Reaction and structure of Ti on Si probed by surface extended energy-loss fine structure and extended appearance potential fine structure

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Abstract
The local structures of the low-temperature (<500 °C) phases of the Ti/Si system have been determined by surface extended energy-loss fine structure (SEELFS) and by extended appearance potential fine structure (EAPFS), two variations of the electron analog of x-ray absorption fine structure (EXAFS). The extended fine structure above the Ti L23 edge has been measured for thin layers of Ti deposited on atomically clean Si surface and annealed at 20 °C (a pure Ti overlayer), 250 °C (a Si-rich Ti overlayer), and 400 °C (a silicide phase). Data were analyzed using standard EXAFS analysis routines including Δl=+1 phase shifts, resulting in peak positions at 2.93±0.02 Å for the two unreacted phases and 2.39±0.04 Å for the silicide phase. These values are in good agreement with the known spacings of bulk Ti and TiSi, respectively. The ratio technique, which avoids phase-shift difficulties, was applied to these measurements showing that the pure Ti overlayer and the Si-rich Ti overlayer have the same Ti–Ti spacing, but the Si-rich Ti overlayer exhibits a higher degree of disorder. We find no evidence for the formation of a Ti–Si bond in the Si-rich Ti overlayer. This first application of the ratio technique to SEELFS and EAPFS demonstrates its usefulness.

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