2. Thesis and Dissertations

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Doping of Vacuum Deposited Zinc Oxide Thin Films
(National Institute of Technology Karnataka, Surathkal, 2013) Palimar, Sowmya; Shivakumar, G. K.; Bangera, Kasturi. V.
The main objective of present work is to study the doping of vacuum evaporated zinc oxide (ZnO) thin films. Initially, optimum conditions required to obtain good quality of undoped ZnO thin films is determined by depositing films using two evaporation sources (boats) namely molybdenum and tungsten and annealing them under different conditions. Compositional analysis of films showed incorporation of boat atoms in to ZnO thin films prepared using molybdenum as well as tungsten boat. A considerable reduction in atomic percentage incorporated boat atoms on annealing was observed in both cases. ZnO thin films obtained under optimum conditions were found to be amorphous in nature with good combination of visible region transmittance of up to 90% and room temperature conductivity of 92 Ω−1cm−1. XPS analysis has shown that the film is approximately stoichiometric with slight oxygen deficiency. From the measurements of activation energy it is observed that ZnO thin film is having two donor levels below the conduction band. Further, ZnO films are doped with third group dopants to improve their n-type conductivity. Investigation has been carried out to know the optimum percentage of dopnat to be added to retain the transmittance of the film. Role of third group elements as n-type dopants in the form of pure metals and metal oxides is studied by doping the film with indium and indium oxide separately. It is observed that contribution of indium oxide dopants is more than indium dopants in improving the conductivity of films. ZnO films were then doped with other two third group oxides, namely gallium oxide and aluminum oxide. From structural, optical and electrical properties of these oxide doped ZnO films it is found that all films have smooth surface with visible region transparency of above 90% and significantly high room temperature conductivity of the order of 103 Ω−1cm−1, which are well suited for the application of transparent electrodes.
Studies on Growth, Optical And Electrical Properties of Doped and Undoped Zinc Oxide Thin Films
(National Institute of Technology Karnataka, Surathkal, 2013) K. K., Nagaraja; Nagaraja, H. S.
In recent years zinc oxide (ZnO) has attracted the researchers because of its excellent optical and electrical properties. The primary aim of this work is to deposit undoped and doped ZnO thin films by sputtering techniques and to study the structural, optical and electrical properties. The thin films were deposited on glass and quartz substrates at room temperature using DC and RF magnetron sputtering. The studies on the effects of annealing on the structural and third-order nonlinear optical properties of ZnO thin films deposited on quartz substrates has been carried out under cw He-Ne laser irradiation at 633 nm wavelength using z-scan technique. The enhanced nonlinear response of the films was observed with the increase in the annealing temperature. X-ray diffraction (XRD) patterns show the appearance of crystalline phases of SiO2 at higher annealing temperatures. The appearance of extraneous phase was confirmed by atomic force microscope (AFM) images and optical transmittance spectra. Multiple diffraction rings due to the refractive index change and thermal lensing were observed when the samples were exposed to laser beam. Also, the films exhibited strong optical limiting properties. Transparent conducting aluminum doped zinc oxide (AZO) films were deposited on glass substrates. Variation of stress values and the lattice parameters confirms the presence of Al3+ ions in the ZnO matrix. Electrical resistivity of the deposited films was found to be as low as 0:5×10−4Ω-cm. XRD patterns of the ZnO/Al/ZnO multilayers show only reflections corresponding to Wurtzite ZnO. The sheet resistance of the multilayers found to decrease with the increase in the Al interlayer thickness. But, the average transmittance in the visible region decreases. The XRD results of manganese doped ZnO (MZO) films deposited using compound targets show the decrease of lattice parameters a and c after doping of Mn into ZnO. The third-order nonlinear optical susceptibility χ(3) is found to be of the order of 10−3 esu for MZO films. Keywords: ZnO; magnetron sputtering; XRD; TCO; AFM; CW laser; NLO
Effect of Pulsed Laser Deposited Ceramic Coatings on Microhardness and Corrosion Behavior of Titanium, Ti6al4v and Inconel
(National Institute of Technology Karnataka, Surathkal, 2013) C, Sujaya; Shashikala, H.D.
