摘要
氧化镉(CdO)是一种非常重要的透明导电氧化物(TCO)材料。但CdO光学带隙较小，可见光透过率较低，这些不足严重限制了其应用范围。利用其它元素掺杂能有效地拓宽其光学带隙，并改善其导电和透光性能。然而目前对掺杂CdO薄膜的研究，还存在着基本性能争议较大、能带特性认识不清楚、光电耦合机制研究较缺乏等问题。
针对上述问题，本文采用过滤阴极真空电弧(FCAD)制备了高质量的铟掺杂氧化镉(CdO:In)薄膜：(1)系统地研究了In掺杂浓度、衬底温度、氧气分压、衬底材料等与其成分、结构和光电等性能之间的内在联系；(2)利用不同气氛热处理改善CdO:In薄膜的光电性能；(3)利用不同理论研究了其能带特性和光电相互耦合机制等。
FCAD制备的CdO:In薄膜呈现多晶立方CdO结构，无In2O3和CdIn2O4等相出现。CdO:In薄膜的晶体结构与其生长条件密切相关：(1) In掺杂浓度和氧气分压的变化导致薄膜择优取向改变；(2)升高衬底温度不仅能促进晶粒长大，同时还能改善晶粒排列有序度；(3)在玻璃衬底上，氧化镁(MgO)过渡层为CdO:In薄膜生长提供外延模板，使其出现多个生长方向；在蓝宝石衬底上，衬底中Al扩散到MgO层中形成MgAl2O4结构，有助于提高CdO:In薄膜的结晶质量。CdO:In薄膜中Cd和In原子主要以氧化态形式存在，存在一定量的氧空位。
CdO:In薄膜具有较优异的光电性能。CdO:In薄膜的导电性能与其制备工艺条件密切相关：(1)随着In掺杂浓度的增加，薄膜电阻率先降低后增加，而电子迁移率先增加后降低；(2)氧气分压的增加使得氧空位减少，因此载流子浓度降低，电子迁移率增加；(3)升高衬底温度使得晶界散射减弱，电子迁移率增加；(4)在玻璃衬底上，MgO薄层能有效地降低CdO:In薄膜的电阻率并提高迁移率；在蓝宝石衬底上，MgO层使得CdO:In薄膜电阻率增大而电子迁移率降低。CdO:In薄膜具有较高的光学透过率和较宽的透光波段范围。230nm厚的CdO:In薄膜在500~1250nm波段的平均透过率超过80%。利用透过率衍生法可以较准确地获得CdO:In薄膜的光学带隙，其值为2.5~3.1eV。采用FCAD技术，在230℃、7mTorr和1.2at.%In掺杂条件下制备的CdO:In薄膜具有最佳的光电性能。
退火处理有利于改善薄膜的导电性能。退火后，未掺杂CdO薄膜晶粒尺寸变大，光学透过率显著提高；而CdO:In薄膜晶格长度萎缩，吸收截止限蓝移。经空气退火后，低掺杂浓度薄膜电阻率降低；高掺杂浓度薄膜电阻率增加。经氮气退火后，CdO:In薄膜电阻率降低而电子迁移率增加。
CdO:In薄膜的能带具有明显的非周期性特征。被广泛应用于获取TCO薄膜光学带隙的Tauc法建立在理想周期性能带假设之上。对于CdO:In薄膜，Tauc法高估了其光学带隙。CdO:In薄膜的能带填充效应约为0.5~1.2eV，而能带变窄效应约为0.1~0.3eV。微扰理论能较好地描述CdO:In薄膜导带色散以及费米能级；适当随机相位理论可较好地计算CdO:In薄膜的能带变窄效应；使用费米能级的电子有效质量计算CdO:In非周期性费米能级获得的结果偏低。
FCAD制备的CdO:In薄膜具有较优异的导电性和透光性，且透光范围与太阳辐射光谱匹配较好，因此适合用作高效多结太阳能电池等新型光电器件的表面电极和窗口材料。
Cadmium oxide (CdO) is an important transparent conductive oxide (TCO)material. However, the optical bandgap of undoped CdO is relatively small inducingthe narrow transparent range, adding with the relatively low visible transparency,which strictly limits its wide applications as a TCO material. Doping cansignificantly widen the optical bandgap, and can also improve the conductivity andthe transparency effectively. Nevertheless, research on doped CdO films isinsufficient. Much controversy on its fundamental properties exists and systematicstudy on its optical and electrical interactions as well as conduction band dispersionsis seriously lack.
