Tunnel resonance resistance

Figure C3.2.16. Dependence of measured resistance in an STM junction consisting of a bare tip a tip with one Xe atom attached, and a tip with two Xe atoms. Note that the Xe atoms facilitate tunnelling (compared to empty space). From Yazdani A, Eigler D M and Lang N D 1996 Off resonance conduction tlirough atomic wires Science 111 1921-4.

MBE supcrlatticc structures also arc vciy promising. These supcrlatticc structures, with periodicities of 50-100 micrometers, show negative resistance characteristics attributed to resonant tunneling into the quantized energy states associated with the narrow potential wells formed by the layers. Detailed studies have shown that the potential well distributions may be controlled and positioned to a precision of a few atomic layers. 

Fig. 3 Current-voltage plots taken at a variety of tunnel gap dimensions. The linear ohmic response suggests the absence of sharp resonant tunneling phenomena occurring in the Pt-lr tip/0.01 M HCI04/Au(111) junction. The gap dimensions were set according to a specific tunnel resistance (/ r), after which the feedback was disengaged to collect the / — V data. The currents were rescaled, (Q, for display purposes. (Reprinted with permission from Ref [29], Copyright 1995 by Elsevier.)...

Film growth under free corrosion conditions requires transfer of electrons through the inner layer to couple oxidation of metal constiments with the reduction of water or dissolved oxygen. The MCM assumes that ionic point defects play the role of electron donors or acceptors [77], electronic conduction being coupled to the transport of ionic defects via resonance (band-to-band) tunneling. The model does not consider electronic conduction to be rate-limiting since in sim electric resistance and electrochemical impedance measurements during oxidation of steels in SCW between 500 and 700°C have shown that the oxide layer specific conductivity is comparable to that on pure iron [82]. 

Negative differential resistance can also be observed for surfaces with localized trap states. A tunnelling electron can become localized for long times in these surface states, when they are in resonance with the Fermi level of the tip. Electrons so localized electrostatically repel other electrons causing a decrease in tunnelling current, referred to as a coulomb blockade. The voltage at which the NDR occurs is a measure of the energy of the localized trap state. 

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