Browsing by Subject "Dielectric constant"

Now showing 1 - 7 of 7

Open Access
Effect of O2/Ar flow ratio and post-deposition annealing on the structural, optical and electrical characteristics of SrTiO3 thin films deposited by RF sputtering at room temperature
(Elsevier, 2015) Goldenberg, E.; Bayrak, T.; Ozgit Akgun, C.; Haider A.; Leghari, S.A.; Kumar, M.; Bıyıklı, Necmi
SrTiO3 (STO) thin films have been prepared by reactive RF magnetron sputtering on Si (100) and UV fused silica substrates at room temperature. The effect of oxygen flow on film characteristics was investigated at a total gas flow of 30 sccm, for various O2/O2 + Ar flow rate ratios. As-deposited films were annealed at 700 °C in oxygen atmosphere for 1 h. Post-deposition annealing improved both film crystallinity and spectral transmittance. Film microstructure, along with optical and electrical properties, was evaluated for both as-deposited and annealed films. Abroad photoluminescence emission was observed within the spectral range of 2.75–3.50 eV for all STO thin films irrespective of their deposition parameters. Upon annealing, the optical band gap of the film deposited with 0% O2 concentration slightly blue-shifted, while the other samples grown at higher oxygen partial pressure did not show any shift. Refractive indices (n) (at 550 nm) were in the range of 2.05 to 2.09, and 2.10 to 2.12 for as-deposited and annealed films, respectively. Dielectric constant values (at 100 kHz) within the range of 30–66 were obtained for film thicknesses less than 300 nm, which decreased to ~30–38 after postdeposition annealing.
Open Access
Interventional MRI: tapering improves the distal sensitivity of the loopless antenna
(Wiley, 2010) Qian, D.; El-Sharkawy, A. M. M.; Atalar, Ergin; Bottomley, P. A.
The "loopless antenna" is an interventional MRI detector consisting of a tuned coaxial cable and an extended inner conductor or "whip". A limitation is the poor sensitivity afforded at, and immediately proximal to, its distal end, which is exacerbated by the extended whip length when the whip is uniformly insulated. It is shown here that tapered insulation dramatically improves the distal sensitivity of the loopless antenna by pushing the current sensitivity toward the tip. The absolute signal-to-noise ratio is numerically computed by the electromagnetic method-of-moments for three resonant 3-T antennae with no insulation, uniform insulation, and with linearly tapered insulation. The analysis shows that tapered insulation provides an ∼400% increase in signal-to-noise ratio in trans-axial planes 1 cm from the tip and a 16-fold increase in the sensitive area as compared to an equivalent, uniformly insulated antenna. These findings are directly confirmed by phantom experiments and by MRI of an aorta specimen. The results demonstrate that numerical electromagnetic signal-tonoise ratio analysis can accurately predict the loopless detector's signal-to-noise ratio and play a central role in optimizing its design. The manifold improvement in distal signal-to-noise ratio afforded by redistributing the insulation should improve the loopless antenna's utility for interventional MRI.
Open Access
Low temperature growth, characterization and applications of rf-sputtered SrTiO3 and BaSrTiO3 thin films
(2016-03) Bayrak, Türkan
Among the several perovskite ferroelectric oxides, SrTiO3 (STO) and BaSrTiO3 (BST) thin lms have attracted signi cant attention due to their potential applications in oxide-based electronics. However, reliability and performance of STO and BST thin lms depend usually on the precise knowledge of microstructure, as well as optical and electrical properties. STO and BST thin lms were deposited at room-temperature on Si (100), UVgrade fused silica, quartz substrates and TiO2 nano bers by radio frequency (RF) magnetron sputtering using di erent plasma power, oxygen mixing ratios (OMRs) and deposition pressure levels. As-deposited thin lms showed amorphous-like nanocrystalline microstructure almost independent of the deposition conditions. In uence of post-deposition annealing at various temperatures of RF sputtered STO thin lms were also investigated. All lms were found to be highly transparent (>75%) in the visible region, and both STO and BST lms exhibited well de ned main absorption edges: the calculated indirect and direct band gaps for STO lms were in the range of 2.32 to 4.55 eV. The refractive index of the STO lms increased with OMR and post-deposition annealing for 3 mTorr deposition for STO, BST and STO annealing study. However, there is no correlation for 5 mTorr deposition. The refractive indices of BST lms were in the range of 1.90-2.07 at 550 nm depending on their deposition conditions. The optical band gap of the BST lms were calculated the ranging in 3.60 to 4.30 eV. Electrical dielectric constant values of the STO thin lms were extracted from frequency or voltage dependent capacitance measurements using micro-fabricated Ag/STO/p-Si device structures. High dielectric constant values reaching up to 100 were obtained. All STO samples exhibited more than 2.5 C/cm2 charge storage capacity and low dielectric loss (less than 0.07 at 100 kHz). Post-deposition annealing at 800oC for 1 h resulted in polycrystalline BST thin lms with increased refractive indices and dielectric constants, along with reduced optical transmission values. Frequency dependent dielectric constants were found to be in the range of 46-72, and the observed leakage current was very small,less than 1 A. Our experimental results show that these low temperature grown STO and BST lms have the potential for various electrical applications.
