Beschreibung: Moritz Brehm
"SiGe Island Growth: New Growth Phenomena in the Stranski-Krastanow Nucleation Mode and their Influence on the Optoelectronic Island Properties "
Silicon-Germanium (SiGe) based nanostructures can be seen as a link between the Si based integrated technology and the emerging field of Nanotechnology. At the same time the SiGe system has established itself as a kind of model system for strain induced growth of quantum dots formed via the Stranski-Krastanow growth mode and, thus, has attracted great interest in the scientific community over the last twenty years. In the Stranski-Krastanow system initially a two-dimensional strained wetting layer is formed on the substrate surface. After the wetting layer has exceeded a critical thickness this strain is relieved in the form of three-dimensional objects, usually called quantum dots or islands. During the course of this thesis such quantum dots, islands and quantum wells were grown by means of molecular beam epitaxy (MBE) and characterized by atomic force microscopy (AFM) and photoluminescence spectroscopy (PL).
The first Chapters of this thesis contain a description of fundamental properties of the SiGe system, e.g. the dependence of the Germanium (Ge) content and the strain in the nanos- tructure on the band structure. Furthermore a short description of the formation process of SiGe islands is presented.
Hereafter, the experimental setups, i.e. the MBE system, AFM and PL are described. The main focus in this Chapter is devoted to details of the experiments that were established during this work.
By utilizing geometrical properties of the MBE growth chamber it was possible to produce four inch SiGe samples on which the epilayer thickness varies from one side of the wafer to the other. By measuring the island morphology (by AFM) and optical properties (by PL) of the islands and the wetting layer along the Ge thickness gradient of four inch Silicon (Si) wafers it was possible to shed more light into the Stranski-Krastanow nucleation dynamics. By measuring the PL transition energy of the electron-hole pairs in the wetting layer as a function of the wetting layer thickness and comparing the results to kp band structure calculations the Ge composition profiles in growth direction could be determined for different capping layer growth temperatures of 300°C, 500°C and 700°C.
In contrast to what was reported in the literature, not small pyramidal islands are the first ones to form on a wetting layer with overcritical thickness at elevated Ge growth temperatures of more than 670° C, but large, multi-faceted dome islands. This phenomenon was explained by combining AFM and PL experiments with ab-initio and finite element calculations that were performed by the group of Leo Miglio in Milano, Italy.
The influence of the Si capping layer growth temperature on the islands and the wetting layer was studied. This is important since the capping layer is absolutely necessary for any kind of optical and electrical applications in order to avoid unwanted surface recombination of the excited electron-hole pairs.
Furthermore a new type of island shapes, named "cupola" islands, formed at elevated growth temperature of 900C was found by performing AFM measurements samples glued on wedge-formed, tilted sample holders. Those cupola islands exhibit extremely steep side facets at the base of the islands with inclination angles of up to 68° with respect to the Si(001) substrate. Such cupola islands are formed under elevated Ge growth temperature of about 900°C.
By performing power dependent PL measurements on quantum dot samples and com- paring the results to band structure calculations performed with the nextnano3 code, it was shown that even in islands with large lateral and vertical island dimensions (˜ 100 nm and ˜20 nm, respectively) significant quantum confinement can exist. This enhanced quantum confinement is a result of the inhomogeneous Ge composition gradient within the dots. Ad- ditionally it is shown that this effect of enhanced quantum confinement is more pronounced in islands grown on flat Si(001) substrates as compared to islands grown on pit-patterned Si substrates.
By tuning the Ge composition gradient during the growth of the structures it is possible to shift the location of the hole wave functions spatially within the islands. This seems to be a promising way to employ new recombination paths of the electron-hole pairs in SiGe islands.