Damage mechanisms in superconductors due to the impact of high energy proton beams and radiation tolerance of cryogenic diodes used in particle accelerator magnet systems

High energy hadron accelerators such as the Large Hadron Collider (LHC) at CERN and its planned upgrade to achieve higher luminosity, the High Luminosity Large Hadron Collider (HL-LHC), require superconducting magnets to provide strong magnetic fields, needed to steer and focus the particle beams at these high energies. During operation the superconducting magnets and their components are exposed to radiation resulting from primary and secondary particles from two main sources of beam losses. During normal operation, steady state losses resulting from interaction of the particle beams with residual gas molecules or from particle debris in interaction points affect the accelerator magnets and their components along the machine. In case of failures, significant parts of the beam can be lost in a short time, resulting in localized damage due to heating from energy deposition, which in turn causes thermo-mechanical stresses and strains. In the HL-LHC, novel focusing superconducting quadrupole magnets will be installed, based on Nb$_3$Sn and located close to the interaction points. Furthermore, the beam intensity will be doubled. Both, steady state losses and the severity of losses due to fast failures scale with the beam intensity. In this thesis, effects of beam losses on accelerator magnet components were studied. Firstly, the effects of high intensity and high energy proton beam impact on the low temperature superconductors (LTS) Nb-Ti, Nb$_3$Sn and tapes based on the high temperature superconductor (HTS) YBCO were studied. An experiment was performed where beam was directed on superconductors in a cryogenic environment in CERN’s HiRadMat facility. The performance of the superconductors was afterwards analyzed for their critical transport current, critical field and temperature, as well as inspected with optical and electron microscopic methods. The experimental setup, the observed damage mechanisms and the subsequent analysis are discussed. Secondly, the powering layout of the magnet circuits foresees the use of cryogenic power diodes, connected in parallel to each magnet, serving as passive protection in case of a quench. The diodes are located in close proximity to the beam axis and are affected by the enhanced radiation levels close to the interaction points. To identify a diode type that can be safely operated during the lifetime of HL-LHC, the radiation hardness of existing LHC-type diodes and prototype diodes, that are expected to be more radiation tolerant were tested. An experiment was set up, which allowed the irradiation and in situ measurements of three different types of diodes at cryogenic temperatures. All prototypes were analyzed for forward and reverse bias voltage characteristics and the temperature dependence while warming up. Their thermal annealing potential could also be evaluated. The experimental setup, the in situ measurements and the subsequent analysis are discussed

Solar spectral irradiance - measurement and application in photovoltaics

This thesis presents the outcome of investigations undertaken in the field of terrestrial spectral solar irradiance characterisation and its impact on photovoltaics. Spectral irradiance has not previously been widely researched in the context of photovoltaic applications. Long-term, natural environment spectral irradiance observations are practically non-existent with availability very limited in terms of covered period, temporal resolution and site location. The work presented concentrates on four major aspects of spectral irradiance: spectroradiometer calibration spectral irradiance calibration transfer standards natural spectral irradiance variability and its impact on photovoltaic device efficiency impact of reference sensor spectral mismatch on accuracy of reference irradiance measurement

Electronic processes in electroluminescent device structures

Electronic processes in two different electroluminescent device structures, the forward biassed metal/thick insulator/semiconductor (MIS) diode and the high field metal/insulator/metal (MIM) panel, are investigated. Models are produced to explain the behaviour of two particular MIS systems which have been studied experimentally. One of these systems is the Au/cadmium stearate/n-GaP structure, where the insulator is deposited using Langmuir-Blodgett (LB) technology. The other is the Au/i-ZnS/n-ZnS structure. In the MIS devices electroluminescence occurs as a result of the recombination of electrons and holes in the semiconductor and so it is necessary to have an efficient minority carrier (hole) injection mechanism. Attention is paid to the impact excitation of the electron gas in the metal by the electrons injected from the semiconductor because this has been proposed by other workers as a process for producing holes in the metal that are energetically capable of entering the semiconductor valence band, provided they can traverse the insulator. The characteristics of the LB film devices are found to be best described by assuming the minority carrier injection to be limited by the hole transport through the insulator. Hopping between interface states on the successive LB layers is proposed as the transport mechanism. However, the device incorporating a II-VI semi-insulator is shown to be more characteristic of hole transport in the insulator valence band and a minority carrier injection which is limited by the supply of holes from the metal. In high field MIM panels the mechanism of electroluminescence is quite different with impurity centres being impact excited or impact ionised by injected electrons and subsequently luminescing. Such devices driven by a dc signal are susceptible to the formation of high current filaments which burn out and result in device failure. A model is developed which predicts that there is a voltage range over which the device can exist in either a low current state or two higher current states and the resultant instability is expected to be destructive. Current-voltage characteristics are produced using this model and their general features are found to be relatively insensitive to material and device parameters. In order to understand the evolution of the electrical state of the MIM device after switch-on, a time dependent theory of system behaviour is also developed. This is particularly important as the devices are usually driven by a pulsed signal. For an homogeneous system the current is found to converge to the lower current state of the steady state characteristic

