A key piece of information in the understanding of quantum dot behaviour is the composition of the dot after any capping and/or annealing processes. It is important to know the composition and uniformity of the dots after they have been embedded in a semiconducting matrix, which usually contains one or more of the elements in the dot. This is a classically difficult analytical problem for any TEM technique. This problem has been approached by developing a model of the interaction between the electron probe and any dot/wettinglayer/matrix configuration. This model predicts the shape of an analytical line scan for any particular configuration and includes the effect of beam broadening on the analysis of dots at different depths within the thin section. The results are compared with STEM analyses of InAs dots in a GaAs matrix.

3-D computer simulations were carried out to investigate the effects of surface energy anisotropy and initial surface roughness on the self-organization of 3-D islands in epitaxial thin film systems. Under the present scheme, the driving force for surface diffusion is the gradient of surface chemical potential, which includes strain energy and surface energy. The simulations demonstrate that the typical sequence of surface evolution is from an initial random surface to a rippled surface, then to irregular islands and then to self-organized islands. Whether the final islands are regular or not depends on both the form of surface energy anisotropy and initial surface roughness. However, the stability and shape transition of the epitaxially strained islands are associated with the form of surface energy anisotropy. The comparisons between the simulations and various experiments were also discussed.

The contribution is mainly focused on the formation and properties of InGaAs islands (quantum dots, QD) in a GaAs matrix, viz. a system showing properties similar to those of other materials systems. Depending on the growth techniques applied (MBE or MOCVD), the islands/dots differ in size, shape, chemical composition, and lattice strain. These parameters influence strongly the optical properties of QD structures, which might consist of single layers or complex 3-dimensional arrays. To reveal the correlation between the morphology/structure and the optical behavior of such systems different analytical methods are available, more effective in some combined applications. The present paper is mainly focused on the possibilities and limitations of transmission electron microscopy (TEM) to analyze the lattice structure of QDs down to the nm-range. Such TEM investigations imply a general problem: to deconvolute separately information on the QD size, shape (e.g., pyramidal or spherical islands), strain and composition. TEM imaging techniques used for such structural analyses will be described, including conventional diffraction contrast and high-resolution electron microscopy, respective examples of which will be presented. For the chemical analysis energy sensitive techniques (e.g., energy filtered images) are appropriate methods. Applications and limitations of such investigations will be discussed.

Options are discussed which allow to distinguish equilibrium arrays of strained islands from kinetically-controlled arrays. 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 small islands and results in the shrinkage of the islands with increasing temperature. The average size of islands decreases tremendously with respect to zero-temperature size at temperatures far below the characteristic energy of island formation. Such a temperature dependence can be the basis for decisive experiments aimed to distinguish between thermodynamic and kinetic effects on the formation of arrays of 2D islands. The theory is able to explain results of high-resolution electron microscopy of submonolayer arrays of InAs/GaAs(001) islands. Their formation is predominantly influenced by thermodynamics.

Up to 1011 cm−2 InAs quantum dots (QD) can be grown on Silicon(001) by molecular beam epitaxy. This very new material system is on the one hand interesting with regard to the integration of optoelectronics with silicon technology on the other hand it offers new insight into the formation of QDs. We report on RHEED, TEM and Raman studies about (in-) coherence of the QDs and on an according to our knowledge so far unknown dewetting transition in this material system. The results are being discussed on the basis of a thermodynamic model, assuming a liquid-like behavior of a strained adlayer.

We report InP self-assembled quantum dots embedded in In0.51Al0.49P grown by metalorganic chemical vapor deposition. Growth parameters are altered to study the InP quantum-dot growth characteristics under various growth conditions. Quantum-dot morphology is characterized using atomic-force microscopy. Also, photoluminescence studies of the light-emitting properties are performed. Direct-bandgap ternary InxAlI−xP (x=˜0.7, ˜0.85) self-assembled quantum dots are also grown and compared with InP quantum dots.

At 650°C, Si freely intermixes with Ge in the dome islands causing a reduction in the strain of the islands and an increase in island size. The shape reversal of Ge/Si domes to pyramids is investigated by analysis of the strain and size changes that occur on an island by island basis. This was carried out for anneal times of 0, 20, 40 and 60 minutes. Transition islands were observed consistent with previous work[1], which are partially domes and partially pyramids. These islands demonstrated a strain gradient, having a slightly lower strain on the side that has transformed to a pyramid. Cross-sectional STEM was then used to show that this strain gradient is associated with a non-uniform Si intermixing in the islands.

