Crystal Structure And Orientation Regulation Of Hafnium Oxide Based Ferroelectric Thin Films

The arrival of Big Data era,along with information explosion and the rapid growth of IOT devices at a compound annual growth rate of 20.9%,has aroused people’s concern on the importance of information memory.Nowadays,the industry and academia has achieved a common goal to seek the high performance memory with low cost,small size and large capacity.Ferroelectric Random Access Memory（FE-RAM）,Phase-change Random Access Memory（PC-RAM）,Magnetic Random Access Memory（MRAM）and Resistance Random Access Memory（RRAM）are four new types of Non-volatile Memory（NVM）.Due to advantages of low power consumption and high reliability,FE-RAM took the lead in industrialization in the 1980s among four new types of Non-volatile Memory（NVM）.However,the traditional perovskite FE-RAM has been stuck at in scaling the size below the 130 nm node for a long time due to the reason that the device performance is greatly reduced with size decreasing and it is difficult to be compatible with the new CMOS technology.The discovery of the ferroelectric properties in doped hafnium oxide guides the development of FE-RAM.The hafnium oxide based ferroelectric thin films have excellent polarization characteristics and high compatibility with CMOS technology,which has a broad application prospect in the field of next generation high-integration and low-power devices.At present,researchers have improved the performance of hafnium oxide and studied the origin of polarization based ferroelectric thin films by doping,changing film thickness and designing capacitor structure.However,the hafnium oxide based ferroelectric thin films prepared by atomic layer deposition process have the polycrystalline structure,which leads to the directional difference in crystal phase and orientation.This problem results in the coexistence of ferroelectric phase and non-ferroelectric phase,which affects the optimization and improvement of hafnium based ferroelectric devices.This paper focuses on the Hf_0.5Zr_0.5O₂（HZO）thin films.In order to obtain a consistent ferroelectric phase structure and a large increase in remanent polarization,it regulates and optimizes the structure and performance of hafnium-based capacitors by combining interface structure design,microstructure characterization and ferroelectric performance testing.By designing a series of ferroelectric capacitor structures with different substrates,different thicknesses and different upper electrodes,the plane,cross-section and interface structures were characterized by spherical aberration（Cs）-corrected transmission electron microscopy（TEM）,and the factors affecting the change of ferroelectric properties were compared and analyzed,revealing the influence of interface and electrode on the structure and orientation of ferroelectric thin films.This article proposed a method to control the structure and properties of hafnium based ferroelectric thin films in ALD deposition process,which provides a new idea for the design of a new generation ferroelectric memory devices.The specific research content is as follows.（1）The effect of NSTO single crystal bottom electrode on the structure and orientation of HZO films.The HZO films with different thickness on NSTO electrode was prepared by ALD deposition process,and Post Metallization Anneal（PMA）and Post Deposition Anneal（PDA）processes were used respectively.The structure and orientation of hafnium-based thin films were regulated by the lattice matching relationship between NSTO and HZO.A textured HZO ferroelectric thin film with preferred orientation（?1??1?1）_HZO//（010）_NSTO,（11?0）_HZO//（100）_NSTO epitaxial relationship was obtained.The regulating effect is closely related to the thickness.When the film thickness is 10 nm,the tensile stress of HZO film relaxes at the upper interface and part of M phase appears at the upper interface.When the film thickness is below 5 nm,HZO film is completely regulated by NSTO.The thin film is affected by surface energy and shows coexistence of O and T phases.（2）The regulation of HZO thin films by LSMO/NSTO single crystal bottom electrode.Based on the experience of Pulsed laser deposition（PLD）and the difference of thermal expansion coefficient of different electrodes,HZO film with appropriate thickness was prepared on LSMO/NSTO bottom electrode by ALD,and the structure and orientation of hafnium based thin film were regulated by chemical reconstruction of interface between LSMO and HZO.We obtained two preferential orientations（100）_HZO//（100）_LSMO,（01?1）_HZO//（010）_LSMO;（110）_HZO//（100）_NSTO,（11?0）_HZO//（010）_NSTOepitaxial texture films.This lays a foundation for further improvement of ferroelectric remanent polarization.（3）The regulation of HZO/LSMO/NSTO by different metal top electrodes.A series of HZO films with different top electrodes and the same LSMO/NSTO bottom electrodes were prepared by ALD deposition.The top electrodes were W,Ir and Ti N respectively.Among them,the capacitor covered by Ir electrode shows the largest remanent polarization value at 60μC/cm².Combined with SADP and COMSOL simulation stress analysis,the stress state of the sample covered by Ir electrode is the best.It will be a new way to regulate the struct of hafnium oxide thin films and improving the ferroelectric properties by selecting suitable top and bottom electrodes.（4）The structure evolution of HZO thin film under the in-situ biasing:The hafnium based ferroelectric capacitor and the in-situ biasing experiment was realized in Cs-TEM.The different crystal phases in HZO film have complex structural transformation process:the transformation from T phase to ferroelectric O phase,corresponding to the macroscopic wake up effect;The transformation from T to M phase is directly related to the macroscopic fatigue behaviour.The paraelectric M phase does not transform under electric field.The phase transition process is affected by stress and depends on the structural orientation of initial state.The structural transition paths and corresponding relationships of M phase,O phase and T phase are proposed.