Aluminum nitride (AlN) is a piezoelectric semiconductor material used for optoelectronic devices, high-frequency acoustic filters, resonators, and piezoelectric transducers for structural health monitoring due to their wide band gap, chemical and mechanical stability, dielectric properties, and relatively large values of its elastic constants. On the other hand, lead zirconate titanate (PZT) ceramics are also used in excitation and detection of acoustic waves in aircraft integrated structures for structural health monitoring and nondestructive testing. Prior to any potential device application it is necessary to characterize the mechanical properties of those materials.
In addition to the polycrystalline materials AlN, PZT, and single crystal lithium niobate (LiNbO3) has been studied in the present thesis in view of its piezoelectric and high-frequency elastic properties. LiNbO3 is currently intensively used for piezoelectric, as well as electro-optic and nonlinear optical applications. Although the focusing of thermal phonon has been studied previously in LiNbO3 and its elastic and piezoelectric constants have been determined in many previous studies, its acoustic properties are still not completely explored.
In the first part of the thesis scanning acoustic microscopy has been used to determine the mechanical properties of AlN thin films and PZT ceramics. AlN thin films were grown using the reactive RF-magnetron sputtering technique. The microstructure, surface morphology, and chemical composition of the AlN thin films were determined. Later on, the Coulomb coupling technique has been applied to determine the acoustic velocities and transport properties of ultrasonic waves in PZT and LiNbO3 in order to assess the feasibility of this technique.
The longitudinal, skimming longitudinal, transversal, and surface acoustic wave velocities and the corresponding elastic constants were determined in AlN as well as in PZT ceramics. AlN does not grow as a single crystal so that LiNbO3 single crystals have been employed to demonstrate the generation and detection of surface acoustic waves (SAW’s) for defect characterization in piezoelectric materials.
In the second part of the thesis, the developed scheme has been applied to image the transport properties of bulk and guided acoustic waves travelling in PZT. A delta pulse, broad band signal excites both longitudinal and transverse bulk waves, and metamorphosis of bulk wave into Lamb waves was sequentially monitored.
In further studies, ultrasonic imaging with high temporal and spatial resolution was conducted on LiNbO3. The imaging is performed with switched sinusoidal excitation and quadrature detection from which the magnitude and phase are derived. The wavelengths of surface skimming longitudinal waves and SAW’s are both determined from the observed phase rotation as a function of position. This technique also used to study the influence of a surface defects on the scattering of SAW propagating on the surface of the LiNbO3 crystal. Artificial defects employed for interaction with the waves were produced by deposition of silver paint on the surface. These defects are both absorptive and scattering. The scattering and attenuation of SAW’s are studied by imaging in vector contrast. The interaction allows a clear differentiation of volume waves skimming the surface and guided waves traveling at the surface. The thesis, hence, describes the use of the local electric field probe technique to study the structure of piezoelectric materials by acoustic methods.