【作者】 白丽娜；

【导师】 连建设；

【作者基本信息】 吉林大学 ， 材料学， 2014， 博士

【摘要】 透明导电氧化物薄膜(TCO)由于其优异的性能，成为科研和工业上的重点研究课题。TCO薄膜具有较低的电阻系数和高的可见光透明度，已经广泛应用于光电子器件之中(例如：太阳能电池，触摸屏，传感器等)。In2O3和ZnO是典型的n型TCO材料。但是，本征材料性能的单一性已经成为了限制材料扩展应用的主要问题，因此急需寻求新的替代材料。目前，掺杂改性是扩展本征材料性能的主要手段之一。其中，Sn掺杂In2O3(ITO)薄膜具有很高的载流子浓度同时保持高的可见光透明度，从而成为应用较为广泛的TCO薄膜材料。因此，理论上研究In2O3和ZnO基半导体氧化物的掺杂改性具有重要的意义。综上所述，本论文从三个基本研究内容出发：一、如何通过调整本征缺陷的含量进一步提高ZnO的光学和催化性能；二、对ZnO进行掺杂改性，研究Cd、Mg、Ca、Y等元素掺杂对体系电子结构和光学性质的影响；三、理论上研究如何进一步提高In2O3的光电特性。本文以理论研究为主，并针对掺杂和碳化ZnO进行必要的实验研究。理论部分采用基于密度泛函理论超软赝势方法的CASTEP模块研究材料的微观电子结构和光学性质，交换关联泛函选用广义梯度近似(GGA)和局域密度近似(LDA)，并在计算中引入位库伦耦合参数U调节计算带隙。本文的实验部分利用X射线衍射(XRD)，X射线光电子能谱(XPS)，拉曼光谱(Raman)和透射电子显微镜(TEM)表征材料的结构性质；利用紫外-可见光分光光度计，光致发光对材料的光学性能进行了检测；材料的光催化活性是通过降解20mg/L的亚甲基蓝(MB)溶液检测的。本论文主要研究成果可分如下四部分内容：1.核壳嵌套结构的非晶碳包覆ZnO(碳化氧化锌)纳米晶先驱体是通过溶胶-凝胶方法合成的。高缺陷的ZnO纳米晶体是通过退火碳化氧化锌先驱体顺利制备的。HRTEM,Raman和XPS分析表明ZnO纳米晶体具有高含量的本征缺陷。大量缺陷的存在导致了以绿光散射为中心，波长范围在420nm~660nm之间强烈的深能级(DL)散射，以及一个相对较弱的紫外可见光(UV)散射。ZnO纳米晶体同样在降解MB溶液时表现出明显的光催化活性的提高。实验和基于密度泛函理论的模拟计算显示出增进的DL散射和改进的光催化活性，将归结于通过两次制备在ZnO纳米晶体中引入的高含量的氧缺乏导致的本征缺陷的存在。2. ZnCdO薄膜是利用脉冲激光沉积法在石英玻璃基底上制备的。通过调节Cd元素的掺杂含量，可以改变Zn1-xCdxO薄膜的禁带宽度。同时，Zn1-xCdxO薄膜的电子结构和光学性质是通过密度泛函理论广义梯度近似考虑位库伦耦合效应研究的，考虑位库伦耦合参数可以更加准确地预测ZnO和Zn1-xCdxO薄膜的带隙值及其带隙变化规律。例如，当x≤0.5时，带隙可以在3.219eV~2.197eV范围内调整，光致发光区间将从紫外光向黄绿光偏移。实验和理论研究表明随着Cd掺杂含量的增加，它的可见光吸收范围是增加的。可调节的光致发光和可见光吸收都表明了ZnCdO薄膜是潜在的光电子应用材料。3.纤锌矿结构ZnXO(X=Mg,Ca,Y)三元合金的电子结构和光学性质是通过第一性原理密度泛函理论考虑位库伦耦合效应计算的。三元合金纤锌矿结构的稳定性是通过晶格常数和内坐标参数的变化趋势分析的。首先，我们计算了纤锌矿ZnMgO三元合金的电子结构。我们发现ZnMgO三元合金的带隙随着掺杂Mg原子摩尔浓度的增加而增大。计算的光学吸收谱，在吸收系数和蓝移趋势的变化上很好的与实验值相匹配。其次，我们计算了纤锌矿ZnCaO三元合金的电子结构。通过比较三种不同Ca掺杂位置的形成能，确定了Ca原子最稳定的掺杂位置是取代晶格中金属Zn原子的晶格位(CaZn)。纤锌矿结构ZnCaO三元合金的带隙随着掺杂Ca原子摩尔浓度的增加而增大。最后，我们计算了纤锌矿ZnYO三元合金的电子结构。当晶格中的金属Zn原子被Y原子取代时，杂质能级为浅施主能级，Y掺杂ZnO可以在保持材料光学透明度的同时提高材料的电导。上述计算结果表明纤锌矿结构ZnXO(X=Mg,Ca,Y)三元合金可以调节本征ZnO的光学性质，使其作为光电材料应用。4. Sn或者Y掺杂In2O3的结构性质和电子结构是通过第一性原理计算的，掺杂改进了材料的光电性能。我们设计了Sn原子在In2O3中可能掺杂位的构型，计算结果表明Sn原子优先取代b-位的In原子。通过比较不同掺杂构型的形成能，我们发现在实验中Sn原子进入晶格间隙位是可能的，本征缺陷氧空位的存在可以导致Sn原子间隙位掺杂。但是，计算结果表明当Y原子取代In原子时，优先取代位是d-位，并且将导致光学带隙的蓝移。Y原子的间隙位掺杂是三重施主掺杂具有低的形成能和缺陷跃迁能，在实验制备中是可能存在的。对于复合缺陷构型的形成能分析表明，本征缺陷氧空位和Y原子间隙位掺杂的复合缺陷在制备过程中是难以形成的。计算结果表明InXO(X=Sn,Y)三元合金可以调节本征In2O3的光学性质，使三元合金可以作为光电材料应用。更多还原

【Abstract】 Transparent conductive oxides thin films (TCO) due to the excellent performance, havebecome the key research projects of academe and industry. Through the low resistivities andhigh optical transparency in visible region, they have been widely used for optoelectronicdevices such as solar cells, flat panel displays, or light emitting diodes. In2O3and ZnObelong to the prototypical n-type TCO. Nevertheless, problems of limited properties havebecome an important issue to expand technological applications, hence the urgent need toseek new materials. In fabrication methodology, doping has become a new trend as itselectronic and optical properties can be greatly improved. The current industry standardn-type In2O3:Sn (ITO) can support very high carrier concentration and still maintain highoptical transparency. Thus, theoretical study of In2O3and ZnO-based semiconductor oxidesis still an essential work.To