Controlling the Polarity of Scandium Aluminum Nitride (ScAlN) Piezoelectric Thin Films by Using Ge Addition

A highly <i>c</i>-oriented wurtzite phased scandium aluminum nitride (ScAlN) piezoelectric thin film could exist in nature as aluminum/scandium (Al/Sc) or nitrogen (N) polar thin films. The utilization of multilayers piezoelectric thin films with different polarity can be advantageous for the performance of the device. For example, the performance of a solidly mounted resonator type bulk acoustic wave (SMR-BAW) can be enhanced by using a stack of N-polar and Al-polar piezoelectric thin films [1]. Since ScAlN-based resonators feature a significant improvement from aluminum nitride (AlN)-based resonators, wurtzite phased ScAlN has been extensively studied and has arose as the future material for many promising electronic applications including radio frequency (RF) filters [2-3]. Polarity control of ScAlN-based thin films is expected to bring further improvement in the performance of the current RF filter by enabling it to function at higher frequency range in 5G communication technology. In terms of polarity control, the addition of germanium (Ge) into AlN have been reported to capable of inversing the polarization direction of non-doped AlN from Al-polar to N-polar [1]. However, there are no report on addition of Ge to inverse the polarity of ScAlN. Therefore, in this study, we investigated of the effect of Ge addition on the polarization direction of ScAlN piezoelectric thin films.<br/><br/>The fabrication of the thin films were conducted by using radio frequency (RF) magnetron sputtering system. The concentration of Sc, Ge and Al was controlled by adjusting the power of cathodes during deposition process. In this study, we examine how co-addition of Sc and Ge into AlN on affect the polarity, the crystal structure, lattice parameters and surface state of each element in the thin films. Positive d₃₃ value indicates that the thin film has Al/Sc-polarity while negative d₃₃ value indicates that the thin film has N-polarity. All Sc_xAl_1-xN thin films exhibited positive d₃₃ values which indicate that the thin films have Al/Sc-polarity. Depending on the concentration and the ratio of Sc and Ge, co-addition of Sc and Ge into AlN resulted in thin films with negative d₃₃ which suggested that the thin films have N-polarity. For example, Sc<i>_x</i>Ge<i>_y</i>Al_{1-<i>x</i>-<i>y</i>}N thin films with <i>x</i> = 0.3 and <i>y</i> = 0.05~0.14 exhibited N-polarity suggesting that co-addition of Sc and Ge into AlN yielded in thin films with inversed polarity. However, addition of Ge was found to unable to inverse the polarity of Sc_0.4Al_0.6N, since the resulting thin films exhibited Al-polarity. Based on these results, it can be inferred that Ge addition can be used not only to control the polarity of AlN but also the polarity of ScAlN, as long as the concentration of Sc is maintained to be less than 40 at.% and Ge concentration is also kept in a certain concentration range. Thus, both concentration and ratio of Sc and Ge are considered to be among the important parameters that control the polarization direction of the resulting thin films.<br/><br/><b><u>Acknowledgement</u></b><br/>This work was supported by JSPS KAKENHI grant number JP21K04168.<br/><br/><b><u>References</u></b>:<br/>[1] Mizuno, T. <i>et al</i>. In <i>2017 19th International Conference on Solid-State Sensors, Actuators and Microsystems (TRANSDUCERS)</i>. 1891.<br/>[2] M. Akiyama, T. Kamohara, K. Kano, A. Teshigashara, Y. Takeuchi, N. Kawahara, <i>Adv. Mater.</i> (2009) 21, 593.<br/>[3] M. Moreira, T. Torndahl, I. Katardajiev, T. Kubart, J. <i>Vac. Sci. Technol. A 33</i> (2015) 021518.