About the Fluorine Chemistry in MCVD: The Influence of Fluorine Doping on SiO2 Deposition

In the fabrication of quartz glass fibers for optical communication, fluorine doping gets increasing importance. In contrast to other dopants, fluorine influences the deposition of SiO₂ in Modified Chemical Vapour Deposition. An equation is derived from considerations of equilibrium chemistry which yields the deposition efficiency of SiO₂ as function of fluorine doping. It is compared with experimental results.     

Fluorine incorporation in silica glass by the MCVD process: Study of fluorine incorporation zone,evaluation of optical properties and structure of the glass

A theoretical approach was made to find out a complete fluorine incorporation zone on a ternary diagram which serves as a useful graphical representation to select the flows of the supplied reagents for incorporation of the suitable amount of fluorine into cladding glass of optical fiber preform made by the MCVD process using CCl₂F₂ as a source of fluorine under oxygen abundance, oxygen deficiency and intermediate oxygen state conditions. The possible mechanism for incorporation of fluorine into cladding glass of optical fiber is also evaluated on the basis of the thermodynamical data. The fluorine incorporation mechanism in silica glass by the MCVD process is found to be dependent on the CCl₂F₂/SiCl₄ ratio in the input gas mixture. Fluorine doping is found to be effective for removing the strained Si–O–Si bonds, which govern the optical transparency in deep ultra-violet (DUV) and vacuum ultra-violet (VUV) regions. The maximum refractive index depression of ?0.5 × 10^?3 is obtained with incorporation of fluorine into silica cladding glass by the MCVD process using CCl₂F₂ as a dopant precursor with suitable flow of SiCl₄ vapor along with O₂ through backward deposition pass. The structure of fluorine doped silica glass preform samples containing 1.70–1.79 mol% fluorine incorporated by the MCVD process based on the analyses of ¹⁹F MAS spectra done by high-resolution ¹⁹F NMR spectroscopy reveal the presence of two distinct types of fluorine environments. The majority of the fluorine environments are formed in SiO_1.5F polyhedral and less abundant species is observed to be highly unusual, yielding a fivefold coordinated silicon of the type SiO₂F polyhedral which become increased with increasing the fluorine content.     

Thermodynamics of fluorine incorporation into silica glass

Fluorine incorporation from the gaseous phase into silica glass was investigated both during the deposition of synthetic silica layers and during the sintering of pre-deposited porous silica layers, using different fluorine-containing compounds. The fluorine content dependence on the gas concentration and the influence of fluorine on the deposition efficiency of silica were determined. The results could be interpreted and quantitatively described by a simplified model of the equilibrium chemistry, valid for different fluorine sources.     

Incorporation of dopants into epitaxial CVD-Silicon layers and incorporation equilibrium

Based on the temperature dependence of the layer growth rate influence on the incorporation of dopants into epitaxial silicon the range of validity of an incorporation equilibrium between dopants in the gas phase and in the layer is discussed. Additional to the borderline case of an infinitely low layer growth rate the incorporation equilibrium is equally important to high layer deposition temperature. For this region deducing the incorporation enthalpy from the temperature dependence of the incorporation of dopants proves adequate.     

Reflectivity measurements of thin SiO2 layers in the soft-X-ray region

Energy and angular dependent reflectivity measurements of differently prepared SiO₂ layers were made in the soft X-ray region in order to study differences in their optical and electronic behaviour caused by the technological process. The optical constants of the layers were determined by fitting the angular dependent reflectivity data with a multilayer model which takes into account the roughness of all interfaces. Detailed knowledge of the polarization level of the primary beam from the synchrotron radiation source is needed for reliable results. The energy dependent reflectivity measurements indicate that high-pressure dry-oxidized layers have a more perfect structure than normal dry-oxidized layers. Independent on the deposition process the Si SiO₂ interface roughness amounts to ± 1 nm. The Si_2p-XANES of Si and SiO₂ were detected.     

Composition of Ge+ and Si+ implanted SiO2/Si layers: Role of oxides in nanocluster formation

X-ray photoelectron spectroscopy (XPS) has been used to examine the atomic content of implanted SiO₂/Si layers. In particular, an XPS analysis permits to identify elemental Ge and Si, as well as GeO₂ precipitations in SiO₂ matrices. The XPS results reveal valuable information not only about the formation mechanism of Ge and Si nanoclusters but also on the annealing kinetics of SiO₂ whose properties are known to be significantly altered during the process of ion implantation and subsequent annealing. The composition of ion beam-modified SiO₂ layers strongly depends on the annealing temperature. With respect to germanium implanted samples a possibility of Ge nanocrystals formation appears at high (above 1000 °C) annealing temperatures. It has been shown that an intermediate step in the Ge oxide formation is necessary for the creation of Ge nanoclusters. Additionally, the presence of a subsurface zone GeO_x (about 100 nm thick) predicted in kinetic three-dimensional lattice simulations has been confirmed. In the case of Si⁺ implanted samples substoichiometric silicon oxide lines in the XPS spectra of a SiO₂ layer for all samples have been observed. No evidence of a line connected to the Si–Si bonding has been observed even at the highest annealing temperatures, at which only stoichiometric SiO₂ has been detected.     

