Shelley A. Scott

Elastically strain-sharing nanomembranes: flexible and transferable strained silicon and silicon?germanium alloys

The emerging field of strained-Si based nanomembranes is reviewed, including fabrication techniques, strain-induced band structure engineering, electronic applications and three-dimensional membrane architectures. Elastic strain sharing between thin heteroepitaxial Si and SiGe films, enabled by techniques that allow release of these films from a handling substrate, creates a new material: freestanding, single-crystal, strained nanomembranes. These flexible nanomembranes are virtually dislocation-free and have many potential new applications. Strain engineering also provides opportunities for massively parallel self-assembly of a wide variety of three-dimensional nanostructures.

Influence of surface chemical modification on charge transport properties in ultrathin silicon membranes.

Ultrathin silicon-on-insulator, composed of a crystalline sheet of silicon bounded by native oxide and a buried oxide layer, is extremely resistive because of charge trapping at the interfaces between the sheet of silicon and the oxide. After chemical modification of the top surface with hydrofluoric acid (HF), the sheet resistance drops to values below what is expected based on bulk doping alone. We explain this behavior in terms of surface-induced band structure changes combined with the effective isolation from bulk properties created by crystal thinness.

Templated-assembly of conducting antimony cluster wires

Wires with meso- and nanoscale widths have been fabricated using a novel assembly technique based on the deposition of Sb clusters on templated surfaces. The template elements are V-grooves etched into the surface of a Si wafer. It is demonstrated that the clusters bounce or slide to the apex of the V-grooves, without significant fragmentation, and that this motion is the underlying mechanism behind the formation of the wires. The flow rate of inert gas into the cluster growth chamber controls the average velocity of the clusters and the morphology and width of the wires. Sb cluster-assembled wires with lengths over 150 µ ma nd widths down to 100 nm have been assembled from ∼40 nm diameter Sb clusters. Electrical contacts to the wire sh av eb een achieved via lithographic alignment of NiCr/Au contacts to the V-grooves prior to deposition.

Fractal electronic devices: simulation and implementation

Many natural structures have fractal geometries that exhibit useful functional properties. These properties, which exploit the recurrence of patterns at increasingly small scales, are often desirable in applications and, consequently, fractal geometry is increasingly employed in diverse technologies ranging from radio antennae to storm barriers. In this paper, we explore the application of fractal geometry to electrical devices. First, we lay the foundations for the implementation of fractal devices by considering diffusion-limited aggregation (DLA) of atomic clusters. Under appropriate growth conditions, atomic clusters of various elements form fractal patterns driven by DLA. We perform a fractal analysis of both simulated and physical devices to determine their spatial scaling properties and demonstrate their potential as fractal circuit elements. Finally, we simulate conduction through idealized and DLA fractal devices and show that their fractal scaling properties generate novel, nonlinear conduction properties in response to depletion by electrostatic gates.

Probing the electronic structure at semiconductor surfaces using charge transport in nanomembranes.

The electrical properties of nanostructures are extremely sensitive to their surface condition. In very thin two-dimensional crystalline-semiconductor sheets, termed nanomembranes, the influence of the bulk is diminished, and the electrical conductance becomes exquisitely responsive to the structure of the surface and the type and density of defects there. Its understanding therefore requires a precise knowledge of the surface condition. Here we report measurements, using nanomembranes, that demonstrate direct charge transport through the π* band of the clean reconstructed Si(001) surface. We determine the charge carrier mobility in this band. These measurements, performed in ultra-high vacuum to create a truly clean surface, lay the foundation for a quantitative understanding of the role of extended or localized surface states, created by surface structure, defects or adsorbed atoms/molecules, in modifying charge transport through semiconductor nanostructures.

