STM investigation of site-controlled SiGe nanoislands

Self-assembly of three-dimensional nanoislands by strained-layer heteroepitaxy has become an important tool for direct synthesis of semiconductor nanostructures, which are of great interest not only for their novel physical properties but also for electronic and opto-electronic device applications. Their synthesis is based on the fundamental morphological instability of strained layers on lattice-mismatched substrates, which leads to the spontaneous formation of self-assembled nanoislands on the surface once a certain critical layer thickness is exceeded. When embedded in a higher band gap matrix material, quantum dots are formed with atomic-like electronic and optical properties. Advanced applications of quantum dots not only require a high degree of uniformity, but also the ability to laterally position the dots precisely on large surface areas in order to be able to contact and address them on an individual basis within complex device architectures. In this research project, ultra-high vacuum scanning tunneling microscopy will be used to investigate and advance the techniqe of lateral positioning of self-assembled SiGe nanoisland on Si wafers by using site-controlled island nucleation on prepatterned non-planar substrate templates. The work will encompass the fabrication of different types on nanopatterned template structures using holographic as well as electron beam lithography and reactive ion etching for pattern transfer. Subsequently, in situ scanning tunneling microscopy will be used for detailed studies of the island deposition process performed using molecular beam epitaxy. The major goal is to elucidate the fundamental mechanisms and structural transformation involved in the site- controlled deposition process and to determine how this process is influenced and controlled by the growth conditions, the geometry and dimensions of the template patterns as well as by the cleaning procedures and buffer layer growth. In addition, we will extend these studies to the case of Si nanoisland growth on prepatterned SiGe buffer layers for which the sign of the misfit strain is reversed and for which a novel growth behavior is expected. To assess our approach for actual device fabrication, selected samples will be studied by x-ray diffraction, transmission electron microscopy as well as luminescence spectroscopy.

In this project, site-controlled growth of silicon-germanium nanoislands on lithographically prepatterned silicon substrate templates was developed and studied using high resolution scanning tunnelling microscopy. Such nanoislands form self-assembled quantum dots in which the electrons are confined to a nanometer-sized region in space. Due to quantization effects, quantum dots exhibit atomic-like electronic and optical properties and provide unique opportunities for realization of novel electronic and opto-electronic devices for a wide range of applications. The synthesis of quantum dots is usually based on heteroepitaxial growth of thin layers on substrates with different atom spacing in the crystal lattice. The resulting lattice-mismatch strain induces the spontaneous formation of nano-sized surface islands once a certain critical layer thickness is exceeded. Practical applications of quantum dots, however, require not only a high degree of size uniformity, but also the ability to precisely position quantum dots at specific locations on large area substrate surface. This is crucial to be able to address them individually within complex device architectures. For positioning of self-assembled quantum dots, in this project epitaxial growth on lithographically pre-patterned silicon substrate templates was employed. The silicon-germanium material system is of great practical importance, because more than 90% of all microelectronic devices are made by silicon technology. The patterned silicon substrate templates were fabricated using electron-beam or holographic lithography and reactive ion etching, and the patterns consisted of structures with feature sizes ranging from 50 to 300 nanometers. Subsequently, silicon buffers layer were deposited, followed by a few atomic layers of germanium, during which surface nanoislands are formed. The experimental studies show that the surface morphology developed during buffer growth decisively influences the position of nanoislands formation on the patterned surfaces. This is due to spontaneous faceting of the non-planar substrate surfaces. By microscopy investigations the involved mechanisms and shape transitions could be clarified. Thus, through optimization of the growth conditions, a site-controlled deposition of germanium nanoislands was achieved.