X-Ray Standing Wave Analysis Of Overlayer-Induced Substrate Relaxation: The Clean And Bi-Covered (110) Gap Surface

The relaxation of the surface P atoms, for both the clean and Bi-covered GaP(110) surface, was studied with x-ray standing wave (XSW) spectroscopy using surface-sensitive x-ray photoelectron as the XSW modulated signal. The photoemission signal of the outermost surface layer is mixed with the signal from the remaining near surface of the underlying substrate, so further analysis is required to calculate the geometry of the relaxation of the surface atoms. We present a general analysis method for extracting the geometry of the surface reconstruction that minimizes the propagation of the uncertainties associated with fitting XSW data. It takes advantage of the fact that the coherent distance may be more accurately determined than the coherent fraction in XSW data analysis. This method makes use of the electron attenuation length, and shows that the relaxation is only weakly dependent on the uncertainties of this parameter. Results indicate that, for the clean GaP surface, P relaxes with a small outward rotational displacement, with the axis of the rotation located at the second-layer Ga site, whereas, for the Bi-covered case, relaxation consists of a rotation in the opposite direction. The magnitude of the contraction is not negligible, and might be important in the interpretation of low-energy electron diffraction data and in ab initio calculations.