The properties and consequences of mobile ionic defects in metal halide perovskites

Metal halide perovskites are a class of semiconductors that are solution processable and have received much research attention over the last decade for the application of photovoltaics and other electronic devices. They differ from conventional semiconductors since in addition to free electrons and holes (electronic charge carriers) they also contain mobile ionic defects (ionic charge carriers) whose concentration can exceed the electronic charge carriers. Efficiency and other performance metrics have improved greatly yet the devices suffer from instability that hinders their wider application in industry. Further understanding of the mechanisms underlying their characteristics is still needed for the eventual commercial success of these material. Using drift-diffusion simulations, as well as time and frequency resolved optoelectronic measurements, I examine the implications of these ionic charge carriers for the fundamental properties of, interpretation of measurements of, and the stability to humidity of, metal halide perovskite electronic devices. Using drift-diffusion simulations, the mobile ionic charge is found to modify the device’s energetic profile during the attainment of thermodynamic equilibrium, doping the perovskite layer. This doping mechanism is shown to be controllable through the choice of materials and through the application of a long timescale pre-biasing voltage. The existence of this ‘interfacial doping’ mechanism is confirmed experimentally using a pulsed-voltage technique. I show that the relative densities of the ionic and electronic charges in the perovskite define which of the species control the device’s electronic properties, this understanding is critical when characterising the material’s properties. To highlight this, I asses the validity of the commonly applied Mott-Gurney law used to find the electronic mobility from current-voltage measurements of single carrier perovskite devices (space-charge-limited current measurements, SCLC). The electronic charge density needs to be approximately five times higher than the ion density for the law to apply. I show that, even using an experimental protocol that limits ionic redistribution during the measurement, determining the electronic mobility from SCLC measurements is difficult or impossible. Both water, and ionic transport are considered to play important roles in the degradation of metal halide perovskites, understanding their interaction may help to develop strategies to improve stability. Using electrochemical impedance spectroscopy interpreted with an equivalent circuit model I show that the activation energy for ionic charge migration decreases as environmental humidity increases. This observation is consistent with previous ab-initio calculations which calculate that the existence of the monohydrate phase decreases the ion transport activation energy. Using these findings, I suggest the process by which perovskites interact with water and forms new phases on the way to degradation. Finally, I show that the addition of Cs to the perovskite stops this decrease in ion migration activation energy. These findings have significant implications and provide valuable additions to the understanding of the degradation of different metal halide perovskites with exposure to water.Open Acces

High-temperature optically activated GaAs power switching for aircraft digital electronic control

Gallium arsenide high-temperature devices were fabricated and assembled into an optically activated pulse-width-modulated power control for a torque motor typical of the kinds used in jet engine actuators. A bipolar heterojunction phototransistor with gallium aluminum arsenide emitter/window, a gallium arsenide junction field-effect power transistor and a gallium arsenide transient protection diode were designed and fabricated. A high-temperature fiber optic/phototransistor coupling scheme was implemented. The devices assembled into the demonstrator were successfully tested at 250 C, proving the feasibility of actuator-located switching of control power using optical signals transmitted by fibers. Assessments of the efficiency and technical merits were made for extension of this high-temperature technology to local conversion of optical power to electrical power and its control at levels useful for driving actuators. Optical power sources included in the comparisons were an infrared light-emitting diode, an injection laser diode, tungsten-halogen lamps and arc lamps. Optical-to-electrical power conversion was limited to photovoltaics located at the actuator. Impedance matching of the photovoltaic array to the load was considered over the full temperature range, -55 C to 260 C. Loss of photovoltaic efficiency at higher temperatures was taken into account. Serious losses in efficiency are: (1) in the optical source and the cooling which they may require in the assumed 125 C ambient, (2) in the decreased conversion efficiency of the gallium arsenide photovoltaic at 260 C, and (3) in impedance matching. Practical systems require improvements in these areas

Current measurement in power electronic and motor drive applications - a comprehensive study

Current measurement has many applications in power electronics and motor drives. Current measurement is used for control, protection, monitoring, and power management purposes. Parameters such as low cost, accuracy, high current measurement, isolation needs, broad frequency bandwidth, linearity and stability with temperature variations, high immunity to dv/dt, low realization effort, fast response time, and compatibility with integration process are required to ensure high performance of current sensors. Various current sensing techniques based on different physical effects such as Faraday\u27s induction law, Ohm\u27s law, Lorentz force law, magneto-resistance effect, and magnetic saturation are studied in this thesis. Review and examination of these current measurement methods are presented. The most common current sensing method is to insert a sensing resistor in the path of an unknown current. This method incurs significant power loss in a sense resistor at high output currents. Alternatives for accurate and lossless current measurement are presented in this thesis. Various current sensing techniques with self-tuning and self-calibration for accurate and continuous current measurement are also discussed. Isolation and large bandwidth from dc to several kilo-hertz or mega-hertz are the most difficult, but also most crucial characteristics of current measurement. Electromagnetic-based current sensing techniques, which are used to achieve these characteristics, are analyzed. Many applications require average current information for control purposes. Different average current sensing methods of measuring average current are also reviewed. --Abstract, page iii

