Browsing by Subject "Defects"

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Open Access
Crystallization of Ge in SiO2 matrix by femtosecond laser processing
(American Vacuum Society, 2012-01-19) Salihoglu, O.; Kürüm, U.; Yaglioglu, H. G.; Elmali, A.; Aydınlı, Atilla
Germanium nanocrystals embedded in a siliconoxide matrix has been fabricated by single femtosecond laser pulse irradiation of germanium doped SiO2 thin films deposited with plasma enhanced chemical vapor deposition. SEM and AFM are used to analyze surface modification induced by laser irradiation. Crystallization of Ge in the oxide matrix is monitored with the optic phonon at 300 cm(-1) as a function of laser fluence. Both the position the linewidth of the phonon provides clear signature for crystallization of Ge. In PL experiments, strong luminescence around 600 nm has been observed.
Open Access
Defect luminescence in some layered binary chalcogenide semiconductors
(Scientific.Net, 2002) Aydınlı, Atilla; Gasanly, N. M.
A number of semiconductors such as GaS, GaSe, GaSSe show layered structure where intralayer bonding is strong and interlayer bonding quite weak. With bandgaps mostly in the visible and the near infrared and high crystal structure anisotropy, such semiconductors offer interesting possibilities for optoelectronic applications In this review, we will summarize the recent developments on the photoluminescent properties of these materials such as luminescence due to donor-acceptor pair recombination. As these materials are undoped, the observed photoluminescence is attributed mostly to defect states present in these materials.
Open Access
Diffraction inspired unidirectional and bidirectional beam splitting in defect-containing photonic structures without interface corrugations
(American Institute of Physics Inc., 2016) Colak, E.; Serebryannikov, A. E.; Usik, P. V.; Özbay, Ekmel
It is shown that strong diffractions and related dual-beam splitting can be obtained at transmission through the nonsymmetric structures that represent two slabs of photonic crystal (PhC) separated by a single coupled-cavity type defect layer, while there are no grating-like corrugations at the interfaces. The basic operation regimes include unidirectional and bidirectional splitting that occur due to the dominant contribution of the first positive and first negative diffraction orders to the transmission, which is typically connected with different manifestations of the asymmetric transmission phenomenon. Being the main component of the resulting transmission mechanism, diffractions appear owing to the effect exerted by the defect layer that works like an embedded diffractive element. Two mechanisms can co-exist in one structure, which differ, among others, in that whether dispersion allows coupling of zero order to a wave propagating in the regular, i.e., defect-free PhC segments or not. The possibility of strong diffractions and efficient splitting related to it strongly depend on the dispersion properties of the Floquet-Bloch modes of the PhC. Existence of one of the studied transmission scenarios is not affected by location of the defect layer.
Open Access
Dynamic control of photoresponse in ZnO-based thin-film transistors in the visible spectrum
(IEEE, 2013-04) Aygun, L. E.; Oruc, F. B.; Atar, F. B.; Okyay, Ali Kemal
We present ZnO-channel thin-film transistors with actively tunable photocurrent in the visible spectrum, although ZnO band edge is in the ultraviolet. ZnO channel is deposited by atomic layer deposition technique at a low temperature (80), which is known to introduce deep level traps within the forbidden band of ZnO. The gate bias dynamically modifies the occupancy probability of these trap states by controlling the depletion region in the ZnO channel. Unoccupied trap states enable the absorption of the photons with lower energies than the bandgap of ZnO. Photoresponse to visible light is controlled by the applied voltage bias at the gate terminal. © 2009-2012 IEEE.
Open Access
Effect of growth pressure on coalescence thickness and crystal quality of GaN deposited on 4H-SiC
(Elsevier, 2010-09-25) Caban, P.; Strupinski, W.; Szmidt, J.; Wojcik, M.; Gaca, J.; Kelekci, O.; Caliskan, D.; Özbay, Ekmel
The influence of growth pressure on the coalescence thickness and the crystal quality of GaN deposited on 4HSiC by low pressure metalorganic vapor phase epitaxy was studied. It was shown that growth pressure has an impact on the surface roughness of epilayers and their crystal quality. GaN coalescence thicknesses were determined for the investigated growth pressures. The GaN layers were characterized by AFM and HRXRD measurements. HEMT structures were also fabricated and characterized. Among the growth pressures studied, 50, 125 and 200 mbar, 200 mbar was found to be most suitable for GaN/SiC epitaxy.
