Browsing by Author "Battal, E."
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Item Open AccessActively tunable thin films for visible light by thermo-optic modulation of ZnO(Wiley-VCH Verlag, 2016) Battal, E.; Okyay, Ali Kemal; Okyay, Ali KemalApplications of active control of light matter interactions within integrated photonics, hyper-spectral imaging, reconfigurable lasers, and selective bio-surfaces have enormously increased the demand for realization of optical modulation covering the spectrum from ultraviolet (UV) up to infrared (IR) wavelength range. In this study, we demonstrate ZnO-based actively tunable perfect absorber operating within UV and visible spectrum with more than 5 nm shift in the resonant absorption wavelength. Using spectroscopic ellipsometry technique, we extract temperature-dependent optical constants of atomic layer-deposited ZnO within 0.3-1.6 and 4-40 μm spectra. We also observe bandgap narrowing of ZnO at elevated temperatures due to lattice relaxation verified by the red-shift of phonon-modes. At around its bandgap, refractive index variations up to 0.2 is obtained and ZnO is shown to exhibit thermo-optic coefficient as high as 9.17 Ã— 10-4 K-1 around the bandgap which is the largest among well-known large bandgap materials. Â© 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. Item Open AccessAn all-ZnO microbolometer for infrared imaging(Elsevier BV, 2014) Kesim, Y. E.; Battal, E.; Tanrikulu, M. Y.; Okyay, Ali Kemal; Okyay, Ali KemalMicrobolometers are extensively used for uncooled infrared imaging applications. These imaging units generally employ vanadium oxide or amorphous silicon as the active layer and silicon nitride as the absorber layer. However, using different materials for active and absorber layers increases the fabrication and integration complexity of the pixel structure. In order to reduce fabrication steps and therefore increase the yield and reduce the cost of the imaging arrays, a single layer can be employed both as the absorber and the active material. In this paper, we propose an all-ZnO microbolometer, where atomic layer deposition grown zinc oxide is employed both as the absorber and the active material. Optical constants of ZnO are measured and fed into finite-difference-time-domain simulations where absorption performances of microbolometers with different gap size and ZnO film thicknesses are extracted. Using the results of these optical simulations, thermal simulations are conducted using finite-element-method in order to extract the noise equivalent temperature difference (NETD) and thermal time constant values of several bolometer structures with different gap sizes, arm and film thicknesses. It is shown that the maximum performance of 171 mK can be achieved with a body thickness of 1.1 μm and arm thickness of 50 nm, while the fastest response with a time constant of 0.32 ms can be achieved with a ZnO thickness of 150 nm both in arms and body. © 2014 Elsevier B.V. All rights reserved. Item Open AccessAtomic-layer-deposited zinc oxide as tunable uncooled infrared microbolometer material(Wiley-VCH Verlag, 2014) Battal, E.; Bolat, S.; Tanrikulu, M. Y.; Okyay, Ali Kemal; Akin, T.; Okyay, Ali KemalZnO is an attractive material for both electrical and optical applications due to its wide bandgap of 3.37 eV and tunable electrical properties. Here, we investigate the application potential of atomic-layer-deposited ZnO in uncooled microbolometers. The temperature coefficient of resistance is observed to be as high as-10.4% K-1 near room temperature with the ZnO thin film grown at 120 °C. Spectral noise characteristics of thin films grown at various temperatures are also investigated and show that the 120 °C grown ZnO has a corner frequency of 2 kHz. With its high TCR value and low electrical noise, atomic-layer-deposited (ALD) ZnO at 120 °C is shown to possess a great potential to be used as the active layer of uncooled microbolometers. The optical properties of the ALD-grown ZnO films in the infrared region are demonstrated to be tunable with growth temperature from near transparent to a strong absorber. We also show that ALD-grown ZnO can outperform commercially standard absorber materials and appears promising as a new structural material for microbolometer-based applications. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. Item Open AccessEffect 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.; Okyay, Ali KemalIn 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. Item Open AccessExperimental and theoretical investigation of phosphorus in-situ doping of germanium epitaxial layers(Elsevier, 2013) Yu, H. -Y.; Battal, E.; Okyay, Ali Kemal; Shim, J.; Park J. -H.; Baek, J. W.; Saraswat, K. C.; Okyay, Ali KemalWe investigate phosphorus in-situ doping characteristics in germanium (Ge) during epitaxial growth by spreading resistance profiling analysis. In addition, we present an accurate model for the kinetics of the diffusion in the in-situ process, modeling combined growth and diffusion events. The activation energy and pre-exponential factor for phosphorus (P) diffusion are determined to be 1.91 eV and 3.75 × 10-5 cm2/s. These results show that P in-situ doping diffusivity is low enough to form shallow junctions for high performance Ge devices. Item Open AccessMetal-dielectric-metal plasmonic resonators for active beam steering in the infrared(Optical Society of America, 2013) Battal, E.; Okyay, Ali Kemal; Okyay, Ali KemalActive beam-steering devices near the optical frequencies have long been sought after due to their applications in communication, defense, and display technologies; however, the challenge lies in achieving actively tunable structures near these frequencies. An