To produce large-size, defect-free single crystals for the next generation of ultra-large scaled integrated circuit devices, a better understanding of the crystallisation process from the melt is required. In particular, the atomic-scale structure of the melt is of interest in a wide temperature range, including the equilibrium and the undercooled state. Therefore, X-ray diffraction (XRD) experiments were performed at the ESRF to determine the structure of molten silicon. In order to access the undercooled regime and to provide high-purity conditions, the containerless electromagnetic levitation technique was employed. This technique uses an rf-electrical current which is sent through a conical coil to produce lift and heat in a small drop (approx. 5 mm in diameter) as shown in Figure 101. It is possible to levitate solid Si electromagnetically by preheating the sample; as molten Si is conducting [1]. So far, only one X-ray diffraction experiment on aerodynamically levitated Si has been performed [2].

The experiments were carried out at beamline ID9, where an intense white X-ray beam is available for energy dispersive measurements. With the X-ray detector positioned at a fixed angle the structure factor S(Q) can be measured as a function of temperature. Temperature is measured by non-contact pyrometry and is controlled either by adjusting the heater power of the rf-generator, or by convectively cooling the sample with a gas stream of high-purity helium. With this setup it was possible to scan a temperature range from 1150°C to 1650°C, including a maximum undercooling of T = 290°C.

S(Q) yields information about the coordination around the Si atoms and helps to determine an improved empirical interatomic potential which can be used for molecular dynamics calculations. The spectra obtained are shown in Figure 102. They clearly show the appearance of a shoulder on the high-Q side of the first maximum. This feature becomes more pronounced and develops into a maximum with increased undercooling. This maximum is characteristic for molten silicon, in contrast to normal metals. The spectra of liquid metals, like, e.g., Ni, do not show such a maximum in agreement with the generally accepted picture that the structure of molten metals corresponds to that of closely packed spheres. In contrast, the silicon melt has a different structure, reminiscent of its tetrahedrally bonded character in the solid state.

References
[1] M. Langen, T. Hibiya, M. Eguchi, I. Egry, J. Crystal Growth, 186, 550 (1998).
[2] S. Ansell, S. Krishnan, J.F. Felten, D. Price, J. Phys., C10, L73 (1998).

Principal Publication and Authors
I. Egry (a), D. Holland-Moritz (a), T. Schenk (a), K.-R. Bauchspieß (a), S. Schneider (a), H. Kimura (b), M. Watanabe (b), K. Izumi (b), T. Hibiya (b), M. Hanfland (c), to be published.

(a) Institut für Raumsimulation, DLR, Köln (Germany)
(b) Fundamental Research Labs, NEC Corporation, Tsukuba (Japan)
(c) ESRF