Silicon Suboxide Research Articles

Phase separation in SiOx films annealed under enhanced hydrostatic pressure

AbstractThe effect of enhanced hydrostatic pressure (HP, (10–12) × 108 Pa) on thermally stimulated phase decomposition of silicon suboxide layers processed at 450–1000 °C was investigated by infrared spectroscopy and photoluminescence measurements. HP stimulates decomposition of non‐stoichiometric SiOx most efficiently at about 450 °C. In spite of enhanced SiOx decomposition, visible photoluminescence appears in HP‐treated samples at higher annealing temperatures in comparison to those annealed under ambient pressure (AP, 105 Pa). Contrary to that, application of HP results in essential enhancement of near‐infrared emission at lower annealing temperatures as compared to processing under AP. This can be related to pressure‐stimulated crystallization of Si inclusions. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Optical properties of silicon nanocrystals in silica: Results from spectral filtering effect, m-line technique, and x-ray photoelectron spectroscopy

The optical properties of silica layers containing silicon nanocrystals are analyzed in terms of spectral filtering in absorbing planar waveguides (cutoff spectra), m-line measurements, and x-ray photoelectron spectroscopy (XPS). The effects of optical dispersion, approximation of weak guiding, and depth dependence of refractive index in a planar waveguide are studied. We compare the measured optical properties of silicon-rich silicon oxide samples with the values estimated by the Bruggeman theory using the XPS structural components. A good agreement between the measured and calculated refractive indices is found. The results for absorption suggest high transparency of the nanoscale-suboxide component in contrast to the corresponding bulk material. The Raman intensity of silicon nanocrystals is proportional to the XPS amount of bulk silicon. The extinction coefficient extracted for the Si component is between the values for crystalline and amorphous silicon. Annealing at higher temperatures decreases the Si component extinction coefficient, which is interpreted as a decrease in the amorphous Si fraction. The XPS method surprisingly suggests a large proportion of silicon suboxide even after annealing at 1200 °C.

Influences of Target-to-Substrate Distance and DepositionTemperature on a-SiOx/Indium Doped Tin Oxide Substrate as aLiquid Crystal Alignment Layer

We present the structural, optical, and electrical properties of amorphous silicon suboxide (a-SiO ) films grown on indium tin oxide glass substrates with a radio frequency magnetron technique from a polycrystalline silicon oxide target using ambient Ar. For different substrate-target distances (d = 8 cm and 10 cm), the deposition temperature effects were systematically studied. For d = 8cm, oxygen content in a-SiO decreased with dissociation of oxygen onto the silicon oxide matrix; temperature increased due to enlargement of kinetic energy. For d = 10 cm, however, the oxygen content had a minimum between 150 C and 200 C. Using simple optical measurements, we can predict a preferred orientation of liquid crystal molecules on a- SiO thin film. At higher oxygen content (x > 1.6), liquid crystal molecules on an inorganic liquid crystal alignment layer of a-SiO showed homogeneous alignment; however, in the lower case (x < 1.6), liquid crystals showed homeotropic alignment.

Control of thermochromic spectrum in vanadium dioxide by amorphous silicon suboxide layer

Vanadium dioxide ( VO 2 ) is a promising candidate for various thermochromic devices. Layering VO 2 film with amorphous silicon suboxide ( a - SiO x ) is an effective approach for controlling the thermochromic spectrum efficiently, since a - SiO x possesses an easily adjustable refractive index over a wide range. However, the crystalline quality of VO 2 is so sensitive to under- and/or over-coated materials that the a - SiO x coatings may be disadvantageous with respect to material suitability. In this study, the controllability of the thermochromic spectrum of VO 2 by employing a - SiO x layer was evaluated under several fabrication conditions and, as a result, it was found that an a - SiO x layer can control thermochromism well without any decline in the stability of VO 2 crystalline growth, which strongly suggests the possibility of expansion in the optical design for efficient thermochromic control.

