Researchers demonstrate breakthrough piezoelectric possibility with Mg-Nb-AlN

Aluminum nitride (AlN) is a very promising piezoelectric material, especially for energy harvesting and microelectromechanical systems, and also for improved film bulk acoustic resonators (FBARs) utilized in next generation smart phones. Akiyama et al. have demonstrated that the piezoelectric coefficient d_33,f, dramatically increases with the addition of scandium (Sc) to AlN films. However, Sc, which is trivalent, is very expensive, so researchers have theorized combining bivalent and tetravalent elements as an alternative. Recently, a group of Japanese researchers have demonstrated that adding a magnesium (Mg) and niobium (Nb) alloy – an inexpensive option – greatly improves AlN d_33,f. The researchers report their findings in Applied Physics Letters.

Lead author Masato Uehara explains that the researchers chose bivalent Mg because of its chemical stability and economic viability. After testing a few elements, they found AlN with Nb and Mg compares well to Sc. The researchers prepared the Mg-Nb-AlN by multiple element co-sputtering using three metal targets. In testing for piezoelectricity, the researchers found marked improvements when Mg and Nb were added simultaneously. Moreover, the optimal piezoelectric effect was at a 1.6 Mg-Nb ratio, while the conventional theory indicated it would be 2. With X-ray photoelectron spectroscopy, the Mg was confirmed as bivalent, while Nb had multiple valences, which supported the 1.6 ratio.

The researchers studied the crystalline structure of Mg-Nb-AlN, which had similar characteristics to (Sc,Al)N, but Uehara says future work should explore in more depth its crystalline structure and in connection to Mg-Nb valences. Uehara highlights that this research demonstrates that the Mg-Nb addition causes a giant increase in the AlN piezocoefficient and that the d_33,f increase is related to the multiple states of Nb.

Source: “Giant increase in piezoelectric coefficient of AlN by Mg-Nb simultaneous addition and multiple chemical states of Nb,” by Masato Uehara, Hokuto Shigemoto, Yuki Fujio, Toshimi Nagase, Yasuhiro Aida, Keiichi Umeda, and Morito Akiyama, Applied Physics Letters (2017). The article can be accessed at https://doi.org/10.1063/1.4990533 .