Optical and electrical properties of indium tin oxide thin films with tilted and spiral microstructures prepared by obli
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Y.J. Park, K.M.A. Sobahan, and C.K. Hwangbo Department of Physics, Inha University, Incheon 402-751, Korea
Y.P. Lee Quantum Photonic Science Research Center, Department of Physics, Hanyang University, Seoul 133-791, Korea
T.G. Kima) School of Electrical Engineering, Korea University, Seoul 136-701, Korea (Received 17 December 2007; accepted 11 June 2008)
The optical and electrical properties of “tilted” and “spiral” indium tin oxide (ITO) thin films are reported. The influence of the flux incident angle on the optical and electrical properties is investigated. When the flux incident angle is increased, both the refractive index and extinction coefficient of the film are decreased, but the resistivity is increased. Thus, the physical properties of the film can be modified over a wide range by adjusting the flux incident angle and substrate rotation scheme. It is suggested that the oblique angle deposition technique provides ITO films with more application possibilities by allowing their optical and electrical properties to be tailored.
I. INTRODUCTION
Indium tin oxide (ITO) has been widely used as a transparent conductor in optoelectronic devices, such as light emitting diodes, solar cells, liquid crystal displayers, etc. ITO thin films can be prepared by a variety of methods, including electron beam evaporation,1 sputtering,2 chemical vapor deposition,3 sol-gel process,4 and spray pyrolysis.5 It has been reported that ITO films could have a high transmittance (∼90%) in the visible range, as well as a low electrical resistivity (∼10−4⍀cm).6 Recently, low-refractive index ITO fabricated by oblique angle deposition (OAD) has been reported to enhance the light extraction from GaN-based light emitting diodes in various ways, such as omnidirectional reflectors,7 conductive distributed Bragg reflectors,8 and graded index antireflection coatings.9 OAD7–17 is a method to deposit thin films with porous microstructures. Figure 1 shows a schematic of OAD. The incident vapor flux arrives at the substrate at an oblique angle ␣. At the initial stage of deposition, the randomly deposited particles cast shadow areas on the a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2008.0312 2500
http://journals.cambridge.org
J. Mater. Res., Vol. 23, No. 9, Sep 2008 Downloaded: 24 Jan 2015
substrate and prevent contact with the incident flux. Because of this self-shadowing effect and limited adatom mobility, the incoming vapor particles deposit on the existing nucleus preferentially, and the columns in the OAD film are also inclined at a column angle . Generally, column angle  is less than the flux incident angle ␣ and follows the empirical tangent rule tan ⳱ 1/2 tan␣ for small ␣15; or the cosine rule  ⳱ ␣ − arcsin[(1 − cos␣)/2] for large ␣.16 By controlling the vapor flux incident angle and substrate rotation, it is possible to tailor the microstructure on the nanometer scale in three dimensions. Spiral posts17 can be fabricated by rotating the substrate holder with the following sequence
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