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Results for the development and detailed analysis of self-organized InAs/InAlGaAs/InP quantum dots suitable for single-photon emission at the $1.55\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{m}$ telecom wavelength are reported. The structural and compositional properties of the system are obtained from high-resolution scanning transmission electron microscopy of individual quantum dots. The system is composed of almost pure InAs quantum dots embedded in quaternary InAlGaAs barrier material, which is lattice matched to the InP substrate. When using the measured results for a representative quantum-dot geometry as well as experimentally reconstructed alloy concentrations, a combination of strain-field and electronic-state calculations is able to reproduce the quantum-dot emission wavelength in agreement with the experimentally determined photoluminescence spectrum. The inhomogeneous broadening of the latter can be related to calculated variations of the emission wavelength for the experimentally deduced In-concentration fluctuations and size variations.

The preadsorption of Ag on Si(111) drastically changes the growth of Ge. In a temperature range from $400{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$ to $650{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$, Ag adsorption on Si leads to the formation of a $\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}$-R ${30}^{\ensuremath{\circ}}$ reconstruction that exhibits a maze-like morphology on the mesoscopic scale, as observed by low-energy electron diffraction (LEED) and low-energy electron microscopy. This maze morphology can be attributed to a surface roughening on an atomic scale, induced by the re-arrangement of top layer atoms during the $7\ifmmode\times\else\texttimes\fi{}7$ to $\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}$-R ${30}^{\ensuremath{\circ}}$ transition. The subsequent deposition of Ge results in the formation of a wetting layer, the evolution of which has been found to be governed by the Ag/Si(111)-$\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}$-R ${30}^{\ensuremath{\circ}}$ template's maze structure, as the latter offers a high density of heterogeneous nucleation sites. Upon further Ge growth, three-dimensional islands with diameters in the micrometer range are formed, which exhibit a large and flat (111) top facet. X-ray photoemission electron microscopy reveals that during Ge growth, Ag is segregating to the surface very efficiently. Grazing-incidence x-ray diffraction and transmission electron microscopy have been used to study the composition, strain state and defect structure of the Ge islands in dependence of the growth temperature. The strain induced by lattice mismatch is found to be largely relaxed (80--90% relaxation) in the investigated growth temperature range from 400 to $600{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$, which is confirmed by high-resolution LEED measurements. As a main relaxation mechanism, the formation of interfacial misfit dislocations has been identified. Interdiffusion of Si into the Ge islands becomes more and more pronounced for increasing growth temperature, whereas the formation of twinned Ge regions can drastically be suppressed at higher temperature.

A transmission electron microscopy (TEM) study of the structure and chemical composition of 0.5 to 3.0 ML (monolayer) CdSe sheets that are buried in a ZnSe matrix is presented. The CdSe layers were grown by migration-enhanced epitaxy at a growth temperature of 280 \ifmmode^\circ\else\textdegree\fi{}C. We find two-dimensional (2D) ${\mathrm{Cd}}_{x}{\mathrm{Zn}}_{1\ensuremath{-}x}\mathrm{Se}$ layers with a total thickness of approximately 3 nm for all samples independent of the nominal CdSe content that contain inclusions (islands) with an enlarged Cd concentration. Plan-view TEM revealed two types of islands: First, small 2D islands with a lateral size of less than 10 nm, and second, large 2D islands with a lateral size between 30 and 130 nm. The combination of two-beam dark-field imaging and the new composition evaluation by lattice fringe analysis (CELFA) procedure allow the precise measurement of the Cd-concentration profiles of the ${\mathrm{Cd}}_{x}{\mathrm{Zn}}_{1\ensuremath{-}x}\mathrm{Se}$ layers. The CELFA evaluation yields a full width at half maximum value of $(10\ifmmode\pm\else\textpm\fi{}1)\mathrm{ML}.$ The most probable origin of the broadening is a strong interdiffusion of Cd and Zn with an additional contribution of the segregation of the Cd atoms. The diffusion length of the Cd diffusion in ZnSe during the growth of the ZnSe cap layer is ${L}_{D}=(3.6\ifmmode\pm\else\textpm\fi{}0.8)\mathrm{ML}$ and the segregation probability is estimated to be $R=(0.6\ifmmode\pm\else\textpm\fi{}0.2).$ It is shown that neither objective lens aberrations nor specimen tilt are the main sources for the observed enormous broadening of the CdSe interlayers.

