Browsing by Subject "Thin film"

Now showing 1 - 14 of 14

Open Access
Atomic layer deposition of zirconium oxide thin film on an optical fiber for cladding light strippers
(TÜBİTAK, 2020) Karatutlu, Ali
Cladding light strippers are essential components in high-power fiber lasers used for removal of unwanted cladding light that can distort the beam quality or even damage the whole fiber laser system. In this study, an Atomic Layer Deposition system was used for the first time to prepare the cladding light stripper devices using a 40 nm thick zirconia layer grown on optical fiber. The thickness of the zirconia coating was confirmed using the Scanning Electron Microscopy (SEM) and the Ellipsometry techniques. The elemental analysis was also performed using the wavelength dispersive X-ray spectroscopy technique. The Raman spectroscopy and XRD data confirm the structure of the atomic layer deposition-grown zirconia thin films to be predominantly amorphous. The cladding light stripper devices formed using the zirconia thin films with the lengths of 8.5 and 15.5 cm were able to strip approximately 30% (~1.5 dB) and 40% (~2.3 dB) of the unwanted cladding light.
Open Access
A baseball-bat-like CdTe/TiO2 nanorods-based heterojunction core–shell solar cell
(Elsevier, 2013) Karaagac, H.; Parlak, M.; Aygun, L. E.; Ghaffari, M.; Bıyıklı, Necmi; Okyay, Ali Kemal
Rutile TiO2 nanorods on fluorine-doped thin oxide glass substrates via the hydrothermal technique were synthesized and decorated with a sputtered CdTe layer to fabricate a core-shell type n-TiO2/p-CdTe solar cell. Absorbance spectrum verified the absorption contribution of both TiO2 and CdTe to the absorption process. The solar cell parameters, such as open circuit voltage, short circuit current density, fill factor and power conversion efficiency were found to be 0.34 V, 1.27 mA cm-2, 28% and 0.12%, respectively. .
Open Access
Co doping induced structural and optical properties of sol-gel prepared ZnO thin films
(Elsevier BV, 2014-11) Gungor, E.; Gungor, T.; Caliskan, D.; Ceylan, A.; Özbay, Ekmel
The preparation conditions for Co doping process into the ZnO structure were studied by the ultrasonic spray pyrolysis technique. Structural and optical properties of the Co:ZnO thin films as a function of Co concentrations were examined. It was observed that hexagonal wurtzite structure of ZnO is dominant up to the critical value, and after the value, the cubic structural phase of the cobalt oxide appears in the X-ray diffraction patterns. Every band-edge of Co:ZnO films shifts to the lower energies and all are confirmed with the PL measurements. Co substitution in ZnO lattice has been proved by the optical transmittance measurement which is observed as the loss of transmission appearing in specific region due to Co2+ characteristic transitions. © 2014 Elsevier B.V. All rights reserved.
Open Access
Effect of gold nanoparticles size on light scattering for thin film amorphous-silicon solar cells
(Elsevier Ltd, 2014-05) Islam, K.; Alnuaimi, A.; Battal, E.; Okyay, Ali Kemal; Nayfeh, A.
In this work, the effect of gold (Au) nanoparticles on the performance of a-Si:H solar cells is investigated experimentally. The solar cell stack is grown on a highly doped p-type Si wafer and consists of 20nm heavily doped p-type a-Si, 500nm undoped a-Si, 20nm heavily doped n-type a-Si and finally 80nm Indium Tin Oxide (ITO) on the top. Au nanoparticles of 10, 20, 50, 80, 100, 200 and 400nm are spin coated on top of the ITO before metallization. The plasmonic effect of the Au nanoparticles allows for additional scattering at the surface thus reducing the overall reflectivity. The larger the nanoparticle size the more scattering is obtained and the median reflectivity drops from about 23% to 18%. The results show an increase in the short-circuit current density (Jsc) and efficiency with increasing nanoparticle size. The Jsc increases from 9.34 to 10.1mA/cm2. In addition, the efficiency increases from 4.28% to 5.01%. © 2014 Elsevier Ltd.
