ELECTRICAL-FIELD EFFECT IN HIGHLY RESISTIVE NBN MICROBRIDGE

We investigate conduction properties and electrical field effect of hi ghly resistive NbN microbridges. The bridge region consists of NbN thi n films which have granular structure with grain size similar to 10nm. The dimension of the microbridge is approximately similar to 200nm in length and similar to 50nm in width. The highly resistive (>100k Ohm) microbridges exhibit nonlinear current-voltage (I-V) characteristics at low temperatures which are similar to those of an array of single e lectron tunneling (SET) junctions. The offset voltage ranges from 2mV to 300mV at 4.2K depending on the bridge resistance and the bridge wid th. Periodic modulation of bridge conductance is observed when externa l gate electric field is applied to the bridges. These results coincid e fairly well with the simulation based on the SET array model.