In polycrystalline silicon (pc-Si) thin-film solar cells, defect passivation is critical to device performance. Isoelectronic or covalently bonded impurities, hydrogenic, extended defects and defects with localized levels in the bandgap (deep level defects) are typically introduced during the fabrication of, and/or are inherent to, pc-Si thin-film solar cells. These defects dramatically affect minority carrier lifetimes. Removing and/or passivating these defects is required to maximize minority carrier lifetimes and is typically done through thermal annealing and passivation techniques. For pc-Si thin-film solar cells on low temperature glass superstrates, rapid thermal annealing (RTA) and hydrogen plasma passivation (hydrogenation) are powerful techniques to achieve effective removal and passivation of these defects. In this thesis, three silicon thin-film solar cells structures on low-temperature glass are subjected to variations in RTA high-temperature plateaus, RTA plateau times, and hydrogen plasma passivation parameters. These solar cells are referred to as ALICIA, EVA and PLASMA. By varying the RTA plateau temperature and time at plateau, the trade-off between extensive dopant diffusion and maximum defect removal is optimized. To reduce the density of point defects and to electrically activate the majority of dopants, an RTA process is shown to be essential. For all three of the thin-film solar cell structures investigated in this thesis, a shorter, higher-temperature RTA process provides the best open-circuit voltage (Voc). Extensive RTA plateau times cause excessive dopant smearing, increasing n = 2 recombination and shunt resistance losses. Hydrogenation is shown to be an essential step to achieve maximum device performance by `healing' the defects inherent to pc-Si thin-film solar cells. If the hydrogen concentration is about 1-2 times the density of oxygen in the cells as measured by secondary ion mass spectroscopy (SIMS), the cells seem to respond best to hydrogenation, with good resultant Voc and short-circuit for all cells investigated in this thesis. The effect of hydrogen passivation on the Voc is spectacular, typically increasing it by a factor of 2 to 3.5. Hydrogen de-bonding from repeated thermal treatments at increasing temperature provides a deeper understanding of what defects exist and the nature of the defects that limit the cell voltage. The variation in RTA and hydrogenation process parameters produces significant empirical insight into the effectiveness of RTA processes for point defect removal, dopant activation, point defect and grain boundary passivation, and impurity passivation. SIMS measurements are used to determine the impurities present in the cells' bulk and the amount of hydrogen available to passivate defects. From the results presented it appears that pc-Si thin-film solar cells on low-temperature glass are a promising, and potentially lower-cost, alternative to Si wafer based cells.

Thin-film solar cells are widely recognised to have the potential to compete with fossil fuels in the electricity market due to their low cost per peak Watt. The Thin-Film Group at the University of New South Wales (UNSW) is engaged in developing polycrystalline silicon (poly-Si) thin-film solar cells on glass using e-beam evaporation technology. We believe our solar cells have the potential of significantly lowering the manufacturing cost compared to conventional, PECVD-fabricated thin-film solar cells. After years of materials research, the focus of the Group??s work is now moving to the metallisation of evaporated solar cells. Minimising various kinds of losses is the main challenge of the cell metallisation procedure, within which the contact resistance is always a big issue. In this thesis, the contact resistance of aluminium contacts on poly-Si thin-film solar cells on glass is investigated. To the best of the author??s knowledge, this is the first ever contact resistance investigation of Al contacts on evaporated poly-Si material for photovoltaic applications. Various transmission line models (TLM) are employed to measure the contact resistance. An improved TLM model is developed to increase the measurement precision and, simultaneously, to simplify the TLM pattern fabrication process. In order to accommodate the particular requirements of poly-Si coated glass substrates, a TLM pattern fabrication process using photolithography is established. Furthermore, a Kelvin sense tester is set up to ensure an accurate measurement of the contact resistance. After establishment of the TLM technique at UNSW, it is successfully tested on singlecrystalline silicon wafer samples. The thermal annealing process of the contacts is also optimised. Then, the general behaviour of Al contacts on uniformly doped poly-Si films (i.e., no p-n junction) is investigated using the verified TLM technique. The long-term stability of the contacts is also studied. This is followed by an investigation of the contact resistance of the back surface field and emitter layers of different types of poly-Si thin-film solar cells. Finally, a novel contact resistance measurement model is proposed that is believed to be able to overcome the measurement bottleneck of the transmission line models.

