Dr. DI Christoph Schelling
"Growth and characterization of self-organized and 'organized' Si and Si1-xGex nanostructures"
The present work is subdivided in three parts. Part I deals with self-organization growth phenomena of Si homoepitaxial and SiGe heteroepitaxial layers on vicinal Si(001) substrates. Part II reports on efforts to model these growth experiments. Finally, part III treats approaches to "organize" growth of Si/SiGe nanostructures with the use of prestructured substrates.
The morphology of unstrained Si and strained SiGe layers grown with molecular beam epitaxy was investigated with atomic force microscopy and scanning tunneling microscopy. It is found that vicinal Si(001) surfaces are inherently unstable during homoepitaxial growth under kinetically limited growth conditions. The evolving morphology depends on many parameters such as the growth rate and temperature, miscut angle and orientation, layer thickness, surface preparation, etc. Surface roughening is found over a wide range in the multi-dimensional parameter space. The instability is observed on wafers with miscuts ranging from 0.2-4°. Such vicinal substrates are commonly used in commercial manufacturing where epitaxial processes are involved. Typical growth parameters where roughening occurs are: Tgrowth=450-625°C; r=0.2-0.8Å/s; tepi>300Å. These are standard growth parameters for day-to-day Si MBE and corroborate the relevance of this work. The morphologies comprise a large variety in structures, e.g. hillocks, ripples and triangular features, in length scales that range from ~70nm to >1µm and in heights up to 40Å. These surface structures show very good long-range ordering up to large scales and exhibit preferential alignment along <110> crystal directions. The temperature dependence of the morphology highlights the importance of kinetics for the structural evolution of the surface at temperatures below 500°C. Smooth homoepitaxial Si layers can be obtained by growing at elevated temperatures or by an annealing step after growth.
Earlier investigations reported on thermodynamic strain-induced step bunching effects in single, strained SiGe layers on vicinal Si(001). The evidence produced in the present work does not support these observations but rather questions the observability of strain-induced step bunching in this material system. It is found instead that small Ge concentrations (<20%) actually smooth the surface under kinetic growth conditions. For higher Ge contents (40-50%), roughening can be pronounced especially on substrates with a large-angle miscut along <100>. But it became apparent in annealing experiments that the square-base hut cluster morphology is the thermodynamically stable state of these surfaces.
From these growth investigations on single layers, it can be concluded that obviously above ~650°C, kinetic processes become fast enough to result in growth very close to thermodynamic equilibrium. The surface growth morphologies of Si and SiGe layers are then primarily determined by energetic factors rather than by kinetic limitations.
The lack of evidence for strain-induced step bunching also queries the Holý model that makes use of inhomogeneous strain fields to explain the oblique roughness correlation found with x-ray methods in Si/SiGe superlattices. New evidence suggests that the strain in the layers actually plays no role.
Part II deals with approaches to model the step bunching phenomenon using extensions of the Schwoebel model. The models are based on an optical analogy: adsorption, reflection and transmission of adatoms at step edges is considered. Computer simulations revealed that step bunches advance slower than single steps and that growth becomes stabilized for high material interchange over step edges.
In the last part, local epitaxial techniques have been explored for their potential to produce "organized" self-assembled nanometer scale structures of high crystalline quality on prepatterned substrates. The experiments revealed that significant mass transport occurs on the Si(001) surface already some 500K below the melting point leading to a smoothing of the surface. Surface transport also provides a negative undercut in oxide masks that turn them into self-aligned shadow masks. Epitaxial nanometer structures with high crystalline quality can be produced with this method as photoluminescence from these structures proves.