Growth and characterization of self-organized and 'organized' Si and Si1-XGeX nanostructures

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 parameters such as the growth rate and temperature, miscut angle and orientation, layer thickness, surface preparation, etc.. The morphologies comprise a large variety in structures and show very good long-range ordering over large scales. The temperature dependence of the morphology highlights the importance of kinetics for the structural evolution of the surface at temperatures below 500 oC. Smooth homoepitaxial Si layers can be obtained by growing at elevated temperatures or by an annealing step after growth. Thermodynamic strain-induced step bunching effects in single, strained SiGe layers on vicinal Si(001) that were quoted in literature were not found. Instead, annealing experiments showed that the square-base hut cluster morphology is the thermodynamically stable state of these surfaces. Evidence suggests that the strain in the layers plays no role in the oblique correlation of roughness found with X-ray methods in Si/SiGe superlattices. Approaches are presented 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. Additionally, local epitaxial techniques have been explored for their potential to produce 'organized' self-assembled nanometer scale structures of high crystalline quality on pre-patterned substrates. The experiments revealed that significant mass transport occurs on the Si(001) surface already some 500 K below the melting point leading to a smoothing of the surface. (author)