Effects of heat treatment on Ag particle growth and optical properties in Ag/SiO 2 glass composite thin films
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Small Ag particles were embedded in SiO2 glass thin films by a multi-target sputtering method. The mean diameter of Ag particles in the as-deposited film with 28.0 at. % of Ag was estimated to be 4.4 nm and it was increased to 24.0 nm when the film was heat-treated at 700 °C for 3 h. The diameter was proportional to the cube root of the heat-treatment time, suggesting that the Ag particles grew in the supersaturated solid solution. In the optical absorption spectra of the heat-treated films, the absorption peak due to the surface plasmon resonance of Ag particles was observed about 410 nm. The peak intensity became large and the full width at half maximum of the absorption band was decreased with increasing the diameter of Ag particles.
I. INTRODUCTION Small metal particles embedded in glasses have attracted much interest as materials for optical devices.1'2 It is known that the third-order optical nonlinearity in the glasses doped with metal particles is strongly enhanced near the surface plasmon resonance frequency.3-4 The concentrations of metal particles in the glasses produced by the conventional melt-quenching technique are low (typically 10~2 wt. %). It is also difficult to eliminate the impurities by the technique. On the other hand, by a sputtering method, the extent of metals embedded in the glass thin film is easily controlled up to high concentrations,5 and a wide variety of metals can be used. Moreover, thin film processes are appropriate for the fabrication of waveguide-based nonlinear devices. Nucleation and growth have been investigated in detail for Ag particles in photosensitive glass.6 However, the mechanism of the Ag particle growth in pure SiO2 glass thin films has not been investigated. In order to control the particle size, it is important to clarify the growth mechanism of the particles in the glass thin films. In this paper, we report on the preparation and characterization of Ag particles/SiO 2 glass composite thin films by a multi-target sputtering method, the growth process of Ag particles, and their optical properties. II. EXPERIMENTAL Ag particles/SiO 2 glass composite thin films (Ag/SiO 2 films) were prepared by a multi-target magnetron sputtering system. The Ag (99.9%), 3 inch in diameter) and the SiO2 (99.99%, 3 inch in diameter) targets were independently manipulated. Ag and SiO2 were deposited alternatively onto SiO2 or GaAs substrates. The substrates were not intentionally heated. 362 http://journals.cambridge.org
J. Mater. Res., Vol. 10, No. 2, Feb 1995 Downloaded: 11 Jan 2015
The concentration of Ag in the films was controlled by both the deposition time and the input power. Sputtering was carried out in Ar gas of 2.0 Pa. Typical applied rf powers of the Ag and SiO2 targets were 20 W and 200 W, respectively. The heat treatment of the asdeposited films was carried out at 200-800 °C in air to investigate the growth of the Ag particles. X-ray diffraction measurements (XRD) were done to examine the crystallinity and the mean diameter of Ag particles. The mean diameter, d, was es
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