Controlled positioning of self-organized SiGe islands

Semiconductor nanostructures have attracted tremendous interest in the past few years because of their special physical properties, and also because of their potential for applications in optoelectronic devices. In such nanostructures the free carriers are confined to a small region of space by potential barriers, and if the size of this region is less than the electron wavelength, the electronic states become quantized at discrete energy levels. The ultimate limit of low-dimensional structures is the quantum dot, in which the carriers are confined in all three directions. The fabrication of quantum dots is challenging, because the small dimensions required are at the limit of lithographic processing techniques. The spontaneous formation of three-dimensional semiconductor islands during hetero-epitaxial growth has emerged as a novel approach for quantum dot formation. The driving force for this growth mode transition is the elastic relaxation of strain energy. Althrough single quantum dots exhibit extremely sharp atomic-like luminescence properties, a considerable inhomogeneous line broadening in dot ensembles occurs due to non-uniformities in their sizes. In this project we aim to control the lateral positioning of Ge-rich islands on Si and/or embedded in Si in regular two-dimensional arrays, resulting in a small dispersion of the island sizes. In order to achieve this goal we propose to use two-dimensional periodic nucleation centers for the islands, thus forcing their lateral ordering. We will combine the advantages of lithographically defined structures, namely their periodicity, with the absence of defects in self-assembled islands. Through lithography and etching, well defined and ordered nucleation sites can be provided by a suitable patterning of Si substrates or strained SiGe epilayers for the subsequent self-organized growth of Ge-rich nanostructures. Through this approach we hope to avoid both the disadvantages of patterning, i.e., sidewall defects, as well as of self-organized growth, i.e., size fluctuations. We will characterize these islands with respect to their shape, size, and composition, using novel analysis techniques, based on different x-ray diffraction and scattering methods, which will be developed further. In particular, we intend to characterize the changes which occur during Si overgrowth. These investigations will be supplemented by transmission electron microscopy and atomic force microscopy studies. The electronic properties of the periodic arrays of Si-capped Gerich islands will be investigated by photoluminescence. Through the improved size and shape uniformity we expect to achieve much more uniform electronic properties of the islands.

Semiconductor nanostructures like so-called quantum wires and quantum dots have attracted a lot of interest in recent years because of their particular physical properties and their potential for novel electronic and optoelectronic devices. These properties result from quantum confinement phenomena, i.e., the properties of these nanostructures do not only depend on their chemical composition but also on their size and shape and can be tuned accordingly. Quantum dots consist typically of about 10000 to 100 000 atoms, and if embedded in a proper matrix material, can be used both for light generation (lasers) as well as for light detectors. In this project tiny Silicon-Germanium (SiGe) islands with typical dimensions of 100 nm diameter and 10 nm height were grown on single crystalline Silicon (Si) substrates. Usually these islands grow on plain substrates with a random distribution of their nucleation sites. Whereas for applications as detectors a random arrangement of islands is not detrimental, for electronic and signal processing applications a high degree of lateral ordering is mandatory, in order to address certain dots individually. In order to arrange such islands regularly, the Si substrates were prepatterned using lithography and etching techniques, similar to the ones employed for the fabrication of electronic devices in semiconductor industry. A periodic two-dimensional array of pits was produced in the Si surface before Ge was deposited. In the course of this project proper growth conditions were found with which Ge islands could be deposited in the pits, forming a regular tow-dimensional pattern. The structural properties of the islands (size, shape, Ge content, strain) were characterized by x-ray scattering techniques. Even if pure Ge is deposited, due to growth temperatures around 600C, intermixing with the Si substrate occurs and actually the islands consist of SiGe alloys. The Ge content increases from the bottom to the top of the islands. These changes of the composition could be determined by a novel x-ray diffraction technique on a nanometer scale