Si Nanodots Research Articles

FeOx and Si nano-dots as dual Li-storage centers bonded with graphene for high performance lithium ion batteries.

A novel design based on both FeOx and Si nano-dots bonded with graphene (FeOx·Si@GNS) as dual lithium-storage centers is reported. They show high performance as anode materials for Li-ion batteries with a remarkable reversible capacity of 1160 mA h g(-1) at 0.2 A g(-1), fast charging/discharging rate, and long cycling life (e.g., a capacity retention of 81.7% at 2.0 A g(-1) after 600 cycles). The origin of these high performances comes from the key factors of the high theoretical specific capacity of FeOx and Si, the shorter Li-ion diffusion distance of both nano-dot structures, fast electron conductivity, and the strain relaxation due to volume variations of both nano-dots bonded with graphene nanosheets during cycles.

Silicon crystallization in nanodot arrays organized by block copolymer lithography

Asymmetric polystyrene-b-polymethylmethacrylate (PS-b-PMMA) block copolymers are used to fabricate nanoporous PS templates with different pore diameter depending on the specific substrate neutralization protocol. The resulting polymeric templates are used as masks for the subsequent deposition of a thin (h = 5 nm) amorphous Si layer by electron beam evaporation. After removal of the polymeric film and of the silicon excess, well-defined hexagonally packed amorphous Si nanodots are formed on the substrate. Their average diameter (d < 20 nm), density (1.2 × 1011 cm−2), and lateral distribution closely mimic the original nanoporous template. Upon capping with SiO2 and high temperature annealing (1050 °C, N2), each amorphous Si nanodot rearranges in agglomerates of Si nanocrystals (d < 4 nm). The average diameter and shape of these Si nanocrystals strongly depend on the size of the initial Si nanodot.

Anneal effect on multipeak photoluminescence properties of porous silicon pillar arrays

Porous silicon pillar array (PSPA) samples with broad photoluminescence (PL) from ~380 to ~800nm were prepared by a hydrothermal etching method and found to be a hierarchical structure of silicon. The broad PL of PSPAs, which arises from four separate origins, could be tuned by varying annealing temperature and protective gas. In addition, the elemental compositions and their valence states change with annealing temperature and ventilation conditions. The results show that as the numbers of Si-Hx and Si-Si-Si bonds in the PSPAs decreased, different change of the intensities of red and pink PL bands. Moreover, at high annealing temperature, the PSPA samples contained only Si–O–Si bonds and only green and blue PL bands were observed. We attribute the pink and red PL bands to quantum confinement and quantum confinement/luminescence center emissions caused by the Si nanodots, and the blue and green emissions to two different luminescence centers associated with oxygen vacancies.

Nanoparticle size determination using optical microscopes

We present a simple method for size determination of nanoparticles using conventional optical microscopes. The method, called through-focus scanning optical microscopy, makes use of the four-dimensional optical information collected at different focus positions. Low partial coherence illumination combined with analysis of through-focus optical content enables nanoparticle size determination with nanometer scale sensitivity. We experimentally demonstrate this using fabricated Si nanodots and spherical gold nanoparticles. The method is economical, as no hardware modifications to conventional optical microscopes are needed. In addition, the method also has high throughput and potential for soft nanoparticle size determination without distortion.

Epitaxial Growth of Nanodots on Si Substrates with Controlled Interfaces and Their Application to Electronics and Thermoelectronics

