Browsing by Author "Butun, S."

Now showing 1 - 11 of 11

Open Access
AlGaN-based high-performance metal-semiconductor-metal photodetectors
(Elsevier BV, 2007-10) Gökkavas, M.; Butun, S.; Tur, T.; Bıyıklı, Necmi; Özbay, Ekmel
Design, structure growth, fabrication, and characterization of high performance AlGaN-based metal–semiconductor–metal (MSM) photodetectors (PD) are reported. By incorporating AlN nucleation and buffer layers, the leakage current density of GaN MSM PD was reduced to 1.96 1010 A/cm2 at a 50 V bias, which is four orders of magnitude lower compared to control devices. A 229 nm cut-off wavelength, a peak responsivity of 0.53 A/W at 222 nm, and seven orders of magnitude visible rejection was obtained from Al0.75Ga0.25N MSM PD. Two-color monolithic AlGaN MSM PD with excellent dark current characteristics were demonstrated, where both detectors reject the other detector spectral band with more than three orders of magnitude. High-speed measurements of Al0.38Ga0.62N MSM PD resulted in fast responses with greater than gigahertz bandwidths, where the fastest devices had a 3-dB bandwidth of 5.4 GHz.
Open Access
E-Beam lithography designed substrates for surface enhanced Raman spectroscopy
(Elsevier BV, 2015-06) Cinel, N. A.; Cakmakyapan, S.; Butun, S.; Ertas, G.; Özbay, Ekmel
Surface Enhanced Raman Spectroscopy (SERS) is a popular method that amplifies weak Raman signals from Raman-active analyte molecules making use of certain specially-prepared metallic surfaces. The main challenge in SERS is to design and fabricate highly repeatable, predictable, and sensitive substrates. There are many fabrication methods that strive to achieve this goal, which are briefly summarized in this paper. The E-beam lithography method is proposed to be superior to the mentioned techniques. In this paper, we review how EBL can be utilized in the preparation of SERS substrates and we discuss the contributions to the field by the Özbay group.
Open Access
Electron beam lithography designed silver nano-disks used as label free nano-biosensors based on localized surface plasmon resonance
(Optical Society of America, 2012-01-20) Cinel, N. A.; Butun, S.; Özbay, Ekmel
We present a label-free, optical nano-biosensor based on the Localized Surface Plasmon Resonance (LSPR) that is observed at the metaldielectric interface of silver nano-disk arrays located periodically on a sapphire substrate by Electron-Beam Lithography (EBL). The nano-disk array was designed by finite-difference and time-domain (FDTD) algorithm-based simulations. Refractive index sensitivity was calculated experimentally as 221-354 nm/RIU for different sized arrays. The sensing mechanism was first tested with a biotin-avidin pair, and then a preliminary trial for sensing heat-killed Escherichia coli (E. coli) O157:H7 bacteria was done. Although the study is at an early stage, the results indicate that such a plasmonic structure can be applied to bio-sensing applications and then extended to the detection of specific bacteria species as a fast and low cost alternative. © 2012 Optical Society of America.
Open Access
'Fairy Chimney'-Shaped Tandem Metamaterials as Double Resonance SERS Substrates
(Wiley-VCH Verlag, 2012-10-12) Cinel, N. A.; Butun, S.; Ertas, G.; Özbay, Ekmel
A highly tunable design for obtaining double resonance substrates to be used in surface-enhanced Raman spectroscopy is proposed. Tandem truncated nanocones composed of Au-SiO2-Au layers are designed, simulated and fabricated to obtain resonances at laser excitation and Stokes frequencies. Surface-enhanced Raman scattering experiments are conducted to compare the enhancements obtained from double resonance substrates to those obtained from single resonance gold truncated nanocones. The best enhancement factor obtained using the new design is 3.86 x 107. The resultant tandem structures are named after Fairy Chimneys rock formation in Cappadocia, Turkey.
Open Access
Integrated AlGaN quadruple-band ultraviolet photodetectors
(IOP Publishing, 2012-04-27) Gökkavas, M.; Butun, S.; Caban, P.; Strupinski, W.; Özbay, Ekmel
Monolithically 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.
Open Access
LSPR enhanced MSM UV photodetectors
(IOP Publishing, 2012-10-18) Butun, S.; Cinel, N. A.; Özbay, Ekmel
We fabricated localized surface plasmon resonance enhanced UV photodetectors on MOCVD grown semi-insulating GaN. Plasmonic resonance in the UV region was attained using 36nm diameter Al nanoparticles. Extinction spectra of the nanoparticles were measured through spectral transmission measurements. A resonant extinction peak around 300nm was obtained with Al nanoparticles. These particles gave rise to enhanced absorption in GaN at 340nm. Spectral responsivity measurements revealed an enhancement factor of 1.5. These results provided experimental verification for obtaining field enhancement by using Al nanoparticles on GaN.
Open Access
Mg-doped AlGaN grown on an AlN/sapphire template by metalorganic chemical vapour deposition
(2006) Yu, H.; Strupinski, W.; Butun, S.; Özbay, Ekmel
The 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.
