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For realization of new informative systems, the memristor working like synapse has drawn much attention. We developed isolated high-density Fe3O4 nanocrystals on Ge nuclei/Si with uniform and high resistive switching performance using low-temperature growth. The Fe3O4 nanocrystals on Ge nuclei had a well-controlled interface (Fe3O4/GeOx/Ge) composed of high-crystallinity Fe3O4 and high-quality GeOx layers. The nanocrystals showed uniform resistive switching characteristics (high switching probability of ~90%) and relatively high Off/On resistance ratio (~58). The high-quality interface enables electric field application to Fe3O4 and GeOx near the interface, which leads to effective positively charged oxygen vacancy movement, resulting in high-performance resistive switching. Furthermore, we successfully observed memory effect in nanocrystals with well-controlled interface. The experimental confirmation of the memory effect existence even in ultrasmall nanocrystals is significant for realizing non-volatile nanocrystal memory leading to neuromorphic devices. [ABSTRACT FROM AUTHOR]

Ge nanodots with a typical size of ∼4 nm and ultrahigh density (>1012 cm-2) were grown on ultrathin SixGe1-x oxide films made by oxidizing Ge wetting layers grown on Si (111)–(7×7) surfaces. The density of the nanodots was independent of the Ge deposition rate, indicating that the chemical reaction between Ge atoms and the oxide films determined the nucleation of Ge nanodots. The size and density of Ge nanodots grown on the ultrathin SixGe1-x oxide films depended on the growth temperature and deposition amount. These dependences indicate that we can form the Ge nanodots with ultrahigh density controllably by using ultrathin SixGe1-x oxide films. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The design and fabrication of nanostructured materials to control both thermal and electrical properties are demonstrated for high-performance thermoelectric conversion. We have focused on silicon (Si) because it is an environmentally friendly and ubiquitous element. High bulk thermal conductivity of Si limits its potential as a thermoelectric material. The thermal conductivity of Si has been reduced by introducing grains, or wires, yet a further reduction is required while retaining a high electrical conductivity. We have designed two different nanostructures for this purpose. One structure is connected Si nanodots (NDs) with the same crystal orientation. The phonons scattering at the interfaces of these NDs occurred and it depended on the ND size. As a result of phonon scattering, the thermal conductivity of this nanostructured material was below/close to the amorphous limit. The other structure is Si films containing epitaxially grown Ge NDs. The Si layer imparted high electrical conductivity, while the Ge NDs served as phonon scattering bodies reducing thermal conductivity drastically. This work gives a methodology for the independent control of electron and phonon transport using nanostructured materials. This can bring the realization of thermoelectric Si-based materials that are compatible with large scale integrated circuit processing technologies. [ABSTRACT FROM AUTHOR]

Reduction of thermal conductivity κ while preserving high electrical conductivity σ in materials continues to be a vital goal in thermoelectric study for the reuse of exhaust heat energy. In the use of an eco-friendly and ubiquitous element, Si as thermoelectric material, high κ value in bulk Si is the essential bottleneck to achieve high dimensionless figure of merit. This is a motivation for many recent studies on reducing κ in Si, by nanostructuring, e.g., using grains/wires with size smaller than the phonon mean free path. However, κ reduction that can be achieved tends to be saturated presumably due to an amorphous limit. Here, we present a nanoarchitecture for defeating the κ amorphous limit while preserving bulk-like σ . This new nanoarchitecture is an assembly of Si nanocrystals with oriented crystals separated by a 1-monolayer amorphous layer with well-controlled nanoscale shaped interfaces. At these interfaces, novel phonon scattering occurs resulting in κ reduction below the amorphous limit. Preservation of bulk-like σ results from the coherency of the carrier wavefunctions among the oriented nanocrystals separated by the ultrathin amorphous layer. The results will bring environmentally-friendly and low-cost thermoelectric Si material compatible with mature LSI process technology and represent guidelines for optimized thermoelectric nanostructures. [ABSTRACT FROM AUTHOR]

Ge thin films were epitaxially grown on Si(1 1 1) substrates using ultrathin SiO 2 film technique called nanocontact epitaxy. The key to this technique was the use of high density spherical nanodots (NDs) as seed crystals for the epitaxial growth of the Ge films. The seed crystal NDs were elastically strain-relaxed without misfit dislocations so that highly crystalline Ge thin films could be formed. The Ge films were ≈140 nm in thickness and were composed of island-shaped domains with flat Ge(1 1 1) surfaces. The domains had an average size of ≈1 μm and flat surfaces with root mean square of a surface roughness of ≈3 nm. The films had a photoluminescence peak at 0.8 eV, which meant that the films were highly crystalline. [ABSTRACT FROM AUTHOR]

