Polymer applications have become more demanding as industry continuously turns to more microscopic parts. Due to the interactions of the polymer chains with the supporting surface and the air interface, the thinner films required for such applications have distinctly different properties than those of the well-defined bulk systems. The goal of the current research is to elucidate the behavior of ultrathin films. Two separate studies were performed on thin films supported on silicon wafer substrates: the first focuses on the viscoelastic cooperativity of thin films, and the second concentrates on the morphological behavior of polymer brush films. For the first study, polymethyl methacrylate films were spin coated onto silicon wafers, and the film thickness was determined using ellipsometry. A series of thin films were examined using techniques such as dielectric analysis and thermal mechanical analysis. The theory of cooperativity, which explains polymeric behavior using the intermolecular and intramolecular forces among polymer chains, was employed to understand the behavior of these thin films. Another type of thin film, a polymer brush, was investigated in the second study. Polymer brushes are formed by chemically bonding one end of many polymer chains to a substrate. The other ends of the chains can interact with the surrounding environment creating a brush-like structure. Constraining one end of a polymer chain alters the behavior of such a thin film. Polymer brushes of the di-block copolymer poly(t-butyl methacrylate) and polystyrene were produced on silicon wafers using spin coating techniques. The effects of both grafting density and solvent washes were analyzed using contact angle analysis and atomic force microscopy. In addition, hydrolysis was successfully performed on existing polymer brush samples to produce polymer brushes of the di-block copolymer polymethyl acrylic acid and polystyrene. / Master of Science

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Barium Strontium Titanate (BST) was deposited on Pt/ZrO2 / SiO2/Si substrates using liquid source metal organic chemical vapor deposition. A stoichiometry series was deposited with various GrII/Ti ratios (0.658 to 1.022) and a temperature series was deposited at 550 to 700°C. The thin films were characterized using transmission electron microscopy. Both series of samples contained cubic perovskite BST and an amorphous phase. The grain size increased and the volume fraction of amorphous phase decreased with increasing deposition temperature. The electrical and microstructural properties improved as the GrII/Ti ratio approached 1 and deteriorated beyond 1. This research demonstrates that BST thin films are a strong candidate for future MOS transistor gate insulator applications.

The present research work mainly focuses on the fabrication and characterization of single and multilayer thin films based on Bismuth Vanadate (BVO) and Bismuth Titanate (BTO). The multi-target laser ablation technique was used to fabricate single layer thin films of BVO, BVN and BTO; and multilayers composed of BVO and BTO in different structures. The fabricated thin films exhibited dense microstructure and a sharp interface with the substrate. The lattice strain, surface roughness and grain size could be varied as functions of composition and individual layer thickness in different structure fabricated. The optical properties were studied by spectroscopic ellipsometry and optical transmission spectra. The various models that were used for ellipsometric data analysis gave an excellent fitting to the experimental data. The optical constants were determined through multilayer analyses of the films. The band gap of these films was studied by spectroscopic ellipsometry and optical transmission. The optical studies carried out on BVO-BTO bilayer indicated the presence of an interfacial layer in between the BVO and BTO layer, whose refractive index was different from that of the individual layers and is attributed to different nature of the interfacial layer. The ferroelectric nature of BVO films was confirmed by P-E hysteresis loop studies under different applied fields and at various probing frequencies. The same was corroborated via the C-V measurements of these BVO films which exhibited butterfly shaped C-V characteristics. Fatigue studies in these films indicated that the switchable polarization is essentially constant through 105 cycles, after which it starts increasing probably due to the ionic conduction in BVO thin films. The dielectric response of undoped and Nb doped BVO as well as BVBT ML thin films were studied over a wide range of temperatures. The BVO films exhibited remarkable dielectric dispersion at low frequencies especially in the high temperature regime. Further, the frequency and temperature dependence of the dielectric, impedance, modulus and conductivity spectra of these films were investigated in detail. The ac conductivity was found to obey well the double power law in case of ML, indicating the different contributions to the conductivity, the low frequency conductivity being due to the short range translational hopping and the high frequency conductivity is due to the localized or reorientational hopping motion. DC leakage conduction in BVO, BVN and BVBT ML thin films was studied over a wide range of temperatures and applied electric fields. The experimental data were analyzed in light of different models to investigate the dc conduction mechanism in these films which were broadly classified into electrode limited and bulk limited conduction processes. In the case of BVO thin films the dc leakage current exhibited an ohmic nature at low electric fields followed by an onset of the space charge limited conduction (>1). Further in case of BVN films, three distinct regions were observed in I-V characteristics signifying different types of conduction processes in these films. In case of BVBT ML thin films, bulk limited PF mechanism was found to determine the conduction behavior at moderate electric fields. At higher electric fields, a trap filled region was observed which was followed by SCL conduction at higher fields. Therefore the present observation indicates the presence of more than one bulk limited conduction process in BVBT ML thin films. BVO thin films exhibiting good structure and dense morphology were successfully prepared on p-type Si by chemical solution decomposition technique. The C-V characteristics were evaluated for Au/BVO/Si MFS structure which showed a typical high frequency feature of a conventional MFIS structure.

