Person:

Golovchenko, Jene

X-ray standing-wave (111) and (111¯) measurements have been performed on the room-temperature and high-temperature phases of a Pb layer on Ge(111). At room temperature, our results support a four-atom–unit-cell surface structure. On passing into the high-temperature phase, we find that there is an increase in the Fourier coefficient along the surface [112¯] direction for the Pb density distribution. Our model, which requires a 0.6-Å in-plane thermal vibration amplitude, is similar to one previously proposed by Hwang and Golovchenko and disagrees with a strictly two-dimensional-liquid interpretation.

X-ray standing-wave measurements and tunneling microscopy have been combined to solve the atomic geometry of the √3×√3R30° honeycomb phase of Bi on Si(111). The standing-wave measurements utilize three different diffracting planes to triangulate the surface position of Bi atoms. The unoccupied surface sites required to completely determine the structure can be deduced from Rutherford-backscattering coverage and low-energy electron-diffraction symmetry arguments. These arguments are completely confirmed by a tunneling-microscope study, which is free of the ambiguities of previous studies. The final result is a 2/3-ML √3×√3R30° structure with Bi atoms in the T1 sites directly above first-layer Si atoms.

Standing waves of x rays have been used to determine the positions of bromine atoms in submonolayer coverages on a (111) silicon surface. In addition to the bromine position normal to the crystal surface its components relative to a plane inclined to the surface are also measured. This information suffices to establish the registration of the surface atoms relative to the crystal below.

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 ≤T≤650 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 2.58±0.04 Å and that, independent of temperature, they sit 1.40±0.01 Å above the top bulk-extrapolated silicon (004) plane. These results provide a critical test for theoretical structure calculations.