Open Access
Electrochromic properties of nanostructured tungsten trioxide (hydrate) films and their applications in a complementary electrochromic device
(Elsevier, 2011-12-26) Jiao, Z.; Wang, J.; Ke, L.; Liu, X.; Demir, Hilmi Volkan; Yang, M. F.; Sun, X. W.
Orthorhombic hydrated tungsten trioxide (3WO(3)center dot H2O) films consisted of nanosticks and nanoparticles were prepared on fluorine doped tin oxide (FTO)-coated substrate by a facile and template-free hydrothermal method using ammonium acetate (CH3COONH4) as the capping agent. Irregular nanobrick films were obtained without capping agent. Due to the highly rough surface, the nanostick/nanoparticle film depicts faster ion intercalation/deintercalation kinetics and a greater coloration efficiency (45.5 cm(2)/C) than the nanobrick film. A complementary electrochromic device based on the nanostick/nanoparticle 3WO(3)-H2O film and Prussian blue (PB) was assembled. As a result, the complementary device shows a higher optical modulation (54% at 754 nm), a larger coloration efficiency (151.9 cm(2)/C) and faster switching responses with a bleaching time of 5.7 s and a coloring time of 1.3 s than a single 3WO(3).H2O layer device, making it attractive for a practical application.
Open Access
Exceptional adaptable MWIR thermal emission for ordinary objects covered with thin VO2 film
(Elsevier Ltd, 2021-01-25) Durna, Yılmaz; Kocer, Hasan; Aydın, Koray; Cakir, Mehmet Cihan; Soydan, Mahmut Can; Odabasi, Oguz; Işık, Halil; Ozbay, Ekmel
Monotonous thermal radiation emitted from an ordinary object can be brought into a dynamic and versatile form that can be shaped according to the application area with the ingenious design of the surface coatings. Building the coatings with phase change materials provides exceptional and surprising properties in terms of tunability, adaptability and multifunctionality. In this paper, we investigate the thermal radiation properties in the MWIR band through comprehensive thermographic measurements and theoretical methods while a thin (similar to 90 nm thick) vanadium dioxide (VO2) layer on the sapphire substrate (VO2 thin film) is placed on different ordinary objects under heating/cooling conditions. It is indicated that the emission of the metal object (low emittance) can be boosted and the emission of the blackbody-like object (high emittance) can be suppressed at the relevant temperatures. The thermal emission of the objects covered with thin VO2 film at high temperatures (>75 degrees C) is determined by only the VO2 thin film, since the VO2 layer is completely metallized and the MWIR radiation of the underlying object is masked. When the actual temperature of the object behaving like a blackbody rises up to 95 degrees C, the temperature detected in the MWIR thermal camera is reduced by more than 20% to approx. 75 degrees C due to the VO2 thin film on this object, providing thermal camouflage. It is experimentally and theoretically revealed that the underlying physical mechanism on these strange results is associated with the drastic change in the infrared optical parameters of the VO2 as a result of the applied temperature. (C) 2020 Elsevier Ltd. All rights reserved.
Open Access
An experimental and first-principles study of the effect of B / N doping in TiO2 thin films for visible light photo-catalysis
(Elsevier, 2013) Uddin, M. N.; Shibly, S. U. A.; Ovali, R.; Saiful, I.; Islam, M. S.; Uddin, M. J.; Gulseren, O.; Bengu, E.; Mazumder, M. M. R.
Thin films of TiO2 and boron-nitrogen (B/N) co-doped TiO 2 on glass substrates have been prepared by a simple sol-gel dip coating route. Titanium (IV) isopropoxide, boric acid and urea have been used as titanium, boron and nitrogen sources, respectively. The films were characterized by X-ray diffraction, X-ray photo-electron spectroscopy, scanning electron microscopy, Raman spectroscopy and UV-vis spectroscopy. The TiO 2 thin films with co-doping of different B/N atomic ratios (0.27-20.89) showed better photo-catalytic degradation ability of methylene blue compared to that of bare-TiO2 under visible light. The TiO 2 film doped with the highest atomic concentration of N showed repeatedly the best photo-catalytic performance. The high activity of co-doped TiO2 thin films toward organic degradation can be related to the stronger absorption observed in the UV-vis region, red shift in adsorption edges and surface acidity induced by B/N doping. Furthermore, several atomic models for B/N doping have been used to investigate the effect of doping on electronic structure and density of states of TiO2 through ab-initio density functional theory calculations. The computational study suggested a significant narrowing of the band gap due to the formation of midgap states and the shift of Fermi-level for the interstitial N model supporting the experimental results. © 2013 Elsevier B.V.