Fe-based materials (Fe, its oxide, its silicide and germanide) have been widely studied for device applications: spintronic or magnetic devices and resistance random access memory using resistive switching properties in iron oxide. Skyrmions recently-observed in ε-FeGe(Si) also led to a large interest in Fe-based materials. In terms of device application, Fe-based epitaxial films on Si substrates, which are low-cost and ecofriendly materials, have drawn much attention. However, epitaxial growth of the films on Si with sharp interfaces is difficult because the interfaces are easily disordered. On the other hand, we have developed an ultrathin SiO2 film technique for epitaxial growth of Si and Fe silicide nanodots (NDs) [1-3]. In this paper, by applying the ultrathin SiO2film technique, we develop the epitaxial growth technique of Fe-based NDs on Si with crystal structure controllability. We also demonstrate the application of these NDs. Clean Si surfaces were oxidized at 600 °C at an oxygen pressure of 2×10-4 Pa to form SiO2 films with a thickness of ~0.3 nm. A few monolayers (ML) of Si or Ge were predeposited on the ultrathin SiO2 films to form ultrahigh density (>1012 cm-2) nanovoids in the ultrathin SiO2films. Excessive Si or Ge atoms deposited subsequently formed epitaxial ultrasmall nuclei on nanovoids that are referred to be Si or Ge nanowindows (NWs). Fe was then deposited on the surfaces to form Fe-based NDs. Fe NDs were epitaxially grown on Si substrates by Fe deposition on the ultrathin SiO2 films with Si NWs at room temperature (RT) [4]. In order to form Fe oxide NDs, we exposed the Fe NDs to 1 atmosphere of dry oxygen [4]. Then, oxidation and silicidation were enhanced at the interfaces between Fe-based NDs and Si substrates, leading to the destruction of prior ND structures. This reaction was due to the existence of Fe(ND)/Si(substrates) interfaces. We also formed epitaxial NDs by deposition of Fe on Ge NWs at RT instead of Si NWs. In this case, the above reaction did not occur during O2exposure due to the lack of Fe/Si interfaces, and Fe oxide NDs were then formed while maintaining prior ND structures [4], demonstrating that reaction between gas and NDs were strongly influenced by the nanometer-sized interface between NDs and substrates. On the other hand, at higher temperature of Fe deposition iron germanide NDs were formed. We formed two kinds of Ge nuclei in NWs by tuning temperature of Ge deposition: flattened strained and spherical strain-relaxed ones. By Fe deposition on these Ge nuclei, ultrahigh density iron germanide NDs were epitaxially grown on Si substrates with sharp interfaces [5]. Their formed crystal structures were determined by the shape and strain status of Ge nuclei at the interface between NDs and substrates. These results demonstrated that intermixing and crystal structures in the Fe-Si-Ge system were well-controlled in this technique. We also demonstrate the application of Fe- and Si- based NDs formed by the above technique to electronics or thermoelectronics. This work was supported in part by a JST-PRESTO program (Y. Nakamura). It was also supported by a Grant-in-Aid for Scientific Research B (25286026) and Exploratory Research (25600016), and by TEPCO Memorial Foundation, Research Grant. HRTEM observations were carried out at the Research Center for UHVEM, Osaka Univ. and AIST Tech.–Kansai.[1] Y. Nakamura, et al., Phys. Rev. B 72, 075404 (2005). [2] Y. Nakamura, et al., J. Appl. Phys. 100, 044313 (2006). [3] S. Amari, et al., J. Appl. Phys. 115, 084306 (2014). [4] H. Hamanaka, et al., J. Appl. Phys. 114, 114309 (2013). [5] Y. Nakamura, et al., J. Appl. Phys. 115 044301 (2014).

Equilibrium shapes of polycrystalline silicon nanodots

This study is concerned with the topography of nanostructures consisting of arrays of polycrystalline nanodots. Guided by transmission electron microscopy (TEM) measurements of crystalline Si (c-Si) nanodots that evolved from a “dewetting” process of an amorphous Si (a-Si) layer from a SiO2 coated substrate, we investigate appropriate formulations for the surface energy density and transitions of energy density states at grain boundaries. We introduce a new numerical minimization formulation that allows to account for adhesion energy from an underlying substrate. We demonstrate our approach first for the free standing case, where the solutions can be compared to well-known Wulff constructions, before we treat the general case for interfacial energy settings that support “partial wetting” and grain boundaries for the polycrystalline case. We then use our method to predict the morphologies of silicon nanodots.