Open Access
Effect of in-material losses on terahertz absorption, transmission, and reflection in photonic crystals made of polar dielectrics
(A I P Publishing LLC, 2015) Serebryannikov, A. E.; Nojima, S.; Alici, K. B.; Özbay, Ekmel
The effect of the material absorption factor on terahertz absorption (A), transmittance (T), and reflectance (R) for slabs of PhC that comprise rods made of GaAs, a polar dielectric, is studied. The main goal was to illustrate how critical a choice of the absorption factor for simulations is and to indicate the importance of the possible modification of the absorption ability by using either active or lossy impurities. The spectra of A, T, and R are strongly sensitive to the location of the polaritonic gap with respect to the photonic pass and stop bands connected with periodicity that enables the efficient combination of the effects of material and structural parameters. It will be shown that the spectra can strongly depend on the utilized value of the material absorption factor. In particular, both narrow and wide absorption bands may appear owing to a variation of the material parameters with a frequency in the vicinity of the polaritonic gap. The latter are often achieved at wideband suppression of transmission, so that an ultra-wide stop band can appear as a result of adjustment of the stop bands having different origin. The results obtained at simultaneous variation of the absorption factor and frequency, and angle of incidence and frequency, indicate the possibility of the existence of wide ranges of tolerance, in which the basic features do remain. This allows for mitigating the accuracy requirements for the absorption factor in simulations and promises the efficient absorption of nonmonochromatic waves and beams with a wide angular spectrum. Suppression of narrowband effects in transmission is demonstrated at rather large values of the absorption factor, when they appear due to either the defect modes related to structural defects or dispersion inspired variations of the material parameters in the vicinity of the polaritonic gap. Comparison with auxiliary structures helps one to detect the common features and differences of homogeneous slabs and slabs of a PhC, which are made of GaAs. © 2015 AIP Publishing LLC.
Open Access
Examination of the temperature related structural defects of InGaN/GaN solar cells
(Academic Press, 2015) Durukan, İ. K.; Bayal, Ö.; Kurtuluş, G.; Baş, Y.; Gültekin, A.; Öztürk, M. K.; Çörekçi, S.; Tamer, M.; Özçelik, S.; Özbay, Ekmel
In this study the effects of the annealing temperature on the InGaN/GaN solar cells with different In-contents grown on sapphire substrate by the Metal Organic Chemical Vapor Deposition (MOCVD) are analyzed by High Resolution X-ray Diffraction (HRXRD) and an Atomic Force Microscope (AFM). The plane angles, mosaic crystal sizes, mixed stress, dislocation intensities of the structure of the GaN and InGaN layers are determined. According to the test results, there are no general characteristic trends observed due to temperature at both structures. There are fluctuating failures determined at both structures as of 350 °C. The defect density increased on the GaN layer starting from 350 °C and reaching above 400 °C. A similar trend is observed on the InGaN layer, too.
Open Access
Excitation dependent recombination studies on SnO2/TiO2 electrospun nanofibers
(Royal Society of Chemistry, 2015) Babu, V. J.; Vempati S.; Ertas Y.; Uyar, Tamer
Poly(vinyl acetate) (PVAc)/TiO2 nanofibers, PVAc/SnO2 nanoribbons and PVAc/SnO2-TiO2 nanoribbons were produced via electrospinning. TiO2 nanofibers and SnO2 nanoribbons were obtained by removal of the polymeric matrix (PVAc) after calcination at 450 °C. Interestingly, PVAc/SnO2-TiO2 nanoribbons were transformed into SnO2-TiO2 nanofibers after calcination under the similar conditions. Fiber morphology and elemental mapping confirmed through SEM and TEM microscope techniques respectively. The X-ray diffraction measurements suggested the presence of anatase TiO2 and rutile SnO2 and both were present in the SnO2-TiO2 mixed system. Systematic photoluminescence studies were performed on the electrospun nanostructures at different excitation wavelengths (λex1 = 325, λex2 = 330, λex3 = 350, λex4 = 397 and λex5 = 540 nm). We emphasize that the defects in the SnO2-TiO2 based on the defect levels present in TiO2 and SnO2 and anticipate that these defect levels may have great potential in understanding and characterizing various semiconducting nanostructures.
