| Abstract |
The scope of the present dissertation is to simulate two different types of nanostructures, namely quantum islands and quantum nanocrystals. The former nanodots are formed during heteroepitaxial growth. We will be focused in the formation of Ge islands on Si(100) substrate. The islands, which will be studied, might have the shape of a pyramid or of a dome. Ge islands formed on a Si(100) surface, which is precovered with a small amount of C, are a special and important class of Si-based nanostructures. This type will also be studied. The latter nanostructures are embedded nanocrystals in a matrix, usually amorphous. These quantum dots are formed by various techniques, such as implantation and laser ablation. A lot of interest exhibits the case of Si nanocrystals in a-SiO2. In the first chapter a historical overview and the main aspects of each type of nanostructure is presented.
The structure, composition and energetics of such nanostructures are studied. In order to do this, Monte Carlo simulations within the empirical potential approach utilizing two different potentials were used. Also, optoelectronic properties of the embedded nanocrystals are extracted with Density Functional Theory (DFT) within the Local Density Approximation (LDA) and Generalized Gradient Approximation (GGA). A few details concerning the methods and the simulational aspects are included in the second chapter.
The bare Ge dots, either pyramids or domes, formed on oSi(100) are firstly investigated. The stress and the composition profiles of thse islands are calculated and compared. Also, from the comparison of those properties in the alloyed and non-alloyed dots many issues, concerning diffusion and intermixing, are interpreted. Volume exchange events and stress-driven intermixing need to be considered for the interpretation of experimental results.
In the next section Carbon-induced Ge dots on Si(100) are studied. Since C atoms affect very much the resulting structures and properties, it is important to know how they are distributed in the surface, or even if they occupy cites in and below the islands. It is found that the dots do not contain C, and that they have a gradual composition profile from SiGe at the bottom to bare Ge at the apex. Also, the effects of the Ge and C coverage are subjects which are investigated.
The case of the embedded Si nanocrystals in a-SiO2 matrix is explored in the next section. The interface structure and its energetics are studied as a function of the nanocrystal size. It is found that the low energy geometries at the interface are Si-O-Si bridge bonds. The reduction of their fraction as the size becomes smaller and the substantial deformation in small nanocrystals, give us the opportunity to give an alternative explanation for the reduction of the optical gap in this size regime.
The size and the interface of such structures are found to play vital role in the photoluminescence. So, it is important to know if they have spherical of faceted shape. In order to examine this, seven planar interfaces of Si/SiO2 with different crystal orientations for the Si substrate are constructed. Minimizing the surface energy of a volume with the calculated energies, a nanocrystal with 42 facets, mainly (100), (110) and (121) orientations, is found as the optimum shape. A comparison between a faceted and a spherical embedded nanocrystal revealed that the former might exist under some thermodynamic conditions.
Finally, the reduction of the band gap, with respect to the quantum confinement model, of the embedded Si nanocrystals is verified from our simulations. Also, besides the Si-O-Si bridge bonds and Si=O double bonds, which are believed to pin the gap of the oxidized nanostructures, we found that distortions are also responsible for the behavior of these nanocrystals. A factor which has poorly been considered in literature.
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Theoretical investigation of the properties of nanostructured semiconducting systems with Monte Carlo simulations and first principles calculations
