【作者】 胡文成；

【导师】 杨传仁；

【作者基本信息】 电子科技大学 ， 微电子学与固体电子学， 2008， 博士

【摘要】 非致冷焦平面阵列（Uncooled Focal Plane Array，UFPA）的热成像系统具有高密度、高性能、低成本、低功耗及小型化的特点，是目前红外热成像领域中最具前途的发展方向。热释电材料是红外焦平面阵列的关键技术之一，因此，这方面材料的理论和性能是当今材料科学和微电子科学领域的研究前沿。本文在热释电材料热释电理论和Ba_xSr_1-xTiO₃（BST）热释电薄膜方面主要进行了研究，通过传统铁电材料的热力学模型，推导了热释电晶体的热释电方程，并对实际晶体进行模拟，得出了相应的拟合结果，并对相应的物理参数进行了描述；在铁电材料热力学理论基础上，引入熵值，在爱因斯坦振动理论基础上，通过替代，建立了热释电晶体的赝热力学模型，并对TGS系列晶体进行拟合，得出的结果比传统热力学模型更接近实际热释电曲线；在设定铁电材料的偶极子和非偶极子所建立的热力学平衡基础上，引入传统的热力学概念，并设定热释电薄膜为三明治结构，建立了热释电薄膜的热力学平衡模型，并对实际热释电薄膜Pb_0．95Er_0．05(Zr_0．52Ti_0．48)O₃（PEZT），Ba_0．8Sr_0．2TiO₃（BST80），Ba_0．66Sr_0．34TiO₃（BST66）和Sr_0．255La_0．03Ba_0．7Nb_1．95Ti_0．05O_5．95／2（SLBNT）等进行拟合，拟合的结果与实际的热释电曲线一致，并对热力学平衡模型的物理参数进行了比较和说明；采用多工位平面磁控溅射装置制备BST薄膜，研究了650℃下退火的薄膜介电-温度特性、电滞回线等性能，剩余极化和饱和电场分别为4．1μC／cm²和60．9kV／cm，晶粒尺寸可达80nm左右，居里点在19℃附近。采用sol-gel法准备了BST薄膜，研究了溶胶浓度及薄膜在不同温度、不同退火时间对BST薄膜表面形貌的影响，得出了适宜的溶胶浓度。同时发现，在长时间退火下，BST薄膜的表面出现比较大的岛状结构，该结构对BST薄膜的性能有不利的影响；通过BST薄膜的介电-温度特性，计算得出了这两种方法制备的BST薄膜的热释电系数-温度曲线，并应用热力学平衡模型进行了模拟，得出了模型中各参数的数值；研究了Sn掺杂BST薄膜，Sn的掺入并未使BST薄膜晶粒尺寸受到影响，但介电常数明显降低。Sn离子存在多种变价离子，且占据ABO₃的B位，因此对空位有一定的钉扎作用，改善了BST薄膜的电性能。研究了La掺杂BST薄膜，La的掺入明显降低了BST薄膜的晶粒尺寸，且介电常数也降低，削弱了BST薄膜的介电性能，但漏电流性能有所提高。进一步研究了BST与La掺杂BST交替生长的BST／BSLaT／BST薄膜，明显增大了晶粒尺寸，兼有BST薄膜的介电性能和La掺杂BST薄膜的漏电流特性；通过BST薄膜的介电-温度特性，计算得出了这三种方法制备的BST薄膜的热释电系数-温度曲线，并应用热力学平衡模型进行了模拟，得出了模型中各参数的数值；在Pt／Ti／SiO₂／Si衬底上制备的BST薄膜，经退火后发现Pt电极的Pt原子向BST薄膜发生扩散，并存在一定的扩散深度。在BST薄膜和Pt层界面处主要是以体扩散为主，而在较远的距离发生晶界扩散。Pt原子在BST薄膜中的扩散严重影响了BST薄膜的电性能；研究了Sol-gel法制备LNO电极工艺和LNO薄膜的性能，实验中发现，在700℃下退火的LNO薄膜在（200）面高度定位，电阻率和方块电阻分别达到0．0037Ω·cm和76Ω／□，完全适用于BST薄膜的底电极；研究了BST薄膜和Sn掺杂BST薄膜在LNO底电极上的沉积，通过与在Pt／Ti／SiO₂／Si衬底上沉积的BST薄膜比较发现：BST薄膜和BSTS薄膜的介电常数和介电损耗均有降低，且漏电流特性明显提高。更多还原

