Surface precipitates formed on annealed LSAT (001) single crystal.

Abstract

LSAT (La0.3Sr0.7)(Al0.65Ta0.35)O3, which has a complex perovskite structure of (A'A'')(B'B'')O3, is expected as an attracting substrates for GaN and high temperature superconductivity oxides solid thin films from a viewpoint of the suitable lattice matching. To grow high quality thin film, it is very important to prepare step-terrace structure on substrates used for thin film growth. For this purpose, a technique of annealing substrates with mirror surface is often used. However, surface precipitates, called surface mounts, are reported to appear after annealing LSAT substrates [1]. In this study, we investigated the surface precipitates formed on annealed LSAT surfaces by TEM/STEM. Further, we directly confirmed the terminated atomic layers at the annealed LSAT surfaces in the area without surface precipitates.Commercially available LSAT single crystal substrates with (001) surfaces (SHINKOSHA CO.,LTD) were used for TEM/STEM observation. After annealing at 1300°C for 30 min in air, the (001) surface structures were observed from [110] direction using cross sectional thin foils. The thin foils were prepared by joining two annealed LSAT (001) surfaces with glue, grinding, polishing and finally Ar ion milling. TEM/STEM observation was conducted by JEOL ARM-200F (a double Cs-corrector type for TEM/STEM) operated at 200kV.Surface mounds were confirmed to appear on LSAT crystal surface after annealing at the annealing condition used in this study. A typical example is shown in Fig.1. shows TEM bright field image taken from the surface area of LSAT (001) after annealing. The observation direction of the image is [110], which is parallel to the annealed surface. Cross sectional images of surface mounts with 300nm was clearly seen as indicated by the arrows in the image. The height of the mounts is around 20nm, and it is noted that the interfaces between the mounts and LSAT surfaces are hollowed into LSAT crystal with the depth about 10nm. Nano diffractometric and EDS analysis have revealed that the mounts are amorphous structure with mainly Al and Sr. So far, the surface mounts has been considered to be SrO precipitation because the mounts easily dissolve into water. After cleaning in water, smooth surfaces for thin film growth can be considered to be obtained. However, the mounts are revealed to be formed on the hollowed surfaces of LAST as shown in Fig.1. To use LSAT annealed surfaces as suitable substrates for thin film growth, the hollowed surface structure should be considered.jmicro;63/suppl_1/i20/DFU049F1F1DFU049F1Fig. 1.TEM bright field image of surface mounds on LSAT (001) annealed surface. Meanwhile, we determined the terminated atomic layers of the surface by carrying out high resolved HAADF-STEM observation at the areas without the mounts. After annealing, the surfaces without any mounts have atomically flat (001) surface structure at an atomic level. To clarify the terminated atomic layers, it must be necessary to assign a position of a unit cell for atomically resolved HAADF-STEM image. To carry out the assignment, we used an ordered domain structure exiting in LSAT crystal [2]. In the ordered domains, the cations of Al and Ta occupy at B-site periodically. Observing the B-site ordered domains by HAADF-STEM, the respective B-site atomic columns can be distinguished separately with contrast difference due to the atomic number of Al and Ta ions. To use the contrast variation, we can precisely assign the unit cell position for the obtained HAADF-STEM image. As a result, it was found that the annealed surfaces were terminated at B-site at the area without any mounts. Further, EDS analysis has revealed that Al/Ta ratio at terminated B-site layer is different from that of crystals, which is Al/Ta ∼ 1. There is a report informing that La ions tend to vapor during annealing LSAT [1]. The formation of the surface mounts is closely related to the vaporization of La ions near surfaces. The mounts was formed from residual ions of Al and Sr. As a result, the surface areas without any mounts can be considered to include Ta more.