Effect of Bragg-Williams disorder on reconstructed polar surfaces of tetrahedrally coordinated compound semiconductors by optically simulated leed patterns

Laser generated optical diffraction patterns obtained from two-dimensional model gratings have been used to simulate the surface reconstructions observed by LEED with the polar surfaces of tetrahedrally coordinated compound semiconductors. The 2 × 2 and $\text{4}\text{3}\text{× 4}\text{3}\text{?30°}$ reconstructions conform ideally with the $\text{1}\text{4}$-monolayer criterion dictated by bonding ionicity Nosker, Mark and Levine, Surface Science 19 (1970) 291] while the 3 × 3, $\text{3}\text{×}\text{3}\text{?30°}$ and $\text{19}\text{×}\text{19}\text{?23.4°}$ reconstructions do not. It is shown that the introduction of Bragg-Williams disorder that preserves long-range order into the latter structures does permit the achievement of conformity with the $\text{1}\text{4}$-monolayer criterion without altering the symmetry of the diffraction pattern. Specifically the intensities of the fractional order beams are reduced relative to those of the integral order beams. Thus all the observed reconstruction LEED patterns can be consistent with the $\text{1}\text{4}$-monolayer criterion, provided account is taken of the insensitivity of LEED to surface defect structure.