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 [l angle][ital u][sup 2][r angle] of symmetric As dimers on a Si(001)-(2[times]1) surface. For sample temperatures in the range 300 K [le][ital T][le]650 K, the results are Debye-like. Above 650 K [l angle][ital u][sup 2][r angle] 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 2.58[plus minus]0.04 A and that, independent of temperature, they sit 1.40[plus minus]0.01 A above the top bulk-extrapolated silicon (004) plane. These results provide a critical test for theoretical structure calculations.