Highly textured nanostructure of pulsed laser deposited IrO 2 thin films as investigated by transmission electron micros
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INRS-E´nergie et Mate´riaux, 1650 Blvd. Lionel-Boulet, C.P. 1020, Varennes, Que´bec J3X 1S2, Canada
D.G. Rickerby European Community-Joint Research Centre, IHCP, 21020 Ispra (Va), Italy (Received 27 December 2000; accepted 25 May 2001)
Highly conductive iridium dioxide (IrO2) thin films have been deposited onto in situ oxidized Si(100) substrates by means of a reactive pulsed laser deposition (PLD) process. The polycrystalline IrO2 films were obtained by ablating a metal iridium target under an optimal oxygen background pressure of 200 mtorr and at different substrate deposition temperatures (Td ) ranging from 350 to 550 °C. Conventional and high-resolution transmission electron microscopy (HRTEM) techniques were used to investigate the micro- and nanostructural changes of the PLD IrO2 films as a function of their deposition temperatures. The microstructure and the morphology of the PLD IrO2 films was found to change drastically from an irregular and loosely packed columnar structure at Td ⳱ 300 °C to a uniform and densely packed columnar structure for higher Td (艌350 °C). For IrO2 films deposited in the 350 艋 Td 艋 550 °C range, HRTEM have revealed the presence of highly textured arrangements of almost spherical IrO2 nanograins (of 3–5 nm diameter, regardless of Td) in the columns (of which diameter was found to increase from 85 ± 15 to 180 ± 20 nm as Td increases from 350 to 550 °C). Lattice resolution and dark-field imaging have pointed out the presence of large IrO2 crystallites made of many similarly oriented nanograins (i.e., under the same Bragg diffraction conditions). Moreover, a high continuity of the lattice planes across the entire crystallite was clearly observed. This latter aspect together with the highly textured nanostructure of the IrO2 films correlate well with their high conductivity (42 ± 6 ⍀ cm for Td 艌 400), which was found to be comparable with that of bulk single-crystal IrO2.
I. INTRODUCTION
Iridium dioxide (IrO2) is the best conductor of the transition metal oxides group that exhibit metallic resistivities (in the 30–100 ⍀ cm range1) at room temperature. These metal oxides (i.e., IrO2, RuO2, OsO2, RhO2, etc.) crystallize in the tetragonal rutile structure, and their particular transport properties are due to the presence of an incomplete d-shell in the band structure of the transition metal. Due to their unique combination of attractive electrical, optical, and electrochemical properties, IrO2 thin films have been used for many applications. These applications include such uses as (i) excellent diffusion barrier and suitable electrode material in ferroelectric NVRAM devices,2,3 (ii) optical switching layers in electrochromic displays,4,5 (iii) sensing material in pH sensors,6,7 and (iv) durable electrodes for chlorine and oxygen evolution.8,9 a)
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J. Mater. Res., Vol. 16, No. 8, Aug 2001 Downloaded: 14 Mar 2015
Iridium oxide thin films have been prepared by various m
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