Doping of AlN

Over a decade ago, the doping of aluminum nitride (AlN) with rare earth metals using scandium (Sc) was demonstrated experimentally. This makes it possible to significantly improve the moderate piezoelectric properties of pure AlN by up to 500%. Since then, various doping elements in the form of both ternary (Ta, Cr) and quaternary (Mg and Hf/Zr) doped AlN have been investigated. So far, however, no combination of materials has been found that has higher piezoelectric constants than Sc-doped AlN (ScAlN) with concentrations of up to 43 at% Sc.


For yttrium (Y)-doped AlN with 40 at% Y, density functional theory (DFT) simulations predict an improvement in piezoelectric coefficients by a factor of 4, which is comparable to ScAlN with 35 at% Sc. However, yttrium is significantly more cost-efficient compared to scandium. In addition, DFT calculations predict a higher phase stability of YAlN (up to 75 at% Y) compared to ScAlN (up to 50 at%) before the material stabilizes in a non-piezoelectric cubic crystal lattice. Thus, YAlN has the potential to outperform ScAlN at higher doping concentrations and achieve significantly higher piezoelectric performance compared to pure AlN.


In this project, the potential of yttrium as a dopant in AlN for applications in bulk acoustic wave (BAW) devices is investigated, with a focus on improving key properties such as piezoelectric coefficients and electromechanical coupling. The YAlN thin layers are deposited with an industrial sputtering system and BAW resonators are produced from them in the modern clean room facilities at ISAS / TU Wien. The developed materials are then compared to ScAlN.