Dopant and Defect Induced Electronic States at In2O3 Surfaces

Abstract

This article reviews the impact of defects and doping on the surface electronic structure of indium oxide, a wide gap oxide semiconductor which under degenerate doping becomes an important transparent conducting material. Topics covered include recent evidence for carrier accumulation at In 2 O 3 surfaces when there is a low bulk doping level and the profound effects of doping on core and valence photoemission spectra. The importance of molecular beam epitaxy in growth of single crystal samples is emphasised. IntroductionTransparent conducting oxides (TCOs) combine the usually orthogonal properties of optical transparency in the visible region with a high electrical conductivity [1][2][3][4]. TCOs already find widespread application as the window electrode in display devices including liquid crystal displays and electroluminescent display devices. A TCO electrode is also an essential component in many designs of solar cell [5,6]. In addition there is a growing interest in "transparent electronics" where TCO materials are employed in a more active role, for example in light emitting diodes [7] or lasers operating in the ultraviolet regime. The TCO market is worth around $1.6 billion per annum, with upward of 80 % of this sum based on LCD displays. Indium oxide (In 2 O 3 ) is amenable to n-type doping with Sn to give a prototype TCO usually-but somewhat misleadingly-referred to as indium tin oxide or ITO. At present ITO dominates the TCO market as the material offers a better combination of conductivity and transparency than alternatives such as doped ZnO or SnO 2 . In principle, doped CdO could achieve better performance [8], but Cd is highly toxic. Despite some recent reports of respiratory disease and even deaths [9] among workers in the ITO industry, indium is generally regarded as relatively

Well-Ordered In Adatoms at the In2O3(111) Surface Created by Fe Deposition

“…This is in accordance with observations made on another non-transition metal oxide (SnO 2 , see [38]), where only surface configurations that lead to a Sn 2+ oxidation state (i.e., a lone pair 5s 2 configurations resembling SnO) were found to be stable. The similarity of the gap state of these two systems has already been pointed out by Egdell [39].…”

Surface point defects on bulk oxides: atomically-resolved scanning probe microscopy

“…47,48 (For a recent review on the surfaces of In 2 O 3 and ITO, see the work by Edgell. 23 ) The non-polar In 2 O 3 (111) surface is the thermodynamically most stable facet of this material. A (1 Â 1) surface with a relaxed, bulk-terminated structure results when an In 2 O 3 single crystal is oxidized under UHV conditions, see the left side of Fig.…”

“…To contrast the misconception that only V O s are relevant, we next discuss Fe 3 O 4 , where, under UHV conditions, the most stable modification of the (001) surface exhibits a reconstruction with ordered subsurface cation vacancies. The last example, In 2 O 3 , is a newcomer in surface science, and one of the materials where the technical relevance far outweighs the available information on how its surfaces look like at the atomic scale; 23 it came as a surprise that reducing the sample results in isolated In adatoms at the surface. Indium oxide is also an interesting example of a post-transition metal oxide, a class of materials that is (with the notable exception of ZnO 8 ) far less investigated than transition metal oxides, but has particularly intriguing properties, e.g., electrical conductivity paired with optical transparency.…”

Resolving the Structure of a Well-Ordered Hydroxyl Overlayer on In2O3(111): Nanomanipulation and Theory

“…Reports regarding replacing ITO because of limited In sources have been recently refuted (In is a byproduct of Zn refinement with an abundance comparable to Ag), and the long-term supply appears to be secure . In addition, the properties of ITO are not met by any other prospective material candidate: ITO is suitable for thin film growth, and it excels over other TCOs such as doped SnO 2 or ZnO . As such, ITO is currently widely used as a contact electrode in liquid crystal displays and solar cells, and recently, it is also of increasing interest as a sensor and catalytic material. , Thus, it seems that indium oxide will remain the TCO material of choice for the foreseeable future.…”