Suppression Of The Aging And Leakage Of BiFeO₃ Thin Films

Superior ferroelectric,piezoelectric properties and low deposition temperature(<600^C)of BiFeO₃ make it promising in the applications of data storage andmicroelectromechanical systemsIt is well known that BiFeO₃ possesses a high Curie temperature(Tc=850^C),which leads to aging that will inevitably occur during the deposition process or thepost annealing treatment The aging has been demonstrated to be due to the formation of defect complexes between the acceptors[(A2+fe3+]and oxygenvacancies[(V_o^2-)"]The local fields associated with defect complexes can provide a driving force for domain backswitching,which in turn leads to degradation of the piezoelectric coefficient,dielectric constant,charge retention and remanent polarization in the agedBiFeO₃ thin films On the other hmad,BiFeO₃ films also suffer from a severe leakage problem,which can decrease the effective voltage used for the electrical measurement Therefore,the films can break down easily,which leads to that the electrical properties of BiFeO₃ thin films are not measured Obviously,both the aging and leakage problems are related to(V02-)",SO the elimination of(V_o^2-)"is an effective route to resolve these two problems simultaneously From the defect chemistry point of viev^,high-valence ion substitution should be an efficient method to eliminate(V_o^2-)"In the paper,W6+doped BiFeO₃ films,as well as Cu2+and w6+codoped BiFeO₃ films were prepared using a metal organic decomposition process combined with layer-by-layer annealing technique The structures and electric properties(especially piezoelectricity)are studied in all these filmsThe bottom electrode materials that have been used in the paper are LaNiO₃,which are benefit for the epitaxial growth of BiFeO₃ films This is because they have the same type of perovskite structure as that of BiFeO₃ with the close lattice constant The effects of the polyethylene glycol content,pretreatment temperature and annealing temperature on the morphology images of LaNiO₃ films are studied,and then the best technological conditions are found out to be O.36g,425℃and 650℃,respectively Highly crystallized(100)-oriented LaNiO₃ film with 100nm thickness is fabricated on Si substrate,the resistivity of which at room temperature is 5¹0-4¹Ã—10-3Ω·cm Therefore,LaNiO₃ is a promising candidate for the electrode The epitaxial(100)-oriented BiFeO₃ film is fabricated on LaNiO₃(100)／Si substrate The film show-s strong diffraction peaks of(100)and(200),without any other peaks.Epitaxial BiFeO₃ films with high valence ions W6+substitution are deposited on LaNiO₃(100)／Si substrates The effects of W6+doping(O～2ï¼…)on the structure,leakage current,ferroelectric and piezoelectric properties of BiFeO₃ thin film areinvestigated The results show that the properties of the BiFeO₃ thin films have been improved with the increase of w6+content(<2ï¼…)For instance,the leakage current as well as the value and asymmetry of coercive field decreasesï¼›the remanent piezoelectric coefficient increases and reaches a maximum 132 pm／V.This should be because W6+doping can reduce the content of(V_o^2-)"effectively and in turn solve the leakage and aging problems simultaneously The remanent piezoelectric coefficient ofthe BiFe₍0.99_W0.01O₃ film is higher than that for the biFe0.995W0.005O₃ film,although the ferroelectric domains in both films can be fully switched This may be due to the COntribution from the irreversible movement of non 180。domain walls being much higher in the former than in the latter With the W6+doping content increased to 2％．several weak peaks are detectable in diffraction pattern and the ferro／icIiezoelectric properties become worse in(100)-oriented BiFe_0.98w_0.02O₃ filmcompared to that of BiFe₍0.99_W0.01O₃ film This should be owing to that the increase of the W6+ doping content will not favor the deaging of BiFeO₃ film,which raises the activation energy for epitaxial growth of BiFel-xWxO3 films to some extent,and hencemore charge defects axe formedBiFe_0.995W_0.005O₃ films axe deposited on LaNiO₃(100)／Si substrates The influences ofthe annealing temperature ranging from 425^C to 500℃on the structure and piezoelectric properties of BiFe_0.995W_0.005O₃ films are investigated The film cancrystallize well even at 450℃low temperature The remanent piezoelectric coefficient． the uniformity of the piezoresponse and the ratio of signal to noise are different,although the domains can be fully switched in all crystallized films The film annealedat 475℃exhibits the maximum value of the remanent piezoelectric coefficient (134pm／V),while the uniformity ofthe piezoresponse and the ratio of signal to noise in which are worse than that ofthe films annealed at 450℃and 500℃This should be due to that non 180。domains and 180。domains coexist in the former,non 180。domains and 180。domains exist in the latter two respectivelyBiFe0.995Cuo.005O₃,BiFe_0.995W_0.005O₃ and BFe0.99Cu0.005Wo.005O₃ films are deposited on ITO／glass and LaNiO₃(100)／Si substrates The different of the structureand piezoelectric properties in these two series films are studied The properties in the polycrystalline films deposited on ITO／glass are poor compared to that on LaNiO₃(100)／Si This is because that more defects in the former make the domains switch hard Relatively speaking,the piezoelectric properties ofthe codoped one in all the films deposited on ITO／glass substrate are best due to the lowest leakage current The films deposited on LaNiO₃(100)／Si substrate all show-epitaxial structure with strong diffraction peaks of(100)and(200)The piezoelectric properties of theBiFe0.995Cuo.005O₃ film is worst in all the films deposited on LaNiO₃(100)／Si substrate due to the most serious aging behavior The ferroelectric domains in both BiFe_0.995W_0.005O₃ and BFe0.99Cu0.005Wo.005O₃ films can be fully switched owing to theefficient deaging effect of W6+substitution The uniformity of the piezoresponse and the ratio of signal to noise in the former are better than that in the latter．However,the remanent piezoelectric coefficient ofthe former is lower than that ofthe latter This isdue to that the content of Cu2+ion in BiFe₍0.99_Cu_0.005W_0.005O₃ films is a bit high,SO the local fields associated with defect complexes limit the irreversible movement of non 180。domain wallsIn a word,the leakage and aging problems of BiFeO₃ films can be solved simultaneously though the substitution ofthe high．valence ion W6+which will makes BiFeO₃ films more promising for the microelectronic devices applications especially in high-temperature piezoelectric devices...