Stoichiometric SiO2 Research Articles

Coordination state and defect evolution in vitreous silica structures formed using Molecular Dynamics under variable cooling conditions

Under ideal circumstances, the structures formed in Molecular Dynamics (MD) simulations of relatively simple glassy systems, such as stoichiometric amorphous SiO2, should be fully coordinated and free of intrinsic type defects (NBO, O3C, Si3C, etc). This has been shown to be not always the case for a variety of reasons, one of which being the use of non-optimized cooling conditions. In order for MD simulations to be accurate, critical effects due to variable cooling cycle parameters must be well understood. This is especially true if the bulk structures' ultimate intended use is for surface structure generation and subsequent analysis or reactionary dynamics interaction with the surrounding environment. Here, we report the results of MD simulations on amorphous silica systems, under variable cooling conditions. Analysis of Si and O coordination states as functions of temperature and cooling condition shows a strong non-linear dependence, suggesting that defect (NBO, O3C, Si3C, etc.) concentrations do not necessarily decrease with longer trajectory times. Analyses of defect clusters found in the structures at low temperatures (<1200K) show variable levels of dynamic activity, highlighted by the formation and subsequent annihilation of certain defect pairs (i.e. OC3 – SiC3 and NBO-SiC5). The results suggest that the types of reactions as well as the extent to which they occur are dependent on thermal history.

Direct synthesis of ultrathin SOI structure by extremely low-energy oxygen implantation

We performed extremely low-energy 16O+ implantation at 10 keV (Rp ∼ 25 nm) followed by annealing aiming at directly synthesizing an ultrathin Si layer separated by a buried SiO2 layer in Si(001) substrates, and then investigated feasible condition of recrystallization and stabilization of the superficial Si and the buried oxide layer by significantly low temperature annealing. The elemental compositions were analyzed by Rutherford backscattering (RBS) and secondary ion mass spectroscopy (SIMS). The crystallinity of the superficial Si layer was quantitatively confirmed by ananlyzing RBS-channeling spectra. Cross-sectional morphologies and atomic configurations were observed by transmission electron microscope (TEM). As a result, we succeeded in directly synthesizing an ultrathin single-crystalline silicon layer with ≤20 nm thick separated by a thin buried stoichiometric SiO2 layer with ≤20 nm thick formed by extremely low-energy 16O+ implantation followed by surprisingly low temperature annealing at 1050∘ C.

ChemInform Abstract: Synthesis, Structure, and Polymorphism of A3LnSi2O7 (A: Na, K; Ln: Sm, Ho, Yb).

AbstractCrystals of K3SmSi2O7 (I) are grown from stoichiometric Sm2O3 and SiO2 in a KF flux (KF‐covered Ag crucible, 900 °C, 12 h).

Luminescence properties of Ce3+ and Tb3+ co-doped SiOxNy thin films: Prospects for color tunability in silicon-based hosts

