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Researchers at the National Institute of Standards and Technology (NIST) and the University of Colorado at Boulder have grown 30-500nm wide and 5-20 micron long gallium nitride nanowires with a high mechanical ‘quality factor’ (Q factor), vibrating between 400,000 and 2.8 million times per second (versus about 32,000 times a second normally for quartz crystals in watches) – see S.M. Tanner et al., Applied Physics Letters. 91 (2007), 203117. The resonators have been enabled by the researchers developing a unique way of growing low-defect-density hexagonal GaN nanowires on a silicon substrate. The simple, inexpensive method is also compatible with existing microelectronics processing techniques, the researchers claim.

 

Picture: Electron micrograph of a GaN nanowire with a high Q factor, vibrating more
than 1 million times per second. Lower right: a stationary nanowire shows
the typical hexagonal shape of the GaN crystals. (Credit: S. Tanner, CU/JILA.)

 

To measure the nanowires’ resonant properties, the researchers observed clumps of nanowires using a scanning electron microscope. The nanowires vibrated when placed on a piezoelectric device stimulated by an electrical signal (they also oscillated when excited directly by an electron beam, due to GaN’s intrinsic piezoelectric ability to covert voltage to mechanical force).

For the nanowire in the image, the Q value (about 38,000) is at least 10 times higher than for other GaN nanowires, carbon nanotubes, and single-crystal silicon microstructures of similar surface-to-volume ratio. The researchers have also measured Q values of more than 1 million in resonating GaN nanowires using feedback (as would occur in a real device).

Q measures the damping of oscillations in a mechanical system as a function of frequency—the higher its Q, the longer it resonates. Ordinarily, Q factors of mechanical resonators (e.g. made from silicon and carbon nanotubes) tend to drop as their diameters shrink, since at the nanoscale even the smallest impurities or defects in the device surface affect its vibrations . “The most interesting thing about these wires is the very high quality factor observed for such a small object,” says NIST researcher and co-author Kris Bertness, who grew the nanowires.

But GaN nanowires have a number of properties that may boost their Q and make them suitable as practical oscillators. For most oscillators, irregularities in a nanostructure’s surface (even adsorbed gas molecules) can change its mass, damping its vibrations and reducing the Q factor. However, the GaN nanowires have extremely flat and smooth surfaces. GaN also has a resonant frequency similar to silicon, but is less susceptible to some sources of noise. Also, GaN has high heat capacity and thermal conductivity, reducing sensitivity to temperature fluctuations.

Because a high Q factor indicates a capacity for stable vibration, the nanowires could be used as oscillators in nano-electromechanical systems for nano-sensors and communications devices. In particular, crystal resonators form an integral part of radio receivers in cell phones (picking out the frequency of the relevant radio signal from noise in the airwaves), but they occupy areas of millimeters squared (much more than the square microns of the corresponding control electronics). Replacing quartz resonators with nanowires could hence decrease cell-phone manufacturing costs, it is claimed.

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