【作者】 潘露璐；

【导师】 连建设；

【作者基本信息】 吉林大学 ， 材料学， 2014， 博士

【摘要】 氧化镉(CdO)是属于II-VI族的一种直接窄带隙n型半导体氧化物，在常温下具有立方NaCl晶体结构。CdO薄膜本身具有大量的本征点缺陷，如氧空位(Vo)和镉间隙原子(Cdi)等缺陷，它们作为浅施主为CdO提供了大量的载流子，使其具有较低的电阻率(10-3~10-4Ω cm)。CdO的禁带宽度(Eg)为2.2eV，对应的波长为550nm，位于太阳光谱在可见光波段能量最强值510nm附近。因此，CdO以其高载流子浓度、低电阻率、可见光区域高透过率等特点，被广泛的应用于透明电极、太阳能电池、光电晶体管和气敏传感器等领域。然而，CdO的禁带宽度过窄极大地限制了其在光电器件领域的应用。为了与其它TCOs薄膜竞争，在提高CdO的电学性能的同时，通过掺杂其它元素或调解工艺参数增加其禁带宽度来提高薄膜在可见光波段的透过率范围，改善其光学性能。本论文对CdO薄膜的光学、电学和气敏性等性质进行了系统的总结，详细介绍了CdO的各种实际应用和制备方法，同时对CdO薄膜的研究方向进行了展望。采用磁控溅射法和脉冲激光沉积技术制备CdO基薄膜，通过调节化学计量比或掺杂过渡元素、稀土元素(Gd、Ce、In)等途径来制备导电性好、可见光区域透过率高的透明CdO薄膜。在此基础上，将Gd和In掺杂的CdO薄膜沉积在p-Si上制备异质结太阳能薄膜电池并研究其电学性能，探究CdO基薄膜作为太阳能薄膜电池的可能性。使用单束飞秒激光技术在CdO薄膜表面诱导出周期纳米条纹结构，并将该微型结构薄膜制成CdO/p-Si异质结。我们希望这些研究能够拓展CdO纳米材料在光电器件和太阳能薄膜电池等领域的应用。利用X射线衍射仪(XRD)、X射线光电子能谱(XPS)、衍射透射电子显微镜(TEM)、高倍透射电子显微镜(HRTEM)、场发射扫描电子显微镜(FE-SEM)及场发射扫描电子显微镜配套的能谱仪(EDS)对薄膜的成分和微观组织进行了分析和观察；利用紫外-可见光分光光度计(UV-VIS)、霍尔效应仪(Hall)对薄膜的光学和电学性能进行了表征；采用电流-电压(I-V)伏安特性曲线对异质结的电学性能进行了表征。具体的实验内容和结果如下：1.利用脉冲激光沉积法(PLD)在普通玻璃衬底上制备未掺杂CdO薄膜，氧气气氛压强为10Pa，衬底温度为室温(25℃)，靶材为纯度99.99%CdO粉末制备的陶瓷靶。研究了不同脉冲激光能量密度(10~25J/cm2)下获得的CdO薄膜的晶体结构、光学和电学性能影响。结果表明所有薄膜为多晶CdO立方晶体结构，沿着[200]方向高度择优生长。随着激光能量密度的下降，薄膜的平均晶粒尺寸变化较大，由108.3nm变为21.1nm。晶粒尺寸的不断减小使得薄膜应力增加并导致薄膜表面Cd和O原子扩散速率改变，薄膜内以氧空位(Vo)为主的缺陷逐渐增多，而这些缺陷能级使得本征CdO薄膜的自由电子浓度增加。拥有最小晶粒尺寸的CdO薄膜在可见光区域内具有高的光学透过率(~92%)，明显变宽的禁带宽度(3.33eV)，高的载流子浓度(1.25×1021cm-3)和低的电阻率(2.8×10-4cm)。使得该实验方法和工艺条件下制备的晶粒尺寸较小的CdO薄膜在太阳能电池、透明薄膜电极和其它光电子设备具有广泛的应用前景。2.通过射频磁控溅射法(RFMS)在普通玻璃衬底上沉积了铈(Ce)和钆(Gd)共掺杂氧化镉透明导电薄膜。气体气氛总压强为10Pa，其中氩气(Ar)气体流量为40sccm，氧气(O2)气体流量为10sccm，衬底温度为400℃，靶材为纯度99.99%的CdO、CeO2和Gd2O3粉末充分混合制备的陶瓷靶。研究了不同Ce和Gd含量对CdO薄膜晶体结构、光学和电学性能的影响。Ce和Gd的掺杂促进所有薄膜沿[200]方向的择优生长，(200)晶面衍射角(θ)逐渐变小，晶格常数(a)逐渐变大。0.4at.%Ce和0.8at.%Gd掺杂的薄膜具有最佳的光电学性能，在可见光区域内具有高的光学透过率(~85%)，光学禁带宽度增大为2.99eV，电阻率达到最小值3.3×10-4Ω cm。当Ce和Gd的掺杂含量进一步增加到0.5at.%和1.0at.%时，晶界散射和电离杂质散射作用的增强导致薄膜的光学和电学性能下降。因而，适当的Ce和Gd掺杂能够改善CdO薄膜的光学和电学性能，使其在透明导电材料方面具有重要的应用价值。3.通过脉冲激光法(PLD)在石英玻璃基底上沉积了钆(Gd)和铟(In)共掺杂氧化镉透明导电薄膜。氧气气氛压强为10Pa，衬底温度为200℃，靶材为纯度99.99%的CdO、In2O3和Gd2O3粉末充分混合制备的陶瓷靶。XRD分析证实所有薄膜具有立方NaCl晶体结构，具有高度(200)择优取向。而XPS分析表明In的掺杂导致了In2O3相的出现。随着Gd和In的掺杂含量的不断增加，薄膜的晶粒尺寸由40nm逐渐减小为23nm。所有薄膜在可见光范围内都有很高的透过率(~85%)。尤其是2at.%Gd和4at.%In掺杂的CdO薄膜在可见光波段的透过率高达92%，其相应的禁带宽度为3.56eV。在相同实验条件下，将这些薄膜沉积在p-Si衬底上制得p-n异质结。样品的I-V特性曲线表明实验制得的异质结具有二极管特性，而且在光照条件下表现出了明显的光伏效应。3at.%Gd和6at.%In掺杂的CdO薄膜表现出最佳的光伏特性。其光电转换效率达到了7.5%，填充因子为63%，饱和电流密度(Jsc)和开路电压(Voc)分别为11.4mA/cm2和1.04V。实验结果表明掺杂Gd和In的CdO薄膜在太阳能薄膜电池方面具备潜在应用价值。4.利用脉冲激光沉积法(PLD)在普通玻璃衬底上制备未掺杂CdO薄膜，氧气气氛压强为10Pa，衬底温度为25℃。将制得的薄膜在马弗炉里进行退火处理，退火温度和退火时间分别为200℃和30min。使用单光束飞秒激光脉冲在CdO薄膜表面上采用不同的脉冲重复频率进行周期纳米条纹结构诱导。输出激光脉冲的中心波长为800nm，脉冲宽度为100fs，重复频率为1~2500Hz可调，单脉冲能量为2mJ。由FE-SEM观察可知，经过2000个脉冲数照射后，CdO薄膜表面出现短周期条纹微型结构，条纹周期为150~170nm。当脉冲数大于2000时，CdO表面开始出现蓬松的纳米泡沫的微型结构。配套积分球的UV-VIS测试表明具有飞秒激光微型结构的CdO薄膜具有宽频高吸收的特性。而Hall效应测试表明，当脉冲数为2000时，CdO薄膜获得最高的载流子浓度为1.9×1021cm-3和最低电阻率为1.3×10-5Ωcm。在此基础上，利用飞秒激光微型结构薄膜来制备的CdO/p-Si异质结，在暗态和光照两种环境下均表现出良好的整流特性。光照条件I-V特性曲线表明异质结具有光伏特性，测得最佳的太阳能电池开路电压Voc为0.55V，短路电流Jsc为6.8mA/cm2，填充因子为29.1%，转换效率为1.1%。飞秒激光微型结构薄膜的宽频高吸收特性使其在很多的光电器件领域，如光伏器件、电荷藕合器件和光探测器等具有潜在的应用前景。更多还原

