Hot carrier degradation in deep submicron n-MOS technologies

With the aggressive scaling of MOS devices hot carrier degradation continues to be a major reliability concern. The LDD technologies, which have been used to minimise the hot carrier damage in MOS devices, suffer from the spacer damage causing the drain series resistance degradation, along with the channel mobility degradation. Therefore, in order to optimise the performance and reliability of these technologies it is necessary to quantify the roles of spacer and channel damages in determining their degradation behaviour. In this thesis the hot carrier degradation behaviour of different generations of graded drain (lightly doped, mildly doped and highly doped) n-MOS technologies, designed for 5V, 3V and 2V operation is investigated. The stress time beginning from microseconds is investigated to study how the damage initiates and evolves over time. A technology dependent two-stage degradation behaviour in the measured transconductance with an early stage deviating from conventionally observed power law behaviour is reported. A methodology based on conventional extraction procedure using the L-array method is first developed to analyse the drain series resistance and the mobility degradation. For 5V technologies the analysis of the damage using this methodology shows a two-stage drain series resistance degradation with early stage lasting about lOOms. However, it is seen that the conventional series resistance and mobility degradation methodology fails to satisfactorily predict degradation behaviour of 3V and 2V technologies, resulting in unphysical decreasing extracted series resistance. It is shown that after the hot carrier stress a change in the universal mobility behaviour for channel lengths approaching quarter micron regime has a significant effect on the parameter extraction. A modified universal mobility model incorporating the effect of the interface charge is developed using the FN stress experiments. A new generalised extraction methodology modelling hot carrier stressed device as series combination of undamaged and damaged channel regions, along with the series source drain resistance is developed, incorporating the modified universal model in the damaged channel region. The new methodology has the advantage of being single device based and serves as an effective tool in evaluating. the roles of series resistance and mobility degradations for technology qualification. This is especially true for the deep submicron regime where the conventional extraction procedures are not applicable. Further, the new extraction method has the potential of being integrated into commercial device simulation tools, to accurately analyse the device degradation behaviour in deep submicron regime

Carbon Monoxide Poisoning Mechanisms and Mitigation Strategies for Polymer Electrolyte Membrane Fuel Cells

Fuel cells represent a viable option for the diversification of renewable energies, necessary to meet the increasing energy demand and reduce the emissions of greenhouse gases (GHGs). Among the challenges polymer electrolyte membrane fuel cells (PEMFCs) face for their deployment, are the adverse effects that the presence of CO provokes in their performance and durability. A deeper understanding of the mechanisms occurring during the poisoning is needed in order to develop more efficient mitigation strategies that contribute to the competitiveness of PEMFCs. In this work, the spatial variations and temporal dynamics occurring in the presence of CO were studied. As a means of mitigating the effects of CO poisoning, short-circuiting strategies were evaluated. First, preliminary studies that evaluated the short-term effects of CO under different operating conditions were presented in a single cell. Next, an array of localised reference electrodes measured directly the anodic overpotential in three different locations of an MEA exposed to CO under galvanostatic control, where self-sustained potential oscillations were observed. This information was complemented by the measurement of the concentration of CO2 in the outlet that followed the evolution of the oxidation of CO. In the following study, a segmented-in-series system combined with different diagnostic techniques including thermal imaging, current interrupt and mass spectrometry was studied in the presence of two different concentrations of CO. The setups studied contributed to show the variations in the coverage of CO throughout the systems, and the effects of the local conditions in the oxidation of CO. These results highlight the importance of the distribution of the poisoning for the design and optimization of mitigation strategies against poisoning. Short-circuiting of the individual cells of a segmented-in-series stack was presented as an alternative, as the different patterns of the shorts (length and frequency) can be adjusted depending on the degree of poisoning, dependant on the location of the cells in the stack

The impact of radiation damage on photon counting with an EMCCD for the WFIRST-AFTA coronagraph

