Significant shear mode softening in a c-axis tilt nanostructured hexagonal thin film induced by a self-shadowing effect

The mechanical properties of tilted AlN fiber structure caused by self-shadowing are investigated using gigahertz vibration induced through the piezoelectricity of AlN itself. We observed significant shear elastic softening in a highly tilted fiber nanostructure, but only a slight decrease in compressive elasticity. Abnormal 1/4 wavelength thickness mode resonance, indicating the decrease in the shear acoustic wave velocity, is reproducibly observed. We attribute this phenomenon to the weak fiber-to-fiber connection seen in a cross-sectional scanning electron microscopy image.