The character of Cd distribution and the morphology of CdSe layers with nominal thicknesses between 0.7 and 3.6 ML in a ZnSe matrix were studied by conventional transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) using plan-view and cross-section TEM samples. The Cd distribution was determined on an atomic scale by two different techniques. The first method is based on the measurement of local lattice parameters from zone-axis HRTEM images. The second technique relies on the evaluation of Fourier amplitudes derived from off-axis lattice fringe images. Continuous ${\mathrm{Cd}}_{x}{\mathrm{Zn}}_{1\ensuremath{-}x}\mathrm{Se}$ layers are observed, which are significantly broadened compared to the nominal thicknesses. The layers contain Cd-rich inclusions (small islands) with a size of less than 10 nm, and regions with a lower Cd concentration. With increasing nominal CdSe thickness, the Cd concentration and the island density increase. In addition, larger islands with a density two orders of magnitude below the small island density and a shape asymmetry are found in the 3.6-ML CdSe layer. The results are discussed with respect to the impact of the averaging effect caused by the finite TEM foil thickness on the measured Cd content in the Cd-rich islands and the surrounding region.

We report a detailed structural and chemical study of buried and free-standing ${\mathrm{In}}_{0.6}{\mathrm{Ga}}_{0.4}\mathrm{As}$ Stranski-Krastanow islands. The layers were grown by molecular-beam epitaxy on GaAs(001) substrates. We investigated two different types of samples with nominal ${\mathrm{In}}_{0.6}{\mathrm{Ga}}_{0.4}\mathrm{As}$ layer thicknesses of 1.5 and 2 nm. The growth was interrupted for 0, 60, or 180 s prior to the deposition of the 10-nm-thick GaAs cap layer. The chemical and structural analyses of the ${\mathrm{In}}_{0.6}{\mathrm{Ga}}_{0.4}\mathrm{As}$ layers were carried out with high-resolution transmission electron microscopy. The chemical morphology of the buried layers was evaluated with the composition evaluation by lattice-fringe analysis (CELFA) method. The free-standing islands were investigated by strain state analysis combined with finite element calculations. The density and size distribution of the islands was obtained by conventional plan-view transmission electron microscopy. We found two types of islands: Coherent islands with a lateral size of approximately 13 nm and large islands (40\char21{}100 nm) showing plastical strain relaxation. The density of the defect-free small islands decreases with increasing duration of the growth interruption whereas the density and size of the large islands increases. A detailed study of the wetting layer with the CELFA method revealed about a 4-nm-thick ${\mathrm{In}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$ layer. The total amount of In contained in the wetting layer decreases with increasing duration of the growth interruption. Composition profiles in growth direction were measured. Their shape is explained by mainly three effects: Segregation of In, incorporation of migrating In into the growing cap layer, and strain-driven migration of In and Ga. An inhomogeneous In concentration increasing from bottom to top is observed in free-standing islands.

The structural properties of BeTe/ZnSe short-period superlattices grown by molecular-beam epitaxy on (001)-oriented GaAs substrates were investigated. Different growth modes were used which influence the morphology and chemical transition at the interfaces. The BeTe/ZnSe superlattices were examined by optical microscopy, conventional and high-resolution transmission electron microscopy, x-ray diffractometry, and atomic force microscopy, with a particular emphasis on the defect generation mechanisms and the effects of different bond configurations at the interfaces. The critical thicknesses largely exceed the theoretical values for the plastic relaxation by misfit dislocations. The mismatch is relaxed by cracks preferentially oriented along one particular 〈110〉 direction under tensile stress conditions. In order to quantify the abruptness of the chemical transition at the interfaces and to determine the layer thicknesses accurately, high-resolution transmission electron micrographs were evaluated by correspondence analysis.

A transmission electron microscopy (TEM) method for simultaneous measurement of indium and nitrogen content in InGaNAs at atomic scale is introduced, tested, and applied to investigate thermal annealing effects on structural properties. Our technique is based on the extraction of strain and chemical sensitive contrast from a single three-beam TEM lattice fringe image by subsequent decomposition into 220 and 020 two-beam fringe images, being free of nonlinear imaging artifacts. From comparison with simulated strain and 020 fringe amplitude, concentration maps and profiles are derived. For this purpose, the Bloch-wave approach is used with structure factors adapted for chemical bonding, static atomic displacements, as well as diffuse losses due to static and thermal disorder. Application to In0.28Ga0.72N0.025As0.975 before and after annealing at 670 ◦C yields dissolution of In-rich islands and N-rich clusters and formation of a quantum well with nearly constant thickness and homogeneous elemental distributions, resulting in symmetric profiles along growth direction. To verify that these structural transitions are indeed correlated with typically observed changes of optical properties during thermal annealing, photoluminescence spectra are presented, revealing an increase in intensity by a factor of 20 and a strong blue shift of 60 meV.

