Strain Control Of Ferroelectric Properties In Orthogonal HfO₂ Thin Films

Due to its excellent dielectric properties,HfO₂materials have been studied since the1970s.In particular,the discovery of ferroelectricity in Si-doped HfO₂thin films in 2011has drawn special attention.The ferroelectric memory prepared by the HfO₂film has the advantages of simple preparation,no pollution,and compatibility with the silicon process;the film still has polarization strength below 5 nm thickness,and has the advantages of ultra-thin film.These advantages make the ferroelectric properties of few-layer HfO₂thin films of particular importance.HfO₂ films have four phase structures.The monoclinic phase is the most stable at room temperature and pressure,but has no ferroelectricity.It was observed experimentally that the ferroelectricity of the HfO₂thin film originates from the metastable orthorhombic phase,and the preparation temperature,doping,stress and other factors can affect its stability.However,the relationship between ferroelectricity,phase structure and interfacial strain remains unknown due to the lack of data on epitaxial thin films composed of a single phase.What is particularly interesting is that with the delamination of the HfO₂film,not only the critical thickness disappears,but its spontaneous polarization increases anomalously,showing an extremely anomalous phenomenon of perovskite-type ferroelectric materials.When the thickness of the HfO₂ferroelectric film is smaller,the strain effect and strain gradient effect become particularly important,and the underlying mechanism is still unclear.Although the preparation of ultrathin HfO₂ferroelectric materials is a great challenge,exploring its stress-strain mechanism will create a possibility for the regulation of its ferroelectric properties.In this paper,a normal strain model,a normal strain gradient model and a shear strain model were constructed,and the first-principles calculations were used to explore the influence and regulation mechanism of normal strain,normal strain gradient and shear strain on the ferroelectric properties of HfO₂.Main work completed:（1）Enhancement effect of strain gradient on ferroelectric properties of HfO₂thin films under large compressive strain.Firstly,a normal strain model was constructed,and the influence of normal strain on the energy of the four phase structures of HfO₂was calculated.It was found that the compressive strain was beneficial to the stable existence of the HfO₂ferroelectric phase.The analysis shows that the normal strain has an obvious regulation effect on the orthorhombic ferroelectricity.It is found that the tensile strain reduces the spontaneous polarization,and the compressive strain increases the polarization strength.On this basis,a normal strain gradient model was constructed to study the effect of normal strain gradient on the ferroelectricity of HfO₂.It is found that when the strain is greater than-2.5%,the polarization strength of the HfO₂film decreases with the increase of the strain gradient.When the strain is less than-2.5%,the polarization strength of the HfO₂film increases with the increase of the strain gradient.The results explain the experimental phenomenon that the polarization strength of HfO₂films increases abnormally with decreasing thickness.（2）Interfacial shear strain effect of depolarization field in HfO₂thin films.A shear strain model was constructed to calculate the effect of shear strain on the ferroelectric properties of HfO₂thin films.We find a ferroelectric monoclinic phase with a similar structure and similar energy to the monoclinic phase,but with in-plane polarization.At the same time,it is found that when the unidirectional shear strain in the xz plane exceeds5%,the out-of-plane polarization can be converted into in-plane polarization,so that the depolarization field of the HfO₂ferroelectric film disappears.Secondly,the unidirectional shear strain in the yz plane and the effect of the biaxial shear strain on the ferroelectric properties of the HfO₂thin film were calculated.Therefore,a normal strain is applied on the basis of the unidirectional shear strain in the xz plane,and it is found that when the shear strain is 3%,a larger tensile strain can reduce the out-of-plane polarization and enhance the in-plane polarization;when the shear strain is 5%,a larger compressive strain can enhance the in-plane polarization.Our work systematically reveals the strain effect and strain gradient effect of HfO₂ferroelectric thin films.Our results can guide the preparation and device design of HfO₂ferroelectric thin films.During the experimental preparation of HfO₂ultrathin films,selecting appropriate external conditions such as substrates and electrodes is conducive to stabilizing the metastable ferroelectric orthorhombic phase in the HfO₂films,and applying appropriate strain and strain gradients through the substrates and electrodes can control the HfO₂ultrathin films.Ferroelectric properties of thin films.As a result,the HfO₂-based ferroelectric memory can be more stable in the process of miniaturization,with higher storage density and more times of reading and writing.