Open Access
Negative refraction by photonic crystals
(Nature, 2003) Cubukcu, E.; Aydin, K.; Özbay, Ekmel; Foteinopoulou, S.; Soukoulis, C. M.
Open Access
Photonic band-gap effect, localization, and waveguiding in the two-dimensional Penrose lattice
(American Physical Society, 2001) Bayındır, Mehmet; Cubukcu, E.; Bulu, I.; Özbay, Ekmel
We report experimental observation of a full photonic band gap in a two-dimensional Penrose lattice made of dielectric rods. Tightly confined defect modes having high quality factors were observed. Absence of the translational symmetry in Penrose lattice was used to change the defect frequency within the stop band. We also achieved the guiding and bending of electromagnetic waves through a row of missing rods. Propagation of photons along highly localized coupled-cavity modes was experimentally demonstrated and analyzed within the tight-binding approximation.
Open Access
Structural, optical and electrical characteristics BaSrTiOx thin films: Effect of deposition pressure and annealing
(Elsevier BV * North-Holland, 2017) Bayrak, T.; Ozgit-Akgun, C.; Goldenberg, E.
Among perovskite oxide materials, BaSrTiOx (BST) has attracted great attention due to its potential applications in oxide-based electronics. However, reliability and efficiency of BST thin films strongly depend on the precise knowledge of the film microstructure, as well as optical and electrical properties. In the present work, BST films were deposited at room temperature using radio frequency magnetron sputtering technique. The impact of deposition pressure, partial oxygen flow, and post-deposition annealing treatment on film microstructure, surface morphology, refractive index, and dielectric constants were studied by X-ray diffraction, scanning electron microscopy, spectrophotometry, ellipsometry, photoluminescence, as well as capacitance-voltage measurements. Well-adhered and uniform amorphous films were obtained at room temperature. For all as-deposited films, the average optical transmission was ~ 85% in the VIS-NIR spectrum. The refractive indices of BST films were in the range of 1.90–2.07 (λ = 550 nm). Post-deposition annealing at 800 °C for 1 h resulted in polycrystalline thin films with increased refractive indices and dielectric constants, however reduced optical transmission values. Frequency dependent dielectric constants were found to be in the range of 46–72. However, the observed leakage current was relatively small, about 1 μA. The highest FOM values were obtained for films deposited at 0.67 Pa pressures, while charge storage capacity values increased with increased deposition pressure. Results show that room-temperature grown BST films have potential for device applications.
Open Access
XPS analysis with external bias: a simple method for probing differential charging
(John Wiley & Sons Ltd., 2004) Ertas, G.; Süzer, Şefik
The XPS spectra of thermally grown oxide layers on Si, Al, W and Hf substrates have been recorded while the samples were subjected to external d.c. voltage bias. The bias induces additional shifts in the measured binding energy differences between the XPS peaks of the oxide and that of the metal substrate in Si and Al (as probed both in the 2p and the KLL Auger regions), but not in W and Hf (as probed in the 4f region). These bias induced shifts are attributed to differential charging between the oxide layer and the substrate, which in turn is postulated to be related to the capacitance and inversely to the dielectric constant of the oxide layer. Accordingly, silicon dioxide with the smallest dielectric constant undergoes the largest differential charging, aluminium oxide is in the middle and no appreciable charging can be induced in the high-k tungsten and hafnium oxides, all of which are ∼6 nm thick. Copyright © 2004 John Wiley & Sons, Ltd.