Influence Of ZrO₂ Seed Layer On The Phase Stability Of HfO₂-based Thin Film From First-principle

With the development of 5G,Artificial Intelligence,and the Internet of Things,the need for high-performance memory has been stimulated by the growth of information.Ferroelectric memory has been widely concerned and used because of its advantages such as high memory density,high read-write speed,low power consumption,radiation resistance and non-volatility.However,the disadvantages of traditional ferroelectric materials limit the development of memory.Therefore,it is necessary to develop a new type of ferroelectric material.Since the ferroelectric properties of doped hafnium oxide(HfO₂)thin films were discovered in 2011,this kind of material has attracted widespread attention rapidly.Most people think that the ferroelectricity of the HfO₂comes from the orthorhombic ferroelectric phase(Pca2₁),which is a metastable phase that cannot exist stably at room temperature and atmospheric pressure.However,there is no unified understanding of how to stabilize the orthorhombic ferroelectric phases and regulate the ferroelectricity of it.In recent years,the interface between the substrate and HfO₂film has been found playing a significant role on its ferroelectric properties.The seed layers,such as the zirconia(ZrO₂)seed layer,between the thin films and the substrates have been proven to be an effective way to realize the interface tuning.However,the microscopic mechanisms of interface tuning induced by the substrates or seed layers on ferroelectric properties of HfO₂-based thin films are still not clear.Therefore,the influence and tuning mechanisms of ZrO₂seed layers on the phase stability and the polarization properties of HfO₂films are systematically by first-principles calculation in this thesis.The following conclusions are obtained:(1)The phase stability of HfO₂ films with orientations of<001>and<111>is studied by first-principles calculation.We found that,for the<001>-orientations,the energy differences between the ferroelectric phase and monoclinic phase of HfO₂films with hafnium,oxygen,and hafnium/oxygen terminations are 0.50 e V,0.20 e V and 0.15-0.20 e V,respectively,It is shown that the surfaces with oxygen and hafnium/oxygen terminations are beneficial to the stability of the ferroelectric phase.The ferroelectric films with four different<111>-orientations have the same phase stability,the energy difference between the ferroelectric phase and the monoclinic phase is 0.63-0.67 e V,which shows the difficulty of stabilizing the ferroelectric phase of<111>-oriented HfO₂films under the only influence of size effect.Therefore,other regulation methods are needed to improve the phase stability of ferroelectric phase of HfO₂based thin films.(2)The effects of the ZrO₂ seed layers on the phase stability and the preferred orientation of the HfO₂thin films were studied.The results show that the[001]-oriented ZrO₂seed layer has negligible effect on the stability of the ferroelectric phase of HfO₂thin films.However,the[100]-oriented ZrO₂seed layer shows positive effects on the ferroelectric phase of HfO₂thin films.And the thinner of the HfO₂thin film,the more stable of the ferroelectric phase.More remarkably,the energy difference between the ferroelectric phase and monoclinic phase of HfO₂thin film is0.03-0.05 e V per Hf-O₂layer with the[111]-oriented ZrO₂seed layer,and the ferroelectric phase maintains well at different thicknesses.The polarization value and switching energy barrier of the most stable<111>-oriented HfO₂ferroelectric film were calculated by means of the Bonn effective charge and the NEB method,respectively.The results show that the ferroelectric polarization magnitude is in accordance with the experimental results,and the polarization switching barrier is effectively reduced,which means the possibility of reducing the coercive field of HfO₂ferroelectric thin films by introducing the seed layers.