Browsing by Subject "Superlattices"
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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 Detecting squeezed phonons through an indirect radiative transition(A I P Publishing LLC, 1997-04-27) MÜstecaplioglu, Ö. E.; Shumovsky, A. S.A model of the effect of the longitudinal optical phonon number distribution on the Rabi oscillations of the photons involved in the associated indirect transition in a semiconductor is presented. It is shown that a faster cavity photon revival rate is obtained given an initial squeezed thermal phonon state. This is the result of pairwise correlations of phonons due to squeezing and also holds true when phonons are in squeezed vacuum or in squeezed number states; It may be possible to use this effect to detect squeezing in phonons. (C) 1997 American Institute of Physics.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 Electrical performance of InAs/AlSb/GaSb superlattice photodetectors(Academic Press, 2016) Tansel, T.; Hostut M.; Elagoz, S.; Kilic A.; Ergun, Y.; Aydınlı, AtillaTemperature dependence of dark current measurements is an efficient way to verify the quality of an infrared detector. Low dark current density values are needed for high performance detector applications. Identification of dominant current mechanisms in each operating temperature can be used to extract minority carrier lifetimes which are highly important for understanding carrier transport and improving the detector performance. InAs/AlSb/GaSb based T2SL N-structures with AlSb unipolar barriers are designed for low dark current with high resistance and detectivity. Here we present electrical and optical performance of such N-structure photodetectors.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 Electronic structure of Ge-Si superlattices grown on Ge (001)(1991) Gülseren, Oğuz; Çıracı, SalimThe authors have studied the electronic energy structure of pseudomorphic Gem/Sin superlattices by using the empirical tight-binding method. Effects of the band offset, sublattice periodicity and the lateral lattice constant on the transition energies have been investigated. They found that GemSin superlattices grown on Ge (001) can have a direct band gap, if m+n=10 and m=6. However, optical matrix elements for in-plane and perpendicular polarized light are negligible for the transition from the highest valence band to the lowest conduction band state at the centre of the superlattice Brillouin zone.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 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 Many-body properties of a disordered charged Bose gas superlattice(Academic Press, 2000) Tanatar, Bilal; Das, A. K.We study some many-body properties of a disordered charged Bose gas (CBG) superlattice-an infinite array of CBG layers each of which containing disorder. The latter is assumed to cause collisions with the charged bosons, the effect of collisions being taken into account through a number-conserving relaxation time approximation incorporated within the random phase approximation (RPA) at T = 0. We go beyond the RPA and include a local-field correction G(q, q z) which is assumed to be collision independent, as an approximation. The resulting density-density correlation function is then used to calculate a number of many-body quantities of physical interest, e.g. (a) collective modes, (b) static structure factor, (c) energy-loss function, (d) plasmon density of states, and (e) groundstate energy. The effects of collisions on these quantities are discussed, and the results are compared with the corresponding results for an electron gas superlattice.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.Item Open Access Plasma modes in layered superconductors(Elsevier B.V., 2005) Askerzade, I. N.; Tanatar, BilalAn expression for the plasmon spectrum in the layered superconductors with arbitrary thickness of planes, which varies within a wide range is obtained. The obtained result can be attractive for the explanation of experimental data on plasmon modes in cuprates and other recently discovered superconductors.Item Open Access Quantum dots on vertically aligned gold nanorod monolayer: plasmon enhanced fluorescence(Royal Society of Chemistry, 2014) Peng, B.; Li, Z.; Mutlugun, E.; Martinez, P. L. H.; Li, D.; Zhang, Q.; Gao, Y.; Demir, Hilmi Volkan; Xiong, Q.CTAB-coated Au nanorods were directly self-assembled into a vertically aligned monolayer with highly uniform hot spots through a simple but robust approach. By coupling with CdSe/ZnS quantum dots, a maximum enhancement of 10.4 is achieved due to: increased excitation transition rate, radiative rate, and coupling efficiency of emission to the far field.Item Open Access Size controlled germanium nanocrystals in dielectrics : structural and optical analysis and stress evolution(2017-08) Bahariqushchi, RahimGroup IV semiconductor nanocrystals, namely silicon and germanium have attracted much interest in the past two decades due to their broad applications in photovoltaic, memory, optoelectronic, medical imaging and photodetection devices. Generally, there are two major features of semiconducor nanocrystals: First, spatial confinement of charge carriers which leads to the significant changes in optical and electronic properties of materials as a function of size. This effect gives the possibility to use the size and shape of the nanocrystals to tune the energy of electronic energy states. Second feature of nanocrystals, is the increased of surface area to volume ratio of the nanocrystal with reducing size. This leads to an enhanced role of the effects related to surface and interface of the nanocrystal. Furthermore, stress on the nanocrystals can lead modification of the band structure as well as in uencing the crystallization of the nanomaterials. Recent works show that measurement and control of the stress can open the way for strain engineering of the electronic band structure, thereby opening the way for new physics and applications. In this thesis, we first carry out a study on the synthesis of germanium embedded in silicon nitride and oxide matrices. In uence of the annealing method as well as germanium concentration on the formation of nanocrystals is discussed. It was found that Ge concentration and annealing play important roles in the formation of the Ge nanocrystals. With crystallographic data obtained from high resolution transmission electron microscopy, quantitative