Coatings are used to modify and increase the functionality of a bulk surface or substrate without modifying the bulk properties of the material. The present work aims at obtaining uniform adhesive coatings of alumina and silicon carbide on different substrates viz., titanium, Ti6Al4V and inconel by pulsed laser deposition technique using Q-switched Nd: YAG laser at low temperature. Processing parameters such as laser fluence, substrate target distance, substrate temperature and target density during deposition were standardized to get adhesive films. Coated films were characterized using scanning electron microscopy, energy dispersive X-ray spectroscopy, spectrophotometer, optical microscope, nanoindentation, surface roughness measurements using 3D optical profilometer, adhesion test. Microhardness and corrosion studies were carried on substrates and after coating. Composite microhardness of ceramic coated substrates was measured using Knoop indenter and its film hardness was separated from composite hardness using a mathematical model based on modified area-law of mixture. Then by including indentation size effect the film hardness was compared with values obtained using nanoindentation method. Composite hardness as well as film hardness of the ceramic coating was found to be higher compared to the substrates. Corrosion behavior of substrates after ceramic coating was studied using 3.5% NaCl solution by potentiodynamic polarization and electrochemical impedance spectroscopy measurements. The Nyquist and the Bode plots obtained from the electrochemical impedance spectroscopy data are fitted by appropriate equivalent circuits. The pore resistance, the charge transfer resistance, the coating capacitance and the double layer capacitance of the coatings were obtained from the equivalent circuit. Alumina coated substrates showed more corrosion resistance than silicon carbide coated substrates. After the corrosion testing, the surface topography of the uncoated and the coated system were examined under scanning electron microscopy. Experimental results confirmed the possibility of using Nd: YAG laser for ceramic film deposition which improves the microhardness and corrosion resistance of the substrate considerably
Study of Doping In Zinc Oxide in Thin Films Grown by Spray Pyrolysis Technique
(National Institute of Technology Karnataka, Surathkal, 2014) Kumar N, Sadananda; Bangera, Kasturi V.; Shivakumar, G. K.
The main objective of present work is to study the effect of doping on the optical and electrical properties of spray deposited zinc oxide (ZnO) thin films. The optimum conditions required to obtain quality of ZnO thin films is determined initially by varying the deposition parameters such as substrate temperature, spray rate and precursor molarity. Later film thickness and annealing temperature is fixed by considering the transmittance and conductivity of the films. ZnO thin films obtained under optimum conditions were found to be crystalline in nature with the combination of visible region transmittance of 78 % and room temperature conductivity of 0.156 S/cm. XPS analysis has been shown that the as-deposited film is oxygen deficient and annealed films are oxygen rich. Further, ZnO films were doped with group V and group III dopants to improve their conductivity. Investigation has been carried out to know the optimum percentage of dopant to be added to retain the transmittance of the films by varying dopant concentration from 0 to 5%. The effect of group V elements as dopants in the form of pure metals has been studied by doping the film with antimony and bismuth separately. A slight improvement in the conductivity of the films was observed with antimony and bismuth doping without changing optical transmittance in the visible region. ZnO film doped with Sb (3% and above) shows ptype conductivity. Role of group III elements as n-type dopants in the form of pure metals is studied by doping the film with aluminium, indium and gallium separately. It has been observed that all the doped films possess transparent and highly conducting properties. The 3% doping concentration and annealing at 450°C for 4hr is the optimum condition to achieve transparent and high conductive ZnO films doped with Al, In and Ga. From the structural, optical and electrical properties of these doped ZnO films it is found that all films have smooth surface with visible region transparency of 80% and significantly high electrical conductivity of the order of 102 S/cm at room temperatures, which are well suited for the transparent electrodes application.
