Minghuang Huang

Semiconductor Nanomembrane Tubes: Three-Dimensional Confinement for Controlled Neurite Outgrowth

In many neural culture studies, neurite migration on a flat, open surface does not reflect the three-dimensional (3D) microenvironment in vivo. With that in mind, we fabricated arrays of semiconductor tubes using strained silicon (Si) and germanium (Ge) nanomembranes and employed them as a cell culture substrate for primary cortical neurons. Our experiments show that the SiGe substrate and the tube fabrication process are biologically viable for neuron cells. We also observe that neurons are attracted by the tube topography, even in the absence of adhesion factors, and can be guided to pass through the tubes during outgrowth. Coupled with selective seeding of individual neurons close to the tube opening, growth within a tube can be limited to a single axon. Furthermore, the tube feature resembles the natural myelin, both physically and electrically, and it is possible to control the tube diameter to be close to that of an axon, providing a confined 3D contact with the axon membrane and potentially insulating it from the extracellular solution.

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.

Straining nanomembranes via highly mismatched heteroepitaxial growth: InAs islands on compliant Si substrates.

Freestanding, edge-supported silicon nanomembranes are defined by selective underetching of patterned silicon-on-insulator substrates. The membranes are afterward introduced into a molecular beam epitaxy chamber and overgrown with InAs, resulting in the formation of InAs islands on flat areas and at the top of the Si nanomembranes. A detailed analysis of sample morphology, island structure, and strain is carried out. Scanning electron microscopy shows that the membrane stays intact during overgrowth. Atomic force microscopy reveals a lower island density on top of the freestanding membranes, denoting a modified wetting or diffusivity in these areas. An observed bending of the membrane indicates a strain transfer from the InAs islands to the compliant substrate. X-ray diffraction and finite-element modeling indicate a nonuniform strain state of the island ensemble grown on the freestanding membrane. A simulation of the bending of the nanomembranes indicates that the islands at the center of the freestanding area are highly strained, whereas islands on the border tend to be fully relaxed. Finally, continuum elasticity calculations suggest that for a sufficiently thin membrane InAs could transfer enough strain to the membrane to allow coherent epitaxial growth, something not possible on bulk substrates.

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.

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

Local-Wetting-Induced Deformation of Rolled-Up Si/Si-Ge Nanomembranes: A Potential Route for Remote Chemical Sensing

We fabricate curled 3-D objects from semiconductor nanomembranes consisting of single-crystal silicon, which is epitaxially grown on silicon-germanium-on-insulator substrates. The curling is caused by relaxing the strain induced by lattice mismatch between silicon (Si) and germanium (Ge). Depending on the lithographically patterned geometries and their orientation with respect to the crystallographic direction, different shapes of tubes can be realized. Particularly interesting are tubes that are not completely closed, or partially open, whose mechanical response is ultraelastic. We demonstrate that applying acetone on such tubes generates a surface stress imbalance between the Si and Si-Ge layers, resulting in detectable shape changes. This mechanism has potential applications in chemical sensing, where the deformable curled structures act as dynamic-aperture reflector antennas. Our simulation suggests the curvature changes induced in the presence of certain chemical, such as acetone, will lead to distinctive far-field radiation patterns in the terahertz (THz) range.

Pattern Formation on Silicon-on-Insulator

The strain driven self-assembly of faceted Ge nanocrystals during epitaxy on Si(001) to form quantum dots (QDs) is by now well known. We have also recently provided an understanding of the thermodynamic driving force for directed assembly of QDs on bulk Si (extendable to other QD systems) based on local chemical potential and curvature of the surface. Silicon-on-insulator (SOI) produces unique new phenomena. The essential thermodynamic instability of the very thin crystalline layer (called the template layer) resting on an oxide can cause this layer, under appropriate conditions, to dewet, agglomerate, and self-organize into an array of Si nanocrystals. Using low-energy electron microscopy (LEEM), we observe this process and, with the help of first-principles total-energy calculations, we provide a quantitative understanding of this pattern formation. The Si nanocrystal pattern formation can be controlled by lithographic patterning of the SOI prior to the dewetting process. The resulting patterns of electrically isolated Si nanocrystals can in turn be used as a template for growth of nanostructures, such as carbon nanotubes (CNTs). Finally we show that this growth may be controlled by the flow dynamics of the feed gas across the substrate.

Mechanical stability of ultrathin Ge∕Si film on SiO2: The effect of Si∕SiO2 interface

We perform two-dimensional linear elastic finite element analysis to investigate the mechanical stability of ultrathin Ge∕Si film grown on or bonded to SiO2, using imperfect interface elements between Si and SiO2 to model Si∕SiO2 interfacial slippage. We demonstrate that the overall composite film is stable when only the tangential slippage is allowed, however, it becomes unstable when normal slippage is allowed: the coherently strained Ge island induces a large local bending of Si layer, and separates the Si layer from the underlying SiO2 forming a void at the Si∕SiO2 interface.

Group IV nanomembranes and nanoepitaxy: new properties via local and global strain engineering

Semiconductor nanomembranes, single-crystal sheets as thin as ten nanometers, offer many opportunities for novel devices and new science. The most interesting involve epitaxy to introduce strain at both local and global levels. Coming into play are membrane thinness, access to both sides of a sheet, transferability, and enhanced compliancy. Advances in Group IV optoelectronics, thermoelectrics, and photonics may be achievable by combining epitaxy with Si and Ge nanomembranes. Nanoepitaxy allows formation of new strained materials, periodic strain lattices, and mix and match membranes with hybrid orientations or compositions.