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.

We have measured extremely large anisotropic thermal vibrations of gallium adatoms on a Si(111) surface using x-ray standing waves. At 830 K the rms amplitude normal to the surface is 0.47±0.02 Å while the parallel amplitude is 0.30±0.02 Å. The temperature dependence of these amplitudes deviates significantly from that expected from Debye theory and suggests a significant thermal softening of the surface bonding at elevated temperatures.

The position of As atoms on a clean Si(111) surface has been determined with x-ray standing waves in ultrahigh vacuum. The As atoms occupy exclusively the top half of the silicon (111) double plane and lie at 0.17 Å above the unrelaxed bulk terminated silicon (111) plane. This value is in good agreement with recent total-energy minimization calculations. A significant but limited stability of As passivated surfaces is observed upon exposure to various ambients. We also present general arguments showing how surface specificity can be achieved with x-ray standing-wave measurements such as the above, where bulk-symmetry site-occupancy rules are broken.

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.

The position of germanium atoms on a Si(111) (7×7) surface prepared in a molecular-beam-epitaxy system have been established using the characteristic fluorescence generated by x-ray standing waves. For submonolayer coverages germanium occupies both types of (111) lattice sites. Measurements of the germanium position relative to planes inclined to the surface do not support recent stacking-fault models for the Si(111) (7×7) surface.

By monitoring the characteristic fluorescent radiation excited by x-ray standing waves we have been able to distinguish the two different kinds of lattice sites in a heteropolar crystal of GaAs. The strong extinction effects that mask the position-dependent fluorescent signal were minimized by adopting a suitable detector geometry.

The presence and properties of x-ray evanescent waves are demonstrated detecting their absorption and their excitation in the vicinity of a target surface. The role of microscopic reversibility in relating the two cases is discussed and practical applications of the phenomena to probe interfaces are suggested.

The position of Ga and As atoms at monolayer coverages of heteroepitaxial GaAs on clean Si(111) have been measured by x-ray standing waves in UHV. Though both As and Ga are incident on the surface As atoms choose to occupy the upper half of the (111) double plane about 5% higher on the average, relative to the Si(111) d spacing, than the bulk silicon position. Ga atoms are exclusively located on the lower half of the (111) plane about 3% higher than the corresponding bulk positions.