Open Access
An experiment to observe the impact of UML diagrams on the effectiveness of software requirements inspections
(IEEE, 2009) Albayrak, Özlem
Software inspections aim to find defects early in the development process and studies have found them to be effective. However, there is almost no data available regarding the impact of UML diagram utilization in software requirements specification documents on inspection effectiveness. This paper addresses this issue by investigating whether inclusion of UML diagrams impacts the effectiveness of requirements inspection. We conducted an experiment in an academic environment with 35 subjects to empirically investigate the impact of UML diagram inclusion on requirements inspections' effectiveness and the number of reported defects. The results show that including UML diagrams in requirements specification document significantly impacts the number of reported defects, and there is no significant impact on the effectiveness of individual i nspections. © 2009 IEEE.
Open Access
Functional carbon and silicon monolayers in biphenylene network
(American Chemical Society, 2022-06-28) Gorkan, T.; Çallıoǧlu, Şafak; Demirci, S.; Aktürk, E.; Ciraci, S.
We investigated the effects of vacancy, void, substitutional impurity, isolated adsorption of selected adatoms, and their patterned coverage on the physical and chemical properties of metallic carbon and silicon monolayers in a biphenylene network. These monolayers can acquire diverse electronic and magnetic properties to become more functional depending on the repeating symmetry, size of the point defects, and on the type of adsorbed adatoms. While a carbon monovacancy attains a local magnetic moment, its void can display closed edge states with interesting physical effects. Adsorbed light-transition or rare-earth metal atoms attribute magnetism to these monolayers. The opening of a gap in the metallic density of states, which depends on the pattern and density of adsorbed hydrogen, oxygen, and carbon adatoms, can be used as the band gap engineering of these two-dimensional materials. The energy barriers against the passage of oxygen atoms through the centers of hexagon and octagon rings are investigated, and the coating of the active surfaces with carbon monolayers is exploited as a means of protection against oxidation. We showed that the repulsive forces exerting even at distant separations between two parallel, hydrogenated carbon monolayers in a biphenylene network can lead to the superlow friction features in their sliding motion. All these results obtained from the calculations using the density functional theory herald critical applications.
Open Access
A glycosaminoglycan mimetic peptide nanofiber gel as an osteoinductive scaffold
(Royal Society of Chemistry, 2016) Tansik, G.; Kilic, E.; Beter, M.; Demiralp, B.; K.Sendur, G.; Can, N.; Ozkan, H.; Ergul, E.; Güler, Mustafa O.; Tekinay, A. B.
Biomineralization of the extracellular matrix (ECM) plays a crucial role in bone formation. Functional and structural biomimetic native bone ECM components can therefore be used to change the fate of stem cells and induce bone regeneration and mineralization. Glycosaminoglycan (GAG) mimetic peptide nanofibers can interact with several growth factors. These nanostructures are capable of enhancing the osteogenic activity and mineral deposition of osteoblastic cells, which is indicative of their potential application in bone tissue regeneration. In this study, we investigated the potential of GAG-mimetic peptide nanofibers to promote the osteogenic differentiation of rat mesenchymal stem cells (rMSCs) in vitro and enhance the bone regeneration and biomineralization process in vivo in a rabbit tibial bone defect model. Alkaline phosphatase (ALP) activity and Alizarin red staining results suggested that osteogenic differentiation is enhanced when rMSCs are cultured on GAG-mimetic peptide nanofibers. Moreover, osteogenic marker genes were shown to be upregulated in the presence of the peptide nanofiber system. Histological and micro-computed tomography (Micro-CT) observations of regenerated bone defects in rabbit tibia bone also suggested that the injection of a GAG-mimetic nanofiber gel supports cortical bone deposition by enhancing the secretion of an inorganic mineral matrix. The volume of the repaired cortical bone was higher in GAG-PA gel injected animals. The overall results indicate that GAG-mimetic peptide nanofibers can be utilized effectively as a new bioactive platform for bone regeneration. © 2016 The Royal Society of Chemistry.
