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The electronic properties of amorphous materials are greatly affected by the density of localized states in the mobility gap of these materials. The exact shape of the density of states (DOS) distribution in amorphous selenium (a-Se) is still unresolved despite decades of research. One of the most commonly employed methods to investigate charge transport properties in high resistivity materials is time-of-flight (TOF) transient photoconductivity experiment. The TOF transient photoconductivity technique is used to measure the induced photocurrent in the external circuit when the sample is photoexcited. Information pertaining to carrier mobility and other carrier parameters are deduced from the shape of the photocurrent. The investigation of the charge transport phenomenon is well known to be a complicated task. Monte Carlo (MC) simulation method has become a standard method for carrier transport studies in amorphous materials. The purpose of this research work is to develop a Monte Carlo simulation model for charge transport in typical TOF transient photoconductivity experiment to investigate the DOS distribution in a-Se. The MC simulations were first performed for relatively simpler models for which theoretical and analytical solutions were available. The MC model developed here is based on simulating the drift of carriers resulting from photogeneration, subject to the influence of an applied electric field and multiple trapping events. The free drift time of photocarriers and their dwell time in the traps are stochastic in nature, in accordance with the probabilities of these events. Electron time-of-flight transient photocurrents were calculated in amorphous selenium as a function of the electric field. The distribution of localized states (DOS) in a-Se has been investigated by comparing the experimentally measured and calculated transient photocurrents. The analysis of multiple-trapping transport has been done by the discretization of a continuous DOS. The DOS distribution has been optimized to produce the best agreement between the calculated and measured transient photocurrents. The resulting DOS has distinct features: A first peak at ~0.30 eV below Ec with an amplitude ~1017 eV1 cm3, a second small peak (or shoulder) at 0.450.50 eV below Ec with an amplitude 10141015 eV1 cm3, and deep states with an integral concentration 10111014 cm3 lying below 0.65 eV, whose exact distribution could not be resolved because of the limitations of the available experimental data. The density of states (DOS) distribution in the vicinity of the valence band mobility edge in vacuum coated a-Se films has been investigated by calculating the MC hole transient photocurrents at different temperatures, and also the dependence of the drift mobility on the temperature and field. The calculated TOF transient photocurrents were compared with experimental data published elsewhere. It is shown that, analogous to electron transport in a-Si:H, the DOS near Ev is a featureless, monotonically decreasing distribution in energy up to Ev + 0.4 eV, without the 0.28 eV peak near the valence band which was thought to control the hole drift mobility. Such a DOS was able to account for hole TOF data reported previously by several authors to date.

The electronic properties of amorphous materials are greatly affected by the density of localized states in the mobility gap of these materials. The exact shape of the density of states (DOS) distribution in amorphous selenium (a-Se) is still unresolved despite decades of research. One of the most commonly employed methods to investigate charge transport properties in high resistivity materials is time-of-flight (TOF) transient photoconductivity experiment. The TOF transient photoconductivity technique is used to measure the induced photocurrent in the external circuit when the sample is photoexcited. Information pertaining to carrier mobility and other carrier parameters are deduced from the shape of the photocurrent. The investigation of the charge transport phenomenon is well known to be a complicated task. Monte Carlo (MC) simulation method has become a standard method for carrier transport studies in amorphous materials. The purpose of this research work is to develop a Monte Carlo simulation model for charge transport in typical TOF transient photoconductivity experiment to investigate the DOS distribution in a-Se. The MC simulations were first performed for relatively simpler models for which theoretical and analytical solutions were available. The MC model developed here is based on simulating the drift of carriers resulting from photogeneration, subject to the influence of an applied electric field and multiple trapping events. The free drift time of photocarriers and their dwell time in the traps are stochastic in nature, in accordance with the probabilities of these events. Electron time-of-flight transient photocurrents were calculated in amorphous selenium as a function of the electric field. The distribution of localized states (DOS) in a-Se has been investigated by comparing the experimentally measured and calculated transient photocurrents. The analysis of multiple-trapping transport has been done by the discretization of a continuous DOS. The DOS distribution has been optimized to produce the best agreement between the calculated and measured transient photocurrents. The resulting DOS has distinct features: A first peak at ~0.30 eV below Ec with an amplitude ~1017 eV1 cm3, a second small peak (or shoulder) at 0.450.50 eV below Ec with an amplitude 10141015 eV1 cm3, and deep states with an integral concentration 10111014 cm3 lying below 0.65 eV, whose exact distribution could not be resolved because of the limitations of the available experimental data. The density of states (DOS) distribution in the vicinity of the valence band mobility edge in vacuum coated a-Se films has been investigated by calculating the MC hole transient photocurrents at different temperatures, and also the dependence of the drift mobility on the temperature and field. The calculated TOF transient photocurrents were compared with experimental data published elsewhere. It is shown that, analogous to electron transport in a-Si:H, the DOS near Ev is a featureless, monotonically decreasing distribution in energy up to Ev + 0.4 eV, without the 0.28 eV peak near the valence band which was thought to control the hole drift mobility. Such a DOS was able to account for hole TOF data reported previously by several authors to date.

