Transparent Conductive Doped-stannate Films And Their Application In All-perovskite Ferroelectric Film Devices

Transparent and conductive oxide(TCO) thin-films such as Sn-doped In₂O₃(ITO), Al-doped ZnO,and Sb-doped SnO₂ are one type of important materials in optoelectronic industries.The unique feature of high optical transmittance in the visible range and low electric resistivity at room temperature(RT) gives them particular functions in flat panel displays,solar cells,touch panels,and so on. However,the developments in devices applications frequently demand improvements in physical properties of TCO films.For example,the full-colored liquid-crystal displays require TCO films with a wider direct allowed band gap,while in surface emitting lasers the films need to possess high optical transmittance in the near infrared wavelength.And for transparent thin-film transistors,single-crystalline TCO films may also be required in order to grow epitaxially with the other functional layers for minimizing the density of defects at the interfaces.All these mean that the TCO films adaptable to specialized applications should be highly expected.On the other hand, the perovskite oxides represent also an important class of materials since they were discovered to have many intriguing physical properties such as the high transition-temperature superconductivity,colossal magnetoresistance,ferroelectric, piezoelectric,magneto-electric,and electro-optic effects.Due to their similar crystal structure,with the advances in film growth,a variety of all-perovskite heterostructures have been proposed and fabricated for devices with improved performance or unconventional functionalities.Thus,to search for high-quality single-crystalline TCO films of the perovskite structure is significant not only for the TCO films themselves but also for the thin-film devices based on all-perovskite heterostructures.In the past years,a few perovskite-type TCO thin-films including the In-,Sb-,and La-doped SrTiO₃,Nb-doped CaTiO₃,and Cd₃TeO₆ have been fabricated and characterized.Ferroelectric thin films have attracted great attention for their physics and potential applications in various microelectronic devices such as nonvolatile ferroelectric random access memories and ferroelectric field-effect transistors,which are also transparent in the visible range due to their wide direct allowed band gap. Furthermore,the ferroelectric films can be treated as p-type semiconductors at reduced film thicknesses,due to possibly the processing-induced element loss(Pb for the PZT,and Bi for the BFO) and oxygen vacancies in the films.Herein,transparent ferroelectric thin-film devices with all-perovskite structures have been highly expected,although the lack of high-quality perovskite-type TCO film was a serious trouble.In this thesis,La_xBa_1-xSnO₃(LBSO) and La_xSr_1-xSnO₃(LSSO) films have been epitaxial grown on SrTiO₃(001) substrates by the pulsed laser deposition(PLD) method.It is revealed that all the films have cubic perovskite structures,excellent optical transmittance in the visible range,and n-type electronic transport with low resistivities at room temperature.The different optical transmittances in the near infrared wavelength endue the doped BaSnO₃(BSO) and SrSnO₃(SSO) with various applications in film devices.Since the lattice constant is very close to the in-plane lattice constant of PZT and BFMO(BFO) films,all-perovskite LSSO/PZT/LSSO and (Pt)/BFMO/LSSO capacitors were fabricated,and their structural,optical,and ferroelectric properties have been investigated.With the reduced ferroelectric film thicknesses,the junctions using heteroepitaxial p-PZT/n-LSSO and p-BFO/n-LSSO were also proposed and examined,and we demonstrated that they show good rectifying characteristics in addition to the high optical transmittance in the visible range.The whole thesis consists of five chapters.Chapter 1:The general review of the history and present research situation of the TCO film is given.First,we briefly review the development of the conventional TCO thin-films including systems,research progress,application and defect,and point out the high-quality single-crystalline TCO films of the perovskite structure is significant not only for the TCO films but also for the thin-film devices based on all-perovskite heterostructures.Next,we make clear the mechanism of optical transmittance and electronic transport in the films based on the theoretical models.In the end of the chapter,we simply review the research progress of ferroelectric thin-film devices and transparent thin-film devices,respectively,and discuss the feasibility and advantage of the transparent ferroelectric film devices with all-perovskite structures.Chapter 2:The film preparation methods and samples measurements are introduced.Especially,the pulsed laser deposition(PLD) method,X-ray diffraction reciprocal space mapping,and the hall measurement are described in details.Chapter 3:La-doped BSO films have been prepared by laser ablation under conditions compatible with many other perovskite oxide films.The epitaxial films show a cubic perovskite structure,good transmittance in the visible range and a high conductivity at room temperature.The transport behavior and conductivity can be controlled by the growth temperature.Comparing to the conventional TCO films,the LBSO films can be attractive as a new TCO with the perovskite structure,and are promising for use in epitaxial all-perovskite transparent devices,due to the wide direct bang gap,large transparent window,and suitable growth parameters.Chapter 4:We probe the effect of various growth parameters and doping concentrations on the structures and properties of LSSO films prepared by PLD. These films can be grown epitaxially on STO(001) at a much lower deposition temperature of 600℃,and possess pseudo-cubic perovskite single-crystalline structures,wide direct band gaps larger than 4.5eV,high conductivities at room temperature,and well optical transmittance even at the wavelength as far as 2300nm. Comparing to LBSO films,LSSO have great advantages for use in transparent thin-film devices due to their lattice constants conformable to PZT and BFO counterparts,lower growth temperature,and well optical transmittance in the near infrared wavelength.Chapter 5:Ferroelectric LSSO/PZT/LSSO and(Pt)/BFMO/LSSO capacitors have been grown epitaxially on STO(001) substrates,and their structural,optical,and ferroelectric properties have been investigated.These capacitors show square polarization-electric field hysteresis loops,well optical transmittance,but larger coercive field,and especially poor fatigue resistance,in contrast to the epitaxial ferroelectric capacitors with p-type LSMO electrodes,which may be related to charge injection and the interface capacitor or built-in potential between PZT(BFMO) and n-type LSSO films.To understand the interface property of the heterostructures,with the reduced ferroelectric film thicknesses,the junctions using heteroepitaxial p-PZT/n-LSSO and p-BFO/n-LSSO were also proposed and examined,and we demonstrated that they show good rectifying characteristics in addition to the high optical transmittance in the visible range.The ideal turn-on voltage determined from staggered energy band diagrams are in good agreement with the experimental values. Herein,we believe the p-n junctions have been built in between PZT(BFO) and LSSO films.