array of metal-dielectric-metal plasmonic resonators is demonstrated as a dynamic beam-steering device to operate at midinfrared wavelengths. We numerically demonstrate continuous-angle beam steering of 8.75° by making use of tunable properties of silicon as the active dielectric. The proposed device achieves a refractive index insensitive divergence angle and it operates in a 650 nm wide spectral window around 10 μm wavelength. The results of this Letter pave the way to exploiting active beam steering in various applications at midinfrared wavelengths. Item Open AccessOxygen partial pressure dependence of magnetic, optical and magneto-optical properties of epitaxial cobalt-substituted SrTiO3 films(OSA - The Optical Society, 2015) Onbaşli, M.C.; Goto, T.; Tang, A.; Pan, A.; Battal, E.; Okyay, Ali Kemal; Dionne G.F.; Ross, C.A.; Okyay, Ali KemalCobalt-substituted SrTiO3 films (SrTi0.70Co0.30O3-δ) were grown on SrTiO3 substrates using pulsed laser deposition under oxygen pressures ranging from 1 μTorr to 20 mTorr. The effect of oxygen pressure on structural, magnetic, optical, and magneto-optical properties of the films was investigated. The film grown at 3 μTorr has the highest Faraday rotation (FR) and magnetic saturation moment (Ms). Increasing oxygen pressure during growth reduced Ms, FR and optical absorption in the nearinfrared. This trend is attributed to decreasing Co2+ ion concentration and oxygen vacancy concentration with higher oxygen partial pressure during growth. © 2015 Optical Society of America. Item Open AccessPlasmonic materials based on ZnO films and their potential for developing broadband middle-infrared absorbers(American Institute of Physics Inc., 2014) Kesim, Y.E.; Battal, E.; Okyay, Ali Kemal; Okyay, Ali KemalNoble metals such as gold and silver have been extensively used for plasmonic applications due to their ability to support plasmons, yet they suffer from high intrinsic losses. Alternative plasmonic materials that offer low loss and tunability are desired for a new generation of efficient and agile devices. In this paper, atomic layer deposition (ALD) grown ZnO is investigated as a candidate material for plasmonic applications. Optical constants of ZnO are investigated along with figures of merit pertaining to plasmonic waveguides. We show that ZnO can alleviate the trade-off between propagation length and mode confinement width owing to tunable dielectric properties. In order to demonstrate plasmonic resonances, we simulate a grating structure and computationally demonstrate an ultra-wide-band (4-15 μm) infrared absorber. © 2014 Author(s). Item Open AccessPlasmonically enhanced hot electron based photovoltaic device(Optical Society of American (OSA), 2013) Atar F.B.; Battal, E.; Aygun L.E.; Daglar, B.; Bayındır, Mehmet; Okyay, Ali Kemal; Okyay, Ali Kemal; Bayındır, MehmetHot electron photovoltaics is emerging as a candidate for low cost and ultra thin solar cells. Plasmonic means can be utilized to significantly boost device efficiency. We separately form the tunneling metal-insulator-metal (MIM) junction for electron collection and the plasmon exciting MIM structure on top of each other, which provides high flexibility in plasmonic design and tunneling MIM design separately. We demonstrate close to one order of magnitude enhancement in the short circuit current at the resonance wavelengths. © 2013 Optical Society of America. Item Open AccessResistive Switching based electro-optical modulation(Wiley, 2014-09-08) Battal, E.; Ozcan, A.; Okyay, Ali Kemal; Okyay, Ali KemalResistive switching enables optical modulation via atomic scale modifications that induce change in the refractive index of active device materials. The formation of filaments and migration of atoms around these filaments between high resistance and low resistance states results in the modulation of the free carrier concentration and, hence, the optical constants of the material. Item Open AccessTriangular metallic gratings for large absorption enhancement in thin film Si solar cells(Optical Society of American (OSA), 2012) Battal, E.; Yogurt, T.A.; Aygun L.E.; Okyay, Ali Kemal; Okyay, Ali KemalWe estimate high optical absorption in silicon thin film photovoltaic devices using triangular corrugations on the back metallic contact. We computationally show 21.9% overall absorptivity in a 100-nmthick silicon layer, exceeding any reported absorptivity using single layer gratings placed on the top or the bottom, considering both transverse electric and transverse magnetic polarizations and a wide spectral range (280 - 1100 nm). We also show that the overall absorptivity of the proposed scheme is relatively insensitive to light polarization and the angle of incidence. We also discuss the implications of potential fabrication process variations on such a device. © 2012 Optical Society of America. Item Open AccessUltrahigh contrast one-way optical transmission through a subwavelength slit(Springer, 2013) Battal, E.; Yogurt, T. A.; Okyay, Ali Kemal; Okyay, Ali KemalWe computationally demonstrate one-way optical transmission characteristics of a subwavelength slit. We comparatively study the effect in single layer and double layer metallic corrugations. We also investigate the effect of a dielectric spacer layer between double corrugations to control the volumetric coupling of plasmon and optical modes. We computationally show unidirectional transmission behavior with an ultrahigh contrast ratio of 53.4 dB at λ = 1.56 μm. Volumetric coupling efficiency through the nanoslit strongly depends on the efficient excitation of both the surface plasmon resonance and metal–insulator–metal waveguide modes. We show that the behavior is tunable in a wide spectral range.