Temperature-dependent properties of wurtzite and rock salt Al_1−xSc_xN for applications in BAW resonators

Abstract

Scandium aluminum nitride (AlxSc1-xN) is known for its tremendous enhancement of piezoelectric properties. This material can improve the energy efficiency of a wide range of applications, for example bulk acoustic wave (BAW) resonators. Such devices heat up during operation and are thus sensitive to temperature-dependent material properties. As a result, device characteristics change with temperature, causing undesirable shifts in achievable frequency and performance. These shifts are well known for AlN-based resonators, where the frequency shifts are compensated without increasing the bandwidth of the frequency filter. For the ternary alloy AlxSc1-xN, many temperature-dependent structural, mechanical and piezoelectric properties have not or have been insufficiently investigated. This review provides an overview of relevant temperature dependent properties of hexagonal wurtzite (x < 0.5) and cubic rock salt AlxSc1-xN (x > 0.5) crystals up to 2000 K. The properties studied are the thermal expansion coefficient (TEC) α, the elastic tensor C ij, the piezoelectric coefficient d 33, the piezoelectric stress coefficient e 33 and the permittivity ε33. Based on these material properties, the derived characteristics for BAW resonators are determined from 0 to 1400 K, which are the resonance frequency f R and the electromechanical coupling coefficient k t 2.