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Increasing demand for high-density memories has necessitated the search for new materials with higher dielectric constants to satisfy the minimum charge storage density requirements. Several materials such as Ta₂O₅, BST¹, BBT² are being investigated to replace the currently used Si based oxide/nitride dielectrics. Among the materials under investigation, Ta₂O₅ is one of the most promising, especially from the fab compatibility point of view. Ta₂O₅ thin films offer a six-fold increase in dielectric constant compared to conventional dielectrics. However, the significant improvement in dielectric constant is offset by higher leakage currents compared to conventional dielectrics. Improvement in both, dielectric and insulating properties is required for the successful integration of Ta₂O₅ thin films into devices. In the current research work, it was demonstrated that by chemically modifying the tantalum pentoxide matrix, significant improvements in its electrical properties can be achieved which would enable the fabrication of a reliable high-density memory device. In the present work, the effects of Al addition on Ta₂O₅ thin films were systematically studied. The structural and electrical properties of these chemically modified thin films were investigated in detail to establish their potential for device applications. The effects on dielectric and insulating characteristics due to incorporation of Al in Ta₂O₅ matrix were studied in capacitor configuration. A metallorganic solution decomposition (MOSD) technique was used to deposit thin films onto Pt coated Si(100) substrates. The capacitors were fabricated by sputter depositing Pt electrodes on the top surface of the films. The dielectric and insulating properties of pure and modified Ta₂O₅ thin films and their dependence on film composition, processing temperature, and the thickness were discussed and an attempt was made to provide theoretical understanding for the experimental observations. The dielectric and insulating properties of Ta₂O₅ were found to be significantly modified by addition of Al. It was observed that Al addition has decreased the leakage currents approximately by an order of magnitude and improved thermal and bias stability characteristics of Ta₂O₅ capacitors. For example, the leakage currents in crystalline pure Ta₂O₅ thin films were found to be 4.5 x 10⁷ A/cm² in a 1MV/cm dc field which decreased to 3.4 x 10⁸ A/cm² for 10% Al modified Ta₂O₅ thin films. A typical dielectric constant of 42.5 was obtained for 10% Al modified Ta₂O₅ thin films. This is significantly higher compared to the commonly reported dielectric constant of 25 to 35 for Ta₂O₅ thin films. This enhancement was attributed to strong (100) orientation exhibited by both pure and modified Ta₂O₅ thin films. The high dielectric constant, low dielectric loss, low leakage currents and low temperature coefficient of capacitance suggest the suitability of Al modified Ta₂O₅ as a capacitor dielectric for future generation DRAM applications. ¹Barium strontium titanate, ²Barium bismuth tantalate / Master of Science

The semiconducting properties and wide bandgaps of some metal oxides have made them useful in sensing applications and printed electronics. Doping of common metal oxides to achieve p-type conductivity and the formation of p-n junctions with them is not feasible. However, nickel oxide (NiO) is known for p-type conductivity due to intrinsic defects. This research is aimed at synthesizing NiO thin films by means of chemical solution methods, and to characterize the films to determine the effects of processing methods and conditions of deposition on the properties of the films with the aim to use the films in light or gas sensors. NiO films were synthesized on glass substrates using chemical bath deposition (CBD), sol-gel spin-coating, and spray pyrolysis. Some conditions of deposition such as the concentration of precursors and the processing temperature of the films were varied and the effects of the variations on the properties of the films were studied using X-ray diffraction (XRD), scanning electron microscopy (SEM), scanning probe microscopy (SPM), ultraviolet-visible spectrophotometry, and Raman spectroscopy. Electrical characterization was carried out using a linear four-point probe and current-voltage measurement systems. The XRD results confirmed that the as-deposited films grown by CBD were hydrated nickel hydroxide. Thermogravimetric analysis (TGA) showed that the transformation of this phase to NiO required annealing at a temperature above 350 degree Celsius. The films were of porous morphology and were made of nanowalls of varying thickness with an average of 77 nm. This shrank to 52 nm after annealing at 450 degrees Celsius. NiO thin films grown by the sol-gel method were granular, and the crystallinity, grain size and electrical conductivity of the films depended on the temperature at which they were processed. Minimum electrical resistivity of 125 Ohm.cm was measured for the film that was dried at 250 degrees Celsius and annealed at 500 degrees Celsius. For the films deposited through spray pyrolysis, the sheet resistance decreased with increasing concentration of precursor and decreased with annealing at 500 degree Celsius for one hour. Generally, the films were polycrystalline, with the most prominent peak in their XRD patterns due to diffraction in the (111) crystallographic plane in the films that were produced by the CBD and spray pyrolysis. However, the (200) peak was most prominent in films that were produced by the sol-gel spin-coating. In all the films, the presence of a Ni-O bond was confirmed by the observation of the Ni-O stretching mode in one-phonon first-order and two-phonon second-order Raman peaks. Spray pyrolysis was found to be the best method for producing transparent and conducting NiO thin films. The current-voltage characteristics of the junction between p-NiO and n-type silicon, showed rectification of two orders of magnitude both in the dark and under illumination. The device was weakly sensitive to solar radiation. / Thesis (PhD (Physics))--University of Pretoria, 2019. / Physics / PhD (Physics) / Unrestricted

