Browsing by Author "Turan, R."
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Item Open Access Atomic layer deposited Al 2O 3 passivation of type II InAs/GaSb superlattice photodetectors(AIP, 2012) Salihoğlu, Ömer; Muti, Abdullah; Kutluer, Kutlu; Tansel, T.; Turan, R.; Kocabaş, Coşkun; Aydınlı, AtillaTaking advantage of the favorable Gibbs free energies, atomic layer deposited (ALD) aluminum oxide (Al 2O 3) was used as a novel approach for passivation of type II InAs/GaSb superlattice (SL) midwave infrared (MWIR) single pixel photodetectors in a self cleaning process (λ cut-off ∼ 5.1 m). Al 2O 3 passivated and unpassivated diodes were compared for their electrical and optical performances. For passivated diodes, the dark current density was improved by an order of magnitude at 77 K. The zero bias responsivity and detectivity was 1.33 A/W and 1.9 × 10 13 Jones, respectively at 4μm and 77 K. Quantum efficiency (QE) was determined as 41 for these detectors. This conformal passivation technique is promising for focal plane array (FPA) applications. © 2012 American Institute of Physics.Item Open Access Bias in bonding behavior among boron, carbon, and nitrogen atoms in ion implanted a-BN, a-BC, and diamond like carbon films(2011) Genisel, M. F.; Uddin, M. N.; Say, Z.; Kulakci, M.; Turan, R.; Gulseren, O.; Bengu, E.In this study, we implanted Nþ and Nþ 2 ions into sputter deposited amorphous boron carbide (a-BC) and diamond like carbon (DLC) thin films in an effort to understand the chemical bonding involved and investigate possible phase separation routes in boron carbon nitride (BCN) films. In addition, we investigated the effect of implanted Cþ ions in sputter deposited amorphous boron nitride (a-BN) films. Implanted ion energies for all ion species were set at 40 KeV. Implanted films were then analyzed using x-ray photoelectron spectroscopy (XPS). The changes in the chemical composition and bonding chemistry due to ion-implantation were examined at different depths of the films using sequential ion-beam etching and high resolution XPS analysis cycles. A comparative analysis has been made with the results from sputter deposited BCN films suggesting that implanted nitrogen and carbon atoms behaved very similar to nitrogen and carbon atoms in sputter deposited BCN films. We found that implanted nitrogen atoms would prefer bonding to carbon atoms in the films only if there is no boron atom in the vicinity or after all available boron atoms have been saturated with nitrogen. Implanted carbon atoms also preferred to either bond with available boron atoms or, more likely bonded with other implanted carbon atoms. These results were also supported by ab-initio density functional theory calculations which indicated that carbon-carbon bonds were energetically preferable to carbon-boron and carbon-nitrogen bonds.Item Open Access Charge retention in quantized energy levels of nanocrystals(Elsevier B.V., 2007) Dâna, A.; Akça, I.; Ergun, O.; Aydınlı, Atilla; Turan, R.; Finstad, T. G.Understanding charging mechanisms and charge retention dynamics of nanocrystal (NC) memory devices is important in optimization of device design. Capacitance spectroscopy on PECVD grown germanium NCs embedded in a silicon oxide matrix was performed. Dynamic measurements of discharge dynamics are carried out. Charge decay is modelled by assuming storage of carriers in the ground states of NCs and that the decay is dominated by direct tunnelling. Discharge rates are calculated using the theoretical model for different NC sizes and densities and are compared with experimental data. Experimental results agree well with the proposed model and suggest that charge is indeed stored in the quantized energy levels of the NCs.Item Open Access Comparison of electron and hole charge-discharge dynamics in germanium nanocrystal flash memories(AIP Publishing, 2008-02) Akça, İmran B.; Dâna, Aykutlu; Aydınlı, Atilla; Turan, R.Electron and hole charge and discharge dynamics are studied on plasma enhanced chemical vapor deposition grown metal-oxide-silicon germanium nanocrystal flash memory devices. Electron and hole charge and discharge currents are observed to differ significantly and depend on annealing conditions chosen for the formation of nanocrystals. At low annealing temperatures, holes