Annealing-induced evolution of optical properties of the multilayered nanoperiodic SiO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>/ZrO<Subscript>2</Subscript> system containing Si nanoclusters

The photoluminescence, infrared absorption, and Raman spectra of amorphous multilayered nanoperiodic a-SiO _x /ZrO₂ structures produced by vacuum evaporation and then annealed at different temperatures (500–1100°C) are studied. It is established that the evolution of the optical properties with increasing annealing temperature is controlled by sequential transformation of Si clusters formed in the SiO _x layers from nonphase inclusions to amorphous clusters and then to nanocrystals. The finally formed nanocrystals are limited in sizes by the thickness of the initial SiO _x layers and by chemical reactions with ZrO₂.     

Resonant inelastic light scattering and photoluminescence in isolated nc-Si/SiO2 quantum dots

Observation at the room temperature the spectra of the resonant inelastic light scattering by the spatially confined optical phonons as well as the excitonic luminescence caused by confinement effects in the ensemble of isolated quantum dots (QDs) nc-Si/SiO₂ is reported. It is shown that the samples investigated are high purity and high crystalline perfection quality nc-Si/SiO₂ QDs without amorphous phase α-Si and contaminants. Comparison between the experimental data obtained and phenomenological model of the strong space confinement of optical phonons revealed the need of the more accurate form of the weighted function for the confinement of optical phonons. It is shown that simultaneous detection of the inelastic light scattering by the confinement of phonons and the excitonic luminescence spectra by the confined electron-hole pairs in the nc-Si/SiO₂ QDs allows selfconsistently to determine more accurate values of the diameter of the nc-Si/SiO₂ QDs.     

A comparison of microstrip models to low temperature co-fired ceramic–silver microstrip measurements

A number of analytical and numerical models for microstrip lines are used for analyzing the propagation loss of CaO–B₂O₃–SiO₂-based low temperature co-fired ceramics (LTCC) subjected to different processing conditions. An optimal microstrip model is identified for the frequency range between 0.5 and 15 GHz and used for the extraction of dielectric loss for this material system. The measurements show that the dominant loss contribution changes from the conductor to dielectric loss as the processing temperature for the LTCC dielectrics is lowered. Comparison with microstructural analysis shows that the increase in dielectric loss is strongly correlated to an increase in the amorphous SiO₂ content in the ceramic matrix.     

Direct View on Nanoionic Proton Mobility

The field of nanoionics is of great importance for the development of superior materials for devices that rely on the transport of charged ions, like fuel cells, batteries, and sensors. Often nanostructuring leads to enhanced ionic mobilities due to the induced space‐charge effects. Here these large space‐charge effects occurring in composites of the proton‐donating solid acid CsHSO₄ and the proton‐accepting TiO₂ or SiO₂ are studied. CsHSO₄ is chosen for this study because it can operate effectively as a fuel‐cell electrolyte at elevated temperature while its low‐temperature conductivity is increased upon nano­structuring. The composites have a negative enthalpy of formation for defects involving the transfer of protons from the acid to the acceptor. Very high defect densities of up to 10% of the available sites are observed by neutron diffraction. The effect on the mobility of the protons is observed directly using quasielastic neutron scattering and nuclear magnetic resonance spectroscopy. Surprisingly large fractions of up to 25% of the hydrogen ions show orders‐of‐magnitude enhanced mobility in the nanostructured composites of TiO₂ or SiO₂, both in crystalline CsHSO₄ and an amorphous fraction.     

Structural,compositional and electrical characterization of Si-rich SiOx layers suitable for application in light sensors