We investigated the electrical properties of non-doped wafer-bonded germanium-on-insulator substrates (Ge(001)-OI) post-annealed in various ambiences using a pseudo-metal-oxide-semiconductor field effect transistor. The transistor operation in the Ge(001)-OI without post-annealing was observed in the n-channel mode with hysteresis in the channel conductance versus gate voltage curves. The n-channel transistor operation was depletion type, with a large threshold voltage shift from the ideal threshold voltage (∼0 eV). Upon post-annealing the Ge(001)-OI, the behavior of the hysteresis and threshold voltage shift caused by interface defects was improved, which strongly depended on the post-annealing condition. Post-annealing in a N2 ambience yielded the most effective improvement, which included some O2 gas. This improvement is presumably because oxygen vacancies near the Ge/SiO2 interfaces are reduced by the oxygen supply during the post-annealing treatment. [ABSTRACT FROM AUTHOR]

Abstract: We develop the epitaxial growth technique of ultrahigh density (>1012cm-2) iron silicide nanodots (NDs) on Si substrates using ultrathin SiO2 films. This technique enables the ND formation of various kinds of iron silicides such as β-FeSi2 and Fe3Si. These NDs nanocontact with Si substrates through limited area of nanowindows in the ultrathin SiO2 films. Due to the nanocontact between NDs and Si substrates, heteroepitaxial growth of high quality NDs on Si substrate is successful. Absorption spectra of individual β-FeSi2 NDs show the property of the indirect transition semiconductor. However, the Si/β-FeSi2 NDs/Si structures exhibit strong photoluminescence near 0.8eV. [Copyright &y& Elsevier]

Abstract: We investigated desorption of chlorine atoms on Si (111)-(7×7) surfaces induced by hole injection from scanning tunneling microscope tips. The hole-induced desorption of chlorine atoms had a threshold bias voltage corresponding to the energy position of the S3 surface band originated in Si backbonds. The chlorine atom desorption rate was almost proportional to the square of the tunneling current. We have discussed possible mechanisms that two holes injected into Si surface states get localized at the backbonds of chlorinated Si adatoms, which induces the rupture of Cl–Si bonds to result in chlorine atom desorption. [Copyright &y& Elsevier]

Abstract: Germanium quantum dots (QDs) were extracted from ultrathin Si x Ge1−x oxide films using scanning tunneling microscope (STM) tips. The extraction was most efficiently performed at a positive sample bias voltage of +5.0V. The tunneling current dependence of the extraction efficiency was explained by the electric field evaporation transfer mechanism for positive Ge ions from QDs to STM tips. Ge QDs (∼7nm) were formed and isolated spatially by extracting the surrounding Ge QDs with an ultrahigh density of >1012 cm−2. Scanning tunneling spectroscopy of the spatially-isolated QDs revealed that QDs with an ultrahigh density are electrically-isolated from the adjacent dots. [Copyright &y& Elsevier]

Injection of tunneling electrons and holes from the probe tips of a scanning tunneling microscope was found to enhance the hopping motion of Cl atoms between neighboring dangling-bond sites of Si dimers on Si(1 0 0)-(2 × 1) surfaces, featured by the rate of hopping linearly dependent on the injection current. The hopping rate formed peaks at sample biases of VS∼+1.25 and −0.85 V, which agree with the peaks in the local density of states spectrum measured by scanning tunneling spectroscopy. The Cl hopping was enhanced at Cl-adsorbed sites even remote from the injection point. The Cl hopping by hole injection was more efficiently enhanced by sweeping the tip along the Si dimer row than by tip-sweeping along the perpendicular direction. Such anisotropy, on the other hand, was insignificant in the electron injection case. All of these findings can be interpreted by the model that the holes injected primarily into a surface band originated from the dangling bonds of Si dimers propagate quite anisotropically along the surface, and become localized at Cl sites somehow to destabilize the Si–Cl bonds causing hopping of the Cl atoms. The electrons injected into a bulk band propagate in an isotropic manner and then get resonantly trapped at Si–Cl antibonding orbitals, resulting in bond destabilization and hopping of the Cl atoms. [Copyright &y& Elsevier]