are seen to charge slower but to escape faster than electrons. They discharge slower than electrons when annealing temperatures are raised. The results suggest that discharge currents are dominated by the interface layer acting as a quantum well for holes and by direct tunneling for elec-trons.Item Open Access Effect of the passivation layer on the noise characteristics of mid-wave-infrared InAs / GaSb superlattice photodiodes(IEEE, 2012) Tansel, T.; Kutluer, K.; Salihoglu, Ö.; Aydınlı, Atilla; Aslan, B.; Arikan, B.; Kilinc, M. C.; Ergun, Y.; Serincan, U.; Turan, R.The authors describe the noise characterization of a mid-wavelength- infrared (MWIR) photodiode based on indium arsenide and gallium antimonide (InAs/GaSb) superlattice (SL), addressing the influence of different passivation layers applied to the surface of the device. The MWIR InAs/GaSb SL design structure is based on p-i-n configuration grown by the molecular beam epitaxy on a (001) n-GaSb substrate. The SiO 2-passivated SL photodiodes demonstrated a Schottky-limited noise up to a bias voltage of -0.1 V where the measured peak responsivity is 1.37 A/W with a cut-off wavelength of 4.9 μm and the specific detectivity as high as 1.23 × 10 12 cm. Hz 1/2 W , demonstrating the high quality of the fabricated MWIR SL photodiodes. The noise measurements exhibited a frequency-dependent plateau (i.e., 1/f noise) for unpassivated and Si 3N 4-passivated samples, whereas 1/f-type noise suppression (i.e., frequency-independent plateau) with a noise current reduction at about 30 Hz of more than one order of magnitude was observed for the SiO 2-passivated ones.Item Open Access Electronic and optical properties of 4.2 lm‘‘N’’ structured superlattice MWIR photodetectors(Elsevier B.V., 2013-01-05) Salihoglu, O.; Hostut M.; Tansel, T.; Kutluer, K.; Kilic A.; Alyoruk, M.; Sevik, C.; Turan, R.; Ergun, Y.; Aydınlı, AtillaWe report on the development of a new structure for type II superlattice photodiodes that we call the ‘‘N’’ design. In this new design, we insert an electron barrier between InAs and GaSb in the growth direction. The barrier pushes the electron and hole wavefunctions towards the layer edges and under bias, increases the overlap integral by about 25% leading to higher detectivity. InAs/AlSb/GaSb superlattices were studied with density functional theory. Both AlAs and InSb interfaces were taken into account by calculating the heavy hole–light hole (HH–LH) splittings. Experiments were carried out on single pixel photodiodes by measuring electrical and optical performance. With cut-off wavelength of 4.2 lm at 120 K, temperature dependent dark current and detectivity measurements show that the dark current is 2.5 10 9 A under zero bias with corresponding R0A resistance of 1.5 104 X cm2 for the 500 500 lm2 single pixel square photodetectors. Photodetector reaches BLIP condition at 125 K with the BLIP detectivity (D BLIP) of 2.6 1010 Jones under 300 K background and 0.3 V bias voltage.Item Open Access Electronic and optical properties of 4.2 μm"N" structured superlattice MWIR photodetectors(Elsevier, 2013) Salihoglu, O.; Hostut M.; Tansel, T.; Kutluer, K.; Kilic A.; Alyoruk, M.; Sevik, C.; Turan, R.; Ergun, Y.; Aydınlı, AtillaWe report on the development of a new structure for type II superlattice photodiodes that we call the "N" design. In this new design, we insert an electron barrier between InAs and GaSb in the growth direction. The barrier pushes the electron and hole wavefunctions towards the layer edges and under bias, increases the overlap integral by about 25% leading to higher detectivity. InAs/AlSb/GaSb superlattices were studied with density functional theory. Both AlAs and InSb interfaces were taken into account by calculating the heavy hole-light hole (HH-LH) splittings. Experiments were carried out on single pixel photodiodes by measuring electrical and optical performance. With cut-off wavelength of 4.2 μm at 120 K, temperature dependent dark current and detectivity measurements show that the dark current is 2.5 × 10 -9 A under zero bias with corresponding R0A resistance of 1.5 × 104 Ω cm2 for the 500 × 500 μm2 single pixel square photodetectors. Photodetector reaches BLIP condition at 125 K with the BLIP detectivity (DBLIP) of 2.6 × 10 10 Jones under 300 K