The Surface Acoustic Wave Propagating Properties In AlN/GaN Structures

In this thesis, the propagating properties of surface acoustic wave (SAW) in AlN/GaN structures were researched by the Adler matrix methods. Considering the mechanical and electrical boundary conditions of AlN/GaN structures, determinantal equations which are used to solve the phase velocities of surface acoustic wave were given from the basic principle of the Adler matrix methods. With solving the determi- nantal equations by the program compiled with MATLAB, SAW propagating properties in AlN/GaN structures were acquired, including the phase velocities and electromechanical coupling coefficients which are varied with frequency,film thickness and c-axis orientation of AlN,metallization conditions of surface and interface.In this thesis, the propagating properties of SAW in ZnO/GaAs structures were also researched with the help of the program. At the same time, the calculation result was contrasted with the result by the effective surface permittivity (ESP) function methods in order to verify the matrix methods. Furthermore, the matrix methods were extended to analyse the structures of multilayers films on substrates.The elastic stiffness tensor,piezoelectric stress tensor,permittivity tensor will be changed when the SAW propagating coordinate system is different from the crystal principal orthogonal coordinate system. In order to analyse the properties of SAW propagating along different directions,we must calculate the right forms of the three tensors in the SAW propagating coordinate system. In this thesis, the right forms of the three tensors were calculated by the Euler transformation methods and the program was designed according the basic principle of the methods.AlN and GaN are semiconductors with piezoelectric properties and have been widely researched in recent years. But the use of GaN in SAW devices is restrained by the donor impurity which is introduced during growth. However, the composite structures of AlN films on GaN substrates can lessen the intrinsic problem of SAW devices on GaN, due to the lower electrical conductivity and larger piezoelectric coefficient of AlN. In addition, the smooth film surface and good crystalline structure can be obtained with the magnetron sputtering methods, due to the little lattice mismatch between AlN and GaN. Furthermore, the composite structures can make the devices complanation and integration, which can be used in new piezoelectric and acousto-optic devices. So the AlN/GaN structures may provide the imagination of future applications in SAW devices.