The Phase Transition And Ferroelectric Properties Of Mg/Sr Doped HfO₂ Thin Films Prepared By The Sol-gel Method

Ferroelectric memory is becoming a powerful competitor for the next generation non-volatile memories on account of its appealing features of high speed,low power consumption,and high reliability.However,the development of ferroelectric memory is limited by the use of traditional ferroelectric materials like PZT,which led to poor compatibility with the standard CMOS process and the difficulty of size miniaturization.In recent decades,it has been found that hafnium dioxide（HfO₂）film is a new type of ferroelectric material with simple structure,controllable composition,stable chemical properties and excellent compatibility with Si-based CMOS integrated technology,and can also obtain high remnant polarization and large coercive field by element doping or controlling the preparation process.The excellent performance of HfO₂-based ferroelectric film makes it suitable for the new generation of high density and low power ferroelectric memory,which has a broad application prospect in information storage and other fields.In this paper,the HfO₂ sol precursor is prepared by the pure water-based sol-gel method.Mg/Sr doped hafnium dioxide（Mg/Sr:HfO₂）films are deposited on p-Si（100）substrate by spin-coating.A W-Mg/Sr:HfO₂-Si（MIS）structure capacitor was formed by sputtering tungsten（W）electrode on the film,and the effects of Mg/Sr doping concentration and film thickness on the structure and properties of the film were investigated in this paper.The film composition was analyzed by X-ray photoelectron spectroscopy（XPS）,the surface roughness was analyzed by atomic force microscopy（AFM）,the phase structure of the film was analyzed by grazing incidence X-ray diffraction（GIXRD）,the thickness,density and roughness of the films were fitted by X-ray reflectance（XRR）data,and the electrical performance was tested by the radiant ferroelectric tester.In this experiment,Mg:HfO₂ and Sr:HfO₂ films with obvious ferroelectricity were successfully prepared,and tungsten electrodes were successfully deposited by DC magnetron sputtering.With the increase of Mg/Sr concentration,the stable grains under room temperature in the film undergo a phase transition from monoclinic,through orthorhombic,and then to cubic.The formation of the monoclinic phase in the film can be completely inhibited by appropriate doping concentration.Ferroelectricity can be obtained when the Mg concentration is in the range of 0.4～3.7 mol%,and when the Mg concentration is 2.3 mol%,excellent ferroelectricity can be obtained.Ferroelectricity can be obtained when the Sr concentration is in the range of 2.0～3.6 mol%,and excellent ferroelectricity can be obtained when the Sr concentration is 3.6 mol%.After 10⁷ cycles of bipolar cyclic electric field loading,the films doped with both elements can maintain certain properties,showing excellent fatigue resistance.When the doping concentration is constant,the crystal structure is related to the film thickness.For the low doping concentration,the small film thickness is favorable to the stability of the orthorhombic/cubic phase at room temperature and inhibits the formation of monoclinic phase.At high doping concentration,the larger film thickness also stabilizes the orthorhombic/cubic grains in the film at room temperature.The results show that ferroelectricity can be obtained for 2.3 mol%Mg:HfO₂ films with film thickness ranging from 13.5 nm to 57.4 nm and 3.6 mol%Sr:HfO₂ films with film thickness ranging from 13.0nm to 58.0 nm,and the relationship between coercive field and film thickness of the films follows the Janovec-Kay-Dunn（JKD）law.