Electron trap states and low frequency noise in tunnel junctions

We present the results of measurements of the low frequency excess, or 1/f, noise in small area Nb/Nb2O5/PbBi and Nb/Nb2O5/PbAuIn tunnel junctions. Our study shows that the low frequency noise in all of these devices arises from fluctuations in the tunnel barrier transmission coefficient due to changes in the barrier shape. These time dependent barrier deformations appear to be due to charge capture and emission by electron traps in the tunnel barrier material. We find that Nb2O5barriers prepared either by Reactive Ion Beam Oxidation (RIBO) or by the Raider-Drake Plasma Oxidation process have one particular type of charge trap which dominates the noise spectrum. The density of these noise sources can be controlled in a variety of ways: Improved processing techniques result in higher quality Current-Voltage characteristics and a simultaneous decrease in the density of noise sources. Further, we find that ion implantation of the barrier with Boron results in a decrease in the density of noise sources. The detailed energy density of states for the noise sources also can be modified by thermal cycling and by application of large bias voltages. We will discuss the implications that our single tunnel junction measurements have for the production of extremely low noise SQUID magnetometers.