We report a synchrotron X-ray absorption spectroscopy study of the average neighborhood of Si near the transition from a-Si:H to μc-Si on wedge-shaped samples prepared by hot-wire CVD in a chamber using a movable shutter. The thickness of the wedge varies from 30 to 160 nm. Nucleation of μc-Si occurs at a critical thickness of approximately 100 nm. X-Ray absorption was measured at the Si K-edge (1.84 keV) by total electron photoemission yield. The absorption oscillations in the EXAFS region are very similar to all along the wedge. Analysis indicates an average tetrahedral first neighbor shell with radial disorder decreasing with crystallization. In the near-edge (XANES) region multiple scattering effects appear at the onset of crystallinity. Unlike single crystal silicon, these effects involve only double scattering within the first neighbor shell, indicating an ill-formed second shell in μc-Si. © 2003 Elsevier Science B.V. All rights reserved.
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