Open Access
Impact of maintainability defects on code inspections
(ACM, 2010) Albayrak, Özlem; Davenport, David
Software inspections are effective ways to detect defects early in the development process. In this paper, we analyze the impact of certain defect types on the effectiveness of code inspection. We conducted an experiment in an academic environment with 88 subjects to empirically investigate the effect of two maintainability defects, i.e., indentation and naming conventions, on the number of functional defects found, the effectiveness of functional defect detections, and the number of false positives reported during individual code inspections. Results show that in cases where both naming conventions and indentation defects exist, the participants found minimum number of defects and reported the highest number of false positives, as compared to the cases where either indentation or naming defects exist. Among maintainability defects, indentation seems to significantly impact the number of functional defects found by the inspector, while the presence of naming conventions defects seems to have no significant impact on the number of functional defects detected. The presence of maintainability defects significantly impacts the number of false positives reported. On the effectiveness of individual code inspectors we observed no significant impact originated from the presence of indentation or naming convention defects. © 2010 ACM.
Open Access
Materials for articular cartilage regeneration
(Bentham Science Publishers B.V., 2012) Tombuloglu, Ayşegül; Tekinay, Ayşe B.; Güler, Mustafa O.
Many health problems remaining to be untreatable throughout the human history can be overcome by utilizing new biomedical materials. Healing cartilage defects is one of the problems causing significant health issue due to low regeneration capacity of the cartilage tissue. Scaffolds as three-dimensional functional networks provide promising tools for complete regeneration of the cartilage tissue. Diversity of materials and fabrication methods give rise to many forms of scaffolds including injectable and mechanically stable ones. Various approaches can be considered depending on the condition of cartilage defect. A scaffold should maintain tissue function within a short time, and should be easily applied in order to minimally harm the body. This review will cover several patents and other publications on materials for cartilage regeneration with an outlook on essential characteristics of materials and scaffolds.
Open Access
The mechanical characterization of two-dimensional materials (WS2, MoS2, and graphene) and the effect of defects on young`s modulus of CVD grown single-layer graphene
(2017-11) Adilbekova, Begimai
With the first isolation of graphene two-dimensional (2D) materials attracted the enormous interest of many researchers. Owing to extraordinary properties and atomic thickness 2D materials have many applications in gas detection, electrodes, energy storage devices, field effect devices, sensors, photodetectors, solar cells, nanocomposites, actuators/ resonators, biological membranes,cancer detectors, piezoresistive pressure sensors, gas impermeable membranes, gas or liquid separation. Despite the atomic thickness, the gapless character of the graphene limits its applications in modern electronic devices. There are two strategies for solving the problem, first one is to open the band-gap in graphene and second is to explore new 2D materials. Generation of defects and applying an electrical field can increase the band-gap of graphene but defects can affect other properties of it. Therefore, there is a need for new analogs and transition metal dichalcogenides (TMDs) are the most promising ones. TMDs drew the attention of many researchers because of the remarkable electronic and optical properties. TMD materials exhibit a semiconducting nature owing to the presence of the band-gap, which is essential for the logical operations. Besides electronic properties, the mechanical properties( Young`s Modulus) play a significant role in applications of 2D materials. 2D materials are most promising candidates for flexible electronic devices, which received enormous interest in recent years. But the applied strain and other external forces can modify the structure of crystalline graphene and TMDs, consequently affect the performance and lifetime of devices. In this work, we aimed to measure the Young`s Modulus of graphene, MoS2 film, MoS2 and WS2 flakes with an Atomic Force Microscope (AFM). In addition, we sought the relation between the defect intensity and Young`s Modulus of graphene. The defects in graphene were generated by Ga+ with different doses in Focused Ion Beam (FIB). We found Young`s Moduli of graphene, MoS2 film, MoS2 and WS2 flakes to be 270 N/m, 330 N/m, 90 N/m and 140 N/m, respectively. These values are lower than those given in a literature, what might be caused by the pre-existence of unwanted defects. Also, it appeared that the introduction of defects leads to the fall in Young`s Modulus values of the graphene.
