【作者】 刘新典；

【导师】 姜恩永；

【作者基本信息】 天津大学 ， 材料物理与化学， 2008， 博士

【摘要】 透明导电氧化物薄膜集对可见光的良好透过率和高的电导率于一身,使得这类材料在太阳电池、液晶及等离子体高清晰显示、智能视窗以及建筑物的节能玻璃窗等诸多领域都获得了广泛的应用。本论文对ITO、ZnO:B和ZnO:Al薄膜的结构、电输运性质和光学性质进行了系统的实验研究,并采用基于密度泛函理论的局域密度近似方法计算了ZnO、ZnO:B和TiO2、TiO2:Nb的电子结构和光学性质。在基片温度T≤100℃时得到的ITO薄膜是具有半导体导电特性的非晶薄膜。基片温度T≥120℃时得到的多晶薄膜在高温( > 100K)时表现为金属导电特性,在较低温度( T < 80K)电阻温度系数变为负值,而薄膜的磁电导均为正值。研究发现,负的电阻温度系数来源于弱局域效应及库仑相互作用对载流子输运的影响,并且,电子-电子散射是样品中主要的非弹性散射过程。在基片温度为150℃下制备了具有金属导电特性的ZnO:B薄膜,通过对样品的磁电阻数据进行分析,发现其中负的部分来源于弱局域效应,而正的部分可用二能带模型很好地描述,并且,低温下载流子的非弹性散射主要是电子-电子散射,而库仑相互作用是样品低温下反常电导率的主要来源。在不同厚度(463.63 nm、203.03 nm和66.85 nm)的ZnO:Al薄膜中观察到的负的磁电阻部分并非来自弱局域效应,而是来源于局域磁矩对传导电子的散射。第一性原理计算显示,ZnO的费米能级位于带隙中,显示出半导体特性,导带底起伏较大,说明其中载流子有效质量较小。ZnO:B的费米能级位于导带,显示了金属导电性,价带顶与费米能级间的能量差大于纯ZnO的带隙,施主能级位于导带,在带隙中未观察到杂质能级,这与实验上得到的ZnO:B的载流子浓度与温度无关相对应。TiO2:Nb的费米能级位于导带,表现出金属特性;价带顶与费米能级间的能量差大于纯TiO2的带隙,这可以解释实验上观察到的当Nb掺入TiO2时的蓝移现象;光学性质计算给出,基本吸收边位于可见光光子能量以上,在高于和低于基本吸收边均有吸收现象,而后者(位于可见光区域)吸收系数比前者的数值低了三个数量级,从而解明了TiO2:Nb薄膜对可见光透明的物理本性。更多还原

【Abstract】 The combination of high optical transparency at the visible wavelengths and high electrical conductivities in transparent conducting oxide films makes the class of materials be widely used in various fields, such as in solar cells, liquid crystal and high definition displays, electrochromic and smart windows, as well as in architectural energy-efficient windows.The structure, electrical transport properties, and optical properties of ITO, B- and Al-doped ZnO films were experimentally studied systematically. The band structures and optical properties of undoped and B-doped ZnO as well as undoped and Nb-doped anatase TiO2 were calculated respectively, using local-density approximation based on the density-functional theory.The amorphous ITO films with semiconductor behaviors were obtained when the substrate temperatures T≤100℃. The polycrystalline films deposited above 120℃( T≥120℃) exhibit metallic characteristics at higher temperatures ( > 100 K), and the temperature coefficients of resistivities become negative below 80 K. The magnetoconductivities of the polycrystalline films are all positive. The negative temperature coefficients of resistivities originate from the weak-localization effect and Coulomb interactions. The electron-electron interaction is the mainly inelastic scattering process in the polycrystalline films.The ZnO:B films deposited at 150℃exhibit metallic characteristics. The magnetoresistances of the films were studied and it is found that the negative components of the magnetoresistances originate from the weak-localization effect and the positive parts can be well described by the double-band model. The electron-electron collisions are the dominant inelastic scattering process. The nontrivial corrections in the temperature dependence of electrical conductivity mainly come from the Coulomb interactions at low temperatures in the films.The negative components of magnetoresistances observed in ZnO:Al films with different thickness do not come from the weak-localization effect, but from the scattering of conduction electrons due to localized magnetic moments.The first-principle calculations were performed with wurtzite ZnO and ZnO:B. For ZnO, the Fermi level locates in the band gap, indicating the semiconductor characteristic. The high dispersion of the bottom of the conduction band in ZnO suggests small effective mass of electron carriers. For ZnO:B, the Fermi level locates in the conduction band, indicating the metallic characteristic. The energy difference between the top of the valence and the Fermi level is larger than the band gap of the pure ZnO. The donor level locates in the conduction band and no impurity band was found in the band gap. This is consistent with the experimental observation that the carrier concentration is independent with temperature.The Fermi level of TiO2:Nb locates in the conduction, showing the metallic characteristic. The energy difference between the top of the valence and the Fermi level is larger than the band gap of the pure TiO2, which may explain the experimentally observed blueshift of absorption edge in anatase TiO2 films with Nb doping. According to the optical properties calculations, the fundamental absorption edge locates above the visible range. Absorption behaviors are observed both above and below the fundamental absorption edge and the absorption coefficient of the former is ~ 3 orders of magnitude weaker than that of the latter, indicating the intrinsic transparency at the visible wavelengths for TiO2:Nb films.更多还原