Transparent Conductive Tin Doped Indium Oxide Thin Films with Silver Additive
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Transparent Conductive Tin Doped Indium Oxide Thin Films with Silver Additive Annette Hultåker1, Jun Lu, Eva Olsson, Gunnar A. Niklasson, and Claes-Göran Granqvist Department of Materials Science, The Ångström Laboratory, Uppsala University, P O Box 534, SE-751 21 Uppsala, Sweden. 1 Corresponding author: Phone: +46 18 471 31 32, Fax: +46 18 500 131, E-mail: [email protected]
ABSTRACT
Thin films of intermixed layers of In2O3:Sn (denoted ITO) and silver were made by reactive dc magnetron sputtering. The silver to indium atomic ratio was 0 x )LOPVZLWKx = 0.01 showed increased luminous transmittance Tlum after annealing at 100 or 200°C, whereas x > 0.01 yielded lowered Tlum. The optical properties could be reconciled with the Maxwell-Garnett effective medium theory assuming that a well-defined portion of the silver was occluded as spheroidal particles. Films with x KDGHQKDQFHGHOHFWULFDOFRQGXFWLYLW\DIWHUDQQHDOLQJDW 200 or 300°C. Transmission electron microscopy displayed columnar features whose character depended on x.
INTRODUCTION
Films of In2O3:Sn (denoted ITO) can combine high luminous transmittance Tlum with an electrical resistivity ρ as low as 10-4 Ωcm. They have important applications in e.g. flat panel displays, solar cells and energy efficient windows [1]. It has been shown recently that a small addition of silver can decrease ρ without excessively lowering Tlum [2-4]. This paper reports initial results of an extensive study of reactively dc magnetron sputtered ITO:Ag films with a silver to indium ratio x of 0 x :HVKRZWKDWDMXGLFLRXVFKRLFHRIx can effect a small increase of Tlum and a concomitant decrease of ρ. The optical properties can be interpreted within effective medium theory in a way that highlights the role of spheroidal silver particles. Microstructural studies by transmission electron microscopy (TEM) revealed columnar features whose character depended on the amount of added silver.
EXPERIMENTS AND RESULTS
Film preparation by sequential sputtering from ITO and silver targets, procedures for annealing heat treatment at different temperatures (TA), optical measurements and composition analysis have been described elsewhere [5]. F1.9.1
Figure 1: Cross-sectional TEM images of (a) pure ITO and (b) ITO:Ag with x = 0.09. Both deposited on glass and annealed at 300°C. TEM analysis was employed using a Jeol 2000 FX II instrument. It was found that the silver had a significant influence on the structure of the films. The pure ITO films had a columnar structure with individual crystals extending from the substrate up to the film surface (Fig. 1a). Figure 1b gives evidence for discrete silver containing layers originating from the sequential deposition of ITO and silver. The ITO was, however, still able to continue the growth of the columnar grains and maintain the crystallographic orientation despite the defects at the positions of the silver containing layers. In addition, the morphology of the ITO grain boundaries was altered with the introduction
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