Silica Buffer Layer Research Articles

Densely packed Ge quantum dots grown on SiO 2/Si substrate

We studied the growing process of Ge dots on silicon substrates covered with an ultrathin silicon dioxide buffer layer which was formed with simple chemical procedure. Uniform and densely packed (10 11 cm −2) quantum dots (QDs) were obtained by optimizing the growth parameter with the MBE method. The influence of temperature, coverage, as well as the post-annealing process, on the epitaxial and non-epitaxial nanodots formation was evaluated. Nano-sized high density quantum dots were also realized with different growing conditions, whose structural and growing mechanism were discussed under the help of SEM and RHEED results.

Properties of indium tin oxide films deposited on unheated polymer substrates by ion beam assisted deposition

The optical, electrical and mechanical properties of indium tin oxide (ITO) films prepared on polyethylene terephthalate (PET) substrates by ion beam assisted deposition at room temperature were investigated. The properties of ITO films can be improved by introducing a buffer layer of silicon dioxide (SiO 2) between the ITO film and the PET substrate. ITO films deposited on SiO 2-coated PET have better crystallinity, lower electrical resistivity, and improved resistance stability under bending than those deposited on bare PET. The average transmittance and the resistivity of ITO films deposited on SiO 2-coated PET are 85% and 0.90 × 10 − 3 Ω cm, respectively, and when the films are bent, the resistance remains almost constant until a bending radius of 1 cm and it increases slowly under a given bending radius with an increase of the bending cycles. The improved resistance stability of ITO films deposited on SiO 2-coated PET is mainly attributed to the perfect adhesion of ITO films induced by the SiO 2 buffer layer.

Improved silica-zirconia sol synthesis for fabrication of a single-mode embedded dielectric channel waveguide with low transmission losses

A novel inorganic-organic hybrid silica-zirconia solgel material, which can generate 10 microm thick film in a single spin-coating process, has been developed and employed in the fabrication of an embedded dielectric channel waveguide on a silica buffer layer of a silicon substrate. The fabricated channel waveguide core had steep ridge walls, good smoothness, and high robustness, and the novel sol synthesis enabled a precise control of the geometrical and optical parameters of the embedded dielectric channel waveguide. In the 1.55 microm telecommunication window, the fundamental modes TE(00) and TM(00) in the embedded channel waveguide had low transmission losses of 0.40 +/- 0.03 dB/cm and 0.59 +/- 0.03 dB/cm, respectively.

Determination of infrared optical parameters of SrTiO 3 thin films from the reflectivity spectrum

Strontium titanate thin films have been prepared at different oxygen pressures and subjected to different post-deposition annealing treatments. The films were deposited on Si substrates with a silica buffer layer. Infrared reflectance measurements were performed to determine the optical parameters, and the spectra were fitted using adequate dielectric function forms for the layers and the substrate. The as-deposited films were found to be amorphous. Their infrared spectra presented bands corresponding to highly damped vibrational modes. As the annealing time and temperature were increased, the strontium titanate layer optical parameters became closer to those corresponding to bulk SrTiO 3 and a strong increase of the silica layer thickness was observed. The silica layer growth was attributed to the gradual oxidation of the Si substrate during the annealing that induced also a blueshift of the silica Si–O asymmetric stretching frequency. This frequency shift was associated to a decrease in the silicon oxide density as the progressive layer oxidation originated a variation of its stoichiometry from SiO 1.5 towards amorphous SiO 2.

Carbon nanotubes synthesis in microwave plasma torch at atmospheric pressure

Well aligned multi-walled carbon nanotubes were synthesized at atmospheric pressure using a microwave plasma torch on silicon substrates with silicon oxide buffer layer and catalyst overlayer in the mixture of argon, hydrogen and methane. Iron or nickel was used as catalysts. The optimum substrate temperature for the deposition on Si/SiO 2/Fe substrates was about 970 K. In this case SEM micrographs of the deposits revealed a presence of vertically aligned nanotubes with the diameters around 15 nm. TEM micrographs showed a presence of amorphous carbon particles in the samples and some defects in the wall structure of the produced nanotubes. In Raman spectra two peaks at 1332 and 1584 cm −1 were observed. The CNTs were also synthesized on the substrates without SiO 2 buffer layer but their quality was lower. The synthesis with Ni instead of Fe catalyst required lower temperature and the alignment of the nanotubes was worse. The deposition process was monitored by optical emission spectroscopy. Atomic lines of hydrogen and argon, an emission of CN due to a presence of nitrogen impurities from atmosphere, a weak molecular band of CH and strong C 2 emission were detected in the spectra.

