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Characterisation of ZnO nanorod arrays grown by a low temperature hydrothermal method
(Taylor & Francis Ltd, 2012) Kahraman, S.; Cetinkara, H. A.; Bayansal, F.; Cakmak, H. M.; Guder, H. S.
In this paper, growth steps of well defined ZnO nanorod arrays deposited on seeded substrates were investigated. To obtain ZnO seed layer on glass substrates, a successive ionic layer adsorption and reaction (SILAR) method was used and then ZnO nanorods were grown on seed layer using a chemical bath deposition (CBD) method. The effects of seed layer and deposition time on morphology, crystallographic structure (e.g. grain size, microstrain and dislocation density) and electrical characteristics of ZnO nanorods were studied. From the SEM micrographs, it could be seen that the ZnO nanorods densely covered the substrate and were nearly perpendicular to the substrate surface. The XRD patterns showed that the ZnO nanorod arrays had a hexagonal wurtzite structure with a preferred orientation along the (002) plane. An increase in deposition time resulted in an increase in the intensity of the preferred orientation and grain size, but a decrease in microstrain and dislocation density. Electrical activation energies of the structures were calculated as 0.15-0.85 eV from current-temperature characteristics. It was concluded that the morphologies of the structures obtained in this study via a simple and fast solution method can provide high surface areas which are important in area-dependent applications, such as solar cells, hydrogen conversion devices, sensors, etc.
Characteristics of ZnO thin films doped by various elements
(Elsevier Science Bv, 2013) Kahraman, S.; Cakmak, H. M.; Cetinkaya, S.; Bayansal, F.; Cetinkara, H. A.; Guder, H. S.
We have investigated the effects of Al, K and Co dopant elements on the properties of ZnO thin films deposited by CBD method on glass substrates. Changing in morphology, structural parameters, ionization energies of impurity levels, absorption behavior and optical band gap values were investigated through scanning electron microscopy (SEM), X-Ray diffraction (XRD), resistance-temperature measurement (R-T) and ultraviolet-visible spectroscopy (UV-vis) techniques. From the SEM observations, various morphologies (rod-like, flower-like and rice-like) were observed. Those morphological variations were attributed to the change in stable growth mechanism of intrinsic ZnO, induced by different atomic radius and different electronegativity of dopants. XRD results indicated that all orientations are well indexed to hexagonal phase crystalline ZnO. The impurity level ionization energy values (Delta E) were estimated as 0.32/0.13/0.07 eV; 0.34/0.15 eV; 0.40/0.13 eV and 0.48/0.22 eV for the Al, K, Co doped samples and i-ZnO, respectively. Optical band gap values were found that the doped samples' were higher than the intrinsic one's. This increasing (blue shift) was attributed to a deterioration which occurred in the lattice of the structures after doping. This effect was also supported by the structural results. (C) 2012 Elsevier B.V. All rights reserved.
Characterization of CBD grown ZnO films with high c-axis orientation
(Elsevier Science Sa, 2012) Kahraman, S.; Bayansal, F.; Cetinkara, H. A.; Cakmak, H. M.; Guder, H. S.
Highly c-axis oriented ZnO films were deposited on seeded glass substrates. Successive ionic layer adsorption and reaction (SILAR) method and chemical bath deposition (CBD) method were used to obtain seed layers and ZnO films. To see the effects of seed layer and deposition time, structural (e.g. grain size, microstrain and dislocation density), morphological, and electrical (e.g. resistivity, activation energy) properties of the films were investigated by scanning electron microscopy, X-ray diffraction, and four point probe method. From the SEM images, resultant structures were found as well defined nanorods nearly perpendicular to the substrate surfaces and densely cover the substrates. The XRD patterns showed that ZnO films have hexagonal wurtzite structure with a preferred c-axis orientation along (002) plane. C-axis orientation was also supported by texture coefficient calculations. The lattice parameters of the structures were determined as a = 3.2268 angstrom, b = 5.2745 angstrom, alpha = beta = 90 degrees and gamma = 120 degrees. From the XRD patterns, it was revealed that, microstrain and dislocation density values of the structures decreased whereas grain size increased. This was attributed to enhancement occurred in lattice structure of the ZnO films. Activation energy values of the films were found in between 0.12 and 0.15 eV from the dark electrical resistivity-temperature characteristics in a temperature range of 300-500 K. (c) 2012 Elsevier B.V. All rights reserved.
A comparative study of Cu2ZnSnS4 thin films growth by successive ionic layer adsorption-reaction and sol-gel methods
(Elsevier Science Sa, 2014) Kahraman, S.; Cetinkaya, S.; Cetinkara, H. A.; Guder, H. S.