Photovoltaics (PV) is fast emerging as an attractive renewable energy technology due to concerns of global warming, pollution and scarcity of fossil fuel supplies. However to compete in the global energy market, solar cells need to be cheaper and more energy efficient. Silicon is the favorite semiconductor used in solar photovoltaic cells because of its ubiquity and established technology, but due to its indirect bandgap silicon is a poor absorber and light emitter. Thin film cells play an important role in low cost photovoltaics, but at the cost of reduced efficiencies when compared to wafer based cells. There remains much untapped potential in thin-film solar cells which this work has attempted to exploit through exploring novel approaches of enhancing the efficiency of thin film cells using the optical properties of sub-wavelength metal nanoparticles. Metals are considered as strong absorbers of light because of their large free-electron density. How can metals improve light trapping in solar cells? This question has raised several eyebrows and this thesis is an attempt to show that metal nanoparticles can be useful in producing efficient solar cells. Subwavelength metal particles support surface modes called surface plasmons when light is incident on them, which cause the particles to strongly scatter light into the underlying waveguide or substrate, enhancing absorption. The process of coupling thin film silicon waveguide modes to plasmonic metals using unpolarised light at normal incidence is applied to silicon-based solar cells and light emitting diodes, and enhanced photocurrent and electroluminescence is realized with potential for further optimisation and improvement. The results from this study correspond to a current increase of up to 19% from planar wafer based cells and up to 33% increase from 1.25 micron thin-film silicon-on-insulator structures for the AM1.5 global spectrum. We also report for the first time an up to twelve fold increase in electroluminescence signal from 95nm thick light-emitting diodes. From the results we conclude that this method which involves simple techniques of nanoparticle deposition and characterization could hold important implications in the improvement of thin-film silicon cell absorption / emission efficiencies where conventional methods of light trapping are not feasible, resulting in promising near-term applications of surface plasmons in photovoltaics and optoelectronics.

Solid Oxide Fuel Cells (SOFCs) are devices that directly convert chemical energy into electrical energy, without proceeding through a Carnot combustion cycle. These devices are based on the usage of solid oxide electrolytes operating at relatively elevated temperatures. Two major hurdles must be overcome in order to decrease the operating temperatures of practical SOFCs. The first relates to reducing ohmic losses within solid electrolytes. The second relates to the need for developing high performance electrodes since electrolyte reaction rates at both anode and cathode are affected detrimentally as operating temperatures fall. This PhD project has focussed on addressing the first hurdle in two innovative ways: 1. the implementation of solid electrolytes with higher ionic conductivity than zirconia, 2. the development of very thin film electrolytes as thick as 5ƒÝm. Several thin films with novel electrode-electrolyte structures were fabricated and evaluated in order to demonstrate the viability of low temperature SOFC operations. Development of such thin films was innovative and challenging to achieve. The approach taken in this work involved fabricating a dense and thin gadolinia doped ceria (10GDC - Gd 10wt%, Ce 90wt%) oxide electrolyte. 10GDC is an electrolyte exhibiting higher conductivities than conventional materials during low temperature operations. A research contribution of this PhD was the demonstration of the deposition of 10GDC thin films using RF magnetron sputtering for the first time. 10GDC thin film electrolytes with thickness in a range between 0.1 to 5ƒÝm were fabricated on 10 yttrium stabilised zirconium (10YSZ) substrates by using a RF magnetron sputterer. The primary parameters controlling 10GDC thin film deposition using this method were explored in order to identify optimal conditions. The fabricated films were subsequently analysed for their morphology, composition and stoichiometry using a variety of methods, including Scanning Electron Microscopy (SEM) with Energy Dispersive X-ray Spectrometry (EDS), optical microscopy, X-ray Photoelectron Spectroscopy (XPS), and X-ray Diffraction (XRD). A preliminary test was conducted in order to examine the function of 10GDC thin film electrolytes together with the cathode and anode substrates at intermediate temperatures (700oC). A complete planar single cell was designed and assembled for this purpose. However, when fully assembled and tested, the cell failed to generate any voltage or current. Consequently, the remainder of the PhD work was focused on systematically exploring the factors contributing to the assembled fuel cell failure. As fabrication failure analysis is seldom reported in the scientific literature, this analysis represents a significant scientific contribution. This analysis proceeded in a series of steps that involved several different methods, including SEM, red dye analysis, surface morphology and cross section analysis of the cell. It was found that pinholes and cracks were present during the fuel cell operating test. Cathode delamination was also found to have occurred during the test operation. This was determined to be due to thermal expansion mismatch between the cathode substrate and the 10GDC electrolyte thin film. A series of suggestions for future research are presented in the conclusion of this work.

