Role of Surfaces and Interfaces for the Electronic Properties of Conducting Oxides
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Role of Surfaces and Interfaces for the Electronic Properties of Conducting Oxides Andreas Klein Institute of Materials Science, Darmstadt University of Technology Petersenstrasse 23, 64287 Darmstadt, Germany ABSTRACT Transparent conductive oxides (TCOs) are generally considered as degenerate semiconductors doped intrinsically by oxygen vacancies and by intentionally added dopants. For some applications a high work function is required in addition to high conductivity and it is desired to tune both properties independently. To increase the work function, the distance between the Fermi energy and the vacuum level must increase, which can be realized either by electronic surface dipoles or by space charge layers. Photoelectron spectroscopy data of in-situ prepared samples clearly show that highly doped TCOs can show surface band bending of the order of 1 eV. It is further shown that the band alignment at heterointerfaces between TCOs and other materials, which are crucial for many devices, are also affected by such band bending. The origin of the band bending, which seems to be general to all TCOs, depends on TCO thin film and surface processing conditions. The implication of surface band bending on the electronic properties of thin films and interfaces are discussed.
INTRODUCTION Transparent conductive oxides (TCOs) as indium oxide (In2O3), tin dioxide (SnO2) and zinc oxide (ZnO) are widely used in electronic and optoelectronic circuits (see e.g. Ref. [1]). It is accepted for these materials that the Fermi level lies in the conduction band for high doping levels [1], which is proven by the high electron concentration of 1019-1021 cm-3 and particularly by the observation of the Burstein-Moss shift of the optical absorption edge [1,2]. Doping is achieved either by a high concentration of oxygen vacancies or by substitution of cations like in Al doped ZnO (ZnO:Al) or Sn doped In2O3 (ITO) or anions like in F doped SnO2 (SnO2:F). In general semiconductors might exhibit a band bending at the surface because of electrically charged surface states from broken chemical bonds at the surfaces (see e.g. Ref. [3]). The surface charge is balanced by charged dopants in the space charge layer. The magnitude of band bending and width of the space charge layer as well as the associated surface charge depend on the doping level of the semiconductor. In most devices the carrier density of the TCO films is optimized to a maximum level which still allows for good optical transparency. Because of this very high doping levels of TCO films the surface and interface behaviour of the TCO films is mostly described in terms of metallic properties, where no band bending can occur. On the other hand some of the applications of TCOs like varistors and gas sensors are based on electronic barriers at grain boundaries or surfaces. In these cases, however, nominally undoped materials are used where the presence of surface band bending is easily understood. Increasing the electron concentration of the samples is accompanied by rising the Fermi level in
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