Piezoelectric and Magnetoelastic Strain in the Transduction and Frequency Control of Nanomechanical Resonators
Abstract
Stress and strain play a central role in semiconductors, and are strongly manifested at the nanometer-scale regime. Piezoelectricity and magnetostriction produce internal strains that are anisotropic and addressable via a remote electric or magnetic field. These properties could greatly benefit the nascent field of nanoelectromechanical systems (NEMS), which promises to impact a variety of sensor and actuator applications. The piezoelectric semiconductor GaAs is used as a platform for probing novel implementations of resonant nanomechanical actuation and frequency control. GaAs/AlGaAs heterostructures can be grown epitaxially, are easily amenable to suspended nanostructure fabrication, have a modest piezoelectric coefficient roughly twice that of quartz, and if appropriately doped with manganese, can form dilute magnetic compounds. In ordinary piezoelectric transducers there is a clear distinction between the metal electrodes and piezoelectric insulator. But this distinction is blurred in semiconductors. An integrated piezoelectric actuation mechanism is demonstrated in a series of suspended anisotype GaAs junctions, notably pin diodes. A dc bias was found to alter the resonance amplitude and frequency in such devices. The results are in good agreement with a model of strain based actuation encompassing the diode’s voltage-dependent carrier depletion width and impedance. A bandstructure engineering approach is employed to control the actuation efficiency by appropriately designing the doping level and thickness of the GaAs structure. Actuation and frequency are also sensitively dependent on the device’s crystallographic orientation. This combined tuning behavior represents a novel type of depletion-mediated electromechanical coupling in piezoelectric semiconductor nanostructures. All devices are actuated piezoelectrically, whereas three techniques are demonstrated for sensing: optical interferometry, piezoresistance and piezoelectricity. Finally, a nanoelectromechanical GaMnAs resonator is used to obtain the first measurement of magnetostriction in a dilute magnetic semiconductor. Resonance frequency shifts induced by field-dependent magnetoelastic stress are used to simultaneously map the magnetostriction and magnetic anisotropy constants over a wide range of temperatures. Owing to the central role of carriers in controlling ferromagnetic interactions in this material, the results appear to provide insight into a unique form of magnetoelastic behavior mediated by holesSimilar works
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Last time updated on 15/07/2013
This paper was published in Caltech Theses and Dissertations.
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