Metal-Oxide-Silicon (MOS) structures containing silicon nanoparticles (SiNPs) in three different gate dielectrics, single SiO_x layer (c-Si/SiNPs-SiO_x), two-region (c-Si/thermal SiO_x/SiNPs-SiO_x) or three-region (c-Si/thermal SiO₂/SiNPs-SiO_x/SiO₂) oxides, were prepared on n-type (100) c-Si wafers. The silicon nanoparticles were grown by a high temperature furnace annealing of sub-stoichiometric SiO_x films (x=1.15) prepared by thermal vacuum evaporation technique. Annealing in N₂ at 700 or 1000 °C leads to formation of amorphous or crystalline SiNPs in a SiO_x amorphous matrix with x=1.8 or 2.0, respectively. The three-region gate dielectric (thermal SiO₂/SiNPs-SiO₂/SiO₂) was prepared by a two-step annealing of c-Si/thermal SiO₂/SiO_x structures at 1000 °C . The first annealing step was carried out in an oxidizing atmosphere while the second one was performed in N₂. Cross-sectional Transmission Electron Microscopy and X-ray Photoelectron Spectroscopy have proven both the nanoparticle growth and the formation of a three region gate dielectric. Annealed MOS structures with semitransparent aluminum top electrodes were characterized electrically by current/capacitance–voltage measurements in dark and under light illumination. A strong variation of the current at negative gate voltages on the light intensity has been observed in the control and annealed at 700 °C c-Si/SiNPs-SiO_x/Al structures. The obtained results indicate that MOS structures with SiO_1.15 gate dielectric have potential for application in light sensors in the NIR–Visible Light–UV range.     

Raman studies of silicon nanocrystals embedded in silicon suboxide layers

Raman spectroscopy is used for the study of SiO _x (x ≈ 1) layers subjected to thermal annealing at the temperatures from 950 to 1200°C to form Si nanocrystals inside the layers. From comparison of the experimental data with the model of spatial confinement of phonons, the volume fractions of the crystalline and amorphous Si phases in the layers are determined. It is established that, as the annealing temperature is increased, the average dimensions of Si nanocrystals increase from 4 to 6.5 nm. This is attributed to the coarsening of nanocrystals due to crystallization of the amorphous Si phase and to the processes of coalescence of neighboring nanocrystals at the highest temperatures of annealing.     

Thermal evolution of the morphology, structure, and optical properties of multilayer nanoperiodic systems produced by the vacuum evaporation of SiO and SiO2

The alternate vacuum evaporation of SiO and SiO₂ from separate sources is used to produce amorphous a-SiO _x /SiO₂ multilayer nanoperiodic structures with periods of 5–10 nm and a number of layers of up to 64. The effect of annealing at temperatures T _a = 500–1100°C on the structural and optical properties of the nanostructures is studied. The results of transmission electron microscopy of the samples annealed at 1100°C indicate the annealing-induced formation of vertically ordered quasiperiodic arrays of Si nanocrystals, whose dimensions are comparable to the a-SiO _x -layer thickness in the initial nanostructures. The nanostructures annealed at 1100°C exhibit size-dependent photoluminescence in the wavelength range 750–830 nm corresponding to Si nanocrystals. The data on infrared absorption and Raman scattering show that the thermal evolution of structural and phase state of the SiO _x layers with increasing annealing temperature proceeds through the formation of amorphous Si nanoinclusions with the subsequent formation and growth of Si nanocrystals.     

Structural transformations and silicon nanocrystallite formation in SiOx films

The results of a comprehensive study by the methods of IR absorption, Raman scattering, photoluminescence (PL), and electron spin resonance (ESR) of SiO_x films prepared by thermal evaporation of SiO in a vacuum are presented. The nature of structural transformations occurring on annealing the films is determined. Annealing in the temperature range 300–600°C gives rise to a PL band at 650 nm, presumably related to structural defects in SiO_x film. Raising the annealing temperature further leads to healing of such defects and quenching of the PL band. Silicon precipitates pass from the amorphous to the crystalline state on being annealed at T _ann=1100°C, which gives rise to a new PL band at 730 nm. ESR spectra of P _b centers were recorded at the interface between randomly oriented silicon nanocrystallites and SiO₂.     

Enhanced charge storage characteristics by ZrO2 nanocrystallites precipitated from amorphous (ZrO2)0.8(SiO2)0.2 charge trapping layer

Charge trapping memory capacitors using (ZrO₂)_0.8(SiO₂)_0.2 film as charge trapping layer and amorphous Al₂O₃ as the tunneling layer and blocking layer were fabricated for nonvolatile semiconductor memory application. The ZrO₂ nanocrystallites with a size of 3–5 nm precipitated from amorphous (ZrO₂)_0.8(SiO₂)_0.2 during rapid thermal annealing at 800 °C can serve as the storage nodes, with which a large hysteresis memory window of 7.5 V at a sweeping gate voltage of 8 V has been achieved. At 150 °C bake temperature, the memory capacitor exhibited an excellent endurance up to 10⁵ write/erase cycles, after which a small charge loss of about 12% was achieved.     

