Effect of Mg and Zr Co-Doping on Piezoelectric AlN Thin Films for Bulk Acoustic Wave Resonators

This paper reports the crystal structures and piezoelectric properties of Mg and Zr co-doped AlN (MgZr-doped AlN) thin films. MgZr-doped AlN thin films on Si (100) substrates were fabricated by using a radio-frequency magnetron reactive cosputtering system. The relations between dopant concentration and the crystal structures measured by X-ray diffraction, transmission electron microscopy, and the piezoelectric constant d(33) measured with a piezometer were investigated. Up to total Mg and Zr concentrations of 34.8 at%, the MgZr-doped AlN films maintained the c-axis-oriented wurtzite structure and were likely formed by substituting Mg and Zr atoms into Al atom sites. The d(33) of the MgZr-doped AlN with a total Mg and Zr concentration (Mg + Zr) of 34.8 at% was about three times that of pure AlN. The experimental results on the relation between total Mg and Zr concentration and the crystal structure and the d(33) were in close agreement with the results of first-principles calculations. Finally, thin film bulk acoustic wave resonators (FBARs) that used MgZr-doped AlN as a piezoelectric thin film were fabricated and compared with the AlN-based FBAR. A total Mg and Zr concentration of 13 at% was found to improve the electromechanical coupling coefficient of AlN from 7.1% (for pure AlN) to 8.5%. The results from this study suggest that the MgZr-doped AlN films have potential as piezoelectric thin films for wideband and high-frequency RF applications.