Browsing by Subject "Polycrystalline"
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Item Open Access Low temperature growth, characterization and applications of rf-sputtered SrTiO3 and BaSrTiO3 thin films(Bilkent University, 2016-03) Bayrak, TürkanAmong 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.Item Open Access Preparation of Al 2O 3and AlN nanotubes by atomic layer deposition(Cambridge University Press, 2012) Ozgit-Akgun, Çagla; Kayacı, Fatma; Dönmez, İnci; Çağatay, Engin; Uyar, Tamer; Bıyıklı, NecmiAl 2O 3 and AlN nanotubes were fabricated by depositing conformal thin films via atomic layer deposition (ALD) on electrospun nylon 66 (PA66) nanofiber templates. Depositions were carried out at 200°C, using trimethylaluminum (TMAl), water (H 2O), and ammonia (NH 3) as the aluminum, oxygen, and nitrogen precursors, respectively. Deposition rates of Al 2O 3 and AlN at this temperature were ∼1.05 and 0.86 Å/cycle. After the depositions, Al 2O 3- and AlN-coated nanofibers were calcinated at 500°C for 2 h in order to remove organic components. Nanotubes were characterized by using X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). AlN nanotubes were polycrystalline as determined by high resolution TEM (HR-TEM) and selected area electron diffraction (SAED). TEM images of all the samples reported in this study indicated uniform wall thicknesses. © 2012 Materials Research Society.Item Open Access Self-limiting low-temperature growth of crystalline AlN thin films by plasma-enhanced atomic layer deposition(2012) Ozgit, C.; Donmez I.; Alevli, M.; Bıyıklı, NecmiWe report on the self-limiting growth and characterization of aluminum nitride (AlN) thin films. AlN films were deposited by plasma-enhanced atomic layer deposition on various substrates using trimethylaluminum (TMA) and ammonia (NH 3). At 185 °C, deposition rate saturated for TMA and NH 3 doses starting from 0.05 and 40 s, respectively. Saturative surface reactions between TMA and NH 3 resulted in a constant growth rate of ∼ 0.86 Å/cycle from 100 to 200 °C. Within this temperature range, film thickness increased linearly with the number of deposition cycles. At higher temperatures (≤ 225 °C) deposition rate increased with temperature. Chemical composition and bonding states of the films deposited at 185 °C were investigated by X-ray photoelectron spectroscopy. High resolution Al 2p and N 1s spectra confirmed the presence of AlN with peaks located at 73.02 and 396.07 eV, respectively. Films deposited at 185 °C were polycrystalline with a hexagonal wurtzite structure regardless of the substrate selection as determined by grazing incidence X-ray diffraction. High-resolution transmission electron microscopy images of the AlN thin films deposited on Si (100) and glass substrates revealed a microstructure consisting of nanometer sized crystallites. Films exhibited an optical band edge at ∼ 5.8 eV and an optical transmittance of > 95% in the visible region of the spectrum. © 2011 Elsevier B.V. All rights reserved.Item Open Access TiO2 thin film transistor by atomic layer deposition(SPIE, 2013) Okyay, Ali Kemal; Oruç, Feyza B.; Çimen, Furkan; Aygün, Levent E.In this study, TiO2 films were deposited using thermal Atomic Layer Deposition (ALD) system. It is observed that asdeposited ALD TiO 2 films are amorphous and not suitable as TFT channel material. In order to use the film as channel material, a post-annealing process is needed. Annealed films transform into a polycrystalline form containing mixed anatase and rutile phases. For this purpose, devices are annealed at 475°C and observed that their threshold voltage value is 6.5V, subthreshold slope is 0.35 V/dec, Ion/Ioff ratios 2.5×106 and mobility value is 0.672 cm2/V.s. Optical response measurements showed that devices exhibits decent performance at ultraviolet region where TiO 2 has band to band absorption mechanism. © 2013 SPIE.