In this work, the role of the nitrogen content, the annealing temperature, and the sample morphology on the luminescence properties of Ce3+ and Tb3+ co-doped SiOxNy thin films has been investigated. An increasing nitrogen atomic percentage has been incorporated in the host matrix by gradually replacing oxygen with nitrogen during fabrication while maintaining the Si content unaltered, obtaining a sequential variation in the film composition from nearly stoichiometric SiO2 to SiOxNy. The study of rare earth doped single layers has allowed us to identify the parameters that yield an optimum optical performance from Ce3+ and Tb3+ ions. Ce3+ ions proved to be highly sensitive to the annealing temperature and the nitrogen content, showing strong PL emission for relatively low nitrogen contents (from 0 to 20%) and moderate annealing temperatures (800–1000 °C) or under high temperature annealing (1180 °C). Tb3+ ions, on the other hand, displayed a mild dependence on those film parameters. Rare earth co-doping has also been investigated by comparing the luminescence properties of three different approaches: (i) a Ce3+ and Tb3+ co-doped SiOxNy single layer, (ii) a bilayer composed of two SiOxNy single layers doped with either Ce3+ or Tb3+ ions, and (iii) a multilayer composed of a series of either Tb3+ or Ce3+-doped SiOxNy thin films with interleaved SiO2 spacers. Bright green emission and efficient energy transfer from either Ce3+ ions or Ce silicates to Tb3+ ions has been observed in the co-doped single layer as a consequence of the strong ion-ion interaction. On the other hand, independent luminescence from Ce3+ and Tb3+ ions has been observed in the Ce3+ and Tb3+ co-doped bilayer and multilayer, providing a good scenario to develop light emitting devices with wide color tunability by varying the number of deposited films that contain each rare earth dopant. Moreover, the optoelectronic properties of Ce3+- and/or Tb3+-doped thin films have been studied by depositing transparent conductive electrodes over selected samples. An electroluminescence signal according to the rare earth transitions is obtained in all cases, validating the excitation of Ce3+ and Tb3+ ions upon electron injection. Also, the main charge transport of injected electrons has been evaluated and correlated with the layer stoichiometry. Finally, a simple reliability test has allowed disclosing the origin of the early breakdown of test devices, attributed to the excessive joule heating at filament currents that occur around a region close to the polarization point.

Characterization of Chemical Vapor Deposited Tetraethyl Orthosilicate based SiO2 Films for Photonic Devices

Silicon has been the choice for photonics technology because of its cost, compatibility with mass production and availability. Silicon based photonic devices are very significant from commercial point of view and are much compatible with established technology. This paper deals with deposition and characterization of SiO2 films prepared by indigenously developed chemical vapor deposition system. Ellipsometry study of prepared films showed an increase in refractive index and film thickness with the increment in deposition temperature. The deposition temperature has a significant role for stoichiometric SiO2 films, FTIR measurement has shown the three characteristics peaks of Si-O-Si through three samples prepared at temperatures 700, 750 and 800 °C while Si-O-Si stretching peak positions were observed to be shifted to lower wavenumber in accordance to the temperature. FESEM analysis has confirmed the smooth surface without any crack or disorder while EDX analysis showed the corresponding peaks of compositional SiO2 films.DOI: http://dx.doi.org/10.5755/j01.ms.22.1.7245

Performance of Arrayed Microring Resonator Sensors with the TriPleX Platform

Integrated-optical (bio) chemical sensors are more and more commercialized, mainly because of their high intrinsic sensitivity in combination with the possibilities they offer for integration in optofluidic devices. The microring resonator (MRR), for example, is a very feasible structure to be used as microarray sensor element for multiplex biosensing. Fabrication of MRRs has been described in the case of the TriPleX platform employing well-defined stacks of stoichiometric Si3N4 and SiO2 material. This platform allows for a wide variety of applications, due to its intrinsic low loss and its transparency for the VIS/NIR wavelength range. In the present paper, we describe the recent achievements in the characterization of arrays of these MRRs regarding temperature sensitivity, refractive index sensitivity and protein immobilization performance. Furthermore, the use of a reference channel/reference MRR is demonstrated in order to show the advantage of compensation of unintended change in temperature or sample composition. The refractive index sensitivity was determined to be 104 nm/RIU and the limit of detection was about 2 × 10-6 RIU. MRRs appeared to behave very comparable (expressed as the coefficient of (intra-array) variation (CV)) regarding the response to temperature (CV≈0.3%) as well as refractive index (CV<0.1%). Furthermore, it is shown that protein immobilization onto the different MRRs of the same arrays can be realized in a comparable way (CV<3%). A good comparability (i.e., a low variation) in these aspects allows for the use of internal referencing in order to compensate for unintended change in temperature or refractive index and also helps to attain a high assay precision in the ultimate biosensing application. Ongoing research is focused on the integration of more functionality on chip and further miniaturization in order to allow for fabrication of complex, though affordable, sensor devices.