Open Access
Nanoantenna coupled UV subwavelength photodtectors based on GaN
(Optical Society of America, 2012) Butun, S.; Cinel, N. A.; Özbay, Ekmel
The integration of nano structures with opto-electronic devices has many potential applications. It allows the coupling of more light into or out of the device while decreasing the size of the device itself. Such devices are reported in the VIS and NIR regions. However, making plasmonic structures for the UV region is still a challenge. Here, we report on a UV nano-antenna integrated metal semiconductor metal (MSM) photodetector based on GaN. We designed and fabricated Al grating structures. Well defined plasmonic resonances were measured in the reflectance spectra. Optimized grating structure integrated photodetectors exhibited more than sevenfold photocurrent enhancement. Finite difference time domain simulations revealed that both geometrical and plasmonic effects played role in photocurrent enhancement. (C) 2012 Optical Society of America
Open Access
The persistent photoconductivity effect in AIGaN/GaN heterostructures grown on sapphire and SiC substrates
(AIP Publishing LLC, 2008) Arslan, E.; Butun, S.; Lisesivdin, S. B.; Kasap, M.; Ozcelik, S.; Özbay, Ekmel
In the present study, we reported the results of the investigation of electrical and optical measurements in Al(x)Ga(1-x)N/GaN heterostructures (x=0.20) that were grown by way of metal-organic chemical vapor deposition on sapphire and SiC substrates with the same buffer structures and similar conditions. We investigated the substrate material effects on the electrical and optical properties of Al(0.20)Ga(0.80)N/GaN heterostructures. The related electrical and optical properties of Al(x)Ga(1-x)N/GaN heterostructures were investigated by variable-temperature Hall effect measurements, photoluminescence (PL), photocurrent, and persistent photoconductivity (PPC) that in turn illuminated the samples with a blue (lambda=470 nm) light-emitting diode (LED) and thereby induced a persistent increase in the carrier density and two-dimensional electron gas (2DEG) electron mobility. In sample A (Al(0.20)Ga(0.80)N/GaN/sapphire), the carrier density increased from 7.59x10(12) to 9.9x10(12) cm(-2) via illumination at 30 K. On the other hand, in sample B (Al(0.20)Ga(0.80)N/GaN/SiC), the increments in the carrier density were larger than those in sample A, in which it increased from 7.62x10(12) to 1.23x10(13) cm(-2) at the same temperature. The 2DEG mobility increased from 1.22x10(4) to 1.37x10(4) cm(-2)/V s for samples A and B, in which 2DEG mobility increments occurred from 3.83x10(3) to 5.47x10(3) cm(-2)/V s at 30 K. The PL results show that the samples possessed a strong near-band-edge exciton luminescence line at around 3.44 and 3.43 eV for samples A and B, respectively. The samples showed a broad yellow band spreading from 1.80 to 2.60 eV with a peak maximum at 2.25 eV with a ratio of a near-band-edge excitation peak intensity up to a deep-level emission peak intensity ratio that were equal to 3 and 1.8 for samples A and B, respectively. Both of the samples that were illuminated with three different energy photon PPC decay behaviors can be well described by a stretched-exponential function and relaxation time constant tau as well as a decay exponent beta that changes with the substrate type. The energy barrier for the capture of electrons in the 2DEG channel via the deep-level impurities (DX-like centers) in AlGaN for the Al(0.20)Ga(0.80)N/GaN/sapphire and Al(0.20)Ga(0.80)N/GaN/SiC heterojunction samples are 343 and 228 meV, respectively. The activation energy for the thermal capture of an electron by the defects Delta E changed with the substrate materials. Our results show that the substrate material strongly affects the electrical and optical properties of Al(0.20)Ga(0.80)N/GaN heterostructures. These results can be explained with the differing degrees of the lattice mismatch between the grown layers and substrates.
Open Access
Solar-blind AlxGa1-xN-based avalanche photodiodes
(American Institute of Physics, 2005) Tut, T.; Butun, S.; Butun, B.; Gokkavas, M.; Yu, H. B.; Özbay, Ekmel
We report the Metalorganic Chemical Vapor Deposition (MOCVD) growth, fabrication, and characterization of solar blind AlxGa1-xN/GaN-based avalanche photodiodes. The photocurrent voltage characteristics indicate a reproducible avalanche gain higher than 25 at a 72 V applied reverse bias. Under a 25 V reverse bias voltage, the 100 mu m diameter devices had a maximum quantum efficiency of 55% and a peak responsivity of 0.11 A/W at 254 nm, and a NEP of 1.89x10(-16) W/Hz(1/2).
Open Access
Thermal tuning of infrared resonant absorbers based on hybrid gold-VO2 nanostructures
(American Institute of Physics Inc., 2015) Kocer H.; Butun, S.; Banar, B.; Wang, K.; Tongay, S.; Wu J.; Aydin, K.
Resonant absorbers based on plasmonic materials, metamaterials, and thin films enable spectrally selective absorption filters, where absorption is maximized at the resonance wavelength. By controlling the geometrical parameters of nano/microstructures and materials' refractive indices, resonant absorbers are designed to operate at wide range of wavelengths for applications including absorption filters, thermal emitters, thermophotovoltaic devices, and sensors. However, once resonant absorbers are fabricated, it is rather challenging to control and tune the spectral absorption response. Here, we propose and demonstrate thermally tunable infrared resonant absorbers using hybrid gold-vanadium dioxide (VO2) nanostructure arrays. Absorption intensity is tuned from 90% to 20% and 96% to 32% using hybrid gold-VO2 nanowire and nanodisc arrays, respectively, by heating up the absorbers above the phase transition temperature of VO2 (68°C). Phase change materials such as VO2 deliver useful means of altering optical properties as a function of temperature. Absorbers with tunable spectral response can find applications in sensor and detector applications, in which external stimulus such as heat, electrical signal, or light results in a change in the absorption spectrum and intensity. © 2015 AIP Publishing LLC.