Chapter Four - In-plane ferroelectric switching of non-polar wurtzite AlScN films using SAW resonators

Since the discovery of the ferroelectric properties of AlScN, the range of possibilities for the development of novel electronic devices and technologies has expanded. However, the realization of all these technologies are based on the implementation of c-plane oriented AlScN films, as AlScN exhibits most desired physical properties along this axis. Thus, the deposition and characterization of c-plane oriented nitrides have been the main focus of recent research activities. However, non-polar a-plane oriented AlScN showed promising results regarding the field surface acoustic wave resonators for frequency filters. In this case, the c-axis is aligned parallel to the surface, which, however, increases the complexity of assessing and characterizing the in-plane properties. This work reports the experimental attempt of characterizing sputtered a-plane Al0.7Sc0.3N film on an r-plane Al2O3 substrate using ferroelectricity. In this context, in-plane ferroelectricity was demonstrated using an a-plane Al0.7Sc0.3N film, in which the current response of a surface acoustic wave resonator (SAW) to an applied voltage signal was measured. The ferroelectric properties were validated by estimating the ferroelectrically active cross-section based on the simulated electric field distribution using the finite-element method (FEM). The ferroelectrically active cross-section was also confirmed by measuring and simulating the frequency response of the admittance of the SAW resonators before and after ferroelectric switching. The derivation of this cross-section using both approaches revealed a remnant polarization value between 0.9 and 1.35 C/m2, which agrees with reported data for c-plane Al0.7Sc0.3N so far, confirming the high degree of c-axis orientation of the film along the surface.