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

High precision electronics are particularly susceptible to swings in resistance that occur in most materials when temperatures change. To make electronics with consistent performance across a wide range of temperatures, near-zero temperature coefficient of resistivity (nz-TCR) materials are needed. Further, as technology shrinks and we approach the angstrom era, methods of depositing nz-TCR materials of sufficient thinness are also necessary. This study demonstrates the design and deposition of such thin films using atomic layer deposition (ALD). Precise composition control is possible due to the self-limiting and highly conformal nature of ALD. Films made include, firstly, a conducting form of titania (TiOx) – typically an insulator, known as black titania, with a conductivity 108 times higher than TiO2. Next, metallic, nanocrystalline ruthenium film was deposited via plasma-enhanced ALD. Then, composites of black titania - ruthenium were made to explore how composition and structure impact TCR. Lastly, films of silicon-doped titanium nitride were also deposited with varying at% silicon. This set of films produced an extreme near-zero temperature coefficient over a wide temperature range. The films were characterized with many methods, including scanning and tunneling electron microscopy, x-ray photoelectron spectroscopy, x-ray diffractometry, spectroscopic ellipsometry, van der Pauw resistivity measurements, and Hall measurements to obtain carrier concentration and carrier mobility. This comprehensive investigation thus reveals the relationship between structure, composition, and TCR, facilitating the future design of nz-TCR materials.

This investigation is based on the topochemical modification of three set of phases: Sr₃Co₂O₅Cl₂, SrO(Sr(Ru_0.5M_0.5)O₃)n (M = Ti, Mn, Fe; n = 1, 2, &infin;), and SrO(SrVO₃)n (n = 1, 2, &infin;). The topochemical reduction of Sr₃Co₂O₅Cl₂ using sodium hydride as a solid state reducing agent results in the formation of a reduced phase containing cobalt centres with an average oxidation state of +2 and an overall composition of Sr₃Co₂O₄Cl₂. The resulting material adopts a structure containing double sheets of square-planar corner-sharing CoO2 units separated by rock salt SrCl layers. Variable-temperature diffraction measurements reveal that these sheets undergo a cooperative Jahn-Teller distortion at T ~ 200 K due to unevenly filled degenerate (d_xy, d_yz) orbitals. This material adopts a magnetic structure in which the moments within each sheet are ordered antiferromagnetically, but the sheets are aligned ferromagnetically. An investigation was carried on the reduction behaviour of Ru-doped Sr(Ru_xFe_1-x)O₃. It was found that the reduction was non-topochemical for values of x > 0.5. For values of 0 < x < 0.5, no single phase precursor material could be formed. For the material with x = 0.5, reduction with CaH₂ produced a new phase with composition Sr(Ru_0.5Fe_0.5)O₂. This material is the first reported instance of Ru²⁺ in an extended transition metal oxide. DFT calculations reveal that, while the iron centres adopt a high-spin configuration, the ruthenium centres are in an intermediate-spin S = 1 configuration. Resulting competing magnetic interactions lead to frustration and lack of ordering. In order to further study the reduction behaviour of extended transition metal oxides containing ruthenium, the reduction of Sr₂(Ru_0.5Fe_0.5)O₄ and Sr₃(Ru_0.5Fe_0.5)₂O₇ was performed using CaH₂ as a solid state reducing agent. In these cases, reduction leads to segregation of the materials into multiple phases adopting closely related structures that differ mainly in their oxygen content. In these materials, the ruthenium centres are preferentially reduced, such that starting from materials containing Ru⁵⁺ and Fe³⁺, materials containing Ru^(3-δ)+ and Fe³⁺ are produced. Similarly, the low-temperature oxidation using CuF₂ as a solid state fluoride source was performed on materials with composition Sr3(Ru0.5M0.5)2O7 (M = Ti, Mn, Fe). In the case of M = Mn and Ti, materials with composition Sr₃(Ru_0.5Fe_0.5)₂O₇F₂ are produced in which the ruthenium centres are oxidised to Ru⁶⁺. For the M = Fe material, oxidation results in partial exchange of O for F and a material with composition Sr₃(Ru_0.5Fe_0.5)₂O_5.5F_3.5 in which the ruthenium centres are oxidised from +5 to +5.5 while the iron centres remain in a +3 oxidation state. While fluorination of the M = Ti leads to increasing itinerant electronic behaviour, fluorination of the M = Mn and Fe materials induces a twisting of the MX₆ octahedra that enables magnetic order to emerge at low temperatures. Finally, reaction of the SrO(SrVO₃)n (n = 1, 2, &infin;) series of phases with CaH₂ results in the formation of phases with composition SrO(SrVO₂H)_n (n = 1, 2, &infin;), the first examples of stoichiometric oxyhydride materials. SrVO₂H is magnetically ordered at room temperature, while the n = 1 and n = 2 materials order at 170 K and 240 K respectively. The high magnetic ordering temperature arises from strong interactions between (d_xy, d_yz) orbitals in a manner analogous to the reduced iron-containing phases SrO(SrFeO₂)_n.

