[Reminder] MSE PhD Dissertation Defense: Naga Phani Aetukuri - Today 3pm CIS-X 101

Department of Materials Science & Engineering

University PhD Dissertation Defense

 

Electrically-Triggered Metallization in Single-Crystal Vanadium Dioxide Thin Film Heterostructures

 

Naga Phani Aetukuri

 

Research Advisor: Professor James Harris

Co-Advisor: Professor Stuart Parkin

Co-Advisor: Professor Paul McIntyre

 

February 6, 2012 @ 3:00 PM

(Refreshments will be served starting at 2:45 PM)

 

Location: Paul G. Allen Auditorium (CIS-X 101)
http://cis.stanford.edu/directions

 

 

Vanadium dioxide (VO₂) is metallic at high temperatures but undergoes a metal to insulator transition (MIT) below ~340 K in bulk crystals that is generally attributed to electron-electron correlation effects.  This transition can also be engendered by the application of modest electric fields, several orders of magnitude below the electric breakdown field, making this phenomenon potentially useful for two or three terminal switches, depending on its speed and origin.

 

We have used pulsed laser deposition to prepare epitaxial single-crystalline thin films of VO₂ that show a sharp change in resistance at the MIT for films as thin as 10nm.  We find that strain induced by underlying layers can modify the transition temperature of these films.  For example, on TiO₂ (001) single-crystal substrates, the transition temperature is reduced to ~290 K, whereas by growing similar films on RuO₂ buffers layers the MIT can be increased up to ~335 K. 

 

A combination of electron-beam and optical lithography was used to fabricate lateral two-terminal nano-devices from VO₂ films deposited directly on TiO₂ substrates.   We have observed reversible and reproducible transitions triggered by electric-field but with a delay time varying from as long as 10 ns to as short as 500 ps.  The delay time is found to decrease with increasing electric field and temperature.  We discuss whether these results indicate the transition is dominated by electronic or by Joule heating effects. In vertical structures with a metallic RuO₂ bottom electrode, we have used a current-sensing atomic force microscope to probe the electric-field-induced MIT in much smaller volumes of VO₂.  Reversible switching between insulating and metallic states was observed at modest electric fields for VO₂ layers as thin as 20 nm. These results demonstrate the possibility of triggering an MIT at low voltages and, therefore, at low energies, which is essential for device applications.