Silicon Nanodot Layers for Photovoltaic Application: Size/Density Control and Electrical Properties

Abstract Fabrication of Si nanodot single layers under ultrahigh vacuum (UHV) conditions is achieved by decomposition and self-organized growth from thermally deposited non-stoichiometric SiO x (x &lt; 2) precursor layers provided with an ultrathin SiO2 capping layer due to phase separation upon appropriate in situ annealing. The kinetics of the thermal decomposition of the constituting Si suboxides (Si n+, n = 0…4) into Si nanodots and the surrounding SiO2 matrix is analyzed in situ by X-ray photoelectron spectroscopy (XPS) as a function of the annealing temperature. The maximum size and the density of the nanodots are varied by adjusting the stoichiometric coefficient x and the layer thickness. Thus, the electronic nanodot properties and the interlayer transport properties can be controlled. For initial compositions (x ranging from 0.9 to 1.4) and layer thicknesses (3 to 10 nm), phase separation was completed at 850 ℃. The phase separation revealed by XPS is directly correlated with electrical properties as derived from atomic force microscopy measurements detecting surface potentials (KFM) and local conductivity (CS-AFM) across individual nanodots.

Production of ordered and pure Si nanodots at grazing ion beam sputtering under concurrent substrate rotation

Abstract Hexagonal array of Si-nanodots is spontaneously generated by low energy Ar+ sputtering of a Si (1 0 0) surface at grazing incidence angle (75°) under continuous rotation of the substrate holder. The effects of rotation speed, beam current density and beam energy on the dot morphology are investigated by atomic force microscopy. The transmission electron microscopy of the dot microstructure shows that the dots are crystalline and are not induced by metal contaminants. Moreover, the dots are found to be capped by negligibly thin amorphous layer.

Microstructure and interdiffusion behaviour of β-FeSi2 flat islands grown on Si(111) surfaces

β-FeSi2 flat islands have been fabricated on ultra-thin oxidized Si(111) surfaces by Fe deposition on Si nanodots. The microstructure and interdiffusion behaviour of the β-FeSi2/Si(111) system at the atomic level were studied by using spherical aberration-corrected high-angle annular dark-field scanning transmission electron microscopy and energy dispersive X-ray spectroscopy. The formed β-FeSi2 flat islands had a disc shape with an average size of 30–150 nm width and 10–20 nm height, and were epitaxically grown on high-quality single-phase Si with a crystallographic relationship (110)β-FeSi2/(111)Si and [001]β-FeSi2/[1\bar 10]Si. Moreover, the heterojunction between the β-FeSi2(110) flat islands and the Si(111) substrate was an atomically and chemically abrupt interface without any irregularities. It is believed that these results are caused by the use of ultra-thin SiO2 films in our fabrication method, which is likely to be beneficial particularly for fabricating practical nanoscaled devices.

Silicon nanodot formation and self‐ordering under bombardment with heavy Bi3 ions

AbstractSi nanodots of high density and hexagonal short‐range order are observed upon normal‐incidence bombardment of hot, crystalline Si with Bi3+ ions having a kinetic energy of a few tens of keV. The heights of nanodots are comparable to their widths of ∼20 nm. The implanted Bi accumulates in tiny Bi nanocrystals in a thin Si top layer which is amorphous due to implantation damage. Light and heavy ions up to Xe cause smoothing of surfaces, but Bi3+ ions considered here have a much higher mass. Atomistic simulations prove that each Bi3+impact deposits an extremely high energy density resulting in a several nanometer large melt pool, which resolidifies within a few hundreds of picoseconds. Experiments confirm that dot patterns form only if the deposited energy density exceeds the threshold for melting. Comparing monatomic and polyatomic Bi ion irradiation, Bi–Si phase separation and preferential ion erosion are ruled out as driving forces of pattern formation. A model based on capillary forces in the melt pool explains the pattern formation consistently.magnified imageHigh‐density Si nanodots are formed by polyatomic Bi ion irradiation of hot Si surfaces. Each impact causes local transient melt pools smaller than the dots. Hexagonally ordered patterns evolve by self‐organization driven by repeated ion‐induced melting of tiny volumes. Homogeneously distributed Bi nanocrystals are found in the a‐Si film. These nanocrystals are related to particularities of the Si–Bi phase diagram. (© 2013 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Surface Potential Based Analytical Modeling of Double Gate MOSFET with Si and Au Nano-Dots Embedded Gate Dielectric for Non-Volatile Memory Applications

