【作者】 王海峰；

【导师】 吴文彬；

【作者基本信息】 中国科学技术大学 ， 凝聚态物理， 2009， 博士

【摘要】 透明导电氧化物（TCO）薄膜以掺Sn的In₂O₃（ITO）、掺Al的ZnO（AZO）、掺Sb的SnO₂（ATO）等为代表,是一种重要的光电信息材料。由于在可见光的范围内具有很高的光透过率,其电阻率又接近金属的数值,这使得TCO薄膜材料成为应用范围广且最具有商业应用价值的功能薄膜材料之一。目前的透明导电薄膜广泛应用于平板显示器、太阳能电池、触摸屏等领域。近年来,随着光电子特殊器件的要求,之前研究并应用的TCO薄膜性能已经越来越不能满足人们的需求,如真彩液晶显示器要求TCO膜具有大的光学禁带宽度、面接触式发射激光器需要TCO在红外光区间（特别是1000nm-1500nm）具有很高的光透射率（而ITO在1000nm-1500nm的波长区间吸收系数已经非常大）等。而对于透明薄膜器件,单晶TCO薄膜可以和其他薄膜相互外延生长,从而能够有效的降低界面缺陷对器件性能带来的影响。所有这些都预示着具有特殊应用价值的TCO薄膜应该得到人们的高度重视和研究。另一方面,钙钛矿型氧化物是一类重要的功能材料,包含有高温超导、巨磁阻、铁电、压电、磁电、光电等众多的物理性能。全钙钛矿薄膜器件由于各层薄膜之间的相互外延生长,很大程度上解决了界面问题,使得器件特性得到了显著的提高,并呈现出了一些独特的功能。因此,寻找并制备具有高质量的钙钛矿结构外延TCO薄膜对TCO材料以及全钙钛矿薄膜异质结都具有着重要意义。铁电薄膜由于其物理机制和潜在的在各种微电子器件如非挥发性动态随机存贮器和铁电场效应管上的应用,受到了很多的关注。由于具有较大的直接禁带宽度,铁电薄膜在可见光范围内是透明的,而当薄膜厚度降低到一定的程度,因为氧空位和离子扩散等存在,其内部存在着比较大的载流子浓度,又可以被当作p型半导体材料,因此全透明铁电薄膜器件被人们寄予极大的期望。然而,高质量n型TCO薄膜的严重缺乏,阻碍了该类薄膜器件的研究进展。我们利用激光脉冲沉积（PLD）系统,基于对BaSnO₃和SrSnO₃进行A位La元素少量掺杂,在SrTiO₃（001）基片上成功外延生长了LBSO和LSSO两种锡酸盐基n型TCO薄膜。该类薄膜具有赝立方钙钛矿单晶结构,以及可见光区的高透光率和室温下10^-3Ωcm量级的电阻率。BaSnO₃和SrSnO₃基TCO薄膜具有不同的近红外光透光率行为,使得它们可以应用于不同的透明薄膜器件中。由于LSSO薄膜的晶胞参数和PZT和BFO非常匹配,我们成功实现了LSSO和PZT、BFO的相互外延生长,制备了全钙钛矿透明铁电电容器。实验表明,我们制备的电容器具有非常好的结构特征和光学性质,并表现出了一些有趣的疲劳特性。而利用较薄的PZT和BFO薄膜当作p型半导体,我们又成功制备了PZT/LSSO和BFO/LSSO透明钙钛矿p-n结,这两种异质结均表现出了优秀的整流特性。本论文总共分五章。第一章主要介绍了透明导电薄膜的研究现状。首先我们回顾了传统TCO薄膜的分类、研究进展、应用领域及其发展要求:一方面从实际应用角度出发,概括了传统TCO薄膜的发展现状和存在的问题,指出了寻找新型TCO薄膜的一些研究方向;另一方面介绍了钙钛矿结构氧化物薄膜的一些物理特性,指明寻找和制备具有钙钛矿结构的TCO薄膜的意义。接着从理论的角度,阐明了TCO薄膜的透明和导电机制。在这些基础上,我们分别回顾了透明薄膜器件和铁电薄膜器件的研究进展,并最终指出全钙钛矿型透明铁电薄膜器件的优越性和制备的可行性。第二章主要介绍了本文实验需要用到的薄膜制备方法和样品测试方法,特别对于几种重要的实验手段如X射线衍射的各种扫描方式,倒易空间图基本原理,从霍尔效应测量结果分析推导薄膜电输运特性等做了详细的介绍。第三章我们对La掺杂的BaSnO₃外延TCO薄膜进行了研究。实验表明该类薄膜具有立方钙钛矿单晶结构,并具有可见光区95%以上的透光率以及室温接近4mΩcm的n型电阻率。相比于传统的TCO薄膜,LBSO薄膜可以满足一些新型光电薄膜器件的宽禁带、钙钛矿单晶结构、大的透光窗口等特殊要求。而相比于其他文献所报道的具有钙钛矿结构的TCO,LBSO薄膜具有和其他钙钛矿功能薄膜更加匹配的沉积参数以及更大的禁带宽度,有利于其在全钙钛矿薄膜器件中的应用。第四章系统研究了沉积参数和掺杂浓度对La掺杂的SrSnO₃外延薄膜的结构、电学和光学性质的影响。实验结果表明,在SrTiO₃（001）基片上生长的LSSO均具有立方钙钛矿外延单晶结构以及4.5eV以上的光学能带宽度,并表现出良好的n型电输运和透光特性。相比于之前研究的LBSO薄膜,LSSO在600℃制备温度下依然可以得到良好的外延单晶结构。另外LSSO薄膜的晶胞参数与PZT、BFO非常匹配,并在红外光波长范围内依然具有良好的透光率,这些都预示着LSSO薄膜可以更好地应用到全钙钛矿透明薄膜器件中。第五章我们成功实现了LSSO和PZT、BFMO（BFO）的相互外延生长,制备了透明全钙钛矿铁电薄膜电容器。实验表明,这两类电容器具有非常好的结构特征和光学性质,并表现出了较大的矫顽场和较差的抗疲劳特性,我们认为这可能和LSSO和铁电薄膜之间形成了空间电荷区有关。为了更深入了解LSSO和铁电薄膜间的界面问题,利用较薄的PZT和BFO薄膜当作p型半导体,我们又成功制备了PZT/LSSO和BFO/LSSO透明钙钛矿p-n结,这两种异质结均表现出了优秀的整流特性,从而验证了我们对铁电电容器疲劳特性的解释。另外通过能带结构分析,我们得到的实验测量数据与理论结果非常符合。更多还原

【Abstract】 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.更多还原