No description available.

This thesis investigates the growth of intermetallic compounds by co-sputtering from single elemental targets. The preliminary work involved constructing a sputtering set-up to grow abinary alloy (Sm-Co) and investigating how to control the composition spread that was obtained. Having achieved this, a larger sputtering flange was built up to grow the ternary Heusler alloy, Co₂MnSi. Co₂MnSi has been predicted to be a half-metallic ferromagnet, which means that there is an energy gap in the minority spin band at the Fermi energy. This leads to 100% spin polarised conduction electrons, which would enable ideal spin-device performance to be obtained. Co₂MnSi is particularly promising because it is predicted to have a large energy gap in the minority band of ~0.4 eV and has the highest Curie temperature among the known Heuslers of 985 K. Initially, Co₂MnSi was grown on a-plane sapphire and stoichiometric films were single phase and highly (110) textured, without the use of a seed layer. They exhibited the bulk value of the saturation magnetisation, Ms and films grown at the highest deposition temperature (715 K) showed the lowest resistivity (47 μΩcm at 4.2 K) and the lowest room temperature coercivity (18 Oe). The spin polarisation of the transport current, Pt of a 400 nm film grown at this deposition temperature was 54%, consistent with measurements on bulk single crystals. Ms decreased with decreasing film thickness indicating a graded disorder. By growing on GaAs (001), which has a similar lattice parameter to Co₂MnSi it was expected that this disordered region would be confined to the first few atomic layers. However, this was not the case because interfacial reactions resulted in the formation of an epitaxial Mn-As region, and a thin interfacial layer that was Co-Ga rich. This prevented the lattice matching of the Co₂MnSi to the GaAs(001) hence hindering epitaxial growth of the Heusler. The reaction zone also meant that films exhibited a Ms slightly below the bulk value. The expected fourfold anisotropy was not obtained for this cubic material, which is most likely due to the anisotropy of the reconstructed GaAs surface. Inspite of this anomalous behaviour, Pt was 55%, similar to the result obtained on sapphire, indicating that either Pt is independent of orientation or that the Heusler surface reconstructs in the same way. Films showed some improved properties to films grown on a-plane sapphire, indicating the potential of growing on this technologically important substrate. With this in mind, pseudo spin valves involving Co₂MnSi as one of the ferromagnetic electrodes were fabricated in both the CIP and CPP configurations. Clear low-field spin-valve contributions were observed at 15 K but the MR values are much lower than that expected from a PSV with a predicted 100% spin polarised electrode.

No description available.

No description available.

