Unraveling the Atomic Mechanism of the Disorder- Order Phase transition from γ-Ga2O3 to β-Ga₂O₃ by in situ Transmission Electron Microscopy

Monoclinic Ga₂O₃ (β-Ga₂O₃) is a semiconductor with a bandgap of 4.7 eV and an estimated breakdown field strength of 8 MVcm⁻¹. It has attracted considerable interest as a promising material for electronic applications such as solar blind UV photodetectors, and high power devices. In addition to the thermodynamically stable monoclinic β-phase, Ga₂O₃ can be stabilized under ambient conditions in a number of metastable structures (α-Ga2O3 , γ-Ga2O3, κ-Ga2O3 and δ-Ga2O3) in which Ga cations exist in both tetrahedral (GaO4) and octahedral (GaO6) coordination. The polymorphic transition between β-Ga₂O₃ and γ-Ga₂O₃has recently attracted particular attention and has become the focus of intense research efforts. These research activities were triggered by the experimental observation that after implantation of Si or Ge, used to selectively n-dope β-Ga₂O_3, a phase transition from β-Ga₂O₃ to γ-Ga₂O₃ was observed. [1,2,3] Recovery of β-Ga₂O₃ was observed after annealing. [1,2] It was also observed that β-Ga₂O₃ is extremely radiation tolerant. Since none of the other crystalline phases were observed to form, the question of a hidden structural relationship between γ-Ga₂O₃ and β-Ga₂O₃ was raised.<br/>In this paper we study the reverse direction, i.e. the disorder-order phase transition from amorphous Ga₂O₃ to γ-Ga₂O₃ and then to β-Ga₂O₃ by <i>in situ</i> transmission electron microscopy. The <i>in situ</i> studies are complemented by <i>ex situ</i> annealing experiments, the results of which are analyzed by X-ray diffraction and high resolution (scanning) transmission electron microscopy. Amorphous Ga₂O₃ deposited at 100°C by molecular beam epitaxy crystallizes at 470°C in the γ phase , which undergoes a phase transition to the β phase above 500°C. Between 500° and 900°C we find a mixture of γ-Ga₂O₃ and the β-Ga₂O₃ coexisting. Above 950°C we find only β-Ga₂O₃. Taking into account the crystallographic symmetry relations, we construct a common lattice of both structures containing a common fcc-type sublattice occupied by oxygen atoms, the cation sites of β-Ga₂O₃ common to both phases, and partially occupied cation sites in the γ-phase corresponding to interstitial sites in the β-phase. We assign the atomic displacements within this lattice that transform γ-Ga₂O₃ (the defective spinel structure with partially occupied cation sites) into perfectly ordered β-Ga₂O₃. By its nature, this is a reconstructive disorder-order phase transition in which atoms exchange positions to the nearest neighbor sites. Few of the exchange processes we derive from crystallographic symmetry relations match those derived by Huang et al. [2,3] from analysis of atomically resolved scanning transmission electron microscopy (STEM) images of implanted β-Ga₂O₃. Our results explain, why disordering of the lattice, e.g. by implantation, causes the formation of γ-Ga₂O₃ and not any of the other phases. This would require a martensitic transition, i.e. shearing of the oxygen sublattice and not simply displacement of cations. It also explains, why annealing causes the reverse phase transition and results in a perfect β-Ga₂O₃ lattice as described by Huang et al.. [2]<br/><br/>[1] A. Azarov, et al. ,Nature Communications 14, 4855 (2023)<br/>[2] L. Huang, et al. APL Mater. 11, 061113 (2023).<br/>[3] L. Huang et al., Appl. Phys. Lett. 122, 251602 (2023),