Atomic Layer Deposited 2D SnS Thin Film and Its Ferroelectric Properties

Various properties that two-dimension (2D) thin film materials possess but have not been turned out are continually being revealed through recent research results. Based on the novel properties that have recently been revealed, further research is being conducted worldwide to apply 2D materials to various fields such as sensors, catalysts, batteries, and solar cells. Among these, transition metal di-chalcogenide (TMDC) materials such as MoS₂ and WS₂ are currently one of the most actively researched materials due to their excellent electrical and physical properties. However, transition metals in TMDC are generally embedded in rare amounts on earth, which cause budget issues during process development. Also transition metals are toxic. So achieve environmentally friendly research is impossible. In addition, they have a higher melting temperature than other metals. Therefore, it is difficult to deposit materials onto flexible substrates used to fabricate flexible, wearable devices. Research for new materials that can overcome this disadvantages is necessary. Therefore, we conducted research about tin sulfide materials that could replace TMDC. Tin sulfide is a material that exhibits a 2D structure. Also tin is abundant on earth, non-toxic, and has a low melting point, making it possible to process it at low temperatures. Among tin sulfides, tin mono-sulfide (SnS) has a structure that exhibits low symmetry, so that ferroelectric property based on the polarization easily appears inside the thin film when an external force or electric field is applied. This makes it possible to fabricate a highly sensitive sensor using SnS. Furthermore, since SnS exhibits ferroelectric property predominantly in the in-plane direction, it is hardly affected by the upper and lower electrodes that may occur when the device thickness is reduced. A variety of processes are used to deposit SnS thin film, including atomic layer deposition (ALD), chemical vapor deposition (CVD), physical vapor deposition (PVD), and mechanical exfoliation. To confirm whether the ferroelectric property changes with the SnS thin film characteristics, we deposited and evaluated SnS thin film using ALD, a process based on a self-limiting reaction mechanism.<br/>In this study, ALD process was proceeded at 150°C using Sn(acac)₂ precursor and H₂S reactant to deposit SnS thin film. Annealing was performed to improve the crystallinity of the SnS thin film after deposition. Annealing was performed in an Ar atmosphere at a temperature of 350°C. X-ray reflectance (XRR) analysis was performed to confirm the thickness and growth rate of the SnS. Growth rate of Sn(acac)₂ SnS thin film was 0.34Å/cycle. X-ray diffraction (XRD) and Raman were utilized to compare the crystallinity and phase of SnS thin film. After annealing, SnS thin film showed higher crystallinity compare with as-deposited SnS thin film. Also single orthorhombic phase was achieved at post annealed SnS thin film. Transmission electron microscope (TEM) was used to confirm the 2D layered structure. Clearly 2D layered structure was showed by annealed SnS thin film. In addition, x-ray photoelectron spectroscopy (XPS) analysis was performed to check the bonding state of SnS thin film. The Sn 3d and S 2p spectra binding energy was similar with other SnS thin film research papers. Finally, positive up negative down (PUND) and piezoresponse force microscopy (PFM) were performed to evaluate the ferroelectric properties. Results of PUND and PFM analysis confirmed that SnS thin film deposited using ALD exhibited ferroelectric property.