Semiconductor materials confined in one or more dimensions exhibit special properties due to the quantum confinement effect. When confined in all three dimensions, a quantum dot (QD) is formed. Because of the confinement, energies of the QDs are discrete and depend on the QD size, enabling control of the absorption. Core/shell structures are somewhat more complicated but have several advantages over the regular core-only QDs. The shell could be used as a core passivation, a protective layer that reduces core oxidation and could increase the charge carrier lifetime, depending on band alignment. This dissertation presents the experimental realization of Ge/Si core/shell quantum dots differing by the size of the core and shell and their optical properties. The material consists of Ge/Si core/shell self-assembled quantum dots produced by a simple magnetron sputter deposition of the Ge/Si/Al2O3 multilayer. Such QDs are uniform in size and ordered in a three-dimensional body-centered tetragonal QD-lattice within Al2O3 matrix. Different deposition parameters were used and their influence on the structure and electrical properties were investigated. Optical properties of the Ge/Si core/shell quantum dots are shown to significantly differ from the properties of pure Ge quantum dots. Influence of the matrix type on growth and assembly of QDs was studied. For that purpose, Ge QDs in silicon carbide and nitride were made. Considering that Ge oxidation is a serious problem in thin films with Ge QDs embedded in oxide matrices, we have shown that SiC and Si3N4 represent a good alternative to reduce the oxidation. However, from the studied samples, the ones with a silicon shell around the core are the best solution. After a detailed analysis of the structure, electrical transport in the films was studied. All thin films have shown a significant current increase under illumination, which suggests a potential application in photodetectors.