Highly piezoelectric co-doped AlN thin films for wideband FBAR applications

In this paper, we report new piezoelectric materials composed of charge-compensated co-doped (Mg, β)_xAl_1-xN (β = Zr or Hf). The effect of the dopant element into AlN on chemical stability, crystal structure, and piezoelectric property of co-doped AlN was determined on the basis of first-principles calculation, and the theoretical piezoelectric property was confirmed by experimentally depositing thin films of magnesium (Mg) and zirconium (Zr) co-doped AlN (Mg-Zr-doped AlN). The Mg-Zr-doped AlN thin films were prepared on Si (100) substrates by a triple-radio-frequency magnetron reactive co-sputtering system. The crystal structures and piezoelectric coefficients (d₃₃) of the films were investigated as a function of their concentrations, which were measured by X-ray diffraction and a piezometer. The investigation results show that d₃₃ of Mg-Zr-doped AlN at total Mg and Zr concentrations (both expressed as x) of 0.35 is 280% larger than that of pure AlN. The experimentally measured parameter of the crystal structure and d₃₃ of Mg-Zr-doped AlN (plotted as functions of total Mg and Zr concentrations) are in very close agreement with the corresponding values obtained by first-principle calculations. Thin film bulk acoustic wave resonators (FBARs) employing (Mg,Zr)_0.13Al_0.87N and (Mg,Hf)_0.13Al_0.87N as a piezoelectric thin film were fabricated, and their resonant characteristics were evaluated. As a result, the measured electromechanical coupling coefficient was found to increase from 7.1% (for pure AlN) to 8.5% for Mg-Zr-doped AlN and 10.0% for Mg-Hf-doped AlN. These results indicate that co-doped (Mg, β)_xAl_1-xN (β = Zr or Hf) films have potential as piezoelectric thin films for wideband RF applications.