WFIRST-AFTA is a 2.4m class NASA observatory designed to address a wide range of science objectives using two complementary scientific payloads. The Wide Field Instrument (WFI) offers Hubble quality imaging over a 0.28 square degree field of view, and will gather NIR statistical data on exoplanets through gravitational microlensing. The second instrument is a high contrast coronagraph that will carry out the direct imaging and spectroscopic analysis of exoplanets, providing a means to probe the structure and composition of planetary systems. The coronagraph instrument is expected to operate in low photon flux for long integration times, meaning all noise sources must be kept to a minimum. In order to satisfy the low noise requirements, the Electron Multiplication (EM)-CCD has been baselined for both the imaging and spectrograph cameras. The EMCCD was selected in comparison with other candidates because of its low effective electronic read noise at sub-electron values with appropriate multiplication gain setting. The presence of other noise sources, however, such as thermal dark signal and Clock Induced Charge (CIC), need to be characterised and mitigated. In addition, operation within a space environment will subject the device to radiation damage that will degrade the Charge Transfer Efficiency (CTE) of the device throughout the mission lifetime. Here we present our latest results from pre- and post-irradiation testing of the e2v CCD201-20 BI EMCCD sensor, baselined for the WFIRST-AFTA coronagraph instrument. A description of the detector technology is presented, alongside considerations for operation within a space environment. The results from a room temperature irradiation are discussed in context with the nominal operating requirements of AFTA-C and future work which entails a cryogenic irradiation of the CCD201-20 is presented

Cryogenic irradiation of an EMCCD for the WFIRST coronagraph: preliminary performance analysis

The Wide Field Infra-Red Survey Telescope (WFIRST) is a NASA observatory scheduled to launch in the next decade that will settle essential questions in exoplanet science. The Wide Field Instrument (WFI) offers Hubble quality imaging over a 0.28 square degree field of view and will gather NIR statistical data on exoplanets through gravitational microlensing. An on-board coronagraph will for the first time perform direct imaging and spectroscopic analysis of exoplanets with properties analogous to those within our own solar system, including cold Jupiters, mini Neptunes and potentially super Earths. The Coronagraph Instrument (CGI) will be required to operate with low signal flux for long integration times, demanding all noise sources are kept to a minimum. The Electron Multiplication (EM)-CCD has been baselined for both the imaging and spectrograph cameras due its ability to operate with sub-electron effective read noise values with appropriate multiplication gain setting. The presence of other noise sources, however, such as thermal dark signal and Clock Induced Charge (CIC), need to be characterized and mitigated. In addition, operation within a space environment will subject the device to radiation damage that will degrade the Charge Transfer Effciency (CTE) of the device throughout the mission lifetime. Irradiation at the nominal instrument operating temperature has the potential to provide the best estimate of performance degradation that will be experienced in-flight, since the final population of silicon defects has been shown to be dependent upon the temperature at which the sensor is irradiated. Here we present initial findings from pre- and post- cryogenic irradiation testing of the e2v CCD201-20 BI EMCCD sensor, baselined for the WFIRST coronagraph instrument. The motivation for irradiation at cryogenic temperatures is discussed with reference to previous investigations of a similar nature. The results are presented in context with those from a previous room temperature irradiation investigation that was performed on a CCD201-20 operated under the same conditions. A key conclusion is that the measured performance degradation for a given proton fluence is seen to measurably differ for the cryogenic case compared to the room temperature equivalent for the conditions of this study

