QUANTITATIVE-ANALYSES OF RHEED PATTERNS FROM MBE GROWN GAAS(001)-2X4 SURFACES

A multislice formalism of Cowley and Moodie [Acta Crystallogr. 10 (195 7) 609] with a recently developed edge-patching method has been applie d to quantitative analyses of RHEED patterns from MBE grown GaAs(001)- 2 x 4 surfaces. The analyses are based on the ordering of visually est imated spot intensities of several observed RHEED patterns from the Ga As(001)-2 x 4 surfaces. The surface structure is proved to be a dimeri zed vacant 2 x 4 reconstruction missing one dimer of every four along the [110] direction, which is consistent with previous STM observation s. The relaxation of the top layer is found to be about delta = + 8.0% (0.113 angstrom). The results give an overall Debye-Waller factor of B = 0.35 angstrom2 and a crystal absorption of alpha = 0.1 for the con ditions at which the observed RHEED patterns were acquired. The effect s of dimer twist on the intensities of RHEED patterns are briefly asse ssed. This indicates that twist of dimers on the GaAs(001)-2 x 4 surfa ce is unlikely. Errors involved in the calculations are also discussed .