Microstructure and Reconstruction of LaAlO 3 {100} and {110} Surfaces
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REM experiments were carried out at 300 kV and 120 kV in transmission electron microscopes (TEMs) under a vacuum of 10.6 to 10.7 torr. Bulk specimens were prepared by cutting LaA103 single crystal sheets into samples with dimensions of about 2.5xlx0.8 ram. The {100} and {110} surfaces were mechanically polished before annealing. The polished samples were annealed at 1500 °C for 10 or 20 h in air. The crystallographic data on LaA103 have been given by Geller and Bala [5] and Berkstresser et al. [6]. The structure is the distorted-perovskite structure with lattice constant a = b = c = 0.3788 nm and a =B = •,= 90.066°. The structure is referred to a face-centered rhombohedral cell, in which the La+• ion locates at (000), the AI+3 ion at (0.5 0.5 0.5), and the 0-2 ions at the face-centers {0.5 0 0}.
Figure 1.An [001] RHEED pattern from an annealed LaA103 (100) surface showing 5×5 surface reconstruction. The inset is the RHEED pattern.
175 Mat. Res. Soc. Syrup. Proc. Vol. 355 ©1995 Materials Research Society
LaAIO 3 {1001 Figure 1 shows a [001] reflection high-energy electron diffraction (RHEED) pattern of the annealed LaA10 3 (100) surface. Two strong superlattice reflections are observed between bulk reflections (10 0 0) and (10 1 0). The two superlattice reflections are separated by 1/5(0 20) and 1/5(020), respectively, from the (10 0 0) and (10 1 0) bulk reflections. On the right-hand side of (10 1 0) reflection, four equally-spaced superlattice reflections are seen. These superlattice reflections correspond to the formation of 5x5 reconstruction on the surface. LaA103 exhibits (1001 twin structures, but the twin angle is as small as 0.130. This twin structure causes the splitting of the superlattice reflections, as seen in figure 1.
Figure 2. REM images of the LaA10 3 (100) surface showing the [010] and [001] faceted steps. The images were recorded with the (10 0 0) specularly reflected beam under the diffracting condition shown in figure 1. Beam azimuth is [001]. Numerous [010] and [001] surface steps were observed on the LaA103 (100) surfaces. Figure 2a is a REM image recorded with the (10 0 0) specularly reflected beam under the diffracting condition shown in figure 1. In the REM image, the 5x5 reconstructed area shows darker contrast due to the reduction of the (10 0 0) reflected beam intensity as the result of exciting the superlattice reflections. The unreconstructed surface areas show brighter contrast. Some of the surface areas are completely covered by the reconstructed surface layer. However, the formation of [010] and [001] steps is unaffected. The faceted steps are seen almost everywhere on the surface. The regions which show brighter and darker contrast sometime share the same terrace, and there is no step between them. Figure 2b shows an REM image of the (100) surface entirely covered by the 5x5 reconstruction layer. Many densely distributed [001] and [010] steps are seen. The larger steps are about 2-3 unit cells in height [7]. When the specimen was annealed at 1500 oC, the surface atoms tend to
