Unraveling the atomic mechanism of the disorder-order phase transition from -Ga₂O₃ to -Ga₂O₃

In this paper, we employ in situ transmission electron microscopy to study the disorder-order phase transition from amorphous Ga2O3 to gamma-Ga2O3 and then to beta-Ga2O3. The in situ studies are complemented by ex situ annealing experiments, of which the results are analyzed by x-ray diffraction and high resolution (scanning) transmission electron microscopy. Amorphous Ga2O3 deposited at 100 degrees C by molecular beam epitaxy crystallizes at 470 degrees C in the gamma phase (Fd (3) over barm), which undergoes a phase transition to the beta phase above 500 degrees C. Between 500 degrees and 900 degrees C, we find a mixture of gamma-Ga2O3 and beta-Ga2O3 coexisting. Above 950 degrees C, we find only beta-Ga2O3. Through our analyses and by considering symmetry relations, we have constructed a coincidence site lattice of both structures containing a common fcc-type sublattice occupied by oxygen atoms, the cation sites of beta-Ga2O3 common to both phases, and partially occupied cation sites in the gamma phase corresponding to the interstitial sites in the beta phase. We assign the atomic displacements within this lattice responsible for transforming the initially disordered spinel structure with partially occupied cation sites into the well-ordered lattice of beta-Ga2O3. We identify this transition as a reconstructive disorder-to-order phase transition, mediated by the exchange of cations to next nearest neighbor sites. Our model not only explains recent observations of the formation of gamma-Ga2O3 during implantation for n-type doping and the subsequent recovery of beta-Ga2O3 following annealing but also holds potential for inspiring understanding in other materials with similar phase transitions.