Gallium Nitride Nanowire Electromechanical Resonators

Gray, Jason Michael

Graduate Thesis Or Dissertation

Gallium Nitride Nanowire Electromechanical Resonators Public Deposited

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Citeable URL: https://scholar.colorado.edu/concern/graduate_thesis_or_dissertations/4m90dv511

Abstract

Nanoscale mechanical resonators are of great interest for high-resolution sensing applications, where the small resonator mass and high quality factor (Q, defined as resonance frequency f₀over full width at half maximum power) lead to unprecedented sensitivity. Here, we investigate gallium nitride (GaN) nanowire (NW) resonators. The single-crystal, c-axis NWs are 5 μm - 20 μm long, with diameters from 50 nm - 500 nm, and grow essentially free of defects. Our initial experiments involve measuring the resonances of as-grown NWs in a scanning electron microscope, where we observe exceptionally high Q values of 10⁴ - 10⁵, one to two orders of magnitude higher than most NWs of comparable size. Using a single NW as a mass sensor, we then demonstrate a sub-attogram mass sensitivity.

To provide a more flexible measurement technique that avoids electron-microscope detection, we fabricate doubly clamped NWs with an entirely electronic drive and readout scheme using a combination of lithographic patterning and dielectrophoresis. An electrostatic gate induces vibration, while readout utilizes the piezoresistivity of GaN. Observed resonances range from 9-36 MHz with Q values typically around 10³ at room temperature and 10^-4 Pa.

We use the behavior of f₀ and Q to sense the NW's local environment, such as the additional sources of energy dissipation not present in the as-grown NWs. By cooling the device to 8 K, Q increases by an order of magnitude to above 10⁴, with a highest value to date of 26,000 under vacuum. We explore additional NW properties through the thermal noise in the NW's mechanical motion and the exponential decay of mechanical motion in the presence of burst drive. Finally, we investigate the low-frequency 1/f parameter noise displayed by f₀. We show that the noise in f₀ is consistent with noise in the NW's resistance leading to temperature noise from local Joule heating, which in turn generates resonance frequency noise. For sensor applications, there will be optimal drive conditions that balance the f₀ noise with the signal-to-noise ratio of the system. With these insights, along with the simple drive and readout technique, these GaN-NW doubly clamped resonators have significant potential for high-resolution sensing applications.

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