Ionic Bonds Control Ferroelectric Behavior in Wurtzite Nitrides

Ferroelectricity enables key integrated technologies from non-volatile memory to precision ultrasound. Wurtzite ferroelectric Al_1-xSc_xN has recently received significant attention because of its enhanced piezoelectric response [1], robust ferroelectricity [2] and Si process compatibility in addition to being the first known ferroelectric wurtzite. Since the discovery of the ferroelectricity of the material system, chemistry, stress, strain, and film thickness have been rigorously investigated to control ferroelectricity in this material system [2-5]. However, the origin and control of ferroelectricity in wurtzite materials is not yet fully understood. Understanding the root cause is important for coercive field engineering and new wurtzite ferroelectric material discovery to achieve low-power electronic applications such as non-volatile memories. <br/><br/>A part of hinderance for understanding the new wurtzite ferroelectric physics comes from the fact that the prior studies of ferroelectricity in Al_1-xSc_xN included explicit and implicit variables such as residual strain state, process parameters, target condition, chamber type, substrate type/treatment, etc. These variables convolute experimental effects, and the resulting data scatter can easily mask important but unrepresented factors such as microstructure and defects. Combinatorial techniques reduce uncontrolled process variables because a single film library can include all samples of interest and also offer significant advantages for rapid screening.<br/><br/>In this talk, we demonstrate that the local bond ionicity, rather than simply the change in tetrahedral distortion, is key to controlling the macroscopic ferroelectric response, according to our combinatorial film synthesis/characterization experiment and computational approach. Across the composition gradient in Sc &lt; 0.35 range and 140 – 260 nm thickness in combinatorial thin films of Al_1-xSc_xN, the pure wurtzite phase exhibits a similar <i>c/a</i> ratio regardless of the Sc content, due to elastic interaction with neighboring crystals. The coercive field and spontaneous polarization significantly decrease with increasing Sc content despite this invariant <i>c/a</i> ratio, due to the more ionic bonding nature of Sc-N relative to the more covalent Al-N bonds, supported by DFT calculations. Based on these insights, ionicity engineering is introduced as an approach to reduce coercive field of Al_1-xSc_xN for memory and other applications and to control ferroelectric properties in other wurtzite materials. <br/> <br/>[1] M. Akiyama, T. Kamohara, K. Kano, A. Teshigahara, Y. Takeuchi and N. Kawahara, <i>Adv. Mater.</i>, 2009, <b>21</b>, 593–596.<br/>[2] S. Fichtner, N. Wolff, F. Lofink, L. Kienle and B. Wagner, <i>J. Appl. Phys.</i>, 2019, <b>125</b>, 114103.<br/>[3] S. Yasuoka, T. Shimizu, A. Tateyama, M. Uehara, H. Yamada, M. Akiyama, Y. Hiranaga, Y. Cho and H. Funakubo, <i>J. Appl. Phys.</i>, 2020, <b>128</b>, 114103.<br/>[4] K. Yazawa, D. Drury, A. Zakutayev and G. L. Brennecka, <i>Appl. Phys. Lett.</i>, 2021, <b>118</b>, 162903.<br/>[5] R. Mizutani, S. Yasuoka, T. Shiraishi, T. Shimizu, M. Uehara, H. Yamada, M. Akiyama, O. Sakata and H. Funakubo, <i>Appl. Phys. Express</i>, 2021, <b>14</b>, 105501.