Browsing by Author "Selamet, Yusuf"

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Applications of transparent conductive indium tin oxide films in automotive and vitrifications industries
(Izmir Institute of Technology, 2009) Tuna, Öcal; Selamet, Yusuf
Due to its unique electrical and optical properties, highly doped n-type Indium tin oxide used for various applications such as smart glass, LCDs, OLEDs, solar cells and car windows. In this study Indium Tin Oxide (ITO) thin films were grown by both DC and RF magnetron sputtering techniques. To know deposition rate of ITO, system was calibrated for both DCMS and RFMS and then ITO were grown on glass substrate with the thickness of 70 nm and 40 nm by changing substrate temperature. The effect of substrate temperature, film thickness and sputtering method on structural, electrical and optical properties were investigated. Wan der Pauw method was used for electrical characterization and to use this method properly, we patterned ITO thin films by photolithography and Ion beam etching techniques. The results show that substrate temperature and film thickness substantially affects the film properties, especially crystallization and resistivity. The thin films grown at the lower than 150 oC showed amorphous structure. However, crystallization was detected with the further increase of substrate temperature. Substrate temperature and film thickness increment were lead to increase band gap of ITO which can be explained by BMS. Band gap of ITO was calculated to be about 3.64 eV at the substrate temperature of 150 oC, and it widened with substrate temperature increment. From electrical measurements the resistivity at room temperature was obtained 1.28*10 and 1.29*10 cm, for DC and RF sputtered films, respectively. We also measured temperature dependence resistivity and the Hall coefficient of the films, and we calculated carrier concentration and Hall mobility.
Characterization of defect structure of epitaxial CdTe films
(Izmir Institute of Technology, 2014-07) Özden, Selin; Selamet, Yusuf
Mercury Cadmium Telluride (HgCdTe) is widely used material for infrared detection. Epitaxial growths carried on Gallium arsenide (GaAs) substrates gained more attention in recent years due to commercially availability of epi-ready wafers. However, large lattice mismatch between the HgCdTe epilayer and GaAs substrates, and Gallium (Ga) diffusion into HgCdTe layers during growth limit the device performance. In order to decrease large lattice mismatch and hereby dislocations formed at HgCdTe epilayer, a closely lattice matched Cadmium Telluride (CdTe) is preffered buffer layer for Molecular Beam Epitaxial (MBE) growth of HgCdTe. This thesis focuses on a study of defects on (211)B CdTe buffer layers grown on (211)B oriented GaAs substrates by MBE. Prior to epitaxial growth of CdTe layers, to understand the effect of wet cleaning procedure on chemical composition of epi-ready GaAs wafers, piranha solution-based wet chemical etching and oxide removal processes using diluted hydrofluoric acid (HF) were performed on undoped 625≤25 μm thick GaAs(211)B wafers. The surfaces of GaAs wafers were investigated by Atomic Force Microscopy (AFM) and Scanning Electron Microscopy (SEM). The variation of As2O3 and Ga2O3 contents on GaAs (211)B wafers studied by Raman spectroscopy. Following the growth of CdTe (211)B epitaxial films, the quality of CdTe layers were investigated in detail by various characterization techniques such as AFM, SEM, Nomarski Microscopy, X-ray Diffraction (XRD), Fourier Transform Infrared Spectroscopy (FTIR) and Raman Spectroscopy. Thicknesses of CdTe layers were calculated via intensity oscillations in the transmittance spectrum of the films.