Effect of Mesoporous Silica Buffer Layer on the Orientation of MFI Zeolite Membranes

Growth of MFI zeolite membranes on porous α‐alumina substrates with and without precoated mesoporous silica buffer layer has been performed by in situ crystallization. A preferentially b‐oriented MFI membrane can be prepared on a mesoporous silica‐coated substrate. A membrane with a randomly oriented and eventually [h0h]‐oriented top layer is obtained on an uncoated α‐alumina substrate. The chemical properties of the substrate are suggested to have significant effects on the membrane orientation.

Infrared characterization of strontium titanate thin films

Strontium titanate thin films have been prepared at different oxygen pressures with various post-deposition annealing treatments. The films were deposited by pulsed laser ablation at room temperature on Si(001) substrates with a silica buffer layer. Infrared reflectance measurements were performed in order to determine relevant film parameters such as layer thicknesses and chemical composition. The infrared reflectance spectra were fitted by using adequate dielectric function forms for each layer. The fitting procedure provided the extraction of the dielectric functions of the strontium titanate film, the silica layer and the substrate. The as-deposited films are found to be amorphous, and their infrared spectra present peaks corresponding to modes with high damping constants. As the annealing time and temperature increases the strontium titanate layer becomes more ordered so that it can be described by its SrTiO3 bulk mode parameters. Also, the silica layer grows along with the ordering of the strontium titanate film, due to oxidation during annealing.

Study of silica–titania films doped with Er and Ag by RBS and ERDA

The photoluminescence (PL) efficiency of Er-doped silica–titania planar waveguides, prepared by sol–gel processing, may be enhanced through the incorporation of neutral Ag metal particles. However, the presence of residual Si–OH groups and the limited reproducibility of the doping process limit the optical activity of the Er 3+ ions. We have used Rutherford backscattering spectrometry (RBS) combined with elastic recoil detection analysis (ERDA) to study the incorporation behaviour of Ag and H atoms during the heat treatments used to densify the films. These were deposited by sol–gel spin-coating on single crystal Si or silica-buffered Si substrates (∼5 μm thick silica buffer layer). The results indicate a homogeneous distribution of Er in the as-deposited films. On the contrary, Ag displays a bimodal in-depth profile centred at the film surface and at the film/substrate interface. Annealing up to 700 °C leads to in-diffusion and concentration of all the Ag species at the latter interface. The H concentration maintains nearly the same values after annealing at 500 and 900 °C, with larger values for temperatures in between. During heat treatment the Er profile remains stable.

Annealing Induced Ordering of SrTiO3 Thin Films Deposited by Laser Ablation Over Si Substrates

Strontium titanate thin films have been prepared at different oxygen pressures and with different post deposition annealing treatments. The films were deposited by laser ablation at room temperature on Si(001) substrates with a silica buffer layer. The as-deposited films are amorphous with smooth and uniform surface. As the annealing temperature increase they become more crystalline, presenting a cubic SrTiO3 phase. The infrared characterization show that the silica layer also grows due to the annealing. The characteristics of this SrTiO3 crystalline depend on the oxygen partial pressure. At low oxygen pressures the annealed films are polycrystalline. As the oxygen pressure increase the films become more textured so that at longer enough annealing times the films become (200) oriented. The results are discussed in terms of atomic diffusion in the films.