Studies on Laser Induced Fluorescence in Subsonic and Supersonic Jet Using Ketone Tracers
(2014) Vikas M, Shelar; Umesh, G.; Ramachandran, G.
The present work focuses on the study of LIF from ketone based tracers for gas flow visualization. For all these studies a frequency quadrupled, Q switched, Nd:YAG laser (266 nm) was used as an excitation source. The quenching effect of oxygen on LIF of acetone, Methyl Ethyl Ketone (MEK) and 3-pentanone was studied quantitatively at low pressures (~700 torr) with oxygen partial pressures upto 450 torr. Nitrogen was used as a bath gas into which these molecular tracers were added in different quantities according to their vapor pressure at room temperature. Further Smoluchowski theory was used to calculate the quenching parameters and compared with the experimental results. The molecular density distribution measurement in turbulent nitrogen jet (Re 3X103), using acetone and MEK tracers was demonstrated. Emitted fluorescence images of subsonic jet flow field were recorded on CMOS camera. The dependence of PLIF intensity on acetone vapor density was used to convert PLIF image of nitrogen jet into the density image on pixel by pixel basis. Instantaneous quantitative density image of nitrogen jet, seeded with acetone was obtained. Arrow head shaped coherent turbulent structures were observed in our experiments. PLIF imaging was used for supersonic jet, using acetone as molecular tracer. For supersonic jet, the fluorescence images were recorded on ICCD camera. Significant decrease in the PLIF intensity due to the presence of oxygen was clearly observed. The dependence of PLIF intensity on binning of images and air pressure was studied. It was demonstrated that even with the presence of the condensation of acetone, one can obtain good quality PLIF images. It was also shown that binning and Gaussian image processing of steady state jet PLIF images can further improve the image quality for quantitative analysis.
Investigation of thirdorder nonlinear optical and all-optical switching properties of some metalorganic materials
(National Institute of Technology Karnataka, Surathkal, 2014) K. B, Manjunatha; Umesh, G.
Nonlinear optical (NLO) materials and their structural engineering have gained growing importance in the field of nonlinear optics due to their large diverse applications in optoelectronics and photonics. In our efforts to identify the structure dependent NLO properties, we have investigated three different classes of organometallic complexes, viz. (i) palladium complexes with Schiff base ligand containing triphenylphosphine, (ii) ruthenium complexes with 1,3 dithiole 2 thione- 4,5-dithiolate (dmit) and triphenylarsine/ triphenylphosphine ligands and (iii) ruthenium complexes of salen/salophene ligands. The solutions of these complexes were prepared by dissolving them into dimethylformamide (DMF) solvent. Solid films were formed by first dispersing the complexes in Polymethylmethacrylate (PMMA) solution and then spin-coating them on glass substrates. For linear optical characterization, UV-Vis absorption spectra and linear refractive indexes were determined for both solution and film form. Third-order nonlinear optical properties of the complexes were determined using Z-scan and Degenerate four wave mixing (DFWM) techniques with laser pulses of 7 ns at the wavelength of 532 nm. Using energy dependent transmission technique, optical power limiting performance of the complexes were studied. Further, all-optical switching behavior of the complexes were investigated using two color pump-probe technique. All complexes exhibit negative nonlinear refractive index (self-defocusing) and nonlinear absorption due to reverse saturable absorption (RSA) which is explained using five-level model. All the complexes investigated by us have shown good optical power limiting as well as all-optical switching behavior. This thesis provides an insight on the role of electron acceptor and electron donor substituent and the effect of !-conjugation and electron delocalization on third-order nonlinear optical properties of the metal-organic complexes. Further, the suitability of such materials for devices such as optical power limiters and all-optical switches is also discussed.