Chalcogenide glasses are a class of covalent amorphous semiconductors with interesting properties. The presence of short-range order and the pinned Fermi level are the two important properties that make them suitable for many applications. With flash memory technology reaching the scaling limit as per Moore’s law, alternate materials and techniques are being researched at for realizing next generation non-volatile memories. Two such possibilities that are being looked at are Phase Change Memory (PCM) and Programmable Metallization Cell (PMC) both of which make use of chalcogenide materials. This thesis starts with a survey of the work done so far in realizing PCMs in reality. For chalcogenides to be used as a main memory or as a replacement to FLASH technology, the electrical switching parameters like switching voltage, programming current, ON state and OFF state resistances, switching time and optical parameters like band gap are to be considered. A survey on the work done in this regard has revealed that various parameters such as chemical composition of the PC material, nature of additives used to enhance the performance of PCM, topological thresholds (Rigidity Percolation Threshold and Chemical Threshold), device geometry, thickness of the active volume, etc., influence the electrical switching parameters. This has motivated to further investigate the material and experimental parameters that affect switching and also to explore the possibility of multi level switching. In this thesis work, the feasibility of using two chalcogenide systems namely Si15Te85-xGex and Ge15Te85-xAgx in the form of amorphous thin films for PCM application is explored. In the process, electrical switching experiments have been carried out on thin films belonging to these systems and the results obtained are found to exhibit some interesting anomalies. Further experiments and analysis have been carried out to understand these anomalies. Finally, the dynamics of electrical switching has been investigated and presented for amorphous Si15Te85-xGex thin films. From these studies, it is also seen that multi state switching/multiple resistance levels of the material can be achieved by current controlled switching, the mechanisms of which have been further probed using XRD analysis and AFM studies. In addition, investigations have been carried out on the electrical switching behavior of amorphous Ge15Te85-xAgx thin film devices and optical band gap studies on amorphous Ge15Te85-xAgx thin films. Chapter one of the thesis, gives a brief introduction to the limitations in existing memory technology and the alternative memory technologies that are being researched, based on which it can be inferred that PCM is a promising candidate for the next generation non volatile memory. This chapter also discusses the principle of using PCM to store data, realization of PCM using chalcogenides, the material properties to be considered in designing PCM, the trade offs in the process of design and the current trends in PCM technology. Chapter two provides a brief review of the electrical switching phenomenon observed in various bulk chalcogenide glasses, as electrical switching is the underlying principle behind the working of a PCM. In the process of designing a memory, many parameters like read/write operation speed, data retentivity and life, etc., have to be optimized for which a thorough understanding on the dependence of electrical switching mechanism on various material parameters is essential. In this chapter, the dependence of electrical switching on parameters like network topological thresholds and electrical and thermal properties of the material is discussed. Doping is an efficient way of controlling the electrical parameters of chalcogenides. The nature of dopant also influences switching parameters and this also is briefly discussed. Chapter three provides a brief introduction to the different experimental techniques used for the thesis work such as bulk chalcogenide glass preparation, preparation of thin amorphous films, measurement of film thickness, confirmation of amorphous nature of the films using X-Ray Diffraction (XRD), electrical switching experiments using a custom made setup, crystallization study using XRD and Atomic Force Microscopy (AFM) and optical band gap studies using UV-Vis spectrometer. Vt is an important parameter in the design of a PCM. Chapter four discusses the dependence of Switching voltage, Vt, on input energy. It is already established that the Vt is influenced by the composition of the base glass, nature of dopants, thickness of films and by the ambient temperature. Based on the results of electrical switching experiments in Si15Te74Ge11 amorphous thin films a comprehensive analysis has been done to understand the kinetics of electrical switching. Chapter five discusses a current controlled crystallization technique that can be used to realize multi-bit storage with a single layer of chalcogenide material. In case of PCM, data is stored as structural information; the memory cell in the amorphous state is read as data ‘0’ and the memory cell in crystalline state is read as data ‘1’. This is accomplished through the process of electrical switching. In order to increase the memory density or storage density, multi-bit storage is being probed at by having multiple layers of chalcogenide material. However, with this technique, the problems of inter-diffusion between different layers cannot be ruled out. In this thesis work, a current controlled crystallization technique has been used to achieve multiple stable resistance states in Si15Te75Ge10 thin films. Chapter six discusses the mechanism behind multi state switching exhibited by certain compositions of Si15Te85-xGex thin films. Crystallization studies on certain Si15Te85-xGex films have been carried out using XRD and AFM to understand the phenomenon of multiple states. The results of these experiments and analysis are presented in this chapter. Chapter seven discusses the results of electrical switching experiments and optical band gap studies on amorphous Ge15Te85-xAgx thin films. Chapter eight gives the conclusion and scope for future work.

