We have investigated the extent to which the emission wavelength of self-assembled InAs/GaAs quantum dots can be controlled by growth parameters using conventional solid source MBE. Changing from conventionally high growth rates to a very low growth rate (LGR) and a relatively high substrate temperature, tunes the photoluminescence (PL) emission from 1.1 μm to 1.3 μm at room temperature. Atomic force micrographs obtained from uncapped samples reveal that these LGRQDs are larger, lower in density and extremely uniform in size. The improved size uniformity is reflected in the reduction of the PL linewidth from 78 meV to 22 meV. Under conditions of high excitation, emission from the ground and two excited states each separated by ∼70 meV is observed. This implies a parabolic confining potential. Time resolved photoluminescence (TRPL) measurements of dots grown under the various growth conditions yield radiative lifetimes which reflect the depth of the confining potential. A comparison of the decay times measured for the excited states show that the relaxation of carriers within the dots cannot be ascribed to phonon effects.

We have investigated the emission properties of InAs/GaAs self-assembled quantum dots in the mid-infrared. The emission relies on the intraband transitions in the valence band of the quantum dots. We first show that third-harmonic generation can be observed. The frequency tripling efficiency is enhanced by the resonances between the pump excitation and the quantum dot intraband transitions. A giant third-order nonlinear susceptibility is measured for one dot plane. The narrow spectral dependence of the nonlinear susceptibility is well explained by simulations which account for the three-dimensional confinement potential. We secondly show that the mid-infrared spontaneous emission between hole confined states can be observed under an interband optical pumping. The spontaneous emission involves transitions between either the ground and excited states or between excited states. These measurements demonstrate the potentiality of self-assembled quantum dots for mid-infrared emission.

Semiconducting iron silicide dots with dimensions ranging between 5 and 100 nm can be obtained by ion implantation on Si wafers and exhibit interesting photo- and electro- luminescent properties.

In our study we use structural and optical characterization as well as theoretical modelling in order to: i) discriminate among intrinsic effects of FeSi2 dots and effects due to lattice damage and Si matrix; ii) identify the range of physical parameters (size, phase, electronic structure) corresponding to the luminescent dots.

We have grown chains of crystalline-Si nanospheres: the crystalline-Si nanospheres of about 10 nm in diameter are supported in amorphous silica and carbon at a nearly equal spacing by a self-organized process. The self-organized phenomenon was attributed to the periodic instability of catalyst on the top of growing wire and oxidization during the growth of nanowires.

Quantum dot structures containing 2 and 7 layers of small coherent InAs clusters embedded into a Si single crystal matrix were grown by MBE. The structure of these clusters was investigated by high resolution transmission electron microscopy. The crystallographic quality of the structure severely depends on the substrate temperature, growth sequence, and the geometrical parameters of the sample. The investigation demonstrates that Si can incorporate a limited volume of InAs in a form of small coherent clusters about 3 nm in diameter. If the deposited InAs layer exceeds a critical thickness, large dislocated InAs precipitates are formed during Si overgrowth accumulating the excess of InAs.

We have applied Atomic Number Contrast Scanning Transmission Electron Microscopy (Z-Contrast STEM) and STEM/EELS (Electron Energy Loss Spectroscopy) towards the study of colloidal CdSe semiconductor nanocrystals embedded in MEH-PPV polymer films. Z-Contrast images are direct projections of the atomic structure. Hence they can be interpreted without the need for sophisticated image simulation and the image intensity is a direct measure of the thickness of a nanocrystal. Our thickness measurements are in agreement with the predicted faceted shape of these nanocrystals.

Our unique 1.3Å resolution STEM has successfully resolved the sublattice structure of these CdSe nanocrystals. In [010] projection (the polar axis in the image plane) we can distinguish Se atom columns from Cd columns.

EELS measurements on individual nanocrystals indicate a significant amount (equivalent to 0.5–1 surface monolayers) of oxygen on the nanocrystals, despite processing in an inert atmosphere. Spatially resolved measurements at 7Å resolution suggest a surface oxide layer.

It is shown that. under appropriate conditions, high-Ge-concentration coherent 3D SiGe islands grown on Si(100) self-embed in a matrix of low-Ge-concentration alloy. The process may be more generally useful for preserving the shape of self-assembled “quantum dot” islands during embedding in a matrix material.

