Philosophiae Doctor - PhD / In this thesis, intrinsic hydrogenated nanocrystalline silicon thin films for solar cells application have been deposited by means of the hot – wire chemical vapour deposition (HWCVD) technique and have been characterised for their performance. It is noticed that hydrogenated nanocrystalline silicon is similar in some aspects (mainly optical) to its counterpart amorphous silicon actually used as the intrinsic layer in the photovoltaic industry. Substantial differences between the two materials have been found however in their respective structural and electronic properties. We show that hydrogenated nanocrystalline silicon retains good absorption coefficients known for amorphous silicon in the visible region. The order improvement and a reduced content of the bonded hydrogen in the films are linked to their good stability. We argue that provided a moderate hydrogen dilution ratio in the monosilane gas and efficient process pressure in the deposition chamber, intrinsic hydrogenated nanocrystalline silicon with photosensitivity better than 102 and most importantly resistant to the Staebler Wronski effect (SWE) can be produced. This work explores the optical, structural and electronic properties of this promising material whose study – samples have been exclusively produced in the HWCVD reactors based in the Solar Cells laboratory of the Physics department at the University of the Western Cape. / South Africa

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M.Sc. (Physics) / Amorhous silicon is one of the most promising materials for large area solar cells for terestrial photovoltaic applications. Unfortunately these cells suffer from two serious problems: the efficiencies drop when laboratory processes are scaled up and the cells degrade after some exposure to sunlight. The exact causes of these two problems are still unknown. In this project some aspects of the above two problems where investigated. The drop in efficiency due to scaling up of laboratory processes can be ascribed to macroscopic inhomogeneities in the film. An investigation was done by changing the chamber geometry and gas flow pattern to establish empirical conditions to obtain films with maximum macroscopic homogeneity. It was found that a uniform electric field above the substrate was the most important factor determining the macroscopic homogeneity of the film. The hydronamic gas flow pattern was of secondary importance. Some techniques to obtain a uniform electric field has been devised. The photo-degradation was investigated by illuminating films of o-Si.H with simulated sunlight for different lenghts of time. The change in the electrical and optical properties of the intrinsic films were determined as function of total photon flux. No change in the optical properties could be detected. The effect of the photo-degradation manifests itself in a drop in the the dark conductivity and photoconductivity. The observed phenomena is explained in terms of photo-induced deep levels in the gap. The Fermi level shifts to the middle of the gap due to these defect states, causing a drop in the free carrier concentration and conductivity. These defect levels increase the absorptiom coefficient in the long wavelength region, but they also decrease the lifetime of the photo-generated carriers. The photo-induced defects were investigated with the CPM-technique. A large part of this project involved the construction and commissioning of the CPM-apparatus. It was found that the light introduced two types of defects at energies 0.5 eV and 0.75 eV below the conduction band edge. The concentration of the defects increases with illumination, but saturates after about 24 hours of illumination. The defects could almost completely be annealed at ISOaC. The photo-degradation of o-Si.H solar cells is ascribed to the reduction in the carrier lifetimes of photo-generated carriers due to recombination at these defect centers.

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Amorphous Silicon(a-Si) films have attracted the attention of several investigators as it is an economical material for devices. One of the problems that is addressed is the doping of these films after they are prepared. In this thesis, we investigated the effects of doping amorphous Sil­icon films(prepared by r.f. sputtering) with Aluminium(Al) by ther­mal diffusion. Amorphous Silicon films have been prepared on glass substrates at optimal process parameters. Then, the a-Si films are coated with Al by vacuum evaporation and subjected to heating in N2 atmosphere in the temperature range 300°C to 600°C for different durations. After etching Al layer, it has been found that some of the films which are heated around 550°C contain filament like polycrystalline regions surrounding islands of a-Si. This structure has been confirmed through Scanning Electron Mi-croscope(SEM) photographs and electrical conductivity measurements. SEM photographs indicate that, bright regions of amorphous mate­rial are surrounded by dark regions of relatively higher conducting boundaries. The electrical conductivity study shows that there is sharp increase in conductivity of Al doped films, which is attributed to the conduct­ing polycrystalUne filament. A simple model has been proposed to explain the variation of con­ductivity of these transformed films, with process parameters and with temperature. Schottky barrier diodes have been fabricated using these trans­formed materials and their characteristics explained.

