新型SnO₂基透明导电薄膜及其二极管的研究

【作者】 黄延伟；

【导师】 张群；

【作者基本信息】 复旦大学 ， 物理电子学， 2010， 博士

【摘要】 透明导电氧化物薄膜(TCO)既具有优良的金属导电性,又具有可见光范围的高透明性,已经成为平板显示器、太阳能电池和透明电子器件中不可或缺的材料。其中ITO(In2O3:Sn)薄膜是目前市场上应用最为广泛的TCO薄膜,随着太阳能电池和平板显示产业的不断发展,铟资源短缺和价格高昂的问题越发突出,寻求ITO替代材料的需求日益加大；目前应用于太阳能电池的透明窗口电极材料大多为近红外高反射TCO薄膜,这就阻止了对占太阳能总能量52%的近红外区域能量的有效且充分的利用；透明电子学的出现对TCO薄膜材料和透明氧化物半导体(TOS)薄膜材料提出了更高的要求,需要光学和电学性能优良的新型TCO和TOS薄膜。针对这些问题和需求,本论文研究开发了新型非晶和多晶掺钨氧化锡(SnO2:W)透明导电薄膜,系统研究了其电学和光学性能及其与脉冲等离子体沉积技术(pulsed plasma deposition, PPD)各种制备参数之间的关系；确立了制备优良光学和电学性能的SnO2:W薄膜的条件；采用更具生产性的sol-gel法研制了SnO2:W薄膜,薄膜不仅表面平整,而且光学透明性得到了很大提高,揭示了其在太阳能电池领域具有潜在的应用价值,同时提出了溶胶配制过程中可能的反应机理；采用第一性原理对SnO2:W进行了理论计算,阐明了W掺杂SnO2:W薄膜具有优良电学性能的机制；研究开发了p型导电NiO:Li透明氧化物半导体薄膜,探索性研制了NiO:Li/SnO2:W透明二极管,揭示了SnO2:W薄膜应用于透明电子器件的可能性。本文利用多种表征手段(如XRD、AFM、XPS、SEM、EDS. IR、Raman、台阶仪、Hall效应测试等)对所有制备的薄膜进行了详尽细致的分析和探讨,研究结果表明：在采用PPD法制备薄膜的过程中,先是采用氧氩共掺气体,在较低温度下制备了非晶SnO2:W薄膜,研究了钨掺杂含量和氧分压对薄膜电学和光学性能等的影响,薄膜的最低电阻率达到2.1×10-3 ohm·cm,对应的载流子浓度和载流子迁移率分别为9.6×1019cm-3和30 cm2V-1s-1,可见光区的平均透射率超过80%,薄膜的方均根粗糙度和平均粗糙度分别为15.7 nm和1 1.9 nm。为进一步降低薄膜的电阻率,采用PPD法室温下在纯氩气体中沉积出SnO2:W薄膜,后经退火处理,薄膜的电学和光学性质均得到明显提高,最低电阻率可达6.7×10-4 ohm·cm,对应载流子浓度和载流子迁移率分别为1.44×1020cm-3和65 cm2V-1s-1,薄膜在可见光和近红外区的透射率分别为86%和85%,研究了钨掺杂含量和退火温度对薄膜电学性能、光学性能、结构和表面粗糙度的影响,并通过分析薄膜中各元素的化合价态讨论了薄膜的导电机理。在SnO2:W薄膜中,W原子替代了SnO2晶格中的Sn原子的位置,没有新的化合物相生成,也没有改变SnO2的金红石结构。w在薄膜中以W6+形式存在,W替代Sn位置贡献了两个自由电子,这是薄膜导电性增加的主要原因,而ITO薄膜中Sn取代In位置只多出一个电子,所以SnO2:W薄膜中W作为掺杂剂的效率较高。即要得到相同载流子浓度的薄膜,SnO2:W中掺W所需的掺杂量比SnO2:Sb, In2O3:Sn或ZnO:Al(其价态差仅为1)等中的掺杂量都更少,这使得引入的离子杂质缺陷减少,载流子散射中心减少,有利于提高薄膜的载流子迁移率。采用sol-gel法,以SnCl2·2H2O和WCl6为先驱物,经过溶胶配制、浸渍提拉、干燥退火等步骤在石英衬底上制备了大面积均匀导电的SnO2:W透明导电薄膜,薄膜最低电阻率为5.8×10-3 ohm·cm,对应载流子浓度和载流子迁移率分别为7.6×1019 cm-3和14.2 cm2V-1s-1,尽管薄膜的电学性质有待于进一步提高,但sol-gel法相对于PPD法具有设备廉价、成膜厚度均匀等的优点,且薄膜在可见光和近红外区的光学透射率达到90%,薄膜的平均粗糙度约为1.8 nm。对于sol-gel制备的薄膜,随着钨掺杂含量的增加,薄膜的XRD衍射峰强度变弱,甚至变宽,当掺杂含量较大时,薄膜中出现微小的非晶颗粒,这使得薄膜的粗糙度减小,在W掺杂含量分别为0,1,3,5 at.%时,薄膜的平均粗糙度分别为1.92 nm,1.87 nm,1.82 nm,1.61 mm。同时也说明采用sol-gel法制备薄膜的过程中,掺杂含量较高使薄膜的结晶性变差。SnO2:W透明导电薄膜的第一性原理计算结果表明,未掺杂的SnO2的价带顶主要源于O的2p态的贡献,导带底主要源于Sn的5s态的贡献；掺杂后SnO2:W的导带底主要源于掺杂原子W的5d态的贡献；W是提高SnO2导电性有利的掺杂元素。以固态反应法制备了p型导电性较好的掺锂氧化镍靶材,采用PPD技术在玻璃衬底上沉积了p型透明导电NiO:Li薄膜,薄膜电导率可达15 S.cm-1,载流子浓度变化范围为2.98×1020 cm-3-2.1×1020 cm-3,迁移率变化范围为0.3-0.422,一般文献报道的p型导电薄膜的载流子浓度的为1018 cm-3、1019cm-3量级甚至更低,可见此法制备的p型NiO:Li薄膜的导电性较好。本文基于前述实验结果探索性地将n型SnO2:W薄膜和p型NiO:Li薄膜结合起来制备透明电子器件中最基本的器件——透明二极管NiO:Li/SnO2:W。介绍了实验室条件下采用PPD法制备具有良好整流特性的透明二极管的方法和步骤,通过研究单层膜的导电性来比较分析二极管的伏安特性,并给出所制备器件的透明性。更多还原

