Browsing by Author "Acar, S."

Now showing 1 - 12 of 12

Open Access
Determination of two-dimensional electron and hole gas carriers in AlGaN/GaN/AlN heterostructures grown by Metal Organic Chemical Vapor Deposition
(Elsevier BV, 2008-02-29) Acar, S.; Lisesivdin, S. B.; Kasap, M.; Özcelik, S.; Özbay, Ekmel
Resistivity and Hall effect measurements on nominally undoped Al0.25Ga0.75N/GaN/AlN heterostructures grown on sapphire substrates prepared by metal organic chemical vapor deposition have been carried out as a function of temperature (20-300 K) and magnetic field (0-1.4 T). Variable magnetic field Hall data have been analyzed using the improved quantitative mobility spectrum analysis technique. The mobility and density of the two-dimensional electron gas at the AlGaN/GaN interface and the two-dimensional hole gas at the GaN/AIN interface are separated by quantitative mobility spectrum analysis. The analysis shows that two-channel conduction is present in nominally undoped Al0.25Ga0.75N/GaN/AlN heterostructures grown on sapphire substrate.
Open Access
The effect of strain relaxation on electron transport in undoped Al0.25Ga0.75N/GaN heterostructures
(Elsevier BV * North-Holland, 2007-11-01) Lişesivdin, S. B.; Yıldız, A.; Acar, S.; Kasap, M.; Özçelik, S.; Özbay, Ekmel
The two-dimensional electron gas (2DEG) transport properties of two-step growth undoped Al0.25Ga0.75N/GaN heterostructures with semi-insulating buffer, grown by MOCVD, were investigated in a temperature range of 20–350 K. Using the quantitative mobility spectrum analysis (QMSA) method, it was shown that significant parallel conduction does not occur in worked structures. In-plain growth axis strains are calculated using the total polarization-induced charge density taken as the sheet carrier density measured from the Hall effect. It was found that the calculated strain values are in good agreement with those reported. Influences of the two-step growth parameters such as growth ramp time, the annealing temperature of the GaN nucleation layer on the mobility, and density of the 2DEG are also discussed.
Open Access
Electronic transport characterization of AlGaN∕GaN heterostructures using quantitative mobility spectrum analysis
(AIP Publishing LLC, 2007-09-06) Lisesivdin, S. B.; Yildiz, A.; Acar, S.; Kasap, M.; Ozcelik, S.; Özbay, Ekmel
Resistivity and Hall effect measurements in nominally undoped Al0.25Ga0.75N/GaN heterostructures grown on sapphire substrate by metal-organic chemical vapor deposition are carried out as a function of temperature (20-350 K) and magnetic field (0-1.5 T). The measurement results are analyzed using the quantitative mobility spectrum analysis techniques. It is found that there is strong two-dimensional electron gas localization below 100 K, while the thermally activated minority carriers with the activation energies of similar to 58 and similar to 218 meV contribute to the electron transport at high temperatures.
Open Access
Energy and environmental policy against climate change in Turkey
(Academic Press, 2018) Acar, S.; Voyvoda, E.; Yeldan, Alp Erinç
Based on the structure of the economy in general but especially focusing on the duality patterns laid out in Chapter 2, this chapter first examines the recent energy and environment statistics in Turkey, and second, evaluates the existing (energy and environmental) policy framework and future potential of these policies in terms of their implications for climate change. We argue that the lack of mitigation at the aggregate national level finds its manifestation in the widening gap across regional GHG emissions as a consequence of the dual economic structure and the lack of differential policies across the regions.
Open Access
Environmental impacts of coal subsidies in Turkey: a general equilibrium analysis
(Elsevier, 2016) Acar, S.; Yeldan, Alp Erinç
In this study we aim at providing an analytical framework for Turkey to study the macroeconomics and environmental impacts of the existing coal subsidization scheme. To this end we develop a regionally differentiated applied general equilibrium model spanning over 2015-2030. Our analytical apparatus focuses exclusively on the fiscal implications as well as the environmental repercussions of the removal of the subsidies on greenhouse gas emissions. With the aid of a set of alternative policy scenarios against a "business as usual" path, we study the regional and sectorial performances of growth, employment, investment and capital accumulation, consumption/welfare and trade balance. Our results indicate that by simple elimination of the coal subsidization scheme, Turkey can reduce its aggregate gaseous emissions by as much as 5% without a significant loss in its GDP.
Open Access
Introduction
(Academic Press, 2018) Acar, S.; Voyvoda, E.; Yeldan, Alp Erinç
This chapter serves as an introduction to the main research questions, background, and objectives of the study. It describes the context (i.e., conceptualization of duality) and the modeling tools for decarbonized sustainable growth pathways as a way of pursuing a regionally-equitable development strategy.
Open Access
Investigating patterns of carbon convergence in an uneven economy: the case of Turkey
(Elsevier B.V., 2018) Acar, S.; Yeldan, A. Erinç
Turkey is known to suffer from severe volatility in its growth patterns, as well as from the uneven sectoral growth and employment. Volatile rates of emissions across sectors are further manifestations of this uneven structure. The purpose of this study is two-fold: first, we check for dynamic patterns of convergence of carbon dioxide (CO2) emissions across sectors; and second, using evidence from panel data econometrics, we search for the determinants of these processes utilizing macroeconomic explanatory variables. We find that, based on various alternate criteria, CO2 emissions display conditional convergence mainly driven by the business cycle. Furthermore, across sectors, high technology activities display convergence over time; and yet, medium technology sectors that constitute the bulk of the aggregate value added display either poorly convergent or divergent trends. These results reveal that much of the emissions convergence is driven by the business cycle rather than the workings of discretionary mitigation policy.