Hole injection from scanning tunneling microscope (STM) tips was found to enhance diffusion of chlorine atoms on Si(1 1 1)-(7×7) surface. The sample bias dependence exhibits a broad peak centered at a sample bias, Vs≈−2.0V and the diffusion rate at the peak bias voltage depends linearly on the injected current. The effect spreads laterally along the surface over a range of ≈10 nm from the position of the tip, but not to extend beyond step edges. These facts are well interpreted by the model that holes injected primarily to a surface band peaking at Vs=−1.8 V propagate along the surface and become localized at Cl sites, leading somehow to destabilization or weakening of the bonds that results in Cl atom movements such as diffusion. [Copyright &y& Elsevier]

The authors investigated the annealing effects on atomic structures and elemental compositions in a stacking structure, Si capping layer on Ge1-xSnx nanodots on Si substrate covered with ultrathin SiO2 film, to clarify the origin of intense photoluminescence at ∼0.8 eV from the structure, using transmission electron microscopy. After the annealing, it was found that decay of Ge1-xSnx nanodots, formation of SiOx precipitates embedded in Si-rich Si1-xGex layer at the Si cap/Si substrate interface, formation of SnO2 nanoparticles on the oxidized surface of the Si capping layer, and morphological change of dislocations in the Si capping layer occur. Reaction products that appear as a result of the movement of dislocations can be related to the origin of intense photoluminescence. [ABSTRACT FROM AUTHOR]

Abstract: Photoabsorption properties of β-FeSi2 nanoislands epitaxially grown on Si(111) and Si(001) have been discussed using photoabsorption nano-spectroscopy based on scanning tunneling microscope. The obtained spectra exhibit clear features around 0.86–0.91eV and around 0.71–0.74eV, which are explained as a direct and an indirect photoabsorption edge of β-FeSi2, respectively. We also observed a blue shift of spectrum obtained from β-FeSi2 nanoislands on Si(111) substrates, compared to those on Si(001) substrates. We attributed the dependence on Si-substrate orientation not to a quantum confinement effect but to an effect of elastic strain in the β-FeSi2 nanoislands epitaxially grown on the substrate. [Copyright &y& Elsevier]

The authors observed a quantum-confinement effect in individual Ge1-xSnx quantum dots (QDs) on Si (111) substrates covered with ultrathin SiO2 films using scanning tunneling spectroscopy at room temperature. The quantum-confinement effect was featured by an increase in the energy band gap of ∼1.5 eV with a decrease in QD diameter from 35 to 4 nm. The peaks for quantum levels of QDs became broader with a decrease in the height-diameter aspect ratio of QDs, demonstrating the gradual emergence of two dimensionality in density of states of quasi zero-dimensional QDs with the QD flattening. [ABSTRACT FROM AUTHOR]

The quantum-confinement effect in two-dimensional β-FeSi2 nanoislands epitaxially grown on Si (111) by codeposition of Fe and Si was observed using scanning tunneling spectroscopy at room temperature. The energy band gaps of the H-terminated β-FeSi2 nanoislands increased by approximately 0.4 eV when island height decreased from 5 to 2 nm. This size dependence was explained by the quantum-confinement effect in β-FeSi2 nanoislands. [ABSTRACT FROM AUTHOR]

• FeGe γ with Nowotny chimney-ladder structure are epitaxially grown on Si substrate. • c -axis lattice constant in FeGe γ is modulated by composition change. • a -axis lattice constant and lattice strain are modulated by interface control. The single phase FeGe γ nanocrystals (NCs) with incommensurate Nowotny chimney-ladder structure were epitaxially grown on Si substrates by reaction between deposited Fe atoms and Ge nanostructured layers on Si. The Ge nanostructured layers worked as blocking layers of the Fe diffusion to Si substrates. We used the two kinds of the Ge nanostructured layers: Ge wetting layers formed by direct Ge deposition on Si and Ge NCs formed by Ge deposition on ultrathin SiO 2 films with ultrasmall nanowindows on Si. In the case of Ge wetting layers, the FeGe γ NCs were directly formed on Si, where there was a -axis strain due to the 2.9% lattice mismatch between FeGe γ and Si. On the other hand, in the case of Ge NCs, formed FeGe γ NCs were elastically-strain-relaxed due to the small contact with Si substrates through nanowindows in the ultrathin SiO 2 films. Furthermore, the c -axis lattice constant in the epitaxial FeGe γ NCs changed due to the variation of the Fe-Ge composition. These results demonstrated that a -axis lattice constant was modulated by strain control of FeGe γ NCs due to the change of contact area with Si substrate and that c -axis lattice constant of FeGe γ NCs with incommensurate Nowotny chimney-ladder structure was modulated by manipulating Fe-Ge composition. These modulation of lattice constants will open a road for manipulating phonon group velocity related to the lattice constant and managing the waste heat in various devices implemented on Si substrates. [ABSTRACT FROM AUTHOR]