Structure, composition, morphology and electrical properties of amorphous SiO x (0 < x < 2) thin films

Thin films of amorphous silicon suboxide (a-SiO x with x < 2) have been deposited by reactive r.f. magnetron sputtering from a polycrystalline-silicon target in an Ar/O 2 gas mixture. Previously, we have shown that highly oxygenated a-SiO x layers are structurally inhomogeneous. In this paper we bring evidences for the deposition pressure influence on the material's physical properties. The layers' composition was investigated by energy dispersive X-ray (EDX) measurements. Fourier transform infra-red (FTIR) spectroscopy and X-ray photoemission spectroscopy (XPS) were used to study the local atomic structure of the deposited layers. The surface morphology was comprehensively characterised by atomic force microscopy (AFM). The electrical characteristics of metal – SiO x – metal sandwich samples are also described in this paper focussing on electrical conductivity and layer capacity. Special attention was paid to an empirical relationship between the internal structure and its electrical properties. Both the surface morphology and the electrical properties are influenced by the internal structure that is designed by modifying the deposition parameters.

Linear and nonlinear optical properties of Si nanocrystals in SiO2 deposited by plasma-enhanced chemical-vapor deposition

Linear and nonlinear optical properties of silicon suboxide SiOx films deposited by plasma-enhanced chemical-vapor deposition have been studied for different Si excesses up to 24at.%. The layers have been fully characterized with respect to their atomic composition and the structure of the Si precipitates. Linear refractive index and extinction coefficient have been determined in the whole visible range, enabling to estimate the optical bandgap as a function of the Si nanocrystal size. Nonlinear optical properties have been evaluated by the z-scan technique for two different excitations: at 0.80eV in the nanosecond regime and at 1.50eV in the femtosecond regime. Under nanosecond excitation conditions, the nonlinear process is ruled by thermal effects, showing large values of both nonlinear refractive index (n2∼−10−8cm2∕W) and nonlinear absorption coefficient (β∼10−6cm∕W). Under femtosecond excitation conditions, a smaller nonlinear refractive index is found (n2∼10−12cm2∕W), typical of nonlinearities arising from electronic response. The contribution per nanocrystal to the electronic third-order nonlinear susceptibility increases as the size of the Si nanoparticles is reduced, due to the appearance of electronic transitions between discrete levels induced by quantum confinement.

High quality passivation for heterojunction solar cells by hydrogenated amorphous silicon suboxide films

In this letter, we report on our investigations of hydrogenated amorphous silicon suboxides (a-SiOx:H) used as a high quality passivation scheme for heterojunction solar cells. The a-SiOx:H films were deposited using high frequency (70MHz) plasma enhanced chemical vapor deposition by decomposition of carbon dioxide, hydrogen, and silane at a substrate temperature of around 155°C. High effective lifetimes of outstanding 4ms on 1Ωcm n-type float-zone material and a surface recombination velocity of ⩽2.6cm∕s have been repeatedly obtained. Optical analysis revealed a distinct decrease of blue light absorption in the a-SiOx:H films compared to commonly used intrinsic amorphous silicon passivation used in heterojunction cells.

Mechanism of apatite formation on silicon suboxide film prepared by pulsed metal vacuum arc deposition

Pulsed metal vacuum arc deposition technology was employed to fabricate SiO x ( x ≤ 2) film at the oxygen partial pressure at 0.6 × 10 −2 Pa. Surface characteristics of the as-deposited SiO x film were studied by X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). Results show that the film is substoichiometric and abundant oxygen vacancies exist in the film. The as-deposited film was immersed in simulated body fluid (SBF) for evaluation of in vitro bioactivity. After incubation in SBF, ball-like apatite particles were found on the film surfaces by scanning electron microscopy (SEM), X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR). The mechanism of apatite deposition on the SiO x film was explained in terms of the silanol functional groups formed on the surface of the film after immersion in SBF. There are abundant active oxygen vacancies in the SiO x film that were attacked by the water molecules in the solution and then formed silanol groups in the film. The silanol groups adsorbed calcium and phosphate ions from the solution to the hydrated surface to form critical apatite cluster. Once the apatite nuclei are formed, they spontaneously grow by consuming calcium and phosphate ions in the surrounding fluid.

