Browsing by Subject "Semiconducting aluminum compounds"
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Item Open Access 45-GHz bandwidth-efficiency resonant-cavity-enhanced ITO-Schottky photodiodes(IEEE, 2001) Bıyıklı, Necmi; Kimukin, I.; Aytür, O.; Gökkavas, M.; Ünlü, M. S.; Özbay, EkmelHigh-speed Schottky photodiodes suffer from low efficiency mainly due to the thin absorption layers and the semitransparent Schottky-contact metals. We have designed, fabricated and characterized high-speed and high-efficiency AlGaAs-GaAs-based Schottky photodiodes using transparent indium-tin-oxide Schottky contact material and resonant cavity enhanced detector structure. The measured devices displayed resonance peaks around 820 nm with 75% maximum peak efficiency and an experimental setup limited temporal response of 11 ps pulsewidth. The resulting 45-GHz bandwidth-efficiency product obtained from these devices corresponds to the best performance reported to date for vertically illuminated Schottky photodiodes.Item Open Access Characterization of AlInN/AlN/GaN heterostructures with different AlN buffer thickness(Springer New York LLC, 2016) Çörekçi, S.; Dugan, S.; Öztürk, M. K.; Çetin, S. Ş.; Çakmak, M.; Özçelik, S.; Özbay, EkmelTwo AlInN/AlN/GaN heterostructures with 280-nm- and 400-nm-thick AlN buffer grown on sapphire substrates by metal-organic chemical vapor deposition (MOCVD) have been investigated by x-ray diffraction (XRD), atomic force microscopy (AFM), photoluminescence (PL) and Hall-effect measurements. The symmetric (0002) plane with respect to the asymmetric (101 ¯ 2) plane in the 280-nm-thick AlN buffer has a higher crystal quality, as opposed to the 400-nm-thick buffer. The thinner buffer improves the crystallinity of both (0002) and (101 ¯ 2) planes in the GaN layers, it also provides a sizeable reduction in dislocation density of GaN. Furthermore, the lower buffer thickness leads to a good quality surface with an rms roughness of 0.30 nm and a dark spot density of 4.0 × 108 cm−2. The optical and transport properties of the AlInN/AlN/GaN structure with the relatively thin buffer are compatible with the enhancement in its structural quality, as verified by XRD and AFM results.Item Open Access Direct magnetic imaging of ferromagnetic domain structures by room temperature scanning hall probe microscopy using a bismuth micro-Hall probe(Japan Society of Applied Physics, 2001) Sandhu, A.; Masuda, H.; Oral, A.; Bending, S. J.A bismuth micro-Hall probe sensor with an integrated scanning tunnelling microscope tip was incorporated into a room temperature scanning Hall probe microscope system and successfully used for the direct magnetic imaging of microscopic domains of low coercivity perpendicular garnet thin films and demagnetized strontium ferrite permanent magnets. At a driving current of 800 μA, the Hall coefficient, magnetic field sensitivity and spatial resolution of the Bi probe were 3.3 × 10-4 Ω/G, 0.38 G/√Hz and ∼ 2.8 μm, respectively. The room temperature magnetic field sensitivity of the Bi probe was comparable to that of a semiconducting 1.2μm GaAs/AlGaAs heterostructure micro-Hall probe, which exhibited a value of 0.41 G/√Hz at a maximum driving current of 2μA.Item Open Access Dual-color ultraviolet metal-semiconductor-metal AlGaN photodetectors(AIP Publishing LLC, 2006) Gökkavas, M.; Bütün, S.; Yu, H.; Tut, T.; Bütün, B.; Özbay, EkmelBackilluminated ultraviolet metal-semiconductor-metal photodetectors with different spectral responsivity bands were demonstrated on a single Alx Ga1-x N heterostructure. This was accomplished by the incorporation of an epitaxial filter layer and the recess etching of the surface. The 11 nm full width at half maximum (FWHM) responsivity peak of the detector that was fabricated on the as-grown surface was 0.12 AW at 310 nm with 10 V bias, whereas the 22 nm FWHM responsivity peak of the detector