Owing to the high natural abundance and non-toxicity of all the constituents, the copper-zinc-tin-sulfide compound has been attracting attention in recent years for the production of cheap solar absorber materials. Solution-based low-cost approaches are being developed for the deposition of this compound. In this comparative study, we have investigated Cu2ZnSnS4 thin films prepared by using successive ionic layer adsorption-reaction and sol-gel methods. X-ray diffraction studies indicated the polycrystalline nature of the films. No secondary phases were observed. The sol-gel grown Cu2ZnSnS4 film was found to have smaller crystallite size and higher microstrain and dislocation density values. Phase purity and good crystalline quality of the studied films were proved through the Raman studies. From the scanning electron microscopy images, sol-gel grown Cu2ZnSnS4 film was found to have more homogenous and smooth morphology. Possible chemical formula of the films was determined. The optical absorption of the sol-gel grown film covered a wider wavelength range by means of absorbing more visible photons. The optical band gap values were estimated to be 1.45 and 1.40 eV for the successive ionic layer adsorption-reaction and sol-gel grown samples, respectively. It was concluded that both methods are effective and can be used to produce high-quality Cu2ZnSnS4 thin films for cheap and green solar cell applications. (C) 2013 Elsevier B. V. All rights reserved.
The effects of coumarin additive on the properties of ZnO nanostructures
(Pergamon-Elsevier Science Ltd, 2013) Kahraman, S.; Cakmak, H. M.; Cetinkaya, S.; Cetinkara, H. A.; Guder, H. S.
In this study, we investigated the effects of coumarin presence in the growth solution on the properties of ZnO. Significant changes/improvements were observed in morphology and crystal structure. From the morphological studies, it was seen that the substrates were uniformly covered with rod/rice-like nano-structures and average diameters of the structures decreased with increasing coumarin content in the growth solution. Also coumarin affected the surface morphology, coverage density and uniformity of the shape of the structures. The X-ray diffraction patterns indicated that all diffraction peaks were well indexed to hexagonal phase crystalline ZnO. The greater intensities of (0 0 2) peaks in the patterns revealed c-axis orientation normal to the substrate. The crystallization of the samples was gradually enhanced with increasing coumarin volume up to 5%. When the coumarin volume exceeded 5%, the crystallization was affected negatively. The electrical resistivity values increased with increasing coumarin content and it was attributed to the decrease in carrier concentration originated by carrier traps at the grain boundaries. (C) 2012 Elsevier Ltd. All rights reserved.
Effects of diethanolamine on sol-gel-processed Cu2ZnSnS4 photovoltaic absorber thin films
(Pergamon-Elsevier Science Ltd, 2014) Kahraman, S.; Cetinkaya, S.; Cetinkara, H. A.; Guder, H. S.
As a promising solar absorber, the Cu2ZnSnS4 compound has been popular recently for the production of green and economical thin-film solar cells owing to the abundancy and non-toxicity of all the constituents. In this study, we have produced Cu2ZnSnS4 films via the sal-gel technique. As a stabilizer, the effects of the diethanolamine on the properties of the films were investigated. The amount of diethanolamine significantly affected the crystal structure, crystallite sizes and phase purity of the films. X-ray diffraction and Raman spectroscopy analyses confirmed the formation of phase-pure CZTS films. It was found that the film produced by using 2 ml of diethanolamine in sol exhibited pure CZTS phase, compact and dense morphology and enhanced photo-sensitivity. Light on/off current ratio of the n-Si/Cu2ZnSnS4 junction was found to be 47 under 100 mW/cm(2) of illumination. Electrical activation energies of the films were investigated and the variations were attributed to delocalized phonon states generating from the presence of other phases and lattice defects. (C) 2013 Elsevier Ltd. All rights reserved.
Effects of different annealing atmospheres on the properties of cadmium sulfide thin films
(Pergamon-Elsevier Science Ltd, 2015) Yucel, E.; Kahraman, S.; Guder, H. S.
Cadmium sulfide (CdS) thin films were prepared on glass substrates by using chemical bath deposition (CBD) technique. The effects of different annealing atmospheres (air and sulfur) on the structural, morphological and optical properties of CdS thin films were studied at three different pH values. Compactness and smoothness of the films (especially for pH 10.5 and 11) enhanced after sulfur annealing. pH value of the precursor solution remarkably affected the roughness, uniformity and particle sizes of the films. Based on the analysis of X-ray diffraction (XRD) patterns of the films, micro-strain and dislocation density values of the sulfur-annealed films (pH 10.5 and 11) were found to be lower than those of air-annealed films. Air-annealed films (pH 10.5, 11 and 11.5) exhibited higher transmittance than sulfur-annealed films in the wavelength region of 550-800 nm. Optical band gap values of the films were found between 2.31 eV and 236 eV. (C) 2015 Elsevier Ltd. All rights reserved.
