Investigation On The Preparation And Properties Of Aliovalent-ion Doped BiFeO₃-based Thin Films

With an ever-expanding demand for data storage, transducers, and microelectro-mechanical （MEMS） systems applications, materials with superior ferroelectric andpiezoelectric responses are of great interest. The lead zirconate titanate （PZT） family ofmaterials has served as the cornerstone for such applications up until now. However, a criticaldrawback of this material is the presence of heavy metal lead with toxicity. Using oflead-based devices goes against the purpose of environmental protection and sustainabledevelopment. Researchers begin to looking for an alternative lead-free material whoseproperties can be comparable to PZT. Ferroelectric BiFeO₃（BFO） has attracted a great deal ofattention because of its superior thin-film ferroelectric properties, which are comparable tothose of the tetragonal, Ti-rich PZT system. Besides the perfect ferroelectric properties, theadvantages for BFO also contain multiferroic and the high Curie point （T_C⁸30°C） which canmake ferroelectrics work under high temperature conditions.The annealing temperatures for BFO thin films are usually ranging from500⁷00°C,which are much lower than its Curie temperature. Therefore, the film should be subjected toaging during the deposition process. Since the switching of domains will be constrained bythe defect complexes associated with the oxygen vacancies and low-valence-ion, aging effectcan be aggravated. This will also impact on the long-time stability of microelectronic devices.From the defect chemistry point of view, high-valence-ion doping can inhibit the formation ofV_O^2-and in turn postpone the aging. Besides, BFO suffers from a serious leakage problem,which is also relation to theV_O^2-. Qi et al. have decreased the leakage current by morethan three orders of magnitude through doping of Ti⁴⁺in BFO thin film. Whereas in ourprevious work, the leakage current of Ti⁴⁺doped BFO film prepared under N2annealingatmosphere is a little higher than pure BFO. Therefore, the influence of processing parameterson the properties for high-valence-ion doped BFO thin films can not be ignored. On the otherhand, there is a great number of Fe²⁺existing in BFO, in turn leading to that the film isnon-stoichiometric. The transfer of electrons between the neighboring Fe²⁺and Fe³⁺could aggravate the leakage current for BFO films. High-valence-ion doping does not eliminate theformation of Fe²⁺. Fortunately, this problem can be improved by doping low-valence-ionswith stable valence state to some extent.In order to optimizing the structures and properties of aliovalent-ion doped BFO films,we prepared series of BFO-based films on different bottom electrodes using a metal organicdecomposition combined with sequential layer annealing process. The effects of thepreheating temperature and annealing atmosphere on the structures and piezoelectricproperties of BFO-based films were investigated. The optimum proportion for high-andlow-valence-ions codoped BFO-based films which possesses a largest piezoelectriccoefficient d33was also be identified. The study contents are listed below:1. BFO-based films doped by various aliovalent-ions were prepared and Ti⁴⁺, Zn²⁺wereselected as the doping ions for further research. The BFO-based film were deposited onITO/glass, Pt（100）/Si and LNO（100）/Si substrates, respectively. The film deposited onITO/glass substrates showed a polycrystalline structure with two strong x-ray diffractionpeaks of （012） and （110） orientations, while films on Pt（100）/Si and LNO（100）/Si are mainlycomposed of （100）-oriented grains.2. BFO films preheated at different temperatures were fabricated on ITO/glass substrates.Comparing the structure evolution upon film thickness for the two BFO films preheated at350and425°C, we can see that （012）-and （110）-oriented grains grew up simultaneously asfilm thickening for BFO film preheated at350°C, while BFO film preheated at425°C inwhich there existing the low-temperature nucleation of （110） grains exhibited a monotonousincrease of I₁₁₀/I₀₁₂as the increase of film thickness and in turn formed a most highlypreferential degree of （110） orientation. Then we tested the structure for BFTO filmspreheated at different temperatures ranging from375to450°C. The BFTO film preheated at425°C also possessed a most highly preferential degree of （110） orientation. Its growth modeof columnar structure can decrease the volume fraction of grain boundaries. Thus, the amountof defects and defects complexes reduced, making the domain switching easy. So a largestremanent piezoelectric response d₃₃of¹37pm/V was obtained in BFTO film preheated at425°C.3. BiFe_1-xTi_xO₃（x=0³%） films were deposited on LNO（100）/Si substrates and prepared under different annealing atmosphere. BiFe_0.98Ti_0.02O₃possessed the largest d₃₃, suggestingthat doping of2%Ti⁴⁺can eliminate oxygen vacancies effectively without obviously negativeinfluence on grain growth. The d₃₃of BiFe_0.98Ti_0.02O₃film annealed in O₂was larger than thatof the one annealed in N₂, which originates from the different defect reactions for the twofilms. In N₂, n-type semiconductor was formed for high-valence-ion doped BFO films, that is,free electrons served as the charge carrier. Evidently, the leakage current of the film wouldincrease and in turn resulting in a weak piezoelectric response. For the case of BiFe_0.98Ti_0.02O₃film annealed in O₂, it preferred to form the cation vacancies which do a little contribution toleakage. Therefore, the film can be polarized uniformly. In addition, all films exhibitedasymmetric piezoelectric responses polarized under positive and negative electric fields. Weexplained this phenomenon based on the self-polarization of high-and low-valence-ionsdoped BFO films.4. We have prepared BiFe_0.98-yTi_0.02Zn_yO₃films with Zn²⁺doping content ranging from0³at.%on ITO/glass substrates. The experimental results indicated that doping of Zn²⁺moderately favored the grain growth of （012） orientation. BiFe_0.98-xTi_0.02Zn_xO₃showed a（012）-preferred structure when x=1.5%. A largest remanent d33of¹23pm/V can be obtainedat x=1.5%, which stemmed from that the Fe²⁺were suppressed as well as the highlypreferential degree of （012） orientation for BiFe_0.965Ti_0.02Zn_0.015O₃film.In sum, we have investigated the effect of low-temperature nucleation and annealingatmosphere on the structure and piezoelectric response for Ti⁴⁺-doped BFO films. Theproperties of BFO-based films were optimized through controlling the process parameters,making them promising for application in high-temperature piezoelectric devices andAFM-tip-based data storage.