【Abstract】 Cadmium Oxide (CdO) is n type and II-VI semiconductor oxide with a directand narrow band gap. At room temperature, it has a cubic NaCl crystal structure. CdOfilm has a lot of intrinsic point defects, such as oxygen vacancies (Vo) and interstitialcadmium atoms (Cdi), they act as shallow donors and provide a lot of carriers, theymake the undoped CdO film has lower resistivity (10-3~10-4Ω cm). CdO has a bandgap of about2.2eV, and it is near the maximum energy of solar spectrum in thevisible spectrum. Therefore, CdO has many unique properties, such as high carrierconcentration, low resisitivity, and high transmittance in visible range and it is widelyused as a transparent conducting electrode, solar cells, phototransistor and gas sensor,etc. However, the optical band gap of CdO is relatively small to strictly limit theadhibition of photoelectric devices. In order to compete with other TCOs, on the basisof improving the conductivity, we increase the transmittance in visible range and theoptical band gap by doping with other elements or adjusting the technologicalparameters.This thesis systematically sums up the optical and electrical properties and gassensitivities of CdO thin film, and introduces the various application developmentsand preparation methods in detail, and provides an outlook of research directions. TheCdO-based films were prepared by the radio frequency magnetron sputtering (RFMS)and pulsed laser deposition (PLD) methods. The good electrical conductivity and hightransmittance in visible range of CdO thin films can be obtained by adjusting theproportion of intrinsic point defects or doping transition elements and rare earthelements, such as gadolinium (Gd), cerium (Ce), and indium (In). In addition, theCdO thin films prepared with Gd and In were deposited on p-type silicon (p-Si)substrates, the p-n heterojunctions were formed. And the probability of the CdO thin film acting as the solar thin-flim cells was investigated. The periodic nano stripestructure on the surface of CdO thin film was induced by the single-beamfemtosecond laser technology, and this film was used to prepare the CdO/p-Siheterojunction diode. We hope these studies can extend the applications inoptoelectronic devices and solar cells for the CdO nano materials. The structure,composition and microstructure of CdO-based thin films were characterized byX-radial Diffractometer (XRD), X-ray photoelectron emission microscope (XPS),transmission electron microscopy (TEM), High Resolution Transmission ElectronMicroscopy (HRTEM), field emission scanning electron microscope (FE-SEM) andEnergy Disperse Spectroscopy (EDS), the optical and electrical properties werecharacterized by Ultraviolet-visible (UV-VIS)Absorption Spectroscopy and HallEffect measurement, and the electrical and photovoltaic propersities of heterojunctiondiodes were characterized by the Current-voltage (I-V) characteristic curves. Themain research contents and results were summarized as follows:1. The undoped cadmium oxide (CdO) thin films were deposited on glasssubstrates by PLD method with different laser