The thermocouple systems used for the measurement of surface temperature in high temperature applications such as advanced aerospace propulsion systems and diesel engine systems are expected to perform in rapidly fluctuating and extremely high heat fluxes corresponding to high temperatures (in excess of 1400 K) and high speed flows. Traditionally, Pt/Pt-Rh based thin film thermocouples have been used for surface temperature measurements. However, recent studies indicated several problems associated with these thermocouples at temperatures exceeding 1000 K, some of which include poor adhesion to the substrate, rhodium oxidation and reaction with the substrate at high temperatures. Therefore, there is an impending demand for thermoelectric materials that can withstand severe environments in terms of temperature and heat fluxes. In this study, thin films of titanium carbide and tantalum carbide as well as two families of conducting perovskite oxides viz., cobaltites and manganates (La(1-x)SrxCoO3, M(1-x)Cax MnO3 where, M=La,Y) were investigated for high temperature thin film thermocouple applications as alternate candidate materials. Thin films of the carbides were deposited by r.f. sputtering while the oxide thin films were deposited using pulsed laser ablation. Sapphire (1102) was used as substrate for all the thin film depositions. All the thin films were characterized for high temperature stability in terms of phase, microstructure and chemical composition using x-ray diffraction, atomic force microscopy and electron spectroscopy for chemical analysis respectively. Electrical conductivity and seebeck coefficients were measured in-situ using a custom made device. It was observed that TiC/TaC thin film thermocouples were stable up to 1373 K in vacuum and yield high and fairly stable thermocouple output. The conducting oxides were tested in air and were found to be stable up to at least 1273 K. The manganates were stable up to 1373 K. It was observed that all the oxides studied crystallize in a single phase perovskite structure. This phase is stable up to annealing temperatures of 1373 K. The predominant electrical conduction mechanism was found to be small polaron hopping. Stable and fairly high electrical conductivities as well as seebeck coefficients accompanied with phase, structure, composition and microstructure stability indicate that these materials hold excellent promise for high temperature thin film thermocouple applications. / Master of Science

Materials processing using microwave radiation is emerging as a novel and innovative technology that has proven useful in a number of applications. It has various advantages over conventional processing, such as; time and energy saving, very rapid heating rates, considerably reduced processing time and temperature, fine microstructures and improved mechanical properties, better product performance, etc. Microwave irradiation has shown great potential for the processing of different semiconductor materials and inorganic solids for various advanced electronic and optoelectronic devices such as solar cells, batteries, supercapacitors, fuel cells etc. This work intends to investigate the effect of microwave radiation on various semiconductor materials and inorganic solids, in particular the changes in their chemical, physical and photoelectrochemical properties after microwave treatment. Microwaves have been used as an alternative method to conventional thermal annealing for post annealing of widely used semiconductors (TiO2, ZnO nanorods), battery materials (lithium aluminium titanium phosphates), and synthesis of materials (ZnO, Ti0.97Pd0.03O1.97). It is found that, in contrast to conventional thermal annealing, microwave treatment of such materials improves the crystallinity without any structural changes by preserving their nanostructure due to the difference in the heating mechanism (volumetric heating). The results demonstrate that microwave processing is a promising alternative method to the traditional conventional sintering for materials processing for advanced electronic and optoelectronic devices. Also the microwave annealing method offers energy savings of up to ~75%, which would make it highly desirable for industrial scale up.

