Duncan Stewart is a research physicist at Hewlett-Packard Laboratories in the Quantum Science Research group.  He received a B.A.Sc. in Engineering Physics from the University of Toronto in 1992 and a Ph.D. in Applied Physics from Stanford University in 1999, where he studied nanoscale electron transport.  He joined HP Labs in 1999, and has since worked extensively on nanoscale hybrid organic/inorganic devices, with efforts in both basic science and applied nanotechnologies.   Basic science efforts focus on the challenging physical and chemical characterization of nanoscale interfaces, particularly organic/inorganic structures.  Nanotechnology contributions include molecular-scale electronic switching, demonstrations of electronically addressed nano-crossbar memory at world-record densities, and the first demonstrations of nano-crossbar logic circuits with transistor-like functionality.  Stewart has published more than 25 reviewed scientific papers and authored more than 35 patent applications in the area of nanoscale electronics and materials.

Abstract:
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 TiOx 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/TiOx/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. 


Slides