Formation of a mixed ordered termination on the surface of LaAlO3(001)

We have investigated the surface termination of LaAlO3(001) at elevated temperatures by low-energy electron microscopy. The terminating layer can be LaO or AlO2. The LaO surface shows a root 5 x root 5R26 reconstruction which can be used as a signature for the LaO termination, while the AlO2 termination is unreconstructed. We find that heating of as-delivered substrates in vacuum or heating substrates which were previously annealed in air or oxygen can lead to a reconstructed surface as observed in diffraction. However, the real-space image shows that the reconstructed areas only cover about one-third of the surface and that the termination is actually an ordered mixture of the reconstructed LaO and the unreconstructed AlO2 terminations. This conclusion is supported by data from atomic force microscopy. We also demonstrate how the disordered mixture of both terminations changes to large LaO islands in the middle of the AlO2 terraces upon heating.