Polarity Inversion of Scandium Aluminum Nitride (ScAlN) Piezoelectric Thin Films via Addition of Element in Group IVB (C, Si, Ge or Sn)

The discovery of scandium aluminum nitride (ScAlN) piezoelectric thin film has successfully brought promising improvement to the performance of modern wireless communication technology, particularly radio frequency (RF) filter [1,2]. The shift toward 6G communication system demands RF filter that can work at much higher frequency and wider bandwidth to allow efficient data transmission. Since integrating multilayers of piezoelectric thin films with different polarities enable an RF filter to function at much higher frequency [3], stacking Al-polar and N-polar ScAlN thin films is expected to be an effective strategy to achieve high-performance RF filter. As a result, controlling the polarity of ScAlN has become paramount for the development of next generation RF filter. Addition of element such as silicon (Si) or germanium (Ge) into AlN was reported to capable of inversing the polarity of aluminum nitride (AlN) [3,4] as well as ScAlN [5,6]. Therefore, in this study, we investigated the effect of addition of each element in group IVB (carbon (C), Si, Ge, tin (Sn)) on the polarization direction of ScAlN thin films.<br/>All thin films were deposited on Si (100) wafer by using radio frequency magnetron sputtering system. Our study reveals that addition of C into ScAlN resulted in thin films with positive piezoelectric coefficient (<i>d</i>₃₃), which means addition of C into ScAlN maintained the polarity of ScAlN to be Al-polar. However, addition of either Si or Ge into ScAlN resulted in thin films with negative <i>d</i>₃₃ and the <i>d</i>₃₃values have similar magnitude with that of ScAlN. This suggested that addition of Si or Ge could inverse the polarity of ScAlN from Al-polar into N-polar, without significantly decreasing the value of <i>d</i>₃₃. In case of Sn, the negative <i>d</i>₃₃that was obtained upon the addition of Sn into ScAlN is an indication that addition of Sn could led to thin films with N-polarity. However, the <i>d</i>₃₃ values of ScSnAlN are lower than that of ScAlN. Our study confirmed that the formation of N-polar ScAlN is largely controlled by both concentration of each additive element (in this case: Si, Ge, Sn) and the concentration of Sc. Effect of addition of each element on polarity distribution, crystal structure and surface states were also investigated in this study.<br/><br/><b><u>Acknowledgement</u></b>: This work was supported by JSPS KAKENHI Grant Number JP21K04168.<br/><br/><b><u>References</u></b>:<br/>[1] M. Akiyama et al, <i>Adv. Mater.</i> (2009) 21, 593.<br/>[2] M. Moreira et al, <i>Vacuum, </i>(2011) 86, 23.<br/>[3] Mizuno, T. et al. <i>2017 19th International Conference on Solid-State Sensors, Actuators and Microsystems</i> (TRANSDUCERS). 1891.<br/>[4] S. A. Anggraini et al, <i>Scientific Reports</i> (2020) 10, 4369.<br/>[5] S. A. Anggraini et al, <i>JSAP Spring Meeting 2021</i>.<br/>[6] S. A. Anggraini et al, <i>JSAP Fall Meeting 2021</i>.