Studies on electrochromatic materials and devices

This thesis investigates electrochromic thin films needed to construct a variable transmission electro chromic device. Such a device is made of 5 layers sandwiched between 2 pieces of glass: two electronic transparent conducting layers, an optically active electro chromic layer (W03), a ion-conducting polymer electrolyte and an ionstorage layer (NiOx, TiOx, VOx, VzTiyOx) . Electrochromic NiOx thin films were produced by R.F. magnetron sputtering and electrodeposition techniques and studied under proton intercalation. A visible transmittance modulation of 0.70 and 0.80 and a visible coloration efficiency of 35 and 100 cm2.C-1 for a thickness of 300 and 200 nm were obtained for sputtered and chemically-deposited NiOx films respectively. Anodic films are extremely porous and soft. Under the mechanical stresses of ionic insertion/extraction they degrade more quickly than the compact nanostructure of physically deposited films. When studied under lithium intercalation, sputtered NiOx films exhibit a nucleation loop observed in cyclic voltammetry indicating the growth of a new phase and are seen to degrade quickly. NiOx films were not seen to be potential candidates for EC applications using Lt intercalation. W03, TiOx and VOx thin films were deposited by R.F. magnetron sputtering and studied under Lt intercalation/deintercalation. Optimised W03 films exhibited good electro chromic properties: a visible transmittance modulation of 0.82 and a visible coloration efficiency of 49 cm2 . C-l for a thickness of 450 nm. Electrochromic properties of TiOx films were seen to not strongly depend on the sputtering process parameters whereas VOx films showed a stronger dependence. TiOx films are able to store a limited quantity of charge Q = 13 mC.cm-2 for thicknesses greater than 13 nm. They are transparent in both charged and uncharged states T V,u and Tv,ch> 0.80, and are stable upon charge insertion/extraction. VOx films can store a much larger quantity of charge Q = 35 mC.cm-2 for a thickness of 70 nm. They are yellow in the uncharged state and bluish in the charged state: Tv,u and Tv,ch > 0.70, and the charge insertion/extraction process is seen to evolve during the initial cycles. Both TiOx and VOx films did not show all the required electrochromic properties for EC applications. The main achievement of this work was the development of highly durable vanadium/titanium mixed oxide thin films. Work was carried out on different VITi ratios using specific deposition techniques developed for that purpose. Films with a vanadium to titanium ratio of about 50 % showed optimum performance characteristics for passive ion storage layer applications. Such layers deposited on ITO exhibited high visible transmittance: Tv,ch > 0.62, and a relatively low visible modulation (0.20), with high storage capacity Q > 40 mC.cm-2 for a thickness of80 nm. The laminated W03IPAAUAlVzTiyOx EC device was assembled and exhibited under specific switching conditions encouraging properties: a visible transmittance modulation > 0.50 over more than 105 cycles

Distribution of Relaxation Times Analysis of High-Temperature PEM Fuel Cell Impedance Spectra

In this study, Distribution of Relaxation Times (DRT) was successfully demonstrated in the analysis of the impedance spectra of High-Temperature Polymer Electrolyte Membrane Fuel Cells (HT-PEMFC) doped with phosphoric acid. Electrochemical impedance spectroscopy (EIS) was performed and the quality of the recorded spectra was verified by Kramers-Kronig relations. DRT was then applied to the measured spectra and polarization losses were separated on the basis of their typical time constants. The main features of the distribution function were assigned to the cell’s polarization processes by selecting appropriate experimental conditions. DRT can be used to identify individual internal HT-PEMFC fuel cell phenomena without any a-priori knowledge about the physics of the system. This method has the potential to further improve EIS spectra interpretation with either equivalent circuits or physical models

The Impact of Radiation Damage on Electron Multiplying CCD Technology for the WFIRST Coronagraph