Magnetic Properties of Cobalt Nanostructures Grown Using Porous Anodic Alumina Membrane
(National Institute of Technology Karnataka, Surathkal, 2016) S, Boominathasellarajan; Nagaraja, H. S.; Chowdhury, Prasanta
In this report, the magnetic nanostructure of Co were grown through the nanopores of Anodic aluminum oxide (AAO) template using both electrochemical and physical vapor deposition (PVD) techniques. Detailed investigations on the fabrication of highly ordered nanoporous AAO template in different electrolyte baths were presented. Ordered domain structure with pore diameter of 40±5 nm was achieved in an oxalic acid electrolyte bath at a temperature of 10 ºC and potential of 40 V. Whereas, in H2SO4 electrolyte bath, the optimized pore diameter was obtained as 22±4 nm while anodizing at a temperature around 3 ºC and at a potential of 25 V. As grown membranes were further processed to vary pore diameters without modifying the interpore separation from 40 to 100±10 nm, and 22 to 45±4 nm through a wet chemical route. Structural investigations revealed that Co nanowires have different crystallographic texture while growing them in different conditions: such as pore diameter of the template, the length of the nanowire, pH of the electrolyte, and electrodeposition modes. These changes in crystallographic structure in Co nanowire were reflected while studied their magnetic properties. Both of these structural and magnetic properties were then correlated with a micromagnetic simulation using OOMMF package and detailed understanding of magnetization reversal processes were presented. These studies were further extended by reducing the aspect ratio between the length to wire diameter less than unity and entered into a nanodot array regime trough PVD techniques using attached ultrathin AAO membrane. The magnetic properties of fabricated both soft and hard magnetic nanodot arrays were studied and the experimental results were presented.
AB Initio Studies of the Ground State Structure and Properties of Boron Carbides and Ruthenium Carbides
(National Institute of Technology Karnataka, Surathkal, 2016) G, Harikrishnan; K. M, Ajith
This work investigates the ground state structure and properties of Boron Carbides (B12C3 and B13C2 stoichiometries) and Ruthenium Carbides (RuC, Ru2C and Ru3C stoichiometries), each belonging to a class of hard materials. Exhaustive crystal structure search using evolutionary algorithm and density functional theory is performed in each of these stoichiometries. The lowest energy structures emerging from the structure search are further relaxed and their ground properties are computed using DFT. The work in B12C3 stoichiometry provides the first independent confirmation using structure search that B11Cp(CBC) is the ground state structure of this stoichiometry. It is established that mechanically and dynamically stable structures with base-centered monoclinic symmetry can be at thermodynamical equilibrium at temperatures up to 660 K in B12C3, raising the possibility of identifying the monoclinic symmetry in experimental measurements. A demonstration of experimentally identifiable signatures of monoclinic symmetry is provided through the computed cumulative infrared spectrum of some of the systems. The work in B13C2 stoichiometry has conclusively solved the long standing problem of the discrepancy between the DFT calculations and the experimental observations over the semiconducting nature of B13C2. The remarkable success of a newly identified 30-atomcell structure in explaining many of the experimental data on B12C3 and B13C2 provides the first definitive evidence that structures with larger unit cells, are associated with crystals of these stoichiometries even at the ground state. The work in Ruthenium Carbide stoichiometries has gathered into a coherent perspective the widely varying structures proposed from experimental reports of synthesis, computational modeling and crystal structure search and provided conclusive structural candidates to be pursued in experiments. The study of the pressure-induced variation of their stability and properties has set indicators and benchmarks for future experimental investigations. The estimation of hardness of all the systems has underlined their importance in many applications, with nearly superhard values for some of them.
Electrical Studies on IIVI Compound Semiconductors for Device Applications
(National Institute of Technology Karnataka, Surathkal, 2016) Shashidhara; Shivashankar, G. K.; Bangera, Kasturi V.
