Large-Area Epitaxial Growth of α-In₂Se₃ and its Layer-Dependent Nonlinear Optical Properties

Non-volatile memories based on ferroelectric materials is one of the most competitive next-generation memories due to its low power consumption, fast read and write speed, non-volatility, and compatibility with CMOS manufacturing processes. However, the critical dimensions of conventional ferroelectric thin films as well as the depolarization and leakage current in conventional ferroelectric dielectrics limit the development of ferroelectric memories. On the other hand, recent development of field effect transistors based on two-dimensional ferroelectric semiconductors offers great potential to outperform conventional ferroelectric memories, in which the polarization states are stored in the semiconducting channels to bypass the limitations of conventional ferroelectric materials. Among all phases of In₂Se₃, two-dimensional α-In₂Se₃ with non-centrosymmetric structure possesses both intercoupled in-plane (IP) and out-of-plane (OOP) ferroelectricity in monolayer, which exhibits great application prospects such as non-volatile memories devices, memristors, and artificial intelligence synapse. However, the coexistence of multiple phases during the growth of In₂Se₃ thin films remains as a challenge to growing uniform and pure α phase In₂Se₃ in a large area and meeting the requirements of large-scale integration.<br/>Herein, we demonstrate the synthesis of large-are α-In₂Se₃ monolayer and multilayer films via both chemical (CVD) and physical (PVD) vapor deposition methods. Characterization techniques such as Raman spectroscopy, piezo-force microscopy (PFM), high-resolution transmission electron microscopy (TEM) and second harmonic generation (SHG) were carried out to identify the single phase and high crystallinity of the films. In particular, we notice that the SHG signal intensity of PVD-grown α-In₂Se₃ decrease gradually with the increase of layers, which is the opposite trend to CVD-grown α-In₂Se₃. This indicates a homostructure of α-In₂Se₃ films was formed in our CVD process compared to the vertical heterostructure of α/β-In₂Se₃ generated from our PVD process. We test the durability of our ferroelectric films. The α-In₂Se₃films maintained the polarization for more than 30 minutes under PFM measurement and the films remained active after 2 months exposure to the air.Finally, we transfer the sample onto Au/Si substrate to study the polarization switching behavior of CVD and PVD-α-In₂Se₃ under external electric field by conduct atomic force microscope (CAFM).