Grown Silicon Dioxide Research Articles

Electrical transport and grain growth in solution-cast, chloride-terminated cadmium selenide nanocrystal thin films.

We report the evolution of electrical transport and grain size during the sintering of thin films spin-cast from soluble phosphine and amine-bound, chloride-terminated cadmium selenide nanocrystals. Sintering of the nanocrystals occurs in three distinct stages as the annealing temperature is increased: (1) reversible desorption of the organic ligands (≤150 °C), (2) irreversible particle fusion (200–300 °C), and (3) ripening of the grains to >5 nm domains (>200 °C). Grain growth occurs at 200 °C in films with 8 atom % Cl–, while films with 3 atom % Cl– resist growth until 300 °C. Fused nanocrystalline thin films (grain size = 4.5–5.5 nm) on thermally grown silicon dioxide gate dielectrics produce field-effect transistors with electron mobilities as high as 25 cm2/(Vs) and on/off ratios of 105 with less than 0.5 V hysteresis in threshold voltage without the addition of indium.

Anodic Oxidation of Silicon

The set up used to grow silicon dioxide anodically on silicon surface has been described and the results obtained are discussed. Such layers have been used in obtaining information about diffused layers, getting planar structures and reducing the thickness of slices by known amounts. The method has certain advantages over techniques like thermal oxidation, sputtering etc. which are dealt in the paper.

Nested potassium hydroxide etching and protective coatings for silicon-based microreactors

We have developed a multilayer, multichannel silicon-based microreactor that uses elemental fluorine as a reagent and generates hydrogen fluoride as a byproduct. Nested potassium hydroxide etching (using silicon nitride and silicon oxide as masking materials) was developed to create a large number of channels (60 reaction channels connected to individual gas and liquid distributors) of significantly different depths (50–650 µm) with sloped walls (54.7° with respect to the (1 0 0) wafer surface) and precise control over their geometry. The wetted areas were coated with thermally grown silicon oxide and electron-beam evaporated nickel films to protect them from the corrosive fluorination environment. Up to four Pyrex layers were anodically bonded to three silicon layers in a total of six bonding steps to cap the microchannels and stack the reaction layers. The average pinhole density in as-evaporated films was 3 holes cm−2. Heating during anodic bonding (up to 350 °C for 4 min) did not significantly alter the film composition. Upon fluorine exposure, nickel films (160 nm thick) deposited on an adhesion layer of Cr (10 nm) over an oxidized silicon substrate (up to 500 nm thick SiO2) led to the formation of a nickel fluoride passivation layer. This microreactor was used to investigate direct fluorinations at room temperature over several hours without visible signs of film erosion.

Magnetic Properties of Co-Doped TiO2 Films Grown on TiN Buffered Silicon Substrates

Co-doped TiO2 thin films are grown on TiN buffered silicon substrates by the pulsed laser deposition method and then hydrogenated. Transmission electron microscopy and high-angle annular dark-field scanning transmission electron microscopy measurements have shown that the TiN buffer layer can suffer a 400°C deposition temperature and prevent the growth of silicon dioxide on silicon. After that, the room temperature ferromagnetism behaviors are observed in the hydrogenated samples, which are measured by the alternating gradient magnetometer. X-ray photoelectron spectroscopy and x-ray absorption fine structure measurements have revealed the existence of cobalt clusters. According to the material analysis, the magnetic behavior after hydrogenation is suggested to be induced by the enhancement of cobalt clusters.