Study of the Leakage Loss in a Silica-on-Silicon Slab Waveguide

A simple and efficient method for studying the leakage loss in a conventional silica-on-silicon planar waveguide is presented. The magnitude of the leakage loss is obtained by calculating the complex propagation constant of the leaky waveguide structure. The dependence of the leakage loss on the thickness of the silica buffer layer (as well as the thickness of the silica upper-cladding layer) is investigated. Numerical results are presented and compared to those calculated with the beam propagation method.

40-Gb/s X-cut LiNbO/sub 3/ optical modulator with two-step back-slot structure

We propose a newly designed X-cut lithium niobate (LiNbO/sub 3/) optical modulator. It has a two-step back-slot structure to satisfy the velocity-matching condition without the buffer layer of silicon dioxide (SiO/sub 2/). Accordingly, this modulator can achieve low drive voltage and low optical insertion loss. In addition, the dc-drift phenomena due to the buffer layer can be suppressed. This structure is fabricated with micromachining technology using excimer laser ablation. The optical 3-dB bandwidth of the fabricated modulator reaches 30 GHz, and the drive voltage is less than 3 V at 1 kHz. From the measurement of the optical eye diagram at 43.5-Gb/s, clear eye openings were obtained. This modulator is sufficient for 40-Gb/s optical transmission systems.

Optimization of microwave properties for ultrahigh-speed etched and unetched lithium niobate electrooptic modulators

Simultaneous phase velocity and characteristic impedance matching of the ultrahigh-speed electrooptic modulators is presented by using the finite-element method (FEM). It is also shown that the dielectric loss in the silica buffer layer is larger than that in the lithium niobate substrate and when these dielectric losses are included, the resulting bandwidth is reduced significantly. It is also shown that for an etched LN structure, it is relatively easier to match both N/sub m/ and Z/sub c/ simultaneously and the resulting optical bandwidth is also greater.

Photochromic coatings including silver halide microcrystals via sol–gel process

Photochromic coatings including silver halide microcrystals were formed on a glass substrate via the sol–gel process. 3-Glycidoxypropyltrimethoxysilane and methyltrimethoxysilane were used as starting materials of the ormosil matrices. The 3-chloropropyltrimethoxysilane and bromophenyltrimethoxysilane were added as halogen sources and silver colloidal dispersions were introduced into the precursor solution. The precursor solution was applied using a spin-coating process on a silica buffer layer which prevents Ag + migration into the glass substrate. The coated glass became transparent and photosensitive after Ag(Cl 1− x Br x ) microcrystals were precipitated in the coatings above 300°C. However, fading reaction did not proceed at room temperature. It was found that the deposition of a sensitizing layer including cuprous ions on the photosensitive layer was very effective in promoting fading reaction at room temperature. The cuprous ions in the sensitizing layer penetrated and diffused into the photosensitive layer during coating and firing. The cuprous ion located near or in the Ag(Cl 1− x Br x ) microcrystals acted as an effective sensitizer.

Optical spectroscopy of Pr 3+ ions in sol–gel derived GeO 2–SiO 2 planar waveguides

A GeO 2–SiO 2 planar waveguide, activated with 1% mol Pr 3+, has been prepared using a dip-coating technique. The GeO 2–SiO 2 film was deposited on a substrate consisting of a silicon wafer with a silica buffer layer. After annealing at 800°C and 900°C the waveguide luminescence and Raman spectra were measured. The Raman spectra show that complete densification is achieved after annealing at 800°C. Analysis of the emission spectra and the decay curves from the 3P 0 and 1D 2 states of Pr 3+ indicates that the non-radiative relaxation due to the rare-earth clustering is less important than in massive silica xerogels.

Deposition of thick TEOS PECVD silicon oxide layers for integrated optical waveguide applications