Abstract Amorphous Selenium (a-Se) was first commercialized for use as a photoconductor in xerography during the middle of the twentieth century. Since then the hole transport properties of a-Se have been studied extensively, however the study of electron transport remains relatively limited. Flat panel digital X-ray detectors using a-Se as a photoconductor have been developed and are being used in mammographic screening. The charge transport properties of the photoconductor layer will in part determine the performance of the flat panel detector. X-ray absorption causes electron-hole pair generation in the bulk of the photoconductor, requiring both electrons and holes to drift across the sample and be collected. If these carriers are lost in the many localized trapping states as they cross the sample, they will not contribute to the image signal resulting in unnecessary radiation exposure to the patient. Eleven a-Se samples were deposited at the University of Saskatchewan varying in thickness from 13 μm to 501 μm. Pure a-Se was chosen to ensure uniformity across the thickness of the samples, that is, to ensure the composition of the film did not change across the thickness. Time of flight transient photoconductivity experiments (TOF) and interrupted field time of flight (IFTOF) measurements were performed to measure the electron drift mobility and lifetime respectively. The product of electron drift mobility μ and lifetime τ, hence the carrier range (μτ) at a given applied electric field. The electron range is an important parameter as this places limits on the practical thickness of the photoconducting layer in a detector. This study also includes an investigation into the effect of the definition of transit time on the calculated drift mobility and analysis of the dispersive transport properties of a-Se. It was observed that as sample thickness (L) increased, electron drift mobility (μ) decreased. In addition electron lifetime (τ) decreased dramatically in samples thinner than 50 μm. Electron range (μτ) was 2.26 × 〖10〗^(-6) cm^2/V in the 147μm sample and 5.46 × 〖10〗^(-8) cm^2/V in the 13 μm sample, a difference of almost two orders of magnitude. The comparison of the half current method and inflection point methods to calculate the transit time of the same TOF curve, shows that the calculated mobility can vary by as much as 24%. This illustrates clearly that it is important to use the same point on the TOF curve to define the transit time. Charge packet dispersion (spread) in the time domain in pure a-Se samples was proportional to L^m where L is the photoconductor thickness and m ~ 1.3, measured at both 1 V/μm and 4 V/μm.

Neste trabalho foram investigados dois sistemas de multicamadas de semicondutores amorfos, a-SI:h/SI IND.1-XC IND.X:h e a-SI:h/a-GE:h, crescidos pela técnica de plasma enhanced chemical vapor deposition (PECVD). A difração de raios x a baixo angulo (SAXRD) foi a técnica utilizada para estudo das propriedades estruturais destas super-redes. Os objetivos deste trabalho foram: a) determinar as propriedades estruturais destes sistemas, no que tange a periodicidade da super-rede, uniformidade em espessura e tipo de interface; b) desenvolver modelos teóricos de simulação das intensidades difratadas; e c) avaliar o processo de difusão e cristalização dos componentes das multicamadas, através de tratamentos térmicos. As multicamadas de a-SI:h/a-SI IND.1-XC IND.X:h foram crescidas variando-se dois parâmetros de deposição: a concentração de metano na mistura gasosa e o tempo de plasma etching de hidrogênio entre deposições consecutivas. A combinação dos resultados de espectroscopia de eletrons auger (aes) e saxrd permitiram avaliar a espessura das interfaces. Interfaces mais abruptas foram obtidas em sistemas crescidos sobre uma camada buffer, tempos de plasma etching de hidrogênio de, pelo menos, 2min, e camadas de a-SI IND.1-XC IND.X:H crescidas com mais alta concentração de CH IND.4 na mistura gasosa e em condições de baixo fluxo de silano. / In this work, two types of amorphous semiconductor multilayers were investigated, a-SI:h/SI IND.1-XC IND.X:h and a-SI:h/a-GE:h, deposited by the Plasma Enhanced Chemical Vapor Deposition (PECVD) method. The Small Angle X-Ray Diffraction (SAXRD) technique was used to study the structural properties of these super-lattices. The aim of this work was: a) to determine the structural properties of these systems, including the periodicity, thickness uniformity and interface sharpness; b) to develop theoretical models to simulate the diffracted intensities; and c) to evaluate the diffusion and crystallization processes of the multilayer components, by means of heat treatments. The a-SI:h/a-SI IND.1-XC IND.X multilayers were deposited, varying two growth parameters: the methane concentration in the gaseous mixture and the intermediate plasma etching time between consecutive depositions. The Auger Electron Spectroscopy (AES) and SAXRD results were combined to evaluate the interface thickness. The sharpest interfaces were obtained on samples deposited on top of a buffer layer, plasma etching times of at least 2 min. and a-SI IND.1-XC IND.X:H layers deposited with a higher CH IND.4. concentration in the gaseous mixture and in conditions of low silane flow.