【Abstract】 Transparent conductive oxide (TCO) thin film is indispensable in flat panel displays, solar cells and transparent optoelectronic devices due to its low resistivity and good optical transmittance. The most widely used TCO in present market is mainly tin-doped In2O3 (ITO). However, the large consumption of indium in increasingly expanding market, especially in liquid crystal displays (LCD) and solar cells, makes it high price and difficult to a stable supply. Simultaneously, one consideration of improving the energy conversion efficiency of solar energy in photovoltaic cells is how to effectively utilize the sun energy in the near infrared (NIR) region because the NIR accounts on 52% of the energy in the air-mass 1.5 global solar irradiance spectrum (300-2500 nm). Moreover, novel TCO and transparent oxide semiconductor (TOS) thin films with high optical and electrical properties are of very important in new transparent electronics.In this dissertation, novel amorphous and polycrystalline tungsten-doped tin oxide (SnO2:W) TCO thin films are developed in terms of above mentioned questions and requirements. Pulsed plasma deposition (PPD) method are used to prepare the SnO2:W thin films, the properties of the films and the dependence on various preparation parameters are systematically investigated. The preparation conditions of SnO2:W thin films with good optical and electrical properties are established. Also, SnO2:W thin films with smooth surface and much better transparency are successfully synthesized by sol-gel method, a much simple and much convenient technique apt to mass production and large area uniform coating. This implies the promising application of the films in solar cells. The possible reactive mechanisms in sol-gel process are proposed. The calculated results show that the bottom of conduction band of SnO2:W is mainly ascribed to W 5d states, which results in high conductive SnO2:W films. To study the transparent diode based on SnO2:W films, novel p-type lithium-doped nickel oxide (NiO:Li) thin films are developed, and NiO:Li/SnO2:W p-n junction is fabricated. It reveals the possibility of SnO2:W thin films in application of transparent electronics. The various measurements for all prepared samples, such as XRD, AFM, XPS, SEM, EDS, IR, Raman, stylus profilometer, four-point probe and Hall system, were applied to analyze and characterize the properties of the films. The main results are as follows:Amorphous SnO2:W thin films were deposited by PPD at (Ar+O2) mixed atmosphere, and the structural, electrical and optical properties have been investigated as functions of tungsten doping content and oxygen partial pressure. The lowest resistivity of 2.1×10-3 ohm·cm was reproducibly obtained, with carrier mobility of 30 cm2V-1s-1 and carrier concentration of 9.6×1019 cm-3 at the oxygen partial pressure of 1.8 Pa. The average optical transmission was above 80% in the visible region from 400 to 700 nm, with the optical band gap ranging from 3.91 to 4.02 eV. The root mean square (RMS) and average roughness were measured to be 15.7 nm and 11.9 nm respectively.Polycrystalline SnO2:W thin films were fabricated on quartz glass substrates by improved method:at pure Ar working gas using PPD with post-annealing. Doping of tungsten can effectively enhance the conductivity of the films while maintaining high transparency. The structural, electrical and optical properties of the films have been investigated with different annealing temperatures and tungsten-doped contents. The lowest resistivity of 6.7×10-4 ohm·cm was obtained, with carrier mobility of 65 cm2V-1s-1 and carrier concentration of 1.44×1020 cm-3 in 3 wt.