Browsing by Subject "Silicon oxides"
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Item Open Access Analysis of Fe nanoparticles using XPS measurements under d.c. or pulsed-voltage bias(2010) Süzer, Şefik; Baer, D. R.; Engelhard, M. H.The impact of solution exposure on the charging properties of oxide coatings on Fe metal-core oxide-shell nanoparticles has been examined by sample biasing during XPS measurements. The Fe nanoparticles were suspended in relatively unreactive acetone and analyzed after particles containing solutions were deposited on SiO2/Si or Au substrates. The particle and substrate combinations were subjected to ±10V d.c. or ±5V a.c., biasing in the form of square wave (SQW) pulses. The samples experienced variable degrees of charging for which low-energy electrons at ∼1eV, 20 μA and low-energy Ar+ ions were used to minimize it. Application of d.c. bias and/or SQW pulses significantly influences the extent of charging, which is utilized to gather additional analytical information about the sample under investigation. This approach allows separation of otherwise overlapping peaks. Accordingly, the O1s peaks of the silicon oxide substrate, the iron oxide nanoparticles, and that of the casting solvent can be separated from each other. Similarly, the C1s peak belonging to the solvent can be separated from that of the adventitious carbon. The charging shifts of the iron nanoparticles are strongly influenced by the solvent to which the particles were exposed. Hence, acetone exhibited the largest shift, water the smallest, and methanol in between. Dynamical measurements performed by application of the voltage stress in the form of SQW pulses provides information about the time constants of the processes involved, which leads us to postulate that these charging properties we probe in these systems stem mainly from ionic movement(s).Item Open Access Charging/discharging dynamics of CdS and CdSe films under photoillumination using dynamic x-ray photoelectron spectroscopy(A I P Publishing LLC, 2010) Sezen, H.; Süzer, ŞefikThin films of CdS and CdSe are deposited on HF-cleaned Si O2 /Si substrates containing ∼5 nm thermally grown silicon oxide. x-ray photoelectron spectroscopy (XPS) data of these films are collected in a dynamic mode, which is based on recording the spectrum under modulation with an electrical signal in the form of ±10 V square-wave pulses. Accordingly, all peaks are twined and shifted with respect to the grounded spectrum. The binding energy difference between the twinned peaks of a dielectric system has a strong dependence on the frequency of the electrical stimuli. Therefore, dynamic XPS provides a means to extract additional properties of dielectric materials, such as effective resistance and capacitance. In this work, the authors report a new advancement to the previous method, where they now probe a photodynamic process. For this reason, photoillumination is introduced as an additional form of stimulus and used to investigate the combined optical and electrical response of the photoconductive thin films of CdS and CdSe using dynamic XPS.Item Open Access Conversion of wooden structures into porous SiC with shape memory synthesis(2011) Dhiman, R.; Petrunin V.; Rana, K.; Morgen P.Synthesis of structured silicon carbide materials can be accomplished using wooden materials as the carbon source, with various silicon impregnation techniques. We have explored the low cost synthesis of SiC by impregnation of carbon from wood with SiO gas at high temperatures, which largely retains the structure of the starting wood (shape memory synthesis). Suitably structured, porous SiC could prove to be an important type of catalyst support material. Shape memory synthesis (SMS) has earlier been tried on high surface area carbon materials. Here we have made an extensive study of SMS on carbon structures obtained from different types of wood. © 2011 Elsevier Ltd and Techna Group S.r.l.Item Open Access Laser induced sponge-like Si in Si-rich oxides for photovoltaics(Optical Society of American (OSA), 2013) Gundogdu, S.; Sungur Ozen, E.; Hübner, R.; Heinig, K.H.; Aydınlı, AtillaWe show that