The amount of traditional energy sources are finite and the ecological impact of continuing to produce energy using fossil fuels will only exacerbate the carbon footprint. It is for these reasons that photovoltaic modules are becoming a larger and more necessary part the world's electricity production paradigm. Photovoltaic (PV) semiconductor modules are grouped into three categories. 'First generation' monocrystalline and polycrystalline silicon modules that consist of pn junctions created via the addition of impurities known as dopants. Almost 85% of solar cells produced at this time are `first generation' and it is the high production costs of silicon PV modules that motivated the search for new methods and materials to use as PV cells. 'Second generation' PV modules consist of semiconductor thin films. The 'second generation' PV modules in production at this time are copper indium gallium diselenide (CIGS), copper indium gallium (CIG), amorphous silicon (a-Si), and cadmium telluride (CdTe). The 'third generation' PV modules consist of dye-sensitized and organic materials. Thin films use less material, have less stringent production parameters and less waste, making thin films cost effective. In this investigation, solar cells were prepared using un-doped Group II-VI semiconductor thin films that exploit differences in bandoffsets to form effective p-n heterojunctions as a viable low cost alternative to doping. The thin films were deposited by thermal evaporation upon glass substrates coated with indium tin oxide (ITO). A layer of aluminum formed the back contact. Various configurations of the solar cells were produced including: ITO/CdS/CdSe/Al, ITO/ZnTe/CdSe/Al, ITO/CdTe/CdSe/Al, ITO/ZnTe/CdTe/CdS/Al. The solar cells produced have been characterized to determine thin film internal resistances, quantum and 'wall-plug' efficiencies, as well as I-V and spectral response. The open circuit voltage, short circuit current density, fill factor, and efficiency of our best devices were 0.26 V, 4.6 mA, 27.5 and 0.4% respectively. Additional device optimization should be possible and should improve these results. Solar cell design based on band-offset is an effective method for predetermining likely PV structures, while future investigation using Group II-VI semiconductor nanowires and nanorods and employing epitaxial films are likely to enhance the efficiency.

