Browsing by Author "Bek, Alpan"
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Item Open Access High performance 15-μm pitch 640 × 512 MWIR InAs/GaSb type-II superlattice sensors(IEEE, 2021-11-18) Oğuz, Fikri; Ülker, E.; Arslan, Y.; Nuzumlali, Ö. L.; Bek, Alpan; Özbay, EkmelWe report the high performance of Mid-wave Infrared Region (MWIR) InAs/GaSb Type-II Superlattice (T2SL) sensors with $640\times512$ format and 15- $\mu \text{m}$ pixel pitch at both Focal Plane Array (FPA) and pixel level. The p-intrinsic-Barrier-n epilayer structure is adopted for this study, which is grown on 620 ± $30~ \mu \text{m}$ thick GaSb substrate and highly-doped GaSb cap layer at the top structure. The mesa type pixels with sizes of $220\,\,\mu \text{m}\,\,\times 220\,\,\mu \text{m}$ have dark currents $7.8\times10$ −12 A at 77 K both of which are equivalent to state-of-the-art values for Type-II Superlattice sensors. The various passivation techniques to lower the dark current are applied and the results are given in terms of dark current. Electro-optical measurements yielded comparable results to literature. After gathering data and optimizing the fabrication conditions, the FPA of 15- $\mu \text{m}$ pitch having $4.92~ \mu \text{m}$ cut-off wavelength ( $\lambda _{\mathrm {c}}$ ) shows 1.6 A/W peak responsivity, Noise Equivalent Temperature Difference (NETD) of 22.6 mK with optics of f/2.3, quantum efficiency larger than 65% and 99.75% operability. The acquired images by using aforementioned FPA device is presented in this paper. With the reduction of dark current, an encouraging imaging performance is obtained which shows the potential of the Type-II Superlattice detectors in 3 rd generation infrared sensors.Item Open Access Low loss optical waveguides and polarization splitters with oxidized AlxGa1-xAs layers(Bilkent University, 1998) Bek, AlpanLow propagation loss waveguides, operating at 1.55 fim optical wavelength, are fabricated utilizing oxidized AhGai_a;As layers. MBE grown multilayer semiconductor heterostructures are characterized before and after oxidation by ellipsometric techniques. In fabrication of optical waveguides, reactive ion etching method is used extensively. Loss measurements are performed, involving a fiber input-coupled laser source setup using Fabry-Perot resonance technique. Propagation loss of an AlGaAs based multilayer rib waveguide with oxidized AhG ai_3,As top layer is observed to reduce from 6 dB/cm to as low as 1 clB/cm lor TM and from 3.7 dB/cm to as low as 0.6 dB/cm for TE polarizations in the presence of metal electrodes on top of the rib. These results arc compared with loss measurements on standard rib waveguides. Polarization splitters are also fabricated with the same material. Effect of the oxide layer on the polarization splitter’s coupling length for TE and TM polarizations are measured. Polarization extinction ratios as high as 12.4 dB are obtained. Polarization extinction ratios are also attempted to be controlled by the use of electro-optic effect in Al.x,.Gai_a;As system. Only AC fields are found to be effective.Item Open Access Microcavity effects in the photoluminescence of hydrogenated amorphous silicon nitride(SPIE, 1998) Serpengüzel, Ali; Aydınlı, Atilla; Bek, AlpanFabry-Perot microcavities are used for the alteration of photoluminescence in hydrogenated amorphous silicon nitride grown with and without ammonia. The photoluminescence is red-near-infrared for the samples grown without ammonia, and blue-green for the samples grown with ammonia. In the Fabry- Perot microcavities, the amplitude of the photoluminescence is enhanced, while its linewidth is reduced with respect to the bulk hydrogenated amorphous silicon nitride. The microcavity was realized by a metallic back mirror and a hydrogenated amorphous silicon nitride - air or a metallic front mirror. The transmittance, reflectance, and absorbance spectra were also measured and calculated. The calculated spectra agree well with the experimental spectra. The hydrogenated amorphous silicon nitride microcavity has potential for becoming a versatile silicon based optoelectronic device such as a color flat panel display, a resonant cavity enhanced light emitting diode, or a laser.Item Open Access Microcavity enhanced amorphous silicon photoluminescence(IEEE, 1997) Serpengüzel, Ali; Aydınlı, Atilla; Bek, AlpanA microcavity enhancement of room temperature photoluminescence (PL) of a hydrogenated amorphous