a sponge-like structure of interconnected Si nanowires embedded in a dielectric matrix can be obtained by laser annealing of silicon rich oxides (SRO). Due to quantum confinement, the large bandgap displayed by these percolated nanostructures can be utilized as a tandem stage in 3rd generation thin-film solar cells. Well passivated by the SiO2 dielectric matrix, they are expected to overcome the difficulty of carrier separation encountered in the case of isolated crystalline quantum dots. In this study PECVD grown SRO were irradiated by a cw Ar+ laser. Raman spectroscopy has been used to assess the crystallinity of the Si nanostructures and thus to optimize the annealing conditions as dwell times and power densities. In addition, Si plasmon imaging in the transmission electron microscope was applied to identify the sponge-like structure of phase-separated silicon. © 2013 Optical Society of America.Item Open Access Lateral overgrowth of germanium for monolithic integration of germanium-on-insulator on silicon(Elsevier, 2015) Hyung Nam J.; Alkis, S.; Nam, D.; Afshinmanesh F.; Shim J.; Park, J.; Brongersma, M.; Okyay, Ali Kemal; Kamins, T.I.; Saraswat, K.A technique to locally grow germanium-on-insulator (GOI) structure on silicon (Si) platform is studied. On (001) Si wafer, silicon dioxide (SiO2) is thermally grown and patterned to define growth window for germanium (Ge). Crystalline Ge is grown via selective hetero-epitaxy, using SiO2 as growth mask. Lateral overgrowth of Ge crystal covers SiO2 surface and neighboring Ge crystals coalesce with each other. Therefore, single crystalline Ge sitting on insulator for GOI applications is achieved. Chemical mechanical polishing (CMP) is performed to planarize the GOI surface. Transmission electron microscopy (TEM) analysis, Raman spectroscopy, and time-resolved photoluminescence (TRPL) show high quality crystalline Ge sitting on SiO2. Optical response from metal-semiconductor-metal (MSM) photodetector shows good optical absorption at 850 nm and 1550 nm wavelength. © 2015 Elsevier B.V. All rights reserved.Item Open Access Matrix density effect on morphology of germanium nanocrystals embedded in silicon dioxide thin films(Materials Research Society, 2011) Alagoz, A. S.; Genisel, M. F.; Foss, Steinar; Finstad, T. G.; Turan, R.Flash type electronic memories are the preferred format in code storage at complex programs running on fast processors and larger media files in portable electronics due to fast write/read operations, long rewrite life, high density and low cost of fabrication. Scaling limitations of top-down fabrication approaches can be overcome in next generation flash memories by replacing continuous floating gate with array of nanocrystals. Germanium (Ge) is a good candidate for nanocrystal based flash memories due its small band gap. In this work, we present effect of silicon dioxide (SiO 2) host matrix density on Ge nanocrystals morphology. Low density Ge+SiO 2 layers are deposited between high density SiO 2 layers by using off-angle magnetron sputter deposition. After high temperature post-annealing, faceted and elongated Ge nanocrystals formation is observed in low density layers. Effects of Ge concentration and annealing temperature on nanocrystal morphology and mean size were investigated by using transmission electron microscopy. Positive correlation between stress development and nanocrystal size is observed at Raman spectroscopy measurements. We concluded that non-uniform stress distribution on nanocrystals during growth is responsible from faceted and elongated nanocrystal morphology.Item Open Access Memory effect by charging of ultra‐small 2‐nm laser‐synthesized solution processable Si‐nanoparticles embedded in Si–Al2O3–SiO2 structure(Wiley-VCH Verlag, 2015) El-Atab, N.; Rizk, A.; Tekcan, B.; Alkis, S.; Okyay, Ali Kemal; Nayfeh, A.A memory structure containing ultra-small 2-nm laser-synthesized silicon nanoparticles is demonstrated. The Si-nanoparticles are embedded between an atomic layer deposited high-κ dielectric Al2O3 layer and a sputtered SiO2 layer. A memory effect due to charging of the Si nanoparticles is observed using high frequency C-V measurements. The shift of the threshold voltage obtained from the hysteresis measurements is around 3.3V at 10/-10V gate voltage sweeping. The analysis of the energy band diagram of the memory structure and the negative shift of the programmed C-V curve indicate that holes are tunneling from p-type Si via Fowler-Nordheim tunneling and are being trapped in the Si nanoparticles. In addition, the structures show good endurance characteristic (>105program/erase cycles) and long retention time (>10 years), which make them promising for applications in non-volatile memory devices. