Microvoids in Polycrystalline Cvd Diamond
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This technique is demonstrated as an excellent tool for identifying, characterizing and quantifying these defects in commercial materials. Light scattering is based on the differences in refractive index of a feature and its surrounding matrix. 7 Thus, it is possible to carry out light scattering experiments directly on samples without staining if appropriate contrast is present.8 For microvoids in diamond the refractive index within the defects (that of a sub-atmospheric pressure of hydrogen gas) is expected to contrast well with diamond's high index of refraction. Unlike dynamic light scattering, which relies on defect fluctuations, the static light scattering experiments described here measure the angular dependence of the scattering from static defects and are thus, applicable to the solid-state. The distribution, size and shape of such defects inferred from the angular scattering patterns provide valuable information on the internal structure of the diamond films. The van der Drift model for growth of a randomly nucleated polycrystalline film 9 , predicts the evolution of a columnar structure where individual grain size increases with film thickness. Such columnar growth has been well documented in the growth of CVD diamond films.' 0 Under conditions which produce a large anisotropy in the growth rate
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Mat. Res. Soc. Symp. Proc. Vol. 588 © 2000 Materials Research Society
of various crystal planes, it is possible for inter-granular void formation to occur. The size of these defects should be related to the magnitude of the anisotropy and may exceed several microns in thicker samples. Such micron scale inter-granular voids, predicted by the van der Drift model, have been observed in CVD in the past using scanning electron microscopy (SEM) (Figure 1 and 2), however their impact on film properties is poorly understood since SEM provides no quantitative data.2' The presence of these defects is difficult to detect and even more difficult to quantify in diamond films. Therefore, little analysis of these features has been performed. The application of small angle light scattering can help to solve this problem. EXPERIMENT A schematic of the light scattering system used in this work is shown in Figure 3. The system consists of a 10 mW single phase Helium-Neon laser at 632.8 nm; a focusing lens and spatial filter system; a sample goniometer, which allows adjustment of the sample with respect to the laser beam; a polarizer to allow detection of either vertically or horizontally polarized light; a series of lens and filters to focus the image on a pinhole aperture, and a photographic detector or silicon photodiode array detector, positioned at an angle, 0, to the incident laser beam. The information obtained is then digitized and sent to a computer for analysis. In order to study of larger scale defects, the light scattering system can be modified to operated at larger incident wavelengths, for example 1.67 or 3.0 gim, however, the current discussion is limited to results at 632.8 nm. All measurements discussed wer
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