Conducting two-phase silicon oxide layers for thin-film silicon solar cells

AbstractWe present optical properties and microstructure analyses of hydrogenated silicon suboxide layers containing silicon nanocrystals (nc-SiOx:H). This material is especially adapted for the use as intermediate reflecting layer (IRL) in micromorph silicon tandem cells due to its low refractive index and relatively high transverse conductivity. The nc-SiOx:H is deposited by very high frequency plasma enhanced chemical vapor deposition from a SiH4/CO2/H2/PH3 gas mixture. We show the influence of H2/SiH4 and CO2/SiH4 gas ratios on the layer properties as well as on the micromorph cell when the nc-SiOx:H is used as IRL. The lowest refractive index achieved in a working micromoph cell is 1.71 and the highest initial micromoph efficiency with such an IRL is 13.3 %.

Distinct processes in radio-frequency reactive magnetron plasma sputter deposition of silicon suboxide films

A detailed investigation of the distinct processes in radio-frequency reactive magnetron plasma sputter deposition of SiOx films in a O2∕Ar atmosphere has been carried out, using the experimental evaluation of the individual growth rates of silicon and oxygen and of the ion impingement on the growth surface. Experimental variables are the total pressure, the oxygen partial pressure necessary to grow layers with 0⩽x⩽2, the RF power, the substrate temperature during deposition and the height of the cathode with respect to the growth surface. The various possible contributions to the silicon and oxygen incorporation on the growth surface have been distinguished and the magnitude of their contribution estimated, including that of sputtered SiO molecules. A model concerning the oxygen coverage on the cathode erosion area during sputtering is discussed, including the transition from the metallic cathode to the poisoned, nonmetallic, cathode.

Embedded structure of silicon monoxide in SiO 2 films

The structure of SiO x ( x = 1.94) films has been investigated using both X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (TOF-SIMS). The SiO x films were deposited by vacuum evaporation. XPS spectra show that SiO 1.94 films are composed of silicon suboxides and the SiO 2 matrix. Silicon clusters appeared only negligibly in the films in the XPS spectra. Si 3O + ion species were found in the TOF-SIMS spectra with strong intensity. These results reveal the structure of the films to be silicon monoxide embedded in SiO 2, and this structure most likely exists as a predominant form of Si 3O 4. The existence of Si–Si structures in the SiO 2 matrix will give rise to dense parts in loose glass networks.

Infrared and Photoluminescence Study of Rapidly Thermally Annealed SiO&lt;sub&gt;x&lt;/sub&gt; Thin Films

Silicon suboxide, SiOx thin films with different oxygen contents (1.15≤x≤1.70) were prepared by thermal evaporation of silicon monoxide at a residual oxygen pressure of 1·10-3 Pa and deposition rates of 0.2, 1.0, 3.0 and 6.0 nm/s. Rapid thermal annealing (RTA) of films was carried out at 1100°C in vacuum for 15 and 30 s and the films were analyzed by infrared (FTIR) and photoluminescence (PL) spectroscopy. In the FTIR spectra of SiOx annealed samples, a blue-shift of the stretching band with initial oxygen content, x, is observed. This band is shifted to a much lower frequency with prolonged RTA time. This behavior can be interpreted in terms of the partial decrease of oxygen content and film density upon annealing in vacuum. With annealing time increase a new band at 1106 cm-1 appears. Therefore, infrared spectra of SiOx films are significantly affected by the oxygen content. PL spectra of these films also change drastically with increasing annealing time. In the PL spectra of films annealed for 15 s two bands are easily visible: broad redorange band at 2.2 eV and a green band at 2.4 eV, while for 30 s annealing only low-energy band exists. Green band is connected with the defects in the SiOx matrix while red-orange band can be deconvoluted into several bands at 2.0, 2.3 and 2.5 eV. Two types of defects can be responsible for the PL band at about 2 eV: defects in a-Si (amorphous silicon) nanoparticles separated during RTA, and nonbridging oxygen hole recombination centers (NBOHC) formed by loosing of oxygen during RTA. The PL band at 2.3 eV is associated with the defects formed at a-Si/SiOx interfaces while a hardly visible band at 2.5 eV is related to the defects connected with the oxygen deficiency formed in the SiOx matrix during RTA.

