## 1 Introduction  
With the rapid development of smart terminals and human information society [1, 2, 3, 4, 5], surface acoustic wave (SAW) devices are comprehensively applied in Radio Frequency (RF) filters, sensors, and microfluidics due to their miniature size, low cost, and steady performance [6, 7, 8]. For the sake of SAW devices with more practical value, researchers continue to explore innovative structures and materials. Furthermore, compared to SAW devices mainly made of bulk piezoelectric substrates, multi-layer substrate has attracted extensive attention due to its high acoustic velocity and suitable piezoelectricity, which combines the merits of the piezoelectric layer, high velocity substrate, and other functional layers [9, 10, 11]. In SAW devices, ZnO and AlN are widely used as piezoelectric film materials, while non-piezoelectric substrates with high acoustic velocity include Si, sapphire (Al\({}_{2}\)O\({}_{3}\)), SiC, diamond, and so on [12]. Among these combinations of piezoelectric films and substrates, ZnO/SiC structure devices have attracted the attention of researchers because ZnO possesses good piezoelectricity and moderate velocity while SiC exhibits a high velocity (-12,500 m/s), low propagation loss, and high thermal conductivity (-490 W\(\cdot\)m\({}^{-1}\cdot\)K\({}^{-1}\)), which is necessary for the fabrication of high performance devices. Moreover, the lattice mismatch between SiC (a = 3.073 A, c = 15.120 A) [13] and ZnO (a = 3.249 A, c = 5.206 A) [14] piezoelectric films is as small as 5%, which simplifies device preparation [15]. Our previous work reported a SAW filter that shows high SAW velocity with a working frequency as high as 6.8 GHz [16]. Nevertheless, acoustic waves propagating from a conventional ZnO/SiC layered structure suffer from low \(K^{2}\). As the calculated results show, the \(K^{2}\) value of the Rayleigh mode is less than 0.8%, and that of the Sezawa mode is no more than 3.2%. The explosive growth of data transmission volume proposes a higher filter requirement, especially wideband filters. For SAW sensor applications, more sensitive sensors have traditionally been targeted. Since \(K^{2}\) is a decisive factor in the filter bandwidth and sensor sensitivity, pursuing a higher \(K^{2}\) is necessary for the needs of wideband filters and high sensitivity sensors. In order to meet the increasing demands of wideband filters in mobile communication, pursuing a higher \(K^{2}\) is necessary. In recent years, many studies have been committed to improving the \(K^{2}\) in multi-layer substrate SAW devices. First, enhancing the piezoelectric response of the piezoelectric film can cause an increase in the \(K^{2}\) of the SAW resonators. Doping is a widely used approach for improving piezoelectric response. Many kinds of elements such as Sc [17], Ti [18], Y [19], and Ta [20] have been studied as dopants for AlN. It is reported that doping Fe [21], Cr, and V [22] can also improve the piezoelectric coefficient of ZnO. However, doping sometimes leads to a low quality factor and decreased acoustic velocity [23]. In addition, developing a new configuration of multi-layer substrate is another promising way to obtain higher \(K^{2}\)[23]. Over the past few years, many imaginative structures have been proven to achieve good performance, including embedding the top electrodes under SiO\({}_{2}\)[10; 24], burying the top electrodes into the grooves on the piezoelectric film [25], optimizing the transducer topology [26], and so on. Recently, a new configuration of hybrid SAW/bulk acoustic wave (BAW) structure has been reported that also shows an enhanced \(K^{2}\)[27]. Since ZnO/SiC structure is a potential structure for building high performance devices, promoting a simpler and more effective way to enhance the \(K^{2}\) of this structure is necessary.