energy density (10~25J/cm2) at a fixedoxygen pressure of10Pa. The experiment temperature was25℃. The targes weremade with the CdO powder, and its purity is99.99%. The microstructures, opticaland electrical performances of the CdO thin films were studied. Results show that allthe films were polycrystalline with a cubic NaCl crystal structure, and they highlygrew along [200] preferred orientation. With the decrease in the laser energy density,the average grain size significantly decreases from108.3nm to21.1nm. The decreasein the grain size leads to the interior stress increasing and the diffusion rates of Cd andO atom changing, and the oxygen vacancies (Vo) increase in the films. These defectlevels can increase the concentrations of free electron. The film with smallest grainsize has high optical transmittance (~92%) in the visible region, the significantlywidened band gap (3.33eV), and the highest carrier concentration (1.25×1021cm-3)and lowest resistivity (2.8×10-4cm). The results show that the CdO film withsmaller grain size has extensive application prospects in solar cells, transparentconducting electrode and other optoelectronic equipment. 2. The cerium (Ce) and gadolinium (Gd) co-doped cadmium oxide thin filmswith different Ce and Gd concentrations were deposited on glass substrates by RFMSmethod at400°C. The total pressure was10Pa, and the gas flow of argon (Ar) andoxygen (O2) was40sccm and10sccm, respectively. The ceramic target material wasprepared by the powder mixture of CdO, CeO2and Gd2O3, the purity of all the poweris99.99%. The microstructures, optical and electrical properties of the CdO filmsprepared were studied at different Ce and Gd concentrations. The doping of Ce andGd can promote the growth of the films along [200] preferred orientation, the (200)diffraction angle slightly shifts towards lower values and its lattice constants increaseas the Cd content increasing. When the concentrations of Ce and Gd reach to0.4at%and0.8at.%, respectively, the as-doposited CdO film has the best optical andelectrical properties. It is highly transparent (~85%) in the visible region, the opticalband gap was2.99eV and lowest resistivity was3.3×10-4Ωcm. However, with afurther increase of Ce and Gd concentrations to0.5at.%and1.0at.%, respectively,the enhanced grain boundary scattering and ionization impurity scattering effects canlead to the degradation of these optical and electrical performances. Therefore, theappropriate doping of Ce and Gd can evidently improve the optical and electricalproperties of CdO, and endow that the as-deposited CdO thin films have importantapplications in transparent conducting materials.3. The gadolinium (Gd) and indium (In) co-doped cadmium oxide thin filmswith different Gd and In concentrations were deposited on quartz glass substrates byPLD method at200°C. The fixed oxygen pressure was10Pa. The ceramic targetmaterial was prepared by the powder mixture of CdO, In2O3and Gd2O3, the purity ofall power is99.99%. XRD analysis confirms that all the films were cubic NaClstructure, and all the films grew along (200) preferred orientation, and XPS analysisindicates that the doping of In induces the formation of In2O3. With the increase inconcentrations of Gd and In, the grain size gradually decreases from40nm to23nm.All the films have high transmittance (~85%) in visible range. When theconcentrations of Gd and In reach to2at%and4at.