Electric Field Cycling Behavior Of Si-doped HfO₂ Ferroelectric Thin Films

Hafnium oxide(HfO2)is a representative high-k material,which has been successfully applied to the large-scale industrial production of CMOS integrated circuit at technology nodes of 65 nm and below.In 2011,T.Boscke et al.from Qimonda company in Germany first reported the ferroelectric and antiferroelectric properties of Si-doped HfO2 thin films.This discovery gives HfO2 more abundant research connotation and new application value.Due to significant advantages such as lead-free,ultra-thin physical thickness,especially excellent Si-based CMOS integration process compatibility,the HfO2-based ferroelectric thin film was rapidly applied to the industrial development of ferroelectric non-volatile memory devices.Major breakthroughs have been made in integration density and memory retention performance.Polarization switching is the physical basis for the realization of "0,1" information reading and writing in ferroelectric memory devices.The stability of polarization switching during field cycling directly determines the service reliability of devices.To this end,the characterization and analysis of polarization switching characteristics have become the focus in the field of HfO2,in addition to the exploration of new doping components,optimization of preparation processes and device integration.A large number of studies have shown that the polarization switching of HfO2-based ferroelectric thin films under cyclic field loading presents complex evolution processes including wake-up,fatigue and subcycling induced split-up effects,etc.Exploring the micro-mechanisms behind them and proposing feasible improvement measures will effectively promote the commercial mass production of HfO2-based ferroelectric memories.At present,a variety of wake-up mechanisms have been proposed,but there is still no general consensus on its dominant mechanism.Meanwhile,the research on fatigue and subcycling behavior is still in its infancy and the mechanisms behind them are still poorly understood.Indepth study of these issues can not only deepen the understanding of HfO2-based ferroelectric materials at the basic physics level,but also provide the necessary scientific basis for the preparation of high-performance HfO2-based ferroelectric memories.In this thesis,the effects of electric field frequency and temperature on the polarization switching dynamics of 5.3 mol%Si:HfO2 ferroelectric thin film were first studied by characterizing the P-E,I-E,C-E curves and FORC switching density distribution,then the temperature dependence of wake-up effect and the effects of electric field frequency,amplitude and temperature on subcycling behavior were systematically studied.The main contents and conclusions are summarized as follows:(1)By testing the evolution of P-E,I-E and C-E curves in the frequency range of 10 Hz50 kHz and the temperature range of 100-350 K,and combining with the change of FORC density distributions,it is confirmed that the polarization switching of Si:HfO2 ferroelectric thin film exhibits significant frequency and temperature dependence.With increasing frequency,the coercive field exponentially increases from 0.75 MV/cm at 10 Hz to 1.34 MV/cm at 50 kHz.With increasing temperature,the coercive field linearly decreases from 1.27 MV/cm at 100 K to 0.93 MV/cm at 350 K.The fitting results of the switching dynamics indicate that the polarization switching of Si:HfO2 ferroelectric film conforms to the nucleation-limited switching model.The switching of domains is determined by the nucleation time and their statistical distribution.(2)The wake-up effect of Si:HfO2 ferroelectric thin films under different testing temperatures(110-350 K)was characterized in detail.Based on the evolution of remanent polarization and internal bias field with the number of cycles,the rate constant of wake-up effect was extracted and the activation energy was calculated to be 26.8±0.9 and 29.5±2.5 meV.The mechanism behind wake-up effect was analyzed by comparing the obtained activation energy with other existing values in HfO2 and other traditional ferroelectric materials,and combining the evolution of the switching density distribution,XPS and C-E curves measured at room temperature.The results suggest that wake-up effect of Si:HfO2 ferroelectric film is dominated by the homogenization movement of oxygen vacancies from interfaces into the film bulk.The work done by asymmetric local electric field during field cycling promotes the movement of oxygen vacancies and the calculated activation energy reflects the barrier height of oxygen vacancy migration,.(3)Based on the coercive field distribution of Si:HfO2 ferroelectric thin film and its frequency dependence,the cause of subcycling induced split-up of switching current peak and its relationship with fatigue were analyzed.Then,the evolution of the subcycling behavior under different electric field frequencies,amplitudes and test temperatures were characterized in detail.The results show that the decrease of the cyclic field amplitude,the increase of frequency and temperature will aggravate the subcycling behavior,resulting in more serious distortion of the P-E hysteresis loop and split-up of the switching current peak in the I-E curve.By comparing the switching density distribution of samples before and after subcycling with the switching behavior of domain units,it is found that the generation of local internal bias fields with different distribution and intensity was the root cause of subcycling behavior.Using the proposed internal bias field generation model based on oxygen vacancy movement and segregation process,the change of polarization state,internal electric field and defect distribution in the thin film during subcycling were analyzed in detail.The dependence of subcycling behavior on the frequency,amplitude and test temperature of cyclic field was explained.