Investigation of the growth of In 2 O 3 on Y-stabilized ZrO 2 (100) by oxygen plasma assisted molecular beam epitaxy A. Bourlange a , D.J. Payne a , R.G. Palgrave a , J.S. Foord a , R.G. Egdell a, ⁎, R.M.J. Jacobs a , A. Schertel b , J.L. Hutchison c , P.J. Dobson d a Department of Chemistry, Chemistry Research Laboratory, Mansfield Road, Oxford, OX1 3TA, UK b Carl Zeiss SMT AG, Carl-Zeiss-Strasse 56, 73447 Oberkochen, Germany c Department of Materials, Parks Road, Oxford OX1 3PH, UK d Oxford University Begbroke Science Park, Sandy Lane, Yarnton, Kidlington, Oxon, OX5 1PF, UK abstract article info Article history: Received 12 August 2008 Received in revised form 13 November 2008 Accepted 19 November 2008 Available online 27 November 2008 Keywords: Indium oxide Molecular beam epitaxy Atomic force microscopy X-ray photoelectron spectroscopy High resolution transmission electron microscopy Thin films of In 2 O 3 have been grown on Y-stabilised ZrO 2 (100) substrates by oxygen plasma assisted molecular beam epitaxy over a range of substrate temperatures between 650 °C and 900 °C. Growth at 650 °C leads to continuous but granular films and complete extinction of substrate core level structure in X-ray photoelectron spectroscopy. However with increasing substrate temperature the films break up into a series of discrete micrometer sized islands. Both the continuous and the island films have excellent epitaxial relationship with the substrate as gauged by X-ray diffraction and selected area electron diffraction and lattice imaging in high resolution transmission electron microscopy. © 2008 Elsevier B.V. All rights reserved. 1. Introduction Stoichiometric indium oxide (In 2 O 3 ) is a transparent insulator. It is amenable to degenerate n-type doping with Sn cations to give so- called indium tin oxide (ITO). ITO is one of a very restricted range of materials which combines the property of optical transparency in the visible region with a high electrical conductivity. The optimal conductivity in ITO is higher than in related materials such as Sb- doped SnO 2 and thus ITO is the transparent conducting oxide of choice in many technological areas [1–4]. Despite the near ubiquitous application of ITO in liquid crystal displays, solar cells and electro- luminescent display devices, little effort has been directed toward growth of high quality single crystal thin films of In 2 O 3 or ITO. Not surprisingly then many aspects of the fundamental physics of In 2 O 3 have remained controversial, including even the magnitude and nature of the bulk bandgap. Absorption measurements on single crystal In 2 O 3 carried in 1967 showed a weak absorption onset at around 2.67 eV, attributed to indirect optical transitions [5], with a stronger absorption onset at 3.75 eV. Nonetheless the bandgap of In 2 O 3 was for many years quoted to be 3.75 eV [6–8]. However the valence band onset in photoemission measurements is less than 3 eV below the Fermi energy [9]. This observation is inconsistent with a bandgap of 3.75 eV unless there is pronounced upward band bending at the surface [10,11]. However it has recently been shown the bandgap is in fact direct, but transitions from states toward the top of valence band into the conduction band are either dipole forbidden or have minimal dipole intensity: this explains the ~1 eV shift between weak and strong optical absorption onsets [12]. To date most work on growth of high quality single crystal In 2 O 3 films has concentrated on deposition of In 2 O 3 on alumina [13] and yttria-stabilised zirconia single crystal substrates by carefully con- trolled pulsed laser deposition (PLD) [14–16] (i.e. “laser” molecular beam epitaxy), although there are some reports of single crystal growth metalloorganic chemical vapour deposition [17] and by molecular beam epitaxy (MBE) [18–20] using conventional indium Knudsen cells and oxygen atom plasma sources. These considerations have prompted us to initiate a programme concerned with growth of In 2 O 3 thin films on cubic zirconia by oxygen plasma assisted MBE. ZrO 2 itself has a low symmetry monoclinic structure at room temperature, but a cubic phase can be stabilized by replacement of some of the Zr (IV) with larger cations such as Ca(II) or Y(III), with concomitant introduction of compensating oxygen vacancies. The face centred cubic fluorite structure of Y-stabilised ZrO 2 belongs to the space group Fm3 m. The lattice parameter of this phase increases with Y doping level. For the minimum Y concentration of around 17% required to stabilize the cubic phase the lattice parameter can be estimated as a =5.1423 Å [21–23], whilst for 28% Y-doping a =5.2100 Å [21]. The Thin Solid Films 517 (2009) 4286–4294 ⁎ Corresponding author. E-mail address: russell.egdell@chem.ox.ac.uk (R.G. Egdell). 0040-6090/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2008.11.134 Contents lists available at ScienceDirect Thin Solid Films journal homepage: www.elsevier.com/locate/tsf