SiO2/TiO2 Composite Film for High Capacity and Excellent Cycling Stability in Lithium‐Ion Battery Anodes

In this study, partially crystalline anodic TiO₂ with SiO₂ well‐distributed througout the entire oxide film is prepared using plasma electrolytic oxidation (PEO) to obtain a high‐capacity anode with an excellent cycling stability for Li‐ion batteries. The micropore sizes in the anodic film become inhomogeneous as the SiO₂ content is increased from 0% to 25%. The X‐ray diffraction peaks show that the formed oxide contains the anatase and rutile phases of TiO₂. In addition, X‐ray photoelectron spectroscopy and energy‐dispersive X‐ray analyses confirm that TiO₂ contains amorphous SiO₂. Anodic oxides of the SiO₂/TiO₂ composite prepared by PEO in 0.2 m H₂SO₄ and 0.4 m Na₂SiO₃ electrolyte deliver the best performance in Li‐ion batteries, exhibiting a capacity of 240 µAh cm^?2 at a fairly high current density of 500 µA cm^–2. The composite film shows the typical Li–TiO₂ and Li–SiO₂ redox peaks in the cyclic voltammogram and a corresponding plateau in the galvanostatic charge/discharge curves. The as‐prepared SiO₂/TiO₂ composite anode shows at least twice the capacity of other types of binder‐free TiO₂ and TiO₂ composites and very stable cycling stability for more than 250 cycles despite the severe mechanical stress.     

Achieving time-of-flight mobilities for amorphous organic semiconductors in a thin film transistor configuration

We observe bulk-like hole transport in amorphous organic semiconductors in a thin film transistor (TFT) configuration. Five different organic hole transporters (HTs) commonly used in organic light-emitting diodes are investigated. When these HTs are deposited on SiO₂ gate dielectric layer, the TFT mobilities are 1–2 orders of magnitude smaller than those obtained from bulk films (3–8 μm) using time-of-flight (TOF) technique. The reduction of hole mobilities can be attributed to the interactions between the organic HTs and the polar SiO bonds on the gate dielectric layer. Detailed temperature dependence studies, employing the Gaussian disorder model, indicate that the SiO₂ gate dielectric contributes between 60 and 90 meV of energetic disorder in the charge hopping manifold. Besides SiO₂ gate dielectric, similar effects can also be observed for other polar insulators including polymeric PMMA and BCB, or HMDS-modified SiO₂. However, when a common non-polar polymer, polystyrene (PS), is employed as the dielectric layer, the dipolar energetic disorder becomes negligible. Holes effectively experience bulk-like transport on the PS gate dielectric surface. TFT mobilities extracted from all five organic HTs are in excellent agreements with TOF mobilities. The present study should have broad applications in the transport characterization of amorphous organic semiconductors.     

Patterning and etching of amorphous teflon films

Amorphous Teflon®, deposited by spin-coating and thermal evaporation, has been studied as a low dielectric constant insulator for high performance interconnects. Since it is difficult to deposit a film on amorphous Teflon because of its inert chemical bonds, plasma etching and Zonyl FSN® fluorosurfactant are used to improve the adhesion of photoresist to amorphous Teflon. Plasma etching is shown to increase surface roughness and change chemical bonds of the amorphous Teflon, resulting in improved adhesion of metals and SiO₂ to amorphous Teflon. While amorphous Teflon cannot be etched by wet chemicals, etching the films in Ar, O₂, or CF₄/O₂ plasma is very effective.     

