Preparation of composition-modulated ALN/Al_2O_3 structure and its application to acoustic wave devices

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The project is aimed at synthesising composition-modulated piezoelectric materials and applying them to acoustic wave devices. The composition-modulated materials are those which are realised by spacially arranging more than two materials of different kind.The result has shown that the composition-modulated material can be regarded as a homogeneous material when the periodicity for the spacial arrangement is less than about 1/5 acoustic wavelengths. However, for an increased periodicity (or a decreased acoustic wavelength), higher order modes propagate and the behaviour of the composition-modulated material becomes complicated. Added to these facts, it is shown that sophisticated arrangements of its constituent materials give the composition-modulated material new properties which do not lie between those of its constituent materials. Experimentally, preparation of piezoelectric AlN films by reactive DC magnetron sputtering in an N_2 or N_2+Ar atmosphere has been attempted. Finally, a ZnO and AlN structure was prepared on a GaAs substrate to develop a monolithic surface acoustic wave device. As was expected from the theoretical analysis, the structure exhibited a large piezoelectric coupling. However, it was found that a technique to prepare very thin piezoelectric AlN fims of less than 0.5<micrn> is needed to develop a composition-modulated structure consisting of thin multi-layers.