<p>Alumina-Zirconia thin films were grown on a range of substrates using dual magnetron sputtering. Film growth was achieved at a relatively low temperature of 450 °C and at higher temperatures up to 810 °C. The films were grown on well-defined surfaces such as silicon (100) but also on industrially relevant substrates such as hardmetal (WC-Co). Radio frequency power supplies were used in combination with magnetron sputtering to avoid problems with target arcing. A range of film compositions were possible by varying the power on each target. The influence of sputtering target were investigated, both ceramic oxide targets and metallic targets being used.</p><p>The phase composition of the as-deposited films was investigated by x-ray diffraction. The pure zirconia films contained the monoclinic zirconia phase, while the pure alumina films appeared either amorphous or contained the gamma-alumina phase. The composite films contained a mixture of amorphous alumina, gamma-alumina and the cubic zirconia phase. In-depth high-resolution electron microscopy studies revealed that the microstructures consisted of phase-separated alumina and zirconia nanocrystals in the case of the nanocomposites. In-situ spectroscopy was also performed to characterise the nature of the bonding within the as-deposited films.</p><p>The oxygen stoichiometry in the films was investigated as a possible reason for the stabilisation of the cubic zirconia phase in the nanocomposite. Ion beam techniques such as Rutherford backscattering scattering and electron recoil detection analysis were used in these studies. The growth of films with ceramic targets led to films that may be slightly understoichiometric in oxygen, causing the phase stabilisation. The growth of films from metallic targets necessitates oxygen rich plasmas and it is not expected that such films will be oxygen deficient.</p><p>Initial attempts were also made to characterise the mechanical properties of the new material with nanoindentation. The nanocomposite appeared to have greater resistance to wear than the pure zirconia film. In doing so, some surface interactions and some material interactions have been studied.</p> / Report code: LIU-TEK-LIC-2006:41

