Toward Thick Piezoelectric HfO2‐Based Films

• Published in: Physica status solidi. PSS-RRL. Rapid research letters Vol. 14; no. 3
• Main Authors: Schenk, Tony, Godard, Nicolas, Mahjoub, Aymen, Girod, Stephanie, Matavz, Aleksander, Bobnar, Vid, Defay, Emmanuel, Glinsek, Sebastjan
• Format: Journal Article
• Language: English
• Published: 01.03.2020
• Subjects: chemical solution deposition; ferroelectric HfO2; piezoelectrics; sol–gel; thick films
• ISSN: 1862-6254; 1862-6270
• DOI: 10.1002/pssr.201900626

For HfO2‐based films, which are mainly prepared by atomic layer deposition, the desired ferroelectric (FE) properties typically vanish while extending the thickness beyond a limit of about 50 nm. Herein, the successful fabrication of a 1 μm‐thick piezoelectric La:HfO2 film is demonstrated using chemical solution deposition, paving the way toward sensor and actuator applications. After identifying the optimal La content, the film thickness is increased from 45 nm to 1 μm. Polarization and strain measurements evidence the persistence of the FE properties at all thicknesses and even a slight improvement due to a better orientation of the polar axis at higher thicknesses. Scanning electron microscopy and X‐ray reflectivity reveal a fine‐grained microstructure and a density of only 80% of the theoretical value, which seems to be a common issue of solution‐deposited HfO2‐based films, based on the performed literature survey. Using tilt‐angle‐dependent X‐ray diffraction scans, homogeneous nucleation is found to be the likely root cause of the observed microstructure. It is suggested that this microstructure issue is key for the optimization of cycling stability of solution‐based films to exploit the full potential of the HfO2 for cost‐efficient, thick piezoelectric films. This work opens a path toward cheap HfO2‐based thin films for piezoelectric applications. Solution‐derived La:HfO2 films are deposited up to 1 μm thickness, and their ferro‐ and piezoelectric activities are successfully demonstrated. Beyond that, a common microstructure issue is identified and emphasized to raise awareness and to spark future optimization effort for this highly application‐relevant fabrication route.