background and -0.3 V bias voltage. © 2012 Elsevier B.V. All rights reserved.Item Open Access A figure of merit for optimization of nanocrystal flash memory design(2008) Dâna, A.; Akca, I.; Aydınlı, Atilla; Turan, R.; Finstad, T. G.Nanocrystals can be used as storage media for carriers in flash memories. The performance of a nanocrystal flash memory depends critically on the choice of nanocrystal size and density as well as on the choice of tunnel dielectric properties. The performance of a nanocrystal memory device can be expressed in terms of write/erase speed, carrier retention time and cycling durability. We present a model that describes the charge/discharge dynamics of nanocrystal flash memories and calculate the effect of nanocrystal, gate, tunnel dielectric and substrate properties on device performance. The model assumes charge storage in quantized energy levels of nanocrystals. Effect of temperature is included implicitly in the model through perturbation of the substrate minority carrier concentration and Fermi level. Because a large number of variables affect these performance measures, in order to compare various designs, a figure of merit that measures the device performance in terms of design parameters is defined as a function of write/erase/discharge times which are calculated using the theoretical model. The effects of nanocrystal size and density, gate work function, substrate doping, control and tunnel dielectric properties and device geometry on the device performance are evaluated through the figure of merit. Experimental data showing agreement of the theoretical model with the measurement results are presented for devices that has PECVD grown germanium nanocrystals as the storage media. CopyrightItem Open Access Formation of silicon nanocrystals in sapphire by ion implantation and the origin of visible photoluminescence(AIP Publishing, 2006) Yerli, S.; Serincan, U.; Dogan, I.; Tokay, S.; Genisel, M.; Aydınlı, Atilla; Turan, R.Silicon nanocrystals, average sizes ranging between 3 and 7 nm, were formed in sapphire matrix by ion implantation and subsequent annealing. Evolution of the nanocrystals was detected by Raman spectroscopy and x-ray diffraction (XRD). Raman spectra display that clusters in the matrix start to form nanocrystalline structures at annealing temperatures as low as 800 degrees C in samples with high dose Si implantation. The onset temperature of crystallization increases with decreasing dose. Raman spectroscopy and XRD reveal gradual transformation of Si clusters into crystalline form. Visible photoluminescence band appears following implantation and its intensity increases with subsequent annealing process. While the center of the peak does not shift, the intensity of the peak decreases with increasing dose. The origin of the observed photoluminescence is discussed in terms of radiation induced defects in the sapphire matrix.Item Open Access High quantum efficiency Type-II superlattice N-structure photodetectors with thin intrinsic layers(SPIE, 2013) Ergun, Y.; Hostut, M.; Tansel, T.; Muti, bdullah; Kilic, A.; Turan, R.; Aydınlı, AtillaWe report on the development of InAs/AlSb/GaSb based N-structure superlattice pin photodiode. In this new design, AlSb layer in between InAs and GaSb layers acts as an electron barrier that pushes electron and hole wave functions towards the GaSb/InAs interface to perform strong overlap under reverse bias. Experimental results show that, with only 20 periods of intrinsic layers, dark current density and dynamic resistance at -50 mV bias are measured as 6×10-3 A/cm2 and 148 ωcm2 at 77K, respectively. Under zero bias, high spectral response of 1.2A/W is obtained at 5 μm with 50% cut-off wavelengths (λc) of 6 μm. With this new design, devices with only 146 nm thick i-regions exhibit a quantum efficiency of 42% at 3 μm with front-side illimunation and no anti-reflection coatings. © 2013 SPIE.Item Open Access In-chip microstructures and photonic devices fabricated by nonlinear laser lithography deep inside silicon(Nature Publishing Group, 2017) Tokel, O.; Turnalı, A.; Makey, G.; Elahi, P.; Çolakoǧlu, T.; Ergeçen E.; Yavuz, Ö.; Hübner R.; Borra, M. Z.; Pavlov, I.; Bek, A.; Turan, R.; Kesim, D. K.; Tozburun, S.; Ilday, S.; Ilday, F. Ö.Silicon is an excellent material for