July 26, 2011 — Georgia Institute of Technology (Georgia Tech) researchers have used zinc oxide nanowires to create a new type of piezoelectric resistive switching device, wherein the write-read access of memory cells is controlled by electromechanical modulation.
These devices, operating on flexible substrates, could create mechanical/biological/electronic interactions to fuel new applications.
The piezoelectrically modulated resistive memory (PRM) devices are based on mechanical strain’s effect on the resistance of piezoelectric semiconducting materials such as zinc oxide (ZnO). This resistance change can then be electronically detected.
A mechanical action in the biological world can be communicated to conventional electronic devices via this nano-wire-based technology, said Zhong Lin Wang, Regents professor in the School of Materials Science and Engineering at the Georgia Institute of Technology. The piezotronic memory cells operate at low frequencies, which are appropriate for the kind of biologically generated signals they will record, Wang said.
Unlike conventional transistors, where current flow is controlled by a gate voltage, these piezotronic memory devices use a piezoelectric charge created by strain to control current. The researchers replaced "the application of an external voltage with the production of an internal voltage," Wang explained. The devices were developed by Wang and graduate student Wenzhuo Wu. Zinc oxide’s dual nature as a semiconductor and piezoelectric material creates piezopotential, which tunes the charge transport across the interface, the researcher added. The research builds on work Wang reported in 2010 on strain-gating piezotronics.
Charge flows normally across the interface until a strain — anything from hand pressure to robotics — creates a voltage barrier to control flow, Wang explained. The piezotronic switching affects current flow in one direction, depending whether the strain is tensile or compressive. The memory stored in the piezotronic devices has both a sign and a magnitude, allowing the information to be read, processed and stored conventionally.
Taking advantage of large-scale fabrication techniques for zinc oxide nanowire arrays, the Georgia Tech researchers have built non-volatile resistive switching memories for use as a storage medium. They have shown that these piezotronic devices can be written, that information can be read from them, and that they can be erased for re-use. About 20 of the arrays have been built so far for testing.
The 500nm-diamter, 50um-long zinc oxide nanowires are produced via physical vapor deposition (PVD) in a high-temperature furnace. The structures are then treated with oxygen plasma to reduce the number of crystalline defects, improving conductivity control, then transferred to a flexible substrate.
"The switching voltage is tunable, depending on the number of oxygen vacancies in the structure," Wang said. The fewer defects, the larger the voltage that will be required to drive current flow.
These piezotronic memory elements provide another component needed for fabricating complete self-powered nanoelectromechanical systems (NEMS) on a single chip. Wang’s research team has already demonstrated other key elements such as nanogenerators, sensors and wireless transmitters.
The research was reported online June 22 in the journal Nano Letters. Access it here: http://pubs.acs.org/doi/abs/10.1021/nl201074a
The work was sponsored by the Defense Advanced Research Projects Agency (DARPA), the National Science Foundation (NSF), the U.S. Air Force and the U.S. Department of Energy.
Courtesy of John Toon, Georgia Institute of Technology. Learn more at gtresearchnews.gatech.edu.
