Clark Ritz

Mechano-electronic Superlattices in Silicon Nanoribbons

Significant new mechanical and electronic phenomena can arise in single-crystal semiconductors when their thickness reaches nanometer dimensions, where the two surfaces of the crystal are physically close enough to each other that what happens at one surface influences what happens at the other. We show experimentally that, in silicon nanomembranes, through-membrane elastic interactions cause the double-sided ordering of epitaxially grown nanostressors that locally and periodically highly strains the membrane, leading to a strain lattice. Because strain influences band structure, we create a periodic band gap modulation, up to 20% of the band gap, effectively an electronic superlattice. Our calculations demonstrate that discrete minibands can form in the potential wells of an electronic superlattice generated by Ge nanostressors on a sufficiently thin Si(001) nanomembrane at the temperature of 77 K. We predict that it is possible to observe discrete minibands in Si nanoribbons at room temperature if nanostressors of a different material are grown.

Ordering of nanostressors on free-standing silicon nanomembranes and nanoribbons

Epitaxial growth of self-assembled quantum dots (QDs) on single- crystal nanomembranes yields organized arrays of QDs via a growth mode mediated by QD-induced strains in the membrane. A crucial aspect of this effect arises because epitaxial growth on thin Si sheets and nanostructures derived from them can occur simultaneously on two surfaces separated only by the 10-nm-scale thickness of the membrane. A QD on one surface of a free-standing membrane causes the nucleation of QDs in specific positions on the opposite surface. Control experiments using molecular beam epitaxy to deposit QDs on a single surface do not yield long-range order. Through-membrane elastic interactions consistent with predictions from finite-element-based mechanics models are observed using synchrotron x-ray microdiffraction. The role of crystallographic anisotropy is evident in finite-element predictions of the strains that bias the nucleation events. The scaling of the dot spacing with membrane thickness is consistent with the spacing of nucleation sites predicted using the mechanical model.

Integrated freestanding single-crystal silicon nanowires: conductivity and surface treatment

Integrated freestanding single-crystal silicon nanowires with typical dimension of 100 nm × 100 nm × 5 µm are fabricated by conventional 1:1 optical lithography and wet chemical silicon etching. The fabrication procedure can lead to wafer-scale integration of silicon nanowires in arrays. The measured electrical transport characteristics of the silicon nanowires covered with/without SiO(2) support a model of Fermi level pinning near the conduction band. The I-V curves of the nanowires reveal a current carrier polarity reversal depending on Si-SiO(2) and Si-H bonds on the nanowire surfaces.

Structure of elastically strain-sharing silicon(110) nanomembranes

Nanomembranes composed of single-crystal, tensilely strained Si(110) and compressively strained SiGe(110) layers have been fabricated from silicon-on-insulator (SOI) substrates. Elastic strain sharing is demonstrated for a trilayer structure consisting of a 12 nm Si/80 nm Si0.91Ge0.09 film epitaxially grown on a 12 nm thick (110) oriented Si template layer that is subsequently released from its handle substrate. X-ray diffraction on the as-grown and released structures confirms a virtually dislocation-free membrane with a tensile strain of 0.23±0.02% in the Si(110) layers after release. Lower growth temperatures in molecular beam epitaxy allow for smoother growth fronts than are possible using chemical vapour deposition.