Effects of the sulfurization temperature on sol gel-processed Cu2ZnSnS4 thin films
(Elsevier Sci Ltd, 2013) Kahraman, S.; Cetinkaya, S.; Podlogar, M.; Bernik, S.; Cetinkara, H. A.; Guder, H. S.
As a promising and alternative solar absorber material, the copper zinc tin sulfide compound (Cu2ZnSnS4) has been drawing attention in recent years for the production of cheap thin-film solar cells owing to the high natural abundance and non-toxicity of all the constituents, a tunable direct-band-gap energy and a large optical absorption coefficient. In addition, to overcome the problem of expensive vacuum-based methods, solution-based approaches are being developed for Cu2ZnSnS4 deposition. In this study, we have produced Cu2ZnSnS4 thin films via the sol gel technique and subsequent sulfurization. The effects of the sulfurization temperature on the structural, morphological, compositional and optical properties of the films were investigated. X-ray diffraction and Raman spectroscopy analyses confirmed the formation of phase-pure CZTS films. The crystallinity of the films increased with an increasing sulfurization temperature. From the surface images and the results of the composition analysis, it was found that the films are uniform, composed of homogenously distributed grains and have compositions with Cu deficit. The values of the optical absorption coefficients for the films were found to be 10(4) cm(-1) based on absorbance spectroscopy. The optical band-gap values were estimated to be between 1.32 and 2.27 eV depending on the sulfurization temperature. (C) 2013 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Effects of thermal oxidation temperature on vacuum evaporated tin dioxide film
(Academic Press Ltd- Elsevier Science Ltd, 2012) Cakmak, H. M.; Cetinkara, H. A.; Kahraman, S.; Bayansal, F.; Tepe, M.; Guder, H. S.; Cipiloglu, M. A.
In order to investigate the effect of thermal oxidation temperature on tin dioxide (SnO2), tin dioxide films were obtained on quartz substrates by vacuum evaporation of tin metal. The films were characterized by X-ray diffraction (XRD) analyses, scanning electron microscopy (SEM), temperature dependent electrical resistivity measurement and optical absorption spectroscopy. The SEM images showed that the films are dense, continuous and are composed of nanoparticles and particle sizes are increased after thermal oxidation. From the X-ray measurement results, the films indicated two strong reflection peaks of tetragonal structure in the orientations of (101) and (200) at 20 = 33.8 degrees and 37.95 degrees, respectively. Intensity of the peaks increased with increasing thermal oxidation temperature. We found resistivity values of about 10(-4) Omega-cm. Optical absorption spectra of the films in the UV-Vis spectral range revealed that optical band gap (E-g) value of the films increases with increasing thermal oxidation temperature. (C) 2012 Elsevier Ltd. All rights reserved.
Electrodeposition and properties of Zn, Zn-Ni, Zn-Fe and Zn-Fe-Ni alloys from acidic chloride-sulphate electrolytes
(Taylor & Francis Ltd, 2009) Karahan, I. H.; Guder, H. S.
Zn, Zn-Ni, Zn-Fe and Zn-Fe-Ni films have been deposited by electrochemical deposition technique onto steel plate substrates. The objective of this study was to characterise the corrosion properties of these alloys in saline solution for the application as new environmentally friendly sacrificial coatings in the protection of steel structures. The morphological and structural properties of the alloys were systematically studied using XRD and SEM techniques. Cyclic voltammetry of the individual metals was performed to help understand the electroplating process of the films. Grain sizes of the films were calculated using Scherrer's formula. Partial substitution of Zn to Fe and Ni leads to an improvement in the corrosion resistance. Compared with other zinc alloys, the Zn-Ni alloy deposit was the noblest.
Fabrication and characterization of Mn-doped CuO thin films by the SILAR method
(Elsevier Sci Ltd, 2013) Gulen, Y.; Bayansal, F.; Sahin, B.; Cetinkara, H. A.; Guder, H. S.
