Thermal vibration amplitudes and structure of As on Si(001)

Using the x-ray standing-wave method, we have measured directly the thermal vibration amplitudes u2 of symmetric As dimers on a Si(001)-(2×1) surface. For sample temperatures in the range 300 K T650 K, the results are Debye-like. Above 650 K u2 varies more rapidly, indicating the onset of defect-mediated processes. We also found that at room temperature the bond length of the As dimers is 2.580.04 and that, independent of temperature, they sit 1.400.01 above the top bulk-extrapolated silicon (004) plane. These results provide a critical test for theoretical structure calculations.