Under certain growth conditions InAs/Si heteroepitaxial growth proceeds via Stranski-Krastanow or Volmer-Weber growth modes depending on the growth parameters. Critical thickness at which three dimensional InAs islands start to appear at the Si(100) surface is within the range of 0.7 - 4.0 monolayers (substrate temperature range is 350°C - 430 °C). Their size depends critically on the growth conditions and is between 5 nm and 80 nm (uncapped islands). Critical lateral size of the coherent (Si capped) dislocation-free island is equal to 2 - 5 nm depending on the island height. Islands having larger size are dislocated. Optical properties of InAs nanoscale islands capped with Si reveal a luminescence band in the 1.3 μm region.

InAs quantum dots (QD) have been shown to extend the optical emission range for III-V heterostructures pseudomorphically grown on GaAs substrates. One of the ways to achieve 1.3 micron emission from the InAs QDs is to overgrow them with an In-containing alloy layer. In the present work we study structural and optical properties of InAs/In(Ga,Al)As QD structures grown by MBE on GaAs substrates using transmission electron microscopy (TEM), photoluminescence (PL), and electroluminescence (EL). We show that the main reason for the increase in the PL wavelength is the phase separation of the In(Ga,Al)As alloy stimulated by the InAs islands and the effective matrix bandgap plays a minor role. As the result of the local composition modulation, the effective volume of a QD is increased due to the enhancement of In concentration in the vicinity of the InAs island. We study how the effective thickness of InAs and InGaAs affect the crystal perfection and PL peak energy. Using the AlGaAs as a matrix also led to the noticeable red shift of the quantum dot PL line as compared to the GaAs matrix. Possible reasons for this unexpected phenomenon are discussed. Characteristics of the GaAs-based vertical cavity surface emitting structures and high power CW RT edge-emitting lasers for the 1.3 micron range using these QDs are be presented.

We have investigated the molecular beam epitaxy (MBE) growth of buried InAs quantum dots on GaAs (001) surface, as a function of deposition temperature, by transmission electron microscopy (TEM) and photoluminescence (PL). We found that the dot growth at low temperature is controlled by diffusion-limited aggregation. As the growth temperature increases, this growth mode is modified by misfit strain, resulting in a narrowing of the size distribution. However, we did not find any evidence for a thermodynamically optimized size. In addition, we found that the optical properties of the quantum dots does not correlate directly with the geometric size of the quantum dots, indicating a complex internal structure for quantum dots grown at high growth temperature.

We propose a novel site-control technique for strained quantum dots (QDs) based on nano-lithography using an STM integrated into a UHV STM/MBE multi-chamber system. A nano-scale deposit was formed on a GaAs surface by applying voltage between the GaAs surface and the tungsten tip of the STM. Since the deposit acted as a nano-mask, the subsequent GaAs growth formed a nano-hole just above the deposit. Subsequent InAs supply produced a QD on the hole site, and no QD was observed in any undesirable regions. We also observed the QD formation processes involved in the technique, based on step-by-step STM observations of the QD formation process. The observation directly revealed an InAs wetting layer formation with 1-ML thickness on the GaAs terraces followed by the QD formation in the Stranski-Krastanow growth mode.

The experimental aspects of rapid-quench scanning tunneling microscopy are discussed. In particular, the effects of sample quenching are investigated in atomic-scale studies of the molecular beam epitaxial growth of GaAs. Implications for the study of the heteroepitaxial system InAs-GaAs are discussed.

The study of the morphology, lateral and vertical correlation of self-organized PbSe quantum dots in Pb1−xEux Te is presented. The samples consist of quantum dot superlattices of n periods of (PbSe/Pb1−xEux Te) grown on PbTe/(111) BaF2 by the S-K mode using MBE. The Pb1−xEux Te spacer thickness was varied from 34.3nm to 312nm and the growth temperature was varied between 335°C and 380°C. Our TEM results show very good lateral and vertical correlation of the dots. The dots form an array with either a/b/c/a/b/c/... or a/a/a/... vertical stacking sequence along the [111] growth direction. The stacking sequence is accurately controlled by the thickness of the Pb1−xEux Te spacer layer thickness and the growth temperature. The best a/b/c/... spatial correlation was obtained for temperatures around 380°C and spacer thickness of ˜40nm. The dots are highly strained and form triangular pyramids with (111) bases and (100) facets as observed by AFM. The shape of the dots also varied with the Pb1−xEux Te spacer thickness. An analysis of the spatial correlation of the dots' size and shape with respect to spacer thickness and growth temperature is presented.

