Preparation And Research On Dielectrical Or Ferroelectrical Properties Of Bismuth-Based Thin Films

In the past 50 years,the microelectronic technology based on silicon dioxide IC has made astounding advances.With the integration degree increasing,the characteristic size of MOS(the most basic cell of IC) has been becoming smaller and smaller,developing from micrometer to nanometer.As a result,many serious problems came into being due to short channel effect and quantum effect.These problems have badly handicapped the further development of microelectronic technology.People are eager to find a High-k material to substitute conventional SiO₂. Except for high dielectric constant,a High-k material should have the following merits:â‘ good phase stability and reliability;â‘¡high potential block;â‘¢good quality of interface and low state density of interface;â‘£low current density of film gate;⑤big shift of conduction band(comparing with Si).But at present,no materials expected to substitute SiO₂ is perfect to accord totally with all of these standards.As one of the potential high-k materials,Bi₂Ti₂O₇ has high dielectric constant and good insulating property.However,because of poor phase stability,the feasibility of application for Bi₂Ti₂O₇ in MOS is not satisfactory.We searched many correlative papers and deemed that doping rare earth element into Bi₂Ti₂O₇ might improve the phase stability of Bi₂Ti₂O₇.Thereupon,we intend to dope Ce³⁺ into Bi₂Ti₂O₇ thin film, so as to achieve better phase stability and other better properties than pure Bi₂Ti₂O₇.The preparation of ferroelectric thin films began in 50s last century.Ferroelectric thin films applied mainly in ferroelectric memories have been attracting much interest due to their excellent ferroelectricity,piezoelectricity,electro-optic effect and nonlinear optic properties.Main ferroelectric memories include FeRAM,FeFET and DRAM.Comparing with ferroelectric blocks,ferroelectric thin films have many merits,such as small size,light weight,facility for integration and low working voltage.Moreover,it is compatible with IC technology and could integrate with Si electrocircuits.The ferroelectric thin films prepared should be up to the following standards:â‘ integration with metallic or conductive oxide electrode layers;â‘¡compatibility with integrated device processing;â‘¢production of device-compatible, high oriented or polycrystalline films and heterostructure with specific properties;â‘£ ability to produce patterned structures,such as superlattices or layered heterostructures;⑤reproducibility of the deposition process;â‘¥simple and low cost deposition with capacity for high deposition rates.Taking the above mentions into account,we prepared the precursor solutions by a chemical solution decomposition(CSD) method and films by spin-coating. Compared with other techniques,CSD method has the advantages of simplifying process,better homogeneity,stoichimetric composition control and low cost.In the research of BCTO thin films,we reported the preparation and synthesis of Bi₂Ti₂O₇ thin films with Cerium-substitutions on Si[100]substrates.We studied the phase stability and electrical properties of BCTO thin films.XRD patterns indicated that Ce³⁺ took a key role in the phase stability of Bi₂Ti₂O₇ as it prevented the phase transition from Bi₂Ti₂O₇ of pyrochlore structure to Bi₄Ti₃O₁₂ of perovskite phase.The temperature of phase transition of (Bi_0.8Ce_0.2)₂Ti₂O₇ thin films with the most Ce³⁺ is highest(1000℃),and is higher than (Bi_0.8La_0.2)₂Ti₂O₇ thin films(about 850℃) with the same quantity of dopant.It indicated that the effect Ce³⁺ took was stronger than La³⁺.This phenomenon could be explained by the theory of charge compensating:when the heat treatment was carried out in deoxidization atmosphere or ordinary atmosphere in the preparation of thin films,the oxygen element escaped in the form of O₂ molecule,so a quantity of oxygen vacancies came into being.Because of the volatility of Bi₂O₃ in the face layer of the thin film,the area of interface between solid and atmosphere increased,which led to the enhancement of the interfacial energy and the aggravation of instability of O^2- in the system.As a result,a lot of oxygen vacancies emerged in the whole system,which destroyed the charge neutrality and made the phase instable.We