Publications

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A time-dependent, two-dimensional model is used to study internal heating effects and possible device failure in ZnO varistors in response to a high-voltage pulse. The physics and qualitative trends discussed here should hold for materials with internal microstructured grain boundaries. Our analysis is based on an electro-thermal, random Voronoi network. It allows for the dynamic predictions of internal failure and to track the progression of hot-spots and thermal stresses. Results here show that application of high voltage pulses can lead to the attainment of Bi2O3 melting temperatures in the grain boundaries and an accelerated progression towards failure. Comparisons between uniform and normally distributed barrier breakdown voltage showed relatively small difference. Physically, this is shown to be associated with the applied bias regime and grain size. It is argued that reduction in grain size would help lower the maximum internal stress. This is thus a desirable feature, and would also work to enhance the hold-off voltage for a given sample size. © 2007 IEEE.

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Electronic components such as bipolar junction transistors (BJTs) are damaged when they are exposed to radiation and, as a result, their performance can significantly degrade. In certain environments the radiation consists of short, high flux pulses of neutrons. Electronics components have traditionally been tested against short neutron pulses in pulsed nuclear reactors. These reactors are becoming less and less available; many of them were shut down permanently in the past few years. Therefore, new methods using radiation sources other than pulsed nuclear reactors needed to be developed. Neutrons affect semiconductors such as Si by causing atomic displacements of Si atoms. The recoiled Si atom creates a collision cascade which leads to displacements in Si. Since heavy ions create similar cascades in Si we can use them to create similar damage to what neutrons create. This LDRD successfully developed a new technique using easily available particle accelerators to provide an alternative to pulsed nuclear reactors to study the displacement damage and subsequent transient annealing that occurs in various transistor devices and potentially qualify them against radiation effects caused by pulsed neutrons.

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Germanium telluride undergoes rapid transition between polycrystalline and amorphous states under either optical or electrical excitation. While the crystalline phases are predicted to be semiconductors, polycrystalline germanium telluride always exhibits p -type metallic conductivity. We present a study of the electronic structure and formation energies of the vacancy and antisite defects in both known crystalline phases. We show that these intrinsic defects determine the nature of free-carrier transport in crystalline germanium telluride. Germanium vacancies require roughly one-third the energy of the other three defects to form, making this by far the most favorable intrinsic defect. While the tellurium antisite and vacancy induce gap states, the germanium counterparts do not. A simple counting argument, reinforced by integration over the density of states, predicts that the germanium vacancy leads to empty states at the top of the valence band, thus giving a complete explanation of the observed p -type metallic conduction.

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This report describes an LDRD-supported experimental-theoretical collaboration on the enhanced low-dose-rate sensitivity (ELDRS) problem. The experimental work led to a method for elimination of ELDRS, and the theoretical work led to a suite of bimolecular mechanisms that explain ELDRS and is in good agreement with various ELDRS experiments. The model shows that the radiation effects are linear in the limit of very low dose rates. In this limit, the regime of most concern, the model provides a good estimate of the worst-case effects of low dose rate ionizing radiation.

Mechanisms for enhanced low-dose-rate sensitivity are described. In these mechanisms, bimolecular reactions dominate the kinetics at high dose rates thereby causing a sub-linear dependence on total dose, and this leads to a dose-rate dependence. These bimolecular mechanisms include electron-hole recombination, hydrogen recapture at hydrogen source sites, and hydrogen dimerization to form hydrogen molecules. The essence of each of these mechanisms is the dominance of the bimolecular reactions over the radiolysis reaction at high dose rates. However, at low dose rates, the radiolysis reaction dominates leading to a maximum effect of the radiation.

In this paper, we describe a rate equation approach that leads to new insights about electrical breakdown in insulating and semiconducting materials. In this approach, the competition between carrier generation by impact ionization and carrier recombination by Auger and defect recombination leads to steady state solutions for the carrier generation rate, and it is the accessibility of these steady state solutions, for a given electric field, that governs whether breakdown does or does not occur. This approach leads to theoretical definitions for not only the intrinsic breakdown field but also other characteristic quantities. Results obtained for GaAs using a carrier distribution function calculated by both a Maxwellian approximation and an ensemble Monte Carlo method will be discussed.

We present a series of electronic structure calculations that demonstrate a mechanism for spontaneous ionization of hydrogen at the Si-SiO{sub 2} interface. Specifically, we show that an isolated neutral hydrogen atom will spontaneously give up its charge and bond to a threefold coordinated oxygen atom. We refer to this entity as a proton. We have calculated the potential surface and found it to be entirely attractive. In contrast, hydrogen molecules will not undergo an analogous reaction. We relate these calculations both to proton generation experiments and to hydrogen plasma experiments.

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