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Simplifying MEMS pressure sensor technology

A new sensor incorporating gallium nitride and zinc oxide nanowires could provide an artificial sense of touch and be used in MEMS systems.

USING THOUSANDS OF nanometre-scale wires, researchers at the Georgia Institute of Technology have developed a sensor device that converts mechanical pressure - from a signature or a fingerprint - directly into light signals that can be captured and processed optically.

The sensor device could provide an artificial sense of touch, offering sensitivity comparable to that of the human skin. Beyond collecting signatures and fingerprints, the technique could also be used in micro-electromechanical (MEMS) systems and biological imaging. And ultimately, it could provide a new approach for human-machine interfaces.

“You can write with your pen and the sensor will optically detect what you write at high resolution and with a very fast response rate,” says Zhong Lin Wang, Regents’ professor and Hightower Chair in the School of Materials Science and Engineering at Georgia Tech. “This is a new principle for imaging force that uses parallel detection and avoids many of the complications of existing pressure sensors.”

Individual zinc oxide (ZnO) nanowires that are part of the

device operate as tiny LEDS when placed under strain from the mechanical pressure. They allow the device to provide detailed information about the amount of pressure being applied. Known as piezo-phototronics, the technology - first described by Wang in 2009 - provides a new way to capture information about pressure applied at very high resolution: up to 6,300 dots per inch.

Piezoelectric materials generate a charge polarisation when they are placed under strain. The piezo-phototronic devices rely on that physical principle to tune and control the charge transport and recombination by the polarisation charges present at the ends of individual nanowires.

Grown on top of a gallium nitride (GaN) film, the nanowires create pixelled light emitters whose output varies with the pressure, creating an electroluminescent signal that can be integrated with on-chip photonics for data transmission, processing and recording.

“When you have a zinc oxide nanowire under strain, you create a piezoelectric charge at both ends which forms a piezoelectric potential,” Wang explains. “The presence of the potential distorts the band structure in the wire, causing electrons to remain in the p-n junction longer and enhancing the efficiency of the LED.» The efficiency increase in the LED is proportional to the strain created.

Differences in the amount of strain applied translate to differences in light emitted from the root where the nanowires contact the GaN film. To fabricate the devices, a low- temperature chemical growth technique is used to create a patterned array of ZnO nanowires on a GaN thin film substrate with the c-axis pointing upward.

Zhong Lin Wang holding the sensor device along with his research team in the background

18 Issue IV 2013

The interfaces between the nanowires and the GaN film form the bottom surfaces of the nanowires. After infiltrating the space between nanowires with a PMMA thermoplastic, oxygen plasma is used to etch away the PMMA enough to expose the tops of the ZnO nanowires. A nickel-gold electrode is then used to

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