In order to further investigate the surface structure, LEED was applied. Figure shows the LEED diffraction patterns recorded at 52 eV (left hand side) and 64 eV (right hand side). The slightly diffuse LEED pattern and spots with blurred contrast indicate a reduced atomic order of the surface. This could be due to surface defects, lack of short range order in the surface reconstruction, e.g., due to missing hydrogen passivation, or due to traces of contaminants such as oxygen. Aluminium containing surfaces are well known for their high affinity to oxygen incorporation . Nevertheless, the LEED patterns clearly exhibit first and half order spots. In addition to the first order spots, both diffraction patterns exhibit half-order spots indicating a 2×1-like surface reconstruction (marked with white circles) and diffused streaks in ×2 direction (indicated by a white arrows). This LEED pattern is very similar to measurements on P-rich and partially hydrogen-covered InP(001) and GaP(001) surfaces: The adjacent rows of buckled P dimers stabilized by one hydrogen atom can be arranged in-phase or outof-phase . The in-phase arrangement results in a p(2×2) unit cell, while the out-of-phase arrangement corresponds to a c(4×2) unit cell. Scanning tunneling microscopy (STM) scans of such P-rich InP(001) and GaP(001) surfaces show that those two domains are randomly distributed . Their superposition leads to the 2×1-like LEED pattern with characteristic streaks in the ×2 half-order (see Fig. in Ref. ). This excludes the β2-2×4 surfaces and is strong evidence that the surface prepared here corresponds to the 2×2-2D-2H reconstruction predicted from ab initio thermodynamics.