Piezoelectric effect and polarization switching in Al$_{1-x}$Sc$_x$N

Abstract

Aluminum nitride is piezoelectric and exhibits spontaneous polarization along the c-axis, but the polarization cannot be switched by applying an electric field. Adding Sc to AlN enhances the piezoelectric properties, and can make the alloy ferroelectric. We perform a detailed first-principles analysis of spontaneous and piezoelectric polarization. Comparisons between explicit supercell calculations show that the virtual crystal approximation produces accurate results for polarization, but falls short in describing the phase stability of the alloy. We relate the behavior of the piezoelectric constant e 33 to the microscopic behavior of the internal displacement parameter u, finding that the internal strain contribution dominates in the Sc-induced enhancement. The value of u increases with scandium concentration, bringing the alloy locally closer to a layered hexagonal structure. Our approach allows us to calculate the ferroelectric switching barrier, which we analyze as a function of Sc concentration and temperature based on Ginzburg-Landau theory.

Strongly temperature dependent ferroelectric switching in AlN, Al1-xScxN, and Al1-xBxN thin films

“…B is advantageous in that it increases both the bandgap and u, which facilitates observation of switching. 12,20 While the mechanism for polarization reversal is not yet understood in these wurtzite-structured ferroelectrics, it is speculated that domain wall motion may be important, as is the case in many other ferroelectrics. Motion of pre-existing domain walls in ferroelectrics is widely recognized to be a thermally activated phenomenon.…”