Epitaxy and stabilization of a metastable single-crystal hexagonal YAlO₃ perovskite via a periodic Y-O monolayer

Chao-Kai Cheng^1*, Lawrence Boyu Young¹, Yen-Hsun Lin¹, Ren-Fong Cai², Chien-Ting Wu³, Chia-Hung Hsu⁴, Jueinai Kwo⁵, Minghwei Hong¹

¹Graduate Inst. Appl. Phys. and Department of Physics, National Taiwan University, Taipei, Taiwan
²Material and Chem. Res. Labs, Industrial Technology Research Institute, Hsinchu, Taiwan
³Taiwan Semiconductor Research Institute, Hsinchu, Taiwan
⁴National Synchrotron Radiation Research Center, Hsinchu, Taiwan
⁵Department of Physics, National Tsing Hua University, Hsinchu, Taiwan

* Presenter:Chao-Kai Cheng, email:f02245005@ntu.edu.tw

Hexagonal YAlO₃ perovskite (H-YAP), which usually formed during the synthesis process of Y₃Al₅O₁₂ (YAG) garnet structure, is known as a metastable phase in the Y₂O₃-Al₂O₃ pseudo-binary system.¹ The formation of YAG, and Y₄Al₂O₉ (YAM) are more thermodynamically favored than forming H-YAP in the conventional chemical synthesis. It is difficult to obtain pure H-YAP phase without forming other phases. YAG and YAM usually coexist with H-YAP.² We have deposited a periodic Y-O monolayer as a seed layer to tailor the substrate surfaces to induce epitaxial growth of H-YAP films, making the metastable H-YAP stable. Atomic layer deposition (ALD) was utilized to grow sub-nano-laminated (snl) Al₂O₃/Y₂O₃ multilayers on GaAs(111)A³ and GaN/sapphire(0001). Pure H-YAP phase with a single-crystal structure was formed after rapid thermal annealing (RTA) to 900°C. The role of a periodic Y-O monolayer on GaAs(111)A has been revealed using scanning transmission electron microscopy (STEM) in conjunction with a spherical aberration-corrected probe, and in-situ reflection high-energy electron diffraction. The periodic Y-O array was observed at the interface of amorphous snl Al₂O₃/Y₂O₃ multilayers and GaAs(111)A, and became the bottom layer of H-YAP, which bond with Ga, the top layer of GaAs. H-YAP film has excellent crystallinity and an atomically sharp interface with the substrate according to the results of X-ray diffraction (XRD) and STEM. The remaining amorphous layer on the top of H-YAP is Al₂O₃, as characterized by energy dispersive X-ray spectroscopy. This indicates that the H-YAP is favored to form even though the total chemical composition of Al₂O₃/Y₂O₃ multilayer is not exactly the same as that H-YAP. The same concept was also applied to the GaN/ sapphire (0001). XRD and STEM revealed that the excellent H-YAP film formed after RTA to 900°C. The results indicate that the atomic-layer thick periodic Y-O array induced the epitaxial H-YAP and stabilized the metastable H-YAP phase. Our results may conduce to novel epitaxial growths by tailoring the substrate surfaces using a foreign atomic-layer thick periodic adatom array, thus opening up a new chapter in the hetero-epitaxy.

The corresponding authors: mhong@phys.ntu.edu.tw (M. Hong), raynien@phys.nthu.edu.tw (J. Kwo), ctwu@narlabs.org.tw (C.-T. Wu), and chsu@nsrrc.org.tw (C. H. Hsu).

References:
1. T. Tachiwaki et al., Solid State Communications 119, 603 (2001).
2. N. J. Hess et al., Journal of Materials Science 29, 1873 (1994).
3. M. Hong et al., Nanomaterials 10(8), 1515 (2020).

Keywords: atomic layer deposition, interfacial monolayer-induced epitaxy, hexagonal perovskite YAlO₃, metastable phase