Ferroelectricity and Large Piezoelectric Response of AlN/ScN Superlattice

Abstract

Based on density functional theory, we investigate the ferroelectric and piezoelectric properties of the AlN/ScN superlattice, consists of ScN and AlN buckled monolayers alternating along crystallographic c-direction. We nd that the polar wurtzite (w-ScAlN) structure is mechanically and dynamically stable, and is more stable than the nonpolar hexagonal at conguration. We show that ferroelectric polarization switching can be possible for epitaxially tensile strained superlattice. Due to the elastic constant C 33 softening along with an increase in e 33 , the piezoelectric coecient d 33 of the superlattice is doubled compared to pure w-AlN. The combined enhancement of Born eective charges (Z 33) and the sensitivity of the atomic coordinates to external strain (∂u 3 ∂η 3) is the origin of large piezoelectric constant e 33. Moreover, we show that epitaxial biaxial tensile strain signicantly enhances the piezo-response, so that d 33 becomes seven times larger than that of w-AlN at 4% strain. The tensile strain results in a huge enhancement in e 33 by increasing Z 33 and ∂u 3 ∂η 3 , which boosts the piezoelectric coecient. As short-period superlattice growth and epitaxial strain are already experimentally demonstrated in wurtzite nitrides, our results show a new more controlled approach

Negative Piezoelectric Coefficient in Ferromagnetic 1H-LaBr2 Monolayer

“…Interestingly, FM 1H-LaBr 2 shows a negative e 11 which is also quite low compared to those of the other materials. To understand the origin of piezoelectric constant, e 11 and e 12 can be decomposed into two parts: 8 , 24 …”

“…A piezoelectric stress coefficient ( e ij ), defined as , where ∂ P i is the induced polarization along the i -direction in response to strain ∂ η j along the j -direction, can be split into two contributions: the ionic part, e ij ion , where ions are allowed to move under an applied strain, and the electronic part (also known as the clamped-ion part) e ij elc , where ions are clamped under applied strain. In many bulk materials, including wurtzite nitrides, 7 , 8 e ij elc is negative but is dominated by positive e ij ion , thus resulting in a positive value of e ij . Generally, a positive longitudinal piezoelectric coefficient is expected as a tensile strain is expected to increase the induced electric polarization.…”

Large Piezoelectric Response and Ferroelectricity in Li and V/Nb/Ta Co-Doped w-AlN

“…We calculate the spontaneous polarization by P 3 = e V k Z * k ,33 ∆u k ,3 , where Z * k ,33 is the 33 (3 represents crystal's c-direction) component of Born effective charge tensor of k-th atom in the supercell, ∆u k ,3 is the atomic displacement along the c-direction of k-th atom with respect to the ideal nonpolar hexagonal configuration, e is the magnitude of an electron's charge and V is the polar supercell's volume [14,43]. The calculated polarization as a function of co-doping and Sc doping concentration is shown in Figure 3(f).…”

“…, where i runs over all the atoms in the supercell, a s is the in-plane lattice parameter of the supercell, and e is the electron charge [43,48,16]. It is convenient to consider an average value of Z 33 and…”

Large Piezoelectric Response and Ferroelectricity in Li and V/Nb/Ta co-doped w-AlN

“…, where Z * k ,33 is the 33 (3 represents crystal's c-direction) component of Born effective charge tensor of k-th atom in the supercell, ∆u k ,3 is the atomic displacement along the c-direction of k-th atom with respect to the ideal nonpolar hexagonal configuration, e is the magnitude of an electron's charge and V is the polar supercell's volume [14,43]. The calculated polarization as a function of co-doping and Sc doping concentration is shown in Figure 3(f).…”