Characterization of GaAs (211) surface for epitaxial buffer growth
(Izmir Institute of Technology, 2014) Polat, Mustafa; Selamet, Yusuf
GaAs (211)B wafer can be used for the growth of CdTe buffer layer by MBE after thermal desorption of oxide presents on its surface. Then, CdTe buffered GaAs (211)B called as composite substrate can be used as a template for the growth of HgCdTe. Thermal desorption can be detrimental to surface in some cases if the structure and constituents of this oxide are not fully understood. In this thesis, HF:H2O and H2SO4:H2O2:H2O chemical treatments were applied to epiready GaAs (211)B samples for the determination of suitability of their usage for CdTe buffer layer growth. Effects of these wet chemical etching processes on the surface of samples are characterized and determined by various kinds of characterization techniques including XRD, XPS, SEM, EDX, AFM, and optical microscope. We also analyzed samples cut from 3" epiready GaAs (211)B wafers to determine their oxide structures, surface roughnesses, crystal qualities, and surface morphologies. Crystal quality of as-received samples measured by RC were about 18-21 arcsec. Amounts of arsenic and gallium oxides were decreased after HF treatment according to XPS results. Gallium rich surface was obtained for samples treated with piranha solution. Surface roughnesses of samples increased after piranha treatment. However, it was determined that others treated with HF had smaller surface roughnesses than asreceived samples.
Characterization of lattice mismatch induced dislocations on epitaxial CdTe films
(Izmir Institute of Technology, 2015-07) Bilgilisoy, Elif; Selamet, Yusuf
Mercury Cadmium Telluride (HgCdTe) is a widely used material for infrared focal plane array applications. In order to produce high quality infrared detecting material, HgCdTe needs to be grown on large area alternative substrates such as GaAs, GaSb, Si or Ge. GaAs is the best choice as an alternative substrate due to its surface polarity and commercially availability of high quality epi-ready wafers. However, there exists a lattice mismatch between HgCdTe and GaAs. To minimize the detrimental effect of the large lattice mismatch between the two materials, Cadmium Telluride (CdTe) is preferred as a buffer layer for HgCdTe IR material. The lattice mismatch between HgCdTe/CdTe and the GaAs substrate results in a large number of misfit dislocations. Dislocation density of the buffer layer limits and reduces the detector device performance. For this reason, the crystal quality and dislocation analysis of CdTe are examined in detail to produce large area and high performance HgCdTe IR devices. The aim of this thesis is the characterization of lattice mismatch induced dislocations on epitaxial CdTe buffer layers. CdTe epilayers which were grown on (211)B GaAs by molecular beam epitaxy (MBE) were subjected to two different etch treatments to quantify the crystal quality and dislocation density. The crystal quality was also obtained by using x-ray diffraction (XRD) measurements. The thicknesses of the samples were measured by ex-situ spectroscopic ellipsometry (SE). The surface morphologies of the CdTe buffer layers were analyzed by atomic force microscopy (AFM), scanning electron microscopy (SEM) and Nomarski microscopy before and after wet chemical etching. Vibrational phonon modes distributions of the as-grown and etched samples were examined by Raman spectroscopy mapping. The “triangle” and “trapezoid” shaped etch pits were compared due to the Everson and Nakagawa etching solutions, respectively. Measured etch pit density (EPD) values of “triangle” etch pits were found in 0.3x108 – 3.8x108 cm-2 range and “trapezoid” shaped etch pits were found in 0.03x108 – 0.6x108 cm-2 range for samples.
Characterization of molecular beam epitaxially crown CdTe layers over GaAs by spectroscopic ellipsometry
(Izmir Institute of Technology, 2014) Günnar, Merve; Selamet, Yusuf
The infrared detectors consist of two main parts that are optical elements and sensing elements. The sensor component is generally formed by semiconductor materials that can detect Infrared (IR) light which cannot be seen by human eye. Mercury Cadmium Telluride (MCT, HgCdTe) is widely used as a sensor material for this purpose. The adjustable bandgap (0-1.5 eV) which corresponds to energies of IR light can be obtained by changing the composition x of cadmium (Cd) in the ternary alloy Hg1-xCdxTe. HgCdTe has very high quantum efficiency for the detectible IR wavelengths in the atmospheric windows. HgCdTe which has a great importance in defense industry as an IR detecting material should be grown with high crystallinity in order to obtain high resolution images even under bad weather conditions. In addition, HgCdTe must be grown uniformly over a large area in order to have large format and high operability focal plane arrays. The defect density of HgCdTe strongly depends on the lattice mismatch between substrate and HgCdTe. In order to reduce the lattice mismatch which causes dislocations in HgCdTe the best suitable option is to grow Cadmium Telluride (CdTe) buffer layer on a substrate before growing HgCdTe. Studies have been focusing on semiconductors which are Gallium Arsenide (GaAs), Silicon (Si) and Germanium (Ge) as alternative substrates for CdTe growth. In this study, the CdTe films grown on (211) oriented GaAs wafers by molecular beam epitaxy (MBE) were characterized by ex-situ spectroscopic ellipsometry (SE). The properties of CdTe films such as thickness, surface roughness and optical constants were characterized by comparison with the growth conditions. It was also investigated that how these properties vary over the film surface. Characterization results were compared to those obtained by atomic force microscopy (AFM), Nomarski microscopy, Fourier transformation infrared spectroscopy (FTIR) and X-ray diffraction (XRD). The temperature dependencies of the optical properties of the material obtained by SE were also investigated.
