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    Finite-difference time-domain study of Si nanorod arrays with UV and green light
    (Korean Physical Soc, 2022) Keles, Filiz; Seo, Hye-Won
    The finite-difference time-domain (FDTD) simulation on the absorption properties of Si nanorods with different spacings at UV and green light has been conducted. It was revealed that the enhanced absorption along the sidewalls of Si nanorods occurred, parallel to the light polarization directions, at UV-light while the green light is literally confined towards the nanorod center which leads to the V-shape absorption. The significant difference in absorption profile at two different illuminations is mainly due to the distinct origin of the light-matter interactions. For the UV light, the polarization induced local electric field in the gap between nanorods is responsible for the results observed. However, the confined prolonged absorption of the green light is attributed to the non-negligible total internal reflection inside Si nanorod waveguide.
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    High performance flexible copper indium gallium selenide core-shell nanorod array photodetectors
    (A V S Amer Inst Physics, 2017) Badradeen, Emad; Brozak, Matthew; Keles, Filiz; Al-Mayalee, Khalidah; Karabacak, Tansel
    In this study, the authors fabricated high performance core-shell nanostructured flexible photodetectors on a polyimide substrate of Kapton. For this purpose, p-type copper indium gallium selenide (CIGS) nanorod arrays (core) were coated with aluminum doped zinc oxide (AZO) films (shell) at relatively high Ar gas pressures. CIGS nanorods were prepared by glancing angle deposition (GLAD) technique using radio frequency (RF) magnetron sputtering unit at room temperature. AZO films were deposited by RF sputtering at Ar pressures of 1.0 x 10(-2) mbar (high pressure sputtering) for the shell and at 3.0 x 10(-3) mbar (low pressure sputtering) to create a top contact. As a comparison, the authors also fabricated conventional planar thin film devices incorporating CIGS film of similar material loading to that of CIGS nanorods. The morphological characterization was carried out by field-emission scanning electron microscope. The photocurrent measurement was conducted under 1.5 AM sun at zero electrical biasing, where CIGS devices were observed to absorb in the ultraviolet-visible-near infrared spectrum. GLAD core-shell nanorod photodetectors were shown to demonstrate enhanced photoresponse with an average photocurrent density values of 4.4, 3.2, 2.5, 3.0, and 2.5 mu A/cm(2) for bending angles of 0 degrees; 20 degrees; 40 degrees; 60 degrees, and 80 degrees, respectively. These results are significantly higher than the photocurrent of most of the flexible photodetectors reported in the literature. Moreover, our nanorod devices recovered their photoresponse after several bending experiments that indicate their enhanced mechanical durability. On the other hand, thin film devices did not show any notable photoresponse. Improved photocurrent of CIGS nanorod devices is believed to be due to their enhanced light trapping property and the reduced interelectrode distance because of the core-shell structure, which allows the efficient capture of the photogenerated carriers. In addition, enhanced mechanical durability is achieved by the GLAD nanorod microstructure on a flexible substrate. This approach can open a new strategy to boost the performance of flexible photodetectors and wearable electronics. (C) 2017 American Vacuum Society.
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    Influence of The Substrate-Target Angle and Sputter Temperature On The Properties of CIGS Thin Films Sputtered From Single Quaternary Target
    (2023) Keles, Filiz; Güçlüer, Furkan
    In this study, Copper Indium Gallium Selenide (CIGS) thin films were successfully sputtered from a single quaternary target onto soda lime glass substrates. The effect of the incident angle of target atoms and sputter temperature on the properties of the films were examined using various techniques. It was found that a higher incident angle of target atoms resulted in a columnar microstructure, while a lower angle produced a solid film. The columnar structure showed improved optical absorption compared to the solid film. The sputter temperature had a greater effect on the crystalline properties of the films, with all films except those sputtered at room temperature showing polycrystalline formation. The films displayed a chalcopyrite structure and acceptable band gaps in the range of 1.1-1.3 eV, regardless of the incident angle and sputter temperature. These results indicate that the optical properties of CIGS thin films can be improved by a small increase in the incident angle of target atoms, without adversely affecting the structural and crystalline properties.
