Piezoelectric Properties of Aluminum Nitride Thin Films

Aluminium nitride (AlN) thin films in the wurtzite crystal structure exhibit intrinsic piezoelectricity, whereby mechanical strain induces an electric charge and conversely an applied electric field produces a dimensional change. This effect arises from the non-centrosymmetric arrangement of Al and N atoms along the c-axis and is enhanced by precise control of film orientation, residual stress and defect density. High-quality c-axis-oriented AlN films, typically deposited by sputtering or chemical vapour deposition, achieve piezoelectric coefficients (d33 and e31) sufficient for acoustic resonators, sensors and actuators across radio-frequency to ultrasonic regimes. Alloying with scandium (Sc) further amplifies the piezoelectric response through lattice softening and increased polarisation, though it must be balanced against phase stability and crystalline quality. Key applications include surface acoustic wave filters, bulk acoustic wave resonators for wireless communications, micro-electromechanical sensors for pressure and inertial sensing, and emerging ferroelectric memory elements. Advances in understanding the relationships between composition, microstructure and electromechanical coupling have driven improvements in device performance, reliability and integration with silicon-based platforms, underscoring the global significance of AlN thin films in both established and emerging technologies.

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Technical terms

Piezoelectricity: Property of certain crystals to generate electric charge under mechanical stress and to deform under an applied electric field.

Wurtzite: Hexagonal crystal structure of AlN characterised by polar c-axis orientation essential for piezoelectric behaviour.

Electromechanical coupling coefficient: Dimensionless parameter (kt, k33) quantifying the efficiency of conversion between electrical and mechanical energy in a piezoelectric material.

Spontaneous polarisation: Built-in electric dipole moment per unit volume in a non-centrosymmetric crystal without external fields, contributing to net piezoelectric response.

Two-dimensional electron gas (2DEG): High-mobility electron layer formed at heterointerfaces due to polarisation differences, critical for high-frequency electronic devices.

References

1. New-Generation Ferroelectric AlScN Materials. Nano-Micro Letters (2024).
2. Stress controlled pulsed direct current co-sputtered Al1−xScxN as piezoelectric phase for micromechanical sensor applications. APL Materials (2015).
3. Implications of heterostructural alloying for enhanced piezoelectric performance of (Al,Sc)N. Physical Review Materials (2018).
4. Improved AlScN/GaN heterostructures grown by metal-organic chemical vapor deposition. Semiconductor Science and Technology (2021).
5. Strong enhancement of piezoelectric constants in ScxAl1−xN: First-principles calculations. AIP Advances (2016).
6. Enhanced electromechanical coupling in SAW resonators based on sputtered non-polar Al0.77Sc0.23N 11 2 ¯ 0 thin films. Applied Physics Letters (2020).