P-N junctions in intermetallic semiconductors

Imperial Users onl

Development and fabrication of improved Schottky power diodes

Reproducible methods for the fabrication of silicon Schottky diodes have been developed for tungsten, aluminum, conventional platinum silicide, and low temperature platinum silicide. Barrier heights and barrier lowering under reverse bias have been measured, permitting the accurate prediction of forward and reverse diode characteristics. Processing procedures have been developed that permit the fabrication of large area (about 1 sq cm) mesageometry power Schottky diodes with forward and reverse characteristics that approach theoretical values. A theoretical analysis of the operation of bridge rectifier circuits has been performed, which indicates the ranges of frequency and voltage for which Schottky rectifiers are preferred to p-n junctions. Power Schottky rectifiers have been fabricated and tested for voltage ratings up to 140 volts

Optoelectronic study of InGaN/GaN LEDs

The quality of light emitting diodes (LEDs) has increased to a point where solid state lighting is becoming fairly common. Despite this, greater understanding of the effect of the device structure and the electric fields within them is helpful to continue improving device efficiency and uniformity and in reducing costs. In this thesis the optical and electronic properties of InGaN/GaN LEDs have been studied with a combination of luminescence spectroscopy, microscopy, conductivity mapping and efficiency measurements.A study was made of the effects of the various electric fields, and the interplay between them, on LED luminescence and conductivity. Cathodoluminescence (CL) mapping shows die to die variation across large wafers revealing the powerful effects of a induced electric field on spectral intensity/position/width, in uncontacted devices. Micron scale spots in the LED material, lower in luminescence intensity and which trap charge, were revealed by CL/EBIC mapping with the origin attributed to cluster point defects in the active region. Depth resolved CL and CL under bias reveal the extent of asymmetry in carrier transport in the p/n type GaN around the active region. LEDs grown with different active region temperature profiles were studied. Devices exposed to high temperature after quantum well growth (2T) were found to have a uniform spatial luminescence and a peak efficiency that is higher and occurs at a lower current density (0.1 W/A @ 1 Acm¯²). By contrast those with a low temperature cap (Q2T) exhibit dark spots in the luminescence, and a lower peak efficiency at a higher current density (0.04 W/A @ 10 Acm¯²). The effect of improvement in LED design and material quality on the device efficiency, uniformity and spectral characteristics was studied. The addition of an Al₀.₂₃Ga.₇₇N electron blocking layer (EBL) was found to reduce the size and strength of the dark spots by about a factor of 2, while an additional In₀.₀₅Ga₀.₉₅N underlayer (UL) removed the dark spots entirely and shifted the luminescence peak by around 100 meV. The effect on the electroluminescence efficiency of the reduction in template dislocation density was found to depend strongly on the drive current density, with defect non-radiative recombination more important at low currents. Overall device efficiency was shown to be improved with an EBL and UL. The most efficient devices were those with the 2T type growth but the relative improvements are larger in LEDs grown with the Q2T method.Together, the results present a number of factors limiting the performance of current LEDs and suggest potential routes for improvement and optimisation.The quality of light emitting diodes (LEDs) has increased to a point where solid state lighting is becoming fairly common. Despite this, greater understanding of the effect of the device structure and the electric fields within them is helpful to continue improving device efficiency and uniformity and in reducing costs. In this thesis the optical and electronic properties of InGaN/GaN LEDs have been studied with a combination of luminescence spectroscopy, microscopy, conductivity mapping and efficiency measurements.A study was made of the effects of the various electric fields, and the interplay between them, on LED luminescence and conductivity. Cathodoluminescence (CL) mapping shows die to die variation across large wafers revealing the powerful effects of a induced electric field on spectral intensity/position/width, in uncontacted devices. Micron scale spots in the LED material, lower in luminescence intensity and which trap charge, were revealed by CL/EBIC mapping with the origin attributed to cluster point defects in the active region. Depth resolved CL and CL under bias reveal the extent of asymmetry in carrier transport in the p/n type GaN around the active region. LEDs grown with different active region temperature profiles were studied. Devices exposed to high temperature after quantum well growth (2T) were found to have a uniform spatial luminescence and a peak efficiency that is higher and occurs at a lower current density (0.1 W/A @ 1 Acm¯²). By contrast those with a low temperature cap (Q2T) exhibit dark spots in the luminescence, and a lower peak efficiency at a higher current density (0.04 W/A @ 10 Acm¯²). The effect of improvement in LED design and material quality on the device efficiency, uniformity and spectral characteristics was studied. The addition of an Al₀.₂₃Ga.₇₇N electron blocking layer (EBL) was found to reduce the size and strength of the dark spots by about a factor of 2, while an additional In₀.₀₅Ga₀.₉₅N underlayer (UL) removed the dark spots entirely and shifted the luminescence peak by around 100 meV. The effect on the electroluminescence efficiency of the reduction in template dislocation density was found to depend strongly on the drive current density, with defect non-radiative recombination more important at low currents. Overall device efficiency was shown to be improved with an EBL and UL. The most efficient devices were those with the 2T type growth but the relative improvements are larger in LEDs grown with the Q2T method.Together, the results present a number of factors limiting the performance of current LEDs and suggest potential routes for improvement and optimisation