Open Access
Modeling the effect of subsurface interface defects on contact stiffness for ultrasonic atomic force microscopy
(AIP Publishing LLC, 2004) Sarioglu, A. F.; Atalar, Abdullah; Degertekin, F. L.
We present a model predicting the effects of mechanical defects at layer interfaces on the contact stiffness measured by ultrasonic atomic force microscopy sAFMd. Defects at subsurface interfaces result in changes at the local contact stiffness between the AFM tip and the sample. Surface impedance method is employed to model the imperfections and an iterative algorithm is used to calculate the AFM tip-surface contact stiffness. The sensitivity of AFM to voids or delaminations and disbonds is investigated for film-substrate combinations commonly used in microelectronic structures, and optimum defect depth for maximum sensitivity is defined. The effect of contact force and the tip properties on the defect sensitivity are considered. The results indicate that the ultrasonic AFM should be suitable for subsurface detection and its defect sensitivity can be enhanced by adjusting the applied force as well as by judicious choice of the AFM tip material and geometry.
Open Access
Optical bistability in one-dimensional doped photonic crystals with spontaneously generated coherence
(2013) Aas, S.; Müstecaplioǧlu O.E.
We investigate optical bistability in a multilayer one-dimensional photonic crystal where the central layer is doped with Λ-type three-level atoms. We take into account the influence of spontaneously generated coherence when the lower atomic levels are sufficiently close to each other, in which case Kerr-type nonlinear response of the atoms is enhanced. We calculate the propagation of a probe beam in the defect mode window using the numerical nonlinear transfer matrix method. We find that Rabi frequency of a control field acting on the defect layer and the detuning of the probe field from the atomic resonance can be used to control the size and contrast of the hysteresis loop and the threshold of the optical bistability. In particular we find that at the optimal spontaneously generated coherence, a three orders of magnitude lower threshold can be achieved relative to the case without the coherence. © 2013 American Physical Society.
Open Access
Photonic band gap effect and localization in two-dimensional Penrose lattice
(IEEE, 2001) Bayındır, Mehmet; Çubukçu, Ertuğrul; Bulu, İrfan; Özbay, Ekmel
A study of photonic bandgap effect and localization in two-dimensional Penrose lattice was performed. Penrose quasicrystals consisted of dielectric rods and defect characteristics of various inequivalent sites of the crystal were investigated. It was observed that the defect characteristics of quasi periodic photonic crystals were different from the periodic case. Localization properties of the defect modes in quasicrystals depended on the position of the removed rod and different defect frequencies could be obtained by removal of rods from various positions.
