Research On HfO₂-based Ferroelectric Memory

Ferroelectric memories,as a new type of non-volatile storage,offer advantages such as nanosecond-level read/write speeds,low power consumption,high erase/write endurance,and non-volatility.Existing ferroelectric memories include Ferroelectric Random Access Memory（Fe RAM）,Ferroelectric Field-Effect Transistor（FeFET）,and Ferroelectric Tunnel Junction（FTJ）,with FeFET and FTJ overcoming the destructive readout limitation of Fe RAM.HafniumOxide（HfO₂）-based FTJ and FeFET,due to their simple structure,CMOS compatibility,and mature manufacturing processes,hold great potential in applications such as embedded non-volatile memory（e NVM）and storage-class memory.However,they still face several challenges in the process of industrial application.The main issue faced by HfO₂-based FTJ is the excessively small Tunneling Electroresistance（TER）,which affects its scalability and peripheral circuit complexity.The reported maximum TER ratio is only200.Furthermore,although there have been numerous studies on improving the storage performance and reliability of HfO₂-based FeFETs,the influence of polarization switching on the effective channel mobility(μ_eff)determining their drive current level has not been explored.Based on these reasons,this paper extensively investigates the influence of various factors,including ferroelectric layer thickness,substrate type,Hf_0.5Zr_0.5O₂（HZO）structure,annealing conditions,and Al₂O₃ passivation layer,on the electrical performance of HZO-based FTJs.Additionally,by utilizing amorphous ZrO₂-like ferroelectric thin films,a voltage-modulated oxygen vacancy operation mechanism-based FeFET-like tunnel junction memory is developed,and its storage performance and reliability are studied.Furthermore,the modulation methods for controlling the performance of amorphous ferroelectric tunnel junction memories are investigated through TCAD simulations.Lastly,the influence of ferroelectric polarization on FeFETs is studied,and the reliability ofμ_eff is evaluated.The main research content and conclusions of this paper are divided into the following four parts:1.Novel HfO₂-based FTJ memoryDifferent structures of HZO FTJs were prepared using atomic layer deposition（ALD）technique,and the influences of ferroelectric film thickness,substrate type,HZO structure,annealing conditions,and Al₂O₃ passivation layer on the performance of HZO FTJ memory were systematically studied through polarization-voltage（P-V）testing,DC characteristics,pulse characteristics,retention performance,and fatigue resistance.Firstly,the storage performance of FTJs with HZO film thicknesses of 5 nm,10 nm,and 15 nm was investigated based on the Si/SiO₂/HZO/Au structure,and the TER ratio increased with the increase in HZO film thickness.Secondly,the influence of substrate on HZO FTJ performance was studied based on the Si/SiO₂/HZO（10nm）/Au and Ge/GeO_x/HZO（10 nm）/Au structures.The TER ratio of devices on Si substrate was higher,but their power consumption was much greater than that of devices on Ge substrate.Then,the effects of HZO structure（HfO₂:ZrO₂ratios of 1:1 and 5:5）and annealing conditions（550 ^oC 30 s and 550 ^oC 60 s）on the FTJ storage performance were compared and investigated based on the Ta N/HZO（10nm）/Ta N structure.For the 1:1 HfO₂:ZrO₂ FTJ,the TER ratio was higher with the annealing condition of 550 ^oC 30 s,while for the 5:5 HfO₂:ZrO₂ FTJ,the TER ratio was higher with the annealing condition of 550 ^oC 60 s.Under the same annealing conditions,the TER ratio of the 1:1HfO₂:ZrO₂ FTJ was higher.Finally,the influence of Al₂O₃ passivation layer on the FTJ storage performance was studied based on the Ge/GeO_x/HZO（10nm）/Ta N and Ge/GeO_x/Al₂O₃（2nm）/HZO（10nm）/Ta N structures,and the results showed that the FTJs with Al₂O₃ passivation layer had a higher TER ratio.All the fabricated devices exhibited excellent retention performance,except for the Ta N/HZO（10nm）/Ta N structure FTJs that showed a reduced storage window after 5000 erase pulse cycles,while other devices exhibited excellent fatigue resistance.2.Novel Amorphous ZrO₂ Ferroelectric Tunnel Junction MemoryAmorphous ZrO₂ ferroelectric tunnel junction（FTJ）memory was fabricated using atomic layer deposition（ALD）technique,and the characteristic of the amorphous structure of ZrO₂dielectric was confirmed by high-resolution