Symmetry in Strain Engineering of Nanomembranes: Making New Strained Materials

Strain in a material changes the lattice constant and thereby creates a material with new properties relative to the unstrained, but chemically identical, material. The ability to alter the strain (its magnitude, direction, extent, periodicity, symmetry, and nature) allows tunability of these new properties. A recent development, crystalline nanomembranes, offers a powerful platform for using and tuning strain to create materials that have unique properties, not achievable in bulk materials or with conventional processes. Nanomembranes, because of their thinness, enable elastic strain sharing, a process that introduces large amounts of strain and unique strain distributions in single-crystal materials, without exposing the material to the formation of extended defects. We provide here prescriptions for making new strained materials using crystal symmetry as the driver: we calculate the strain distributions in flat nanomembranes for two-fold and four-fold elastically symmetric materials. We show that we can controllably tune the amount of strain and the asymmetry of the strain distribution in elastically isotropic and anisotropic materials uniformly over large areas. We perform the experimental demonstration with a trilayer Si(110)/Si((1-x))Ge(x)(110)/Si(110) nanomembrane: an elastically two-fold symmetric system in which we can transfer strain that is biaxially isotropic. We are thus able to make uniformly strained materials that cannot be made any other way.

Influence of surface properties on the electrical conductivity of silicon nanomembranes

Because of the large surface-to-volume ratio, the conductivity of semiconductor nanostructures is very sensitive to surface chemical and structural conditions. Two surface modifications, vacuum hydrogenation (VH) and hydrofluoric acid (HF) cleaning, of silicon nanomembranes (SiNMs) that nominally have the same effect, the hydrogen termination of the surface, are compared. The sheet resistance of the SiNMs, measured by the van der Pauw method, shows that HF etching produces at least an order of magnitude larger drop in sheet resistance than that caused by VH treatment, relative to the very high sheet resistance of samples terminated with native oxide. Re-oxidation rates after these treatments also differ. X-ray photoelectron spectroscopy measurements are consistent with the electrical-conductivity results. We pinpoint the likely cause of the differences.PACS: 73.63.-b, 62.23.Kn, 73.40.Ty

Formation of electrically conducting mesoscale wires through self-assembly of atomic clusters

Bi and Sb clusters deposited from an inert gas aggregation source have been used to form cluster-assembled wires on unpassivated, and SiO/sub 2/ passivated, V-grooved Si substrates. V-grooves (4-7 /spl mu/m in width, 6 /spl mu/m-1 mm in length) were prepared using optical lithography and anisotropic etching in KOH solution. The effectiveness of the surface templating technique was demonstrated by scanning electron microscope analysis carried out after deposition. When Sb clusters were deposited onto SiO/sub 2/ passivated substrates, the surface coverage was seen to vary from <20% on the unpatterned (normal-to-beam) surface (which is required to be nonconducting) to >100% at the apexes of the V-grooves used to promote growth of the wire. Sb wires produced with this technique currently have minimum widths of /spl sim/400 nm and lengths of /spl sim/1 mm. Electrical contacts can be positioned within the V-grooves prior to cluster deposition, thus enabling the initial onset of conduction and subsequent I(V) characteristic of a wire to be monitored in vacuum.

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.

Silicon Nanomembranes Incorporating Mixed Crystal Orientations

The desire for increased processor speed leads to a demand for high-carrier-mobility CMOS devices. The complementary nature of CMOS, with both n-type and p-type channels, means that the lowest-mobility channel will limit the device speed. In the conventional (001) orientation of Si, the hole mobility is dramatically less than the electron mobility (1), (2), and hence serves as a bottleneck in CMOS performance. To compensate for the lower hole mobility, it is customary to fabricate the p-type device regions 3-10x larger than the n-type regions, consuming an undesirably large quantity of device real estate. The current drive imbalance between n-type and p-type channels can be minimized, thus negating the need for disproportionately large p-type regions, by fabricating mixed regions of Si(110) (high hole mobility) and Si(001) (high electron mobility) on a single substrate; so-called hybrid-orientation technology (HOT) (1). We fabricate a mixed-crystal-orientation material in flexible membrane form, using Si nanomembrane (SiNM) transfer and overgrowth, to produce a “quilt” of Si(001) and Si(110).