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 [u(2)] of symmetric As dimers on a Si(001)-(2X1) surface. For sample temperatures in the range 300 K less than or equal to T less than or equal to 650 K, the results are Debye-like. Above 650 K [u(2)] 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+/-0.04 Angstrom A and that, independent of temperature, they sit 1.40+/-0.01 Angstrom A above the top bulk-extrapolated silicon (004) plane. These results provide a critical test for theoretical structure calculations.