Highly Piezoelectric Co-Doped AlN Thin Films for Wideband FBAR Applications

We report piezoelectric materials composed of charge-compensated co-doped (Mg, beta)(x)Al1-xN (beta = Zr or Hf) thin films. The effect of the dopant element into AlN on the crystal structure, and piezoelectric properties of co-doped AlN was determined on the basis of a first-principles calculation, and the theoretical piezoelectric properties were 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 using a triple-radio-frequency magnetron reactive co-sputtering system. The crystal structures and piezoelectric coefficients (d(33)) were investigated as a function of the concentrations, which were measured by X-ray diffraction and a piezometer. The results show that the d(33) of Mg-Zr-doped AlN at total Mg and Zr concentrations (both expressed as x) of 0.35 was 280% larger than that of pure AlN. The experimentally measured parameter of the crystal structure and d(33) of Mg-Zr-doped AlN (plotted as functions of total Mg and Zr concentrations) were in very close agreement with the corresponding values obtained by the first-principle calculations. Thin film bulk acoustic wave resonators (FBAR) employing (Mg, Zr)(0.13)Al0.87N and (Mg, Hf)(0.13) Al0.87N as a piezoelectric thin film were fabricated, and their resonant characteristics were evaluated. The measured electromechanical coupling coefficient increased 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, beta)(x) Al1-xN (beta = Zr or Hf) films have potential as piezoelectric thin films for wideband RF applications.