【Abstract】 Nowadays,measurement technique for thermal imaging system with uncooledfocal plane array （UFPA） is one of the most important research directions in the field ofinfrared thermal imaging system due to its excellent performances such as high density,low cost,low power losses,miniaturization,and so on.Pyroelectric materials are thekey technology of UFPA,and the theory and characteristics of pyroelectric materialshave gained more and more attentions of materials and micro-electronics science.This dissertation is aimed to deal with the theory of pyroelectric materials and thecharacteristics of pyroelectric Ba_xSr_1-xTiO₃ （BST） thin films.Based on thethermodynamics theory of Devonshire,the polarization equation of pyroelectric crystalwith the temperature was established under zero electric field.The equation of thepyroelectric coefficient with the temperature was established by the definition ofpyroelectric coefficient and the derivation of polarization equation.The simulationresults showed that the model and expression formula for pyroelectric crystals are inaccordance with the experimental results of triglycine sulfate （TGS） and glycocyaminedoped TGS （GTGS） pyroelectric crystals.The state of pyroelectric crystals in cubicphase to be considered is assumed to approach the state of the single crystal.When thecubic phase transits into the tetragonal phase with temperature decreasing,the entropyand spontaneous polarization will change.Based on the Landau thermodynamic theory,pyroelectric equation of pseudo thermodynamics model is established based on themodified classical oscillation equation,and the simulating curves described by pseudothermodynamics model are in accordance with those of the experimental results of TGSseries pyroelectric crystals.In the thermodynamic equilibrium theory of pyroelectricpolycrystalline thin films,the unit cell of pyroelectric polycrystalline thin films in theferroelectric phase to be considered is assumed as a dipole and is a non dipole at itsparaelectric phase.The phase transition between ferroelectric and paraelectric phases ishypothesized as an equilibrium of the dipole and the nondipole.Based on thethermodynamic equilibrium theory,an expression formula about the spontaneouspolarization and temperature is suggested.Considering the sandwich structure of pyroelectric polycrystalline thin films,an equation about the relations betweenpyroelectric coefficient and temperature was established.Ferroelectric Ba_0.8Sr_0.2TiO₃thin films were deposited on Pt/Ti/SiO₂/Si substrates via a modified RF magnetronsputtering by introducing the revolution of workholders.XRD,AFM and electricalmeasurements were used to characterize BST thin films annealed at differenttemperatures.Smooth and dense surface with homogeneous grains （about 80 nm） wasobserved.The electrical measurement results showed BST films annealed at 650℃havehigher dielectric constant,lower loss tangent,lower leakage current and higherbreakdown voltage.The curves of the temperature dependence of dielectric constant indifferent frequencies exhibit Curie transition at temperature around 19℃.The remnantpolarization and the coercive field are 4.1mC/cm² and 60.9kV/cm,respectively.BSTThin films were deposited on Pt/Ti/SiO₂/Si substrates with approximately 300 nmthickness by sol-gel method.Qualitative film analysis was performed with XRD andAFM.The results indicated that surface roughness of the film increases with theconcentrations of precursor solutions and that the BST thin films fabricated by lowconcentrations precursor solutions were characterized by a small surface roughness witha crack-free uniform microstructure.In addition,the dielectric and ferroelectricproperties of BST films were measured.BST thin films doped with La and Sn wereprepared on the Pt/Ti/SiO₂/Si substrate.The results of XRD and AFM showed thatdopant La causes decrease grain size of BST thin films obviously,and Sn-doped BSTthin films was similar to BST films in size.La and Cd doped were decrease thetunability of BST thin films and Sn doped increased it,which may be explained bystress,electronegativity and oxygen vacancies factors.All the doped BST thin filmsimproved the leakage current characteristic.BST/BSLaT/BST multiplayer （ML） filmswere prepared by sol-gel method on the Pt/Ti/SiO₂/Si substrate.XRD and AFM showedthat dopant La causes decrease grain size of BST thin films obviously,and ML filmswere similar to BST films in size.The dielectric constant vs temperature curve of MLfilms becomes sharper comparing with that of BST films near the phase transition point,which indicated that the pyroelectric coefficient is propitious to be enhanced.The MLfilms also improve the leakage current characteristics of BST thin films and the linearrelationship of the log J versus E^1/2 curves showed in a good agreement with theSchottky thermionic emission model.Barium strontium titanate (Ba_0.65Sr_0.35TiO₃) ferroelectric thin films have been prepared by sol-gel method on Pt/Ti/SiO₂/Si substrate.The XRD pattern determines the films are a polycrystalline perovskite structure andAFM image shows that the crystallite size and the RMS are 90nm and 19.2nm,respectively.In this paper,we focus our discussion on the diffusion of Pt in BST films.The X-ray photoelectron spectrum （XPS） images show that Pt consisting in BST thinfilms is the simple substance state,and the Auger electron spectroscopy （AES） analysesthe Pt concentration in different depth profiles of BST thin films,the result displays thatthe Pt diffusion in BST thin films divided into two regions,near the BST/Pt interface,the diffusion type is volume diffusion,and far from the interface correspondingly,thediffusion type becomes grain boundary diffusion.We also use the previous researcher’sresult to verify our conclusion.LaNiO₃ thin films were fabricated on Si substrates underdifferent annealing temperatures.The XRD pattern showed that the films annealed at700℃were single perovskite phase and highly （200） oriented.The average grain sizeand RMS of the sample are 83.67nm and 5.06nm,respectively.It was also revealed thatthe electrical property of the films mainly depended on the crystallite size of LNO.Theresistivity and sheet resistance of the LNO thin films annealed at 700℃are 0.0037Ω·cm and 76Ω/□,respectively.Sn-doped （Ba,Sr）TiO₃ （BSTS） and BST thin films havebeen deposited on highly （200） oriented LaNiO3 （LNO） thin films.AFM imagesexhibited that the dopant Sn did not decrease the crystalline grain size of BST thin films.The structures of the BST and Sn-doped thin film,presented the higher intensity （110）and （200） peaks,while the latter was distinctly induced by LNO layer.Obviously,Sn-doped BST thin films on LNO/Si substrate were decrease the dielectric constant andthe dielectric loss,which is favourable to potentially improve the figures of merits （F_M）of pyroelectric materials.The BSTS thin films on LNO layer also displayed an excellentleakage current property comparing with the BST thin film on Pt/Ti/SiO₂/Si andLNO/Si substrates.更多还原