For integrated optical waveguides, based on a silicon substrate technology, it is necessary to insulate the dielectric optical medium from the substrate due to its light absorption characteristics. Silicon oxide is the most common material used as insulator layer (buffer), in addition, it is a MOS compatible material. The silicon oxide buffer layer, used in this work, was obtained by PECVD, using TEOS as silicon source, in a home made cluster tool system. This buffer layer has suitable optical properties and the PECVD deposition technique allows us to have an effective control of the refractive index. The buffer layer must be thick (∼3 μm) to avoid optical leakage to the silicon substrate. Due to the high mechanical stress (causing film cracking) characteristic of the TEOS PECVD silicon oxide, deposited in our lab, it was not possible to obtain an oxide thickness higher than 0.7 μm per deposition step. In order to achieve a silicon oxide thickness up to 4.2 μm with controlled refractive index, we developed a multilayer process, which consists of successive deposition steps of a thin silicon oxide layer (0.5 μm), followed by an RTA process after each deposition step. These films were analyzed by ellipsometry, FTIR, RBS, stress measurements and GIXR techniques. The main results are: thickness about 4 μm; constant refractive index (1.44±0.02) in all silicon oxide layers; reduction of O–H bond contamination; low surface roughness and uniform stoichiometry along the silicon oxide layers. The buffer layer was covered by an optical thin film, LPCVD silicon nitride, resulting in a reduction of the total residual stress.

Frequency-dependent propagation characteristicsofcoplanar waveguide electrode on 100 GHz Ti:LiNbO 3 optical modulator

The microwave propagation characteristics of a coplanar waveguide electrode in the frequency range 45 MHz – 110 GHz were studied in order to evaluate the factors which limit the performance of a high-speed ridged Ti:LiNbO3 optical modulator. Since the effective microwave index is constant in the higher frequency range, neither structure nor material dispersions of the modulator are observed. In addition to the conductor loss, the dielectric loss also increases above 20 GHz. The dielectric loss is shown to originate not only in the LiNbO3 substrate, but also in the silicon oxide buffer layer.

Fabrication and characterisation of a CdS-doped silica-on-silicon planar waveguide

A planar silica-titania waveguide doped with CdS nanocrystallites has been fabricated on a silicon substrate. A passive nanoporous host layer and a pure silica buffer layer, of thickness 2.2 and 7.5 µm, respectively, were deposited by the sol-gel technique. Crystallites were then precipitated in the porous matrix by soaking in a CdF2 solution followed by reaction in H2S gas, and a sputtered layer of silica was used to seal the structure. The absorption spectrum of an equivalent film indicates crystallites with radii up to ~26 A, while energy dispersion spectroscopy indicates a dopant concentration of ~0.5 volume %. The third order optical nonlinearity (Kerr effect) was measured using an m-line technique, yielding a value of –5 × 10-9 cm2/kW.

Rare-earth doped glass waveguides prepared by pulsed laser deposition

Rare-earth (Er,Yb) doped glass films are prepared by pulsed laser deposition onto different substrates: Si, fused quartz and Si with a silicon oxide buffer layer. Films grown in the presence of an oxygen pressure higher than6 × 10−2 mbar have a refractive index close and slightly higher than that of the bulk material thus proving that the density of the films is high. Films thicker than 1 μm are multimode waveguides with a refractive index of1.562±0.003 and show losses higher than 10 dB/cm which seem to be related to surface roughness due both to the substrate quality and the deposition process.

Single-mode SiON/SiO2/Si optical waveguides prepared by plasma-enhanced chemical vapor deposition

Abstract We present the characteristics of low-propagation-loss single-mode SiON / SiO2 / Si planar optical waveguides using plasma-enhanced chemical vapor deposition (PECVD). Using a thermal annealing process and a thick silicon dioxide buffer layer, we get an improvement on the propagation loss by the amounts of 1.24 dB /cm and 1.1 dB /cm, respectively. Optical waveguides with propagation loss lower than 0.3 dB /cm can be achieved.

Mode Coupling Between Dielectric and Semiconductor Planar Waveguides

Computer modeling studies on four-layer silicon-clad planar dielectric waveguides indicate that the attenuation and mode index behave as exponentially damped sinusoids when the silicon thickness is increased. This effect can be explained as a periodic coupling between the guided modes of the lossless structure and the lossy modes supported by the high refractive index silicon. Furthermore, the attenuation and mode index are significantly altered by conductivity changes in the silicon. An amplitude modulator and phase modulator have been proposed using these results. Predicted high attenuations in the device may be reduced significantly with a silicon dioxide buffer layer.