Surface precipitates formed on annealed LSAT (001) single crystal

Kazuki Ohashi¹, Shunsuke Okada¹, Katsuhiro Sasaki¹

¹Department of Quantum Engineering, NAGOYA University, Furo-cho, Chikusa-ku, Nagoya, 464-8603, JAPAN.

|November 1, 2014

Summary

This study examined the surface precipitates, or mounts, that form on LSAT (001) single crystal substrates after annealing. Using TEM and STEM techniques, the researchers found that these mounts are amorphous structures composed mainly of Al and Sr. They form in hollowed regions of the surface, likely due to La ion vaporization during annealing. In areas without mounts, the surface is terminated at the B-site of the perovskite structure, with a distinct Al/Ta ratio compared to the bulk crystal. The study highlights the importance of understanding surface morphology for using LSAT as a suitable substrate for thin film growth.

Area of Science:

Materials science and crystal growth
Thin film deposition techniques
Surface characterization in solid-state physics

Background:

LSAT (La0.3Sr0.7)(Al0.65Ta0.35)O3 is a perovskite-type material considered a promising substrate for GaN and high-temperature superconducting thin films due to its lattice compatibility. Prior research has shown that preparing a step-terrace structure on substrates is essential for growing high-quality thin films. Annealing is a common method to achieve such structures. However, a knowledge gap exists in understanding the formation and composition of surface precipitates—known as surface mounts—that appear after annealing LSAT. These mounts may affect the suitability of LSAT as a substrate. This paper investigates the structure and composition of these mounts and the atomic-level termination of annealed LSAT surfaces.

Purpose Of The Study:

The study aimed to investigate the surface precipitates formed on annealed LSAT (001) single crystal substrates and to determine the atomic termination of the surface in regions without these precipitates. The motivation stems from the need to understand how annealing affects the surface structure of LSAT, which is critical for its use as a substrate in thin film growth. By using TEM/STEM techniques, the researchers sought to clarify the morphology, composition, and structural implications of the surface mounts and to identify the atomic layers terminating the annealed surface.

Main Methods:

The study employed transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) to analyze the surface precipitates and atomic termination of LSAT (001) substrates. Commercial LSAT single crystals were annealed at 1300°C for 30 minutes in air. Cross-sectional thin foils were prepared by joining two annealed surfaces, followed by grinding, polishing, and Ar ion milling. TEM bright field imaging and high-angle annular dark field (HAADF) STEM were used to observe surface mounts and determine atomic termination. Nano diffractometry and energy-dispersive X-ray spectroscopy (EDS) were applied to analyze the composition of the mounts and the terminated layers.

Main Results:

Surface mounts were observed on LSAT (001) surfaces after annealing at 1300°C for 30 minutes. These mounts had a height of approximately 20 nm and were found to be amorphous structures composed mainly of Al and Sr. The mounts formed in hollowed regions of the LSAT surface, with a depth of about 10 nm. EDS analysis confirmed the amorphous nature and elemental composition of the mounts. In regions without mounts, HAADF-STEM revealed that the annealed surface was terminated at the B-site of the perovskite structure. The Al/Ta ratio at the terminated layer was different from the bulk crystal, with an approximate ratio of 1. The results suggest that La ion vaporization during annealing contributes to the formation of surface mounts from residual Al and Sr.

Conclusions:

The study confirmed the presence of surface mounts on annealed LSAT (001) substrates, which are amorphous and composed of Al and Sr. These mounts form in hollowed regions of the surface, likely due to La ion vaporization. In areas without mounts, the surface is terminated at the B-site of the perovskite structure, with a distinct Al/Ta ratio compared to the bulk crystal. The findings suggest that the hollowed surface structure should be considered when using LSAT as a substrate for thin film growth. The authors propose that the formation of mounts is closely related to La ion vaporization and the redistribution of Al and Sr ions.

Keywords:

LSAT surface mounts perovskite substrate characterization thin film growth substrates annealing effects on crystal surfaces

Frequently Asked Questions

Surface mounts are amorphous precipitates composed mainly of Al and Sr, formed on LSAT surfaces after annealing at 1300°C for 30 minutes.

HAADF-STEM imaging and contrast analysis of ordered B-site domains were used to identify the atomic termination layers in regions without surface mounts.

The terminated layer has an Al/Ta ratio of approximately 1, differing from the bulk crystal due to La ion vaporization during annealing.

La ion vaporization during annealing leads to the formation of surface mounts from residual Al and Sr ions, as reported in prior studies.

The hollowed structure forms beneath surface mounts and suggests that the surface morphology must be considered for LSAT's use as a thin film substrate.

The findings suggest that the hollowed surface structure and surface mounts should be considered to optimize LSAT for thin film growth.

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