Electroluminescence in epitaxial thin film zns and znse

The application of the metalorganic chemical vapour deposition technique to the production of II-VI compound semiconductor electroluminescent devices is discussed. Both low field MIS minority carrier injection devices and high field impact excitation structures are considered, and comparisons are drawn with more commiercially orientated electroluminescent displays. The epitaxial growth of ZnS and ZnSe onto (100) orientated GaAs substrates, using the reactions between dimethyl zinc and the hydrides HgS and H2Se, is described. Details are given of a novel epitaxial MISi device processing technology, in which a ZnS I-layer also acts as an etch-stop, thus enabling chemical removal of the GaAs substrate. Metal electrodes deposited directly onto the ZnS and ZnSe allow the electrical and electroluminescent characteristics of these epitaxial II-VI compound layers to be investigated in the absence of any influence from the substrate material. X-ray diffraction and reflection high energy electron dififraction confirm that the structures are epitaxial and of excellent crystallinity. It is demonstrated in an electron beam induced current study that conduction in the epitaxial MIS devices is highly uniform, and this is manifested in a uniform spatial distribution of electroluminescence. A description is given of high field impact excitation electroluminescent devices, in which the ZnS layer is doped with manganese during MOCVD growth. The spatial distribution of EL in these devices is shown to be non-uniform, and thus indicative of filamentary conduction in the ZnS:Mn, in accordance with a recently proposed dielectric breakdown model of instability. It is demonstrated that the transient characteristics of the epitaxial structures correlate with those of commercial polycrystalline devices, and are also consistent with the predictions of a dynamic model of instability. As a result of filamentary conduction, both epitaxial and polycrystalline devices are prone to degradation through localised dielectric breakdown. These breakdown events generally result in a gradual erosion of the active electrode area, although, under certain operating conditions, mobile filaments can cause rapid destruction of epitaxial structures. The columnar microstructure of sputtered devices appears to prevent such filament mobility, and it is concluded that, although filamentary conduction is a result of the carrier injection mechanism and is independent of the crystallinity, the associated damage is strongly influenced by the microstructure of the device

Sub-Hz line width diode lasers by stabilization to vibrationally and thermally compensated ULE Fabry-Perot cavities