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    Investigation of Enhanced Light Absorption in GaN Nanorod Arrays by Finite-Difference Time-Domain Method
    (Korean Vacuum Society, 2022) Keles, Filiz; Kim, Il-Hwan; Seo, Hye-Won
    Finite-difference time-domain simulation of GaN nanorod arrays with linearly polarized ultraviolet (UV) (325 nm) and visible (513 nm) light has been carried out. Enhanced light absorption of nanorods with confined electric field in the surrounding gap was observed. To identify the rationale of this finding, we studied their three-dimensional graphical profile and then realized that 1) modal resonance is developed with absorption gain in the UV region and 2) reflection at the interface with substrate leads to a subtle increase of absorption of visible light. By solving the eigenvalue equation, we also confirmed that TM11 mode of leaky-mode resonance is responsible for the observed modal absorption of UV light. © 2022, Korean Vacuum Society. All rights reserved.
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    Modulated 3D light absorption profile in GaN nanorod arrays
    (Elsevier, 2021) Keles, Filiz; Seo, Hye-Won
    The depth dependent UV-light absorption profile of GaN nanorods with different lattice arrays and filling factors was studied using finite-difference time-domain (FDTD) methods. By comparing to the results from LambertBeer's law with Maxwell-Garnett effective medium theory, we identified the quantitative contribution from nano-scattering effect on the light absorption in the nanorod arrays. The FDTD study of graphical 3D profile of light absorption and electric field intensity was parallelly conducted to investigate the origin of the nano scattering. We found that the coupled electric field in the gap regions led to the larger absorption crosssection of the nanorod arrays, which is attributed to the distorted depth profile of the light absorption.
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    Self-anti-reflective density-modulated thin films by HIPS technique
    (Iop Publishing Ltd, 2017) Keles, Filiz; Badradeen, Emad; Karabacak, Tansel
    A critical factor for an efficient light harvesting device is reduced reflectance in order to achieve high optical absorptance. In this regard, refractive index engineering becomes important to minimize reflectance. In this study, a new fabrication approach to obtain density-modulated CuInxGa((1-x)) Se-2 (CIGS) thin films with self-anti-reflective properties has been demonstrated. Density-modulated CIGS samples were fabricated by utilizing high pressure sputtering (HIPS) at Ar gas pressure of 2.75. x 10(-2) mbar along with conventional low pressure sputtering (LPS) at Ar gas pressure of 3.0. x 10(-3) mbar. LPS produces conventional high density thin films while HIPS produces low density thin films with approximate porosities of similar to 15% due to a shadowing effect originating from the wide-spread angular atomic of HIPS. Higher pressure conditions lower the film density, which also leads to lower refractive index values. Density-modulated films that incorporate a HIPS layer at the side from which light enters demonstrate lower reflectance thus higher absorptance compared to conventional LPS films, although there is not any significant morphological difference between them. This result can be attributed to the self-anti-reflective property of the density-modulated samples, which was confirmed by the reduced refractive index calculated for HIPS layer via an envelope method. Therefore, HIPS, a simple and scalable approach, can provide enhanced optical absorptance in thin film materials and eliminate the need for conventional light trapping methods such as anti-reflective coatings of different materials or surface texturing.
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    Sputtered Mo-bilayer thin films with reduced thickness and improved electrical resistivity
    (Iop Publishing Ltd, 2019) Keles, Filiz; Atasoy, Yavuz; Seyhan, Ayse
    In this study, Mo-bilayer film, the thickness of which was reduced to approximately 270 nm with a very low resistivity of 14 mu Omega.cm, was successfully grown by DC magnetron sputter. The Mo-bilayer, whose bottom and top layers were obtained by high pressure sputter (HPS) and low pressure sputter (LPS) respectively, demonstrates good adhesivity and crystalline properties, together with high reflectance. In order to obtain Mo-bilayer with these improved properties, we first determined the optimal growth temperature and pressure parameters by checking the structural and electrical properties respectively of Mo-single layers. As a result, we achieved a deposit of Mo-bilayer thin film that can be used as a good back contact layer in solar cell applications, both in terms of material cost saving and its superior properties, even at such low thickness.