Gallium Nitride Nanowires in Selected-Area Growth Arrays Measured via Optical Bragg Scattering for Mass Sensing

We report the use of optical Bragg scattering to measure the mechanical resonance frequencies and quality factors (Q) of gallium nitride (GaN) nanowires (NWs) in selected-area growth arrays with applications as attogram-level mass sensors. The GaN NWs are grown by catalyst-free molecular beam epitaxy on silicon (111) wafers. Hexagonal arrays of approximately 100 GaN NWs with pitch spacings of 350-1100 nm have been prepared. Optical Bragg scattering from these arrays has been measured to be in good agreement with theoretical designs. The NWs contained in such arrays have diameters ranging from 100-300 nm and lengths from 3-10 μm. A HeNe laser operating at 633 nm and 5 mW of optical power is used to perform Bragg scattering homodyne detection to passively read out the thermally induced Brownian mechanical motion of the NWs. The first order cantilever-mode mechanical resonance frequencies of these NWs have been measured to be between 2-12 MHz. Our readout scheme allows the simultaneous detection of all lowest order mechanical resonances in a given array, enabling the simultaneous monitoring of roughly 100 mass sensors in a 10-100 μm² area. Additionally, we investigate correlations among NW resonances.