Neste trabalho foram investigados dois sistemas de multicamadas de semicondutores amorfos, a-SI:h/SI IND.1-XC IND.X:h e a-SI:h/a-GE:h, crescidos pela técnica de plasma enhanced chemical vapor deposition (PECVD). A difração de raios x a baixo angulo (SAXRD) foi a técnica utilizada para estudo das propriedades estruturais destas super-redes. Os objetivos deste trabalho foram: a) determinar as propriedades estruturais destes sistemas, no que tange a periodicidade da super-rede, uniformidade em espessura e tipo de interface; b) desenvolver modelos teóricos de simulação das intensidades difratadas; e c) avaliar o processo de difusão e cristalização dos componentes das multicamadas, através de tratamentos térmicos. As multicamadas de a-SI:h/a-SI IND.1-XC IND.X:h foram crescidas variando-se dois parâmetros de deposição: a concentração de metano na mistura gasosa e o tempo de plasma etching de hidrogênio entre deposições consecutivas. A combinação dos resultados de espectroscopia de eletrons auger (aes) e saxrd permitiram avaliar a espessura das interfaces. Interfaces mais abruptas foram obtidas em sistemas crescidos sobre uma camada buffer, tempos de plasma etching de hidrogênio de, pelo menos, 2min, e camadas de a-SI IND.1-XC IND.X:H crescidas com mais alta concentração de CH IND.4 na mistura gasosa e em condições de baixo fluxo de silano. / In this work, two types of amorphous semiconductor multilayers were investigated, a-SI:h/SI IND.1-XC IND.X:h and a-SI:h/a-GE:h, deposited by the Plasma Enhanced Chemical Vapor Deposition (PECVD) method. The Small Angle X-Ray Diffraction (SAXRD) technique was used to study the structural properties of these super-lattices. The aim of this work was: a) to determine the structural properties of these systems, including the periodicity, thickness uniformity and interface sharpness; b) to develop theoretical models to simulate the diffracted intensities; and c) to evaluate the diffusion and crystallization processes of the multilayer components, by means of heat treatments. The a-SI:h/a-SI IND.1-XC IND.X multilayers were deposited, varying two growth parameters: the methane concentration in the gaseous mixture and the intermediate plasma etching time between consecutive depositions. The Auger Electron Spectroscopy (AES) and SAXRD results were combined to evaluate the interface thickness. The sharpest interfaces were obtained on samples deposited on top of a buffer layer, plasma etching times of at least 2 min. and a-SI IND.1-XC IND.X:H layers deposited with a higher CH IND.4. concentration in the gaseous mixture and in conditions of low silane flow.

Resistive random access memory (RRAM) is a non-volatile memory technology based on resistive switching in a dielectric or semiconductor sandwiched between two different metals. Also known as memristors, these devices are potential candidates for a next-generation replacement for flash memory. In this thesis, bipolar resistive switching is reported for the first time in solution-deposited zinc-tin-oxide (ZTO). The impact of the compliance current on device operation, including the SET and RESET voltages, pre-SET, RESET and post-RESET currents, the resistance ratio between the low and high resistance states, retention, and the endurance, is investigated for an isolated Al dot/ZTO/Ir blanket device and for Al/ZTO/Pt crossbar RRAM devices. A gradual forming process is devised to improve device stability and performance. It is found that the device performance depends critically on the compliance current density that is used to limit the breakdown conduction during the SET operation. In addition, it was found that the conduction and switching mechanisms are consistent with the filament model of formation and rupture of conductive filaments. / Graduation date: 2012

Fabrication techniques and process integration considerations for amorphous oxide semiconductor (AOS) thin-film transistors (TFTs) constitute the central theme of this dissertation. Within this theme three primary areas of focus are pursued. The first focus involves formulating a general framework for assessing passivation. Avoiding formation of an undesirable backside accumulation layer in an AOS bottom-gate TFT is accomplished by (i) choosing a passivation layer in which the charge neutrality level is aligned with (ideal case) or higher in energy than that of the semiconductor channel layer charge neutrality level, and (ii) depositing the passivation layer in such a manner that a negligible density of oxygen vacancies are present at the channel-passivation layer interface. Two AOS TFT passivation schemes are explored. Sputter-deposited zinc tin silicon oxide (ZTSO) appears promising for suppressing the effects of negative bias illumination stress (NBIS) with respect to ZTO and IGZO TFTs. Solution-deposited silicon dioxide is used as a barrier layer to subsequent PECVD silicon dioxide deposition, yielding ZTO TFT transfer curves showing that the dual-layer passivation process does not significantly alter ZTO TFT electrical characteristics. The second focus involves creating an adaptable back-end process compatible with flexible substrates. A detailed list of possible via formation techniques is presented with particular focus on non-traditional and adaptable techniques. Two of the discussed methods, "hydrophobic surface treatment" and "printed local insulator," are demonstrated and proven effective. The third focus is printing AOS TFT channel layers in order to create an adaptable and additive front-end integrated circuit fabrication scheme. Printed zinc indium aluminum oxide (ZIAO) and indium gallium zinc oxide (IGZO) channel layers are demonstrated using a SonoPlot piezoelectric printing system. Finally, challenges associated with printing electronic materials are discussed. Organic-based solutions are easier to print due to their ability to "stick" to the substrate and form well-defined patterns, but have poor electrical characteristics due to the weakness of organic bonds. Inorganic aqueous-based solutions demonstrate good electrical performance when deposited by spin coating, but are difficult to print because precise control of a substrate's hydrophillic/hydrophobic nature is required. However, precise control is difficult to achieve, since aqueous-based solutions either spread out or ball up on the substrate surface. Thickness control of any printed solution is always problematic due to surface wetting and the elliptical thickness profile of a dispensed solution. / Graduation date: 2013