Browsing by Subject "MOCVD"
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Item Open Access Analysis of defect related optical transitions in biased AlGaN/GaN heterostructures(2010) Bengi, A.; Lisesivdin, S.B.; Kasap, M.; Mammadov, T.; Ozcelik, S.; Özbay, EkmelThe optical transitions in AlGaN/GaN heterostructures that are grown by metalorganic chemical vapor deposition (MOCVD) have been investigated in detail by using Hall and room temperature (RT) photoluminescence (PL) measurements. The Hall measurements show that there is two-dimensional electron gas (2DEG) conduction at the AlGaN/GaN heterointerface. PL measurements show that in addition to the characteristic near-band edge (BE) transition, there are blue (BL) and yellow luminescence (YL) bands, free-exciton transition (FE), and a neighboring emission band (NEB). To analyze these transitions in detail, the PL measurements were taken under bias where the applied electric field changed from 0 to 50 V/cm. Due to the applied electric field, band bending occurs and NEB separates into two different peaks as an ultraviolet luminescence (UVL) and Y4 band. Among these bands, only the yellow band is unaffected with the applied electric field. The luminescence intensity change of these bands with an electric field is investigated in detail. As a result, the most probable candidate of the intensity decrease with an increasing electric field is the reduction in the radiative lifetime. © 2010 Elsevier Ltd. All rights reserved.Item Open Access Analysis of the mosaic defects in graded and non graded InxGa1‐xN solar cell structures(Süleyman Demirel Üniversitesi, 2017) Kars Durukan, İ.; Öztürk, M. K.; Özçelik, S.; Özbay, EkmelIn this study, graded (A) InxGa1‐xN (10.5 ≤ x ≤ 18.4) and non graded (B) InxGa1‐xN (13.6 ≤ x ≤ 24.9) samples are grown on c‐oriented sapphire substrate using the Metal Organic Chemical Vapour Deposition (MOCVD) technique. The structural, optical and electrical features of the grown InGaN/GaN solar cell structures are analyzed using High Resolution X‐Ray Diffraction (HRXRD), Photoluminescense (PL), Ultraviolet (UV), current density and potential (JV) measurements. According to the HRXRD results; it is determined that the InGaN layer of the graded structure has a lower FWHM (Full width at half maximum) value. From the PL measurements, it is observed that the GaN half‐width peak value of the graded sample is narrower and the InGaN peak width value of the graded sample is larger. From UV measurements, that the graded sample has a greater band range. JV measurements determine that the performance of the graded structure is higher.Item Open Access Analyzing the AlGaN/AlN/GaN heterostructures for HEMT applications(American Scientific Publishers, 2018-03) K. Durukan, İ.; Akpınar, Ö.; Avar, C.; Gultekin, A.; Öztürk, M. K.; Özçelik, S.; Özbay, EkmelIn the present work, we have reported two high-electron-mobility transistor structures (HEMTs) with different thick AlN buffer deposited on sapphire substrates by metal-organic chemical vapor deposition (MOCVD). AlN buffer layer has a critical important for the device performance therefore its effect of hererostructures analyzed using mosaic model. AlGaN/AlN/GaN heterostructures were examined by X-ray diffraction (XRD) and Atomic-force microscope (AFM). In order to evaluate the mosaicity of GaN, AlGaN, AlN layers in the structures, mosaic model was also used. These layers of lateral and vertical crystal size, dislocations, tilt and stain are investigated with HR-XRD device by Vegard and William Hall (WH) semi-experimental methods. According to the XRD results; As the buffer layer thickness decreases, the FWHM values of symmetric (002) and asymmetric peaks (105) of epitaxial layers increase. The thicker buffer layer makes the structure more crystalized. According to the AFM results the lower thickness AlN buffer has more frequent pits and hillocks and has more rough. As a result, the 520 nm thick AlN buffer layer showed a better structural performance.Item Open Access Contributions of impurity band and electron-electron interactions to magnetoconductance in AlGaN(Taylor & Francis, 2010-06-30) Tasli, P.; Yildiz, A.; Kasap, M.; Özbay, Ekmel; Lisesivdin, S. B.; Ozcelik, S.Low temperature electrical measurements of conductivity, the Hall effect and magnetoconductance were performed on a degenerate AlGaN sample. The sample exhibited negative magnetoconductance at low magnetic fields and low temperatures, with the magnitude being systematically dependent on