This thesis follows an investigation into the effects of radiation damage on the e2v CCD201-20; the detector baselined for use in the WFIRST coronagraph imaging and spectroscopy camera systems (hereafter, WFIRST CGI). The CCD201 is an EM-CCD, a variant of traditional CCD technology that is well suited for operation in light starved conditions. Despite successful implementation on many ground-based instruments, the technology has yet to be used within a space environment and therefore has low technological maturity compared to the standard CCD counterpart. Improvement of the technological maturity rested upon in-depth investigations into the effect of radiation damage on the CCD201, which in turn could be used to estimate the End Of Life (EOL) performance of the instrument and de-risk the utilisation of EM-CCDs for the mission. An in-depth radiation campaign was completed whereby multiple CCD201s were irradiated to multiple fluence levels at both room temperature and the nominal operating temperature of the mission (165 K). Performance was measured prior to and following each irradiation, including measurements of low-signal Charge Transfer Inefficiency (CTI), dark current and Clock Induced Charge (CIC). Significant performance differences were noted between the room temperature and cryogenic irradiation case, indicating that cryogenic irradiations are instrumental to accurate EOL performance estimates. CTI was identified as the key limitation to CGI science performance, and so attention then turned to amelioration strategies focused on improving performance in the presence of radiation damage, including trap pumping and narrow-channel modelling. The results presented in this thesis have helped lead to the adoption of the CCD201-20 for the WFIRST mission, have provided key insight into the differences between room temperature and cryogenic irradiations, have advanced the “trap pumping” technique for use on EM-CCDs and presented the properties on the dominant traps that impact CTI for radiation damaged CCDs. The findings are not only useful for the WFIRST CGI, but for any future space mission that will utilise EM-CCD technology in an environment where radiation has the potential to degrade science performance

Multicomponent Alloys for Biomedical Applications

Titanium alloys are considered to be the most advanced materials for orthopedic implants due to the favorable combination of mechanical properties, low density, tissue tolerance, high strength-to-weight ratio, good resistance to corrosion by body fluids, biocompatibility, low density, nonmagnetic properties, and the ability to join with the bone. This is the reason why we decided to assess the resistance of two titanium alloys currently used for orthopedic implants, namely, Ti6Al7Nb and Ti6Al4V, as reference, to cyclic fatigue by dynamic tests with crevice corrosion stimulation. According to the results obtained, the examined electrochemical quantities, the visual and SEM observations, and EDX analysis reveal better corrosion behavior of the prostheses made of Ti6Al4V—anodized series compared to prostheses made of Ti6Al7Nb. The further comparison of two explanted proximal modules, made of Ti6Al7Nb and Ti6Al4V, to the same type of prostheses evaluated by cyclic fatigue dynamic tests with crevice corrosion stimulation reveals that there are significant similarities, in particular with regard to the electrolyte diffusion, deposition of products and corrosion. Cation extraction tests which were carried out for Ti6Al7Nb prostheses that have undergone particular surface treatments show significant differences depending on the surface treatment and demonstrate that orthopedic implant materials are not “inert.

Microplastics in urban stormwater systems of Western Sydney

This study assessed the extent of microplastic (MP) pollution in the stormwater catchments of Western Sydney. Studies on microplastics in Australian stormwater systems and the lack of generally accepted methods for sample collection and the isolation and quantification of microplastics, were identified as opportunities for exploration. One of the objectives of this study was to identify and develop an acceptable method for separating microplastics from water samples. A novel procedure was developed to collect microplastics by filtering stormwater using a purpose-built single sieve (48.5 μm) mini-filtering device and cascade filtration setup, which included four steel filters with pore sizes of 48.5, 170, 2500 and 5000 μm. Additionally, the six most commonly used microplastic separation methods were selected to assess their organic matter degradation efficiency and polymer degradation potential. This research also presents the first results regarding microplastics pollution in Western Sydney stormwater catchments. Sample collection and analysis were carried out in two steps: preliminary sampling and secondary sampling. Preliminary sampling was carried out in the urban lake of Woodcroft using the mini-filtration device to test the practicality of the pre-identified procedures. Woodcroft and Wattle Grove were selected as the study areas for secondary sampling. Similar microplastics concentrations were observed in secondary sampling for both sites. From a comparison of the data obtained in this study with those in the literature, it was apparent that the stormwater originating from these two urban catchments was considerably contaminated with microplastics. This was attributed to anthropogenic activities in urban areas. Microplastic particles in stormwater can adversely impact aquatic life present in the receiving water bodies. Also, the presence of microplastics could suggest the presence of nanoplastics in urban stormwater. These findings have implications for urban stormwater management and highlight the need for comprehensive and in-depth studies to evaluate micro- and nanoplastics in the inland water bodies of Australia