Browsing by Author "Turhan, S."

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Open Access
The effect of GaN thickness inserted between two AlN layers on the transport properties of a lattice matched AlInN/AlN/GaN/AlN/GaN double channel heterostructure
(Elsevier BV, 2014-01-31) Tülek, R.; Arslan, E.; Bayraklı, A.; Turhan, S.; Gökden, S.; Duygulu, Ö.; Kaya, A.; Fırat, T.; Teke, A.; Özbay, Ekmel
One AlInN/AlN/GaN single channel heterostructure sample and four AlInN/AlN/GaN/AlN/GaN double channel heterostructure samples with different values of the second GaN layer were studied. The interface profiles, crystalline qualities, surface morphologies, and dislocation densities of the samples were investigated using high resolution transmission electron microscopy, atomic force microscopy, and high-resolution X-ray diffraction. Some of the data provided by these measurements were used as input parameters in the calculation of the scattering mechanisms that govern the transport properties of the studied samples. Experimental transport data were obtained using temperature dependent Hall effect measurements (10-300 K) at low (0.5 T) and high (8 T) magnetic fields to exclude the bulk transport from the two-dimensional one. The effect of the thickness of the second GaN layer inserted between two AlN barrier layers on mobility and carrier concentrations was analyzed and the dominant scattering mechanisms in the low and high temperature regimes were determined. It was found that Hall mobility increases as the thickness of GaN increases until 5 nm at a low temperature where interface roughness scattering is observed as one of the dominant scattering mechanisms. When GaN thicknesses exceed 5 nm, Hall mobility tends to decrease again due to the population of the second channel in which the interface becomes worse compared to the other one. From these analyses, 5 nm GaN layer thicknesses were found to be the optimum thicknesses required for high electron mobility. (C) 2013 Published by Elsevier B.V.
Open Access
Fair allocation of personal protective equipment to health centers during early phases of a pandemic
(Elsevier, 2022-05) Dönmez, Zehranaz; Turhan, S.; Karsu, Özlem; Kara, Bahar Y.; Karaşan, Oya
We consider the problem of allocating personal protective equipment, namely surgical and respiratory masks, to health centers under extremely limited supply. We formulate a multi-objective multi-period non-linear resource allocation model for this problem with the objectives of minimizing the number of infected health workers, the number of infected patients and minimizing a deprivation cost function defined over shortages. We solve the resulting problem using the ε-constraint algorithm so as to obtain the exact Pareto set. We also develop a customized genetic algorithm to obtain an approximate Pareto frontier in reasonable time for larger instances. We provide a comparative analysis of the exact and heuristic methods under various scenarios and give insights on how the suggested allocations outperform the ones obtained through a set of rule-of-thumb policies, policies that are implemented owing to their simplicity and ease-of-implementation. Our comparative analysis shows that as the circumstances get worse, the trade-off between the deprivation cost and the ratio of infections deepens and that the proposed heuristic algorithm gives very close solutions to the exact Pareto frontier, especially under pessimistic scenarios. We also observed that while some rule-of-thumb policies such as a last-in-first-receives type policy work well in terms of deprivation costs in optimistic scenarios, others like split policies perform well in terms of number of infections under neutral or pessimistic settings. While favoring one of the objectives, these policies typically fail to provide good solutions in terms of the other objective; hence if such policies are to be implemented the choice would depend on the problem characteristics and the priorities of the policy makers. Overall, the solutions obtained by the proposed methods imply that more complicated distribution schemes that are not induced by these policies would be needed for best results.