A new NbN multilayer technology has been developed on 3 inch diameter R-plane sapphire substrates, for combining on-chip fast RSFQ circuits with GHz bandwidth optical links. The circuits take advantage of two high quality (110) NbN layers sputtered epitaxially on sapphire at 600°C and selectively patterned: a 400 nm thick layer (LL-250 nm at 6K) acts for the ground-plane and microbridge photodetectors are made of a 3.5-8 nm thick NbN epilayer with T, above 11 K. Innovative dielectrics formed of 10 nm thick MgO sputtered on top of 200 nm S O z layers are found to improve significantly the superconductivity of NbN junction electrode lines deposited below 300°C. Good quality, hysteretic 2 pm2 area, NbN/MgO/NbN junctions with high J, (up to 50 kA/cm2) are obtained with very large gap voltage (6.20 mV) and low sub-gap leakage current (V,,, > 15 mV) at 4.2 K. At 11 K such junctions are found self-shunted (J,-10 kA/cm2) with RJ, above 0.5 mV and with low J, spread in arrays. J, can be adjusted (reduced) without any detrimental effect on the junction quality or spread by annealing at 250°C.
In order to improve the capabilities of the electron-beam-induced current method, a technique based on scanning transmission electron-beam-induced current has been developed. It is shown that it enables the direct correlation of structural defects with their electrical activity. It implies the fabrication of ultrathin Schottky diodes (thickness ≤600 nm). From an approximate theoretical model it was inferred that the spatial resolution reaches about 200 nm in our experimental conditions. Experimental data are obtained on electron-grade and on upgrade metallurgical grade polycrystalline silicon, grown by the heat exchange method on which the behavior of carbon at grain boundaries and defects has been studied by transmission electron microscopy, high-resolution electron microscopy, and electron energy loss. There is a good agreement between the experimental data on electrical activity and the calculated approximation. The present method shows that the electrical activity is mainly related to the presence of impurities. Carbon seems to trap recombining impurities and specifically oxygen. Isolated dislocations are always electrically active whereas ‘‘clean’’ twin boundaries are not active. The activity at boundaries is always localized on extrinsic dislocations or on precipitates. Asymmetric profiles are also observed on boundaries and stacking faults. This was related to the existence of segregated zones lying on one side of these defects which are likely to act as diffusion barriers.
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