Analysis of Intermediate State Ferroelectric Properties of 30% ScAlN, Impacts on Piezoelectric Performance, and Ferroelectric Dead Layer Effect in PMUT Devices.

The ferroelectricity of scandium-doped aluminium nitride (ScAlN) provides important advantages, such as control over the direction of electromechanical response, over traditional aluminium nitride used commonly in piezoelectric micromachined ultrasound transducers (PMUTs). Current literature surrounding the effect of intermediate polarization states on the piezoelectric properties of ferroelectric PMUT devices is limited, and this paper provides an experimentally obtained catalogue of the intermediate polarization states between the as-deposited N-polar and oppositely switched Al-polar states, comments on the polarization magnitudes and subsequent impact on PMUT resonant frequency and electromechanical coupling coefficient keff2 and observes an asymmetrical response between the two saturated polarization states. It is found that the device polarized in the N-polar direction features improved keff2, exhibiting a V-shaped response over polarization magnitude, with poorer performance in the fully Al-polar direction. The existence of a ferroelectrically dead layer within the ScAlN film, which retains its piezoelectric properties but is unable to be ferroelectrically switched, is experimentally examined. By comparison with finite-element simulations, a model containing a fixed N-polar dead layer equivalent to approximately 10% of the ScAlN film thickness and located at the bottom interface provides the closest agreement with the measured polarization and keff2 asymmetries. The estimated thickness and location are obtained from device-level measurements and model comparison rather than direct structural characterization. Additionally, the propagation of the ferroelectric switching through the intermediate states is shown to be axial (thickness direction) in nature, matching well with previous research.