Influence of the annealing temperature on the photoluminescence of Er-doped SiO thin films

Er-doped amorphous silicon suboxide thin films were prepared by the coevaporation method. The Er concentration was varied from 0.4to6at.% and the samples were annealed at different temperatures up to 900°C. The samples exhibit a broad photoluminescence band in the visible range. Both energy and intensity of this band were dependent on the annealing temperature. For as-deposited films and samples annealed below 500°C, this band was assigned to defects in the oxide films. For higher annealing temperatures, this photoluminescence band shifted to higher wavelengths and was correlated to the appearance of amorphous silicon clusters. Two narrow bands in the near-infrared range at 0.98 and 1.54μm were also observed for the annealed samples. The intensity of these Er-related luminescence was maximal for an annealing temperature equal to around 700°C. The effective absorption cross section of Er was dependent on the annealing temperature and was equal to 6.6×10−16cm2 for the sample annealed at 700°C. The strong Er-related photoluminescence is discussed in terms of a coupling phenomenon between Er3+ ions and spatially confined amorphous silicon clusters which act as sensitizers. The existence of a low annealing temperature to obtain the best Er-related photoluminescence is also discussed.

Atomic scale model interfaces between high-khafnium silicates and silicon

Hafnium silicates $({\mathrm{Hf}}_{y}{\mathrm{Si}}_{1\ensuremath{-}y}{\mathrm{O}}_{2})$ are being considered as high-$k$ gate dielectrics in field-effect transistors as a compromise between high permittivity and thermal stability on silicon during complementary metal-oxide-semiconductor processing. Using hafnon $(\mathrm{Hf}\mathrm{Si}{\mathrm{O}}_{4})$ as a prototypical hafnium silicate, we have explored model abrupt interfaces at the atomic scale, employing various hafnon surfaces on Si(100) and Si(110) via a silicon suboxide interfacial layer. Relative stabilities are computed using molecular dynamics and geometry optimization at the density-functional theory level. The immiscibility of $\mathrm{Hf}{\mathrm{O}}_{2}$ and $\mathrm{Si}{\mathrm{O}}_{2}$ means that the interface is intrinsically metastable, so that we predict a poor-quality interface under oxygen-poor conditions. On the other hand, the interface is stabilized under oxidizing conditions, leading eventually to the growth of the interfacial layer into the Si substrate. We find evidence that Si(110) resists oxidation more than Si(100). Structural features such as $\mathrm{Hf}\ensuremath{-}\mathrm{O}\ensuremath{-}\mathrm{Si}$ and $\mathrm{Si}\ensuremath{-}\mathrm{O}\ensuremath{-}\mathrm{Si}$ bridges are found to be common to all defect-free interfaces under all conditions. $4\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$ is the minimum thickness of the defect-free suboxide interfacial layer.

Apatite Formation on Silicon Suboxide Film Prepared by Pulsed Metal Vacuum Arc Depositon

In this paper, pulsed metal vacuum arc deposition technology was employed to fabricate SiO x film on Ti-6Al-4V substrates with the oxygen partial pressure at 0.2∼0.5x10 -2 Pa. X-ray photoelectron spectroscopy and atomic force microscopy were used to characterize the as-deposited SiO x film. Simulated body fluid (SBF) immersion tests were performed for the in vitro bioactivity evaluation of the as-deposite film. The results obtained from the scanning electron microscopy, X-ray diffraction and Fourier transform infrared demonstrated the formation of apatite on the SiO x film after incubation in SBF solution. The bioactivity maybe related to the plenty of oxygen vacancies in the film, which are active and hydrolyze in the solution to form Si-OH groups. It is necessary to induce the formation of apatite.

A study of the growth of Lu2O3 on Si(001) by synchrotron radiation photoemission and transmission electron microscopy

