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

Enhancement of piezoelectricity in w-AlN is desired for many devices including resonators for next-generation wireless communication systems, sensors, and vibrational energy harvesters. Based on density functional theory, we show that Li and X (X = V, Nb, and Ta) co-doping in 1Li:1X ratio transforms brittle w-AlN crystal to ductile, along with broadening the compositional freedom for significantly enhanced piezoelectric response, promising them to be good alternatives to expensive Sc. Interestingly, these co-doped w-AlN also show quite large spontaneous electric polarization (e.g., about 1 C/m² for Li_0.125X_0.125Al_0.75N) with the possibility of ferroelectric polarization switching, opening new possibilities in wurtzite nitrides. An increase in piezoelectric stress constant (e₃₃) with a decrease in elastic constant (C₃₃) results in an enhancement of piezoelectric strain constant (d₃₃), which is desired for improving the performance of bulk acoustic wave (BAW) resonators for high-frequency radio frequency (RF) signals. Also, these co-doped w-AlN are potential lead-free piezoelectric materials for energy harvesting and sensors as they improve the longitudinal electromechanical coupling constant (K₃₃²), transverse piezoelectric strain constant (d₃₁), and figure of merit (FOM) for power generation. However, the enhancement in K₃₃² is not as pronounced as that in d₃₃ because co-doping increases dielectric constant. The longitudinal acoustic wave velocity (7.09 km/s) of Li_0.1875Ta_0.1875Al_0.625N is quite comparable to that of commercially used piezoelectric LiNbO₃ or LiTaO₃ in special cuts (about 5-7 km/s) despite the fact that the acoustic wave velocities, important parameters for designing resonators or sensors, decrease with co-doping or Sc concentration.