Defect reduction study of molecular beam epitaxially grown CdTe thin flims by ex-situ annealing
(Izmir Institute of Technology, 2015-10) Bakali, Emine; Selamet, Yusuf
Molecular Beam Epitaxy (MBE) grown CdTe thin films were annealed in this study to decrease the number density of defects. For annealing, a system was designed and constructed. During anneals; anneal temperature, anneal time, anneal cycle and hydrogen gas effects were analyzed. The effects of annealing parameters were analyzed by Scanning Electron Microscope (SEM), Atomic Force Microscope (AFM), Everson Etch method, resonance Raman spectroscopy and Photoluminescence measurement. In our studies, dislocation density decreased for 5 min. annealing when annealing temperature increased. Dislocation density decreased with increasing annealing time. Besides, dislocation density decreased when cycle number increased. Te precipitation decreased with annealing. Raman mode at 144 cm-1 was investigated and that mode was decided as Te E mode. Also I2LO/ILO ratio decreased with increasing annealing temperature and annealing time. I2LO/ILO ratio were approached to 1 at 80oK due to so called ‘resonance Raman scattering’. Extra peaks were also observed by Raman scattering. On the surface, small pits occurred when annealing temperature increased. Surface roughness decreased with increasing cycle number.
Development of electrically conductive and anisotropic gel-coat systems using CNTs
(Elsevier Science Sa, 2013) Yardimci, Atike Ince; Tanoglu, Metin; Selamet, Yusuf
Electrical conductivity of an unsaturated thermoset polyester based gel-coat system containing 0.05 wt.% of carbon nanotubes (CNTs) was investigated. The CNTs used were synthesized by chemical vapor deposition method by methane decomposition and Raman characterization showed that they were mostly single walled and high quality. To disperse CNTs in the gel-coat resin, 3-roll milling technique was used. It was found that as the CNTs are added to gel-coat system, resistivity value decreases significantly while neat gel-coat showed a high resistivity. By the application of an AC electrical field during curing process, it was attempted to align CNTs in the gel-coat resin and an electrically anisotropic polymer was obtained. (C) 2012 Elsevier B.V. All rights reserved.
Effect of CNT incorporation on PAN/PPy nanofibers synthesized by electrospinning method
(Tubitak Scientific & Technological Research Council Turkey, 2020) Ince Yardimci, Atike; Tanoglu, Metin; Yilmaz, Selahattin; Selamet, Yusuf
In this study, carbon nanotubes (CNTs) added polyacrylonitrile/polypyrrole (PAN/PPy) electrospun nanofibers were produced. Average diameters of the nanofibers were measured as 268 and 153 nm for 10 and 25 wt% of PPy contents, respectively. A relatively higher strain to failure values (23.3%) were observed for the low PPy content. When as-grown CNTs (1 and 4 wt%) were added into the PAN/PPy blends, disordered nanofibers were observed to form within the microstructure. To improve the interfacial properties of CNTs/PAN/PPy composites, CNTs were functionalized with H2SO4/HNO3/HCl solution. The functionalized CNTs were well dispersed within the nanofibers and aligned along the direction of nanofibers. Therefore, beads formation on nanofibers decreased. The impedance of the nanofibers was found to decrease with the PPy content and CNT addition. These nanofibers had a great potential to be used as an electrochemical actuator or a tissue engineering scaffold.