Thin films of un-doped and Mn-doped CuO nanostructures have been deposited on glass substrates via the SILAR method. The morphological, compositional, structural and optical properties of the films have been investigated by scanning electron microscopy, energy dispersive spectroscopy, X-ray diffraction analysis and UV vis spectrophotometry. The analyzed results indicate that the obtained films consist of plate-like nanostructures. From the EDS analysis it is seen that Mn-doping concentration affects the shapes of the nanostructures. XRD results show that all of the films have monoclinic structure. From the room temperature UV-viS analysis it is found that the optical band gap of the films increases with increasing Mn-doping concentrations. (C) 2013 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Growth of homogenous CuO nano-structured thin films by a simple solution method
(Elsevier Science Sa, 2011) Bayansal, F.; Kahraman, S.; Cankaya, G.; Cetinkara, H. A.; Guder, H. S.; Cakmak, H. M.
This paper describes a simple, low temperature and cost effective solution method to synthesize homogenous cupric oxide (CuO) nano-structured thin films. By this method dense and continuous CuO films with good crystallinity can be prepared in a short time, e.g., in 15 min. The scanning electron microscopy illustrated that the synthesized CuO plate-like nanostructures have RMS thicknesses of 100 nm. The XRD measurements showed that the synthesized CuO nanostructures have a high crystallinity with monoclinic crystal structure preferentially in ((1) over bar 1 1) and (1 1 1) directions. From the temperature dependant dark electrical resistivity measurements the ionization energies of the impurity levels and thermal band gap energies of the films were calculated. (C) 2010 Elsevier B.V. All rights reserved.
Influence of coumarin as an additive on CuO nanostructures prepared by successive ionic layer adsorption and reaction (SILAR) method
(Elsevier Science Sa, 2013) Bayansal, F.; Sahin, B.; Yuksel, M.; Biyikli, N.; Cetinkara, H. A.; Guder, H. S.
The effect of coumarin doping during the growth of CuO nanostructures by SILAR method has been studied. It was found that coumarin consider ably influences the growth process, manipulates the band-gap and modifies the crystallite size of the films. XRD experiments evidenced that with higher coumarin concentrations in the growth solution, the microstrain and dislocation density increased, while the crystallite size of the films decreased. SEM images revealed that the thicknesses of the plate-like nanostructures decreased with increasing coumarin concentration. By UV/vis spectrophotometer analysis it is found that the coumarin concentration affects both the optical band gap and the transmission rate: both the band gap and spectral transmittance values of the films decreased for higher coumarin content. (C) 2013 Elsevier B. V. All rights reserved.
Nano-structured CuO films prepared by simple solution methods: Plate-like, needle-like and network-like architectures
(Elsevier Sci Ltd, 2012) Bayansal, F.; Cetinkara, H. A.; Kahraman, S.; Cakmak, H. M.; Guder, H. S.
Two simple, safe and environmental friendly chemical solution methods which are suitable to mass production, applicable at low temperatures and cost effective, were used to prepare homogenous nano-structured cupric oxide (CuO) films. By using different types of substrates (glass, quartz, Cu foil and Si) and techniques various types of CuO nanostructures (plate, needle and network like) were produced. The products were characterized by scanning electron microscopy, X-ray diffraction and temperature dependant dark electrical resistivity measurements in order to investigate the effects of substrate on morphology, crystallographic structure and electrical properties of the films. From these characterizations it was seen that there are significant differences in the morphological, crystallographic, structural and electrical properties of the nanostructures that were produced by different techniques and/or on different substrates. Besides, growth mechanisms of CuO films have been investigated. (C) 2011 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
On the effects of air-exposure-time and time-dependency of Pb/p-Si Schottky diodes
(Elsevier Science Bv, 2010) Cetinkara, H. A.; Guder, H. S.
The effects of air-exposure-time and time-dependency (ageing) on the Pb/p-Si Schottky diodes have been investigated by room temperature I-V and C-V measurements. The barrier height (BH) values between I-V and C-V measurements of the air-exposed samples have been found to be higher than those of the reference diode, and associated with the passivation of the intrinsic surface states on the cleaned Si surface. The ideality factor and the BH values of the air-exposed samples have reached to saturation after ten days air-exposure and the case has been interpreted as the saturation of the oxide layer thickness. The ageing has increased the ideality factors while decreasing the BHs of the samples with increasing ageing time. Both of the parameters have reached to the saturation at 720 h after metal deposition. The carrier concentrations of the air-exposed samples have been considerably lower than that of the reference diode and decreased with increasing exposure-time to air and ageing. This result has been attributed to the passivation of the acceptors by hydrogen atoms diffused into the semiconductor and so the surface Fermi level unpinning. The BH values determined from C-V measurements have reached to equilibrium BH values at 720 h after metal deposition as determined from I-V measurements. (C) 2010 Elsevier B.V. All rights reserved.