temperature. The measured magnetoconductance was compared with models proposed previously by Sondheimer and Wilson [Proc. R. Soc. Lond. Ser. A 190 (1947) p. 435] and Lee and Ramakrishan [Rev. Mod. Phys. 57 (1985) p. 287]. Data were analyzed as the sum of the contribution of a two-band and electron-electron interactions to the magnetoconductance, applying these models to describe the observed behavior. Least-squares fits to the data are presented. In the sample, magnetoconductance can be explained reasonably well by assuming these contributions to the measured magnetoconductance. It was found that theoretical and experimental data were in excellent agreement.Item Open Access Effect of reactor pressure on optical and electrical properties of InN films grown by high-pressure chemical vapor deposition(Wiley - V C H Verlag GmbH & Co. KGaA, 2015) Alevli, M.; Gungor, N.; Alkis, S.; Ozgit Akgun, C.; Donmez, I.; Okyay, Ali Kemal; Gamage, S.; Senevirathna, I.; Dietz, N.; Bıyıklı, NecmiThe influences of reactor pressure on the stoichiometry, free carrier concentration, IR and Hall determined mobility, effective optical band edge, and optical phonon modes of HPCVD grown InN films have been analysed and are reported. The In 3d, and N 1s XPS spectra results revealed In-N and N-In bonding states as well as small concentrations of In-O and N-O bonds, respectively in all samples. InN layers grown at 1 bar were found to contain metallic indium, suggesting that the incorporation of nitrogen into the InN crystal structure was not efficient. The free carrier concentrations, as determined by Hall measurements, were found to decrease with increasing reactor pressure from 1.61×1021 to 8.87×1019 cm-3 and the room-temperature Hall mobility increased with reactor pressure from 21.01 to 155.18 cm2/Vs at 1 and 15 bar reactor pressures, respectively. IR reflectance spectra of all three (1, 8, and 15 bar) InN samples were modelled assuming two distinct layers of InN, having different free carrier concentration, IR mobility, and effective dielectric function values, related to a nucleation/interfacial region at the InN/sapphire, followed by a bulk InN layer. The effective optical band gap has been found to decrease from 1.19 to 0.95 eV with increasing reactor pressure. Improvement of the local structural quality with increasing reactor pressure has been further confirmed by Raman spectroscopy measurements. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.Item 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, EkmelThe 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.Item Open Access Examination of the temperature related structural defects of InGaN/GaN solar cells(Academic Press, 2015) Durukan, İ. K.; Bayal, Ö.; Kurtuluş, G.; Baş, Y.; Gültekin, A.; Öztürk, M. K.; Çörekçi, S.; Tamer, M.; Özçelik, S.; Özbay, EkmelIn this study the effects of the annealing temperature on the InGaN/GaN solar cells with different In-contents grown on sapphire substrate by the Metal Organic Chemical Vapor Deposition (MOCVD) are analyzed by High Resolution X-ray Diffraction (HRXRD) and an Atomic Force Microscope (AFM). The plane angles, mosaic crystal sizes, mixed stress, dislocation intensities of the structure of the GaN and InGaN layers are determined. According to the test results, there are no general characteristic trends observed due to temperature at both structures. There are fluctuating failures determined at both structures as of 350 °C. The defect density increased on the GaN layer starting from 350 °C and reaching above 400 °C. A similar trend is observed on the InGaN layer, too.Item Open Access Forward tunneling current in Pt/p-InGaN and Pt/n-InGaN Schottky barriers in a wide temperature range(Elsevier, 2012-07-27) Arslan, E.; Çakmak, H.; Özbay, EkmelThe current-transport mechanisms of the Pt contacts on p-InGaN and n-InGaN were investigated in a wide temperature range (80-360 K) and in the forward bias regime. It was found that the ideality factor (n) values and Schottky barrier heights (SBHs), as determined by thermionic emission (TE), were a strong function of temperature and Φb0 show the unusual behavior of increasing linearly with an increase in temperature from 80 to 360 K for both Schottky contacts. The tunneling saturation ( JTU(0)) and tunneling parameters (E 0) were calculated for both Schottky contacts. We observed a weak temperature dependence of the saturation current and a fairly small dependence on the temperature of the tunneling parameters in this temperature range. The