Innovative microRNA purification based on surface properties modulation

The increasing interest in circulating microRNAs (miRNAs) as potential non-invasive cancer biomarkers has prompted the rapid development of several extraction techniques. However, current methods lack standardization and are costly and labor intensive. In light of this, we developed a microRNA solid-phase extraction strategy based on charge and roughness modulation on substrate surfaces. PECVD treated silicon oxide (PECVD-SO) and thermally grown silicon oxide (TG-SO) surfaces were functionalized with positively charged 3-aminopropyltriethoxysilanes (APTES) and neutral poly(ethylene glycol) silanes (PEG-s) mixed in different proportions to modulate the density of net positive charges and the roughness of the substrate. Characterization of the surfaces was performed by atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS) and s-SDTB (sulfosuccinimidyl-4-o-(4,4-dimethoxytrityl) butyrate) assay in order to investigate the surface morphology and chemical composition, respectively. Adsorption and elution efficiency were assessed by fluorescence microscopy by means of synthetic fluorescently labeled microRNAs. We identified PECVD-SO functionalized with 0.1% APTES and 0.9% 21–24 units long PEG-s as a promising surface able to selectively bind microRNAs and release them in the presence of a basic buffer (pH=9) compatible with downstream analyses. MicroRNA integrity was assessed by reverse transcription and real-time PCR and confirmed by electrophoresis (Agilent 2100 Bioanalyzer), while binding competition from circulating DNA and proteins was excluded by fluorescence analyses and real-time PCR. On the contrary, total RNA slightly decreased miRNA adsorption. In conclusion, we showed an innovative and easy solid-state purification method for circulating miRNAs based on charge interaction, which could pave the path to future diagnostic and prognostic assays feasible as a routine test.

Effects of SiO2 hard masks on si nanophotonic waveguide loss for photonic device integration

As the basic building block for photonic device integration, silicon nanophotonic waveguide requires low-loss propagation for high-performance ultra-compact photonic device. We experimentally study silicon dioxide hard masks grown by two different methods, i.e., thermal oxidation and plasma-enhanced chemical vapor deposition (PECVD) for silicon nano-waveguides fabrication and their effects on the propagation loss. It is found that the denser and smoother quality of thermally grown silicon dioxide increases the etch selectivity against silicon and reduces the line edge roughness transferred to the silicon nano-waveguide sidewalls, hence resulting in a lower loss as compared with the PECVD silicon dioxide hard mask. With thermally grown silicon dioxide as a hard mask, the silicon nano-waveguides loss can be halved for a 650-nm-wide nano-waveguide, and the loss is comparable with a waveguide fabricated with a resist etch mask.

Compressive elastic moduli and polishing performance of non-rigid core/shell structured PS/SiO2 composite abrasives evaluated by AFM

The core/shell structured polystyrene (PS)/SiO2 composite microspheres with different silica shell morphology were synthesized by a modified Stöber method. As confirmed by transmission electron microscopy (TEM), the rough discontinuous shell consisted of separate SiO2 nanoparticles for composite-A, while the smooth continuous one was composed of amorphous silica network for composite-B. Atomic force microscopy (AFM) was employed to probe the compressive Young's moduli (E) and chemical mechanical polishing (CMP) performances of the as-prepared PS/SiO2 composite microspheres. On the basis of the Hertzian contact mechanics, the calculated E values of the PS microspheres, composite-A and composite-B were 2.9±0.4, 5.1±1.2 and 6.0±1.2GPa, respectively. Compared to traditional abrasives, thermally grown silicon oxide wafers after polished by the core/shell PS/SiO2 composite abrasives obtained a lower root mean square roughness and a higher material removal rate value. In addition, there is an obvious effect of shell morphology of the composites on oxide CMP performance and structural stability during polishing process. This approach would provide a basis for understanding the actual role of organic/inorganic core/shell composite abrasives in the material removal process of CMP.

Hydrogen-atom-mediated electrochemistry

Silicon dioxide thin films are widely used as dielectric layers in microelectronics and can also be engineered on silicon wafers. It seems counterintuitive that electrochemical reactions could occur on such an insulator without relying on tunnelling current. Here we report electrochemistry based on electron transfer through a thin insulating layer of thermally grown silicon dioxide on highly n-doped silicon. Under a negative electrical bias, protons in the silicon dioxide layer were reduced to hydrogen atoms, which served as electron mediators for electrochemical reduction. Palladium nanoparticles were preferentially formed on the dielectric layer and enabled another hydrogen-atom-mediated electrochemistry, as their surfaces retained many electrogenerated hydrogen atoms to act as a 'hydrogen-atom reservoir' for subsequent electrochemical reduction. By harnessing the precisely controlled electrochemical generation of hydrogen atoms, palladium-copper nanocrystals were synthesized without any surfactant or stabilizer on the silicon dioxide layer.

