Application Of New-generation Ferroelectrics Materials In Field Effect Transistor

Field-effect memories based on ferroelectric materials are one of the most promising non-volatile memory technologies.Typically,the finding of ferroelectricity in hafnia-based materials open the door to scalable ferroelectric field-effect transistor.However,the underlying ferroelectric mechanism is not clear as yet and the endurance still needs to be improved,limiting their potential applications.Herein,a reasonably good endurance performance was achieved by epitaxially integrating hafnia-based materials with ZnO.However,our analyses infer that the ferroelectricity of hafnia-based systems possibly have an extrinsic origin.On the other hand,building spin field-effect transistors based on ferroelectrics by inversely control the spin texture through polarization switching,is another possible way for non-volatile memory applications.Here we disclose the general route to realize persistent spin textures in ferroelectric materials towards pin field-effect transistors,by taking tetragonal BiFeO₃ which has a strong spin-orbit coupling as a model system.The main conclusions are listed below:Metastable rhombohedral Hf_0.5Zr_0.5O₂ epilayer was stabilized on c-plane ZnO with thickness up to 35 nm.Domain matching epitaxy facilitates the accommodation of epitaxial strain,and allows the formation of rhombohedral Hf_0.5Zr_0.5O₂.Hysteresis was clearly observed in the capacitance-voltage curve of the Au/Hf_0.5Zr_0.5O₂/ZnO capacitor,demonstrating the reversible control of channel charge through polarization switching.The endurance performance of the device is reasonably good,and a considerable memory window is still detectable after 10¹⁰ switching cycles.Our density functional calculations reveal that the charged oxygen vacancies play an essential role in stabilizing the rhombohedral phase.Moreover,we show that the incorporation of charged oxygen vacancies results in the shift of the capacitance-voltage curve.There has been no consensus on whether the hafnia-based materials have intrinsic ferroelectricity till now.Taking into account the role of charged oxygen vacancies,we proposed a new extrinsic origin of ferroelectricity in hafnia-based systems.Starting from the classical measurement method of ferroelectricity(the integral of the current with time),we demonstrate that the drift of charged oxygen vacancies under electric filed can generate a polarization-field loop.In most reports,oxygen-poor growth or treatment is of vital importance for the realization of ferroelectric behaviors in hafnia-based materials.Such observation can be perfectly explained by the extrinsic ferroelectricity which is originated from the drift of charged oxygen vacancies.Tetragonal BiFeO₃ was epitaxially integrated with a-plane ZnO and a metal-ferroelectric-semiconductor capacitor was demonstrated.We also explored the possible applications of tetragonal BiFeO₃ as a ferroelectric Rashba semiconductor.First principles calculations reveal a strong bulk Rashba effect in tetragonal BiFeO₃,and a considerable Rashba coefficient of 0.31 e V×?is derived.More importantly,our calculations reveal that a persistent spin texture is stabilized on the Z-?-X plane,as a result of the ferroelectric polarization generated unidirectional spin-orbit field.The realization of persistent spin texture has important implications for future spin field-effect transistors.