results indicate that the dominant mechanism of the charge transport across the Pt/p-InGaN and Pt/n-InGaN Schottky contacts are electron tunneling to deep levels in the vicinity of mixed/screw dislocations in the temperature range of 80-360 K.Item Open Access InGaN green light emitting diodes with deposited nanoparticles(Elsevier BV, 2007) Butun, B.; Cesario J.; Enoch, S.; Quidant, R.; Özbay, EkmelWe grew an InGaN/GaN-based light-emitting diode (LED) wafer by metal-organic chemical vapor deposition (MOCVD), fabricated devices by optical lithography, and successfully deposited ellipsoidal Ag nano-particles by way of e-beam lithography on top. The diodes exhibited good device performance, in which we expected an enhancement of the radiated intensity by the simulations and emission measurements. The obtained results showed the feasibility of plasmon-assisted LED emission enhancement.Item Open Access InGaN/GaN LED yapıların mozaik yapı analizi(Osman Sağdıç, 2018) Durukan, İ. K.; Öztürk, M. K.; Özçelik, S.; Özbay, EkmelBu çalışmada, MOCVD yöntemi ile üretilen InGaN/GaN ışık yayan diyot (LED) yapılarının mozaik yapıları analiz edildi. Safir alttaş üzerine biriktirilen, farklı In kompozisyonuna sahip InGaN/GaN bariyer tabakasının mozaik yapı üzerine etkisi Yüksek Çözünürlü X Işını Kırınımı (HR-XRD) yöntemi ile karakterize edildi. Amacımız mozaik yapı hesaplarından yararlanarak LED yapısının kalitesini arttırmaktır. Günümüzde LED' lerin geniş bir kullanım alanına sahip olmasından dolayı sektörde büyük bir pazar payı bulunmaktadır. Özellikle InGaN gibi, GaN bazlı LED' ler arsenik bazlı LED' lerden daha yüksek güç, sıcaklık ve frekans aralıklarında çalışabilirler. Ancak GaN tabanlı LED' ler hala yüksek kusur yoğunluklarına sahiptir. Bu nedenle çalışmalarımızda mozaik yapı analizi yapıldı. Hesaplamalarımızda Vegard ve William Hall yarı deneysel metotları kullanıldı. Kompozisyonun, kenar ve vida türü kusurların azalmasına yol açan önemli bir faktör olduğunu söyleyebiliriz.Item Open Access Microstructural analysis with graded and non-graded indium in InGaN solar cell(American Scientific Publishers, 2017) Durukan, I. K.; Öztürk, M. K.; Çörekçi, S.; Tamer, M.; Baş, Y.; Özçelik, S.; Özbay, EkmelIn this study are graded and non graded InGaN/GaN samples grown on c-oriented sapphire substrate using the Metal Organic Chemical Vapour Deposition (MOCVD) technique. The structural and morphological properties of the grown InGaN/GaN solar cell structures are analyzed using High Resolution X-ray Diffraction (HRXRD), atomic force microscopy (AFM). Each structures c and a lattice parameters strain, biaxial strain, hydrostatic strain, stress, lattice relax, tilt angle, mosaic crystal size, dislocation densities of GaN and InGaN layers are determined by XRD measurements. In accordance with these calculations, the effect of graded structure on the defects, are discussed. As a dramatic result; although values of full width at half maximum (FWHM) are broad, a considerable decrease at dislocations is noticed. The AFM observations have revealed that the two dimensional growth of the graded sample is more significant and its roughness value is lower. JV measurements shown that the performance of the graded structure is higher. It is determined that all test results are consistent with each other.Item Open Access Ultraviolet-visible nanophotonic devices(Bilkent University, 2010) Bütün, BayramRecently in semiconductor market, III-Nitride materials and devices are of much interest due to their mechanical strength, radiation resistance, working in the spectrum from visible down to the deep ultraviolet region and solar-blind device applications. These properties made them strongest candidates for space telecommunication, white light generation, high power lasers and laser pumping light emitting diodes. Since, like other semiconductors, there have been material quality related issues, ongoing research efforts are concentrated on growing high quality crystals and making low p-type ohmic contact. Also, in light emitting device applications, similar to the visible and infrared spectrum components, there are challenging issues like high extraction efficiency and controlled radiation. In this thesis, we worked on growth and characterizations of high quality (In,Al)GaN based semiconductors, fabricating high performance photodiodes and light emitting diodes. We studied different surface modifications and possibilities of obtaining light emitting diode pumped organic/inorganic hybrid laser sources