microelectronics and integrated photonics 1-3, with untapped potential for mid-infrared optics 4 . Despite broad recognition of the importance of the third dimension 5,6, current lithography methods do not allow the fabrication of photonic devices and functional microelements directly inside silicon chips. Even relatively simple curved geometries cannot be realized with techniques like reactive ion etching. Embedded optical elements 7, electronic devices and better electronic-photonic integration are lacking 8 . Here, we demonstrate laser-based fabrication of complex 3D structures deep inside silicon using 1-μm-sized dots and rod-like structures of adjustable length as basic building blocks. The laser-modified Si has an optical index different to that in unmodified parts, enabling the creation of numerous photonic devices. Optionally, these parts can be chemically etched to produce desired 3D shapes. We exemplify a plethora of subsurface - that is, 'in-chip' - microstructures for microfluidic cooling of chips, vias, micro-electro-mechanical systems, photovoltaic applications and photonic devices that match or surpass corresponding state-of-the-art device performances.Item Open Access Indium rich InGaN solar cells grown by MOCVD(Springer New York LLC, 2014) Çakmak, H.; Arslan, E.; Rudziński, M.; Demirel, P.; Unalan, H. E.; Strupiński, W.; Turan, R.; Öztürk, M.; Özbay, EkmelThis study focuses on both epitaxial growths of InxGa 1-xN epilayers with graded In content, and the performance of solar cells structures grown on sapphire substrate by using metal organic chemical vapor deposition. The high resolution X-ray and Hall Effect characterization were carried out after epitaxial InGaN solar cell structures growth. The In content of the graded InGaN layer was calculated from the X-ray reciprocal space mapping measurements. Indium contents of the graded InGaN epilayers change from 8.8 to 7.1 % in Sample A, 15.7-7.1 % in Sample B, and 26.6-15.1 % in Sample C. The current voltage measurements of the solar cell devices were carried out after a standard micro fabrication procedure. Sample B exhibits better performance with a short-circuit current density of 6 mA/cm2, open-circuit voltage of 0.25 V, fill factor of 39.13 %, and the best efficiency measured under a standard solar simulator with one-sun air mass 1.5 global light sources (100 mW/cm2) at room temperature for finished devices was 0.66 %.Item Open Access Laser-slicing of silicon with 3D nonlinear laser lithography(OSA, 2017) Tokel, Onur; Turnalı, Ahmet; Çolakoğlu, T.; İlday, Serim; Borra, M. Z.; Pavlov, Ihor; Bek, A.; Turan, R.; İlday, Fatih ÖmerRecently, we have showed a direct laser writing method that exploits nonlinear interactions to form subsurface modifications in silicon. Here, we use the technique to demonstrate laser-slicing of silicon and its applications.Item Open Access Light harvesting with Ge quantum dots embedded in SiO2 or Si3N4(A I P Publishing LLC, 2014) Cosentino, S.; Ozen, E. S.; Raciti, R.; Mio, A. M.; Nicotra, G.; Simone, F.; Crupi, I.; Turan, R.; Terrasi, A.; Aydınlı, Atilla; Mirabella, S.Germanium quantum dots (QDs) embedded in SiO2or in Si3N4have been studied for light harvesting purposes. SiGeO or SiGeN thin films, produced by plasma enhanced chemical vapor deposition, have been annealed up to 850°C to induce Ge QD precipitation in Si based matrices. By varying the Ge content, the QD diameter can be tuned in the 3-9 nm range in the SiO2matrix, or in the 1-2 nm range in the Si3N4matrix, as measured by transmission electron microscopy. Thus, Si3N4matrix hosts Ge QDs at higher density and more closely spaced than SiO2matrix. Raman spectroscopy revealed a higher threshold for amorphous-to-crystalline transition for Ge QDs embedded in Si3N4matrix in comparison with those in the SiO2host. Light absorption by Ge QDs is shown to be more effective in Si3N4matrix, due to the optical bandgap (0.9-1.6 eV) being lower than in SiO2matrix (1.2-2.2 eV). Significant photoresponse with a large measured internal quantum efficiency has been observed for Ge QDs in Si3N4matrix when they are used as a sensitive layer in a photodetector device. These data will be presented and discussed, opening new routes for application of Ge QDs in light harvesting devices. © 2014 AIP Publishing