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.Item Open Access Nanoscale tribology of graphene grown by chemical vapor deposition and transferred onto silicon oxide substrates(Cambridge University Press, 2016) Demirbaş, T.; Baykara, M. Z.We present a comprehensive nanoscale tribological characterization of single-layer graphene grown by chemical vapor deposition (CVD) and transferred onto silicon oxide (SiO2) substrates. Specifically, the nanotribological properties of graphene samples are studied via atomic force microscopy (AFM) under ambient conditions using calibrated probes, by measuring the evolution of friction force with increasing normal load. The effect of using different probes and post-transfer cleaning procedures on frictional behavior is evaluated. A new method of quantifying lubrication performance based on measured friction coefficient ratios of graphene and SiO2 is introduced. A comparison of lubrication properties with mechanically-exfoliated graphene is performed. Results indicate that CVD-grown graphene constitutes a very good solid lubricant on SiO2, reducing friction coefficients by ∼ 90% for all investigated samples. Finally, the effect of wrinkles associated with CVD-grown graphene on measured friction values is quantitatively analyzed, with results revealing a substantial increase in friction on these structural defects.Item Open Access Practical multi-featured perfect absorber utilizing high conductivity silicon(Institute of Physics Publishing, 2016) Gok, A.; Yilmaz, M.; Bıyıklı, N.; Topallı, K.; Okyay, Ali KemalWe designed all-silicon, multi-featured band-selective perfect absorbing surfaces based on CMOS compatible processes. The center wavelength of the band-selective absorber can be varied between 2 and 22 μm while a bandwidth as high as 2.5 μm is demonstrated. We used a silicon-on-insulator (SOI) wafer which consists of n-type silicon (Si) device layer, silicon dioxide (SiO2) as buried oxide layer, and n-type Si handle layer. The center wavelength and bandwidth can be tuned by adjusting the conductivity of the Si device and handle layers as well as the thicknesses of the device and buried oxide layers. We demonstrate proof-of-concept absorber surfaces experimentally. Such absorber surfaces are easy to microfabricate because the absorbers do not require elaborate microfabrication steps such as patterning. Due to the structural simplicity, low-cost fabrication, wide spectrum range of operation, and band properties of the perfect absorber, the proposed multi-featured perfect absorber surfaces are promising for many applications. These include sensing devices, surface enhanced infrared absorption applications, solar cells, meta-materials, frequency selective sensors and modulators. © 2016 IOP Publishing Ltd.Item Open Access Raman and TEM studies of Ge nanocrystal formation in SiOx: Ge/SiOx multilayers(Wiley, 2007) Dana, Aykutlu; Aǧan, S.; Tokay, S.; Aydınlı, Atilla; Finstad, T. G.Alternating germanosilicate-siliconoxide layers of 10-30 nm thickness were grown on Si substrates by plasma enhanced chemically vapor deposition (PECVD). The compositions of the grown films were determined by X-ray photoelectron spectroscopy measurements. The films were annealed at temperatures varying from 670 to 1000°C for 5 to 45 minutes under nitrogen atmosphere. High resolution cross section TEM images, electron diffraction and electron energy-loss spectroscopy as well as energy-dispersive X-ray analysis (EDAX) data confirm presence of Ge nanocrystals in each layer. The effect of annealing on the Ge