A study of (111) oriented epitaxial thin films of In 2 O 3 on cubic Y-doped ZrO 2 by synchrotron-based x-ray diffraction
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Didier Wermeille XMaS CRG Beamline, European Synchrotron Radiation Facility, 38043 Grenoble Cedex 9, France; and Department of Physics, University of Liverpool, Liverpool, L69 7ZE, United Kingdom
Roger A. Cowley Department of Physics, Clarendon Laboratory, University of Oxford, Oxford OX1 3PU, United Kingdom (Received 8 February 2012; accepted 26 March 2012)
Reciprocal space mapping using synchrotron-based x-ray diffraction has been used to study the effects of strain and strain relaxation in (111) oriented thin films of In2O3 on cubic Y-stabilized ZrO2 over a range of epilayer thicknesses between 35 and 420 nm. Maps around the epilayer (1026) reflection show that the 35-nm film is highly strained with a lateral periodicity close to that of the substrate, while the 420-nm film is almost completely relaxed. Analysis of the map for the former sample leads to an estimate of 0.31 for the Poisson ratio for In2O3. The mosaic spread deduced from transverse scans through the epilayer (444) and (666) reflections increases from 0.1° for the 35-nmthick film to 0.3° for the 420-nm-thick film. These changes are discussed in relation to the morphological changes observed by atomic force microscopy. I. INTRODUCTION
Tin-doped indium oxide is a prototypical transparent conducting oxide (TCO) with important applications as a window electrode in liquid crystal displays, electroluminescent displays, and many designs of solar cell.1,2 Self evidently the band gap of a TCO is one of the most critical properties in determining applications in optoelectronic devices, but nonetheless the magnitude and nature of the band gap of In2O3 has aroused considerable controversy in recent years. It was established only in 2008 that In2O3 has a direct but dipole-forbidden band gap of around 2.9 eV, with a fully allowed optical onset just less than 1 eV higher in energy.3 The relatively low value of the band gap coupled with a strongly dispersing conduction band gives rise to a charge neutrality level that lies above the conduction band minimum, which in turn ensures that In2O3 has a high intrinsic dopability. As a consequence, tin-doped indium oxide remains the TCO of choice in many areas of application in spite of large increases in the price of indium following the surge of sales in liquid crystal television and computer monitor screens in the last decade. Renewed interest in the basic physical properties of In2O3 has prompted attempts to grow high quality In2O3 a)
Present address: Pacific Northwestern National Laboratory, Chemical and Material Sciences Division, Material Sciences, K8-87, Richland, WA 99352 USA Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2012.162 J. Mater. Res., Vol. 27, No. 17, Sep 14, 2012
single crystal thin films by molecular beam epitaxy (MBE).4–8 Indium oxide itself adopts the body-centered cubic bixbyite structure, which belongs to the space group I 3a with a lattice parameter a 5 10.117 Å. The structure may be regarded as a 2 2 2 superstructure of fluorite but with 25% of th
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