fabricated on the recess-etched surface was 0.1 AW at 254 nm with 25 V bias. Both detectors exhibited excellent dark current characteristics with less than 10 fA leakage current. © 2006 American Institute of Physics.Item Open Access The effect of growth conditions on the optical and structural properties of InGaN/GaN MQW LED structures grown by MOCVD(Gazi University Eti Mahallesi, 2014) Cetđn, S.; Sağlam, S.; Ozcelđk, S.; Özbay, EkmelFive period InGaN/GaN MQW LED wafers were grown by low pressure MOCVD on an AlN buffer layer, which was deposited on a c-plane (0001)-faced sapphire substrate. The effect of growth conditions, such as the well growth time, growth temperatures, and indium flow rate on the properties of MQW structures were investigated by using high resolution X-ray diffraction and room temperature photoluminescence. By increasing growth temperature, the emission wavelengths showed a blue-shift while it red-shifted via an increase in the indium flow rate. The emission wavelength can be tuned by way of changing the well growth time of the samples. ©2014 Gazi University Eti Mahallesi. All rights reserved.Item Open Access Fabrication of high-speed resonant cavity enhanced schottky photodiodes(Institute of Electrical and Electronics Engineers, 1997-05) Özbay, Ekmel; Islam, M. S.; Onat, B.; Gökkavas, M.; Aytür, O.; Tuttle, G.; Towe, E.; Henderson, R. H.; Ünlü, M. S.We report the fabrication and testing of a GaAs-based high-speed resonant cavity enhanced (RCE) Schottky photodiode. The top-illuminated RCE detector is constructed by integrating a Schottky contact, a thin absorption region (In0.8Ga0.92As) and a distributed AlAs-GaAs Bragg mirror. The Schottky contact metal serves as a high-reflectivity top mirror in the RCE detector structure. The devices were fabricated by using a microwave-compatible fabrication process. The resulting spectral photo response had a resonance around 895 nm, in good agreement with our simulations. The full-width-at-half-maximum (FWHM) was 15 nm, and the enhancement factor was in excess of 6. The photodiode had an experimental setup limited temporal response of 18 ps FWHM, corresponding to a 3-dB bandwidth of 20 GHz.Item Open Access High-performance ALGaN-based visible-blind resonant cavity enhanced Schottky photodiodes(Materials Research Society, 2003-04) Kimukin, İbrahim; Bıyıklı, Necmi; Kartaloğlu, Tolga; Aytür, Orhan; Özbay, EkmelWe have designed, fabricated and tested resonant cavity enhanced visible-blind AlGaN-based Schottky photodiodes. The bottom mirror of the resonant cavity was formed with a 20 pair AlN/AlGaN Bragg mirror. The devices were fabricated using a microwave compatible fabrication process. Au and indium-tin-oxide (ITO) thin films were used for Schottky contact formation. ITO and Au-Schottky devices exhibited resonant peaks with 0.153 A/W and 0.046 A/W responsivity values at 337 nm and 350 nm respectively. Temporal high-speed measurements at 357 nm resulted in fast pulse responses with pulse widths as short as 77 ps. The fastest UV detector had a 3-dB bandwidth of 780 MHz.Item Open Access High-performance solar-blind AlGaN photodetectors(IEEE, 2004) Özbay, Ekmel; Bıyıklı, Necmi; Kimukin, İbrahim; Tut, Turgut; Kartaloğlu, Tolga; Aytür, OrhanHigh-performance aluminum gallium nitride (AlGaN)-based solar-blind (SB) photodetectors were demonstrated using different device structures. The Al x-Ga1-xN layers structure were grown by metalorganic chemical vapor deposition (MOCVD) on sapphire structures. n+ and p+ ohmic contacts on GaN were formed with non-annealed titanium (Ti)/aluminum (Al) and nickel (Ni)/ gold (Au) alloys. Spectral UV photoresponse measurements confirmed the solar-blind response of the devices.Item Open Access High-performance solar-blind photodetectors based on AlxGa 1_xN heterostructures(IEEE, 2004) Özbay, Ekmel; Bıyıklı, Necmi; Kimukin, I.; Kartaloglu, T.; Tut, T.; Aytür, O.Design, fabrication, and