transmission electron microscopy（HRTEM）.The amorphous ZrO₂ FTJ memory device was tested using a DC power supply,and a clear hysteresis phenomenon was observed,showing positive voltage for switching on and negative voltage for switching off.Pulse voltage measurements were performed on the amorphous ZrO₂ FTJ,and a tunneling electroresistance（TER）ratio of approximately 460was achieved under the conditions of 2.5 V/100 ns write pulse and-1.5 V/100 ns erase pulse,with a read voltage of 0.1 V.This TER ratio is the highest reported value among HfO₂-based FTJs and provides a new direction for HfO₂-based FTJs.The device maintained a TER ratio greater than 400 for up to 10⁴ seconds and remained above 400 even after 10⁴ erase pulse cycles,without observing any wake-up effect.Furthermore,the ferroelectric polarization characteristics of the amorphous ZrO₂ FTJ were demonstrated through P-V tests,where the±Pr values were±1.29μC/cm² and±0.60μC/cm² under±2.5 V and±1.5 V voltage scans at a frequency of 1 k Hz,respectively.The device also exhibited fatigue endurance with more than 10⁶ cycles under±2.5 V erase pulses.Finally,the working mechanism of the amorphous ferroelectric FTJ was verified through TCAD simulation,which showed that the voltage-driven oxygen vacancy modulation effectively controlled the tunneling probability by modulating the static potential distribution and tunneling path in ZrO₂.3.Simulation Study on Performance Modulation of Amorphous Ferroelectric Tunnel Junction MemoryBased on the working mechanism of voltage-modulated oxygen vacancies,a simulation study was conducted to investigate the influence of dipole density(N_dipole),ferroelectric layer thickness(T_FE),interface layer thickness(T_ox),substrate doping concentration（N_d）,and top electrode work function on the performance of tunnel junction memory.The following conclusions were drawn:a higher dipole density led to a larger on-current,smaller off-current,and higher TER ratio,which aligns with the requirements of cross-point memory arrays for high on-current and TER ratio.Increasing the thickness of the ferroelectric layer and reducing the thickness of the interface layer resulted in a higher TER ratio.Decreasing the substrate doping concentration led to a smaller on-current and higher TER ratio,requiring a trade-off between on-current and switching ratio in cross-point array applications.As the top electrode work function increased,the current decreased,the TER ratio initially increased and then decreased.For Si substrate-based ferroelectric tunnel junction memory,a Ti N top electrode was found to be the optimal choice for achieving a high TER ratio.4.Study on the Influence of Ferroelectric Polarization States on Effective Carrier Mobility in HZO FeFETStable HZO（Hafnium-ZirconiumOxide）Ferroelectric Field-Effect Transistor（FeFET）devices were fabricated,and their electrical properties were investigated.DC voltage measurements were performed to test the non-volatile storage characteristics of HZO FeFET.During the reverse sweep process,the HZO FeFET exhibited significant counterclockwise hysteresis,with a hysteresis window of 1.02 V observed within a DC scan voltage range of-2 V to 2 V.Pulse voltage measurements were carried out on the non-volatile HZO FeFET memory,and a storage window greater than 1 V was achieved under the influence of+4.5V/-7.8 V 1μs write/erase pulse voltages.The effective carrier mobility(μ_eff)decreased after the write and erase pulse operations,which was related to the capture/release of oxygen vacancies in the SiO₂ layer.Following a series of write pulses（V_G ranging from 3.5 V to5.25 V）,the threshold voltage of the HZO FeFET gradually shifted towards the negative direction,andμ_eff decreased gradually.After a series of erase pulses（V_G ranging from-6.5V to-8 V）,the threshold voltage moved gradually towards the positive direction,andμ_effincreased gradually but remained lower than the initial state’sμ_eff.Under the influence of+4.5 V/-7.8 V 1μs write/erase pulse operations,μ_eff remained stable within 10⁴ s and maintained stability throughout 10⁵ cycles.However,degradation began after 10⁶ cycles,indicating that the carrier mobility is one of the factors contributing to fatigue degradation.This study provides new insights for low-power devices and suggests strategies for improving the durability of HZO FeFETs.