Recent advances in two-dimensional ferroelectric materials

With the continuous progress of microelectronics, electronic devices are developing towards miniaturization and integration. However, Moore's Law has been challenged when the device size gradually shrinks. The difficulties encountered in highly integrated devices mainly come from the following aspects. (1) Physical limit problem: When the device size is smaller than the electron wavelength of the transistor (usually less than 10 nanometers), the quantum effect and tunneling effect will occur. The performance and stability of the device are affected. (2) Thermal effect: The power density of the device will increase with the decrease of the device size, which can lead to overheating of the device. Therefore, searching for new materials has become one of the hotspots in the field of materials science. Ferroelectric materials are one kind of important functional materials, which have a novel intrinsic physical mechanism and rich characteristics (such as piezoelectricity, pyroelectricity, nonlinear optical effect, etc.), so that it has a wide range of application prospects in electronic devices and information storage technology. However, with the trend of highly integrated electronic devices, the surface effect and size effect cannot be ignored, which restrict the application of bulk ferroelectric materials in modern industry. Therefore, two-dimensional ferroelectric materials without finite size effects and with stable spontaneous polarization have attracted great attention. In this review, the development and research status of two-dimensional ferroelectric materials is discussed. Two-dimensional ferroelectric materials are classified. (1) Non-van der Waals ferroelectric materials: Including perovskite oxides, Bi2O2Se and HfO2 materials. By shielding the depolarization field effect or enhancing the driving force, they can be made to exhibit ferroelectric properties in the two-dimensional limit. (2) van der Waals ferroelectric materials: Including indium copper thiophosphate, indium selenide, tin telluride, molybdenum telluride and tungsten telluride. Their ferroelectric properties mainly come from charge transfer or electron hybridization, and they realize polarization reversal through ion shift or atomic distortion. This review also presents the application and latest research progress of two-dimensional ferroelectric material innovative devices, including (1) ferroelectric field effect transistors (FFEs): Ferroelectric materials usually have the property of self-polarization. When using ferroelectric materials instead of traditional oxides as the dielectric layer of FETs, the polarization induced electric field can adjust the channel conductivity, thereby achieving non-volatile ferroelectric storage. (2) Ferroelectric tunnel junctions (FTJs): The FTJ is a heterostructure consisting of a ferroelectric thin film as a barrier layer, with electrodes sandwiched on both sides. The atomically thin van der Waals ferroelectric materials are also suitable materials for tunneling barrier layers of FTJs. (3) Ferroelectric synapses: Ferroelectric materials have unique nonvolatile polarization characteristics, and the plasticity of polarization is very similar to that of biological synapses. Although significant research progress has been made, ferroelectric synapses are still in the stage of principle verification and further research is needed in the future. Finally, an outlook on the field of 2D ferroelectric materials is provided. In general, the exploration of 2D ferroelectric materials is still in its early stage. Further work includes establishing and improving theoretical models, developing efficient preparation methods, delving into the durability and stability of 2D ferroelectric devices, and studying and analyzing the interface effects between two-dimensional ferroelectric materials and other materials.