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Radiation-induced property changes in materials originate from the energy transfer from an incoming particle to the existing lattice, displacing atoms. The displaced atoms can cause the formation of extended defects including dislocation loops, voids, or precipitates. The non-equilibrium defects created during damage events determine the extent of these larger defects and are a function of dose rate, material, and temperature. However, these defects are transient and can only be probed indirectly. This work presents direct experimental measurements and evidence of irradiated non-equilibrium vacancy formation, where in situ positron annihilation spectroscopy was used to prove the generation of non-equilibrium defects in silicon.

The transition from insulator to conductor can be realized in some materials but requires modification of both the arrangement of atoms and their electronic configurations. This is often achieved by doping. Here we reveal a different mechanism the lattice may adopt to induce such a transition. Experiments showed the surprising finding that limited exposure to sub-bandgap light caused a permanent transition from an insulator state to a conductor state in the insulating oxide Ga2O3, with 9-orders of magnitude increase in electronic conduction. Furthermore, annealing up to 400 C did not suppress or decrease the induced conductivity. Photoexcitation by light-induced modification in the charge state of defects and subsequent lattice distortion around them was suggested to be the underlying mechanism behind this transition. Density functional theory calculations confirmed that modifying the charge state of defects leads to redistribution of the localized electrons and massive structural distortion in the surrounding lattice, causing large shifts in the density of states and introducing new states with shallower energy levels. Both experimental and theoretical results revealed the introduction of new stable shallow energy levels, explaining the mechanism behind the transition from an insulator to a conductor state by light. We suggest that this mechanism may occur in other wide bandgap metal oxides leading to drastic modification in their electronic properties.

Positron annihilation spectroscopy provides a sensitive means of non-destructive characterization of materials, capable of probing single atom vacancies in solids with 10−7 sensitivity. We detail here the development of a magnetically guided, variable energy, pulsed positron beam designed to conduct depth-dependent defect studies in metals, semiconductors, and dielectrics, which will be the first of its kind in the United States. The design of the target stage provides capabilities for measurements during in situ annealing up to 800 °C and incorporates a new approach to minimize the background due to energetic backscattered positrons. The developed beam at Bowling Green State University provides a powerful tool for characterization of thin films, devices, and ion irradiated materials.

A new nondestructive accelerator based x‐ray fluorescence (AXRF) approach has been developed to identify heavy metals in large‐volume samples. Such samples are an important part of the process and waste streams of U.S Department of Energy sites, as well as other industries such as mining and milling. Distributions of heavy metal impurities in these process and waste samples can range from homogeneous to highly inhomogeneous, and non‐destructive assays and imaging that can address both are urgently needed. Our approach is based on using high‐energy, pulsed bremsstrahlung beams (3–6.5 MeV) from small electron accelerators to produce K‐shell atomic fluorescence x‐rays. In addition we exploit pair‐production, Compton scattering and x‐ray transmission measurements from these beams to probe locations of high density and high atomic number. The excellent penetrability of these beams allows assays and images for soil‐like samples at least 15 g/cm2 thick, with elemental impurities of atomic number greater than app...

The hardness of CdTe single crystals doped with 1.2 × 1017 and 2 × 1018 In cm−3 varies with treatments that vary the concentrations of electrons or holes. The hardness increases with increasing concentrations of either electrons or holes. Illumination increases or decreases the hardness depending on the position of the Fermi level in the unilluminated crystal. These effects are explained by the interaction of dislocations with point defects on the basis of a model with two types of dislocations, one having acceptor, the other donor properties. Die Harte von mit 1,2 × 1017 und 2 × 1018 In cm−3 dotierten CdTe-Einkristallen andert sich mit Behandlungen, welche die Elektronen- oder Locherkonzentrationen andern. Die Harte vergrosert sich, wenn sowohl die Konzentration der Elektronen als auch die der Locher zunimmt. Belichtung verursacht entweder Zu- oder Abnahme der Harte in Abhangigkeit von der Lage des Ferminiveaus. Diese Vorgange werden erklart mit einem Modell mit zwei Typen von Versetzungen, eine mit Donator-, die andere mit Akzeptoreigenschaften.

The Hall effect of nearly pure CdTe and CdTe doped with 1.2 × 1017 and 2 × 1018 In cm−3 was measured as a function of the cadmium pressure both at high temperatures where the crystals were in equilibrium with the atmosphere and at room temperature on crystals quenched after the anneal. In the almost pure crystals, a fraction of the electrons resulting from a Cd excess at high temperatures disappear after cooling due to precipitation of the native donors. Transmission electron microscopy shows the presence of Cd precipitates and/or intrinsic dislocation loops, indicating interstitial Cd as the precipitating native donor. For the indium doped crystals the properties of quenched crystals depend on the cooling procedure, different results being obtained for cooling the crystal before the Cd pool establishing the Cd pressure, or vice versa. At high pcd the difference is attributed to co-precipitation of indium and cadmium as an In-Cd alloy in the former case but not in the latter. At medium pcd the difference is due to a difference in the rate of cooling, cadmium vacancies precipitating in one case but not in the other. Es wurde der Halleffekt von nahezu reinem CdTe und CdTe, das mit 1,2 × 1017 und 2 × 1018 In cm−3 dotiert war, in Abhangigkeit vom Kadmiumdruck sowohl bei hohen Temperaturen, wo sich die Kristalle im Gleichgewicht mit dem Dampf befinden, als auch bei Zimmertemperatur an Kristallen, die nach dem Tempern abgeschreckt waren, gemessen. In den nahezu reinen Kristallen verschwindet ein Teil der aus einem Kadmiumuberschus bei hohen Temperaturen resultierenden Elektronen nach dem Abkuhlen infolge Ausscheidung von Eigenstorstellen. Transmissionselektronenmikroskopie zeigt die Anwesenheit von Cd-Aussheidungen und/oder Eigen-Versetzungsringen, was auf Zwischengitterkadmium als ausscheidendem Eigendonator hinweist. Fur die indiumdotierten Kristalle hangen die Eigenschaften der abgeschreckten Kristalle von dem Abkuhlungsverfahren ab, wobei unterschiedliche Ergebnisse erhalten werden, wenn der Kristall vor dem Kadmium, das den Cd-Druck aufrechterhalt, abgekuhlt wird oder umgekehrt. Bei hohem pcd wird der Unterschied im ersten Fall der gleichzeitigen Ausscheidung von Indium und Kadmium, als In-Cd-Legierung zugeschrieben. Im zweiten Fall tritt keine Indiumausscheidung auf. Bei mittlerem pcd wird der Unterschied durch die unterschiedliche Abkuhlungsgeschwindigkeit hervorgerufen, wobei Kadmiumleerstellen nur prazipitieren, wenn der Kristall langsam abgekuhlt wird.

A new process is described for achieving the current‐voltage characteristics of a ZnO varistor with a breakdown voltage less than 20 V. The structure has been made by using Ag contacts containing oxides of Bi and Pb which serve both as a diffusion source for grain and/or grain boundary doping, and as metallic contacts on polycrystalline ZnO substrate. The structure was also made by sputtering a layer of various metal oxides such as Bi2O3, Sb2O3, Co3O4, etc. and then evaporating contact metals. The device shows a nonlinear current‐voltage characteristic when annealed at 800–900 °C in air. Annealing in N2 ambient however, results in linear I‐V characteristics. The presence of certain critical additives such as Bi2O3 was found essential for the development of nonlinearity. Based on these observations and the known defect structure of pure ZnO, a defect model which takes into account the role of oxygen in developing the nonlinear behavior of the proposed device is presented.