient localization of excitons. The localizing potential related to Shockley-type stacking-fault pairs is found to be much shallower.