We achieved a 0.5 Hz optical beat note line width with ~ 0.1 Hz/s frequency drift at 972 nm between two external cavity diode lasers independently stabilized to two vertically mounted Fabry-Perot (FP) reference cavities. Vertical FP reference cavities are suspended in mid-plane such that the influence of vertical vibrations to the mirror separation is significantly suppressed. This makes the setup virtually immune for vertical vibrations that are more difficult to isolate than the horizontal vibrations. To compensate for thermal drifts the FP spacers are made from Ultra-Low-Expansion (ULE) glass which possesses a zero linear expansion coefficient. A new design using Peltier elements in vacuum allows operation at an optimal temperature where the quadratic temperature expansion of the ULE could be eliminated as well. The measured linear drift of such ULE FP cavity of 63 mHz/s was due to material aging and the residual frequency fluctuations were less than 40 Hz during 16 hours of measurement. Some part of the temperature-caused drift is attributed to the thermal expansion of the mirror coatings. High-frequency thermal fluctuations that cause vibrations of the mirror surfaces limit the stability of a well designed reference cavity. By comparing two similar laser systems we obtain an Allan instability of 2*10-15 between 0.1 and 10 s averaging time, which is close to the theoretical thermal noise limit.Comment: submitted to Applied Physics

ADVANCED TERMINATION STRUCTURES FOR HV POWER SEMICONDUCTOR DEVICES

Thesis is in the field of power electronic devices. They operate in power conversion system as power switches able to impose the ON/OFF condition. A power device present two macroscopic areas: 1) active area; 2) termination area. The first one is responsible of conduction during the On-state of the device; while the second one mainly contributes to withstand the voltage rate during the Off-state condition. The actual trend of power devices tends to a technological scaling to increase the switching frequency and reduce the costs. As consequence, the percentage of the die area occupied by the termination is even more growing since its dimension is related to the voltage rate. This introduce the necessity to develop new termination design able to sustain the same voltage rate with a reduced consumption of area. At the same time, the new designs must also guarantee the required standard in term of reliability and ruggedness consolidated in classical designs. The scaling has also effects on the active area, where current density is even more growing leading to reliability problem from the thermal point of view. The design of new termination structure, as well as, reliability analysis of the active area have been the main focus of my third year research activities. Two new termination structure have been designed by means of 2D TCAD simulations. The new design realize an improvement of the classical Junction Termination Extension (JTE) technique to sustain a voltage rate of 1.2 kV. JTE design offers the possibility to considerably reduce the occupation of area but present great limitations in term of reliability. JTE needs of optimizing a low-doped P-region to maximize the breakdown voltage of the device. The critical point is that the breakdown voltage is strongly affected by the doping profile of the low-doped region. The breakdown stability is guaranteed only around the optimal value of the doping concentration. A deviation from the optimal value of about 7-8% already produces an inacceptable degradation of the breakdown capability. Since technological process can be subjected to fluctuation or/and contamination of external impurity able to modify the doping profile have led the JTE design to be less attractive for industry. In my activity two innovative two innovative JTE-based terminations have been presented providing a well precise optimization methodology to maximize the breakdown voltage. Both designs have been developed in order to increase the reliability of the device guaranteeing the breakdown stability in a wide range of doping concentration of the low-doped P-region. The first one design exploits the action of a special passivation layer named SIPOS; while the second one is made combining both JTE and a classical Floating Filed Ring technique. The performances of both terminations are than compared with that an advanced Floating Field Ring structure appropriately optimized. Termination ruggedness has been evaluated by means of Unclamped Inductive Switching simulations as the capacitance of power absorption until the failure event. Therefore, current crowding phenomena occurring in avalanche condition are deeply analyzed together with its relation with the Negative Differential Resistance branch on the I-V avalanche curve. During the third year I spent three months period to the Franhoufer Institute (ISIT). My research was focused on aspects regarding technological process of power devices. During this period I realized an emulation process flow of a Floating Field Ring termination for a 600V Punch-Through IGBT. The reliability of the active area was analyzed by means of Short-Circuit test. It is an industrial test able to evaluate the capacitance of power absorption during the Short-Circuit condition of a device. During the Short-Circuit, the device is driven in conduction at high voltage and the current is limited only by the internal resistance. The influence of design parameters on the Short-Circuit capability of a FS-IGBT device has been analyzed. A commercial device has been experimentally characterized by means of static curves tracker, Inductive Load Switching test and Short-Circuit test. The Short-Circuit capability analysis was led with a simulation approach by means of 3D TCAD electro-thermal simulations. The physical models of the elementary cell of the IGBT device have been calibrated to fit the characteristics of the commercial device at different temperatures. An innovative design has been proposed to increase the Short-Circuit capabilit