InGaN quantum dots were grown by metal-organic vapor phase epitaxy using a phase-separation process based on spinodal and binodal decomposition. Uncapped structures were grown which show InGaN phases with two different In contents on the surface. The high-In-content phase accumulates to huge islands while the low-In content phase forms flat meander and quantum-dot-like structures on the surface. The dissolution of the high-In-containing phase is very sensitive to the growth temperature of the GaN capping while there is no significant change of the InGaN quantum dot structures. The samples were investigated with transmission and scanning electron microscopy, photoluminescence measurements, and x-ray diffraction. A narrow growth window concerning the InxGa1−xN composition was found in which the formation of quantum dots takes place. This growth window is in good agreement with the InGaN miscibility gap calculated for a strained InGaN layer. A detailed theoretical discussion is presented and the quantum dot and island formation will be explained by a strain-modified spinodal and binodal decomposition model.

Surface science studies of thin praseodymium oxide films grown on silicon substrates are of high interest in view of applications in such different fields as microelectronics and heterogeneous catalysis. In particular, a detailed characterization of the growth and the final structure of the films are mandatory to achieve a fundamental understanding of such topics as oxygen mobility and defect structure, and their role for the electronic and chemical properties. In this paper, the MBE growth of praseodymium oxide films on Si(111) substrates was investigated at low-deposition rates (0.06 nm/min) and low-oxygen partial pressures (p(O-2)

Electron holography in a transmission electron microscope was applied to measure the phase shift $\ensuremath{\Delta}\ensuremath{\varphi}$ induced by Au clusters as a function of the cluster size. Large $\ensuremath{\Delta}\ensuremath{\varphi}$ observed for small Au clusters cannot be described by the well-known equation $\ensuremath{\Delta}\ensuremath{\varphi}={C}_{E}{V}_{0}t$ (${C}_{E}$, interaction constant; ${V}_{0}$, mean inner Coulomb potential (MIP) of bulk gold; and $t$, cluster thickness). The rapid increase of the Au MIP with decreasing cluster size derived from $\ensuremath{\Delta}\ensuremath{\varphi}$ can be explained by the compressive strain of surface atoms in the cluster.

The vertical and lateral ordering of stacked CdSe quantum dot layers embedded in $\mathrm{Zn}{\mathrm{S}}_{x}{\mathrm{Se}}_{1\ensuremath{-}x}$ has been investigated by means of grazing incidence small angle x-ray scattering and transmission electron microscopy. Different growth parameters have been varied in order to elucidate the mechanisms leading to quantum dot correlation. From the results obtained for different numbers of quantum dot layers, we conclude on a self-organized process which leads to increasing ordering for progressive stacking. The dependence on the spacer layer thickness indicates that strain induced by lattice mismatch drives the ordering process, which starts to break down for too thick spacer layers in a thickness range from $45\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}80\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$. Typical quantum dot distances in a range from about $110\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}160\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$ have been found. A pronounced anisotropy of the quantum dot correlation has been observed, with the strongest ordering along the $[1\overline{1}0]$ direction. Since an increased ordering is found with increasing growth temperature, the formation of stacking faults as an additional mechanism for quantum dot alignment can be ruled out.

We report on transmission electron microscopy (TEM) and photoluminescence (PL) spectroscopy measurement of mass transport and segregation in InAs Stranski-Krastanow layers grown on GaAs(001) by molecular beam epitaxy at growth temperatures of 480 and 530 °C. Plan-view TEM reveals regularly shaped islands with a density of 7.8×10 1 0 cm - 2 (480 °C) and 1.5×10 1 0 cm - 2 (530 °C), respectively. Uncapped islands were investigated by strain state analysis of electron wave functions reconstructed from high-resolution TEM images. In-concentration profiles of the islands were obtained by the measurement of lattice-parameter profiles of the islands and the application of finite-element calculations. We find that the islands contain Ga-atoms with a percentage of 50% (480 °C) and 67% (530 °C). The capped InAs-layers were investigated with PL and TEM. In agreement with TEM, PL indicates a smaller and deeper potential well of the islands grown at 480 °C. Concentration profiles of the wetting layers were measured with TEM using the composition evaluation of lattice fringe images method, clearly revealing segregation profiles. The obtained segregation efficiency of In-atoms is 0.77′0.02 (480 °C) and 0.82′0.02 (530 °C). As an explanation for the strong mass transport of Ga from the substrate to the islands we show that the segregation of In atoms during the growth of the binary InAs can lead to the generation of vacancies in the metal sublattice. The vacancies are filled by Ga-atoms migrating along the surface or by a diffusion of the vacancies from the wetting layer and the islands into the GaAs buffer, leading to a unidirectional diffusion of Ga atoms from the buffer into the Stranski-Krastanow layer.