This report summarizes the work on breakdown modeling in nonuniform geometries by the ionization coefficient approach. Included are: 1) fits to primary and secondary ionization coefficients used in the modeling; 2) analytical test cases for sphere-to-sphere, wire-to-wire, corner, coaxial, and rod-to-plane geometries; a compilation of experimental data with source references; comparisons between code results, test case results, and experimental data. A simple criterion is proposed to differentiate between corona and spark. The effect of a dielectric surface on avalanche growth is examined by means of Monte Carlo simulations. The presence of a clean dry surface does not appear to enhance growth.3

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The effect of compressive and tensile plane-stress loading on formation energies and electronic properties of vacancies and divacancies in silicon are studied by first-principles approach for in-plane strains up to 0.7%. It is demonstrated that contributions to defect formation energies from the elastic lattice relaxation and from the band structure modification respond to stress in a different manner, leading to noticeable different behaviour of formation energies for different charges states. The most stable vacancy charge states at different Fermi level are shown to be sensitive to strain magnitude and sign. This results in the strain-induced shifts and even disappearance of some of thermal ionization levels of vacancies and divacancies in the band gap.

Two different computer program packages based on the self-consistent local-spin-density approximation- and -are employed in this study of substitutional copper Cu Si and monovacancies V Si in silicon, including the effects of their charge state. The programs differ in the types of basis sets and pseudopotentials they use, each with their own relative merits, while being similar in overall quality. This approach aims to reduce uncertainty in the results, particularly for small or subtle effects, where the risk is greatest that the conclusions are affected by artifacts specific to a particular implementation. The electronic structures of the two defects are closely related, hence they are expected to behave in a similar manner. For both defects structural distortions resulting in lower point group symmetries than T d (the highest possible) are found. This is in good agreement with the results of previous studies of V Si . Much less is known about symmetry-lowering effects for Cu Si ; however, the electronic levels of Cu Si have been measured accurately, while those for V Si are less accessible. Calculating them is a challenging task for theory. The strategy we adopt, based purely on comparing total energies of supercells in different charge states, with and without model defects, reproduces the three known levels for Cu Si reasonably well. Satisfactory results are also obtained for V Si , so far as can be judged for this more complex case.

In this paper we study the effect of chemical environment and elastic strains, which can arise in layered heterostructures due to the lattice parameter mismatch, on the vacancy formation energy in random Si-Ge compounds. Ab initio calculations demonstrate a number of simple trends characterizing the vacancy formation energy dependence on vacancy charge, the number of Ge atoms in its neighbourhood and on the magnitude of elastic strains. The obtained parameters of vacancy-germanium interaction indicate, in particular, a tendency for preferential vacancy accumulation in SiGe region of Si/SiGe/Si layered structures, which is confirmed here by Monte- Carlo simulation of high-temperature vacancy annealing and agrees well with recent experimental observations.