Kinetic and Strain-Induced Self-Organization of SiGe ...

PREFACE iii
Abstract
A kinetic growth instability of silicon-germanium molecular beam epitaxy (SiGe-MBE) was
used to generate periodic Si-ripple-templates. By increasing the substrate miscut to 4 ◦ the
ripple period could be tuned to smaller periods towards the nanometer scale. At Si growth
rates of 0.2 ˚A/s a ripple pattern with few defects develops within a small temperature window
around 425 ◦ C. For our vicinal Si(001) substrates with 4 ◦ miscut along [110] this step-bunching
instability provides undulations of about 100 nm periodicity and 4 nm amplitude.
In the first part of this work several aspects of self-organized growth in the Si/SiGe system
could be addressed. The special morphology of step-bunching was used to investigate the
interplay between kinetics, surface energy and strain. The step-bunching templates provide
a one-dimensional pattern with preferable nucleation sites for SiGe-islands along the ripple
flanks, and thus allow us to combine kinetic and strain-driven self-organization phenomena
in the Si/SiGe heterosystem. At moderate temperatures around 425 ◦ C strain-driven ridges,
which decorate the ripple flanks perpendicular to the step bunches, were observed. The
whole morphology is only made up with (001)-surfaces and {105}-facets. These {105}-faceted
ridges appear to be energetically preferable for geometrical reasons at slopes, which are close
to {1 1 10}-planes. These structures show a fair degree of ordering entirely based on self-
organization. Furthermore, such morphological features are the basis for understanding the
mechanism of Ge-dot nucleation on 2D pit-patterned Si-templates.
In the second part magnetotransport measurements on p-modulation doped Si/SiGe het-
erostructures grown on top of a step-bunched Si-buffer are discussed. The short-scale peri-
odic height fluctuations of the Si-buffer form well-defined undulations in the SiGe-channel.
This should increase scattering in the remotely p-doped quantum well (p-MODQW). Thus
an asymmetry in mobility perpendicular and parallel to the undulations is expected, which
might help to uncouple the different scattering mechanisms. These, namely alloy scattering
and interface-roughness related scattering, are conversely discussed as predominant hole-
mobility limiting factors for p-modulation doped structures. Although still at the beginning,
the first measurements confirm a decreased low-temperature mobility across the undulations
by a remarkable factor of two. Further measurements combined with additional modeling
are expected to provide a new approach toward settling the long-lasting dispute on the hole-
mobility-limiting scattering mechanisms in p-MODQW structures.