Electric Field Effect Surface Passivation for Silicon Solar Cells

Effective reduction of front surface carrier recombination is essential for high efficiency silicon solar cells. Dielectric films are normally used to achieve such reduction. They provide a) an efficient passivation of surface recombination and b) an effective anti-reflection layer. The conditions that produce an effective anti-reflection coating are not necessarily the same for efficient passivation, hence both functions are difficult to achieve simultaneously and expensive processing steps are normally required. This can be overcome by enhancing the passivation properties of an anti-reflective film using the electric field effect. Here, we demonstrate that thermally grown silicon dioxide is an efficient passivation layer when chemically treated and electrically charged, and it is stable over a period of ten months. Double layers of SiO2 and SiN also provided stable and efficient passivation for a period of a year when the sample is submitted to a post-charge anneal. Surface recombination velocity upper limits of 9 cm/s and 19 cm/s were inferred for single and double layers respectively on n-type, 5 Ωcm, Cz-Si.

Characterization of very low thermal conductivity thin films

Characterization of thermal transport in nanoscale thin films with very low thermal conductivity (<1 W m−1 K−1) is challenging due to the difficulties in accurately measuring spatial variations in temperature field as well as the heat losses. In this paper, we present a new experimental technique involving freestanding nanofabricated specimens that are anchored at the ends, while the entire chip is heated by a macroscopic heater. The unique aspect of this technique is to remove uncertainty in measurement of convective heat transfer, which can be of the same magnitude as through the specimen in a low conductivity material. Spatial mapping of temperature field as well as the natural convective heat transfer coefficient allows us to calculate the thermal conductivity of the specimen using an energy balance modeling approach. The technique is demonstrated on thermally grown silicon oxide and low dielectric constant carbon-doped oxide films. The thermal conductivity of 400 nm silicon dioxide films was found to be 1.2 W m−1 K−1, and is in good agreement with the literature. Experimental results for 200 nm thin low dielectric constant oxide films demonstrate that the model is also capable of accurately determining the thermal conductivity for materials with values <1 W m−1 K−1.

Initial Phase of Photoelectrochemical Conditioning of Silicon in Alkaline Media: Surface Chemistry and Topography

Oxidation and dissolution phenomena of Si(111) in alkaline electrolyte are investigated by a combination of photoelectrochemistry, scanning probe microscopy (SPM), transmission electron microscopy (TEM) and in-system synchrotron radiation photoelectron spectroscopy (SRPES). The surface topography in the initial anodic potential regime shows the formation of mesoscale pores with widths in the range 300–500 nm and partial surface oxidation. The surface chemistry assessment by SRPES shows patchy silicon oxide growth, suboxides, and remnants of the former hydrogen terminated surface areas. The use of the obtained self-organized nanostructures for application in nanoemitter photocatalytic solar cells is discussed. The necessary requirements regarding the total surface area of electrocatalysts needed to sustain the current density due to light-induced excess minority carriers in conjunction with the exchange current density of the considered heterogeneous catalysts is discussed.

Conduction mechanisms in thermal nitride and dry gate oxides grown on 4H-SiC

The charge conduction mechanisms in Metal-Oxide-Semiconductor (MOS) capacitors formed on n-type 4H-silicon carbide (SiC) using thermally grown silicon dioxide (SiO2) as gate dielectrics are analyzed. The possible conduction mechanisms have been identified in the whole measurement range. At high electric fields, the charge conduction is dominated by Fowler–Nordheim tunneling. In addition, trap assisted tunneling and ohmic type conduction are considered to explain the cause of leakages detected at intermediate and low oxide electric fields. Various electronic parameters are extracted. The oxide breakdown strengths are higher than 8MV/cm. Fowler–Nordheim tunneling barrier height was found to be 2.74eV for nitride oxides and 2.54eV for dry oxides at high electric field regions and the trap energy level extracted using trap assisted tunneling emission model was estimated to be about 0.3eV for both oxides. The possible contribution of the Poole–Frenkel effect to the conduction mechanism was also considered, and it was found that it does not play a dominant role.