The atmosphere we live in contains various kinds of chemical species, natural and artificial, some of which are vital to our life, while many others are more or less harmful. The vital gases like oxygen, humidity have to be kept at adequate levels in the living atmosphere, whereas the hazardous and toxic gases like hydrocarbons, H2, volatile organic compounds, CO2, CO, NOx, SO2, NH3, O3 etc should be controlled to be under the designated levels. The measurement technology necessary for monitoring these gases has emerged, particularly as organic fuels and other chemicals have become essential in domestic and industrial life. In addition to other applications, environmental pollution monitoring and control has become a fundamental need in the recent years. Therefore, there has been an extensive effort to develop high-performance chemical sensors of small size, rugged construction, light weight, true portability, and with better sensing characteristics such as high sensitivity, fast response and recovery times, low drift, and high degree of specificity. Among the various types of gas sensors studied, solid state gas sensors based on semiconducting metal oxides are well established, due to their advantages over the other types, and hence cover a wide range of applications. However, the widespread application of these sensors has been hindered by limited sensitivity and selectivity. Various strategies have been employed in order to improved the performance parameters of these sensors. This thesis work has two major investigations, which form two parts of the thesis. The first part of this thesis describes the efforts to improve the sensing behaviour of one of the extensively studied metal oxide gas sensors, namely, ZnO, through a novel, ultrasonic-nebulised spray pyrolyis synthesis method, employing an aqueous combustion mixture (NSPACM). The second part of the thesis deals with the ideal of gas detection by optical means through the reversible phase transformation between V2O5 and V6O13 deposited by metalorganic chemical vapor deposition(MOCVD). The introductory chapter I deals with basics of chemical sensors and the characteristic sensing parameters. Different types of gas sensors based on the phenomena employed for sensing are discussed, with an emphasis on semiconducting metal oxide gas sensors. The importance of material selection for solid state gas sensors, depending on the purpose, location, and conditions of operation are discussed, supporting the assertion that semiconducting metal oxides are better suited to fulfill all the requirements of modern gas sensors. Some of the effective methods to improve performance parameters including the influence of grain size, microstructure, and surface doping are described., followed by the motivation of the present thesis. The part I of the thesis is based on the resistive semiconducting metal oxide, where the system investigated was ZnO. Part one comprises Chapters 2, 3 and 4. In Chapter 2, a brief introduction to the material properties of ZnO, followed by various synthesis techniques are discussed. An overview of spray pyrolysis and combustion synthesis is followed by the details of the method employed in the present study, namely NSPACM, which is based on the above two methods, for the formation of ZnO films. A detailed description of the film deposition system built in house is presented, followed by the deposition procedure and the parameters used. Thermal study of the combustion mixture and non-combustion precursor shows the importance of the fuel, along with oxidizer, in forming the film. The films formed using combustion mixture are found to be polycrystalline, whereas films formed without combustion were found to have preferred crystallographic orientation even on an amorphous substrate, which is explained on the basis of minimization of surface energy. The observed unique microstructure with fine crystallite size and porous morphology is attributed to the combustion method employed, which is interesting from the point of view of gas sensing. Chapter 3 concerns the gas sensing study of these ZnO films. The design of the home made gas sensing system is explained in detail. The study of electrode characteristics is followed by the important steps in gas sensing measurements. ZnO gas sensors were mainly studied for their selectivity between aliphatic and aromatic hydrocarbons. The results show two regions of temperature where the sensitivity peaks for aliphatic hydrocarbons, whereas aromatic hydrocarbons show a single sensitive region. This observation can pave the way for imparting selectivity. Possible reasons for the observed behavior are mentioned. Chapter 4 describes the chemical and physical modifications done to ZnO thin films by doping with catalysts, and through the use of x-y translational stage for large-area deposition.. Homogenous distribution of catalysts achieved by the NSPACM synthesis procedure, determined by the x-ray elemental mapping, is discussed. The addition of catalysts improved the sensing both because of catalytic effects and by promoting preferred crystallographic orientation, with Ni addition showing the better effects. The use of the x-y stage in producing the films with high orientation, which improved the gas sensing behavior, is explained. Part II of the thesis comprises Chapters 5,6 and 7, and describes a detailed study of V2O5 and V6O13 thin films deposited by MOCVD for optical sensing of chemical species. In Chapter 5, a brief introduction to chemical vapor deposition is given, followed by the importance of the characteristics of CVD precursors – in particular, the importance of their thermal behavior in film formation. This is followed by the importance of vapor pressure and partial pressure studies in the MOCVD of oxides of a multivalent metal such as vanadium. Various techniques of measuring vapor pressure are listed, followed by the details of the method used in the present study employing rising temperature thermogravimetry, based on the Langmuir equation. Thermogravimetric analysis performed, both at atmospheric as well as at low pressure, using commercial and home made apparatus, respectively is discussed. A detailed description of the home made setup is also presented. Chapter 6 describes the application of the vapor pressure and partial pressure studies to the deposition of films using MOCVD. Here, a detailed description of the vanadium oxide phase diagram and the stability of various phases is presented, which points the importance of precise parameter control during the deposition to obtain pure phases. The details of the CVD setup, followed by the procedure and parameters of deposition, are presented. The films deposited at various deposition temperatures, analyzed using XRD and SEM, are discussed. The effect of temperature on the growth is explained. The effect of vapor pressure is studied by varying the precursor vaporizer temperature, with a growth temperature maintained invariant. The influence of the amount of precursor on film growth, with a particular crystalline orientation and phase content, is explained followed by the description of the deposition of pure phases of V2O5 and V6O13 through the optimization of CVD parameters. Chapter 7 deals with the optical study of the films deposited by the above method. Here, the importance of two phases of vanadium oxide, V2O5 and V6O13, to the proposed gas sensing action, is presented. Their structural similarity in terms of polyhedral arrangement in the ab plane can be the basis of a reversible phase change. The difference in the optical transmittance in two phases forms the basis for the optical method for chemical sensing. The details of the laser-based optical sensing setup, its, design and the detection method, are explained. Studies on hydrocarbon sensing with vanadium, pentoxide films are also presented. The novelty in using reversible chemical transformation of a material system for detection of reducing and oxidizing gases in the ambient gases is discussed. Chapter 8 provides a summary of the present thesis, together with the main conclusions. The work reported in this thesis has been carried out by the candidate as part of the Ph.d training programme. She hopes that this would constitute a worthwhile contribution towards the understanding and subsequent application of ZnO and oxides of vanadium(V2O5 and V6O13) as novel gas sensors which will be useful for environmental protection, as well as for safety in industrial an domestic sectors.

No description available.

An automated ellipsometer was used to study field-induced optical anisotropy in anodic niobium oxide films. The oxide films were found to change from the optically isotropic state to the optically anisotropic state when an electric field was applied normal to the film surface. The anisotropic refractive indices of the oxide films decreased quadratically while the thickness of the films increased quadratically with the applied field. The quadratic electro-optic coefficients were determined. The changes in refractive indices and in thickness of the oxide films were found to be independent of time. Field recrystallization of the anodic niobium oxide films was investigated using a scanning electron microscope. The results are compared with those reported for anodic tantalum oxide films in the published literature. / Applied Science, Faculty of / Electrical and Computer Engineering, Department of / Graduate

