DIRECTED INELASTIC HOPPING OF ELECTRONS THROUGH METAL-INSULATOR-METALTUNNEL-JUNCTIONS

We have used the metal/amorphous silicon/metal tunnel junction as a mo del system to explore the role of localized states in electron transpo rt through thin insulating layers. We measured the tunneling conductan ce as a function of temperature T, bias voltage V, and barrier thickne ss d. The data show marked deviations from the classical WKB tunneling theory in the limit of low T and V with d intermediate between the de cay length in the barrier and the Mott variable range hopping length. The data are instead consistent with directed inelastic hopping along statistically rare but highly conductive ''chains'' of localized state s. The most effective chains for a given set of Conditions (T,V,d) con tain a definite number of localized states, N > 1, configured in a nea rly optimal way in space and energy. The conductance of the lowest-ord er hopping channel (all chains with N=2) exhibits the characteristic v oltage and temperature dependences G(2)(hop)(V) proportional to V-4/3, and G(2)(hop)(T) proportional to T-4/3, respectively, as predicted by theory. Higher-order channels (N > 2) also conform to the theoretical predictions remarkably well. The physical nature of these highly cond uctive channels and their implications for conduction through thick tu nnel barriers and thin dielectrics is discussed.