Low surface energy and corrosion resistant ultrathin TiSiC disk overcoat

Ultra-thin films of titanium silicon carbide (TiSiC) were deposited by magnetron sputtering (using Ti2SiC3 targets) to form protection overcoats (OCs) onto magnetic recording media of hard disk drives. The chemico-physical properties (composition, optical constants, electrical resistivity, mass density, and surface energy) of titanium silicon nitride (TiSiN) films were measured and correlated to their OC performances in terms of protection against Si oxidation, Co corrosion, and Co diffusion. Performances of TiSiC OCs were compared to those of silicon carbide (SiC), silicon nitride (SiN), and TiSiN OCs. It was found that Ti incorporation into SiC and SiN considerably densifies the films, reduces their surface energy, and renders them more metallic. 25 Å thick TiSiC OC forms stable protecting barriers than can sustain hydrolysis conditions without growth of surface silicon oxide or cobalt diffusion or oxidation in the underlying recording magnetic medium. Overall, TiSiC OCs outperformed TiSiN, SiC, and SiN OCs as disk protection layers. We could directly correlate good protection against surface silicon oxide formation with film's lower surface energy, and good protection against Co diffusion with film's higher mass density.

PECVD Al2 O3 /a-Si:B as a dopant source and surface passivation

In this study, a two-layer system consisting of a thin aluminum oxide (Al2O3) and a heavily boron-doped amorphous silicon (a-Si:B) is described. The double layer can be applied as a boron-dopant source in high-temperature diffusion as well as for surface passivation afterwards. The influence of different Al2O3 thicknesses and diffusion processes is investigated regarding the boron diffusion, surface passivation, and optical characteristics of this layer stack. Using a 5-nm thick Al2O3 film, a stable surface passivation as well as significant boron diffusion through the Al2O3 is possible. The best surface passivation is achieved for the annealing process with the highest thermal budget (950 °C for 60 min). Investigations by spectral ellipsometry prove the growth of silicon dioxide during the high-temperature diffusion process and indicate that the remaining boron-rich silicon layer is highly absorbing even for near-bandgap, near-infrared light. To improve future solar cells using this layer as the rear-side coating, the thickness of this layer has to be reduced.

Recombination and Optical Properties of Wet Chemically Polished Thermal Oxide Passivated Si Surfaces

Silicon solar cells typically feature textured surfaces on the front side to increase light absorption. An unwanted side effect of the texture is an increase in surface recombination compared with smoother surfaces. On the rear side of the solar cell, light absorption is not an issue; therefore, planar surfaces are used to decrease surface recombination. In processing, a planar surface can be achieved by wet chemical single-side etching of previously textured surfaces, resulting in a smoothed rear surface. This study investigates surface passivation of these chemically polished surfaces in dependence on the degree of smoothness. Surface passivation is achieved by thin thermally grown silicon oxides. Special focus is set on injection dependence and the influence of postmetallization annealing. Measured optical properties of different surfaces are compared with different optical simulation models. Finally, recombination and optical properties are connected to solar cell performance of fabricated passivated emitter and rear cells.

Effects of Annealing on Chemical-Vapor Deposited PureB Layers

Chemical-vapor-deposited pure boron (PureB) layers can be used as a source of p-type boron dopants for thermal diffusion into silicon during a drive-in anneal. In this work, the effect of thermally annealing PureB layers is investigated in terms of surface morphology and electrical properties. The presence of a few nanometer-thick PureB layer on the Si surface was found to increase the silicon oxide growth rate by several factors during annealing in an oxygen-containing atmosphere. The oxide thickness was dependent on the initial PureB layer thickness and oxygen concentration during anneal. In an oxygen-limited atmosphere, the final thickness is insensitive to the anneal temperature as the reaction is diffusion-limited and, after oxide removal, a hydrophilic boron rich layer remains on the Si surface. With a high oxygen concentration, the boron is depleted by an oxidation of the boron-doped silicon resulting in a lower surface concentration and higher sheet resistance. A reaction mechanism involving the oxidation of Si-B compounds to form B2O3 is proposed to explain the experimental observations. With solar cell and other photodiode applications in mind, the sheet resistance and carrier lifetime measurements were performed and show that a one-step oxidation process can simultaneously drive-in the dopants and form an oxide passivation layer.

