Pulsed Laser Deposition and Crystallization of Transparent Conducting Thin Films
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Laser 02
•
._1111iii•YH eat er
X•= 1.06 4m or 0.532 um
Lens
Tage
Fig. 1. Top view of the PLD experimental arrangement. The distance between the target and the substrate was approximately 3.5 cm. Both the target and substrate were rotated during deposition. The substrate could be heated during deposition up to 450 'C. A turbo pump was used to evacuate the chamber to a pressure of 2 x 10.3 Pa. A mass flow controller in conjunction with a mechanical roughing pump regulated any desired background 02 pressure. operated at 5 Hz and the target and substrate were counter-rotated during deposition. A turbo pump was used to evacuate the chamber to 2 x 10-3 Pa. A mass flow controller in conjunction with a mechanical pump regulated chamber pressure when a 02 background pressure was desired. Deposition of samples was performed in vacuum (2 x 10-3 Pa) or with a background oxygen pressure of 10 or 20 Pa. Substrate temperatures were varied from ambient to 400'C during deposition. Some samples were also annealed in vacuum or in oxygen at 450'C for one hour after deposition. Post-deposition analysis of the films included x-ray diffraction (XRD), visible and ultraviolet (UV) transmittance, x-ray photoelectron spectroscopy (XPS) , and electrical conductivity measurements. RESULTS Using Sn targets Composite thin films with high optical transmission and low electrical conductivity were obtained using pure Sn targets and a background oxygen pressure during deposition. Optical transmission spectra in the visible region for different laser wavelengths and background pressures are shown in Fig. 2. These transmission spectra have been normalized to the transmission of the substrate. When deposition was performed in vacuum (2 x 10-3 Pa), the wavelength dependence of the film transmittance followed the predictions from electromagnetic theory for a thin metallic film. For comparison, the theoretical curve for a metal thin film with transmission at 700 nm normalized to that of the sample deposited in vacuum is also shown in Fig. 2. As the background oxygen pressure was increased, the optical transmittance increased and the functional dependence on wavelength changed, indicating that the films were no longer metallic. Annealing samples in the same oxygen atmosphere as deposition improved transmittance, but only marginally.
218
100 80
S60 S40 H
20
400
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-E-
10 Pa 02 (1064 nm))
700
Wavelength (nm) Vacuum (1064 nm) -0-20 Pa 02 (1064 inm) -W
--
Metal (theory)
--
20 Pa 02 (532 nm)
Fig. 2. Effect of background oxygen pressure on transmittance of films deposited by PLD using a Sn target and laser wavelengths of 1.06 utm and 0.532 utm. Substrates were maintained at 400'C during deposition and samples were annealed in the same atmosphere as used for deposition for one hour at 450°C. The transmission spectra have been normalized to the transmission of the substrate. The theoretical prediction for the optical transmission of a metal thin film was normalized to the transmission at X = 700 nm of the sample deposite
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