analysis of stress state of germanium nanocrystals have been done by analyzing Raman peak shift of embedded nanocrystals taking into account the phonon confinement effect. Finally, using stressors as buffer layers, superlattices of Ge nanosheets were studied to understand the effects of the stressors on the stress state of Ge nanocrystals. We demonstrate that it is possible to tune the stress on the Ge nanocrystals from compressive to tensile. Finally we showed a three dimensional Ge quantum solid that can be used in optoelectronic applications.Item Open Access Spin confinement in the superlattices of graphene ribbons(AIP Publishing, 2008) Topsakal, M.; Sevinçli, H.; Çıracı, SalimBased on first-principles calculations, we showed that repeated heterostructures of zigzag graphene nanoribbons of different widths form multiple quantum well structures. Edge states of specific spin directions can be confined in these wells. The electronic and magnetic state of the ribbon can be modulated in real space. In specific geometries, the absence of reflection symmetry causes the magnetic ground state of whole heterostructure to change from antiferromagnetic to ferrimagnetic. These quantum structures of different geometries provide unique features for spintronic applications.Item Open Access Stress evolution of Ge nanocrystals in dielectric matrices(Institute of Physics Publishing, 2018) Bahariqushchi, R.; Raciti, R.; Kasapoǧlu, A. E.; Gür, E.; Sezen, M.; Kalay, E.; Mirabella, S.; Aydınlı, AtillaGermanium nanocrystals (Ge NCs) embedded in single and multilayer silicon oxide and silicon nitride matrices have been synthesized using plasma enhanced chemical vapor deposition followed by conventional furnace annealing or rapid thermal processing in N2 ambient. Compositions of the films were determined by Rutherford backscattering spectrometry and x-ray photoelectron spectroscopy. The formation of NCs under suitable process conditions was observed with high resolution transmission electron microscope micrographs and Raman spectroscopy. Stress measurements were done using Raman shifts of the Ge optical phonon line at 300.7 cm-1. The effect of the embedding matrix and annealing methods on Ge NC formation were investigated. In addition to Ge NCs in single layer samples, the stress on Ge NCs in multilayer samples was also analyzed. Multilayers of Ge NCs in a silicon nitride matrix separated by dielectric buffer layers to control the size and density of NCs were fabricated. Multilayers consisted of SiN y :Ge ultrathin films sandwiched between either SiO2 or Si3N4 by the proper choice of buffer material. We demonstrated that it is possible to tune the stress state of Ge NCs from compressive to tensile, a desirable property for optoelectronic applications. We also observed that there is a correlation between the stress and the crystallization threshold in which the compressive stress enhances the crystallization, while the tensile stress suppresses the process.Item Open Access Surface recombination noise in InAs / GaSb superlattice photodiodes(IOP Institute of Physics Publishing, 2013) Tansel, T.; Kutluer, K.; Muti, A.; Salihoglu, Ö.; Aydınlı, Atilla; Turan, R.The standard Schottky noise approach alone is not sufficient to describe the noise mechanism in an InAs/GaSb superlattice photodetector at reverse negative bias. The additional noise identified appears at surface activation energies below 60meV and is inversely proportional to the reverse bias. In order to satisfactorily explain the experimental data, we hereby propose the existence of a surface recombination noise that is a function of both the frequency and bias. The calculated noise characteristics indeed show good agreement with the experimental data.Item Open Access Theoretical investigation of InAs/GaSb type-II pin superlattice infrared detector in the mid wavelength infrared range(2013) Kaya, U.; Hostut, M.; Kilic, A.; Sakiroglu, S.; Sokmen I.; Ergun, Y.; Aydınlı, AtillaIn this study, we present the theoretical investigation of type-II InAs/GaSb superlattice p-i-n detector. Kronig-Penney and envelope function approximation is used to calculate band gap energy and superlattice minibands. Variational method is also used to calculate exciton binding energies. Our results show that carriers overlap increases at GaSb/InAs interface on the higher energy side while it decreases at InAs/GaSb interface on the lower energy side with increasing reverse bias due to shifting the hole wavefunction toward to the GaSb/InAs interface decisively. Binding energies increase with increasing electric field due to overall overlap of electron and hole wave functions at the both interfaces in contrast with type I superlattices. This predicts that optical absorption is enhanced with increasing electric field. © 2013 American Institute of Physics.Item Open Access Thiol passivation of MWIR Type II superlattice photodetectors(SPIE, 2013) Salihoğlu, Ömer; Muti, Abdullah; Aydınlı, AtillaPoor passivation on photodetectors can result in catastrophic failure of the device. Abrupt termination of mesa side walls during pixel definition generates dangling bonds that lead to inversion layers and surface traps leading to surface leakage currents that short circuit diode action. Good passivation, therefore, is critical in the fabrication of high performance devices. Silicondioxide has been the main stay of passivation for commercial photodetectors, deposited at high temperatures and high RF powers using plasma deposition techniques. In photodetectors based on III-V compounds, sulphur passivation has been shown to replace oxygen and saturate the dangling bonds. Despite its effectiveness, it degrades over time. More effort is required to create passivation layers which eliminate surface leakage current. In this work, we propose the use of sulphur based octadecanethiol (ODT), CH3(CH2)17SH, as a passivation layer for the InAs/GaSb superlattice photodetectors that acts as a self assembled monolayer (SAM). ODT SAMs consist of a chain of 18 carbon atoms with a sulphur atom at its head. ODT Thiol coating is a simple process that consist of dipping the sample into the solution for a prescribed time. Excellent electrical performance of diodes tested confirm the effectiveness of the sulphur head stabilized by the intermolecular interaction due to van der Walls forces between the long chains of ODT SAM which results in highly stable ultrathin hydrocarbon layers without long term degradation. © 2013 SPIE.