Keiichi Umeda

Giant increase in piezoelectric coefficient of AlN by Mg-Nb simultaneous addition and multiple chemical states of Nb

Aluminum nitride (AlN) is one of piezoelectric materials, which are eagerly anticipated for use in microelectromechanical systems (MEMS) applications such as communication resonators, sensors, and energy harvesters. AlN is particularly excellent in generated voltage characteristics for the MEMS rather than oxide piezoelectric materials such as lead zirconium titanate Pb(Zr, Ti)O3. However, it is necessary to improve the piezoelectric properties of AlN in order to advance the performance of the MEMS. We dramatically increased the piezoelectric coefficient d33 of AlN films by simultaneously adding magnesium (Mg) and niobium (Nb). The d33 of Mg39.3Nb25.0Al35.7N is 22 pC/N, which is about four times that of AlN. The d33 is increased by Mg and Nb simultaneous addition, and is not increased by Mg or Nb single addition. Interestingly, the Nb has multiple chemical states, and which are influenced by the Mg concentration.

Germanium aluminum nitride thin films for piezo-MEMS devices

This paper reports on piezoelectric polarity inversion of AlN thin films by doping. The germanium doped aluminum nitride (GeAlN) thin films are grown by RF magnetron sputtering. Doping amount is varied from 0 to 23%. The maximum piezoelectric polarity inversion from Al polarity to N polarity is observed at 3% of Ge where the piezoelectric d33 value changes from + 7.8 to −7.7 pC/N. The bulk acoustic wave (BAW) resonator with the stack of both N polarity and Al polarity layers is demonstrated. The B AW resonator is excited at 2nd overtone mode at 2.58 GHz. Finally, the mechanism of polarity inversion of GeAlN is discussed and first-principal calculations of GeAlN system are presented.