Study On Ferroelectric Properties Of Zirconium-doped Hafnium Oxide Film Based On Oxygen Vacancy Engineering

In recent decades,with the development of cloud technology and internet,the emerging application such as 5G,artificial intelligence and intelligent vehicles have put forward higher requirements for data storage in terms of read-write speed,power consumption,storage capacity,device reliability and other aspects.For traditional memory,although dynamic random-access memory（DRAM）can reach read-write speed,it has high power consumption,low capacity,high cost,and is volatile storage.Flash memory（NOR Flash and NAND Flash）has low read-write speed,and the storage density is limited by the process.There is a pressing need in the market for memory products that can meet the requirements of new applications,which makes the research of emerging memory become an urgent demand.In the emerging memory,ferroelectric memory is considered as one of the future development directions of memory because of its non-volatile nature,and the characteristics of fast reading and writing speed,long life and low power consumption.Since 2007,hafnium oxide has been used as a high dielectric constant material in the mass production of complementary metal oxide semiconductor（CMOS）integrated circuits,mainly for gate oxides of high-performance field-effect transistors.Researchers discovered the existence of an orthorhombic non-central symmetric ferroelectric phase in hafnium oxide in 2011,prompting the world’s research on the ferroelectric memory based on hafnium oxide.In order to realize high performance ferroelectric capacitor and ferroelectric field effect transistors based on ferroelectric zirconium-doped hafnium oxide film(Hf_0.5Zr_0.5O₂),the thesis studies the following aspects:the fabrication and characterization of high quality hafnium oxide based ferroelectric film;the study of the performance of hafnium oxide based ferroelectric capacitor based on oxygen vacancy engineering;the modulation of substrate effect on the hafnium oxide based ferroelectric capacitors;the realization of high performance ferroelectric field effect transistors.The specific research contents are as follows:At first,high-quality ferroelectric Hf_0.5Zr0.₅O₂ films were obtained by stepwise process optimization.In this study,in order to obtain atomic flat oxide surface,we grown bottom electrode metal Ti N and Hf_0.5Zr0.₅O₂ films by atomic layer deposition technique.The Hf_0.5Zr0.₅O₂ ferroelectric capacitance was verified by a variety of material characterization and electrical characterization techniques.In the first step,we verified the growth rate,chemical composition,and surface roughness of the ferroelectric films.The hafnium zirconium atomic ratio of deposited Hf_0.5Zr0.₅O₂ film is 1:1,the surface roughness is about0.1 nm,and has good uniformity.In the second step,the crystal structure analysis of the crystallized Hf_0.5Zr0.₅O₂ film was performed by Glancing Angle X-ray diffraction.It showed that the 10 nm-thick Hf_0.5Zr0.₅O₂ film with 600 ^oC annealing had the highest o/t-phase portion.In the third step,the ferroelectricity and reliability of the 10 nm-thick Hf_0.5Zr0.₅O₂ film were characterized.The remnant polarization（2P_r）is about 25μC/cm²,and the device endurance can reach 10⁷ cycles.Next,we studied the effect of oxygen vacancies on the phase transition of ferroelectric Hf_0.5Zr0.₅O₂ films from first-principles calculations.Based on the thermodynamic calculation of Gibbs free energy,the gate stacking structure with oxygen scavenging technique was designed by using the metals of La and Y.These metals have the ability to get oxygen from Hf O₂ during in high temperature environment.The oxygen vacancy engineering was proposed to enhance the ferroelectricity and device reliability in the Hf_0.5Zr0.₅O₂ film.The oxygen vacancy engineering consists of a two-step process:the oxygen scavenging process and the oxygen vacancy passivation process.The results show that the oxygen scavenging process with Y as the oxygen scavenging metal can effectively generate a maximum 2.85%oxygen vacancy concentration at the Hf_0.5Zr_0.5O₂ interface,which can stabilize the ferroelectric phase and improve the ferroelectricity of the Hf_0.5Zr0.₅O₂ film.With the process temperature below 400 ^oC,the achieved ferroelectric capacitor shows 2P_r=43μC/cm²,endurance can reach up to 2×10⁹ cycles,and the data retention can reach the standard of more than 10 years at 85 ^oC.Then,we investigated the modulation of the substrate on the Hf_0.5Zr0.₅O₂ phase transition and ferroelectric properties.The results show that the influence of the substrate on the crystalline phase transition and the ferroelectricity of Hf_0.5Zr0.₅O₂ film is as follows:the thermal expansion coefficient of the substrate material,the presence of interface layer between Hf_0.5Zr0.₅O₂ and substrate,and the crystallinity of the substrate.Compared to the Ti N and Si substrates,the thermal expansion coefficient of Ge substrate is small(5.9×10^-6/^oC),and there is no interfacial layer at the interface between the Hf_0.5Zr_0.5O₂ and substrate.By combining the advantages of oxygen vacancy engineering and Ge substrate,the 10 nm-thick Hf_0.5Zr_0.5O₂ film shows an o/t-phase portion of 97%and excellent ferroelectricity（2P_r=73μC/cm²）.By the thickness scaling of the Hf_0.5Zr_0.5O₂ film on the Ge substrate from 10nm to 5.5 nm,the ultra-thin Hf_0.5Zr_0.5O₂ ferroelectric capacitor of shows 2P_r=40μC/cm²and high endurance of 2×10⁹ cycles under the operating voltage of 2 V.Finally,based on theory of the ferroelectric field effect transistor,we calculated that the maximum memory window of the 10 nm-thick HZO FeFET under the ideal conditions is about 2.6 V.Combined with oxygen vacancy engineering,we successfully experimentally achieved a 10 nm-thick HZO FeFET with a memory window of 1.6 V,endurance of 10⁵cycles,and data retention of 10 years of 85 ^oC.The results show that oxygen vacancy engineering can not only enhance the ferroelectricity of HZO ferroelectric film with the metal-ferroelectric material-dielectric layer-semiconductor（MFIS）structure,but also have the modulation effect of interface layer thickness.It can suppress or scale the thickness of the interface layer during the anneal process.This method is beneficial to reducing the depolarization field and improving the endurance of the device.