<p>In this Thesis the magnetic and structural properties of thin epitaxial Fe, Co, and Ni films are discussed. Some of the in-situ prepared samples were used to characterize the degree of circular polarization of the newly installed beamline D1011 at MAX-lab. By means of x-ray magnetic circular dichroism (XMCD) and utilizing the associated magneto optic sum rules, the orbital (<i>m</i><i>l</i>) and spin (<i>m</i><i>s</i>) moments are determined directly in <i>m</i><i>B</i>/atom with elemental specificity. The extended x-ray absorption fine structure (EXAFS) measurements yield site specific information on the local crystallographic structure.</p><p>These measurements were performed using the circular x-rays of several beamlines. The influence of the degree of spatial source coherence <i>l</i><i>spat</i> of the x-rays was characterized by means of Fresnel diffractometry. A correlation between enhanced XAS white line intensities and higher values of <i>l</i><i>spat</i> was established for 20 ML Fe, Co, and Ni films on Cu(100).</p><p>The degree of circularly polarized x-rays (<i>P</i><i>c</i>) at beamline D1011 at MAX-lab was characterized by studying Fe films on Cu(100) by means of XMCD. The maximum value of <i>P</i><i>c</i> is experimentally determined to <i>P</i><i>c</i> =0.85.</p><p>The Au/Co/Au trilayer system was studied as a function of Co thickness, temperature, and Au cap thickness. A 10 mono-layer (ML) Co film, with an Au cap of 20 Å, shows a spin reorientation transition (SRT) from an in-plane to an out-of-plane easy direction as the temperature is lowered from 300 K to 200 K. The magnetic properities of these Co films are very different to what is found for bulk samples due to, in particular, the broken symmetry at the interfaces.</p><p>The thickness dependent spin reorientation transition in the Fe/Ag(100) system was characterized by means of XMCD and EXAFS measurements. 3 ML Fe films show an out-of-plane easy direction with an 125% enhanced orbital moment as compared to the 25 ML Fe in-plane film. Simulations of the Fe <i>L</i>-edge EXAFS indicate the bulk Fe bcc structure for film thicknesses of 6-25 ML Fe. For 3 ML Fe strong deviations from this bcc phase is observed.</p><p>Ultrathin Co films deposited on flat and vicinal Cu(111) in the thickness region 1-25 ML were studied by means of XMCD and scanning tunneling microscopy (STM). The vicinal Cu(111) Co deposition leads to the formation of elongated islands preferentially oriented along the step edges. In connection to this particular Co growth mode we observe an increase of both the orbital and the spin moment on the vicinal Cu(111) of about 25% relative to what was observed for Co on flat Cu(111).</p>

<p>In this thesis the magnetic and structural properties of ultra-thin 3d transition metals films have been investigated, in particular Fe, Ni and Co films. X-ray Magnetic Circular Dichroism (XMCD) has provided element specific spin (m_s ) and orbital (m_l ) moments per atom by utilizing the magneto optic sum-rules. Element specific hysteresis curves have been measured by means of X-ray Resonant Magnetic Scattering (XRMS), and the local crystallographic structure has been investigated using Extended X-ray Absorption Fine Structure (EXAFS). </p><p>By performing XMCD on Fe/Ag(100) we observe a spin reorientation from in-plane to out-of-plane as the Fe thickness is lowered. At temperatures below 300K it occurs around 5-7 mono layers (ML) of Fe. While reorienting the magnetization out-of-plane the orbital moment increases with 125% but only a minor increase (5%) of the spin moment is observed. Extended X-ray Absorption Fine Structure (EXAFS) measurements indicate that films 6 ML and thicker have a bulk-like bcc structure. For the thin out-of-plane films, the local crystallographic structure is more complicated. </p><p>The spin reorientation of the Au/Co/Au tri-layer system has been studied as a function of temperature, Co layer and Au cap thickness. An unexpected behavior of the orbital moment upon spin reorientation is found in these systems. An ex-situ prepared sample shows a smooth spin reorientation from an in-plane to an out-of-plane easy magnetization direction as the temperature is lowered from 300K to 200K. In-situ prepared samples have also been investigated and a novel phase diagram has been identified. The Au/Co interface has been explored during the Au capping by means of photoemission measurements. </p><p>In the bi- and tri-layer system of Fe and Ni we have been able to manipulate the spin reorientation by varying the Fe and Ni thickness. A novel non-collinear interlayer exchange interaction for 3d ferro magnets in direct contact has been discovered for a set of samples. This exchange interaction is found to be strongly dependant on the preparation conditions.</p>

