High-Q Gallium Nitride Nanowire Resonators With Piezoresistive Readout

Abstract

We report on the fabrication and piezoresistive readout of doubly-clamped c-axis GaN nanowire (NW) mechanical resonators. As-grown GaN-NW resonators have demonstrated exceptional mechanical quality factors (Q, defined as resonance frequeny over full width at half maximum power) in the range of 104–105 [1]. This work confirms the NWs can retain this high Q even after removal from the growth substrate and placement on lithographically-defined test structures, with a highest Q to date of 26,000 at 10−5 Pa and 8 K. Wires range from 100–500 nm in diameter and 15–18 micrometers in length. We fabricate the devices using a combination of lithographic patterning and dielectrophoresis to suspend NWs over 8 or 10 micrometer gaps. We deposit an electrostatic gate ∼1 micrometer away from the NW to induce vibration, with readout utilizing the piezoresistivity of GaN. Observed resonances range from 9–36 MHz, consistent with a Young’s modulus of roughly 300 GPA. At room temperature and under vacuum, Q for these fabricated devices is typically around 103, significantly lower than the 104–105 range of the as-grown wires. However, the larger Q can be recovered by cooling the substrate. We find that by ∼10 K, Q increases by an order of magnitude to above 104. We will discuss the temperature dependence, as well as fabrication and processing effects, on the NW resonance and Q.