In this paper we present an analytical study of Double Gate (DG) MOSFET memory devices with Si and Au nanocrystal embedded gate dielectric stacks. We considered an undoped long channel DG MOSFET, having a multilayer SiO2 (5 nm)–Si/Au nc embedded Si3N4 (6 nm)–SiO2 (7 nm) gate dielectric. From a quasi-1-D analytic solution of the Poisson equation, the potential and the electric fields in the substrate and the different layers of the gate oxide stack were derived. Thereafter following a trap-like model using the WKB approximation, we have investigated the Fowler Nordheim tunneling currents from the Si inversion layer to the embedded nanocrystal states in such devices. We evaluated the write-erase characteristics, gate tunneling currents, threshold voltage shifts and the output characteristics of the NVM devices from our analytical model. The performance of the nanoparticle embedded DGMOS memory device was compared with that of a DG SONOS NVM of similar dimensions. From the studies, the nc embedded devices showed better performance than the conventional SONOS DGMOS NVM. Among the two ncs compared the nc-Au embedded device emerged as the better performer in terms of higher charge density, faster charging, higher threshold voltage shift and better charge retention.

Formation of Si or Ge nanodots in Si3N4 with in-situ donor modulation doping of adjacent barrier material

Conventional doping of small nanodots does not provide majority carriers due to self-purification effects and much increased ionisation energies of dopants presenting point defects. Adjacent barrier layers to Si-rich Si3N4 can be doped by excess Si in-situ with the segregation anneal for Si nanodot formation. Donor doping of AlxGa1−xN (0 ⩽ x ⩽ 1) with Si is an established process. Material properties and process compatibility of AlxGa1−xN render it suitable as barrier for Si nanodot superlattices. Ab-initio calculations showed that Ge also works as a donor in AlN, extending the material range to Ge and SiGe nanodots in Si3N4.

Structural properties of Si/SiO2 nanostructures grown by decomposition of substoichiometric SiOxNy layers for photovoltaic applications

AbstractThe structural properties of crystalline Si nanodots embedded in a SiO2 matrix are investigated with respect to the exploitation of quantum confinement effects (QCE) in Si solar cells. The nanostructures are grown on crystalline Si (c‐Si) wafers by decomposition of substoichiometric SiOxNy layers with various [O]/[Si] ratios. Cross‐sectional high‐resolution transmission electron microscopy investigations reveal the formation of separated single crystalline nanodots with diameters below 5 nm inside the SiOxNy volume and directly on the c‐Si wafer. The density and diameter of the nanodots decreases with increasing [O]/[Si] ratio, leading to inter‐dot distances above 10 nm for [O]/[Si]&gt;1.3. Photoluminescence (PL) spectra are blue‐shifted relative to the Si bulk PL, which is in good agreement with theoretical QCE models. It is found that for observing the PL signal the nanodots must be covered by a SiO2 shell to reduce charge carrier recombination via defects at the nanodot surface. This requires an [O]/[Si] ratio &gt;0.5 for which the inter‐dot distance becomes too large for charge carrier transport between the nanodots. It is concluded that a better control over the nanodot formation at high [O]/[Si] ratios has to be achieved before QCE can be successfully applied in Si solar cell devices.

Functionalization of Silicon Crystal Surface by Energetic Cluster Ion Bombardment

We report the creation of a functional nanostructure on a Si crystal surface by 200 keV C60(++) cluster ion bombardment (CIB). We found that the modified layer produced by CIB includes two sublayers with different nanostructures. The top 24-nm-thick sublayer is an agglomeration of 5-nm-sized amorphous Si nanodots (a-Si NDs). The deeper 10-nm-thick sublayer is a transient layer of disordered Si as an interface between the a-Si top sublayer and the bulk Si(100). The top a-Si sublayer and the nc-Si transient layer are formed by the local heating effect and shock wave effect, respectively, induced by the cluster ion impacts. The photoluminescence (PL) spectra of the CIB-modified Si samples revealed an emission line centered at a photon energy of 1.92 eV. The absorption spectra of the modified samples exhibit enhanced light absorption at this photon energy. The parameters of the PL line require ascribing the emission origin to the quantum-confinement-induced optical transitions in the a-Si nanodots. The core-shell structure of a-Si NDs is confirmed by detection of an additional PL line centered at 2.5 eV. Analysis of the Rutherford backscattering (RBS) and the PL spectra implies the existence of -Si--O- bonds in the nanodot outer shells, which are responsible for the additional PL line. The obtained results demonstrate the valuable potential of CIB for the controllable fabrication of Si surface nanostructures, which is attractive for optoelectronics and nanoelectronics. The obtained results elucidate the evolution of structure modification occurring in silicon due to the injection of energetic C60 cluster ions with an energy of hundreds of keV.

