CPM Seminar

Electrons and Ions: The Future of Nanoscale Electronics

Duncan Stewart

Hewlett-Packard Laboratories

Titanium metal is widely used as a top metal contact for nanoscale molecular electronic devices, where it has been assumed to form a few-atom-thick Ti carbide overlayer. Using a vacuum delamination technique we expose and analyze chemically pristine buried titanium/organic monolayer interfaces from devices that have displayed ‘molecular electronic switching’. We establish that under many conditions the titanium instead forms a few-nanometer-thick Ti oxide overlayer. Both TiO₂ and reduced TiO_x species exist — this mixed stoichiometry Ti oxide is responsible for the electronic switching.

In the field of ‘conventional’ nano-electronics, oxide based electrical-resistance switches are pursued for next generation nonvolatile random access memories (R-RAMs). However, the metal/oxide/metal switching mechanism is poorly understood. We demonstrate in Pt/TiO_x/Pt nanocrosspoint devices that the switching is channeling (on) and recovering (off) the Schottky barrier at the Pt/TiO₂ interface due to the creation and drift of positively charged oxygen vacancies under electric field. Engineered oxygen vacancy profiles predictively control the switching polarity and conductance to support a general physics model of switching in these devices.

Nanoscale switches that combine such ionic and electronic dynamics have the potential to both transform the non-volatile RAM memory market and provide disruptive new electronic logic functions, including synapse-like devices for neuromorphic computing.