The incorporation of Cr into gallium arsenide grown by the LEC method

To investigate the incorporation of Cr into LEC GaAs, we have grown nine Cr-doped GaAs crystals where the dopant is labelled with radio-tracer ⁵¹Cr. A thermodynamic model is developed to describe the segregation behaviour in which the distribution coefficient of the Cr is a function of the arsenic activity in the melt. The analysis shows that either the congruent melt is ˜ 2% richer in Ga than the stoichiometric composition or, much more probably, that a significant amount of Ga is lost into the B₂O₃ encapsulant. The latter postulate is shown to be consistent with recent studies by Emori et al. [Japan. J. Appl. Phys. 24 (1985) L291].     

Electronic mechanism of thermal destruction of radiation-induced E'-centers in crystalline and glassy SiO2

The thermal decay regularities for radiation-induced E'-centers in crystalline and glassy SiO₂ were investigated. The results obtained point out that the destruction of E'-centers can be described as ionization process of deep centers in electric field. In terms of used model, the electric field and electron–vibration coupling parameters are sensitive to structural disorder. The most weak electron–phonon coupling in E'-centers is observed for amorphous systems.     

Effects of fluorine incorporation on the properties of amorphous carbon/p-type crystalline silicon heterojunction diodes

Heterojunction diodes fabricated by plasma enhanced chemical vapor deposition of hydrogenated amorphous carbon (a-C:H) and fluorine-doped amorphous carbons (a-C:H:F) on p-type silicon are analyzed in terms of their electronic and photovoltaic properties. Their structural and optical properties were identified by Raman spectroscopy, X-ray photoelectron spectroscopy, ellipsometry, and UV-VIS transmittance. The nature of the heterojunction is confirmed by the rectifying current-voltage characteristic of carbonaceous deposits/p-Si junction. The diodes show a behavior dependent on the amount of the fluorine content. The photovoltaic behavior of the junction is investigated as a function of both fluorine incorporation and thermal treatment of the a-C:H:F films after the deposition. Better photovoltaic effects were observed from annealed a-C:H:F heterojunction structures. The optical and structural characterization performed on films after the thermal treatment indicates that this behavior is most likely due to an extended graphitization.     

Electroluminescence from Er and Yb co-doped silicon dioxide layers: The excitation mechanism

The excitation mechanism of photo- (PL) and electroluminescence (EL) of erbium ions co-implanted with ytterbium into the SiO₂ layer of light emitting MOS devices (MOSLED) was investigated. Ytterbium implanted and annealed samples exhibit the blue and near infrared electroluminescence. The blue electroluminescence at 470 nm appears due to cooperative up-conversion emission in the Yb³⁺-Yb³⁺ system, and the near infrared EL at 975 and 1025 nm corresponds to transitions from the multiple state ²F_5/2 to the ²F_7/2 ground state in the Yb³⁺ ions. The Er implanted SiO₂ exhibits the luminescence in the blue-green and infrared region. The green and blue peaks correspond to radiative transitions from the ²H_11/2 or ⁴S_3/2 energy levels and from the ²H_9/2 or ⁴F_5/2 energy levels to the ⁴I_15/2 ground state, respectively. We have found that the energy transfer from Yb³⁺ to Er³⁺ ions exists only during photoluminescence excitation. The electroluminescence investigation shows the cooperative up-conversion in the Er³⁺-Yb³⁺ system.     

On the Chemical Mechanism of the Zinc Diffusion into GaAs from SiO2 Spin-on Films

The chemical processes which take place during annealing were studied in both model substances and thin films deposited onto GaAs wafers by thermal analysis, X-ray powder diffraction, IR spectroscopy, XPS and TEM. It was found that at 600°C only about 45% of the film was changed to SiO₂ glass. At 700 °C begins the formation of zinc silicates. In this form the zinc seems to be scarcely able to diffuse into GaAs. Obviously, the exothermic DTA effect at 800 °C arises from this zinc silicate formation connected with a recrystallization of the amorphous film.     

