Composition and reconstruction of the GaP{100}-(4x2) surface

The composition and reconstruction of the GaP{100}-(4 x 2) surface has been studied by time-of-flight scattering and recoiling spectrometry (TOF-SARS) and low energy electron diffraction (LEED). Time-of-filight spectra of scattered and recoiled neutrals plus ions were collected as a function of crystal azimuthal rotation angle delta and primary beam incident (polar) angle alpha. Compositional analysis of the surface was obtained from 4 keV Ne+ scattering and recoiling spectra. Structural analysis was obtained from the azimuthal anisotropy of the delta-scans and the features of the alpha-scans using both 4 keV Ne+ and Kr+ for scattering and recoiling. The azimuthal delta-scans were simulated by means of a shadow cone focusing model and a classical ion trajectory model. The totality of this data shows that the (4 x 2) reconstruction of GaP is a Ga missing-row-trimer P dimer (MRTD) structure in which every fourth Ga[0(1) over bar1$] row is missing, the Ga atoms are trimerized along the [011] azimuth, and the 2nd-layer P atoms exposed in the [0(1) over bar1$] troughs are dimerized. This MRTD model is consistent with all of the data, is autocompensated, and has Ga intratrimer spacings of 3.2 +/- 0.2 Angstrom and P intradimer spacings of 2.7 +/- 0.2 Angstrom. The results of the simulations suggest that the two end Ga atoms of the trimers are relaxed downward by a minimum of 0.2 Angstrom. A missing-row-dimer (MRD) model, similar to that proposed for the GaAs (4 x 2) structure, in which every fourth Ga <011) row is missing and Ga dimers form along [011], was also considered. This MRD model is inconsistent with large portions of the experimental data and the simulations. The results are discussed in terms of the differences in the chemical bonds of phosphorus and arsenic.