Dynamical process of KrF pulsed excimer laser crystallization of ultrathin amorphous silicon films to form Si nano-dots

Molecular dynamics (MD) simulations based on the Tersoff potential have been developed to study the laser-induced crystallization of amorphous silicon (a-Si) film with ultrathin thickness to form size-controllable Si nano-dots. The influences of laser fluence and a-Si film thickness on the crystallization process were discussed. Classic nucleation theory was used to explain the results of the MD simulations. The constrain effect of a-Si films thickness on the formation of Si nano-dots was evaluated accordingly.

Nonlinear Optical Properties of Si Nanodot in ${\rm Al}_{2}{\rm O}_{3}$ Matrix

The linear and the third-order nonlinear optical properties associated with intersubband transitions are studied for spherical Si nanodot embedded in an amorphous matrix. The effect of radiation intensity and the change in the size of the dot are investigated on the nonlinear photoabsorption coefficient, refractive index change, and the third-order nonlinear optical susceptibility for transitions from ground state (1s-1p) and excited states (1p-1d, 2s-2p). The oscillator strength is also calculated for these transitions along with the photoelectric cross sections from the initial 1s and 2s states of the dot. It is noted that a decrease in the size of the dot leads to blue shift in peak positions of photoabsorption coefficient, refractive index change, and third-order nonlinear susceptibility involving transitions from the ground and excited states.

Strain Effects on Avalanche Multiplication in a Silicon Nanodot Array

Strain effects on avalanche multiplication in a one-dimensional Si nanodot array have been theoretically studied. Compressive strain has two competing effects of the band-gap narrowing and the level-separation widening. The former reduces the ionization threshold and the latter reduces the impact ionization rate for higher energy region. Larger carrier multiplication factor is observed under compressive strain condition.

Strain Effects on Avalanche Multiplication in a Silicon Nanodot Array

Strain effects on avalanche multiplication in a one-dimensional Si nanodot array have been theoretically studied. Compressive strain has two competing effects of the band-gap narrowing and the level-separation widening. The former reduces the ionization threshold and the latter reduces the impact ionization rate for higher energy region. Larger carrier multiplication factor is observed under compressive strain condition.

Origin of strong white electroluminescence from dense Si nanodots embedded in silicon nitride.

Strong white electroluminescence (EL) from SiN-based devices containing Si nanodots with a density of more than 4.6×10(12)cm(2) was investigated. The white EL illustrates enhanced light emission with increasing applied voltage and can be divided into two components, a dominant peak at ~710 nm and weak one at ~550 nm, which are close to those of the PL spectra optically pumped by the 325 and 488 nm lines, respectively. Based on the PL characteristics, we propose that the dominant EL band arises from the band-to-band recombination in the dense Si nanodots where quantum confinement plays a decisive role in the light emission, whereas the weak EL band originates from the radiative Si dangling bond (K0) centers in the silicon nitride matrix.

Nonlithographic Fabrication of Crystalline Silicon Nanodots on Graphene

We report a nonlithographic fabrication method to grow uniform and large-scale crystalline silicon (Si) nanodot (c-SiNDs) arrays on single-layer graphene by an ultrathin anodic porous alumina template and Ni-induced Si crystallization technique. The lateral height of the template can be as thin as 160 nm and the crystallization of Si can be achieved at a low temperature of 400 °C. The effects of c-SiNDs on graphene were studied by Raman spectroscopy. Furthermore, the c-SiNDs/graphene based field effect transistors were demonstrated.