characterization of high-performance AI xGa1-xN-based photodetectors for solar-blind applications are reported. AlxGa1-xN heterostructures were designed for Schottky. p-i-n, and metal-semicondnctor-metal (MSM) photodiodes. The solar-blind photodiode samples were fabricated using a microwave compatible fabrication process. The resulting devices exhibited extremely low dark currents. Below 3 fA, leakage currents at 6-V reverse bias were measured on p-i-n samples. The excellent current-voltage (I-V) characteristics led to a detectivity performance of 4.9×1014 cmHz1/2W -1. The MSM devices exhibited photoconductive gain, while Schottky and p-i-n samples displayed 0.09 and 0.11 A/W peak responsivity values at 267 and 261 nm, respectively. A visible rejection of 2×104 was achieved with Schottky samples. High-speed measurements at 267 nm resulted in fast pulse responses with greater than gigahertz bandwidths. The fastest devices were MSM photodiodes with a maximum 3-dB bandwidth of 5.4 GHz.Item Open Access High-speed 1.55 μm operation of low-temperature-grown GaAs-based resonant-cavity-enhanced p-i-n photodiodes(American Institute of Physics, 2004) Butun, B.; Bıyıklı, Necmi; Kimukin, I.; Aytur, O.; Özbay, Ekmel; Postigo, P. A.; Silveira, J. P.; Alija, A. R.The 1.55 μm high-speed operation of GaAs-based p-i-n photodiodes was demonstrated and their design, growth and fabrication were discussed. A resonant-cavity-detector structure was used to selectively enhance the photoresponse at 1.55 μm. The bottom mirror of the resonant cavity was formed by a highly reflecting 15-pair GaAs/AlAs Bragg mirror and molecular-beam epitaxy was used for wafer growth. It was found that the fabricated devices exhibited a resonance of around 1548 nm and an enhancement factor of 7.5 was achieved when compared to the efficiency of a single-pass detector.Item Open Access High-speed GaAs-based resonant-cavity-enhanced 1.3-μm photodetector(SPIE, 2000) Özbay, Ekmel; Kimukin, İbrahim; Bıyıklı, Necmi; Gary, T.High-speed photodetectors operating at 1.3 and 1.55 μm are important for long distance fiber optic based telecommunication applications. We fabricated GaAs based photodetectors operating at 1.3 μm that depend on internal photoemission as the absorption mechanism. Detectors using internal photoemission have usually very low quantum efficiency. We increased the quantum efficiency using resonant cavity enhancement effect. Resonant cavity enhancement effect also introduced wavelength selectivity which is very important for wavelength division multiplexing based communication systems. The top-illuminated Schottky photodiodes were fabricated by a microwave-compatible monolithic microfabrication process. The top metal layer serves as the top mirror of the Fabry-Perot cavity. Bottom mirror is composed of 15 pair AlAs/GaAs distributed Bragg reflector. We have used transfer matrix method to simulate the optical properties of the photodiodes. Our room temperature quantum efficiency measurement and simulation of our photodiodes at zero bias show that, we have achieved 9 fold enhancement in the quantum efficiency, with respect to a similar photodetector without a cavity. We also investigated the effect of reverse bias on quantum efficiency. Our devices are RC time constant limited with a predicted 3-dB bandwidth of 70 GHz.Item Open Access High-speed solar-blind photodetectors with indium-tin-oxide Schottky contacts(American Institute of Physics, 2003) Bıyıklı, Necmi; Kimukin, I.; Kartaloglu, T.; Aytur, O.; Özbay, EkmelAlGaN/GaN-based high-speed solar-blind photodetectors were discussed. Current-voltage, spectral responsivity, and high-frequency response characterizations were performed. Breakdown voltages larger than 40 V were obtained. A maximum responsivity of 44 mA/W at 263 nm was measured. True solar-blind detection was also ensured.Item Open Access