Ferroelectricity in Ultrathin Films and 2D Crystals of van der Waals Semiconductors

Prof. Liu’s research has in recent years been expanded into the area of ferroelectricity, focusing on physics and potential applications of ultrathin ferroelectric materials and devices including ferroelectric tunnel junctions (FTJs), ferroelectric semiconductor field-effect transistors, and ferroelectric Schottky diodes. The focus of work in this area is to explore the effects of thickness reduction and the presence of the mobile charges on the properties of ferroelectric materials using the devices as tool for exploring the fundamental physics of ferroelectricity. These devices are considered to be technologically relevant for memories and computing in memories that are important for semiconductor microelectronics. 

We have been investigating ultrathin Hf1-xZrxO2 prepared by CMOS compatible atomic layer deposition or molecular beam epitaxy (in collaboration with other groups). The main focus of the work in this direction is to explore the formation of the oxygen vacancies and other defects in the thinnest films of Hf1-xZrxO2 which are both needed for the occurrence of ferroelectricity but impact negatively the performance of the devices such as (FTJs). We are also investigating quantum tunneling through a ferroelectric barrier of Hf1-xZrxO2 which dominates the electrical transport through the barrier only at low temperatures as well as the possibility of achieving ferroelectric Josephson tunnel junctions.

Two-dimensional (2D) crystals of van der Waals layered semiconductors such as CuInP2S6 and In2Se3 were found to be ferroelectric in recent years. The thickness reduction and the presence of mobile charge in these 2D materials were found to impact strongly their ferroelectric properties, which however give rise to novel device possibilities. In addition to ferroelectric Josephson tunnel junctions, we have been exploring the interplay between ferroelectricity and superconductivity in heterostructures of CuInP2S6 and other van der Waals layered. The former are predicted to exhibit the Josephson diode effect due to the spontaneous breaking of time-reversal symmetry, potentially useful for future quantum technologies.

Recent publications:

1. Justin R. Rodriguez, William Murray, Kazunori Fujisawa, Seng Huat Lee, Alexandra L. Kotrick, Yixuan Chen, Nathan Mckee, Sora Lee, Mauricio Terrones, Susan Trolier-McKinstry, Thomas N. Jackson, Zhiqiang Mao, Zhiwen Liu, and Ying Liu, “Electric field induced metallic behavior in thin crystals of ferroelectric α-In2Se3,” Appl. Phys. Lett. 117, 052901 (2020).

2. Jinyuan Yao, Yongtao Liu, Shaoqing Ding, Yanglin Zhu, Zhiqiang Mao, Sergei V. Kalinin, and Ying Liu, “Ferroelectric Schottky diodes of CuInP2S6 nanosheet,” Appl. Phys. Lett. 123, 14 (2023).

3. Maria Hilse, Justin Rodriguez, Jennifer Gray, Jinyuan Yao, Shaoqing Ding, Derrick Shao Heng Liu, Mo Li, Joshua Young, Ying Liu, and Roman Engel-Herbert. "Mixed polytype/polymorph formation in InSe films grown by molecular beam epitaxy on GaAs (111) B." npj 2D Mater. Appl. 9, 19 (2025).