Electronic Surface Properties of Transparent Conducting Oxides: An Ab Initio Study
We investigate the surface properties of the transparent conducting oxides In2O3, SnO2, and ZnO using density functional theory and quasiparticle calculations based on many-body perturbation theory. We employ the repeated-slab supercell method. An energy
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Abstract We investigate the surface properties of the transparent conducting oxides In2 O3 , SnO2 , and ZnO using density functional theory and quasiparticle calculations based on many-body perturbation theory. We employ the repeated-slab supercell method. An energy alignment of valence and conduction states via the electrostatic potential is applied to determine ionization energies and electron affinities for various surface orientations and terminations of the oxides. In addition, surface energies for different orientations of bixbyite In2 O3 are calculated. We find a strong influence of surface orientation and preparation techniques on these fundamental quantities.
1 Introduction Transparent conducting oxides (TCOs) like In2 O3 , SnO2 , and ZnO are routinely used as transparent electrodes in photovoltaic and optoelectric devices [1] as well as in transparent electronics based on doped oxides [2, 3]. They are transparent in almost the entire range of the solar spectrum and usually exhibit a high electron conductivity [4–6]. They are also used in silicon (Si) photonics and Si-based solar cells [7]. Electronic properties of their surfaces like ionization energy and electron affinity are frequently used to predict natural band discontinuities at the interfaces with other materials such as Si [8, 9]. The existence of surface or interface states within the fundamental gap can lead to electron-hole recombination and limit the efficiency of the device. Consequently, these parameters are of great interest, but due to sample preparation problems, rather poorly known. Modern theoretical approaches can help to address these questions.
B. H¨offling () European Theoretical Spectroscopy Facility (ETSF) and Institut f¨ur Festk¨orpertheorie und -optik, Friedrich-Schiller-Universit¨at Jena, Max-Wien-Platz 1, 07743 Jena, Germany e-mail: [email protected] W.E. Nagel et al. (eds.), High Performance Computing in Science and Engineering ’12, DOI 10.1007/978-3-642-33374-3 11, © Springer-Verlag Berlin Heidelberg 2013
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B. H¨offling and F. Bechstedt
Density functional theory (DFT) is known to underestimate the fundamental band-gap of semiconductors, therefore many-body effects have to be taken into account correctly to describe the electronic properties of oxides [10–13]. We use modern quasiparticle (QP) calculations based on many-body perturbation theory [12, 14] to predict the electronic bulk properties of the body-centered cubic (bcc) bixbyite as well as the rhombohedral (rh) geometry of In2 O3 , the most favored rutile (rt) geometry of SnO2 , and wurtzite (wz) structure ZnO. We combine the results with DFT ground-state calculations to obtain surface energies, ionization potentials and electron affinities for various surface orientations and terminations of the TCOs. Due to the large cell size required for surface calculations and the high computational cost of quasiparticle methods, massively parallel machines are required to perform the calculations. The underlying theoretical and computational methods are
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