Regents professor Zhong Lin Wang holds an array of piezoelectrically modulated resistive memory (PRM) cells on which metal electrodes have been patterned using lithography. Georgia Tech Photo: Gary Meek |
Taking advantage of the unique properties of zinc oxide nanowires,
researchers have demonstrated a new type of piezoelectric resistive switching
device in which the write-read access of memory cells is controlled by
electromechanical modulation. Operating on flexible substrates, arrays of these
devices could provide a new way to interface the mechanical actions of the
biological world to conventional electronic circuitry.
The piezoelectrically modulated resistive memory (PRM) devices take
advantage of the fact that the resistance of piezoelectric semiconducting
materials such as zinc oxide (ZnO) can be controlled through the application of
strain from a mechanical action. The change in resistance can be detected
electronically, providing a simple way to obtain an electronic signal from a
mechanical action.
“We can provide the interface between biology and electronics,”
says Zhong Lin Wang, Regents professor in the School of Materials Science
and Engineering at the Georgia Institute of Technology. “This technology,
which is based on zinc oxide nanowires, allows communication between a
mechanical action in the biological world and conventional devices in the
electronic world.”
The research was reported online in Nano Letters.
In conventional transistors, the flow of current between a source and a
drain is controlled by a gate voltage applied to the device. That gate voltage
determines whether the device is on or off.
The piezotronic memory devices developed by Wang and graduate student
Wenzhuo Wu take advantage of the fact that piezoelectric materials like zinc
oxide produce a charge potential when they are mechanically deformed or
otherwise put under strain. These PRM devices use the piezoelectric charge
created by the deformation to control the current flowing through the zinc
oxide nanowires that are at the heart of the devices—the basic principle of
piezotronics. The charge creates polarity in the nanowires—and increases the
electrical resistance much like gate voltage in a conventional transistor.
“We are replacing the application of an external voltage with the
production of an internal voltage,” Wang explains. “Because zinc
oxide is both piezoelectric and semiconducting, when you strain the material
with a mechanical action, you create a piezopotential. This piezopotential
tunes the charge transport across the interface—instead of controlling channel
width as in conventional field effect transistors.”
The mechanical strain could come from mechanical activities as diverse as
signing a name with a pen, the motion of an actuator on a nanorobot, or
biological activities of the human body such as a heart beating.
“We control the charge flow across the interface using strain,”
Wang explains. “If you have no strain, the charge flows normally. But if
you apply a strain, the resulting voltage builds a barrier that controls the
flow.”
An array of piezoelectrically modulated resistive memory (PRM) cells is shown being studied in an optical microscope. Georgia Tech Photo: Gary Meek |
The piezotronic switching affects current flowing in just one direction,
depending on whether the strain is tensile or compressive. That means the
memory stored in the piezotronic devices has both a sign and a magnitude. The
information in this memory can be read, processed, and stored through
conventional electronic means.
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 zinc oxide nanowires, which are about 500 nm in dia. and about 50
microns long, are produced with a physical vapor deposition process that uses a
high-temperature furnace. The resulting structures are then treated with oxygen
plasma to reduce the number of crystalline defects—which helps to control their
conductivity. The arrays are then transferred to a flexible substrate.
“The switching voltage is tunable, depending on the number of oxygen
vacancies in the structure,” Wang says. “The more defects you quench
away with the oxygen plasma, the larger the voltage that will be required to
drive current flow.”
The piezotronic memory cells operate at low frequencies, which are
appropriate for the kind of biologically-generated signals they will record,
Wang says.
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.
“We are taking another step toward the goal of self-powered complete
systems,” Wang says. “The challenge now is to make them small enough
to be integrated onto a single chip. We believe these systems will solve
important problems in people’s lives.”
Wang believes this new memory will become increasingly important as devices
become more closely connected to individual human activities. The ability to
build these devices on flexible substrates means they can be used in the body—and
with other electronic devices now being built on materials that are not
traditional silicon.
“As computers and other electronic devices become more personalized and
human-like, we will need to develop new types of signals, interfacing
mechanical actions to electronics,” he says. “Piezoelectric materials
provide the most sensitive way to translate these gentle mechanical actions
into electronic signals that can be used by electronic devices.”
