Niedrigdimensionale Halbleiterstrukturen

Through this project several PhD theses, devoted to the growth and the characterization of semiconductor nanostructures were supported at the University of Linz. This project has helped to foster collaborative research in this area in Austria. The investigations were aimed at the controlled growth of thin semiconductor layers with different composition (semiconductor heterostructures) using epitaxial techniques. In such structures, the motion of the electrons is restricted to a plane, i.e. to two dimensions. This leads to completely modified properties with respect to bulk materials. Using epitaxial growth, apart from layers also tiny islands with dimensions of the order of 10 nanometers can be deposited. In such islands the carrier motion is restricted in all three dimensions of the space. This leads e.g. to completely new optical properties which makes such islands suitable for lasers in the mid- infrared. We achieved a very homogeneous and ordered growth of such islands. Their structural properties with respect to their chemical composition as well as their strain status were investigated using a novel x-ray technique (nanotomography). Controlled epitaxial growth allows for the deposition of perfectly single-crystalline layers, which are only few nanometer thick on semiconductor substrates. Such layers have electrical, optical and magnetic properties which do not only depend on their chemical composition but also on their layer thicknesses. Thus their properties can be modified at will. In our project such layers, consisting of Si/SiGe, as well as layers of PbTe/PbSe/PbEuTe. were grown and characterized. It turned out that apart from the layer thicknesses also their strain status influences drastically their properties. In order to determine this strain novel methods were developed which are based on the use of x-ray of high intensity as offered by synchrotron sources. Furthermore specific electrical and optical properties of the heterostructures were investigated as a function of their composition. It turned out that the optical properties of Si/SiGe multilayers in the mid-infrared region of the electromagnetic spectrum make them quite useful for sensitive radiation detection. In heterostructures which contain EuTe, magnetic ordering was observed below a certain temperature. The transition temperature depends both on the EuTe layer thickness as well as on the strain status of the EuTe layers. This behaviour was explained with simulations based on theoretical considerations. Both in Si/SiGe as well as in PbSe/PbEuTe controlled growth conditions led to the realization of islands with nanometer dimensions. In multilayers the vertical and lateral positions of such islands could even be correlated to each other, which opens interesting possibilities for addressing individual quantum dots.