II-VI compounds are vital materials for high performance optoelectronic devices such as photovoltaic, light detecting, light emitting diodes and laser diodes in the blue-green to ultraviolet spectral range. The direct band gap ranging over entire visible region and high absorption coefficient of these materials are main features that make them very attractive for such applications. Studies carried out so far on these materials have provided valuable insight into both fundamental and application aspects. In thin film form, these compounds are of interests for optoelectronic applications. However, the improvement in the efficiency of these devices was rather slow, due to the difficulty in doping and lack of control over the defects. These compounds are known to exhibit high electrical resistivity with low intrinsic carrier concentration and low carrier mobility in the thin film form. In addition, the selfcompensation effects involving defects restrict the conductivity of the films. An effective utilization of these compound semiconducting thin films can only be possible by improving the carrier density and/or mobility of the carriers. Detailed investigations are needed to achieve these improvements. In this thesis, an attempt was made to understand the electrical transport properties in some of the technologically important II-VI compound heterojunctions. As a first step, a detailed study including structural, compositional, and electrical characterizations were carried out on CdTe, CdSe, ZnTe, and ZnSe films grown by vacuum evaporation. Further, the effect of substrate temperature on the properties of these compounds was assessed. It was found that as the substrate temperature increased above room temperature, the composition and crystalline quality improved and hence, the electrical conductivity. Among the four compounds studied, CdTe had high resistivity; therefore, more attention was paid to improve its electrical conductivity through doping with indium and by adding excess Te. The CdTe films were further annealed in air and vacuum to study the effect of annealing on electrical properties of the films. Indium doped films showed n type conductivity and tellurium rich films showed p-type conductivity. The CdTe films showed improvement in electrical conductivity with increasing dopant concentration.Further, four different combinations of heterojunctions (p-CdTe/n-ZnSe, nCdSe/p-ZnTe, n-CdTe/p-Si, and p-CdTe/n-Si) were fabricated using the condition obtained in the first step. The heterojunctions were evaluated using current-voltage (IV) and capacitance-voltage (C-V) characterizations. To identify the dominant conduction mechanism in the heterojunctions, I-V curves were fitted to various models. Series resistance and leakage current were found to affect the characteristics of the junctions. Further, C-V measurements showed that the interfaces had large defect density and contributed to the measured capacitance along with space charges. The results of all the above mentioned studies are presented and analyzed in the present thesis. The doping studies were successful to certain extent, but did not result in drastic improvement in the device characteristics. However, the studies gave a good insight into the behavior of these compounds, which will help in improving the device suitability of the compound semiconductor thin films. Further research is required to improve electrical properties of the films by minimizing the defects, which control the film characteristics, by suitable passivation steps.
Investigations of Naphthalimide and Pyrene Derivatives Based Organic Light Emitting Diodes
(National Institute of Technology Karnataka, Surathkal, 2016) Ulla, Hidayath; Satyanarayan, M. N.; Umesh, G.
Organic light-emitting devices (OLEDs) have been subject of intensive studies due to their great potential in display and lighting applications. The key point in OLEDs development for display and lighting applications is to find out materials emitting pure colours of red, green and blue with excellent emission efficiency and high stability. The efficiency of an OLED can be improved by balancing the transport of charge carriers in the device. Generally, this is done by including hole transporting and electron transporting materials in the OLED architecture. This thesis is focused on the study of electron transport materials for OLEDs based on novel naphthalimide and pyrene derivatives. This work improved the understanding on the physics of electron transport materials and the results of this study may be helpful for developing new devices and their applications. We observed that the naphthalimide derivatives have low-lying LUMO levels making them possess good electron-transporting and hole-blocking properties. The derivatives emitted in blue region with good chromaticity. In addition, they have high thermal stability and capable of forming good morphological films. Studies on the device properties of naphthalimide derivatives showed that they could be used as blue emitting electron transport materials. Also, the derivatives as host material in red OLEDs showed better performance than standard host material Alq3 in terms of both chromaticity and efficiency. Investigations on the OLEDs based on novel pyrene derivatives as green emitting electron transport materials showed better performance than Alq3. Temperature dependent electrical studies on the electron only devices showed that the pyrene derivatives have better electron mobilities than Alq3.

2. Thesis and Dissertations

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