LEED Studies of the (0001) Face of α-Alumina

The (0001) face of α‐Al₂O₃ has been studied with low‐energy electron diffraction. A surface etched with hot phosphoric acid showed the two‐dimensional hexagonal 1×1 lattice expected of the bulk (0001) plane. This 1×1 lattice coincides with the lattice of aluminum atoms in this plane. Heating a freshly etched crystal to some temperature below 1400°C produced a surface with any of the following structures: the 1×1, a $(\sqrt{3}\times \sqrt{3})$ rotated 30°, a $\text{(3}\sqrt{3}\text{×3}\sqrt{3})$ rotated 30°, or a [(31)^1/2×(31)^1/2] rotated $\pm {\tan }^{\text{−1}}(\sqrt{3}\text{/11)}$ . Because of their rotations, all three superstructures were in registry with the 1×1. The $\pm {\tan }^{\text{−1}}(\sqrt{3}\text{/11)}$ , almost ±9°, rotations of the (31)^1/2 structure represent two domains rotated in opposite senses; each domain was 10⁻³ to 10⁻⁵ cm across. Above 1400°C, the entire surface was converted to the (31)^1/2 structure. Once formed, all the observed structures were stable down to room temperature if the incident electron energy was kept below about 150 eV. Electron bombardment at above 150 eV converted all the superstructures to the 1×1 at crystal temperatures above 700°C. Above 1000°C, the (31)^1/2 structure was regenerated fast enough so that the electron beam could not destroy it. The $\sqrt{3}$ and $3\sqrt{3}$ structures were never observed to return once they were removed. In contrast, the (31)^1/2 structure could be converted to the 1×1 either by irradiation with electrons while the crystal was held at 800°C, or by in situ etching at 1000°C with silicon, and regenerated by heating to 1200°C. The (31)^1/2 structure is probably characteristic of the uncontaminated (0001) face.