LLC.Item Open Access Low dark current N structure superlattice MWIR photodetectors(SPIE, 2014) Salihoğlu, O.; Muti, Abdullah; Turan, R.; Ergun, Y.; Aydınlı, AtillaCommercially available read out integrated circuits (ROICs) require the FPA to have high dynamic resistance area product at zero bias (R0A) which is directly related to dark current of the detector. Dark current arises from bulk and surface contributions. Recent band structure engineering studies significantly suppressed the bulk contribution of the type-II superlattice infrared photodetectors (N structure, M structure, W structure). In this letter, we will present improved dark current results for unipolar barrier complex supercell superlattice system which is called as "N structure". The unique electronic band structure of the N structure increases electron-hole overlap under bias, significantly. N structure aims to improve absorption by manipulating electron and hole wavefunctions that are spatially separated in T2SLs, increasing the absorption while decreasing the dark current. In order to engineer the wavefunctions, we introduce a thin AlSb layer between InAs and GaSb layers in the growth direction which also acts as a unipolar electron barrier. Despite the difficulty of perfect lattice matching of InAs and AlSb, such a design is expected to reduce dark current. Experiments were carried out on Single pixel with mesa sizes of 100 × 100 - 700 × 700 μm photodiodes. Temperature dependent dark current with corresponding R0A resistance values are reported.Item Open Access Low-frequency noise behavior at reverse bias region in InAs/GaSb superlattice photodiodes on mid-wave infrared(SPIE, 2013) Tansel, T.; Kutluer, K.; Muti, Abdullah; Salihoğlu, Ömer; Aydınlı, Atilla; Turan, R.We describe a relationship between the noise characterization and activation energy of InAs/GaSb superlattice Mid- Wavelength-Infrared photodiodes for different passivation materials applied to the device. The noise measurements exhibited a frequency dependent plateau (i.e. 1/f-noise characteristic) for unpassivated as well as Si3N4 passivated samples whereas 1/f-type low noise suppression (i.e. frequency independent plateau) with a noise current reduction of more than one order of magnitude was observed for SiO2 passivation. For reverse bias values below -0.15V, the classical Schottky-noise calculation alone did not appear to describe the noise mechanism in a SL noise behavior, which shows a divergence between theoretically and experimentally determined noise values. We identify that, the additional noise appears, with and without passivation, at the surface activation energy of < 60 meV and is inversely proportional to the reverse bias. This is believed to be caused by the surface dangling-bonds (as well as surface states) whose response is controlled by the applied reverse bias. The calculated noise characteristics showed a good agreement with the experimental data. © 2013 SPIE.Item Open Access Matrix density effect on morphology of germanium nanocrystals embedded in silicon dioxide thin films(Materials Research Society, 2011) Alagoz, A. S.; Genisel, M. F.; Foss, Steinar; Finstad, T. G.; Turan, R.Flash type electronic memories are the preferred format in code storage at complex programs running on fast processors and larger media files in portable electronics due to fast write/read operations, long rewrite life, high density and low cost of fabrication. Scaling limitations of top-down fabrication approaches can be overcome in next generation flash memories by replacing continuous floating gate with array of nanocrystals. Germanium (Ge) is a good candidate for nanocrystal based flash memories due its small band gap. In this work, we present effect of silicon dioxide (SiO 2) host matrix density on Ge nanocrystals morphology. Low density Ge+SiO 2 layers are deposited between high density SiO 2 layers by using off-angle magnetron sputter deposition. After high temperature post-annealing, faceted and elongated Ge nanocrystals formation is observed in low density layers. Effects of Ge concentration and annealing temperature on nanocrystal morphology and mean size were investigated by using transmission electron microscopy. Positive correlation between stress development and nanocrystal size is observed at Raman spectroscopy measurements. We concluded that non-uniform stress distribution on nanocrystals during growth is responsible from faceted and elongated nanocrystal morphology.Item Open Access Multiscale self-asssembly of silicon quantum dots into an anisotropic three-dimensional random network(American Chemical Society, 2016) Ilday, S.; Ilday, F. O.; Hübner R.; Prosa, T. J.; Martin, I.; Nogay, G.; Kabacelik, I.; Mics, Z.; Bonn, M.; Turchinovich, D.; Toffoli, H.; Toffoli, D.; Friedrich, D.; Schmidt, B.; Heinig, K.