deemed that the theory above should explain why the phase transition occured.Considering the fact that the cerium ionic radius of 1.03(?) is quite close to that of Bi³⁺(1.02(?)),we deposited BCTO films,in which some of Bi ions were substituted with Ce ions in order to eliminate the oxygen vacancies to keep charge neutrality.By studying the leakage current and dielectric constants of(Bi_1-xCe_x)₂Ti₂O₇ thin films,the optimal value of x was finally determined to be between 0.12 and 0.16,so particular researches on phases,morphologies,densities of leakage current,C-V curves,dielectric constants,dielectric losses of(Bi_0.88Ce_0.12)₂Ti₂O₇ and (Bi_0.84Ce_0.16)₂Ti₂O₇ thin films were carried out.The cubic structure lattice constant of (Bi_0.88Ce_0.12)₂Ti₂O₇ and(Bi_0.84Ce_0.16)₂Ti₂O₇ thin films were worked out to be 20.62(?) and 20.53(?) respectively,using the(444) peaks in the XRD patterns.Both of the films had smooth and compact surfaces,and most of the crystal grains had the same shape and homogeneous size.We calculated the dielectric constants,fix charge density(N_fc) and the widths of memory windows of BCTO(0.12) thin film.The BCTO(0.12) film had not only high dielectric constants,but also low loss factors between 2～100kHz. Under following conditions as 700℃annealing temperature,100kHz frequency and -3V applied bias voltage,the value of dielectric constant and loss factor of BCTO(0.12) thin film are 214 and 0.059,respectively.XPS studies were carried out for BCTO(0.12) and Bi₂Ti₂O₇ thin films.Compared with Bi₂Ti₂O₇ thin films,it could be observed clearly that the location of Bi4f peaks of BCTO(0.12) thin film shifted by 0.57eV to high energy level.The band gap energy value for BCTO(0.12)thin films was 3.36eV by a UV-vis recording spectrophotometer,which was higher than that of BTO thin films.BCTO(0.16) thin films were also studied deep.Under following conditions as 700℃annealing temperature,100kHz frequency and -3V applied bias voltage,the value of dielectric constant and loss factor of BCTO(0.16) thin films are 168 and 0.04,respectively.The research on BCTO thin films above indicated that the doping of Ce was effective for the chemical stability of the Bi₂Ti₂O₇ phase.In addition,BCTO thin film had not only good insulating properties and resistance to breakdown,but also high dielectric constant and low dielectric.It is promising to be used as a High-k material in advanced MOS and DRAM.In the research of K_0.5Bi_0.5TiO₃(KBT) thin films,we reported the preparation and synthesis of KBT thin films on Si[100]substrates and Pt/TiO₂/SiO₂/Si substrates using CSD method with "annealing layer by layer".We studied the phase, morphologies,densities of leakage current,memory windows,hysteresis loops and temperature dependence of the capacitance and the dielectric loss.XRD patterns indicated that the films crystallized very well,presenting a single phase of perovskite structure and random orientation.There were no evident differences between the two KBT thin films on different substrates.By AFM we could find that the KBT films on the two different substrates possessed closely packed quadrilateral column grains and had relatively smooth surfaces with no cracking or voids.Furthermore,the grain sizes were almost the same,about 170nm.The root mean square surface roughness of KBT film on Pt was a little higher(～6.6nm) compared with that of film prepared on Si(～6.2nm).The emphasis was put on the research of ferroelectricity,and the width of memory windows were discussed.The thickness of SiO₂ interface was worked out to be 3nm.The impact of holes injection on the memory ability was also discussed.Both MFS and MFM structures showed excellent insulating properties.The maximum memory window was 3.1V at±8V, which was twice as large as that of the Al/SrBi₂Ta₂O₉/HfO₂/Si structure.The Curie temperature of the films measured from the capacitance-temperature curve was about 350℃,which was smaller than that of KBT ceramic.With the temperature increasing, the dielectric loss began to decrease and then increased.These results above showed that high quality KBT films could be prepared on Si and Pt/TiO₂/SiO₂/Si substrates by CSD method with "annealing layer by layer".The KBT film prepared had good insulating properties and ferroelectric memory ability.It is considered to be used in FeFET.