Browsing by Author "Lişesivdin, S. B."

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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 structure of β-Si3N4 crystals with substitutional icosagen group impurities
(National Institute of Optoelectronics, 2017) Kutlu, E.; Narin, P.; Atmaca, G.; Sarıkavak-Lişesivdin, B.; Lişesivdin, S. B.; Özbay, Ekmel
The β-Si3N4 crystals are widely used in industrial and electronics areas. Therefore, β-Si3N4 has drawn the attention of researchers for many years. In this study, effects of icosagen group impurity atoms in the IIIA group on the electronic properties of the β-Si3N4 crystal were analyzed by using the density functional theory. As a result of these analyses, it was determined that the electronic properties of the crystal change significantly. Basic electronic characteristics for pure β-Si3N4 crystal and icosagen group impurity β-Si3N4 crystals, such as band structures, densities of states, binding energies, and formation energies were investigated. We identified that the band gap of the β-Si3N4 crystal was affected significantly by the impurity, and this change was varying linearly in line with the formation energy for the impurity cases. As a result of calculations, the Al-impurity was found to be the lowest-energy impurity state.
Open Access
A first principles investigation of the effect of aluminum, gallium and indium impurities on optical properties of β-Si3N4 structure
(Elsevier GmbH, 2017) Narin, P.; Kutlu, E.; Atmaca, G.; Lişesivdin, S. B.; Özbay, Ekmel
In this study, effects of some impurity atoms included in IIIA group such as Al, Ga, and In on the optical properties of the β-Si3N4 structure have been discussed. The calculations were made using Density Functional Theory (DFT) in 0–15 eV range and local density approximation (LDA) as the exchange-correlation. Using the real and the imaginary parts of the complex dielectric function, the basic optical properties of β-Si3N4 such as dielectric coefficient, refractive index, absorption, reflection coefficients have been investigated. As a result of the calculations, it is determined that optical properties of structure have been significantly changed with doping.
Open Access
Negative differential resistance observation and a new fitting model for electron drift velocity in GaN-based heterostructures
(Institute of Electrical and Electronics Engineers, 2018) Atmaca, G.; Narin, P.; Kutlu, E.; Malin, T. V.; Mansurov, V. G.; Zhuravlev, K. S.; Lişesivdin, S. B.; Özbay, Ekmel
The aim of this paper is an investigation of electric field-dependent drift velocity characteristics for Al0.3Ga0.7N/AlN/GaN heterostructures without and with in situ Si3N4 passivation. The nanosecond-pulsed current-voltage ( {I}-{V} ) measurements were performed using a 20-ns applied pulse. Electron drift velocity depending on the electric field was obtained from the {I}-{V} measurements. These measurements show that a reduction in peak electron velocity from \text {2.01} \times \text {10}^{\text {7}} to \text {1.39} \times \text {10}^{\text {7}} cm/s after in situ Si3N4 passivation. Also, negative differential resistance regime was observed which begins at lower fields with the implementation of in situ Si3N4 passivation. In our samples, the electric field dependence of drift velocity was measured over 400 kV/cm due to smaller sample lengths. Then, a well-known fitting model was fitted to our experimental results. This fitting model was improved in order to provide an adequate description of the field dependence of drift velocity. It gives reasonable agreement with the experimental drift velocity data up to 475 kV/cm of the electric field and could be used in the device simulators.
Open Access
SiC substrate effects on electron transport in the epitaxial graphene layer
(Springer, 2014) Arslan, E.; Çakmakyapan S.; Kazar, Ö.; Bütün, S.; Lişesivdin, S. B.; Cinel, N. A.; Ertaş, G.; Ardalı, Ş.; Tıraş, E.; Jawad-ul-Hassan,; Özbay, Ekmel; Janzén, E.
Hall effect measurements on epitaxial graphene (EG) on SiC substrate have been carried out as a function of temperature. The mobility and concentration of electrons within the two-dimensional electron gas (2DEG) at the EG layers and within the underlying SiC substrate are readily separated and characterized by the simple parallel conduction extraction method (SPCEM). Two electron carriers are identified in the EG/SiC sample: one highmobility carrier (3493 cm2 /Vs at 300 K) and one low-mobility carrier (1115 cm2 /Vs at 300 K). The high mobility carrier can be assigned to the graphene layers. The second carrier has been assigned to the SiC substrate.
Open Access
Strain calculations from hall measurements in undoped Al 0.25Ga0.75N/GaN HEMT structures
(American Institute of Physics, 2007) Lişesivdin, S. B.; Yıldız, A.; Kasap, M.; Özbay, Ekmel
The transport properties of undoped Al0.25Ga0.75N/GaN HEMT structures grown by MOCVD were investigated in a temperature range of 20 K-350 K. With Quantitative Mobility Spectrum Analysis (QMSA) method; it was found that, all conduction in undoped Al0.25Ga0.75N/GaN HEMT structures belong to the two dimensional electron gas (2DEG). With the acception of Hall sheet carrier density is the total polarization induced charge density, strains of 2DEG interfaces were calculated. Calculated strain values are in good agreement with the literature. Effects of the growth parameters of the nucleation layers of samples on the mobility and density of the 2DEG are listed.