NEW MATERIAL FOR ELIMINATING LINEAR ENERGY TRANSFER SENSITIVITIES IN DEEPLY SCALED CMOS TECHNOLOGIES SRAM CELLS

As technology scales deep in submicron regime, CMOS SRAM memories have become increasingly sensitive to Single-Event Upset sensitivity. Key technological factors that impact Single-Event Upset sensitivity are gate length, gate and drain areas and the power supply voltage all of which impact transistor's nodal capacitance. In this work, I present engineering requirement studies, which show for the first time, the tread of Single-Event Upset sensitivity in deeply scaled SRAM cells. To mitigate the Single-Event Upset sensitivity, a novel approach is presented, illustrating exactly how material defects can be managed in a way that sets electrical resistance of the material as desired. A thin-film high-resistance value ranging from 2kΩ/-3.6MΩ/, and TCR of negative 0.0016%/˚C is presented. A defect model is presented that agrees well with the experimental results. These resistors are used in the cross-coupled latches; to decouple the latch nodes and delay the regenerative action of the cell, thus hardening against single even upset (SEU)

Analysis on Supercapacitor Assisted Low Dropout (SCALDO) Regulators

State-of-the-art electronic systems employ three fundamental techniques for DC-DC converters: (a) switch-mode power supplies (SMPS); (b) linear power supplies; (c) switched capacitor (charge pump) converters. In practical systems, these three techniques are mixed to provide a complex, but elegant, overall solution, with energy efficiency, effective PCB footprint, noise and transient performance to suit different electronic circuit blocks. Switching regulators have relatively high end-to-end efficiency, in the range of 70 to 93%, but can have issues with output noise and EMI/RFI emissions. Switched capacitor converters use a set of capacitors for energy storage and conversion. In general, linear regulators have low efficiencies in the range 30 to 60%. However, they have outstanding output characteristics such as low noise, excellent transient response to load current fluctuations, design simplicity and low cost design which are far superior to SMPS. Given the complex situation in switch-mode converters, low dropout (LDO) regulators were introduced to address the equirements of noise-sensitive and fast transient loads in portable devices. A typical commercial off-the-shelf LDO has its input voltage slightly higher than the desired regulated output for optimal efficiency. The approximate efficiency of a linear regulator, if the power consumed by the control circuits is negligible, can be expressed by the ratio of Vo/Vin. A very low frequency supercapacitor circulation technique can be combined with commercial low dropout regulator ICs to significantly increase the end-to-end efficiency by a multiplication factor in the range of 1.33 to 3, compared to the efficiency of a linear regulator circuit with the same input-output voltages. In this patented supercapacitor-assisted low dropout (SCALDO) regulator technique developed by a research team at the University of Waikato, supercapacitors are used as lossless voltage droppers, and the energy reuse occurs at very low frequencies in the range of less than ten hertz, eliminating RFI/EMI concerns. This SCALDO technique opens up a new approach to design step-down, DC-DC converters suitable for processor power supplies with very high end-to-end efficiency which is closer to the efficiencies of practical switching regulators, while maintaining the superior output specifications of a linear design. Furthermore, it is important to emphasize that the SCALDO technique is not a variation of well-known switched capacitor DC-DC converters. In this thesis, the basic SCALDO concept is further developed to achieve generalised topologies, with the relevant theory that can be applied to a converter with any input-output step-down voltage combination. For these generalised topologies, some important design parameters, such as the number of supercapacitors, switching matrix details and efficiency improvement factors, are derived to form the basis of designing SCALDO regulators. With the availability of commercial LDO ICs with output current ratings up to 10 A, and thin-prole supercapacitors with DC voltage ratings from 2.3 to 5.5 V, several practically useful, medium-current SCALDO prototypes: 12V-to-5V, 5V-to-2V, 5.5V-to-3.3V have been developed. Experimental studies were carried out on these SCALDO prototypes to quantify performance in terms of line regulation, load regulation, efficiency and transient response. In order to accurately predict the performance and associated waveforms of the individual phases (charge, discharge and transition) of the SCALDO regulator, Laplace transform-based theory for supercapacitor circulation is developed, and analytical predictions are compared with experimental measurements for a 12V-to-5V prototype. The analytical results tallied well with the practical waveforms observed in a 12V-to-5V converter, indicating that the SCALDO technique can be generalized to other versatile configurations, and confirming that the simplified assumptions used to describe the circuit elements are reasonable and justifiable. After analysing the performance of several SCALDO prototypes, some practical issues in designing SCALDO regulators have been identified. These relate to power losses and implications for future development of the SCALDO design

