【作者】 程玲；

【导师】 胡广达；

【作者基本信息】 济南大学 ， 材料物理与化学， 2011， 硕士

【摘要】 BiFeO₃由于其具有良好的铁电和压电性，沉积温度较低（低于600℃），在数据存储和做电子机械系统应用上有很大发展前景。众所周知，BiFeO₃具有高居里点温度（Tc=850℃），所以在薄膜沉积和快速退火过程中不可避免要发生老化现象。BiFeO₃薄膜中的老化现象已经被证明与受主缺陷（A2+Fe3+）’和氧空位（Vo2）’’g形成缺陷对（A2+Fe3+）—（VO2-）’’有关。缺陷对所形成的局部电场阻碍了铁电畴的翻转，从而导致了在老化的BiFeO₃薄膜中压电和介电常数、电荷保持能力、剩余极化等部将被降低。另一方面，BiFeO₃薄膜同时遭受严重的漏电问题，这使加在薄膜上的有效电压大大降低了。因此，薄膜很容易被击穿，对薄膜的电学测试影响很大，很难测得好的性能。很明显，老化和漏电部与(V_o^2-)’’有关，所以消除(V_o^2-)’’是同时解决这两个问题的有效途径。从缺陷化学的角度上来分析，采用高价离子掺杂是一种消除(V_o^2-)’’含量的有效方法。本论文采用金属有机分解法，通过层层退火工艺制备了掺W6+以及Cu2+和w6+共掺的BiFeO₃薄膜，研究了其结构和电学性能（尤其压电性）。主要研究内容如下本论文所选用的主要电极材料为LaNi（）3，其与BiFeO₃均为钙钛矿结构且晶格常数相近，非常有利于BiFeO₃薄膜的生长。在si衬底上研究了不同的聚乙二醇含量，预处理温度和退火温度对LaNi（）3薄膜表面形貌的影响，选择出来最佳的工艺条件分别为O.36g，425℃，650℃。用此工艺制备了厚度为150nm的f100)取向的kiNi（）3薄膜，结晶度非常好，并且没有杂峰的产生，电阻率为5×10-4～1×10-3Ω·cm，非常适合作电极材料。随后在LaNi（）3（100）／si衬底上制备了（100）取向的Bifeo₃外延膜。薄膜中的（100）和（200）衍亍射峰均很强并且没有任何其他衍射峰产生。在LaNi（）3（100）／si衬底上制备了掺杂高价离子W6+的BiFeO₃的外延膜，研究了不同W6+掺量（O～2％）对BF0薄膜的结构、漏电、铁电及压电性的影响。结果发现随着W6+掺量的增加（<2％），BiFeO₃薄膜的性质有所改善，比如漏电降低，矫顽场变小并且对称性变好，剩余压电系数变大，最大可达132pm／v。这是由于w6+掺杂有效的减少了（VO2-）’’含量从而达到了同时降低漏电和去老化的作用。Bife_0.995w_0.005O₃薄膜和：BiFe_0.99w_0.01O₃薄膜中的铁电畴均可被完全翻转，但由于两薄膜中非180。畴含量的不同，所以后者的压电系数比前者大。当掺量增加到2％摩尔时，所制备的（100）^取向的BiFe_0.98w_0.02O₃外延膜中有少数做弱杂峰的出现并且铁电性和压电性开始减弱。这是由于W6+掺量过高限制了晶粒的生长，从而产生了更多缺陷，使W6+掺杂抑制BiFeO₃薄膜老化的作用下降。在LaNi（）3（100）／si衬底上制备了不同退火温度的BiFe_0.995w_0.005O₃薄膜，研究了退火温度对BiFe_0.995w_0.005O₃薄膜的结构，压电性能的影响。研究发现薄膜在450℃退火温度下就能结晶并且晶化程度很好。在．450℃、475℃、500℃的退火温度下BiFe_0.995w_0.005O₃薄膜的铁电畴均可以被极化翻转。但是压电响应的大小、均匀性和信噪比有所差别，475℃退火的BiFe_0.995w_0.005O₃薄膜剩余压电系数最大，可达134pm／v。但是它的压电响应均匀性和信噪比不如450℃和500℃退火制备的BiFe_0.995w_0.005O₃薄膜的好。这主要由于前者主要是非180。畴和180。畴的共存，后两者分别主要含有非180。畴和180。畴。在IT0／glass和kLNi（）3（100）／si衬底分别制备了BiFe_0.995cu_0.005O₃，BiFe_0.995w_0.005O₃，Fe_0.99cu_0.005w_0.005O₃薄膜，研究了这两个系列薄膜的结构和压电性能的区别。结果发现在IT0／glass衬底上制备的多晶膜中，很多铁电畴没有被翻转，性能部比较差，共掺的薄膜漏电较低，所以极化相对最好。在LaNi（）3（100）／si衬底上制备的外延膜中均具有很强的（100）和（200）衍射峰。其中，BiFe_0.995cu_0.005O₃薄膜中由于老化严重，所以薄膜的压电性最差。BiFe_0.99cu_0.005w_0.005O₃和BiFe_0.995w_0.005O₃薄膜由于W6+的掺杂抑制了老化，铁电畴均可被完全翻转，前者的压电响应均匀性和信噪比均比后者好，但由于cu2+的掺量选择过高，所以前者中的非180。畴的畴壁运动受到缺陷对形成的局部电场的限制，所以压电系数不如后者的大。总之，本论文通过高价离子W6+的掺杂解决了薄膜的漏电和老化问题，这将使BiFeO₃薄膜在做电子器件尤其高温压电器件应用上具有更广阔的发展前景。更多还原

【Abstract】 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 5¹0-4¹×10-3Ω·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～2％）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（<2％）For instance，the leakage current as well as the value and asymmetry of coercive field decreases；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更多还原