Diffraction of He at the reconstructed Si(100) surface

We present a study of the Si(100) surface using thermal-energy-He diffraction. We find that the clean surface reconstructs with a primary (2 × 1) two-domain periodicity and with substantial additional intensity indicating a secondary reconstruction. We are unable to order the latter using several annealing procedures and consistently obtain diffuse streaks extending across the reciprocal net at the half-order positions. We interpret this reciprocal net in terms of a disordered $c(2\mathrm{}\times 4)$ structure which in addition contains $p(2\mathrm{}\times 2)$ and possible $c(2\mathrm{}\times 2)$ regions. The half- and integer-order intensities vary with incident angle and wavelength in a manner qualitatively describable by semiclassical rainbow scattering. The differential cross section for incoherent scattering has structure similar in angle to the coherent intensities suggesting weak energy losses or surface disorder. Specular intensity scans indicate that the surface is extremely rough on the scale of the He wavelength, with large phase shifts, significant shadowing, and a breakdown of the uniform-attractive-well approximation. The extreme roughness makes a simple global fitting procedure unlikely and the lack of an ordered surface suggests that it will be difficult to fit calculations with a model structure.