Open Access
Experimental study of two-step growth of thin AlN film on 4H-SiC substrate by metalorganic chemical vapor deposition
(2010) Yu H.; Ozturk, M.; Demirel P.; Cakmak H.; Buyuklimanli, T.; Ou W.; Özbay, Ekmel
We report growth optimizations of the thin AlN film on (0001) 4H-SiC substrates by metalorganic chemical vapor deposition. The influence of growth conditions, such as growth temperature and the V/III molar ratio, on the material quality of AlN film is studied. The surface morphology and crystalline quality of the epitaxial layers are investigated by atomic force microscope, X-ray diffraction, and transmission electronic microscope. A new approach is demonstrated to improve the crystalline quality of a 100 nm-thick AlN film by the use of a 5 nm-thick low temperature AlN nucleation layer. Compared to a conventional AlN layer directly grown on SiC substrate at high temperature, the surface morphology of two-step AlN film is remarkably improved along with a decreasing of defect density, leading to the improvement of crystalline quality for the subsequently grown GaN layer. The mechanisms of crystalline quality improvement by use of a low temperature AlN nucleation layer are also investigated and discussed.
Open Access
Fluorescent aerogel films for TNT sensing
(2009) Yıldırım, Adem
Silica aerogels are unique materials with extraordinary properties such as, high porosity, large surface area and low refractive indices. Due to these properties, they can be applied to a wide range of areas including, insulation, catalyst support, sensors and dielectric materials. However, until know because of their poor mechanical properties and costly production a few applications of aerogels were realized. Ambient pressure drying method is a promising way to produce low cost aerogels and thus expanding the realized application areas of aerogels. This method is based on lowering the surface tension on the gel network, in order to minimize the collapse of the gel during drying. For this purpose gel surface can be modified to make it hydrophobic. In the first part of this work, ambient pressure production of fluorescent aerogel thin films are described. The produced fluorescent films were characterized to identify their morphological, optical and surface properties. The gel was produced by using methlyltrimethoxysilane (MTMS) to produce hydrophobic gel. A porphyrin derivative (TCPPH2) was simply mixed with the sol before gelation for the fluorescence property. After gelation and aging the produced gels are homogenized and spin coated on glass substrates. The produced films were found to be highly porous (60.3-77.1%), fluorescent and transparent in visible region (82-89%). In the second part, sensing performances of the films were examined by using the common explosive trinitrotoluene (TNT). All films show fluorescence quenching based sensing against TNT exposure. The quenching efficiency of the films is highly thickness dependent. For the thinnest film (120 nm) the quenching efficiency was found to be 8.6% in 10 seconds and for the thickest film (1100 nm) film 2.1% in 10 seconds.
Open Access
Improved efficiency of thin film a-Si:H solar cells with Au nanoparticles
(Institute of Electrical and Electronics Engineers Inc., 2013) Islam, K.; Alnuaimi, A.; Okyay, Ali Kemal; Nayfeh, A.
In this work, the effect of Au nanoparticles on the performance of a-Si:H solar cells is investigated experimentally. Au nanoparticles of 10, 20, 50, 80, 100, 200 and 400 nm are spin coated on ITO before metallization. The results show an increase in the Jsc and efficiency with increasing nanoparticle size. The Jsc increases from 9.34 mA/cm2 to 10.1 mA/cm2. In addition, the efficiency increases from 4.28% to 5.01%. © 2013 IEEE.
