Mechanical stability of ultrathin Ge/Si film on SiO2: the effect of Si/SiO2 interface

Journal ArticleWe 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

Bending of nanoscale thin Si film induced by growth of Ge islands: hut vs. dome

Journal ArticleWe perform atomistic simulations to compute bending of freestanding nanoscale thin Si film induced by strained Ge islands. We show that a larger Ge dome island can induce smaller bending than a smaller hut island and explain the surprising experimental observation for growth of Ge islands on patterned silicon-on-insulator substrate (SOI) with Si template layer thinned down to nanometer scale. This counterintuitive bending behavior is caused by strain sharing between the film and the ultra thin substrate

Nanostressors and the nanomechanical response of a thin silicon film on an insulator

Journal ArticlePseudomorphic three-dimensional Ge nanocrystals (quantum dots) grown on thin silicon-on-insulator substrates can induce significant bending of the silicon template layer that is local on the nanometer scale. We use molecular dynamics simulations and analytical models to confirm the local bending of the Si template and to show that its magnitude approaches the maximum value for a freestanding membrane. The requisite greatly enhanced viscous flow of SiO2 underneath the Si layer is consistent with the dependence of the viscosity of SiO2 on shear stress

Seeing the atomic orbital: first-principles study of the effect of tip termination on atomic force microscopy

Journal ArticleWe perform extensive first-principles calculations to simulate the topographical atomic-force-microscope image of an adatom on the Si(111)-(7 X 7) surface, demonstrating the feasibility of imaging not only the atoms but also the atomic orbitals. Our comparative study of tip terminations shows that two subatomic features can appear for a single adatom when it is imaged by a Si(001)-type tip having two dangling bonds on its apex, while only one feature would appear if it were imaged by a Si(111)-type tip having one dangling bond on the apex. The key condition for seeing the atomic orbitals is to bring the tip so close to the surface that the angular-dependent force dominates the tip-surface interaction

Bending of nanoscale ultrathin substrates by growth of strained thin films and islands

Journal ArticleMechanical bending is ubiquitous in heteroepitaxial growth of thin films where the strained growing film applies effectively an "external" stress to bend the substrate. Conventionally, when the deposited film is much thinner than the substrate, the bending increases linearly with increasing film thickness following the classical Stoney formula. Here we analyze the bending of ultrathin (nanometer range) substrates induced by growth of coherently strained thin films. The behavior deviates dramatically from the classical linear dependence: when the film thickness becomes comparable to the substrate thickness the bending decreases with increasing film thickness. This complex bending behavior can be understood by considering evolution of strain sharing between the film and substrate. We demonstrate experimentally such counterintuitive bending of a nanoscale thin Si substrate induced by a coherently strained Ge film, in the form of islands, grown on silicon-on-insulator substrate. Larger dome islands, representing a thicker film, induce much less bending of the substrate than smaller hut islands, representing a thinner film, in direct contrast to their behavior on thick Si. We explain these observations by properly considering the island shape and strain relaxation within the island

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 SiO2 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–SiO2 and Si–H bonds on the nanowire surface

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.Keywords: voids (solid), composite materials, slip, elemental semiconductors, island structure, bending, semiconductor thin films, interface structure, silicon compounds, germanium, silicon, finite element analysis, mechanical stabilit

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