Amorphous selenium based digital radiography has attracted much attention because of selenium's high X-ray absorption and excellent charge transport properties, and the ability to be created thick (typically 100 to 1000 micron) uniform layers over a large area (typically 30 cm X 30 cm) at low processing temperatures (typically at around 50 degree C substrate temperature). In this work, the excess noise in amorphous selenium has been studied. A number of device parameters were altered to study the noise characteristics, such as the metal of the electrodes, bulk material composition, device volume, surface conditions and substrate temperature. All the samples had a transverse geometry with 20 to 200- micron thick layers of amorphous selenium electroded with metal at the top and at the bottom. Sample devices were fabricated by conventional vacuum deposition.<p> Noise power was measured over a limited bandwidth of 1 kHz. The fluctuations for one sample amounted to 1% of the bias current. The excess noise was mainly 1/<i>f</i> noise with the slope ranging from -0.77 to -1.4. Interpretation of the noise spectra was complicated due to the samples' highly non-linear I-V relation and long time transients.<p> The metals of the electrode clearly showed a large effect on both the magnitude and shape of the noise spectrum. Of the metals studied, aluminum produced the least normalized noise and platinum the most. The addition of arsenic caused a decrease in the normalized noise. An additional 0.2% (% wt.) arsenic decreased the 1/<i>f</i> noise magnitude by more than a decade, but did not change the slope. The addition of chlorine did not affect the noise magnitude. Amorphous selenium is quite vulnerable to stress and in particular, external mechanical stress causes crystallization. The surface of the sample was gently abraded, applying the least possible amount of stress to the selenium layer. A change in the surface condition before the top electrode was deposited showed that a roughened surface decreased the noise magnitude substantially. These results strongly indicate that the noise is controlled by the metal-semiconductor interface.<p> Noise characteristics in multilayered samples were examined. The p-i-n and n-i-p structures consisted of 200 micron i-layer with 2 to 6 micron p- and n-like layers. The noise fluctuation in the current are typical of 1/<i>f</i> noise showing a power-law spectrum with slopes between -0.9 to -1.1. These samples showed a substantial decrease in the noise power compared to single layer samples; the additional n-like and p-like layers acted as carrier sources so that the current was not controlled by the metal interface. Hence, the measurements are closer to the intrinsic noise of a-Se. After exposure to 14 R (Roentgen) of X-rays, the normalized noise decreased by a factor of 1.6 for the n-i-p structure.

Amorphous selenium based digital radiography has attracted much attention because of selenium's high X-ray absorption and excellent charge transport properties, and the ability to be created thick (typically 100 to 1000 micron) uniform layers over a large area (typically 30 cm X 30 cm) at low processing temperatures (typically at around 50 degree C substrate temperature). In this work, the excess noise in amorphous selenium has been studied. A number of device parameters were altered to study the noise characteristics, such as the metal of the electrodes, bulk material composition, device volume, surface conditions and substrate temperature. All the samples had a transverse geometry with 20 to 200- micron thick layers of amorphous selenium electroded with metal at the top and at the bottom. Sample devices were fabricated by conventional vacuum deposition.<p> Noise power was measured over a limited bandwidth of 1 kHz. The fluctuations for one sample amounted to 1% of the bias current. The excess noise was mainly 1/<i>f</i> noise with the slope ranging from -0.77 to -1.4. Interpretation of the noise spectra was complicated due to the samples' highly non-linear I-V relation and long time transients.<p> The metals of the electrode clearly showed a large effect on both the magnitude and shape of the noise spectrum. Of the metals studied, aluminum produced the least normalized noise and platinum the most. The addition of arsenic caused a decrease in the normalized noise. An additional 0.2% (% wt.) arsenic decreased the 1/<i>f</i> noise magnitude by more than a decade, but did not change the slope. The addition of chlorine did not affect the noise magnitude. Amorphous selenium is quite vulnerable to stress and in particular, external mechanical stress causes crystallization. The surface of the sample was gently abraded, applying the least possible amount of stress to the selenium layer. A change in the surface condition before the top electrode was deposited showed that a roughened surface decreased the noise magnitude substantially. These results strongly indicate that the noise is controlled by the metal-semiconductor interface.<p> Noise characteristics in multilayered samples were examined. The p-i-n and n-i-p structures consisted of 200 micron i-layer with 2 to 6 micron p- and n-like layers. The noise fluctuation in the current are typical of 1/<i>f</i> noise showing a power-law spectrum with slopes between -0.9 to -1.1. These samples showed a substantial decrease in the noise power compared to single layer samples; the additional n-like and p-like layers acted as carrier sources so that the current was not controlled by the metal interface. Hence, the measurements are closer to the intrinsic noise of a-Se. After exposure to 14 R (Roentgen) of X-rays, the normalized noise decreased by a factor of 1.6 for the n-i-p structure.