Open Access
Role of zinc interstitials and oxygen vacancies of ZnO in photocatalysis: a bottom-up approach to control the defect density
(Royal Society of Chemistry, 2014-06-09) Kayaci, F.; Vempati S.; Donmez, I.; Bıyıklı, Necmi; Uyar, Tamer
Oxygen vacancies (VOs) in ZnO are well-known to enhance photocatalytic activity (PCA) despite various other intrinsic crystal defects. In this study, we aim to elucidate the effect of zinc interstitials (Zn i) and VOs on PCA, which has applied as well as fundamental interest. To achieve this, the major hurdle of fabricating ZnO with controlled defect density requires to be overcome, where it is acknowledged that defect level control in ZnO is significantly difficult. In the present context, we fabricated nanostructures and thoroughly characterized their morphological (SEM, TEM), structural (XRD, TEM), chemical (XPS) and optical (photoluminescence, PL) properties. To fabricate the nanostructures, we adopted atomic layer deposition (ALD), which is a powerful bottom-up approach. However, to control defects, we chose polysulfone electrospun nanofibers as a substrate on which the non-uniform adsorption of ALD precursors is inevitable because of the differences in the hydrophilic nature of the functional groups. For the first 100 cycles, Znis were predominant in ZnO quantum dots (QDs), while the presence of VOs was negligible. As the ALD cycle number increased, VOs were introduced, whereas the density of Zni remained unchanged. We employed PL spectra to identify and quantify the density of each defect for all the samples. PCA was performed on all the samples, and the percent change in the decay constant for each sample was juxtaposed with the relative densities of Znis and VOs. A logical comparison of the relative defect densities of Znis and VOs suggested that the former are less efficient than the latter because of the differences in the intrinsic nature and the physical accessibility of the defects. Other reasons for the efficiency differences were elaborated.
Open Access
Roma hukukunda alım-satım sözleşmesinde maldaki maddi ayıplar nedeniyle satıcının sorumluluğu
(İnönü Üniversitesi Hukuk Fakültesi, 2011) Emiroğlu, Haluk
Roma hukukunda alım-satım sözleşmesine konu olan malın ayıplı olması durumunda satıcının sorumlu tutulması ilk kez ius civile tarafından düzenlenmiştir. Söz konusu düzenlemeye göre, satıcının ayıplardan sorumlu tutulabilmesi için, alım-satım konusu malın mülkiyetinin devren kazanılması için kullanılan mancipatio ya da in iure cessio sırasında özel bir teminat verilmesi gerekirdi. Söz konusu teminatın mülkiyetin devir işlemleri sırasında verilmemesi ya da hiç verilmemesi durumunda satıcının sorumluluğu yoluna gidilemezdi. Bu düzenleme, satıcının stipulatio ile ayıplardan sorumlu olacağına ilişkin taahhüdünün geçerli olarak kabul edilmesiyle geliştirilmiş ve actio redhibitoria’nın kabul edilmesiyle son halini almıştır. Klasik hukuk dönemine rastlayan ve Aedilis Curulis tarafından yürürlüğe konan bu düzenlemeye göre, alıcı, alım-satım konusu köle ya da hayvanın ayıplı çıkması durumunda edimlerin iadesini 6 ay içinde actio redhibitoria ile talep edebileceği gibi, 1 yıl içinde alım-satım bedelinin indirilmesini de dava edebilirdi. Iustinianus döneminde ise, actio redhibitoria’nın, sadece köle ve hayvanlar için değil, alım-satım konusu olan tüm mallar için kullanilabileceğinin düzenlendiği görülmektedir.
Open Access
Seed layer assisted hydrothermal deposition of low-resistivity ZnO thin films
(Materials Research Society, 2017) Chubenko, E.; Bondarenko, V.; Ghobadi, Amir; Ulusoy, Gamze; Topallı, Kağan; Okyay, Ali Kemal
In this work, we describe the combination of hydrothermal and atomic layer deposition (ALD) for growing low-resistivity ZnO polycrystalline continuous films. The effect of the thickness of ALD seed layers on the morphology of the hydrothermal ZnO films was studied. It was shown that ZnO films hydrothermally deposited on very thin seed layer consist of separate nanorods but in the case of 20 nm seed layer ZnO films transform to uniform continuous layers comprising of closely packed vertically aligned crystallites. Photoluminescence spectra were shown to exhibit broad band behavior in the visible range, corresponding to radiative recombination processes via oxygen defects of ZnO crystalline lattice, and narrow band in the UV region, associated with band-to-band recombination processes. It was shown that the resistivity of the obtained ZnO films is decreased gradually with the increase of ZnO films thickness and determined by the presence of crystal lattice defects in the seed layer.