<p>Spectrally selective solar absorber using composite Ni-NiO as coating materials was studied. Samples were prepared by dc magnetron sputtering unit named <i>Rulle</i>, which is a miniature copy of an industrial roll-coater unit. Using asymmetric location of the oxygen nozzele, it is possible to form the desired metallic concentration distribution along the sputtering zone under optimized conditions. This distribution can be transferred into a graded film profile by moving the substrate, obtaining good spectral selectivity. For specified mechanical settings (such as locations of gas sprays, target and pump positions etc.), the ratio of used oxygen flow to the corresponding critical oxygen flow, <b>RO</b>, is a dimensionless parameter to control the zone specification. The optimal value is around 0.80 for the <i>Rulle</i>. Optimized zone shows properties with two main parts: the metallic composite part of varied nickel volume fraction and the dielectric part. Two parts of the sputtering zone can form a graded absorbing layer with the right ratio of base and middle layer by the moving substrate technique. Distribution of normalized conductivity, <i>NC</i>, along the absorbing sputtering zone is a simple and good specification of zone property. Profile of graded film prepared by the moving substrate technique can be tailored according to <i>NC</i> distribution. XRD and XPS study confirms the <i>NC</i> results. Simulation reveals that absorption should mainly rely on the intrinsic, but less on the interference mechanism. Used metallic volume fraction of Ni-NiO is 0.3 for main absorbing layer. The front surface reflection loss due to high refractive index can be reduced by adding a layer with low refractive index on the top. Simulation shows that three-layer coatings are a good and simple coating structure. High solar absorptance of 0.97 has been achieved with low thermal emittance of 0.05 by theoretical simulation as well as experimentally prepared samples.</p>

Iron-cobalt alloys have been extensively studied as potential hard disk drive write head materials due to their potentially high saturation flux densities (~2.4T), low coercivities and ease of deposition. Iron-cobalt plating solutions have, however, been shown to have stability issues, necessitating that they be used at low pH or that a stabilizing agent be added to the solution. The purpose of this thesis is to evaluate the stability of a dibasic ammonium citrate plating solution and to characterize the deposits which result from its use. The plating solutions are found to be less stable than previously claimed. The solutions are oxidized by dissolved oxygen, which leads to a valence change in the iron ions and eventually the formation of iron oxide/hydroxide precipitates. These effects are exacerbated by heating or the application of a voltage across the solution. Deposits plated from the solution are fine grained (<40nm) and compact through their thickness. While normally deposited as the equilibrium BCC phase, metastable phases are deposited at elevated temperatures, high pH or in the absence of a stabilizing agent. A metastable phase which is isomorphous to α-Mn is deposited at elevated temperatures. This phase transforms to the BCC phase when annealed at >174ºC and is highly textured. Its presence is detrimental to deposit coercivity. / Materials Engineering

The focus of this thesis involves development of highly transparent, n-channel, accumulation- mode thin-film transistors employing a zinc tin oxide (ZTO) channel layer. ZTO-based transparent thin-film transistors (TTFTs) show improved device performance compared to ZnO-based TTFTs. An estimated peak effective mobility for these devices as high as ~100 cm² V⁻¹sec⁻¹ has been observed, although effective mobilities in the range of 20-50 cm²V⁻¹sec⁻¹ are more common. This performance inconsistency may be due, in part, to the large device dimensions employed in developmental test structures and/or to shadow mask misalignment. Typical drain current on-to-off ratios are > 10⁶. Variation in the post-deposition annealing cycle is found to be an effective means to control the threshold voltage and to improve device performance. Optical characterization of these devices indicates ~84% transparency in the visible spectrum as viewed through the source/drain. Another aspect of this thesis research involves the utilization and extension of quantitative polycrystalline TFT device models with the intention of guiding the design and optimization of future TFTs. In particular, subthreshold conduction is assessed in order to estimate the bulk (and/or grain boundary) and interface trap densities. This leads to a consideration of threshold voltage and channel mobility extraction, as well as establishment of the turn-on voltage, V[subscript turn-on] Finally, a third aspect of this thesis research involves a new radio-frequency (RF) magnetron sputtering system, custom-designed and constructed at OSU by Chris Tasker. Contributions to the development of this tool include assisting in the design and implementation of the computer-controlled interlocks utilized for operation of the tool. The experimental flexibility of this new tool is discussed with respect to its applicability in the design and fabrication of future TTFTS. / Graduation date: 2004