Preparation And Ferroelectric Properties Of Two-dimensional Ga₂Se₃ Single Crystals

Ferroelectric materials have spontaneous polarization in a certain temperature range,and the polarization direction can be reversibly reversed by the applied electric field.As the development of electronic devices with high-speed and miniaturized,ferroelectric materials are required to be thin light and high-density integrated.However,traditional ferroelectric materials suffer from the size effect,so the miniaturization of devices encounters the bottleneck.In recent years,two-dimensional ferroelectric materials have gradually attracted the attention of researchers.Two dimensional ferroelectric materials still have stable and strong spontaneous polarization at the atomic scale,which is conducive to the miniaturization of devices and can greatly improve the integration.Combined with their unique semiconductor characteristics,they can be applied to semiconductor field-effect transistors,nano optoelectronic devices,neural synapse simulation and other emerging fields.At present,a range of two-dimensional ferroelectric materials have been predicted by theoretical calculation,but few of them have been verified by experiments.Therefore,it is very meaningful to find new two-dimensional ferroelectric materials and expand the two-dimensional ferroelectric material system,especially the high temperature two-dimensional ferroelectric materials.Since 2017,theory predicted that monolayer In₂Se₃has both in-plane and out of plane ferroelectricity at room temperature,many important experimental works on ferroelectricity of In₂Se₃ have been reported.Ga₂Se₃,a III₂VI₃ compound,has also been theoretically predicted to have stable ferroelectricity at room temperature in 2017.In 2018,Liu et al.reported that the?-?phase transition occurs around 998 K,indicating Ga₂Se₃ has the potential as high temperature ferroelectric.However,the preparation of Ga₂Se₃ and its ferroelectric properties have not been reported.In this thesis,high-quality Ga₂Se₃ single crystal nanosheets,for the first time,were prepared by physical vapor deposition(PVD).The high temperature ferroelectricity of Ga₂Se₃was verified by piezoelectric force microscope(PFM).The specific work is as follows:(1)High-quality Ga₂Se₃ single crystal nanosheets were prepared by PVD method.By adjusting the growth parameters,Ga Se_x nanosheets with smooth surface,size of?10?m and thickness of?10 nm were successfully prepared.Through the characterization of the composition and structure,it is found that the prepared nanosheets have cubic phase of defect zincblende structure and high crystal quality,which is a good preparation for the further study of ferroelectric properties.(2)Ferroelectric properties of Ga₂Se₃ single crystal nanoflakes.The PFM characterizations show that the nanoflakes have intrinsic ferroelectric domains and these domains can be reversed by the voltage bias.The single point test of phase-voltage(P-E)shows that switching of the phase signals is close to180°,these results confirm that the Ga₂Se₃nanoflakes have ferroelectric properties.In addition,the P-E hysteresis test shows that the Ga₂Se₃nanosheets still have a switching of the phase signals of nearly 180°when the temperature is up to 200?,indicating that Ga₂Se₃ has high temperature ferroelectric properties.(3)Polarization-dependent electrical transport properties of Ga₂Se₃ nanosheets.The resistance switching voltage coincides with the coercive voltage,indicating the resistance switching can be induced by the polarization reversal.In addition,the I_D–V_DS characteristics of Ga₂Se₃ based semiconductor field-effect transistors are measured by Keithley 4200A-SCS analyzer.It is found that the switchable diode effect can be obtained after bias poling,which is mainly attributed to the modulation of interface barrier by polarization reversal.The above pioneering work,including the preparation of 2D Ga₂Se₃ single crystal nanosheets by PVD and the verification of the high temperature ferroelectric properties of 2D Ga₂Se₃,demonstrating that 2D Ga₂Se₃ has great application potential in polarization-related nanodevices.