Super-oxidation of silicon nanoclusters: magnetism and reactive oxygen species at the surface

Oxidation of silicon nanoclusters depending on the temperature and oxygen pressure is explored from first principles using the evolutionary algorithm, and structural and thermodynamic analysis. From our calculations of 90 SinOm clusters we found that under normal conditions oxidation does not stop at the stoichiometric SiO2 composition, as it does in bulk silicon, but goes further placing extra oxygen atoms on the cluster surface. These extra atoms are responsible for light emission, relevant to reactive oxygen species and many of them are magnetic. We argue that the super-oxidation effect is size-independent and discuss its relevance to nanotechnology and miscellaneous applications, including biomedical ones.

Quantum Confined Intense Red Luminescence from Large Area Monolithic Arrays of Mesoporous and Nanocrystal-Decorated Silicon Nanowires for Luminescent Devices

We report intense red luminescence from mesoporous n+-Si(100) nanowires (NWs) and nanocrystal-decorated p-Si NWs fabricated using electroless metal assisted chemical (MAC) etching. n+-Si NWs are composed of a labyrinthine network of silicon nanocrystals in a random mesoporous structure. p-type Si(100) NWs exhibit solid core structure, with a surface roughness that contains surface-bound nanocrystals. Both mesoporous n+-Si NWs and rough, solid p-Si NWs exhibit red luminescence at ∼1.7 and ∼1.8 eV, respectively. Time-resolved photoluminescence (PL) measurements indicated long (tens of μs) radiative recombination lifetimes. The red luminescence is visible with the naked eye and the red light is most intense from mesoporous n+-Si NWs, which exhibit a red-shift in the emission maximum to 1.76 eV at 100 K. The red PL from monolithic arrays of p-type NWs with nanocrystal-decorated rough surfaces is comparatively weak, but originates from the surface bound nanocrystals. Significant PL intensity increase is found during excitation for mesoporous NWs. X-ray photoelectron spectroscopy identifies a stoichiometric SiO2 on the rough p-Si NWs with a SiOx species at the NW surface. No distinct oxide is found on the mesoporous NWs. The analysis confirms that long life-time PL emission arises from quantum confinement from internal nanoscale crystallites, and oxidized surface-bound crystallites, on n+- and p-Si NWs respectively.

Physical characterization of ultrathin silicon oxynitrides grown by Rapid Thermal Processing aiming to MOS tunnel devices

Oxynitrides were grown in a homemade Rapid Thermal Processor (RTP) using a low mass quartz carrier, to obtain thin oxynitrides over large areas of 3 inches silicon p-type wafers. Layers with thickness varying from 0.97 to 2.39 nm with uniformity better than 0.4%, were obtained at 700 and 850°C, in a mixed ambient of nitrogen and oxygen (4N2:3O2 in volume). The nitrogen concentration was obtained with the aid of X-ray photoelectron spectroscopy (XPS) and was 0.6 at%. On the other hand, the Si/O ratio in the oxynitride was approximately 1.9, indicating an almost stoichiometric SiO2 with a small amount of nitrogen. In addition, using the 16O(α, α) elastic-scattering signal at 3.039MeV, the planar concentration of oxygen was 5.5×1015cm2 for the oxynitride grown at 850°C during 40s.

Atmospheric Plasma Deposition of SiO2 Films for Adhesion Promoting Layers on Titanium

This paper evaluates the deposition of silica layers at atmospheric pressure as a pretreatment for the structural bonding of titanium (Ti6Al4V, Ti15V3Cr3Sn3Al) in comparison to an anodizing process (NaTESi process). The SiO2 film was deposited using the LARGE plasma source, a linearly extended DC arc plasma source and applying hexamethyldisiloxane (HMDSO) as a precursor. The morphology of the surface was analyzed by means of SEM, while the characterization of the chemical composition of deposited plasma layers was done by XPS and FTIR. The long-term durability of bonded samples was evaluated by means of a wedge test in hot/wet condition. The almost stoichiometric SiO2 film features a good long-term stability and a high bonding strength compared to the films produced with the wet-chemical NaTESi process.

