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TCOs are wide band gap (Eg) semiconducting oxides, with conductivity in the range 102 – 1.2106 (S). The conductivity is due to doping either by oxygen vacancies or by extrinsic dopants. In the absence of doping, these oxides become very good insulators, with > 1010 -cm. Most of the TCOs are n-type semiconductors. The electrical conductivity of n-type TCO thin films depends on the electron density in the conduction band and on their mobility: =ne, where is the electron mobility, n is its density, and e is the electron charge. The mobility is given by: where is the mean time between collisions, and m* is the effective electron mass. However, as n and are negatively correlated, the magnitude of is limited. Due to the large energy gap (Eg > 3 eV) separating the valence band from the conducting band, the conduction band can not be thermally populated at room temperature (kT~0.03 eV, where k is Boltzmann’s constant), hence, stoichiometric crystalline TCOs are good insulators. To explain the TCO characteristics, various population mechanisms and several models describing the electron mobility were proposed. Some characteristics of the mobility and the processes by which the conduction band is populated with electrons were shown to be interconnected by electronic structure studies, e.g., that the mobility is proportional to the magnitude of the band gap. In the case of intrinsic materials, the density of conducting electrons has often been attributed to the presence of unintentionally introduced donor centers, usually identified as metallic interstitials or oxygen vacancies that produced shallow donor or impurity states located close to the conduction band. The excess or donor electrons are thermally ionized at room temperature, and move into the host conduction band. However, experiments have been inconclusive as to which of the possible dopants was the predominant donor. Extrinsic dopants have an important role in populating the conduction band, and some of them have been unintentionally introduce. Thus, it has been conjectured in the case of ZnO that interstitial hydrogen, in the H+ donor state, could be responsible for the presence of carrier electrons. In the case of SnO2, the important role of interstitial Sn in populating the conducting band, in addition to that of oxygen vacancies, was conclusively supported by first-principle calculations of Kiliç and Zunger. They showed that Sn interstitials and O vacancies, which dominated the defect structure of SnO2 due to the multivalence of Sn, explained the natural nonstoichiometry of this material and produced shallow donor levels, turning the material into an intrinsic n-type semiconductor.10 The electrons released by these defects were not compensated because acceptor-like intrinsic defects consisting of Sn voids and O interstitials did not form spontaneously. Furthermore, the released electrons did not make direct optical transitions in the visible range due to the large gap between the Fermi level and the energy level of the first unoccupied states. Thus, SnO2 could have a carrier density with minor effects on its transparency.10 The conductivity is intrinsically limited for two reasons. First, n and cannot be independently increased for practical TCOs with relatively high carrier concentrations. At high conducting electron density, carrier transport is limited primarily by ionized impurity scattering, i.e., the Coulomb interactions between electrons and the dopants. Higher doping concentration reduces carrier mobility to a degree that the conductivity is not increased, and it decreases the optical transmission at the near-infrared edge. With increasing dopant concentration, the resistivity reaches a lower limit, and does not decrease beyond it, whereas the optical window becomes narrower. Bellingham et al.29 were the first to report that the mobility and hence the resistivity of transparent conductive oxides (ITO, SnO2, ZnO) are limited by ionized impurity scattering for carrier concentrations above 1020 cm-3. Ellmer also showed that in ZnO films deposited by various methods, the resistivity and mobility were nearly independent of the deposition method and limited to about 210-4 cm and 50 cm2/Vs, respectively. , In ITO films, the maximum carrier concentration was about 1.51021 cm-3, and the same conductivity and mobility limits also held . This phenomenon is a universal property of other semiconductors. , Scattering by the ionized dopant atoms that are homogeneously distributed in the semiconductor is only one of the possible effects that reduces the mobility. The all recently developed TCO materials, including doped and undoped binary, ternary, and quaternary compounds, also suffer from the same limitations. Only some exceptional samples had a resistivity of 10-4 cm. In addition to the above mentioned effects that limit the conductivity, high dopant concentration could lead to clustering of the dopant ions, which increases significantly the scattering rate, and it could also produce nonparabolicity of the conduction band, which has to be taken into account for degenerately doped semiconductors with filled conduction bands. |
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2楼2011-03-16 16:51:22
3楼2011-03-18 12:15:11
zhengbiju1833(金币+5, 翻译EPI+1): 挺好，你继续翻译吧。都翻译完。金币都归你 2011-03-18 18:52:17
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先翻两段试试，如果LZ觉得好久继续，不好的话就此打住。 TCO（透明导电氧化物）是能带很宽的半导体化合物，电导率为XXX。它们的高电导率是通过氧空穴掺杂或外部掺杂而获得的，如果没有掺杂，它们便是非常好的绝缘体。 因为价带和导带之间能带间隙较宽，室温下不能通过热方法填充，因此TCO是极好的绝缘材料。为了解释TCO的特征，研究者已经提出了多种填充机制和模型以期解释电子迁移率。研究发现，有些迁移率特征和导带的填充过程与电子结构研究无关，如迁移率与能带的级别成比例。 |
4楼2011-03-18 12:37:33
