Directed Self-Assembly of Ge Nanostructures on Very High Index, Highly Anisotropic Si(hkl) Surfaces NANO LETTERS

ABSTRACT Families of very high-index planes, such as those which bifurcate spontaneously to form a hill-and-valley structure composed of opposing facets, provide natural templates for the directed growth of position-controlled self-organized nanostructures with shapes determined by the facet width ratio R. For example, deposition of a few ML of Ge on Si(173 100 373), corresponding to R(113/517) ) 1.7, results in a field of 40-nm-wide Ge nanowires along [72 187 84] with a uniform period of 60 nm. The controlled fabrication of nanostructures and surface nanopatterns is a crucial step underlying all fields of nanotechnology. For example, nanoscale architectures on Si surfaces are presently of intense interest due to their compatibility with conventional microelectronic device processing, and hence, their potential for future generations of novel electronic devices. Current strategies to achieve uniform nanostructural arrays include both top-down and bottom-up approaches. In the former case, nanostructure fabrication is generally accomplished through conventional optical lithography techniques in which feature sizes are limited by the radiation wavelength, 1 while bottom-up methodologies have been utilized primarily in self-assembly techniques such as film growth in the strain-induced Stranski-Krastanov (SK) mode. A major bottleneck in both tactics, however, is the lack of control in one or more of the following: size, shape, position, and/or areal density distributions. Ge growth on Si(001) has been studied extensively as a model SK system due to a combination of the relatively large lattice constant mismatch, 4.2%, as well as compatibility of Ge with Si device processing. A variety of Ge nanostructures including pyramids (huts), domes, and elongated islands are obtained as a function of temperature, growth rate, and film thickness. 2-4 Some progress in controlling the density, size, and spatial uniformity of these structures has been achieved using stacked multilayer growth, 5 ion implantation, 6 and lithographically-assisted deposition