High-speed widely-tunable >90% quantum-efficiency resonant cavity enhanced p-i-n photodiodes(IEEE, 1998) Bıyıklı, Necmi; Kimukin, İbrahim; Aytür, Orhan; Gökkavas, M.; Ulu, G.; Mirin, R.; Christensen, D. H.; Ünlü, M. S.; Özbay, EkmelWidely-tunable high-speed resonant cavity enhanced p-i-n photodiodes were designed, fabricated and tested for operation around 820 nm. The structure was grown by solid-source MBE on GaAs substrates and features high-reflectivity Bragg mirrors made of quarter-wave Al0.20Ga0.80As/AlAs stacks. Photoresponse and photospectral measurements were carried out. The tuning of the resonance wavelength within the Bragg mirror's upper and lower edges was observed. Quantum efficiency greater than 90% was demonstrated.Item Open Access Highly directive radiation and negative refraction using photonic crystals(Institute of Physics Publishing, 2005) Özbay, Ekmel; Bulu, I.; Aydin, K.; Caglayan H.; Alici, K. B.; Guven, K.In this article, we present an experimental and numerical study of certain optical properties of two-dimensional dielectric photonic crystals (PCs). By modifying the band structure of a two-dimensional photonic crystal through its crystal parameters, we show how it is possible to confine the angular distribution of radiation from an embedded omnidirectional source. We then demonstrate that the anomalous band dispersions of PCs give rise to completely novel optical phenomena, in particular, the negative refraction of electromagnetic waves at the interface of a PC. We investigate the spectral negative refraction, which utilizes a transverse magnetic (TM)-polarized upper band of a PC, in detail and show that a high degree of isotropy can be achieved for the corresponding effective index of refraction. The presence of nearly a isotropic negative refractive index leads to focusing of omnidirectional sources by a PC slab lens, which can surpass certain limitations of conventional (positive refractive) lenses. These examples indicate the potential of PCs for photonics applications utilizing the band structure.Item Open Access Hot electron effects in unipolar n-type submicron structures based on GaN, AlN and their ternary alloys(The Institution of Engineering and Technology, 2003) Sevik, C.; Bulutay, C.The authors present an analysis of impact ionisation (II) and related hot electron effects in submicron sized GaN, AlN and their ternary alloys, all of which can support very high field regimes, reaching a few megavolts per centimetre (MV/cm). The proposed high field transport methodology is based on the ensemble Monte Carlo technique, with all major scattering mechanisms incorporated. As a test-bed for understanding II and hot electron effects, an n+-n-n+ channel device is employed having a 0.1 μm thick n-region. The time evolution of the electron density along the device is seen to display oscillations in the unintentionally doped n-region, until steady state is established. The fermionic degeneracy effects are observed to be operational especially at high fields within the anode n+-region. For AlxGa1-xN-based systems, it can be noted that due to alloy scattering, carriers cannot acquire the velocities attained by the GaN and AlN counterparts. Finally, at very high fields II is shown to introduce a substantial energy loss mechanism for the energetic carriers that have just traversed the unintentionally doped n-region.Item Open Access Hydrodynamic approach for modelling transport in quantum well device structures(Institute of Physics Publishing Ltd., 1998) Besikci, C.; Tanatar, Bilal; Sen, O.A semiclassical approach for modelling electron transport in quantum well structures is presented. The model is based on the balance equations governing the conservation of particle density, momentum and energy with Monte Carlo (MC) generated transport parameters. Three valleys of the conduction band, size quantization in the Γ valley, and