-H.; Turan, R.Multiscale self-assembly is ubiquitous in nature but its deliberate use to synthesize multifunctional three-dimensional materials remains rare, partly due to the notoriously difficult problem of controlling topology from atomic to macroscopic scales to obtain intended material properties. Here, we propose a simple, modular, noncolloidal methodology that is based on exploiting universality in stochastic growth dynamics and driving the growth process under far-from-equilibrium conditions toward a preplanned structure. As proof of principle, we demonstrate a confined-but-connected solid structure, comprising an anisotropic random network of silicon quantum-dots that hierarchically self-assembles from the atomic to the microscopic scales. First, quantum-dots form to subsequently interconnect without inflating their diameters to form a random network, and this network then grows in a preferential direction to form undulated and branching nanowire-like structures. This specific topology simultaneously achieves two scale-dependent features, which were previously thought to be mutually exclusive: good electrical conduction on the microscale and a bandgap tunable over a range of energies on the nanoscale. © 2016 American Chemical Society.Item Open Access N structure for type-II superlattice photodetectors(American Institute of Physics, 2012-08-14) Salihoglu, O.; Muti, A.; Kutluer, K.; Tansel, T.; Turan, R.; Ergun, Y.; Aydınlı, AtillaIn the quest to raise the operating temperature and improve the detectivity of type II superlattice (T2SL) photodetectors, we introduce a design approach that we call the "N structure." N structure aims to improve absorption by manipulating electron and hole wavefunctions that are spatially separated in T2SLs, increasing the absorption while decreasing the dark current. In order to engineer the wavefunctions, we introduce a thin AlSb layer between InAs and GaSb layers in the growth direction which also acts as a unipolar electron barrier. Unlike the symmetrical insertion of AlSb into GaSb layers, N design aims to exploit the shifting of the electron and hole wavefunctions under reverse bias. With cutoff wavelength of 4.3 mu m at 77 K, temperature dependent dark current and detectivity measurements show that the dark current density is 3.6 x 10(-9) A/cm(2), under zero bias. Photodetector reaches background limited infrared photodetection (BLIP) condition at 125 K with the BLIP detectivity (D-BLIP*) of 2.6 x 10(10) Jones under 300 K background and -0.3 V bias voltage.Item Open Access N-structure based on InAs/AlSb/GaSb superlattice photodetectors(Academic Press, 2015) Hostut, M.; Alyoruk, M.; Tansel, T.; Kilic, A.; Turan, R.; Aydınlı, Atilla; Ergun, Y.We have studied the theoretical and experimental properties of InAs/AlSb/GaSb based type-II superlattice (T2SL) pin photodetector called N-structure. Electronic properties of the superlattice such as HH-LH splitting energies was investigated using first principles calculations taking into account InSb and AlAs as possible interface transition alloys between AlSb/InAs layers and individual layer thicknesses of GaSb and InAs. T2SL N-structure was optimized to operate as a MWIR detector based on these theoretical approaches tailoring the band gap and HH-LH splitting energies with InSb transition layers between InAs/AlSb interfaces. Experimental results show that AlSb layers in the structure act as carrier blocking barriers reducing the dark current. Dark current density and R0A product at 125 K were obtained as 1.8 × 10-6 A cm-2 and 800ωcm2 at zero bias, respectively. The specific detectivity was measured as 3 × 1012 Jones with cut-off wavelengths of 4.3 μm at 79 K reaching to 2 × 109 Jones and 4.5 μm at 255 K. ©2014 Elsevier Ltd. All rights reserved.