Preparation of Fine-Grained Silicon-Nitride Ceramics and their Characterization by Depth-Sensing Indentation Tests
(Polish Acad Sciences Inst Physics, 2015) Sahin, O.; Guder, H. S.; Uzun, O.; Sahin, E.; Sopicka-Lizer, M.; Gocmez, H.; Artunc, E.
Both pressureless-sintered and dense, fine-grained silicon nitride ceramics were produced from mechanochemically activated nitride-based precursors. Scanning Electron Microscopy (SEM), Transmition Electron Microscopy (TEM), X-Ray Diffraction (XRD) and an ultra-low load microhardness tester were used to characterize these ceramics. Depth-sensing indentation (DSI) tests in the range of 200-1800 mN were performed on the silicon nitride ceramic to determine dynamic hardness (H-d) and reduced elastic modulus (E-r) values. These values were deduced by analyzing the unloading segments of the DSI curves. It was found that both H-d and E-r exhibits a significant indentation load dependence. Nix-Gao (NG) model was used to analyze the dynamic hardness data in the calculation of the load independent hardness value.
Structural and optical properties of Ba-doped CdO films prepared by SILAR method
(Academic Press Ltd- Elsevier Science Ltd, 2014) Sahin, B.; Gulen, Y.; Bayansal, F.; Cetinkara, H. A.; Guder, H. S.
Nanostructure materials have opened a new discussion in the field of semiconductor device technology because material properties could be changed by changing the crystal morphology and size. In this article, Ba doped CdO films were prepared by SILAR method. The crystal morphology, structure and optical properties of the films were characterized by scanning electron microscopy, X-ray diffraction and ultraviolet-visible spectroscopy, respectively. From the SEM analysis it is seen that Ba-doping concentration affects the shapes of the nanostructures. XRD analysis showed that the films have poly-crystalline structures. The room temperature UV-vis analysis showed that the optical band gap of the CdO films was firstly decreased then increased with the increasing barium doping concentration. (C) 2013 Elsevier Ltd. All rights reserved.
Synthesis and characterization of undoped and tin-doped ZnO nanostructures
(Springer Heidelberg, 2012) Kahraman, S.; Bayansal, F.; Cakmak, H. M.; Cetinkara, H. A.; Guder, H. S.
In this paper, undoped and tin-doped ZnO nanostructures were grown onto non-conductive substrates by a simple solution method. Structural, morphological, optical and electrical properties of the structures were investigated with respect to tin concentration. From XRD studies, all the ZnO nanostructures were found as hexagonal wurtzite type structures growing preponderantly oriented with c-axis normal to the substrate. An increase in tin content resulted in a decrease in grain size, whereas the dislocation density increases. SEM observations indicated that all the structures were textured throughout the substrates without any cracks or pores. The influence of incorporation of tin on surface morphology of the samples was clearly seen. Average diameter of the nanostructures decreased with increasing tin content. Absorption spectra of the structures revealed that the band gap of the films increases with increasing tin concentration. It is found that the tin-doped samples have higher average transmittance than the undoped one. The 1 % tin-doped sample exhibited similar to 80 % average transparency, which was the best transparency among the doped samples. Electrical measurements showed that resistivity of the structures increased with increasing dopant concentration. This increasing was attributed due to a decrease in carrier concentration caused by carrier traps at the grain boundaries.
Vickers and Knoop Indentation Microhardness Study of ?-SiAION Ceramic
(Polish Acad Sciences Inst Physics, 2011) Guder, H. S.; Sahin, E.; Sahin, O.; Gocmez, H.; Duran, C.; Cetinkara, H. Ali
In this paper, analysis and interpretation of mechanical property measurements of beta-SiAION ceramic were reported. Indentation microhardness of beta-SiAION ceramic was measured using the Knoop and Vickers indenters. The analysis of the Vickers indentation microhardness data reveals the reverse indentation size effect that is the apparent microhardness increases with increasing applied indentation test load. However, the Knoop indentation microhardness data exhibit indentation size effect that is the apparent microhardness increases with decreasing applied test load. The experimental Knoop microhardness data was analyzed using Meyer's law, elastic-plastic deformation model, proportional specimen resistance model, and Hays-Kendall's model. As a result, modified proportional specimen resistance model is found to be the most effective one for the load-independent (Hu) microhardness determination of the SiAION ceramic. It was seen that different models used to analyze the data obtained from the Vickers indentation do not give the same intrinsic hardness value. We also present the calculation of the Young modulus, E, of the beta-SiAION ceramic.

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