Simple Cleaning and Conditioning of Silicon Surfaces with UV/Ozone Sources

Within this paper the applicability of three methods to clean and condition the surface of silicon wafers by an ultra-thin oxide layer were tested: two UV/O3 sources (an Hg vapor lamp and a high efficiency excimer module) as well as a wet chemical oxidation in HNO3.It has been demonstrated that organic residues resulting from wafering and masking the surface during alkaline texturing can be removed by UV/O3 (Hg vapor lamp) exposure. Moreover the use of UV/O3 (excimer) and HNO3 oxides result in an improvement of passivation quality and emitter saturation current in combination with Al2O3/SiNx or AlN/SiNx passivation stacks. For n-type silicon wafers with a 70Ω/sq. boron emitter and Al2O3/SiNx passivation very low j0e values of 49 fA/cm2 are achieved by conditioning with UV/O3 or HNO3.Another important application of ultra-thin SiOx layers is as tunnel oxides for the rear side of the TOPCon (Tunnel Oxide Passivated Contact) cell concept [1,2]. High iVOC values exceeding 720mV on planar samples and 710mV on textured samples are obtained with UV/O3 grown tunnel oxides after annealing. The structural properties of the SiOx layers that were analyzed by XPS and compared to the iVOC data, indicate that a minimum oxide layer thickness of approx. 1.4nm and a high amount of oxygen rich sub oxides species (Si2O3, close to the structure of stoichiometric SiO2) are required to obtain a good interface passivation stable up to annealing temperatures of 900°C.

Atmospheric pressure PECVD based on a linearly extended DC arc for adhesion promotion applications

In the aircraft industry, an increasing demand for adhesion promotion layers for titanium based materials is observed to replace polluting and unhealthy substances. Plasma enhanced chemical vapour deposition (PECVD) at atmospheric pressure (AP) seems to be favourable for the deposition of thin silica-like layers. In this work, a linearly extended and scalable DC arc plasma source is used for PECVD of silica layers on Ti6Al4V. The influence of the precursor chemistry is evaluated for HMDSO and TEOS precursors.Using HMDSO, hydrophobic almost stoichiometric SiO2 coatings with a carbon content of 3at.-% to 5at.-% are deposited on the titanium alloy substrate. For TEOS, hydrophilic carbon-free stoichiometric SiO2 coatings are deposited. The dynamic deposition rate for TEOS is 3 times lower than that for HMDSO (up to 640nm•m/min). For the intended coating of slightly curved substrates, the film properties and deposition rates are studied for different working distances from 20mm up to 60mm. For 60mm the deposition rate is reduced by a factor 6 compared to 20mm, also the index of refraction of the film decreases from 1.45 at 20mm distance to 1.39 at 60mm. With increasing distance, the particle size and concentration on the surface increases and leads to less dense structures than in the case of shorter distances.The wedge test is used to evaluate the adhesion properties of the layer. The HMDSO-based coatings exhibit a good long-term durability and a high bonding strength comparable to that of the currently used standard pre-treatment, but without the use of harmful substances.

Parasitic amorphous on single-domain crystal: Structural observations of silicate glass-ceramics

Glass-ceramics (GCs) are materials obtained from the crystallisation of functional phases in glass, and have a structure that the crystallised phase embedded in the glass matrix. Glass-forming oxides are commonly added to the functional phases to improve the stability of precursor glass; however, the issue of glass-ceramics permitting the presence of residual phases resulting from addition is required to be clarified. To elucidate this issue, we prepared ‘perfectly surface-crystallised’ GC consisting of fresnoite-type Sr2TiSi2O8 from a non-stoichiometric glass and performed texture/morphology observations. Numerous SiO2-rich binodal-like nanospheres (~10 nm) were parasitic on the fresnoite single-crystal domains. The parasitic texture is considered to form via the following process: (i) binodal-type phase separation into stoichiometric fresnoite (crystalline matrix) and SiO2-rich phases (amorphous nanoparticles) and (ii) single-domain formation by surface crystallisation in the matrix. Furthermore, in terms of texture, the resulting GC differs from the GCs reported to date, i.e., inverse GC.

