Emergence of ferroelectricity in ZrO2 thin films on TiN/Si featuring high temperature sputtering method

Abstract

ZrO2 has attracted attention as HfO2 and Hf1−xZrxO2 to nonvolatile memory applications because of its ferroelectric nature in orthorhombic phase, lower cost, and excellent thermal stability. Ferroelectricity in ZrO2 has been reported experimentally using several deposition methods. However, the fabrication of a ferroelectric ZrO2 thin film in the ultra-thin region of below 5 nm on TiN/Si remains a challenge. In this study, we examined the formation of ferroelectric ZrO2 ultrathin films on TiN/p+-Si using a high-temperature sputtering method while optimizing the involved substrates and processes.We clarified the importance of the underlying substrate selection to control the crystalline phase of ZrO2 thin films through comparing several types of substrates. TiN/p+-Si was found to be the most appropriate substrate for realizing ferroelectric ZrO2 films. Further, we demonstrated that plasma oxidation treatment improves the ferroelectricity and reduces the leakage current component of the 11-nm-thick ZrO2 films. Subsequently, we discussed the conditions for ferroelectricity by summarizing the switching polarization (PSW) of ZrO2 as functions of the ZrO2 thickness and sputtering temperature and realized the appearance of ferroelectricity in a 4 nm thick ZrO2 film with a PSW of 0.53 μC/cm2 and endurance properties until ∼108 cycles.