silicon (a-Si:h) was performed. A quantum confinement model was developed to describe the occurrence of the PL in the bulk a-Si:H. According to the model, small a-Si clusters are in a matrix of a-Si:H. The regions with Si-H, having larger energy gaps due to strong Si-H bonds, isolate these clusters, and form barrier regions around them. The PL originates from these a-Si clusters.Item Open Access The operation of a novel hot electron vertical cavity surface emitting laser(SPIE, 1998) Balkan, N.; O'Brien-Davies, A.; Thoms, A. B.; Potter, R. J.; Poolton, N.; Adams, M. J.; Masum, J.; Bek, Alpan; Serpengüzel, Ali; Aydınlı, Atilla; Roberts, J. S.The hot Electron Light Emission and Lasing in Semiconductor Heterostructures devices (HELLISH-1) is novel surface emitter consisting of a GaAs quantum well, within the depletion region, on the n side of Ga 1-xAlxAs p- n junction. It utilizes hot electron transport parallel to the layers and injection of hot electron hole pairs into the quantum well through a combination of mechanisms including tunnelling, thermionic emission and diffusion of 'lucky' carriers. Super Radiant HELLISH-1 is an advanced structure incorporating a lower distributed Bragg reflector (DBR). Combined with the finite reflectivity of the upper semiconductor-air interface reflectivity it defines a quasi- resonant cavity enabling emission output from the top surface with a higher spectral purity. The output power has increased by two orders of magnitude and reduced the full width at half maximum (FWHM) to 20 nm. An upper DBR added to the structure defines HELLISH-VCSEL which is currently the first operational hot electron surface emitting laser and lases at room temperature with a 1.5 nm FWHM. In this work we demonstrate and compare the operation of UB-HELLISH-1 and HELLISH-VCSEL using experimental and theoretical reflectivity spectra over an extensive temperature range.Item Open Access SERS-based trace detection by size and shape controlled noble metal particles with high benefit-cost ratio(Digital Library, 2021-08-01) Demirtaş, Özge; Khan, Ghazanfar Ali; Esmaeilzad, Nasim Seyedpour; Iftikhar, R. M. Faheem; Öztürk, İ. Murat; Demir, Ahmet Kemal; Ahmed, Waqqar; Bek, AlpanSERS has become a highly preferred method with the effect of advances in instrument technology and potential for application to many areas from medicine to art conservation for the detection of trace-level environmental and biological analytes. However, the effects of substrate variation and the details of the enhancement mechanism still of concern even much progress has been achieved in recent years. Therefore, it is important to control the size and shape of the materials for maximize the enhancement factor and ensure the reproducibility of the substrates by considering SERS uncertainty principle. In this study, we present novel SERS substrate preparation methods that are highly promising in terms of both signal enhancement and reproducibility with high benefit-cost ratio.Item Open Access Shape and deposition angle control of silver film-over-nanosphere SERS substrates(Institute of Physics Publishing, 2021-10-06) Esmaeilzad, Nasim Seyedpour; Demirtaş, Özge; Demir, Ahmet Kemal; Bek, AlpanThin metallic films on dielectric nanospheres are demonstrated to have a high potential for the fabrication of cost-effective SERS substrates. In addition to the morphological advantages that nanospheres offer for attaining a high density of hot spots, possessing shape adjustability by uncomplicated thermal treatment makes them an attractive platform for tuneable SERS substrates. Furthermore, when combined with the oblique angle metal deposition technique, adjustable gaps at a high density and adjustable shape of metal films, such as Ag films, can be achieved on nanospheres. Applying small changes in deposition angle can provide means for fine adjustment of the Raman enhancement factor (EF), resulting in EF up to 108 measured using crystal violet dye molecule as a Raman analyte. This practice paves the way for the fabrication of high EF SERS substrates at a reasonable cost using a monolayer of self-organized nanosphere patterns. An ultra-thin Ag film coated at 5° tilt is shown to be an excellent substitute for a film deposited at 0° with double the thickness. There is a strong agreement between the experimental results and finite-elements-method-based Maxwell simulations exhibiting expected field enhancements up to 109 at a tilt angle of 5°.