Rare-earth oxides are among the materials which are presently studied as possible replacements of amorphous silicon dioxide as gate insulators in nanometric Si devices; in fact, they generally exhibit high values of the dielectric constant (“high κ”), a necessary requirement to obtain a high capacitance with layer thickness greater than the value below which tunneling currents become unacceptably high. Lu2O3 is one of the rare-earth oxides which may have the required properties in view of its quite high values of κ and forbidden band gap. Since the envisaged dielectric layers are only a few nanometers thick, a description and a physical understanding of the atomic and electronic structure of the interface are of great importance. In this paper, we report a study by synchrotron radiation photoemission and transmission electron microscopy of the growth of Lu2O3 on Si(001). Thanks to the high spectral and spatial resolution, we provide clear evidence of a rather complex structure in which all silicon suboxides and SiO2 are present at the same time, along with a silicatelike phase and Lu2O3 itself; moreover, some grains and both crystalline and amorphous portions are present. In the photoemission experiment, the contribution of the Si surface to the core level line shapes has been taken into account; in the electron microscopy measurements, we present line scans on the nanometer scale of O, Si, and Lu concentrations and a Fourier transform discussion of the structure of the crystalline portions of the overlayer. The valence band discontinuity, which is measured in situ and is relative to the structurally well characterized interface, is found to be 3.16±0.16eV. These findings are discussed in relation to the suitability of Lu2O3 as a high-κ dielectric and in the context of available theoretical predictions of thermodynamic stability versus the formation of silicon oxide, silicates, and silicides and of the band discontinuity problem.

Swift heavy ion induced modifications of silicon (sub) oxide nitride layer structures

Irradiation of a bilayer consisting of a deuterated silicon nitride and a silicon sub-oxide film with heavy ions in the electronic stopping regime results in incorporation of nitrogen and deuterium in the sub-oxides. This experimental observation is discussed within the model, originally established to understand the loss of hydrogen from hydrogenated organic and inorganic materials. This and other reported observations of transport within the bilayer emphasize the role of trapping and permeation in the model.

Room-temperature atmospheric oxidation of Si nanocrystals after HF etching

The effect of HF etching of the silicon oxide shell covering the surface of Si nanocrystals (NCs) on the subsequent room-temperature atmospheric oxidation of Si-NCs has been investigated by means of photoluminescence measurements, Fourier transform infrared spectroscopy, and electron paramagnetic resonance spectroscopy. After the HF etching, the surface of Si-NCs is found to be H terminated. The HF etching also restructures the surface of Si-NCs. This leads to a decrease in the incorporation of O during subsequent oxidation, which finally results in silicon suboxide $\mathrm{Si}{\mathrm{O}}_{1.9}$. In contrast, without the HF etching stoichiometric $\mathrm{Si}{\mathrm{O}}_{2}$ is formed. A smaller ratio of O to Si in the silicon oxide results in a higher density of defects. This contributes to a more significant oxidation-induced decrease in the intensity of photoluminescence from Si-NCs after the HF etching than without the HF etching.

Stabilizing Zigzag Single-Walled Silicon Nanotubes and Tailoring the Electronic Structures by Oxygen Atoms: First-Principles Studies

The effects of oxygen atoms in stabilizing zigzag single-walled silicon nanotubes (SWSiNTs) have been studied by using first-principles calculations within density functional theory. The incorporation of oxygen atoms in the form of silicon monoxides into the (8,0)SWSiNT are found to not only stabilize the SWSiNT but also tailor the electronic structures from semiconducting to metallic. These findings will promote the fabrication and the utilization of silicon suboxide nanotubes which may find potential applications in nanoscale electronics and optoelectronics.

Optical emission studies of the gas phase during the silicon suboxide deposition by reactive r.f. magnetron-sputtering

Silicon oxide (a-SiO) is one of the most used silicon-based materials in optoelectronic and microelectronic technology. It is well-known that the electronic properties are linked to the material structure, which depends on the deposition technique and on the details of the deposition. Silicon suboxide (a-SiO x 0 < x < 2) layers were prepared by r.f. magnetron-sputtering from a polycrystalline silicon target in a well defined oxidation environment. Optical Emission Spectroscopy (OES) was employed to study the plasma used in SiO x depositions. Theoretical calculations performed in the frame of electron impact excitation mechanism for argon and atomic oxygen corroborated with electron impact dissociation of the molecular oxygen have shown the strong influence of the electron temperature on the rate coefficients of photons' production. Correlation functions between the OES signals assigned to silicon and oxygen atoms from plasma (gas phase) and the SiO x layer composition (infrared and energy dispersive X-ray investigated) have been found. Based on these functions, the OES plasma monitoring is proposed as a tool to control in-situ and in direct time the SiO x layer composition.