January 2008

EEEL and University of Colorado Researchers Demonstrate Nanowire Oscillators with Record Q

Nanowires grown by researchers in EEEL’s Optoelectronics Division have a mechanical quality factor at least 10 times higher than reported values for other nanoscale devices, and comparable to that of commercial quartz crystals. EEEL’s Kris Bertness and colleagues have developed the means to grow c-axis hexagonal gallium nitride (GaN) nanowires featuring low defect density (free of dislocations and stacking faults) and high photoluminescence intensity. Professor Charles Rogers’ group at the University of Colorado in Boulder, in collaboration with EEEL, demonstrated high Q factors in wires that are 30 to 500 nm in diameter and 5 to 20 micrometers long, having resonance frequencies between 400 kHz and 2.8 MHz. To measure the resonance properties, the as-grown nanowires on pieces of silicon substrate were attached to the top of a piezoelectric stack in a scanning electron microscope, and an ac signal was applied. The output signal from the SEM secondary electron detector as a function of applied frequency was used to determine the resonance frequencies and Qs of the nanowires. Q near room temperature for various nanowires ranged from 2,700 to above 60,000, with a typical wire exhibiting a Q of 38,000, at least 10 times higher than previously reported values for a-axis oriented GaN nanowires, carbon nanotubes, and single-crystal silicon microstructures of similar surface-to-volume ratio. The Q values observed are comparable to those of commercial quartz crystal resonators used in feedback oscillators. Positive feedback to the piezoelectric stack caused oscillations of the nanowires with closed loop Q exceeding 106. Undoped nanowires also oscillated when excited directly by an electron beam, apparently due to the localized charging causing a piezoelectric deformation of the wire.

GaN nanowires have a range of properties that may contribute to the improved Q over resonators of similar size made from other materials. First, they have crystallographically flat surfaces (as opposed to conventional micromachined silicon resonators) and improved surface conditions. Second, GaN oscillators have a resonant frequency similar to those of silicon of the same size, but they are less susceptible to gas sorption/desorption noise. Third, GaN has high heat capacity and thermal conductivity, reducing its sensitivity to temperature fluctuations. Other advantages are that GaN nanowires are piezoelectric and are grown on silicon, making them compatible with existing microelectronics processing methods and suggesting their increasing importance in nanoelectromechanical (NEMS) systems. In addition to NEMS applications, Optoelectronics Division researchers are investigating applications in near-field optical microscopy, bio-medical sensing, and solid-state lighting.

Contact:  Kris Bertness (EEEL), phone 303-497-5069