nanocrystal formation in multilayers was investigated by Raman spectroscopy and Transmission Electron Microscopy (TEM). As the annealing temperature is raised to 850°C, single layer of Ge nanocrystals observed at lower annealing temperatures is transformed into a double layer with the smaller sized nanocrystals closer to the substrate SiO2 interface.Item Open Access Selective-area high-quality germanium growth for monolithic integrated optoelectronics(Institute of Electrical and Electronics Engineers, 2012-03-02) Yu, H. Y.; Park, J. H.; Okyay, Ali Kemal; Saraswat, K. C.Selective-area germanium (Ge) layer on silicon (Si) is desired to realize the advanced Ge devices integrated with Si very-large-scale-integration (VLSI) components. We demonstrate the area-dependent high-quality Ge growth on Si substrate through SiO 2 windows. The combination of area-dependent growth and multistep deposition/hydrogen annealing cycles has effectively reduced the surface roughness and the threading dislocation density. Low root-mean-square surface roughness of 0.6 nm is confirmed by atomic-force-microscope analysis. Low defect density in the area-dependent grown Ge layer is measured to be as low as 1 × 10 7cm -2 by plan-view transmission-electron-miscroscope analysis. In addition, the excellent metal-semiconductor-metal photodiode characteristics are shown on the grown Ge layer to open up a possibility to merge Ge optoelectronics with Si VLSI.Item Open Access SiGe nanocrystal formation in PECVD grown SiOx/Si/Ge/Si/SiOx multilayers(World Scientific Publishing, 2009) Ağan, S.; Aydınlı, AtillaWe have studied alternating gennanium-silicon-silicon oxide layers of 41 nm thickness grown on Si substrates by plasma enhanced chemically vapor deposition. The compositions of the grown films were detennined by X-ray photoelectron spectroscopy. The films were annealed at temperatures varying from 700 to 950 °C for 7.5 minutes under nitrogen atmosphere. High resolution cross section TEM images, electron diffraction and electron energy-loss spectroscopy as well as energy-dispersive X-ray analysis (EDAX) confinn presence of Ge nanocrystals in each layer. The effect of annealing on the Ge nanocrystal fonnation in multi layers was investigated by Raman spectroscopy and TEM.Item Open Access Visible photoluminescence from SiOx films grown by low temperature plasma enhanced chemical vapor deposition(Pergamon Press, 1995) Timofeev, F. N.; Aydınlı, Atilla; Ellialtioglu, R.; Turkoglu, K.; Gure, M.; Mikhailov, V. N.; Lavrova, O. A.a-SiOx films of varying stoichiometry have been prepared by low temperature plasma enhanced chemical vapor deposition. The majority of films showed photoluminescence (PL) and films prepared in a narrow range of gas flows exhibited much stronger PL after annealing. Peak PL energies ranging from the ultraviolet to the near infrared have been observed. PL, infrared and X-ray diffraction on selected samples indicate formation of Si clusters in the films. The effects of annealing on the PL properties of the films have been found to depend on initial stoichiometry of the films. © 1995.Item Open Access Wafer bonded capacitive micromachined underwater transducers(IEEE, 2009-09) Olcum, Selim; Oǧuz, Kaan; Şenlik, Muhammed N.; Yamaner F. Y.; Bozkurt, A.; Atalar, Abdullah; Köymen, HayrettinIn this work we have designed, fabricated and tested CMUTs as underwater transducers. Single CMUT membranes with three different radii and 380 microns of thickness are fabricated for the demonstration of an underwater CMUT element. The active area of the transducer is fabricated on top of a 3″ silicon wafer. The silicon wafer is bonded to a gold electrode coated glass substrate wafer 10 cm in diameter. Thermally grown silicon oxide layer is used as the insulation layer between membrane and substrate electrodes. Electrical contacts and insulation are made by epoxy layers. Single CMUT elements are tested in air and in water. Approximately 40% bandwidth is achieved around 25 KHz with a single underwater CMUT cell. Radiated pressure field due to second harmonic generation when the CMUTs are driven with high sinusoidal voltages is measured. ©2009 IEEE.