%, respectively, the as-depositedCdO thin film shows high transmittance (92%) in visible range and widened band gap (3.56eV). Under the same experimental conditions, these thin films weredeposited on p-Si substrates, and the CdO/p-Si thin film heterojunctions were formed.The Current-Voltage (I-V) characteristic curves show that all the heterojunctions werediodes and have obvious photovoltaic effects in illumination. The CdO thin filmdoped with3at.%Gd and6at.%In shows best photovoltaic properties. Thephotoelectric conversion efficiency reaches7.5%, the fill factor is63%, the Amaximum saturation current density (Jsc) and open circuit voltage (Voc) are11.4mA/cm2and1.04V, respectively. These results show that the CdO film doped withGd and In has potential applications in solar thin-film cells.4. The undoped cadmium oxide thin films were deposited on glass substrates byPLD method at25℃with the laser energy density of10J/cm2. The fixed oxygenpressure was10Pa. The prepared films were put in the muffle furnace for annealingtreatment at200℃for30min. The treated CdO films were ablated by thesingle-beam femtosecond laser with different pulse repetition frequency to induce theperiodic nanometer stripe structure. The central wavelength of exported laser pulsewas800nm, the pulse width was100fs, and the adjustable repetition frequency was1~2500Hz, single pulse energy was2mJ. The FE-SEM images show that the CdOfilm was irradiated by2000pulses, its surface appears short periodic stripemicrostructure, stripe cycle is about150~170nm. When the pulse numbers weregreater than2000pluses, fluffy nano foam microstructure can be seen on the surfaceof CdO thin films. UV-VIS measurement with an integrating sphere shows that theseCdO films have the broadband and high absorption properties. And Hall Effect testsshow that the CdO thin films ablated over2000pulses have the highest carrierconcentration of1.9×1021cm-3and the lowest resistivity was1.3×10-5Ω cm. On thisbasis, the CdO film with femtosecond laser micro structure was uses to prepare theCdO/p-Si heterojunction. The Current-Voltage (I-V) characteristic curves show goodrectification characteristics under dark and in illumination, respectively. The I-Vcharacteristic curves in illumination show photovoltaic characteristics, the maximumopen circuit voltage (Voc) is0.55V, the short circuit current (Jsc) is6.8mA/cm2. Thefill factor is29.1%, and the conversion efficiency is1.1%. The broadband and high absorption properties for the femtosecond laser micro structure make these thin filmshave potential application prospects in many optoelectronic devices, such asphotovoltaic devices, charge-couple device and optical detector.更多还原