To solve the aforementioned problems in CdO films, high quality indium dopedcadmium oxide (CdO:In) films were deposited by filtered cathodic arc deposition(FCAD) in this study.(1) The effects of indium doping concentration, substratetemperature, oxygen partial pressure, and substrate material on film structural,electrical, and optical properties were systematically studied.(2) Subsequently, theimprovements of electrical and optical properties of CdO:In films via annealing indifferent atmospheres were also investigated.(3) Finally, the electrical and opticalinteractions, as well as the conduction band dispersions, were studied by usingdifferent theoretical models.
It is observed that all of the CdO:In films are phase-pure with featuresassignable to cubic CdO. No In2O3, CdIn2O4or other phases is observed. Thecrystalline structure of CdO:In films is dependent on the growth parameters:(1) Thevariation of In concentration and oxygen pressure induces the change of thepreferred growth orientations in CdO:In films.(2) Increasing substrate temperaturecan not only increase the grain size, but also can improve the alignment of grains.(3)The MgO buffer layers on glass substrates provide the template for CdO growth,which induces the diverse growth orientations of CdO. On sapphire substrates, theAl atoms in the substrates diffuse into the MgO buffer layers and new MgAl2O4structure forms, resulting in the improvement of crystal quality of CdO:In films. TheCd and In atoms in the films are oxidized and a small content of oxygen vacanciesexists in the CdO:In films.
CdO:In films show excellent electrical conductivity and optical transparency.The conductivity is closely related to the growth parameters:(1) With increasing Inconcentration, the resistivity decreases at first and then increases slightly, and theelectron mobility increases at first and subsequently decreases.(2) Increasing oxygen pressure reduces the content of oxygen vacancies in the films, resulting inthe decrease of carrier concentration, which leads to the increase of electronmobility.(3) As the substrate temperature increases, the grain boundary scatteringweakens, which results in the increase of electron mobility.(4) The MgO bufferlayers on glass substrates reduce the resistivity of CdO:In films and improve themobility effectively. On the contrary, on sapphire substrates, the MgO buffer layersincrease the resistivity and cause the decrease of electron mobility. CdO:In filmsexhibit excellent optical transmittance and wide transparent wavelength range. Meantransmittance of230nm thick CdO:In films is over80%in500~1250nmwavelength range. By using the derivative of transmittance, the optical bandgaps ofCdO:In films are about2.5eV~3.1eV. It has been observed that optimized CdO:Infilms can be obtained at230℃and7mTorr with1.2at.%In doping by filteredcathodic arc deposition.
The electrical properties of CdO:In films can be improved significantly byannealing. After annealing, the grains of undoped films grow up and its opticaltransmittance improves. For CdO:In films, the crystal length of films decreases andthe absorption edge blue shifts. After annealing in air, the resistivity in the lowdoping level films decreases, while in high doping level films it increases. Whenannealing in nitrogen, the resistivity of CdO:In films decreases and the Hall mobilityincreases.
The valence and conduction bands of CdO:In show evident nonparabolicity.The extensively misused Tauc relation to obtain optical bandgap is based onassuming parabolic valence and conduction bands. For CdO:In films, of whichobvious nonparabolic valence and conduction bands are already observed, the Taucrelation overestimates the optical bandgaps. The band filling effect in these CdO:Infilms is about0.5~1.2eV, while the bandgap renormalization is about0.1~0.3eV.The dispersion of conduction bands and the Fermi energy in CdO:In films calculatedby using Kane’s two bands perturbation theory agree well with the experimentalresults. Appropriate random phase theory can describe the bandgap renormalizationeffectively in these arc-grown CdO:In films. Simply using the effective mass at theFermi level to calculate the nonparabolic Fermi energy would underestimate theresults for CdO:In films.
These indium doped cadmium oxide films deposited by filtered cathodic arcdeposition show excellent electrical conductivity and optical transparency. It isimportant to note that the transparent wavelength region of these indium dopedcadmium oxide films agrees well with the solar spectra. Therefore, these highquality indium doped cadmium oxide films are potentially suitable for high effeciency multijunction solar cells and other photovoltaic devices.
引文
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