The volume, index-of-refraction, and stress changes induced in vitreous silica by irradiating with 18-keV electrons have been measured over the dose range from 2.5×1010 to 3×1012 rad. The volume and index measurements were performed using a new technique based on producing a spatial modulation of irradiated and unirradiated areas which is used as an optical phase grating. Comparison of the volume change and stress measurements shows there are two components to the compaction; one component, which dominates below doses of 2×1011 rad, is associated with stress generation, while the other component, which dominates at higher doses, proceeds without generating stress. In addition, the relationship between the index change and volume change observed for electron irradiation is different from that obtained by neutron irradiation and hydrostatic compression. The index-vs-volume change data can be interpreted using the Lorentz-Lorenz formalism and Neuman strain-optical constants, and provides further evidence of the two different compaction processes. The measured stress saturates at 1.5×108 N/m2 at approximately 6×1011 rad (0.24% volume change) and decreases with subsequent irradiation. In contrast the volume and index changes saturate at approximately 2×1012 rad (1.4% volume change).
The memory retention characteristics of silicon-nitride-oxide-silicon nonvolatile memory devices are found to be strongly thermally activated. A model is developed based on thermal emission of charge from traps. This model accurately predicts the threshold voltage decay of transistors stored in varying thermal environments. The model is demonstrated to be accurate over 7 decades of time and for temperatures between −40 and 200 °C.
The discharge behavior of silicon-oxide-nitride-oxide-semiconductor nonvolatile memory transistors is investigated for a range of programming and storage temperatures spanning −55 °C to 200 °C. A number of empirical observations strongly limit the nature of the mechanisms that govern charge injection and decay. Both electrons and holes contribute to the charge storage properties of the transistors, and the decay properties of both are thermally activated with a continuous distribution of activation energies (trap depths). Charge decay, for both charge states, is negligibly limited by mechanisms other than those which are strongly thermally activated. The programming temperature, relative to the storage temperature, significantly impacts the retention time of the excess electron state, while not affecting the long term decay of the excess hole state. The experimental results also have significant implications regarding proper retention screening techniques and nonvolatile ROM programming techniques.
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