Investigation On Pyroelectric Theory And Characteristics Of BST Thin Films

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/â–¡,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.