Post deposition UV-induced O2 annealing of HfO2 thin films

UV-assisted annealing processes for thin oxide films is an alternative to conventional thermal annealing and has shown many advantages such as low annealing temperature, reducing annealing time and easy to control. We report in this work the deposition of ultra-thin HfO₂ films on silicon substrate by two CVD techniques, namely thermal CVD and photo-induced CVD using 222 nm excimer lamps at 400 °C. As-deposited films of around 10 nm in thickness with refractive indices from 1.72 to 1.80 were grown. The deposition rate measured by ellipsometry was found to be about 2 nm/min by UV-CVD, while the deposition rate by thermal CVD is 20% less than that by UV-CVD. XRD showed that the as-deposited HfO₂ films were amorphous. This work focuses on the effect of post deposition UV annealing in oxygen on the structural, optical and electrical properties of the HfO₂ films at low temperature (400 °C). Investigation of the interfacial layer by FTIR revealed that thickness of the interfacial SiO₂ layer slightly increases with the UV-annealing time and UV annealing can convert sub-oxides at the interface into stoichiometric SiO₂, leading to improved interfacial qualities. The permittivity ranges in 8–16, are lower than theoretical values. However, the post deposition UV O₂ annealing results in an improvement in effective breakdown field and calculated permittivity, and a reduction in leakage current density for the HfO₂ films.     

Structural characterization of oxide layers thermally grown on 3C-SiC films

The oxidation of 3C-SiC films deposited on off-oriented Si(001) substrates by reactive magnetron sputtering has been studied. The oxidation was carried out using dry conditions at a temperature of 1200°C. The composition of the oxide layer was investigated by Auger electron spectroscopy (AES). The oxide layer was found to contain no C except for the region very close to the interface, and the stoichiometry was found to be close to that of SiO₂. Cross-sectional transmis-sion electron microscopy (XTEM) showed the oxide layer to be completely amorphous, dense, and homogeneous with a uniform thickness. High-resolution XTEM imaging showed an atomically sharp SiO₂/SiC interface.     

Si/SiO2@Graphene Superstructures for High-Performance Lithium-Ion Batteries

The superstructure composed of various functional building units is promising nanostructure for lithium-ion batteries (LIBs) anodes with extreme volume change and structure instability, such as silicon-based materials. Here, a top-down route to fabricate Si/SiO₂@graphene superstructure is demonstrated through reducing silicalite-1 with magnesium reduction and depositing carbon layers. The successful formation of superstructure lies on the strong 3D network formed by the bridged-SiO₂ matrix coated around silicon nanoparticles. Furthermore, the mesoporous Si/SiO₂ with amorphous bridged SiO₂ facilitates the deposition of graphene layers, resulting in excellent structural stability and high ion/electron transport rate. The optimized Si/SiO₂@graphene superstructure anode delivers an outstanding cycling life for ≈1180 mAh g⁻¹ at 2 A g⁻¹ over 500 cycles, excellent rate capability for ≈908 mAh g⁻¹ at 12 A g⁻¹, great areal capacity for ≈7 mAh cm⁻² at 0.5 mA cm⁻², and extraordinary mechanical stability. A full cell test using LiFePO₄ as the cathode manifests a high capacity of 134 mAh g⁻¹ after 290 loops. More notably, a series of technologies disclose that the Si/SiO₂@graphene superstructure electrode can effectively maintain the film between electrode and electrolyte in LIBs. This design of Si/SiO₂@graphene superstructure elucidates a promising potential for commercial application in high-performance LIBs.     

Comparison between the properties of amorphous and crystalline Ta2O5 thin films deposited on Si

In this study, the structural and electrical properties of amorphous and crystalline Ta₂O₅ thin films deposited on p-type Si by low pressure metalorganic chemical vapour deposition from a Ta(OC₂H₅)₅ source have been investigated. The as-deposited layers are amorphous, whereas crystalline Ta₂O₅ (hexagonal phase) was obtained after post-deposition O₂-annealing at 800°C. Physico-chemical analysis of our layers shows that the O₂-treatment leads to the growth of a thin (1 nm) interfacial SiO₂ layer between Ta₂O₅ and Si but, contrary to other studies, was not sufficient to reduce the level of carbon and hydrogen contaminants. Crystalline Ta₂O₅ shows better leakage current properties than amorphous Ta₂O₅. The conduction mechanism in amorphous Ta₂O₅ is clearly attributed to the Poole–Frenkel effect with a barrier height separating the traps from the conduction band of 0.8 eV. For crystalline Ta₂O₅, the situation remains unclear since no simple law can be invoked due to the presence of the SiO₂ interlayer: a double conduction process based on a tunnelling effect in SiO₂ followed by a trap-modulated mechanism in Ta₂O₅ may be invoked. From capacitance–voltage measurements, the permittivity was found to be 25 for amorphous samples, but values ranging from 56 to 59 were found for crystalline layers, suggesting a high anisotropic character.     

