1 Introduction Zinc oxide (ZnO) has been one of the most developed compound semiconductors in the past three decades [1, 2]. It is composed of abundant elements and biocompatible while offering several attractive properties, including a wide bandgap energy (i.e., 3.37 eV at room temperature), a high exciton binding energy (i.e., 60 meV), a decent electron mobility (i.e., around 200 cm2/V·s), and relatively high piezoelectric coefficients (e.g., e33 = 0.89–1.19 C/m2) [1, 2].