This thesis examines melting in superheated silicon films in contact with SiO₂ under pulsed laser irradiation. An excimer-laser pulse was employed to induce heating of the film by irradiating the film through the transparent fused-quartz substrate such that most of the beam energy was deposited near the bottom Si-SiO₂ interface. Melting dynamics were probed via in situ transient reflectance measurements. The temperature profile was estimated computationally by incorporating temperature- and phase-dependent physical parameters and the time-dependent intensity profile of the incident excimer-laser beam obtained from the experiments. The results indicate that a significant degree of superheating occurred in the subsurface region of the film. Surface-initiated melting was observed in spite of the internal heating scheme, which resulted in the film being substantially hotter at and near the bottom Si-SiO₂ interface. By considering that the surface melts at the equilibrium melting point, the solid-phase-only heat-flow analysis estimates that the bottom Si-SiO₂ interface can be superheated by at least 220K during excimer-laser irradiation. It was found that at higher laser fluences (i.e., at higher temperatures), melting can be triggered internally. At heating rates of 10¹⁰ K/s, melting was observed to initiate at or near the (100)-oriented Si-SiO₂ interface at temperatures estimated to be over 300K above the equilibrium melting point. Based on theoretical considerations, it was deduced that melting in the superheated solid initiated via a nucleation and growth process. Nucleation rates were estimated from the experimental data using Johnson-Mehl-Avrami-Kolmogorov (JMAK) analysis. Interpretation of the results using classical nucleation theory suggests that nucleation of the liquid phase occurred via the heterogeneous mechanism along the Si-SiO₂ interface.

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LD2668 .T4 1966 C434 / Master of Science

Four different trivalent and tetravalent metal phthalocyanine systems (chlorogallium, chloroindium, vanadyl, and titanyl phthalocyanines) were used singly to prepare thin films (0.05-2.0 micron thickness) on gold, optically transparent substrates. The photoelectronic properties of these electrodes could be modified either by altering the growth conditions (i.e. rate of sublimation, cleanliness of substrate) or by dosing the thin films with either hydrogen or oxygen at elevated temperatures (150°C). The properties of these thin films were monitored by electron microscopy, UV-visible spectrophotometry, X-ray and Ultra-violet surface spectroscopies, and a variety of electrochemical and photoelectrochemical techniques. All four systems behaved in a manner similar to a p-type semiconductor when prepared at rapid rates (10-20 A/min) on gold substrates. In the dark, for contacting redox couples with Eᵒ’ values negative of +0.6V, the phthalocyanine electrodes showed negligible dark currents. Upon illumination, the photoelectrodes only produced positive photopotentials. Chlorogallium phthalocyanine thin films could be made to produce both positive and negative photopotentials when grown at slow rates (1-5 A/min) on clean, gold substrates. These chlorogallium phthalocyanine electrodes regained the properties of a p-type semiconductor after being dosed with oxygen for 48 hours at 150°C. X-ray Photoelectron Spectroscopy confirmed the presence of a high concentration of oxygen at the surface of all of the p-type phthalocyanine electrodes. The oxygen may accept electron density from the phthalocyanine macrocycle to cause the Fermi level to move down in energy toward its valence band edge. Dosing the film with hydrogen caused the electrode to exhibit its original intrinsic characteristics. This variability in electrical properties as a function of gas dopant may lead to the development of a sensitive gas sensing device. Ultra-violet Photoelectron Spectroscopy, as well as molecular orbital calculations, were applied to the chlorogallium phthalocyanine system to determine the molecular orbital contributions to its valence and conduction bands. Photoelectrochemical cells made from electrodes of chlorogallium and vanadyl phthalocyanines exhibited power conversion efficiencies in excess of 0.1%. The vanadyl and titanyl phthalocyanine electrodes were also effective catalysts for the photoreduction of H⁺ to H₂.