The Characteristics of the Surface Acoustic Wave in Diamond-Based Layered Structures

This study focuses on surface acoustic wave (SAW) characteristics of a multi-layered diamond-based structure. In this thesis, the phase velocity, coupling coefficient, temperature coefficient, and electrode reflectivities were investigated between the Rayleigh mode and higher-order modes of SAW in a diamond-based layered structure using finite element method analysis. SAWs in the diamond-layered structure combined with aluminum nitride or zinc oxide film possesses a high phase velocity and a large electromechanical coupling coefficient, which can be used to design high frequency and low loss SAW component for communication or sensor applications.
In addition to the conventional aluminum nitride or zinc oxide with its c-axis perpendicular to the diamond substrate, a-axis perpendicular structures were also investigated in this thesis. The results showed that the SAW propagating in a-axis-oriented aluminum nitride or zinc oxide diamond-based structures exhibited a higher phase velocity and larger electromechanical coupling coefficient with a thinner piezoelectric film thickness, which is a promising candidate for designing state-of-the-art SAW devices.
The effects of a buffer layer on SAW characteristics are also an important in this thesis. The quality of the piezoelectric film, which is determined by the buffer layer, can significantly affect the quality of excited SAW characteristics. In this section, the thicknesses and mechanical properties of various buffer layers will be calculated to estimate the SAW characteristics propagating in different diamond-based layered structures. The optimal thickness for different buffer layers will be determined and discussed.
A large temperature coefficient of the diamond-based structure limits its applicable potentiality. Nowadays, amorphous films have been utilized to compensate the large temperature coefficient of the diamond-based structure. However, the use of an amorphous film to compensate the temperature coefficient of structure may yield a lower effective electromechanical coupling coefficient. By employing the opposite values of temperature coefficient between AlN and ZnO, this study purpose two novel temperature-stable three-layered structures, (100) AlN/(100) ZnO/diamond and (100) ZnO/(100) AlN/diamond for high-velocity and large-coupling coefficient applications.