Ferroelectric random access memories (FRAM) are considered as future memories due to high speed, low cost, low power, excellent radiation hardness, nonvolatility, and good compatibility with the existing integrated circuit (IC) technology. The non-volatile FRAM devices are divided into two categories, based on reading technique: destructive readout (DRO) FRAM and non-destructive readout (NDRO) FRAM. Lead zirconate titanate (PZT) is recently considered as one of the most promising materials for DRO FRAM devices due to its excellent ferroelectric properties. There are remarkable advances in the applications of PZT thin films, but the direct integration into high density CMOS devices is restricted by high processing temperatures. Hence, it is desirable to lower processing temperature and develop novel high temperature electrode-barrier layers for achieving high density DRO FRAM devices. The NDRO FRAM devices have been developed mainly using metal-ferroelectric-semiconductor (MFS) and metal-ferroelectric-metal-insulator-semiconductor (MFMIS) structure. This devices use the remanent polarization of ferroelectric films to control the surface conductivity of a silicon substrate. The problem of the NDRO FRAM is that the actual electric field applied to ferroelectric films is very small compared to the external electric field, because of the large depolarization field in the MFS structure and the high capacitance ratio of ferroelectric capacitor and SiO2 capacitor in series in the MFMIS structure. Since the typical ferroelectric films show very high dielectric constant over 400, it is desired to develop ferroelectric films with low dielectric constant and low coercive electric field. This research is primarily focused on developing low temperature processing and high temperature electrode-barrier layers for DRO FRAM application, and exploiting novel ferroelectric materials for NDRO FRAM application. The low temperature processing was achieved by a novel sol-gel processing, which takes advantage of in-situ electrode template layer, rapid heating-treatment without pyrolysis step, and molecularly modified precursors. The PZT films with various composition were also investigated as a function of Ti content. In order to study the integration issues for these PZT films, a substrate was constructed as Pt/TiN/TiSi₂/poly-Si, which represents a scheme of capacitor in high density DRO FRAM devices. The ferroelectric films were incorporated into the substrate, and their ferroelectric properties were investigated as a function of annealing temperature. Excellent ferroelectric properties were observed for the thin films processed at a low temperature of 500 °C as contacting between top Pt and bottom polysilicon. The other approach we have taken to overcome the integration problems in high density DRO FRAM devices is to develop high temperature electrode barrier layers. In this research, Pt/IrO2/Ir hybrid layers were prepared on poly-Si substrate as high temperature electrode-barriers. The PZT films fabricated on the Pt/IrO₂/Ir/poly-Si substrates exhibited good ferroelectric properties and outstanding fatigue properties after high temperature processing. It was observed from Auger electron spectroscopy (AES) profiles that the hybrid oxide electrode minimized fatigue problem by reducing the oxygen vacancies entrapment at the electrode/ferroelectric interfaces. This results indicated that Pt/IrO₂/Ir high temperature electrode-barrier layers promise to solve major problems of PZT integration into high density DRO memory devices. For the NDRO FRAM devices, Sr₂Nb₂O₇ and La₂Ti₂O₇ thin films were prepared on Pt-coated silicon, Si(100), and Pt/IrO₂/SiO₂/Si substrates by metalorganic deposition (MOD) technique. The Sr₂Nb₂O₇ and La₂Ti₂O₇ thin films showed the dielectric constant values of 48 and 46, respectively. However, no ferroelectricity was observed at room temperature, which might be attributed to extremely small grains. Extensive studies on preparation and properties of Sr₂(Ta_1-xNb_x)O₇ (STN) both in bulk and thin film form were carried out as a function of composition. The STN films exhibited small dielectric constant of around 46, irrespective of the composition. / Ph. D.

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Room temperature elastic properties of thin supported TiC films, deposited on silicon and silicon carbide substrates and of single Rh-based alloy crystals, Rh3Nb and Rh3Zr, are investigated by the Surface Brillouin Scattering (SBS) technique. Velocity dispersion curves of surface acoustic waves in TiC films of various thicknesses, deposited on each substrate (Si and SiC) were obtained from SBS spectra. Simulations of SBS spectra of TiC thin hard films on germanium, silicon, diamond and silicon substrates have been carried out over a range of film thickness from 5 nm to 700 nm. The simulations are based on the elastodynamic Green's functions method that predicts the surface displacement amplitudes of acoustic phonons. These simulations provide information essential for analysis of experimental data emerging from SBS experiments. There are striking differences in both the simulated and experimental SBS spectra depending on the respective elastic properties of the film and the substrate. In fast on slow systems (e.g. TiC on silicon), the Rayleigh mode is accompanied by both broad and sharp resonances; in slow on fast systems (e.g TiC on SiC), several orders of Sezawa modes are observed together with the Rayleigh mode. The velocity dispersion of the modes has been obtained experimentally for both situations, allowing the elastic constants of the films to be determined. Effects of two deposition conditions, RF power and substrate bias, on the properties of the films are also considered. Platinum metal group alloys are promising candidates for future ultra high temperatures gas turbines materials due to their excellent high-temperature properties. In the present work, room temperature elastic properties of single crystals of Rh3Nb and Rh3Zr are investigated. SBS spectra for a range of wave vector directions from the (001) surface have been acquired in order to determine the angular variation of the surface acoustic wave velocities and the longitudinal wave threshold within the Lamb shoulder. The elastic stiffnesses of the specimens were determined using two approaches; one approach involves a least-square fit of the experimental data to calculated results and the other is an analytical approach which involves the 2 c minimization of secular equations for the Rayleigh surface acoustic wave and the longitudinal wave threshold velocities in the [100] and [110] directions on the (001) surface of a cubic crystal. Results from the two methods were in good agreement.

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