Qualifying silicon-germanium electronics for harsh radiation environments

The objective of this thesis is to investigate the robustness of Silicon-Germanium Heterojunction Bipolar Transistors (SiGe HBTs) to radiation-induced damage. The work described in this document delves into both total ionizing dose (TID) and single-event effect (SEE) mechanisms. Background information is provided for the general operation of SiGe HBTs and basic radiation effects (generic and specifically for SiGe HBTs). Four unique investigations are covered in this work: the first two focus on TID effects for high dose environments and to investigate enhanced-low-dose-rate-sensitivity, and the latter two studies investigate advances in hardening SiGe HBT profiles and methods to conduct SEE experiments using pulsed-lasers in place of highly energetic ionized particles.Ph.D

Measurement techniques for the characterization of radio frequency gallium nitride devices and power amplifiers

The rapid growth of mobile telecommunications has fueled the development of the fifth generation (5G) of standards, aiming to achieve high data rates and low latency. These capabilities make use of new regions of spectrum, wider bandwidths and spectrally efficient modulations. The deployment of 5G relies on the development of radio-frequency (RF) technology with increased performance. The broadband operation at high-power and high-frequency conditions is particularly challenging for power amplifiers (PA) in transmission stages, which seek to concurrently maximize linearity and energy efficiency. The properties of Gallium Nitride (GaN) allow the realization of active devices with favorable characteristics in these applications. However, GaN high-electron mobility transistors (HEMTs) suffer from spurious effects such as trapping due to physical defects introduced during the HEMT growth process. Traps dynamically capture and release mobile charges depending on the applied voltages and temperature, negatively affecting the RF PA performance. This work focuses on the development of novel measurement techniques and setups to investigate trapping behavior of GaN HEMTs and PAs. At low-frequency (LF), charge dynamics is analyzed using pulsed current transient characterizations, identifying relevant time constants in state-of-the-art GaN technologies for 5G. Instead, at high-frequency, tailored methods and setups are used in order to measure trapping effects during the operation of HEMTs and PAs in RF modulated conditions. These RF characterizations emulate application-like regimes, possibly involving the control of the device’s output load termination. Therefore, an innovative wideband active load pull (WALP) setup is developed, using the acquisition capabilities of standard vector-network-analyzers. Moreover, the implications of performing error-vector-magnitude characterizations under wideband load pull conditions are studied. Finally, an efficient implementation of a modified-Volterra model for RF PAs is presented, making use of a custom vector-fitting algorithm to simplify the nonlinear memory operators and enable their realization in simulation environments

Doctor of Philosophy

dissertationCu(In,Ga)Se2 (CIGSe), CuZnSn(S,Se)4 (CZTSSe), etc., are the potential chalcogenide semiconductors being investigated for next-generation thin film photovoltaics (TFPV). While the champion cell efficiency of CIGSe has exceeded 20%, CZTSSe has crossed the 10% mark. This work investigates the effect of laser annealing on CISe films, and compares the electrical characteristics of CIGSe (chalcopyrite) and CZTSe (kesterite) solar cells. Chapter 1 through 3 provide a background on semiconductors and TFPV, properties of chalcopyrite and kesterite materials, and their characterization using deep level transient spectroscopy (DLTS) and thermal admittance spectroscopy (TAS). Chapter 4 investigates electrochemical deposition (nonvacuum synthesis) of CISe followed by continuous wave laser annealing (CWLA) using a 1064 nm laser. It is found that CWLA at ~ 50 W/cm2 results in structural changes without melting and dewetting of the films. While Cu-poor samples show about 40% reduction in the full width at half maximum of the respective x-ray diffraction peaks, identically treated Cu-rich samples register more than 80% reduction. This study demonstrates that an entirely solid-phase laser annealing path exists for chalcopyrite phase formation and crystallization. Chapter 5 investigates the changes in defect populations after pulse laser annealing in submelting regime of electrochemically deposited and furnace annealed CISe films. DLTS on Schottky diodes reveal that the ionization energy of the dominant majority carrier defect state changes nonmonotonically from 215±10 meV for the reference sample, to 330±10 meV for samples irradiated at 20 and 30 mJ/cm2, and then back to 215±10 meV for samples irradiated at 40 mJ/cm2. A hypothesis involving competing processes of diffusion of Cu and laser-induced generation of In vacancies may explain this behavior. Chapter 6 compares the electrical characteristics of chalcopyrite and kesterite materials. Experiments reveal CZTSe cell has an order of magnitude higher net carrier concentration and saturation current density, whereas five times smaller shunt resistance and depletion width at equilibrium compared to CIGSe. The TAS measurements suggest that the dielectric freezeout occurs at relatively higher temperatures (~ 150 K) and lower frequencies (< 1 MHz) for CZTSe cell. Both sample types show a broad DLTS signal, possibly indicating a parallel recombination process with carrier emissions