The Investigation Of Novel Amorphous Ferroelectric-like Dielectric Field-effect Transistor

As a new type of non-volatile memory,ferroelectric memory has the advantages of nanosecond operation speed,low power consumption,non-volatility and good anti-irradiation performance,etc.And it is very suitable for the field of embedded memory.In2011,German researchers proposed and discovered that the doped hafnium oxide（Hf O₂）films has ferroelectricity,which made ferroelectric memory gain a high degree of attention.However,after ten years of research,doped Hf O₂ films have not yet achieved industrial applications.The main problems it faces are high operating voltage and poor endurance performance.In addition,doped Hf O₂ ferroelectric films with polycrystalline structure cannot be further scaled in advanced nano-node.In order to solve these problems,researchers from Xidian University proposed for the first time that ferroelectric properties can be achieved based on amorphous gate dielectrics,and then non-volatile memory devices can be obtained,which provides a new choice for high-performance embedded memory and in-memory computing.In this paper,the ferroelectric-like behavior and memory properties of amorphous ferroelectric-like films were studied in detail.Atomic layer deposition（ALD）was used to fabricate amorphous aluminum oxide Al₂O₃（3 nm）and the Hf O₂（2 nm）gate dielectric field effect transistor.Compared with the doped Hf O₂ ferroelectric thin film,it achieves lower operating voltage and better endurance performance.The main research contents and conclusions of this paper are divided into the following three parts:1.Study of amorphous ferroelectric-like thin filmThe amorphous Al₂O₃ and Hf O₂ films were fabricated by ALD technology,and their polarization-voltage（P-V）characteristics were systematically tested and characterized.The effects of thickness,annealing temperature,electrode/substrate material and oxygen content on ferroelectric-like properties of amorphous films were investigated.The results show that the thickness and annealing temperature have no effect on the ferroelectric-like properties of amorphous films.The amorphous films with Ta N electrode and Ge substrate has the strongest ferroelectric-like properties,and the ferroelectric-like properties of amorphous film increase with the increase of ozone processing time.Compared with the amorphous Al₂O₃film,the polarization value of the amorphous Hf O₂ film is larger.Amorphous Al₂O₃ and Hf O₂ films both achieve 10⁷ cycles endurance with no P_r decay.There was no wake-up effect during fatigue process.Based on the experimental results,a switching model of oxygen vacancy voltage modulation and negative charge dipoles is proposed to explain its physical mechanism.2.Study of amorphous ferroelectric gate dielectric field effect transistorThe amorphous Al₂O₃ gate dielectric field effect transistor and the amorphous Hf O₂ gate dielectric field effect transistor were fabricated by ALD technology.The ferroelectric-like properties of amorphous Al₂O₃ gate dielectric and amorphous Hf O₂ gate dielectric field effect transistors were demonstrated by TEM,P-V and capacitance-voltage（C-V）tests.Clockwise DC hysteresis curves were obtained for both transistors.3 nm thickness amorphous Al₂O₃ gate dielectric field effect transistors obtained a memory window of 0.7 V with 100 ns program/erase pulses at±5.5 V/100 ns.The amorphous Hf O₂ gate dielectric field effect transistor with a thickness of 3 nm can obtain a memory window of 0.64 V when a program/erase voltage of±3 V is applied to the gate.Both transistors obtained endurance characteristics for 10⁶ cycles and no wake-up occurred during cycling.In order to reduce the device size,an amorphous Al₂O₃ gate dielectric Fin FET was fabricated,and a clockwise DC hysteresis curve was obtained.Its DC hysteresis curve remains stable for 500 cycles.The stability of its DC memory window is demonstrated.A memory window of 0.64 V was obtained when a program/erase voltage of±3 V was applied to the gate.And the retention characteristics of the amorphous Al₂O₃ gate dielectric Fin FET were tested.After 2×10 s under a program/erase pulse of±6 V/1 ms,the memory window remained 24%of the initial memory window.The application prospect of amorphous ferroelectric-like gate dielectric field effect transistor in memory field is proved.3.Study of synaptic characteristics of amorphous ferroelectric gate dielectric field effect transistorThe synaptic plasticity of amorphous Al₂O₃ gate dielectric field effect transistors with thicknesses of 3 nm and 6 nm was tested.Including short-term plasticity（STP）,long-term plasticity（LTP）and spike-time dependent plasticity（STDP）.After a single pulse,the post-synaptic current decays to its initial state within 1μs.When the number of pulses increased,the post-synaptic current could be modulated to a higher state,and the post-synaptic current did not decrease to the initial state after the pulse was removed.The simulation of STDP is also realized for the amorphous Al₂O₃ gate dielectric field effect transistor.By changing the pulse time interval,the effect of different pulse time intervals on synaptic plasticity was investigated.The results showed that as the interval decreased,the post-synaptic current gradually increased with the increment of the number of pulses,and the short-term plasticity changed to long-term plasticity.A three-layer fully connected SNN based on the LIF model is used to recognize handwritten digits.A decent learning accuracy（>80%）can be achieved.It is demonstrated that the amorphous ferroelectric-like gate dielectric field effect transistor is an ideal choice for neural synapse applications.