Browsing by Author "Biyikli, Necmi"

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Open Access
A performance-enhanced planar Schottky diode for Terahertz applications: an electromagnetic modeling approach
(Cambridge University Press, 2017) Ghobadi, Amir; Khan, Talha Masood; Celik, Ozan Onur; Biyikli, Necmi; Okyay, Ali Kemal; Topalli, Kagan
In this paper, we present the electromagnetic modeling of a performance-enhanced planar Schottky diode for applications in terahertz (THz) frequencies. We provide a systematic simulation approach for analyzing our Schottky diode based on finite element method and lumped equivalent circuit parameter extraction. Afterward, we use the developed model to investigate the effect of design parameters of the Schottky diode on parasitic capacitive and resistive elements. Based on this model, device design has been improved by deep-trench formation in the substrate and using a closed-loop junction to reduce the amount of parasitic capacitance and spreading resistance, respectively. The results indicate that cut-off frequency can be improved from 4.1 to 14.1 THz. Finally, a scaled version of the diode is designed, fabricated, and well characterized to verify the validity of this modeling approach.
Open Access
Size-controlled conformal nanofabrication of biotemplated three-dimensional TiO2 and ZnO nanonetworks
(Nature, 2013) Ceylan, Hakan; Ozgit-Akgun, Çağla; Erkal, Turan S.; Donmez, İnci; Garifullin, Ruslan; Tekinay, Ayse B.; Usta H.; Biyikli, Necmi; Güler, Mustafa O.
A solvent-free fabrication of TiO2 and ZnO nanonetworks is demonstrated by using supramolecular nanotemplates with high coating conformity, uniformity, and atomic scale size control. Deposition of TiO2 and ZnO on three-dimensional nanofibrous network template is accomplished. Ultrafine control over nanotube diameter allows robust and systematic evaluation of the electrochemical properties of TiO2 and ZnO nanonetworks in terms of size-function relationship. We observe hypsochromic shift in UV absorbance maxima correlated with decrease in wall thickness of the nanotubes. Photocatalytic activities of anatase TiO2 and hexagonal wurtzite ZnO nanonetworks are found to be dependent on both the wall thickness and total surface area per unit of mass. Wall thickness has effect on photoexcitation properties of both TiO2 and ZnO due to band gap energies and total surface area per unit of mass. The present work is a successful example that concentrates on nanofabrication of intact three-dimensional semiconductor nanonetworks with controlled band gap energies.
Open Access
ZnO nanostructures on electrospun nanofibers by atomic layer deposition/hydrothermal growth and their photocatalytic activity
(Materials Research Society, 2014) Kayaci, Fatma; Vempati, Sesha; Ozgit-Akgun, Cagla; Biyikli, Necmi; Uyar, Tamer
A hierarchy of nanostructured-ZnO was fabricated on the electrospun nanofibers by atomic layer deposition (ALD) and hydrothermal growth, subsequently. Firstly, we produced poly(acrylonitrile) (PAN) nanofibers via electrospinning, then ALD process provided a highly uniform and conformai coating of polycrystalline ZnO with a precise control on the thickness (50 nm). In the last step, this ZnO coating depicting dominant oxygen vacancies and significant grain boundaries was used as a seed on which single crystalline ZnO nanoneedles (average diameter and length of ∼25 nm and ∼600 nm, respectively) with high optical quality were hydrothermally grown. The detailed morphological and structural studies were performed on the resulting nanofibers, and the photocatalytic activity (PCA) was tested with reference to the degradation of methylene blue. The results of PCA were discussed in conjunction with photoluminescence response. The nanoneedle structures supported the vectorial transport of photo-charge carriers, which is crucial for high catalytic activity. The enhanced PCA, structural stability and reusability of the PAN/ZnO nanoneedles indicated that this hierarchical structure is a potential candidate for waste water treatment.