*2.2. Characterization Methods*

The surface structure of AlN and AlScN thin films is investigated using a critical dimension scanning electron microscope (CDSEM, Hitachi), which allows loading and imaging of 8-inch wafers. The crystal structures of AlN and AlScN films are examined by performing XRD *θ*-2*θ* scans and *ω* scans in a Rigaku SmartLab diffractometer (9 kW, Hypix detector) with Cu K*α* radiation, a Ge(220)x2 monochromator and soller slit of 5◦ .

Transmission electron microscopy analysis of the Si/AlN/AlScN stack is conducted on a cross-section specimen, which is prepared using the focused ion beam (FIB) method and milled down to electron-transparency (FEI DualBeam Helios600 FIB-SEM). The nanoscopic structural and chemical analyses are performed on a probe CS-corrected Titan³ G2 60-300 microscope operating at 300 kV and a JEOL JEM-2100 transmission electron microscope (thermionic source LaB6, acceleration voltage 200 kV) for selected area electron diffraction (SAED). The elemental distribution across the Si/AlN/AlScN interfaces is probed by energy dispersive X-ray spectroscopy (EDS) mapping using a Super-X EDS detector on the Titan³ microscope.

The piezoelectric coefficient *d*33, *<sup>f</sup>* is characterized using a double beam laser interferometer (DBLI) from aixACCT systems, which allows automatic measurement with an 8-inch wafer. As the piezoelectric coefficient depends on the ratio of the electrode size to the substrate thickness [38], the *d*33, *<sup>f</sup>* shown in this work has been calibrated to its geometry independent value.