We report on our efforts to selectively place chemical beam epitaxy grown Stranski-Krastanov InAs dots on GaAs patterns. The pattern profiles for placement depends on the growth conditions of a GaAs buffer layer grown on the lithographic patterns. Because we seek to suppress dot formation on (100)-oriented surfaces outside of the features, we investigated the effects buffer layer growth conditions have on dot nucleation using reflected high energy electron diffraction, atomic force microscopy and photo-luminescence. We conclude that buffer conditions favorable for patterns have negligible effect on the dots formed on the (100)- oriented surface, and that selective dot placement can be engineered by As pressure, InAs deposition and buffer growth conditions.

The growth mode of CdSe layers grown by migration enhanced epitaxy between ZnSe barriers has been investigated. In situ reflection high-energy electron diffraction shows a gradual transition to a three-dimensional growth mode which, however, is not accompanied by a change of the surface lattice constant. High-resolution transmission electron micrographs reveal a strong Cd diffusion, leading to ternary ZnCdSe quantum wells. Furthermore. composition fluctuations perpendicular to the growth direction on a nanometer scale are found already prior to the beginning of the growth mode transition. In the case of heterostructures containing a CdSe layer that has undergone the growth mode transition, micrographs show Cd-rich quantum dots with diameters of around 8 nm and heights of around 1.5 nm within a ternary quantum well. By spatially resolved photoluminescence the emission from single quantum dots could be observed. The polarization dependence of the emission from single dots indicates an asymmetric shape of the dots with certain preferential orientations along the [110] and [110] directions.

We have used in-situ scanning tunneling microscopy (STM) to study the morphological changes to InAs quantum dots (QD's) caused by overgrowth of a GaAs capping layer. We demonstrate that the InAs islands are highly unstable during GaAs overgrowth and that this morphological instability confirms that significant cation intermixing occurs during overgrowth. In particular, the anisotropic nature of the volume redistribution indicates that In surface diffusion is the specific mechanism of In transfer away from the islands. Images taken after 10 monolayers or more of overgrowth reveal that more rapid GaAs overgrowth better preserves the 3D-islands morphology.

Standard rate equation models of island formation in the InAs/GaAs(001) system have been reassessed in terms of new experimental evidence from real time in-situ reflectance anisotropy spectroscopy (RAS) measurements. These measurements have revealed the behaviour and role of the wetting layer in the modified Stranski-Krastanov growth mode during molecular beam epitaxial growth showing that it can continue to significantly increase in thickness following the onset of islanding. The presence of two dimensional (2D) islands, which act as precursors to three dimensional (3D) islands (the quantum dots) in conventional models, does in principle allow an extension of the “wetting layer”. However, it has been found necessary to extend the standard model to include extra terms that allow material to be incorporated into (and detach from) the wetting layer and which cannot convert to 3D islands. With this improved model, it is found possible to achieve agreement with the RAS measurements.

The present work is concentrated on the investigation of the initial stages of growth of Ge nanoparticles embedded in an amorphous A1203 matrix on suitable substrates. The growth technique is based on self-organization processes related to the balance of the interface energies involved. Combined data from complementary optical techniques (absorption, Raman scattering, spectroscopic ellipsometry) give evidence of a behaviour which can be ascribed to the existence of a wetting layer, not detectable by conventional transmission microscopy.

A three dimensional finite element method is used to simulate the kinetic process of island formation in quantum dot superlattices. Depending on the thickness of spacer layers and interruption time, top layer islands can be fully vertically aligned with the same morphology as the buried islands, or can be partially vertically aligned with increasingly less uniform and regular arrangement, or can be alternately misaligned with increasingly uniform and regular arrangement.

Pure Ge epitaxially grown on Si (100) at high temperatures forms typically 100 nm lateral size islands on top of a 3–4 monolayer thick wetting layer. In stacked layers of Ge dots pronounced vertical alignment is observed if the thickness of the Si spacer layers is smaller than about 50 nm. Pregrowth of a small amount of C on Si substrate induces very small 10 nm size Ge quantum dots after deposition of about 2 to 3 monolayers Ge. Photoluminescence (PL) studies indicate a spatially indirect radiative recombination mechanism with the no-phonon line strongly dominating. Strong confinement shift in the 1–2 nm high and 1Onm lateral size dots results in low activation energies of 30 meV, which causes luminescence quenching above 50K.

For large stacked Ge islands with 13 nm thin Si spacer layers we observe a significantly enhanced Ge dot-related PL signal up to room temperature at 1.55μm wave length. This is attributed to a spatially indirect transition between heavy holes confined within the compressively strained Ge dots and two-fold degenerated A state electrons in the tensile strained Si between the Ge stacked dots.