Implications of heterostructural alloying for enhanced piezoelectric performance of (Al,Sc)N

Abstract

An understanding of the heterostructural implications on alloying in the aluminum nitride-scandium nitride system (Al 1−x Sc x N) can highlight opportunities and design principles for enhancing desired material properties by leveraging nonequilibrium states. The fundamental thermodynamics, and therefore composition-and structuredependent mechanisms, underlying property evolution in this system have not been fully described, despite significant recent efforts driven by interest in enhanced piezoelectric performance. Practical realization of these enhanced properties, however, is hindered by the strong driving thermodynamic driving force for phase separation in the system, highlighting the need for increased study into the role of heterostructural alloying on the thermodynamics and composition-structure-property relationships in this system. With this need in mind, ab initio computed alloy thermodynamics and properties are compared to combinatorial thin-film synthesis and characterization to develop a more complete picture of the structure and property evolution across the Al 1−x Sc x N composition space. The combination of structural frustration and a flattened free-energy landscape lead to substantial increases in electromechanical response. The energy scale of alloy metastability is found to be much larger than previously reported, helping to explain difficulties in achieving homogeneous materials with high scandium concentration. Scandium substitution for aluminum softens the wurtzite crystal lattice, and energetic proximity to the competing hexagonal boron-nitride structure enhances the piezoelectric stress coefficient. Overall, this work provides insight into the understanding of the structure-processing-property relationships in the Al 1−x Sc x N system, suggests material design strategies for even greater property enhancements, and demonstrates the increased property tunability and underexplored nature of nonequilibrium heterostructural alloys.

First-Principles Study of Piezoelectric Properties and Bonding Analysis in (Mg, X, Al)N Solid Solutions (X = Nb, Ti, Zr, Hf)

“…On the other hand, a lower elastic constant C 33 can also contribute in the increasing d 33 . In fact, the calculated C 33 (17,19,20) is in good agreement with the experimental results qualitatively, where the Young’s modulus value of Sc-doped AlN was found to be lower than that of pure AlN. 4 When 40 mol % of Sc is added into AlN, d 33 increases dramatically and the ratio of the lattice constant ( c / a ) decreases from ≈1.6 to ≈1.4.…”

“…The origin of the dramatic improvement of the piezoelectric properties for Sc-doped AlN has been investigated by theoretical calculations. 17−20 Doping Sc into AlN causes an increase in Z 33 and d u /dε which leads to an increase in e 33 . On the other hand, a lower elastic constant C 33 can also contribute in the increasing d 33 .…”

Thin Film Synthesis of Semiconductors in the Mg–Sb–N Materials System

“…In addition, the AlN layer was well-aligned and well known, with the c-axis oriented along the substrate normal and the layer thickness set to around 100 nm. Growth conditions for AlN in this deposition system can be found elsewhere [29]. This protective capping layer increased the lifetime of the nitride sputtered samples from a few hours to a few months.…”

Significant Enhancement of Piezoelectric Response in AlN by Yb Addition

“…The piezoelectric properties of aluminum nitride (AlN)-based wurtzite solid solutions have been investigated experimentally and theoretically [ 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 , 10 , 11 ]. In particular, scandium (Sc)-doped AlN (ScAlN) exhibits high piezoelectricity and has been widely considered for use in high-frequency filters, sensors, and microelectromechanical devices [ 12 , 13 ].…”

“…The piezoelectric constant, d 33 , can be approximated as d 33 ≈ e 33 / C 33 , where e 33 is the piezoelectric stress constant and C 33 is the elastic constant. Computational results have indicated that elastic softening (decrease of C 33 ) can be caused by the addition of other elements into AlN [ 4 , 7 , 9 , 11 ]. Furthermore, the lattice parameter ratio ( c/a ) of the wurtzite structure decreases as the quantity of additional elements increases [ 1 , 2 , 9 , 16 ].…”