There are a great demand and progress of surface-acoustic-wave devices of various types which use a surface acoustic wave propagated near the surface of an elastic solid. One of the reasons of the development is that a surface acoustic wave travels so slowly as 10.sup.-5 times the speed of an electromagnetic wave and hence enables an extreme reduction in size of the device. Another reason is that a surface acoustic wave which travels near the surface of a solid can be readily picked up from any point of the propagation path. A further reason is that since energies are concentrated near the surface of a solid, the device can be used as a device which utilizes an interaction between light and a carrier of a semiconductor or a nonlinearity due to the high energy concentration. A still further reason is that the device can be fabricated by a circuit integration technology and hence readily combined with integrated circuits to provide a new device.
FIGS. 1 and 2 show structures of the prior art surface-acoustic-wave devices. Reference numeral 1 denotes a piezoelectric substrate made from lithium niobate (LiNbO.sub.3) and having a 132.degree. Y-cut surface, 2 is a semiconductor substrate made of silicon which is cut by a crystalline surface substantially equivalent to the (100)-surface, 3 is a piezoelectric layer made from zinc oxide (ZnO) whose crystalline surface substantially equivalent to the (0001)-surface is parallel to the said cut surface of the silicon substrate 2, and 4 and 5 are comb-shaped electrodes which are provided on the lithium niobate substrate 1 or on the zinc oxide layer 3, with their splits interdigitating each other. For example, the electrode 4 is used as input electrode and the electrode 5 is used as output electrode.
A surface acoustic wave excited and entered by the input electrode 4 travels along the surface of the lithium niobate substrate 1 or of the zinc oxide layer 3 and is picked up from the output electrode 5.
If a Rayleigh wave is used as said surface acoustic wave, the device of FIG. 1 provides a so large value as 5.5% square K.sup.2 of the electromechanical coupling coefficient K which is one of the most important factors of the device's nature. This advantage increases the demand of the device in various technical fields. However, since the substrate is made from a single material, the device of FIG. 1 involves such a drawback that the electromechanical coupling coefficient K is fixed by the crystalline axis direction of the substrate and the propagated direction of a surface acoustic wave.
In FIG. 2, however, if a Sezawa wave is propagated in the [011]-axis direction of the silicon substrate 2, the device may have a flexible K.sup.2 characteristic and a larger electromechanical coupling coefficient K by selecting a thickness h.sub.1 of the zinc oxide layer 3 which is obtained by an analysis. For example, if the thickness h.sub.1 is selected so as to satisfy the relation .omega.h.sub.1 =8000 (where .omega. is the angular frequency of the surface acoustic wave), K.sup.2 becomes 6.05%, approximately. The device of FIG. 2, however, is expensive because it needs an increased thickness of the zinc oxide layer which is normally fabricated by a sputtering technology.