Theoretical evidence of the piezoelectric property enhancement for ScY- or CaTi-codoped wurtzite AlN

Abstract

Improving the piezoelectric coefficient of w-AlN ensuring its structural stability is a serious challenge both experimentally and theoretically. Here, the stability and piezoelectric characteristics of ScY- or CaTi-codoped w-AlN are studied in detail using first-principles calculations. The analysis of formation enthalpy and AIMD thermodynamic simulations show that the ordered alloy has good structural stability, and it is confirmed that the stability of the conventionally ordered codoped structure is better than that of the disordered structure. The piezoelectric coefficients of (ScY)_xAl_1−xN and (CaTi)_xAl_1−xN alloys were successfully predicted to be improved several times compared to pure w-AlN. Among them, the piezoelectric strain constant d₃₃ of (CaTi)_0.5Al_0.5N (17.42 pC N⁻¹) is 2 times larger than that of (ScY)_0.5Al_0.5N (8.70 pC N⁻¹) when the dopant concentration x is 0.5. It was verified that the greater polarization of (CaTi)_xAl_1−xN along the c-axis originates from the greater lattice distortion and charge redistribution after doping. Furthermore, the weak covalent binding of Al–N after doping is another reason for the improvement of the piezoelectric strain constant d₃₃. This work provides theoretical support for further improving the piezoelectric performance of w-AlN thin films and their applications in acoustic devices.