Strain-Induced Metastable Phase Stabilization in Ga 2 O 3 Thin Films

Yaobin Xu, Ji Hyeon Park, Zhenpeng Yao, Chris Wolverton, Manijeh Razeghi, Jinsong Wu*, Vinayak P. Dravid

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

12 Scopus citations

Abstract

It is well known that metastable and transient structures in bulk can be stabilized in thin films via epitaxial strain (heteroepitaxy) and appropriate growth conditions that are often far from equilibrium. However, the mechanism of heteroepitaxy, particularly how the nominally unstable or metastable phase gets stabilized, remains largely unclear. This is especially intriguing for thin-film Ga 2 O 3 , where multiple crystal phases may exist under varied growth conditions with spatial and dimensional constraints. Herein, the development and distribution of epitaxial strain at the Ga 2 O 3 /Al 2 O 3 film-substrate interfaces is revealed down to the atomic resolution along different orientations, with an aberration-corrected scanning transmission electron microscope. Just a few layers of metastable α-Ga 2 O 3 structure were found to accommodate the misfit strain in direct contact with the substrate. Following an epitaxial α-Ga 2 O 3 structure of about couple unit cells, several layers (4-5) of transient phase appear as the intermediate structure to release the misfit strain. Subsequent to this transient crystal phase, the nominally unstable κ-Ga 2 O 3 phase is stabilized as the major thin-film phase form. We show that the epitaxial strain is gracefully accommodated by rearrangement of the oxygen polyhedra. When the structure is under large compressive strain, Ga 3+ ions occupy only the oxygen octahedral sites to form a dense structure. With gradual release of the compressive strain, more and more Ga 3+ ions occupy the oxygen tetrahedral sites, leading to volumetric expansion and the phase transformation. The structure of the transition phase is identified by high-resolution electron microscopy observation, complemented by the density functional theory calculations. This study provides insights from the atomic scale and their implications for the design of functional thin-film materials using epitaxial engineering.

Original languageEnglish (US)
Pages (from-to)5536-5543
Number of pages8
JournalACS Applied Materials and Interfaces
Volume11
Issue number5
DOIs
StatePublished - Feb 6 2019

Keywords

  • aberration-corrected scanning transmission electron microscopy
  • metastable phase
  • misfit strain
  • α-Ga O
  • κ-Ga O

ASJC Scopus subject areas

  • Materials Science(all)

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