Anli Ding

The laser ultrasound (LU) technique has been used to determine dispersion curves for surface acoustic waves (SAW) propagating in AlScN/Al₂O₃ systems. Polar and non-polar Al_0.77Sc_0.23N thin films were prepared by magnetron sputter epitaxy on Al₂O₃ substrates and coated with a metal layer. SAW dispersion curves have been measured for various propagation directions on the surface. This is easily achieved in LU measurements since no additional surface structures need to be fabricated, which would be required if elastic properties are determined with the help of SAW resonators. Variation of the propagation direction allows for efficient use of the system’s anisotropy when extracting information on elastic properties. This helps to overcome the complexity caused by a large number of elastic constants in the film material. An analysis of the sensitivity of the SAW phase velocities (with respect to the elastic moduli and their dependence on SAW propagation direction) reveals that the non-polar AlScN films are particularly well suited for the extraction of elastic film properties. Good agreement is found between experiment and theoretical predictions, validating LU as a non-destructive and fast technique for the determination of elastic constants of piezoelectric thin films.

Al 1−x Sc x N (AlScN) is known for its large elastic and piezoelectric constants and thus is a favorable material for applications in novel radio frequency (RF) components. We investigated a-plane AlScN on r-plane Al 2 O 3 (AlScN(11-20)/Al 2 O 3 (1-102)). The surface acoustic wave (SAW) propagation properties of AlScN(11-20)/Al 2 O 3 (1-102) and AlScN(11-20)/Al 2 O 3 (1-102) were analyzed using finite element method (FEM) simulations and the results were compared. Rayleigh-type and Sezawa-type wave modes were identified and the acoustic parameters such as phase velocity, electromechanical coupling coefficient, and reflectivity were evaluated. An increased effective coupling of 5.5 % was detected for Rayleigh-type waves on AlScN(11-20)/Al 2 O 3 (1-102). The Sezawa-type modes show an even higher coupling up to 6.2 %. Furthermore, we detected increased reflectivity of AlScN(11-20) films compared to c-plane AlScN(0001). Our results reveal the potential of using a-plane AlScN for increasing the electromechanical coupling, which is needed for the upcoming piezo-acoustic filter requirements.

1 µm thick sputtered Al1-xScxN films were used to fabricate SAW resonators with 2-20 µm wavelength and the frequency response was measured in the range of 20 °C to 140 °C. Extracted phase velocity dispersion curve was in good agreement with the simulated dispersion curve by FEM model. Moreover, for each investigated temperature, the corresponding phase velocity shift dispersion curves as a function of normalized thickness were shown in detail for each resonator layer (Si substrate, AlScN piezoelectric layer, and Pt electrode), and the independent thermal influence of the Si, AlScN, and Pt were evaluated. The temperature coefficients of frequency (TCF) of Al1-xScxN (x = 0, 0.14, 0.32) layer was calculated independently and compared with the combined TCFs of the resonators.

It has been found that introducing Sc into AIN to form Al _1-x Sc _x N can lead to a significant increase in piezoelectric response, making Al _1-x Sc _x N a promising material for electroacoustic devices, such as frequency filters for mobile communications. 1 μm thick highly c-axis oriented Al _1-x Sc _x N thin films were deposited on 100 mm Si(001)(x = 0, 0.14, 0.23, and 0.32)and on Al ₂ O ₃ (0001)(x = 0.14 and 0.23)substrates using reactive pulsed-DC magnetron sputtering and SAW resonators with wavelengths λ = 2-24 μm were fabricated. The effective electromechanical coupling k _eff ² of the resonators improved with increasing Sc concentration, for instance, the λ = 2 μm AIN (f _s = 2.08 GHz)and Al _0.68 Sc _0.32 N (f _s = 1.83 GHz)resonators exhibited coupling of 0.6 % and 2.6 %, respectively. Furthermore, 3 % increase in resonant frequency and 9.6 % increase in coupling were observed when comparing λ = 2 μm resonators based on Al ₂ O ₃ to those based on Si substrates. The temperature coefficient of frequency (TCF)was found to reach up to -27.4 ppm/K at x = 0.23 using Si substrates, while the equivalent resonators based on Al ₂ O ₃ substrates exhibited lower TCF of -26.1 ppm/K. Thermal hysteresis errors were calculated to be ~1%. Employing Al ₂ O ₃ substrates for λ = 2 μm Al _0.77 Sc _0.23 N-based resonators showed reduced TCF, higher resonant frequency, and higher coupling coefficient when compared to the same design resonators on Si substrates.

Reactive pulsed DC magnetron co‐sputtering is used to grow piezoelectric aluminum nitride (AlN) and aluminum scandium nitride (AlScN) thin films on Si(001) substrates. By using grazing incidence X‐ray diffraction (GIXRD), scanning electron microscopy (SEM), and piezoresponse force microscopy (PFM), we investigate how the microstructure and the presence of misoriented grains affect the piezoelectric properties of the material. N₂concentration and target‐to‐substrate distance are finely tuned to achieve thin films without misoriented grains, resulting in Al_0.87Sc_0.13N thin films with low roughness, high degree ofc‐axis orientation, homogenous polarity, and piezoelectric coefficientd₃₃ = −12.3 pC/N.