The effects of oxidizers on the diameters of the carbon naotubes grown by chemical vapor deposition method
(Izmir Institute of Technology, 2012) Bülbül, Gülay; Selamet, Yusuf
This thesis was focused on growing high quality and small-diameter carbon (C) nanotube (NT) on Fe/Al2O3/SiO2/Si thin film catalyst by ethylene decomposition thermal chemical vapor deposition (CVD) method in the presence of a weak oxidizer (CO2). Moreover, the importance and functional properties of the oxidizer in pretreatment and CNT growth were determined. At first, it was worked in different growth conditions to examine the effects of CO2 in CNT growth. The main parameters were pretreatment time, CO2 gas flow rates during pretreatment and growth, growth temperature. Pretreatment and growth times were kept at 15 min. each and CO2 introduced 5 and 10 min. during pretreatment stage prior to the CNT growth with the flow rates 8:2, 10:2, 10:8 and 10:10 sccm, respectively. Additionally, three different growth temperatures; 750 oC, 760 oC and 770 oC were studied. Secondly, pretreatment time was kept at 15 min. The effects of CO2, which was sent in the system at 8:2, 10:2, 10:8 and 10:10 sccm ratios, on density and height of the catalyst particles were investigated. At last, all catalyst particles and CNTs obtained from the experiments were analyzed by several characterization techniques such as AFM, EDX, SEM, STEM and Raman Spectroscopy, respectively. The optimal values of amount and introduction time of CO2, the ratio of CO2 in growth to that in pretreatment were identified. Moreover, the relation between currently obtained catalyst particles and previously being grown CNTs on them were determined. It was observed that using the appropriate amount of CO2 in pretreatment and growth process positively affected the catalyst sizes and CNT diameter distributions.
Grawth and electrical characterization of high purity carbon nanotubes
(Izmir Institute of Technology, 2009) Kır, Serap; Selamet, Yusuf
This thesis work is focused on growing high purity vertically aligned Carbon nanotubes by ethylene gas thermal chemical vapor deposition method on very thin Cobalt and Iron catalyst thin films deposited on to Si/SiO2/Al2O3 substrates by DC magnetron sputtering. In this study, the effective parameters were changed to grow aligned CNTs. Hence, the vertically aligned CNTs were performed and also the ideal parameters were determined for this kind of growth mechanism. The effect of support layer was examined for three different hydrocarbon gas ratios. SiO2 and Al2O3 were used as support layers between metal catalyst thin films and Si substrate. Two kinds of samples were compared; one of them had only Al2O3 and the other one includes both Al2O3 and SiO2 layers. As a result, the sample that had both oxide layers gave better results for density of CNTs on the surface of samples. Moreover, types of catalyst material effect also were examined on growth of CNTs for three different temperatures. For this aim, the performance of Fe and Co catalyst thin films was compared. According to our results, Fe was more reactive with ethylene gas than Co catalysts and also, the density of CNTs was increased by using Fe as a catalyst material. Hydrogen pretreatment time was performed for another significant effect. Seven different time parameter which were 0, 5, 10, 15, 20, 25, 30 minutes, were examined. The density and diameters of catalysts particles were compared for these different treatment times. Finally, the electrical characterization of CNTs was performed. The resistance of CNTs was measured by using two point contact technique. Moreover, the interaction between resistance of CNTs and humidity was examined.
