Experimental Study of Electron Transport through Nanometer-Scale Metal-Oxide Junctions

Abstract

This work presents results of an experimental study of electron transport through few nanometer-scale metal oxide junctions of two types:First, we have measured transport properties of Nb/Al/Nb junctions fabricated using thermal oxidation or rf-plasma oxidation at various conditions, with rapid thermal post-annealing (RTA) to improve junction endurance in electric fields in excess of 10 MV/cm. The results indicate that such junctions may combine high field endurance (corresponding to at least 10^10 write/erase cycles in floating-gate memories) and high current density (corresponding to 30-ns-scale write/erase time) at high voltages, with very low conductance (corresponding to retention time scale ~0.1 s) at low voltages. We discuss the improvements necessary for the use of such junctions in advanced floating-gate memories.Second, we have studied resistive bistability (memory) effects in junctions based on several metal oxides, with a focus on sample-to-sample reproducibility which is necessary for the practical use of such junctions, in particular as crosspoint devices of hybrid CMOS/nanoelectronic circuits. Few-nm-thick layers of NbOx, CuOx and TiOx have been formed by thermal and plasma oxidation, at various deposition and oxidation conditions, both with or without rapid thermal post-annealing. The resistive bistability effect has been observed for all these materials, with particularly high switching endurance (over 1000 switching cycles) obtained for single-layer TiO2 junctions, and the best reproducibility reached for multi-layer junctions of the same material. Fabrication optimization has allowed us to improve the OFF/ON resistance ratio to about 1000, though the sample-to-sample reproducibility is so far still lower than that required for large scale integration.