% tungsten-doped films annealed at 800℃in air. The average optical transmittance is about 87% in the visible region from 400 to 700 nm, and about 90% in the near-infrared region from 700 to 2500 nm.XPS analysis shows that In SnO2:W thin film, the Sn 3d spectrum consists of a doublet with binding energies of 486.4 eV for Sn 3d5/2 and 494.9 eV for Sn 3d3/2, corresponding to the binding energies of the Sn4+ ion in SnO2. For the spectrum of W 4f doublet, the binding energies of 4f7/2 and 4f5/2 are 35.2 eV and 37.4 eV, respectively, which implies that tungsten atoms were fully oxidized to W6+ ions. From the XRD results, no additional peaks due to impurities phase were observed and this shows W substituted Sn lattice site and provide more electrons. This indicates that W doping can contribute more free carrier concentration than other dopant such as Sb when the doping concentration are equal. In other words, for the same carrier concentration there needs fewer dopant of W in SnO2:W than in other common doped oxides such as SnO2:Sb, In2O3:Sn or ZnO:Al, whose valence differences between the dopant and substituted ion are only 1. This may also be an influence factor in the process of obtaining high mobility because fewer dopant can largely reduce the ionized impurity scattering centers.Transparent conductive SnO2:W films were also synthesized on quartz glass substrates by sol-gel dip-coating method with the precursors SnCl2·2H2O and WCl6. It was found that the films were highly transparent and the average optical transmission was about 90% in the visible and near infrared region from 400 to 2500 nm. The lowest resistivity of 5.8×10-3 ohm·cm was obtained, with the carrier mobility of 14.2 cm2V-1s-1, and carrier concentration of 7.6×1019 cm-3. The structural properties, surface morphology and chemical states for the films were investigated. Although the electrical properties of films need further improve, sol-gel method is relatively cheap and easy to operate, and the prepared films possess excellently transparency and low average roughness of about 1.8 nm.XRD analysis reveals that the peak intensity of sol-gel prepared SnO2:W films became weak and even wide, makes the crystallinity of the films worse, as tungsten doping content increased. When tungsten content is large, amorphous particles appeare in the films, makes the roughness decrease. The average roughness of the SnO2:W films is 1.92 nm,1.87 nm,1.82 nm,1.61 nm respectively, corresponding to the tungsten doping content of 0,1,3,5 at.%.The calculated results showed that the valence bands of tin oxide were composed mainly of O 2p states and the conduction bands consisted mainly of Sn 5 s states for undoped films. However, the bottom of conduction band is changed with W doping. The bottom of conduction band of SnO2:W is due to W 5d states. W is potential doping elements which result in high conductive SnO2 films.The p-type conductive NiO:Li targets were prepared by solid reaction method. By using these targets, p-type conductive NiO:Li transparent thin films were developed on the glass substrates by PPD. The electrical, optical and structural properties were characterized by four-probe method, Hall system, spectrophotometer and AFM. The conductivity of 15 S·cm-1, with carrier mobility ranging from 0.30 cm2V-1s-1 to 0.42 cm2V-1s-1, carrier concentration from 2.98×1020 cm-3 to 2.1×1020 cm-3 film was obtained, while some literatures report the carrier concentration is about 1018,1019 orders of magnitude or lower.The p-n heterojunction diodes of NiO:Li/SnO2:W were fabricated, based on the experimental results of n-type SnO2:W and p-type NiO:Li thin films. The I-V characteristic curves of p-n junctions were analyzed through investigating on the conductivity of each single layer. The rectifying current-voltage characteristics were obtained. The whole diode shows an average visible transmission of about 40%.更多还原