the lowest two subbands in the quantum well are considered by taking the detailed intersubband dynamics into account. The transport parameters of the model are extracted from steady-state MC simulations based on an improved formulation of two-dimensional polar optical phonon scattering including screening effects. The predictions of the proposed model have been found to be in excellent agreement with those of the ensemble MC simulations under both time varying and spatially nonuniform fields. The calculated transport parameters which are of interest for device modelling are presented as a function of the electron energy for the AIGaAs/GaAs quantum well. The model serves as an accurate semiclassical alternative to costly ensemble MC simulations for studying the transport in quantum well structures and for the modelling and optimization of submicron devices based on these structures, such as modulation doped field-effect transistors (MODFETs).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 Raman scattering from confined phonons in GaAs/AlGaAs quantum wires(Academic Press, 1998) Bairamov, B. H.; Aydınlı, Atilla; Tanatar, Bilal; Güven, K.; Gurevich, S.; Mel'tser, B. Ya.; Ivanov, S. V.; Kop'ev, P. S.; Smirnitskii, V. B.; Timofeev, F. N.We report on photoluminescence and Raman scattering performed at low temperature (T = 10 K) on GaAs/Al 0.3Ga 0.7As quantum-well wires with effective wire widths of L = 100.0 and 10.9 nm prepared by molecular beam epitaxial growth followed by holographic patterning, reactive ion etching, and anodic thinning. We find evidence for the existence of longitudinal optical phonon modes confined to the GaAs quantum wire. The observed frequency at ω L10 = 285.6 cm -1 for L = 11.0 nm is in good agreement with that calculated on the basis of the dispersive dielectric continuum theory of Enderlein† as applied to the GaAs/Al 0.3Ga 0.7As system. Our results indicate the high crystalline quality of the quantum-well wires fabricated using these techniques. © 1998 Academic Press.Item Open Access Solar-blind A1GaN-based p-i-n photodiodes with low dark current and high detectivity(IEEE, 2004) Bıyıklı, Necmi; Kimukin, I.; Aytur, O.; Özbay, EkmelWe report solar-blind AlxGal1-xN-based heterojunction p-i-n photodiodes with low dark current and high detectivity. After the p+ GaN cap layer was recess etched, measured dark current was below 3 fA for reverse bias values up to 6 V. The device responsivity increased with reverse bias and reached 0.11 A/W at 261 nm under 10-V reverse bias. The detectors exhibited a cutoff around 283 nm, and a visible rejection of four orders of magnitude at zero bias. Low dark current values led to a high differential resistance of 9.52 × 1015 Ω. The thermally limited detectivity of the devices was calculated as 4.9 × 1014 cm · Hz1/2W-1. © 2004 IEEE.Item Open Access Study of wet oxidized AlxGa1-xAs for integrated optics(Institute of Electrical and Electronics Engineers, 1999) Bek, A.; Aydınlı, Atilla; Champlain, J. G.; Naone, R.; Dagli, N.An investigation of wet oxidized AlxGa1-xAs layers in integrated optical applications is reported. Refractive index and thickness shrinkage of wet oxidized AlxGa1-xAs layers are measured using spectroscopic ellipsometry. A Cauchy fit to the refractive index is found in the wavelength range between 0.3 and 1.6 μm. The refractive index at 1.55 μm is found to be 1.66±0.01 with little dispersion around 1.55 μm. Very low loss single-mode waveguides with metal electrodes showing very low polarization dependence of loss coefficient are fabricated using wet oxidized AlxGa1-xAs layers as upper cladding. Optical polarization splitters are also designed and fabricated from the same type of waveguides taking advantage of increased birefringence. Designs utilizing wet oxidized AlxGa1-xAs are compared with conventional designs using only compound semiconductor heterostructures.