Epitaxy and stabilization of a metastable single-crystal hexagonal YAlO3 perovskite via a periodic Y-O monolayer
Chao-Kai Cheng1*, 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).

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