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Vibrational modes of GaN nanowires in the gigahertz range



Ward L. Johnson, Sudook A. Kim, Roy H. Geiss, Colm Flannery, Kristine A. Bertness, Paul R. Heyliger


Monocrystalline nanowires offer an attractive basis for resonant nanoelectromechanical systems (NEMS) for a variety of applications, including mass and force sensing, and, in each of these applications, there are advantages to operation at frequencies above the megahertz range. To assist in establishing an understanding of the vibrational modes at such frequencies, this study presents Brillouin-light-scattering measurements and finite-element (FE) analysis of vibrational spectra in the range of 5 GHz to 45 GHz of an array of monocrystalline GaN nanowires with hexagonal cross sections. The largest spectral peaks with acoustic angular wavenumbers in the range of 4/microns to ~15/microns were determined to arise from modes with relatively large transverse displacements, consistent with inelastic light scattering arising predominantly from surface ripple. These detected modes have finite frequencies in the limit of zero wavenumber, corresponding to transverse standing waves. At higher wavenumbers, the data and analysis indicate that elasto-optic scattering within the nanowires becomes more significant, and associated narrow peaks emerge in the spectra. Analysis of the spectra is substantially complicated by the presence of a wide distribution of nanowire diameters that is only approximately determined. The FE calculations reveal a variety of modal displacement patterns, including simple flexural, higher-order flexural, approximately "plane-strain," approximately longitudinal, and torsional.


acoustic modes, Brillouin light scattering, finite element modeling, GaN nanowires, phonons, vibrational modes


Johnson, W. , Kim, S. , Geiss, R. , Flannery, C. , Bertness, K. and Heyliger, P. (2012), Vibrational modes of GaN nanowires in the gigahertz range, Nanotechnology, [online], (Accessed April 23, 2024)
Created November 13, 2012, Updated November 10, 2018