Browsing by Author "Strupinski, W."
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Item Open Access AlGaN quadruple-band photodetectors(IEEE, 2009) Gökkavas, Mutlu; Bütün, Serkan; Caban, P.; Strupinski, W.; Özbay, EkmelQuadruple back-illuminated ultraviolet metal-semiconductor-metal photodetectors with four different spectral responsivity bands were demonstrated. The average of the full-width at half-maximum (FWHM) of the quantum efficiency peaks was 9.98 nm.Item 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, EkmelThe 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.Item Open Access The influence of substrate surface preparation on LP MOVPE GaN epitaxy on differently oriented 4H-SiC substrates(Elsevier BV * North-Holland, 2008) Caban, P.; Kosciewicz, K.; Strupinski, W.; Wojcik, M.; Gaca, J.; Szmidt, J.; Ozturk, M.; Özbay, EkmelThe influence of surface preparation and off-cut of 4H-SiC substrates on morphological and structural properties of GaN grown by low-pressure metalorganic vapor phase epitaxy was studied. Substrate etching has an impact on the surface roughness of epilayers and improves its crystal quality. The GaN layers were characterized by atomic force microscopy (AFM) and high-resolution X-ray diffractometry (HRXRD) measurements. It was observed that on-axis 4H-SiC is most suitable for GaN epitaxy and that substrate etching improves the surface morphology of epilayer.Item Open Access Integrated AlGaN quadruple-band ultraviolet photodetectors(IOP Publishing, 2012-04-27) Gökkavas, M.; Butun, S.; Caban, P.; Strupinski, W.; Özbay, EkmelMonolithically integrated quadruple back-illuminated ultraviolet metalsemiconductormetal photodetectors with four different spectral responsivity bands were demonstrated on each of two different Al xGa 1-xN heterostructures. The average of the full-width at half-maximum (FWHM) of the quantum efficiency peaks was 18.15nm for sample A, which incorporated five 1000nm thick epitaxial layers. In comparison, the average FWHM for sample B was 9.98 nm, which incorporated nine 500nm thick epitaxial layers.Item Open Access Mg-doped AlGaN grown on an AlN/sapphire template by metalorganic chemical vapour deposition(2006) Yu, H.; Strupinski, W.; Butun, S.; Özbay, EkmelThe growth of high-performance Mg-doped p-type Al xGa 1-xN (x = 0.35) layers using low-pressure metal-organic chemical vapour deposition on an AlN/sapphire template is reported. The influence of growth conditions on the p-type conductivity of the Al xGa 1-xN (x = 0.35) alloy was investigated. It was found that the p-type resistivity of the AlGaN alloy demonstrates a marked dependence on the Mg concentration, V/III ratio and group III element flow rate. A minimum p-type resistivity of 3.5 Ω cm for Al xGa 1-xN (x = 0.35) epilayers was achieved. A Ni/Au (10 nm/100 nm) ohmic contact was also fabricated and a specific contact resistivity of 8.1 × 10 -2 Ω cm 2 was measured. © 2006 WILEY-VCH Verlag GmbH & Co. KGaA.Item Open Access Resonance broadening and tuning of split ring resonators by top-gated epitaxial graphene on SiC substrate(AIP Publishing LLC, 2013) Cakmakyapan, S.; Sahin, L.; Pierini, F.; Strupinski, W.; Özbay, EkmelSplit ring resonators (SRRs) are subwavelength structures that are able to localize and enhance the electromagnetic wave. Controlling the plasmonic resonance behavior of metallic nanostructures, such as SRRs, plays an important role in optoelectronics and nanophotonics applications. Electrically tunable carrier concentration of graphene provides hybrid devices, where the plasmonic structures and graphene are combined. In this paper, we report the design, fabrication, and measurement of a device comprising a SRR array on epitaxial graphene. We obtained resonance broadening and tuning of split ring resonators by utilizing an epitaxial graphene transistor with transparent top-gate.Item Open Access Step-edge-induced resistance anisotropy in quasi-free-standing bilayer chemical vapor deposition graphene on SiC(AIP Publishing, 2014) Ciuk, T.; Cakmakyapan, S.; Özbay, Ekmel; Caban, P.; Grodecki, K.; Krajewska, A.; Pasternak, I.; Szmidt, J.; Strupinski, W.The transport properties of quasi-free-standing (QFS) bilayer graphene on SiC depend on a range of scattering mechanisms. Most of them are isotropic in nature. However, the SiC substrate morphology marked by a distinctive pattern of the terraces gives rise to an anisotropy in graphene's sheet resistance, which may be considered an additional scattering mechanism. At a technological level, the growth-preceding in situ etching of the SiC surface promotes step bunching which results in macro steps similar to 10 nm in height. In this report, we study the qualitative and quantitative effects of SiC steps edges on the resistance of epitaxial graphene grown by chemical vapor deposition. We experimentally determine the value of step edge resistivity in hydrogen-intercalated QFS-bilayer graphene to be similar to 190 Omega mu m for step height h(S) = 10 nm and provide proof that it cannot originate from mechanical deformation of graphene but is likely to arise from lowered carrier concentration in the step area. Our results are confronted with the previously reported values of the step edge resistivity in monolayer graphene over SiC atomic steps. In our analysis, we focus on large-scale, statistical properties to foster the scalable technology of industrial graphene for electronics and sensor applications. (C) 2014 AIP Publishing LLC.Item Open Access Voltage contrast X-ray photoelectron spectroscopy reveals graphene-substrate interaction in graphene devices fabricated on the C-and Si-faces of SiC(American Institute of Physics Inc., 2015) Aydogan, P.; Arslan, E.; Cakmakyapan, S.; Özbay, Ekmel; Strupinski, W.; Süzer, ŞefikWe report on an X-ray photoelectron spectroscopy (XPS) study of two graphene based devices that were analyzed by imposing a significant current under +3 V bias. The devices were fabricated as graphene layers(s) on hexagonal SiC substrates, either on the C- or Si-terminated faces. Position dependent potential distributions (IR-drop), as measured by variations in the binding energy of a C1s peak are observed to be sporadic for the C-face graphene sample, but very smooth for the Si-face one, although the latter is less conductive. We attribute these sporadic variations in the C-face device to the incomplete electrical decoupling between the graphene layer(s) with the underlying buffer and/or substrate layers. Variations in the Si2p and O1s peaks of the underlayer(s) shed further light into the electrical interaction between graphene and other layers. Since the potential variations are amplified only under applied bias (voltage-contrast), our methodology gives unique, chemically specific electrical information that is difficult to obtain by other techniques.