A unique process of fabricating a strained layer GexSi1-x on insulator is demonstrated. Such strained heterostructures are useful in the fabrication of high-mobility transistors. This technique incorporates well-established silicon processing technology e.g., ion implantation and thermal oxidation. A dilute GeSi layer is initially formed by implanting Ge+ into a silicon-on-insulator (SOI) substrate. Thermal oxidation segregates the Ge at the growing oxide interface to form a distinct GexSi1-x thin-film with a composition that can be tailored by controlling the oxidation parameters (e.g. temperature and oxidation ambient). In addition, the film thickness can be controlled by implantation fluence, which is important since the film forms pseudomorphically below 2×1016 Ge/cm2. Continued oxidation consumes the underlying Si leaving the strained GeSi film encapsulated by the two oxide layers, i.e. the top thermal oxide and the buried oxide. Removal of the thermal oxide by a dilute HF etch completes the process. Strain relaxation can be achieved by either of two methods. One involves vacancy injection by ion implantation to introduce sufficient open-volume within the film to compensate for the compressive strain. The other depends upon the formation of GeO2. If Ge is oxidized in the absence of Si, it evaporates as GeO(g) resulting in spontaneous relaxation within the strained film. Conditions under which this occurs have been discussed along with elaborated results of oxidation kinetics of Ge-ion implanted silicon. Rutherford backscattering spectrometry (RBS), ion channeling, Raman spectroscopy and scanning electron microscopy (SEM) were used as the characterization techniques.

Adherent Cu films were electrodeposited onto polycrystalline W foils from purged solutions of 0.05 M CuSO4 in H2SO4 supporting electrolyte and 0.025 M CuCO3∙Cu(OH)2 in 0.32 M H3BO3 and corresponding HBF4 supporting electrolyte, both at pH = 1. Films were deposited under constant potential conditions at voltages between -0.6 V and -0.2 V vs Ag/AgCl. All films produced by pulses of 10 s duration were visible to the eye, copper colored, and survived a crude test called "the Scotch tape test", which stick the scotch tape on the sample, then peel off the tape and see if the copper film peels off or not. Characterization by scanning electron microscopy (SEM), energy dispersive X-ray (EDX) and X-ray photon spectroscopy (XPS) confirmed the presence of metallic Cu, with apparent dendritic growth. No sulfur impurity was observable by XPS or EDX. Kinetics measurements indicate that the Cu nucleation process in the sulfuric bath is slower than in the borate bath. In both baths, nucleation kinetics do not correspond to either instantaneous or progressive nucleation. Films deposited from 0.05 M CuSO4/H2SO4 solution at pH > 1 at -0.2 V exhibited poor adhesion and decreased Cu reduction current. In both borate and sulfate baths, small Cu nuclei are observable by SEM upon deposition at higher negative overpotentials, while only large nuclei (~ 1 micron or larger) are observed upon deposition at less negative potentials.

Transient electroluminescence (EL) was used to measure the onset of emission delay in OLEDs based on transition metal, phosphorescent bis[3,5-bis(2-pyridyl)-1,2,4-triazolato] platinum(ΙΙ) and rare earth, phosphorescent Eu(hfa)3 with 4'-(p-tolyl)-2,2":6',2" terpyridine (ttrpy) doped into 4,4'-bis(carbazol-9-yl) triphenylamine (CBP), from which the carrier mobility was determined. For the Pt(ptp)2 doped CBP films in OLEDs with the structure: ITO/NPB (40nm)/mcp (10nm)/65% Pt(ptp)2:CBP (25nm)/TPBI (30nm)/Mg:Ag (100nm), where NPB=N, N'-bis(1-naphthyl)-N-N'-biphenyl-1, 1'-biphenyl-4, MCP= N, N'-dicarbazolyl-3,5-benzene, TPBI=1,3,5-tris(phenyl-2-benzimidazolyl)-benzene, delayed recombination was observed and based on its dependence on frequency and duty cycle, ascribed to trapping and de-trapping processes at the interface of the emissive layer and electron blocker. The result suggests that the exciton recombination zone is at, or close to the interface between the emissive layer and electron blocker. The lifetime of the thin films of phosphorescent emitter Pt(ptp)2 were studied for comparison with rare earth emitter Eu(hfa)3. The lifetime of 65% Pt(ptp)2:CBP co-film was around 638 nanoseconds at the emission peak of 572nm, and the lifetime of neat Eu(hfa)3 film was obtained around 1 millisecond at 616 nm, which supports the enhanced efficiency obtained from the Pt(ptp)2 devices. The long lifetime and narrow emission of the rare earth dopant Eu(hfa)3 is a fundamental factor limiting device performance. Red organic light emitting diodes (OLEDs) based on the rare earth emitter Eu(hfa)3 with 4'-(p-tolyl)-2,2":6',2" terpyridine (ttrpy) complex have been studied and improved with respect performance. The 4.5% Eu(hfa)3 doped into CBP device produced the best power efficiency of 0.53 lm/W, and current efficiency of 1.09 cd/A. The data suggests that the long lifetime of the f-f transition of the Eu ion is a principal limiting factor irrespective of how efficient the energy transfer from the host to the dopant and the antenna effect are.