Phase Transformation And Dielectric Properties Of Calcium Doped Hafnium Oxide Films Prepared By Chemical Solution Deposition Method

With the continuous development of microelectronic products toward miniaturization and intelligence,the demand for high integration,high performance and low power consumption of integrated circuits is becoming increasingly urgent.Since the discovery of ferroelectricity in silicon doped hafnium oxide?Si:HfO₂?thin films in 2011,HfO₂-based thin films attracted broad attention due to the fact that they are high-k,simple binary oxides,lead-free,and have excellent CMOS compatibility.Such competitive advantages made it a promising candidate to replace traditional perovskite type ferroelectric materials and it will have broad application in non-volatile ferroelectric memories such as FeFET and FeRAM.In this paper,the calcium doped hafnium oxide?Ca:HfO₂?thin films were prepared on a heavily doped p-type silicon substrate by spin-coating.The impact of annealing temperature,doping concentration and film thickness on phase transition and electrical properties were studied in detail.The thermogravimetric analysis?TGA?and differential scanning calorimetry?DSC?were carried out to examine the thermal behavior of precursor xerogel.X-ray photoelectron spectroscopy?XPS?was used to identify the content and chemical states of elements in Ca:HfO₂ films.The evolution of the crystal structure of the thin film was observed by grazing incidence X-ray diffraction?GIXRD?.The results of thickness and relative density of our samples were obtained by fitted results of X-ray reflectance?XRR?curve.An atomic force microscope?AFM?was utilized observe the surface morphology of Ca:HfO₂ thin film.The polarization-electric field?P-E?and current density-electric field?J-E?measurements were carried out using a Radiant ferroelectric tester.According to the test results,ferroelectric Ca:HfO₂ thin films were successfully prepared by an inorganic sol-gel method.The annealing temperature of the Ca:HfO₂ thin film is determined as 700?due to the fact that a relative lower annealing temperature is beneficial to the stabilization of the high symmetrical phase and the reduction of leakage current.The results show that the crystal structure and correlative properties of the Ca:HfO₂ thin film are mainly controlled by Ca concentration and film thickness.With the Ca content increases from0 mol%to 8.9 mol%,a transition from paraelectric over ferroelectric and then back to paraelectric behaviors is visible,which was resulted from a phase transition from monoclinic phase to higher symmetric phases.The maximum value of remanent polarization reached 10.5?C/cm² in a woken-up 4.8 mol%Ca:HfO₂ thin films.Moreover,GIXRD results show a overall tendency that the percentage of high symmetric phases in Ca:HfO₂ films is increased with decreasing film thickness.From a thermodynamic point of view,it could be explained by a fact that monoclinic to cubic/orthorhombic phase transition temperature decreases with a reduction in film thickness.In addition,with the Ca concentration increases,the critical transition thickness between the monoclinic phase and the cubic/orthorhombic phase increases subsequently.When the Ca concentration is 8.9 mol%,the film can still be completely stabilized into a cubic phase even the thickness reached 44.7 nm.And a relative permittivity of 44 was calculated through the corresponding linear P-E curve.