Growth and characterization of aluminum doped transparent and conductive zinc oxidethin flims
(Izmir Institute of Technology, 2010) Ataç, Derya; Selamet, Yusuf
This thesis focuses on fabrication, characterization and understanding physical properties of transparent and conductive Al doped zinc oxide (ZnO) thin films. Films were deposited by magnetron sputtering technique, using separate ZnO and Al targets. SiO2 and glass (microscope slides) were used as substrates. Growths were performed at room temperature in Ar environment at a constant pressure of 3 mTorr. Films were characterized by atomic force microscope, x-ray diffractometer, scanning electron microscope, UV-vis spectroscophotometer and four point probe electrical measurements. Using transmission data, band gaps of the films and using four point probe measurements, resistivities of films were calculated. Firstly properties of pure ZnO films were studied. They were found to be highly transparent; however their resistivity was very high that we could not measure with our instrument. Therefore, ZnO films were uniformly doped with Al. It was seen that decreasing Al content was improving electrical and optical properties. Al concentration of the films was decreased firstly by decreasing deposition power of Al. After that, content was further decreased by depositing stacks of doped and undoped layers (modulation doping). Following that, modulation doped films were deposited with applying RF bias power to substrates. All films were annealed at 300oC for 1 hour in vacuum. The lowest resistivity obtained in this study was 1.68x10-3 .cm. Transmittance and band gap of the corresponding film were 80% and 4.1 eV respectively. The film was fabricated by modulation doping accompanied with substrate bias of 10 W, followed by annealing at 300oC in vacuum.
Growth and characterization of carbon nanotubes by thernal chemical vapor deposition method
(Izmir Institute of Technology, 2008) Aksak, Meral; Selamet, Yusuf
This thesis work is focused on producing carbon nanotubes (CNTs) by methane gas thermal chemical vapor deposition method on very thin Cobalt, Iron, and Nickel catalyst thin films deposited onto SiO2/Si substrates by DC magnetron sputtering. This thesis is also devoted to understanding some parameters affecting the growth of CNTs; such as catalyst material, temperature, and catalyst layer thickness effects In this study, CNT growth was performed on directly Si substrates, which was observed that the growth was too difficult and requiring very high temperatures. Hence, very thin catalyst films were deposited on SiO2/Si substrates, and the CNT growth was observed. The temperature effect was also examined. When the growth temperature was increased, the average diameters of the CNTs were decreased up to a critical temperature, but after this point the average diameter of CNTs were increased. This effect was studied systematically by utilizing Fe and Co catalyst thin films and with the help of Raman spectroscopy and Scanning Electron Microscopy results.Catalyst thickness effect was also examined. For this aim, Ni catalyst thin films with three different thicknesses; 0.7 nm, 1.4 nm, and 6 nm, were utilized. It was observed that CNTs were grown well on 0.7 and 1.4 nm thick Ni films, while there was a little growth on 6 nm thick Ni film. The roughness analysis of 0.7 nm and 1.4 nm thick Ni films were also done. Some of as-grown CNTs were also examined by X-ray diffraction method, and the results were compared one another.
Influence of Ni thin flim structural properties over graphene growth by CVD
(Izmir Institute of Technology, 2013) Özçeri, Elif; Selamet, Yusuf
This thesis work focused on the effect of polycrystalline Nickel (Ni) TM thin film structure on the growth graphene by chemical vapor deposition (CVD). TM films were deposited by magnetron sputtering technique on Si/SiO2 substrates. To grow 1-2 layer graphene on Ni thin film catalyst by methane decomposition thermal CVD method was carried out using various growth parameters. To reduce the TM film surface roughness and grow larger size graphene layers on Ni film, Si/SiO2 substrates were coated by a thin Al2O3 buffer layers and Cr adhesive layers by magnetron sputtering. Ni film crystal structure and surface roughness, which affected the number of graphene layers, were examined by X-ray Diffraction (XRD) and Atomic Force Microscopy (AFM) techniques, respectively. The thickness and columnar structure of the films were measured from Surface Profiler and Scanning Electron Microscopy (SEM) images. Ni films were annealed at 800 oC, 900 oC and 950 oC in order to improve their crystal quality and to evaluate the effect of the crystallinity on graphene growth at atmospheric pressure. Samples were studied using XRD and AFM also to assess their crystal quality after the annealing process. It was observed that the calculated grain sizes depended on the film thickness and the annealing temperature. Surface roughness of the films was increased by increasing film thickness. A sole thin Al2O3 buffer layer reduced the surface roughness significantly. However, sole Cr adhesive layer or Cr/Al2O3 buffer layers did not reduce the surface roughness, but increased the crystallinity of Ni films in (111) direction. Argon, Hydrogen or a mixture of these two gases was added to methane during graphene growth at ambient pressure by CVD. The Raman spectroscopy was utilized in order to determine the number of the layers and quality of graphene growth over the Ni catalyst film.