SiO2-B2O3 xerogels: The problem of boron leaching

Borosilicate materials are of interest for many technological applications. Whereas sol-gel techniques are well suited for the preparation of these materials, the nominal concentration of boron is often not maintained, and a considerable amount of boron is lost during the preparation procedure. In this work we deal with the synthesis of borosilicate systems in order to analyse the problem of the boron loss. Different sol-gel precursors of B₂O₃ were explored and organically modified silicon alcoxide were tested, in addition P₂O₅ co-doping was proposed. IR spectroscopy, differential Scanning Calorimetry, Thermogravimetry and ICP-OES analyses were performed. Sol-gel synthesis of SiO₂-B₂O₃ xerogels with a high boron content (> 18 wt.%) was successfully obtained by employing trimethoxyboroxine and 3-aminopropyltriethoxysilane.     

Nb redeposition on Si during plasma etching of Nb/SiO2/Si layers investigated by RBS

The redeposition of electrode material (Ni, Cr, Al) and material of a Nb layer on silicon surface during plasma etching of Nb, SiO₂ and Si by a CF₄ plasma has been investigated by Rutherford Back Scattering spectrometry. It is shown that a steady state exists, what causes a Nb contamination of etched Si surfaces. Obviously this steady state concentration (Nb)_ads is influenced by additional redeposition of non-etchable electrode material as Ni, Cr and Al.     

On the kinetics of nitrogen incorporation in GaP LPE layers using NH3 vapour doping

The partial substitution of N for P in GaP has a considerable effect on the luminescence quantum efficiency of GaP epitactic layers. The incorporation of N in GaP liquid phase epitactic layers using NH₃ vapour doping has been studied at different H₂ partial pressures in argon gas. It is shown that the incorporation of N in different ambients is conveniently described by just one parameter. The reaction of NH₃ with Ga is found to be an equilibrium process. The N concentration in the layer is directly proportional to the GaN concentration in the melt. From the correlation of the dissociation of NH₃ with the formation of GaN it is concluded that the NH₃ pressure, as measured in the exit gas, is not representative of the NH₃ pressure at the melt. The dissociation of NH₃ is found to be strongly retarded by H₂ owing to adsorption of H₂ on the surface of reactor materials which catalyse the dissociation.     

Heterogeneous equilibria and distribution of elements in the MCVD process for making optical fiber

Thermodynamic calculations have been performed on systems relevant to doped silica optical fiber processing: SiCl₄---GeCl₄---POCl₃---O₂, SiCl₄---BBr₃---O₂, O₂---H₂O---CH₄---SOCl₂---SiO₂ and O₂---H₂O---CH₄---CF₄---SiO₂ by using the SOLGASMIX-PV computer program. Equilibrium distributions of Ge, P, Si, B and H species are given.     

Peculiarities of the cross-section in chemically prepared SnO2- and SnO2:Sb layers

Layers of pure as well as Sb-doped tin dioxide have been prepared using intentionally simple modification of CVD technique. By electron microscopy of cross-section, at least two sublayers have been observed in samples of thickness of a few micrometers in both of the cases. Their formation is thought to be associated with small perturbation of thermodynamical equilibrium arising due to the layer growth.     

The influence of HF on the incorporation of silicon into the erionite-like AlPO4-17 molecular sieve

The synthesis of SAPO-17 with high silicon content (up to 18 weight-%) succeeded with the addition of HF to the reaction mixtures. The characterization of the samples with chemical analysis, XRD, and adsorption measurements show the incorporation of silicon into tetrahedrally coordinated framework positions. Spectroscopic methods (IR, ¹⁹F MAS NMR) demonstrate the presence of fluorine in the as-synthesized samples. The effect of the fluorine consists in (i) the improvement of the silicon incorporation into framwork positions, (ii) the stabilization of the as-synthesized samples and (iii) the charge compensation of the template ion by F⁻.     

Lattice constants and site preference in the system Ni2SiO4—Co2SiO4

Most important supposition for a possible site preference in a olivine solid-solutions are besides the different crystal chemical properties of M cations (size, electronegativity, crystal field stabilization energy) the differences between the both octahedral coordinated sites M1 and M2 of the olivine structure. Using this regularities the site preference in the system Ni Co olivines is predicted. If site preferences are existing, it is possible to deduce deviations of the Vegard's rule in the system Ni₂SiO₄—Co₂SiO₄ using geometrically-structurally considerations. The plot of experimentally determined lattice parameters in this solid solutions system shows the existing of the proposed deviations of the Vegard's rule and evidences the site preferences of Ni²⁺ in the M1- and Co²⁺ in the M2 octahedra only by determination of lattice parameters.