sum up, this thesis has three research purposes: Firstly, how to adjust the optical andcatalytic properties by adjusting concentration of the intrinsic defects of ZnO; Secondly,doping ZnO with Cd, Mg, Ca and Y elements, we try change the electronic and opticalproperties; Finally, how to further improve the electronic and optical properties of In2O3arestudied by using the first-principles. The theoretical calculations are performed on CASTEPcode which is based on density functional theory (DFT) using ultrasoft pseudopotentialsmethod. Exchange and correlation effects are described by the generalized gradientapproximation (GGA) and the local density approximation (LDA). The on-site Coulombinteraction is used, which can improve the GGA (or LDA) to predict the electronic propertiesand band gap. Experimental works are done to verify or support the theoretical results.Examinations with x-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS), Ramanspectroscopy, transmission electron microscope (TEM) are used to investigate the structureproperties; Photoluminescence (PL) spectra and UV-Visible spectrophotometer are recordedto investigate the optical properties; Photocatalytic activities are estimated by thedegradation of20mg/L Methylene Blue (MB) solution.The main results obtained in the thesis are divided into four parts as following: 1. Core-shell nested structure of amorphous carbon coated on crystalline ZnO(carbonized ZnO) is synthesized by sol-gel method as precursor. Disordered ZnOnanoparticles are successfully prepared by annealing the carbonized ZnO precursor. HRTEM,Raman and XPS analyses show that the ZnO nanoparticles contain high concentration ofoxygen-deficiency defects. The ZnO nanoparticles exhibit an abnormal intense deep levelemission centered at green luminescence and spreading over a wide wave length range of420nm~660nm, and a negligible UV emission in its photo luminescence spectrum. Theyalso show an enhanced photocatalytic activity in degradation of MB solution. Experimentsand theoretical simulation based on density functional theory show that the intense deeplevel emission and improved photocatalytic ability should attributed to their abundantoxygen-deficiency defects in the ZnO nanoparticles introduced by the two-step preparationmethod.2. Zn1-xCdxO thin films are deposited on quartz substrate by pulse laser deposition. Byvarying Cd concentration, the band gap of Zn1-xCdxO films can be adjusted in a wide range.Simultaneity, the electronic structure and optical properties of Zn1-xCdxO alloys areinvestigated by the density functional theory with a combined generalized gradientapproximation plus Hubbard U approach, which precisely predicts the band-gaps of ZnO andZn1-xCdxO alloys. For example, the band gap of Zn1-xCdxO alloys can be adjusted in a widerange from3.219eV for ZnO to2.197eV for Zn0.5Cd0.5O, which produces differentemissions from ultraviolet to Kelly light in their photoluminescence spectra. Both theexperimental results and theoretical simulation reveal that with increasing Cd concentrationin