Two-step Fabrication of Large Area SiO2/Si Membranes

Two-step fabrication technique of SiO 2 /Si membrane combining the deep local etching of double side polished and thermally oxidized silicon wafer in tetramethylammonium hydroxide (TMAH) water solution and SF 6 /O 2 reactive ion etching is presented in this study. The influence of temperature on stress and deformations of membrane was simulated using Solid Works software. The study of influence of photomask opening size on etching rate shows that TMAH etching rate V = 0.44 mm/min is higher for the biggest opening, whereas for smaller openings the etching rate is evidently decreased. It was revealed that TMAH during long etching time smoothly affects thermally grown silicon dioxide film as surface roughness R a increases from 0.558 μm to 0.604 μm. SF 6 /O 2 reactive etching rate is smoothly dependent on deep opening size when plasma power density varies from 0.25 W/cm 2 to 1.0 W/cm 2 . DOI: http://dx.doi.org/10.5755/j01.ms.18.4.3090

Analysis of Phosphorus-Doped Silicon Oxide Layers Deposited by Means of PECVD as a Dopant Source in Diffusion Processes

In order to increase the conversion efficiencies of silicon solar cells, advanced cell structures with selectively doped areas have received increasing interest. There is a strong need to separate the contacted diffusion profiles from the noncontacted. On the one hand, a high dopant concentration in the contact regime reduces the series resistance losses mainly due to lowered contact resistance. Additionally, recombination is reduced by shielding the minority charge carriers from surface at the contact. On the other hand, a low dopant concentration in the noncontact regime reduces the recombination losses and optimizes the spectral response of the cell. In this paper, phosphorus-doped silicon oxide layers are used as a diffusion source for tube furnace diffusion processes. It is shown that the sheet resistance of the diffused area is controlled by the silane gas flow during the deposition of phosphorus-doped silicon oxide. In order to analyze the influence of the diffused areas on the saturation current densities, symmetrical carrier lifetime samples are prepared. Therefore, a stack system consisting of a thermally grown silicon dioxide and silicon nitride is used for passivation purposes on textured samples.

Detailed investigation of the structural and passivation properties of silicon oxynitrides for silicon solar cells

AbstractWe investigated the structural and passivation properties of hydrogenated amorphous silicon oxynitrides (a ‐SiOxNy:H) prepared by plasma enhanced chemical vapor deposition. Using energy dispersive X‐ray analysis and X‐ray photoelectron spectroscopy, we determined the stoichiometry x = [O]/[Si] and y = [N]/[Si] in the whole range extending from a ‐Si:H (x, y =0) to SiO2 (x ∼2, y ∼0). To analyze the thermal stability, test samples were subjected to high temperature firing in the range 850 to 950 °C. Charge carrier lifetime measurements revealed that Si‐rich a ‐SiOxNy:H feature excellent surface passivation on p‐type CZ silicon in the as‐deposited state. After firing, a ‐SiOx/a ‐SiNy:H stacks with near‐stoichiometric a ‐SiOx showed an outstanding thermal stability. A fine tracing of the density and spectral position of the Si‐O‐Si stretching vibration by means of the Fourier transform infrared spectroscopy revealed valuable information on the incorporation of oxygen and nitrogen atoms during deposition. Accordingly, we found that a high density of Si‐O‐Si bonds approaching the characteristic frequencies of thermal SiO2 and silica (1075 to 1080 cm–1) is clearly correlated to the interface passivation quality. Due to hydrogen migration from an a ‐SiNy:H cap layer induced by firing, the a ‐SiO2/a ‐SiNy:H system feature passivation properties close to thermally grown silicon oxide (© 2012 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Photoluminescence enhancement of porous silicon particles by microwave‐assisted activation

AbstractPhotoluminescence (PL) of porous silicon (PSi) particles can be significantly enhanced in some organic solvents (i.e., ethanol or dimethyl sulfoxide) under microwave irradiation. Fourier transform infrared spectra, dynamic‐light‐scattering measurements, and scanning electron microscopy had been adopted to explore the mechanism of PL enhancement of PSi particles under microwave irradiation, which is attributed to the formation of higher porosity and the growth of silicon oxide by microwave‐assisted wet etching. Compared with that fabricated by ultrasonication, smaller luminescent PSi nanoparticles (average size ∼60 nm) with stronger orange‐red fluorescence (PL quantum yield ∼14.8%) and higher dispersibility can be large‐scale prepared for cellular imaging and drug delivery in biomedical applications.