We have found a shape transformation of InGaAs quantum dots formed via a fractional monolayer deposition technique on GaAs (001) surfaces. This is evidenced by the bimodal quantum dot height (peaked at 8.5 nm and 14.5 nm) and aspect ratio (peaked at 0.18 and 0.26) distributions. The lateral size, height, and aspect ratio all become convergent, suggesting a simultaneous quantum dot size equalization and shape stabilization. Photoluminescence peaks red shift as a consequence of dot growth, and their line-widths become smaller due to dot shape stabilization and size equalization. A record low inhomogeneous broadening of 18.4 meV at a wavelength of 1180 nm (4 K) is obtained for vertically-aligned, shape-stabilized, and size-equalized InGaAs dots.

Photoreflectance measurements have been performed in the 0.8–1.5 eV photon energy range and at temperatures from 80 to 300 K on stacked layers of InAs/GaAs self-assembled quantum dots (QDs) grown by Atomic-Layer Molecular Beam Epitaxy. The spectral features due to the QD optical response were analyzed by using lineshape models characteristic of modulation spectroscopy of confined systems. The dependence of the ground state transition energy on the number of stacked QD layers is investigated and it is shown that Coulomb interaction can account for the observed different behavior of the ensemble optical response of QD families characterized by different morphologies and coexisting in the same sample.

Two simple and effective far-field-optics-based methods capable of isolating photoluminescence peaks of different individual self assembled quantum dots (SAQD's) with nanometer scale precision are presented. By using these methods, we performed the temperature and electric field dependent studies on the optical properties of SAQD's. We found temperature induced inter-dot carrier transfer among neighboring quantum dots (QD's) and observed the quantum confined stark effect (QCSE).

Nanoscale TiSi2 islands are formed by electon beam deposition of a few monolayers of titanium followed by in situ annealing at high temperatures (800–1000°C). The typical island sizes were ˜10 nm. Electrical characteristics of these islands were probed using UHV-STM. I-V spectroscopies on these islands show single electron tunneling effects such as Coulomb blockade and Coulomb staircase at room temperature.

The strain distribution inside and in the vicinity of coherently strained self-organized islands has been investigated by high-resolution x-ray diffraction (HRXRD). Finite element method (FEM) calculations were carried out in order to calculate the strain field, which was then used to simulate x-ray reciprocal space maps on the basis of kinematical scattering theory. For Si0 75Ge0.25 islands an abrupt increase in the Ge-concentration at about one third of the island height has been found. This behavior can be attributed to different nucleation stages during growth. Highly strained buried CdSe quantum dots (QDs) strongly influence the surrounding ZnSe matrix. From reciprocal space maps and FEM simulations we were able to estimate the shape and size of the islands. The results are in agreement with transmission electron microscopy (TEM) and UHV atomic force microscopy (AFM) data.

Novel strain-relief mechanisms for Ge/Si(100) coherent islands were identified. Ge/Si(100) islands were grown at temperatures, T, between 450 and 650°C for effective Ge coverages between 3.5 and 14.0 monolayers. The mean dome size increased and island dislocation was delayed as T increased. For T > 600°C, very large hut clusters populated a peak in island size distributions distinct from and between those of huts and domes. For T > 550°C, large dome clusters may form trenches at their base which extend well into the Si substrate. Increasing T reduced the island size for trench formation. Cross-sectional energy dispersive xray nanoanalysis confirmed that Si diffuses into the Ge clusters at T ≥ 550°C. Si interdiffusion was responsible for the increase in dome size and delay of dislocation with increasing T and the existence of very large hut clusters. AFM cross-sections indicate that there is a linear trench depth dependence on island size. A simple atomistic elastic model suggests that this experimentally observed self-limiting trench depth is kinetic rather than energetic in origin.

Using liquid phase epitaxy from Bi solution, a by its nature a near equilibrium growth process, we study the kinetics of island formation in the heteroepitaxial system SiGe/Si(001) as dependent on growth temperature, growth rate and composition (which also determines the lattice misfit between layer and substrate). As a main result island formation can be described by classical nucleation theory, moreover, it can be described as any other crystallization process such as solid state crystallization of amorphous silicon or crystallization from a melt, provided that the limited size the islands can grow into is correctly considered. In consequence, after an incubation time period that depends on the growth temperature, islands nucleate and cover the substrate surface with time. The activation energy of island nucleation is 0.84±0.13eV. The coverage with islands depends only on the undercooling and is independent of the cooling rate in case near equilibrium growth conditions are maintained. In these cases the islands have the shape of truncated pyramids with four {111}– side facets and a base width λ that only depends on the misfit f (λ ∝ 1/f2). Deviations from the equilibrium growth stage at high growth rates (thus higher growth driving forces) result in the formation of a higher density of smaller islands with smaller facet angles. At higher growth rates, some kinetic influences begin to appear indicated by the additional appearance of shallower pyramids with four {115}– facet side faces.