Zn1-xCdxO alloys, their absorption coefficients in visible light range are evidentlyenhanced. The adjustable photoluminescence emission and enhanced visible light absorptionendow Zn1-xCdxO alloys potential applications in optoelectronic and photocatalytic fields.3. The electronic and optical properties of wurtzite ZnXO (X=Mg,Ca,Y) ternary alloysare calculated by using first-principles based on the framework of generalized gradientapproximation with introducing the on-site Coulomb interaction. The structures of thesealloys in wurtzite phase are analyzed by examining their lattice constants and internalstructural parameter u. Firstly, the electronic structure of wurtzite ZnMgO ternary alloys iscalculated. The calculated band gap of wurtzite ZnMgO ternary alloys shows a significantexpansion with increasing Mg concentration. The calculated optical absorption spectra alsoshow good agreement with the experimental spectra in their shape and shift trend withvarying Mg concentration. Secondly, the electronic structure of wurtzite ZnCaO ternary alloys is calculated. By comparing the formation energies of three different Ca doping sites,it is found that Ca at Zn site (CaZn) is the most favored doping site. The calculated band gapof wurtzite ZnCaO ternary alloys shows a significant increase with increasing Caconcentration. Finally, the electronic structure of wurtzite ZnYO ternary alloys is calculated.It has been found that replacement of Zn by Y should generate a shallow donor. The Y-dopedZnO would be a material with enhanced mobility and hence improved electrical conductivitywithout sacrificing optical transparency. Therefore, the theoretical results show that thewurtzite ZnXO (X=Mg,Ca,Y) ternary alloys are the potential candidate alloys foroptoelectronic materials.4. In2O3doped with yttrium or stannum shows improved optoelectronic efficiency. Herethe structural properties and electronic structures of Sn or Y-doped In2O3are investigated byfirst-principles calculations. Various doping sites of Sn in In2O3system have been modeled.The energy favorable site of Sn atom in In2O3is found to be on the b-site of In atom. Bycomparing the formation energies of different configuration, it is found that the doping of Snatom into interstitial sites (Sni) is experimentally possible due to the fluctuation in energy,and the intrinsic oxygen vacancies (VO) in In2O3favorite the formation of Sni. However, wefound that substitutional YInof d-site is more stable site, and could lead to the blue-shift ofoptical band gap. The Yiis treble donors with low formation energy and defect transitionenergy, it is hence likely to incorporate during growth. The complex defect configurationsare calculated. By comparing the formation energies of complex defect configurations, it isdetermined that the Yi-VOconfiguration also has high formation energy, and this suggests anunfavorable role during prepared process. Therefore, the theoretical results show that theInXO (X=Sn or Y) ternary alloys are the potential candidate alloys for optoelectronicmaterials.更多还原