Open Access
Modeling for green growth: environmental policy in a dualistic peripheral economy
(Academic Press, 2018) Acar, S.; Voyvoda, E.; Yeldan, Alp Erinç
The chapter introduces the main components of the applied general equilibrium model to represent the dual structure of Turkey. It discusses the distinguishing features and its contribution to the modeling literature with its unique emphasis on duality, regionalization and social relevance. It further conceptualizes a Social Accounting Matrix for the Regionally Fragmented Dual Economy to accommodate Turkish macro-level data. Given the theoretical structure of the CGE model, the main data sources of the modeling paradigm are introduced and tabulated within the discipline of Walrasian general equilibrium. The unique contribution of the chapter is its accommodation of regional differentiation and of dualistic labor markets and preparation for the database in social accounting matrix format. This chapter purports to extend the traditional neoclassical (Walrasian) growth modeling based on one-sector depictions of the aggregate economy with a balanced growth path notion towards the steady state, and as such, we aim to contribute to the empirics of the traditional growth paradigm via questioning the long run equilibrium path way towards a balanced steady state.
Unknown
Patterns of growth in dual economies: challenges of development in the 21st century
(Academic Press, 2018) Acar, S.; Voyvoda, E.; Yeldan, Alp Erinç
This chapter attempts to examine patterns of growth in dual economies within the context of challenges of development for the 21st century. To this end, we first study the broad contours of growth and adjustments in the global economy, before, during, and after the 2008–09 global crisis. Here four general patterns emerge, namely, stagnation of wage incomes; declining investment effort; consequent decline in the rates of growth of productivity; and a compensatory rise in corporate and household indebtedness. The chapter continues with a deeper investigation of the structural sources of deindustrialization and widening duality in both labor markets and technological diffusion. Finally, we link all these ideas with the macroeconomics of global climate change and implications on the feedback effects of the economy-resource use-environmental degradation nexus.
Open Access
Policy analysis: toward a green development pathway for the peripheral world
(Academic Press, 2018) Acar, S.; Voyvoda, E.; Yeldan, Alp Erinç
This chapter uses the analytical model described in Chapter 4 to quantitatively investigate the set of viable macroeconomic policy instruments for combating climate change, and addresses a set of duality characteristics of the Turkish economy. The chapter initially characterizes the business-as-usual pathway over the first half of the 21st century (until 2040), and then studies a strategic policy scenario to attain multiple objectives of reduced informalization, more equitable regional social welfare, and environmental abatement. The results indicate that by pursuing a coherent macroeconomic strategy toward taxing fossil fuel use and earmarking fiscal revenues for expansion of the renewables sector strategically located within a regional development program, Turkey can mitigate gaseous emissions and expand its income and employment within a more equitable and sustained development pathway.
Embargo
Practical SERS substrates by spray coating of silver solutions for deep learning-assisted sensitive antigen identification
(Elsevier BV, 2025-02-20) Şahin, F.; Demirel Şahin, G.; Çamdal, A.; Akmayan, İ.; Özbek, T.; Acar, S.; Önses, Mustafa Serdar
Surface-enhanced Raman spectroscopy (SERS) has long been recognized for its rapid and sensitive detection capabilities; however, challenges persist in practical fabrication of the substrates and interpreting complex data. Herein, we propose a deep learning (DL) assisted SERS approach to enable rapid and sensitive detection of analytes on practical yet highly effective substrates prepared by direct spray-coating of a nanoparticle-free true solution of a reactive Ag ink and on-site thermal annealing mediated generation of nanostructures. This design ensured homogeneous distribution of Ag nanostructures throughout the entire substrate, significantly increasing the number of hotspots and enhancing the Raman signals, thereby achieving an impressive analytical enhancement factor of ~1010 in a reproducible and consistent manner. The diagnostic utility of this platform was demonstrated by detecting the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) spike (S) protein in both buffer and saliva, with detection limits of 74.3 pg/mL and 7.43 ng/mL, respectively. The DL-assisted SERS not only accurately identified the presence or absence of viral antigen, but also automatically quantified the viral load. This automatic identification achieved an outstanding accuracy of ~99.9 %, highlighting the exceptional performance of the proposed platform. This simple, cost-effective, scalable, and ultra-sensitive DLassisted SERS platform offers significant opportunities for early and precise detection in a range of analytical scenarios
Open Access
Scattering analysis of 2DEG carrier extracted by QMSA in undoped Al0.25Ga0.75N/GaN heterostructures
(Institute of Physics Publishing Ltd., 2007) Lisesivdin, S. B.; Acar, S.; Kasap, M.; Ozcelik, S.; Gokden, S.; Özbay, Ekmel
Hall effect measurements on undoped Al0.25Ga0.75N/GaN heterostructures grown by a metalorganic chemical vapour deposition (MOCVD) technique have been carried out as a function of temperature (20-350 K) and magnetic field (0-1.5 T). Magnetic field dependent Hall data were analysed using the quantitative mobility spectrum analysis (QMSA) technique. The mobility and density within the two-dimensional electron gas (2DEG) at the Al 0.25Ga0.75N/GaN interface and within the underlying GaN layer were successfully separated by QMSA. Mobility analysis has been carried out using both the measured Hall data at a single field and the extracted data from QMSA. Analysis of the temperature-dependent mobility of 2DEG extracted from QMSA indicates that the interface roughness and alloy disorder scattering mechanisms are the dominant scattering mechanisms at low temperatures while at high temperatures only polar optical phonon scattering is the dominant mechanism. Al0.25Ga0.75N/GaN interface related parameters such as well width, deformation potential constant and correlation length were also accurately obtained from the fits of the simple analytical expressions of scattering mechanisms to the 2DEG mobility.