Open Access
Low - temperature self - limiting growth of III - nitride thin films by plasma - enhanced atomic layer deposition
(American Scientific Publishers, 2012) Bıyıklı, Necmi; Ozgit, C.; Donmez, I.
We report on the low-temperature self-limiting growth and characterization of III-Nitride thin films. AlN and GaN films were deposited by plasma-enhanced atomic layer deposition (PEALD) on various substrates using trimethylaluminum (TMA), trimethylgallium (TMG) and triethylgallium (TEG) as group-III, and ammonia (NH3) as nitrogen precursor materials. Self-limiting growth behavior, which is the major characteristic of an ALD process, was achieved for both nitride films at temperatures below 200 °C. AlN deposition rate saturated around 0.86 Å/cycle for TMA and NH3 doses starting from 0.05 and 40 s, respectively, whereas GaN growth rate saturated at a lower value of 0.56 Å/cycle and 0.48 Å/cycle for TMG and TEG doses 0.015 s and 1 s, respectively. The saturation dose for NH3 was measured as 90 s and 120 s, for TMG and TEG experiments, respectively. Within the self-limiting growth temperature range (ALD window), film thicknesses increased linearly with the number of deposition cycles. At higher temperatures (≥225 °C and ≥350 °C for AlN and GaN respectively), deposition rate became temperature-dependent, with increasing growth rates. Chemical composition and bonding states of the films deposited within the self-limiting growth regime were investigated by X-ray photoelectron spectroscopy (XPS). GaN films exhibited high oxygen concentrations regardless of the precursors choice, either TMG or TEG, whereas low-oxygen incorporation in AlN films was confirmed by high resolution Al 2p and N 1s spectra of AlN films. AlN films were polycrystalline with a hexagonal wurtzite structure regardless of the substrate selection as determined by grazing incidence X-ray diffraction (GIXRD). GaN films showed amorphous-like XRD signature, confirming the highly defective layers. High-resolution transmission electron microscopy (HR-TEM) images of the AlN thin films revealed a microstructure consisting of several-nanometer sized crystallites, whereas GaN films exhibited sub-nm small crystallites dispersed in an amorphous matrix.
Open Access
A plasmonic enhanced photodetector based on silicon nanocrystals obtained through laser ablation
(Institute of Physics Publishing, 2012-10-18) Alkis, S.; Oruç, F. B.; Ortaç, B.; Koşger, A. C.; Okyay, Ali Kemal
We present a proof-of-concept photodetector which is sensitive in the visible spectrum. Silicon nanocrystals (Si-NCs) obtained by laser ablation are used as the active absorption region. Si-NC films are formed from a polymeric dispersion. The films are sandwiched between thin insulating films to reduce the electrical leakage current. Furthermore, Ag nanoparticles are integrated with the photodetector to enhance the visible response using plasmonic effects. The measured photocurrent is resonantly enhanced, which is explained in terms of enhanced local fields caused by localized plasmons. The UV-vis spectrum of Ag nanoparticles is also measured to verify the resonance.
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, Necmi
We 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.
Open Access
Thickness dependence of solar cell efficiency in transition metal dichalcogenides MX2 (M: Mo, W; X: S, Se, Te)
(Elsevier, 2020) Özdemir, Burak; Barone, V.
Bulk transition metal dichalcogenides are indirect gap semiconductors with optical gaps in the range of 0.7–1.6 eV, which makes them suitable for solar cell applications. In this work, we study the electronic structure, optical properties, and the thickness dependence of the solar cell efficiencies of MX2 (M: Mo, W; X: S, Se, Te) with density functional theory and GW + BSE. Through this analysis, we find a change in solar cell efficiency trends at slab thicknesses of 3 μm. For thin films solar cells (thicknesses smaller than 3 μm), the tellurides present the highest efficiencies (about 20% for a 100 nm thick slab). In contrast, for thicknesses greater than 3 μm, our results indicate that a maximum solar cell efficiency can be achieved in WS2. For instance, a 100 μm slab of WS2 presents a solar cell efficiency of 36.3%, making this material a promising candidate for solar cell applications.