Quantum confinement of volume plasmons and interband transitions in germanium nanocrystals

The plasmonic properties of individual quantum-sized Ge nanocrystals (NCs) were observed and systematically analyzed by aberration-corrected scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS). For this purpose, Ge NCs embedded in an SiO${}_{2}$ matrix with controllable size, density, and structure were fabricated using magnetron sputtering. The size dependence of the Ge plasmon energies in the size range of 5--9 nm is shown to be well depicted by the so-called medium quantum confinement (QC) model, with an effective mass of 0.57${m}_{0}$ (contrary to expectations of a stronger quantum effect). In the very low-loss region of the EEL spectra, an apparent blue shift of the ${E}_{2}$ interband transition peak up to 2 eV and a strong reduction in the oscillator strength were measured for the NCs in the size range of 4--6 nm. It indicates for this smaller size range a transition to a QC regime where the band structure and the density of states are modified dramatically. These trends are explained by a combination of low-loss and core-loss EELS results, which show that the Ge NCs are surrounded uniformly by nearly stoichiometric SiO${}_{2}$. This local chemistry is shown to provide an infinite potential barrier and to confine electrons and holes in the spherically shaped Ge NCs. In addition to pure QC effects in the Ge NCs, the SiO${}_{2}$ matrix thus plays an important role in the strength of the observed QC and interband transitions.

Room temperature plasma oxidation in DCSBD: A new method for preparation of silicon dioxide films at atmospheric pressure

In this paper a new process for the preparation of thin silicon dioxide (SiO2) film is presented: the oxidation of c-Si (1 1 1) surface in atmospheric pressure plasma at room temperature. Diffuse coplanar surface barrier discharge (DCSBD) at atmospheric pressure in air and oxygen atmosphere has been used. The oxidation rate and the thickness of oxidized layers were estimated by ellipsometry. The structure and the chemical composition of oxidized layers were investigated by infrared reflection absorption spectroscopy (IRRAS), X-ray photoelectron spectroscopy (XPS) and energy dispersive X-ray (EDX) analysis. Scanning electron microscopy (SEM) was used to observe the morphology of the layer surface. It was found that stoichiometric SiO2 layers were obtained with oxidation rates comparable to thermal oxidation.

FT IR spectroscopy of silicon oxide layers prepared with perchloric acid

Chlorine oxidation is important methods for improvement of many properties such as passivation of mobile oxide charge, breakdown strength or enhancement of the minority-carrier lifetime in the underlying silicon. In this study we consider effects influencing the density of SiO2 layers formed by three different methods: thermal oxidation at 850°C, low temperature oxidation method by use of nitric acid – HNO3 (NAOS) and HClO4 afterward passivated with KCN/HCN solutions. Thicknesses of SiO2 layers determined by both capacitance–voltage (C–V) and XPS revealed fast oxidation rate compared with samples prepared by thermal oxidation. FT IR measurement showed that all absorption spectra are almost similar. Higher absorption of the sample prepared in HClO4 was observed. No SiCl bonds were visible. Calculated atomic density of the SiO2 layer obtained from IR measurements was lowest for sample formed in HClO4. Chlorine oxidation results in higher oxidation rate (higher thickness) and formation of stoichiometric SiO2 layer with lower density. Following KCN/HCN passivation causes formation of SiN, SiO and SiOH bonds at the expense of SiCl bonds.

