Phononic Crystal Surface Structures on Gallium Arsenide
A series of computational studies of surface phononic crystals on cubic gallium arsenide are explored to elucidate the behaviour of surface acoustic waves in such systems. The enhanced understanding of surface acoustic wave propagation in phononic crystals may contribute to practical application in quantum well devices. A wet-etched phononic crystal waveguide in gallium arsenide with approximately two micron deep inclusions is studied numerically and compared experimentally for controlled surface acoustic wave propagation. The computed filter response of the phononic crystal confirmed that the phononic crystal was an effective stop band filter in the interval of 400 MHz and 450 MHz. An L1 linear defect waveguide with a stepped funnel entrance design is shown to perform well at a surface acoustic wave frequency of 410.344 MHz and in agreement to simulated results. Next, surface acoustic waves in two-dimensional phononic crystals consisting of shallow void inclusions are studied via its eigenmodes. For the  propagation direction on a (001) gallium arsenide half-space, the conventional Rayleigh wave modes, the layered substrate-associated Sezawa and Lamb modes, the high frequency longitudinal surface waves and bulk waves exhibit hybridization and modal interaction. The longitudinal and vertical shear bulk wave dispersions are observed to be significant thresholds for surface acoustic waves on a shallow phononic crystal exemplified by strong attenuation and mode anticrossings while the shear horizontal wave functions as a more weakly interacting sound line. However, surface modes with low attenuation and exceptional surface boundedness do exist above these sound cones similar to those in thin film coated substrates. These surface modes consist of either Rayleigh-Sezawa or Rayleigh-longitudinal hybridized modes. Phononic viscosity augmentation of stiffness, finite substrate depth and off-angle with respect to  propagation are contrasted for surface acoustic wave propagation. Finally, finite-width, defect-free waveguides on gallium arsenide are explored in contrast to earlier linear defect waveguides. The phononic crystal itself provides the medium of the waveguide core, whereas the cladding is provided by the surrounding gallium arsenide substrate. A finite-width waveguide shows promise for further development in charge and spin transport on gallium arsenide.