The Phase Transformation And Dielectric Properties Of Y Doped HfO₂ Films Prepared By Sol-gel Method

With the development of microelectronic integrated circuit products toward high performance and high density,HfO₂-based thin films have been successfully used as the gate dielectric in advanced microprocessors and DRAM devices owing to its high permittivity,wide band gap,good thermal and chemical stability,standard semiconductor process compatibility and other features.In recent years,the novel ferroelectric?FE?properties have been observed in HfO₂-based thin film.It is expected to break the bottleneck in ferroelectric memories by replacing the traditional perovskite type ferroelectrics with HfO₂-based thin films.It will have broad application prospects in the field of integrated ferroelectric devices such as non-volatile ferroelectric memories.In this paper,the yttrium doped hafnium oxide?Y:HfO₂?thin films were prepared by using a unique sol-gel method.The thermal behavior of the precursor solution was investigated by thermogravimetric analysis?TGA?and differential scanning calorimetry?DSC?.The crystal structure and the thickness of the Y:HfO₂ thin films were determined by grazing incidence X-ray diffraction?GIXRD?and X-ray reflectivity?XRR?measurements,respectively.The yttrium content and chemical bonding features of the films were determined using X-ray photoelectron spectroscopy.The Y:HfO₂ film was integrated into metal-insulator-semiconductor?MIS?structure capacitors for electrical measurements.The polarization-electric field?P-E?and current density-electric field?J-E?curves were measured using a Radiant ferroelectric tester.The results show that the stabilization of monoclinic phase was inhibited with higher Y doping concentration,thinner film thickness and lower annealing temperature.For 1 mol%,4mol%and 6.5 mol%yttrium doped HfO₂ thin films annealed at 850^?C,the critical thickness were 5 nm,15 nm and²0 nm,respectively.For 1.5 mol%yttrium doped HfO₂ thin films annealed at 700^?C,the critical thickness is about 16 nm.For 2 mol%yttrium doped HfO₂ thin films annealed at 700°C,the critical thickness can be extended to a value slightly lower than25 nm.For 16 nm Y:HfO₂ thin films?1.5 mol%?,the linear P-E curves can be observed,indicating that the thin films show dielectric property and its relative permittivity is 27.For 25nm Y:HfO₂ thin films?2 mol%?,a typical ferroelectric hysteresis can be observed.A transition of the polarization behavior from apparent ferroelectric-type to linear dielectric-type was observed for films with thickness increasing from 25 nm to 80 nm,which is correlated to the dominant crystal structure change from high-symmetry phase to monoclinic phase evidenced by grazing incidence X-ray diffraction analysis.With thickness increasing from 25 nm to 80nm,the m-phase fraction increases from 40.6%to 71.3%,accompanied by decrease of the remanent polarization from 14.2?C/cm² to 1.2?C/cm² and decrease of the relative permittivity??_r?from 52 to 29.When the film surface being cleaned using Ar ion sputtering for 60 s,the atomic ratio of Hf with respect to O?Hf/O?is slightly higher than 1:2 of stoichiometric Hf O₂,suggesting oxygen deficiency in the sample.The percentage of oxygen vacancies existed in Y:HfO₂ films is 3.9%.The generation of oxygen vacancies can be explained by the substitution of Hf by trivalent Y in the HfO₂ lattice and annealing under a nitrogen atmosphere.The leakage current density of Y:HfO₂ thin films is 10^-6,10^-7 A?cm^-2 order of magnitude at an applied electric field of 1 MV/cm,indicating that the Y:HfO₂ thin films are smooth and dense which meets the requirements for the use of electronic components.