Chemical and phase compositions of silicon oxide films with nanocrystals prepared by carbon ion implantation

The chemical and phase compositions of silicon oxide films with self-assembled nanoclusters prepared by ion implantation of carbon into SiO x (x < 2) suboxide films with subsequent annealing in a nitrogen atmosphere have been investigated using X-ray photoelectron spectroscopy in combination with depth profiling by ion sputtering. It has been found that the relative concentration of oxygen in the maximum of the distribution of implanted carbon atoms is decreased, whereas the relative concentration of silicon remains almost identical over the depth in the layer containing the implanted carbon. The in-depth distributions of carbon and silicon in different chemical states have been determined. In the regions adjacent to the layer with a maximum carbon content, the annealing results in the formation of silicon oxide layers, which are close in composition to SiO2 and contain silicon nanocrystals, whereas the implanted layer, in addition to the SiO2 phase, contains silicon oxide species Si2+ and Si3+ with stoichiometric formulas SiO and Si2O3, respectively. The film contains carbon in the form of SiC and elemental carbon phases. The lower limit of the average size of silicon nanoclusters has been estimated as ∼2 nm. The photoluminescence spectra of the films have been interpreted using the obtained results.

Investigation of surface pre-treatments for the structural bonding of titanium

This paper evaluates wet-chemical pre-treatments (alkaline etching, anodising) and a plasma treatment for structural bonding of titanium (Ti6Al4V). The main objective of this study is the comparison of the applicability of the plasma pre-treatment to wet-chemical treatments on titanium for structural bonding. In this context, an atmospheric pressure plasma device was used to deposit thin functional films from hexamethyldisiloxane (HMDSO) precursor on titanium.X-ray photoelectron spectroscopy (XPS) was employed to assess the chemical composition of the surface after different pre-treatments on the titanium substrate, while the morphology and the film thickness were investigated with scanning electron microscopy (SEM). The adhesion properties on titanium were evaluated by means of a wedge test in hot/wet conditions. After bonding tests the fracture surface and the failure loci were analysed.Using a long arc plasma generator and HMDSO precursor almost stoichiometric SiO2 coatings were obtained on the titanium substrate. These coatings exhibit good long-term durability and bond strength compared to an alkaline etching in the wedge test. The investigated anodising process leads to oxide layers revealing a highly porous nanostructure. In contrast to the alkaline etching, the plasma derived coatings and the oxide layer produced by the anodising process exhibit a higher micro, respectively nano roughness, and hence a better long-term durability.

Fabrication and Characterization of Single-Crystal Metal-Assisted Chemically Etched Rough Si Nanowires for Lithium-Ion Battery Anodes

Silicon nanowires were fabricated by a metal assisted chemical (MAC) etching process and routes toward ohmic contacting of substrates for Li-ion battery anode application were developed. Si nanowire layers are comprised of wires that are single crystal with rough outer surfaces. The nanowires are epitaxial with the underlying Si(100) substrate, maintain equivalent doping density and crystal orientation, and are coated with a stoichiometric SiO2. Electrical backside contacting using an In-Ga eutectic allows low-resistance ohmic contacts to low-doped nanowire electrodes for electrochemical testing.

Formation of light-emitting nanostructures in layers of stoichiometric SiO2 irradiated with swift heavy ions

Thermally grown SiO2 layers have been irradiated with 700-MeV Bi ions with doses of (3–10) × 1012 cm−2. It is found that, even after a dose of 3 × 1012 cm−2, a photoluminescence band in the region of 600 nm appears. Its intensity levels off at a dose of ∼5 × 1012 cm−2. The nature of the emission centers is studied by the methods of infrared transmission, Raman scattering, X-ray photoelectron spectroscopy, ellipsometry, and the reaction to passivating low-temperature anneals. It is established that irradiation brings about a decrease in the number of Si-O bonds with a relevant increase in the Si-Si bonds. It is assumed that the photoluminescence is caused by nanostructures containing an excess Si and/or having a deficit of O. The reaction of reduction of SiO2 proceeds in ion tracks due to high levels of ionization and heating within these tracks. The dose dependence is used to estimate the diameter of a track at 8–9 nm.