Secure authentication of one’s identity is a major challenge in a modern society due to the increasing popularity of mobile devices (such as smart phones) that can not only store the owner’s personal information but also allow banking transactions. A biometrics-based authentication system has attracted a great attention owing to its relatively high-security level and convenience. Current fingerprint recognition systems do not meet the required security level: optical sensors are hard to miniaturize and easily deceived, and capacitive detectors often fail to recognize the patterns by contamination. The ultrasound technology with pMUT (piezoelectric micromachined ultrasound transducer) is one of the most promising technologies to realize such a highly-secure biometrics-based authentication system for mobile electronics. The performance of pMUT is directly determined with the electromechanical property of the piezoelectric layer. However, using conventional piezoelectric materials such as AlN, ZnO, and PZT, it is difficult to generate high power ultrasound that can penetrate into the skin to see veins. Therefore, it is highly desirable to integrate single crystalline relaxor-ferroelectrics, so-called giant piezoelectric materials, on Si substrate. In this talk, I will discuss the issues on the epitaxial integration of Pb(Mg,Nb)O₃-Pb(Zr,Ti)O₃ thin films on Si.