Diffusion-Controlled growth of Ge nanocrystals in SiO<Subscript>2</Subscript> films under conditions of ion synthesis at high pressure

The growth of Ge nanocrystals in SiO₂ films is studied in relation to the dose of implanted Ge⁺ ions and the annealing temperature at a pressure of 12 kbar. It is established that the dependences of the nanocrystal dimensions on the content of Ge atoms and the annealing time are described by the corresponding root functions. The nanocrystal radius squared is an exponential function of the inverse temperature. The dependences correspond to the model of the diffusion-controlled mechanism of nanocrystal growth. From the temperature dependence of the nanocrystal dimensions, the diffusion coefficient of Ge in SiO₂ at a pressure of 12 kbar is determined: D = 1.1 × 10^–10 exp(–1.43/kT). An increase in the diffusion coefficient of Ge under pressure is attributed to the change in the activation volume of the formation and migration of point defects. Evidence in favor of the interstitial mechanism of the diffusion of Ge atoms to nanocrystal nuclei in SiO₂ is reported.     

Structure–Property Correlations in CoFe–SiO2 Nanogranular Films Utilizing x-Ray Photoelectron Spectroscopy and Small-Angle Scattering Techniques

A quantitative structure–property correlation study of thin films consisting of CoFe nanoparticles embedded in SiO₂ is presented, comparing film microstructure and chemistry with measured magnetic properties. SiO₂ was fully percolated for all films with > ~50% SiO₂ by volume, and decreasing CoFe-nanoparticle size and separation with increasing SiO₂ resulted in a transition to superparamagnetic behavior. Partial oxidation of transition-metal elements is observed by x-ray photoelectron spectroscopy, and evidence for interparticle magnetic interactions can be resolved in soft x-ray resonant small-angle scattering experiments, highlighting the need for additional detailed and quantitative studies in this class of soft magnetic materials.     

Structural Determinants of the Sign of the Pyroelectric Effect in Quasi‐Amorphous SrTiO3 Films

The magnitude and direction of the permanent electric polarization in the non‐crystalline, polar phase (termed quasi‐amorphous) of SrTiO₃ in Si\SiO₂\Me\SrTiO₃\Me, (Me = Cr or W), Si\SrRuO₃\SrTiO₃, and Si\SrTiO₃ layered structures were investigated. Three potential sources of the polarization which appears after the material is pulled through a temperature gradient were considered: a) contact potential difference; b) a flexoelectric effect due to a strain gradient caused by substrate curvature; and c) a flexoelectric effect due to the thermally induced strain gradient that develops while pulling through the steep temperature gradient. Measurements show that options a) and b) can be eliminated from consideration. In most cases studied in this (Si\SrTiO₃, Si\SiO₂\Me\SrTiO₃\Me, M = Cr or W) and previous works (Si\BaTiO₃, Si\BaZrO₃), the top surface of the quasi‐amorphous phase acquires a negative charge upon heating. However, in Si\SrRuO₃\SrTiO₃ structures the top surface acquires a positive charge upon heating. On the basis of the difference in the measured expansion of the upper and lower surfaces of the SrTiO₃ layer in the presence and absence of SrRuO₃, we contend that the magnitude and direction of the pyroelectric effect are determined by the out‐of‐plane gradient of the in‐plane strain in the SrTiO₃ layer while pulling through the temperature gradient.     

The Substrate Effects on Kinetics and Mechanism of Solid-Phase Crystallization of Amorphous Silicon Thin Films

The substrate effects on solid-phase crystallization of amorphous silicon (a-Si) films deposited by low-pressure chemical vapor deposition (LPCVD) using Si₂H₆ gas have been extensively investigated. The a-Si films were prepared on various substrates, such as thermally oxidized Si wafer (SiO₂/Si), quartz and LPCVD-oxide, and annealed at 600 °C in an N₂ ambient for crystallization. The crystallization behavior was found to be strongly dependent on the substrate even though all the silicon films were deposited in amorphous phase. It was first observed that crystallization in a-Si films deposited on the SiO₂/Si starts from the interface between the a-Si and the substrate, so called interface-induced crystallization, while random nucleation process dominates on the other substrates. The different kinetics and mechanism of solid-phase crystallization is attributed to the structural disorderness of a-Si films, which is strongly affected by the surface roughness of the substrates.