Magnetic characterization of expanded austenite phase formed on nitrogen ion implanted 316 stainless steel alloy
(Izmir Institute of Technology, 2015) Karataş, Özgün; Öztürk, Orhan; Selamet, Yusuf
Austenitic stainless steels (SSs) are technologically important alloys and highly resistant to corrosion in a variety of environments. Nevertheless, these materials have a few drawbacks; they are rather soft materials and susceptible to wear. Correspondingly, an improvement of the surface properties is often desirable. Ion beam techniques are widely used to enhance surface properties of these alloys. Surface modification of austenitic SSs by nitrogen ion beams at moderate substrate temperatures near 400 ºC, leads to the formation of a high N content phase. This phase, known as an expanded austenite phase, γN, creates a hard and wear resistant layer on the stainless steel. Additional property of this phase is related to its magnetic structure due to the large amount of nitrogen insertion and corresponding lattice expansion. In the current study, new data corresponding to structural and magnetic nature of the expanded austenite layers on austenitic 316 SS by low-energy, high-flux nitrogen ion implantation are presented. Phase and compositional analyses, surface topography and magnetic features of the nitrogen ion implanted layers were studied by a combination of experimental techniques involving XRD, SEM, AFM, MFM, VSM and MOKE. Nitrogen implantations were performed for 30, 90 and 240 minutes of processing time, at a fixed temperature near 400 °C. Relatively low-energy (0.7 keV) and high-flux (2 mA/cm2) ion beam conditions were carried out during the implantation. Combination of the aforementioned techniques provides strong evidence for the formation of the γN phase with mainly ferromagnetic characteristics. MFM imaging reveals stripe-like domain structures of the nitrogen ion implanted layers. Both VSM and MOKE analyses display hysteresis loops of the layers. Ferromagnetism in the γN layers are manifested by MFM, M and MOKE analyses. Ferromagnetic structure is linked to large lattice expansions 0 due to high nitrogen contents at. . s an interstitial impurity, nitrogen dilates fcc lattice of 316 SS i.e. Fe-Fe distance is increased, which strongly influences the magnetic interactions.
Optimization of carbon nanotube properties by controlled amount oxidizers
(Izmir Institute of Technology, 2011) Söylev, Deniz; Selamet, Yusuf
Finally, all data were subjected through a statistical analysis. From this study we have seen that the majority of the samples had Log-Normal diameter distributions by adjusting oxidizer amount and pretreatment timing. We were able to control CNT diameters within very narrow diameter ranges.This thesis work focuses on growing high quality carbon nanotubes using ethylene as hydrocarbon gas in thermal chemical vapor deposition method in presence of weak oxidizers. We carried out experiments to study the ratio of amount of CO2, O2 and H2O in pretreatment to growth and the timing of the metal-oxide to metal conversion in presence of the oxidizers.Firstly, the effects of CO2 addition were investigated on a variety of growth conditions to study the controlled diameter growth of CNTs. The main growth parameters controlled in this work in the CNT growth were CO2 level, pretreatment time and temperature. Pretreatment times of 15, 10, 5, 2 minutes were studied. CO2 flow rates were send in system during both pretreatment time and growth time at 10:1, 10:2, 5:1, 5:2, 2:1 and 2:2 sccm ratios, respectively. We also studied two different growth temperatures at 740 oC and 760 oC.Secondly, the influence of O2 for effective CNT growth was investigated by utilizing different pretreatment times and O2 levels. Pretreatment times of 15, 10, 5, 2 minutes were studied. O2 gas flow rates during pretreatment time and growth time were studied as 5:1, 5:2, 2:2, 2:1, 2:0.5, respectively. As a result, we obtained small diameters with the pretreatment time of 15 min. in the presence of O2.Thirdly, the effects of H2O vapor for effective CNT growth on Fe/Al2O3/SiO2 were examined. During the experiments, pretreatment time was kept at a constant value of 15 min. and H2O vapor content in Ar was varied. The results of examined analyses show that the best results were obtained at the growth 60 o C (A) / 60 o C (A).