Microstructural Evolution of TiO 2 Sol-Gel Thin Films
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MICROSTRUCTURAL
EVOLUTION OF TiO2 SOL-GEL THIN FILMS
J.L. KEDDIE and E.P. GIANNELIS Department of Materials Science and Engineering, NY 14853.
Cornell University,
Ithaca,
ABSTRACT Backscattering spectrometry has been used to determine the density of TiO2 sol-gel films. The density of the as-deposited films relative to that of anatase varies slightly with respect to deposition conditions and is approximately 0.5. Annealing at temperatures as high as 750°C increases the relative density to 0.7 with concomitant decreases in the O:Ti ratio and H content but it does not result in complete densification. Film densities are consistently higher for films annealed under dynamic vacuum compared to those in air. INTRODUCTION AND THEORY Electronic and optical properties of thin films are strongly influenced by their microstructure, porosity and chemical composition [1]. Quantitative measurements of film composition and density, although a difficult task, are of the utmost importance in assessing the potential technological applications of sol-gel derived thin films [2]. In this paper we report the use of backscattering spectrometry to determine the density of as-deposited and annealed TiO2 sol-gel films. The combination of backscattering and forward recoil spectrometry represents a powerful diagnostic technique capable of determining film stoichiometry, impurity levels and density. A detailed compositional analysis of the films will be communicated in a future paper. In Rutherford Backscattering spectrometry (RBS) the energy of 2 rebounding He + ions after impinging on a thin film is measured by an energy sensitive (silicon surface barrier) detector. The energy of the elastically backscattered ions is indicative of the mass of the target nuclei, resulting 2 in elemental identification. Furthermore, He + backscattered by a nucleus significantly below the surface of the film will emerge with less energy than one scattered by the same nucleus at the surface [3]. 3 The density (atoms/cm ) for individual atoms is calculated by N - Y " cos
/
'Q'a't
(1)
where Y is the integrated peak count, $ is the angle of the beam from the film normal, Q is the total number of incident particles 0 is the detector solid angle, and a is the scattering cross section. The scattering cross section is given by a -
2
(e zZ/4E)
2
4
[sin- (0/2)
- 2(m/14)
2
+
... ]
(2)
where m, z and M, Z are the mass and atomic number for He ions and the target nuclei respectively, and e is the scattering angle as defined in Figure 1. Chemical compositions can be calculated with an accuracy of 0.1%, while the uncertainty of areal densities is approximately 3% [4]. EXPERIMENTAL A stock TiO2 sol with molar ratios TI:PrOH:H 2 0:HCI equal to
Mat. Res. Soc. Symp. Proc. Vol. 180. 01990 Materials Research Society
426
Film
Incident 2.2 MeV 4
-
-
--- --------
He+ Beam
Nuclear Particle Detector
Scattering Angle
Figure 1. Experimental geometry for RBS. 1:11.4:1:1.8 was prepared by adding a solution of deionized H2 0 in propyl alcohol to an acidified so
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