The Investigation Of New Ferroelectric Field Effect Transistors And Ferroelectric Materials

Ferroelectric field effect transistor（Fe FET）can break through the Boltzmann limit due to negative capacitance effect and achieves logic devices with ultra-steep subthreshold swing for low power consumption applications.Meanwhile,Fe FET can also realize nonvolatile storage function by utilizing the spontaneous polarization of ferroelectric materials.Hence,Fe FET is expected to be used in high hashrate and low power consumption chips with computing in memory architecture.In recent years,thanks to the discovery of hafnium based ferroelectric films,which is compatible with CMOS standard process,Fe FET is expected to rapidly enter the stage of large scale fabrication,so it has become one of the current topics of materials and devices in More Moore era.In this dissertation,the capacitance peak characteristics and hysteresis optimization characteristics of negative capacitance field effect transistors（NCFETs）were experimentally investigated.Amorphous ZrO₂ ferroelectric-like transistor was fabricated by the research of the interface of amorphous ZrO₂ ferroelectric-like films.The main research results are as follows.1.The investigation of gate capacitance peak effect of NCFETNCFETs with ferroelectric gate dielectric HfZrO_x（HZO）were fabricated on Ge substrates by front-gate process and systematically characterized.Annealing at 450℃,compared to traditional MOSFETs,NCFETs obtained 22%improvement of on-state current and the hysteresis-free transfer characteristics,the negative differential resistance（NDR）effect and the sudden drop of subthreshold swing characteristics.The transconductance as a function of gate voltage(V_GS)curves for NCFETs show obvious spikes with forward and reverse sweeping of V_GS.Meanwhile,the curves of gate capacitance with V_GS exhibit the unique peak phenomenon of NCFETs.The statistical results show that the larger the gate capacitance peak of NCFET is,the larger the on-state current and the maximum transconductance are.The results have a guiding significance to the performance optimization of NCFETs in the future.2.The investigation of hysteresis characteristics optimization of NCFETThe influence of the match between ferroelectric capacitance(C_FE)and dielectric capacitance(C_MOS)on the hysteresis characteristics of NCFETs was investigated by changing the thickness of HZO and dielectric films.With the decrease of HZO thickness,|C_FE|increases and the hysteresis of NCFETs disappears.Meanwhile,the enhancement effect of negative capacitance on device performance gradually weakens.However,the thin HZO film is beneficial to improve the process temperature of device.By changing passivation time to modulate the thickness of dielectric films,the on-state currents of NCFETs with 40min and 60 min passivation time achieve 35.6%and 18.7%development compared with traditional devices,respectively.Hence the capacitance matching is beneficial to the hysteresis optimization and on-current improvement of NCFETs.3.The investigation of the interface of amorphous ferroelectric-like filmsDoped HfO₂ based Fe FETs have broad application prospects in computing in memory and low power consumption logic field.However,HfO₂ ferroelectric materials require high crystallization temperature,which is not conducive to the integration of CMOS back-end process.Meanwhile,the variations of device-to-device become more obvious due to the multi-domain properties of HfO₂ ferroelectric materials with the scaling of devices.Amorphous ZrO₂ ferroelectric-like films have the advantage of stable ferroelectric-like characteristics,low crystallization temperature,no grain boundaries and CMOS back-end process compatibility.However,the origin of the ferroelectric-like properties of amorphous ZrO₂ films is not clear.Therefore,the mechanism of amorphous ZrO₂ ferroelectric-like properties is investigated by modulating the interface layer of metal-dielectric and semiconductor-dielectric.The results show that the interface layer between Ti N or Ta N metal and ZrO₂ can generate mobile ions（oxygen vacancy and oxygen negative charge）,contributing to ferroelectric-like properties.By investigating the influence of ZrO₂ thickness on ferroelectric-like characteristics,it is find that the polarization intensity of the capacitor decreases obviously with the increase of ZrO₂ thickness.There are few or no movable oxygen vacancies in dielectric layer and the thick ZrO₂ films hinder the migration of movable oxygen vacancy in the dielectric layer.The ferroelectric-like model of amorphous films is constructed based on the experimental results,so the mechanism of mobile ions induced ferroelectric-like properties is explained from a theoretical perspective.In the addition,by modulating the interface of semiconductor-dielectric,the results show that compared to Si and Si Ge substrates,the ferroelectric-like characteristics of MOS capacitance on Ge substrate is obvious due to small band gap of Ge.4.The investigation of amorphous ZrO₂ ferroelectric-like FETsZrO₂ ferroelectric-like FETs were designed and fabricated based amorphous ZrO₂ferroelectric-like films,which can be verified experimentally as ultra-steep subthreshold swing devices and neuromorphic synaptic devices.Due to surface potential amplification of mobile ions in dielectrics,amorphous ZrO₂ ferroelectric-like FETs obtain steep subthreshold swing of 25 m V/decade and 19 m V/decade with the forward and reverse sweeping of V_GS,respectively.Moreover,the symmetric spike-timing dependent plasticity（STDP）function of ZrO₂ ferroelectric-like FETs is demonstrated.Based on ZrO₂ ferroelectric-like FETs,a spike neural network（SNN）architecture is mimicked to execute online and offline training on the Mixed National Institute of Standards and Technology（MNIST）dataset.The accuracy rate of online training and offline training is 87%and 94%after 100 cycles,respectively.