Junctional areas

An interesting aspect of friction is the manner in which the area of contact changes as sliding occurs. This change may be measured either by conductivity, proportional to if, as in the case of metals, it is limited primarily by a number of small metal-to-metal junctions, or by the normal adhesion, that is, the force to separate the two substances. As an illustration of the latter, a steel ball pressed briefly against indium with a load of IS g required about the same IS g for its subsequent detachment [37]. If relative motion was set in, a value of S was observed and, on stopping, the normal force for separation had risen to 100 g. The ratio of 100 IS g may thus be taken as the ratio of junction areas in the two cases. 

Lp Pi 50 pm. and the reverse saturation current would be 17 x 10 = 17 pA for a square centimeter of junction area. Typical reverse saturation currents are about one thousand times greater as a result of generation—recombination currents in the depletion region (9). As the reverse voltage bias increases, the field increases in the depletion region until avalanche breakdown occurs, resulting in the characteristic shown in Figure 7. 

The AV node lies directly above the insertion of the septal leaflet of the tricuspid valve and anterior to the ostium of the coronary sinus. It is part of the AV junction area, which is divided into three regions. The transitional cells, or nodal approaches, connect the atrial myocardium to the compact portion of the AV node. The slowest conduction time occurs within the AV node [8]. At its distal end, the compact portion of the AV node enters the central fibrous body, becoming the penetrating portion, or His-bundle [9]. 

Two samples have been studied. Both are tunnel junctions made of Al/Al oxide/Al, using the double angle evaporation technique [7], In sample A (made by C. Wilson), the bottom and top Al films are 50 nm thick. The bottom electrode was oxidized for 2 hours in pure 02 at a pressure of 500 mTorr. The junction area is 15 pm2. In sample B (made by L. Spietz), the films are 120 nm and 300 nm thick, oxidation was for 10 min, and the junction area is 5.6 pm2. 

For the fabrication of stacked junctions we used focused ion beam technique. This technique has been developed for fabrication of both, the short [9] down to submicron scale [10] and the long junctions with a length of several tens microns [17]. For fabrication we used conventional FIB machine of Seiko Instr. Corp., SMI 9800 (SP) with Ga+-ion beam. The four leads were attached outside the junction area. The contact Ag pads were ablated and annealed before the FIB processing to avoid diffusion of Ga-ions into the junction body. The example of a short stack fabricated by FIB technique is shown in Fig. 1. Typically we had slightly overdoped stacked Bi2Sr2CaCu2C>8+8 structures with <5 0.25. They have Tc = 77K, pc(300K) = 10-12 Ohm cm, JC(4.2K) 1 kA/cm2. 

FIGURE 3.9. I(V) characteristics at 295 K and 77 K for Langmuir-Blodgett monolayer of fatty acid (CH3(CH2)nCOOH) on Al. Junction area is 0.2 cm2. Reprinted with permission from Ref.69 Polymeropoulos et al., J. Chem. Phys. 69, 1836-1847 (1978). Copyright 1978 American Institute of Physics. 

Figure 20. Example of double barrier magnetic tunnel junctions used in thermally assisted switching double free layer and weakly pinned storage layer (left) and minor TMR loops of double barriers junctions (right). Solid squares prior to pulse heating, open triangles after heating pulse and reversing the pinned layer in a 60 Oe easy axis external field. The junction area is 1 pm2, and a TMR of 25% and a resistance x area product of300 Opm2 are obtained.

Electronic properties of the nanojunctions nowadays are intensively investigated theoretically [15, 16]. In these articles it has been found that atomic structure of junction essentially influences on conductance type, conductance is suppressed when disorder in the junction area increases, and even quantum dots can be formed therein [15]. 

Sullivan and Eigler (42) developed a technique for removing the damaged surface layer from the p-n junction area of germanium diodes without masking. The germanium diode is placed inside a hairpin-shaped platinum cathode and a stream of 0.1% KOH is allowed to flow down between the cathode and the germanium. Surface tension confines the electrolyte to the junction area. A polished surface is usually completed in 1 to 2 min at 1. 5 amp/cm. ... 

Recently CMP of an interlevel dielectric SiO, deposited using evaporation techniques, was carried out for superconducting circuits. "CMP of the interlevel dielectric film allows the junction area to be greatly reduced, thereby increasing the speeds of operation of the circuits, by eliminating the need to open vias to make contact to the top electrode. ITie key to the success was in (a) the precise control to terminate the CMP within 200 nm after reaching the endpoint and (b) the ability to planarize a few hundred-micron-wide features. 

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