## 1 Introduction  
With the recent rapid development of more advanced wireless sensors for the Internet of Things (IoT) and wearable electronic devices, the demand for devices that can harvest various forms of ambient energy, such as mechanical vibration [1], acoustic waves [2], human biomechanical movement [3], water waves [4], environmental wind [5], solar radiance [6], and even waste heat [7] has become increasingly urgent. Among the various energy harvesting technologies recently developed, piezoelectric energy harvesting technology is expected to play an important role as a potential energy source to power numerous sensors and devices in the near future. Piezoelectric technology converts mechanical energy into electrical energy by exploiting the piezoelectric effect in certain materials. Zinc oxide (ZnO) is a piezoelectric material frequently used for the fabrication of energy harvesting devices due to its large piezoelectric constant and superior film quality [8]. It is also environmentally friendly and biocompatible because it contains no toxic elements such as lead. In general, lead-free ZnO-based piezoelectric materials are available for wearable piezoelectric devices. Moreover, the lead-free materials are particularly advantageous in transducers for the underwater and medical imaging applications mainly due to their relatively low density as well as lower acoustical impedance [9; 10]. Many research teams worldwide have tried to develop ZnO-based energy harvesting devices while making an effort to maximize their piezoelectric performance. Prof. [PERSON] and his team from Georgia Institute of Technology proposed a method of fabricating vertically integrated structures to serve as nanogenerators using piezoelectric ZnO nanowires, polymethyl methacrylate (PMMA), and Au electrodes [11]. The vertically integrated structures can be fabricated using conventional complementary metal-oxide-semiconductor (CMOS) process technology. They are also more durable than other device structures such as laterally integrated or radially integrated nanogenerators [12; 13]. Their fabrication seems to be cost-effective because all that is required to fabricate these micro-energy generators is simply the stacking of thin film layers. Similarly, high-efficiency piezoelectric ZnO-based nanogenerators have also been developed for energy harvesting applications by forming ZnO/AlN-stacked layers [14] and adopting free-carrier-modulated ZnO:N piezoelectric thin films [15]. Moreover, there have been attempts to improve the piezoelectric materials or the device performance by adopting new electrodes or forming a p-n junction to block the screening effect based on energy band considerations [15; 16; 17; 18]. The screening effect means that the piezoelectric potential (piezopotential) induced through the piezoelectric ZnO film is canceled by free electrons that exist inherently in a semiconductive ZnO material or electrode. This reduces the net potential across the ZnO film, thus degrading device performance. Therefore, to maximize the device output performance, it is essential to suppress the screening effect more effectively by controlling the ZnO free carrier density [15]. Some research teams have used insulators or other semiconductor materials to form a good potential barrier to increase output voltages [19]. Other research using AlN or various oxides (MoO\({}_{\mathrm{x}}\), Cu\({}_{2}\)O, and NiO) has been reported [14; 16; 18; 20]. Despite such intensive research, only few studies were published using SiO\({}_{2}\) as an insulator [21]. SiO\({}_{2}\), an excellent insulating material, is electrically reliable, biocompatible, non-toxic, cheap, and easy to fabricate. Therefore, it seems highly desirable to adopt SiO\({}_{2}\) as an insulating interlayer in ZnO-based piezoelectric devices. In this paper, we present a study of various ZnO/SiO\({}_{2}\)-stacked thin film structures for flexible micro-energy harvesting devices.