Structure of Al/Si(111) Interfaces - a Photoemission Extended X-Ray-Absorption Fine-Structure Study

The atomic geometries of Al/Si(111) interfaces for 1.1 and 2.2 monolayer (ML) aluminum coverages at room temperature have been determined by photoemission extended x-ray-absorption fine structure (PEXAFS). The clean Si(111) surface was prepared by vacuum cleaving, which is known to have a (2 x 1) reconstruction. For 1.1 and 2.2 ML Al/Si(111) interfaces, Si 2p and Al 2p PEXAFS data were acquired. The data were analyzed by Fourier analysis and curve-fitting procedures using the theoretical backscattering phase function of McKale et al. plus absorber phase function of Teo and Lee. For the clean Si(111)-(2 x 1) surface, the first Si-Si bond length is determined as 2.40 +/- 0.04 angstrom, which is slightly larger than its bulk value (2.35 angstrom). For Al/Si(111) interfaces, the average of Si-Si and Si-Al nearest-neighbor distances was determined as 2.47 +/- 0.04 angstrom from the Si 2p PEXAFS. For the 1.1 ML Al/Si(111) interface, the nearest-neighbor Al-Si bond length, derived from Al 2p PEXAFS, is 2.49 +/- 0.04 angstrom, which interface, the nearest-neighbor Al-Si bond length, derived from Al 2p PEXAFS, is 2.49 +/- 0.04 angstrom, which is in excellent agreement with the Al-Si bond length for the Al/Si(111)-(square-root 3 x square-root 3) interface as reported in the literature. For the 2.2 ML Al/Si(111) interface, Al 2p PEXAFS yielded an Al-Al bond length of 2.94 +/- 0.05 angstrom, which is close to its value in Al metal.