Discovery of New Polymorphs of Gallium Oxides with Particle Swarm Optimization-Based Structure Searches

Gallium oxide (Ga2O3) has attracted significant research interest for next-generation high-efficiency power devices because of its unique electronic properties such as ultra-wide band gap, high breakdown electric field, and large Baliga's figure of merit. Ga2O3 crystallizes in a series of reported polymorphs including beta-, alpha-, epsilon-, kappa-, gamma-, and delta-Ga2O3, the structures of some of which are still in serious controversy. A polymorph structure search study of Ga2O3 is herein performed by combining particle swarm optimization with first-principles energetic calculations. In addition to producing the predominant experimental known phases of beta-, alpha-, and kappa-Ga2O3, two new polymorphs are found with space group P1 over bar and Pmc2(1) consisting of four- and five-fold coordinated Ga. They show comparable energy with beta- and alpha-Ga2O3 with the energy difference of several meV per atom, and exhibit robust phonon stability. Similarly, the new phases show quite wide band gaps and small electron effective masses by comparing it with other known phases. The Pmc2(1) phase shows a calculated spontaneous polarization of 0.277 C m(-2), close to that of epsilon/kappa-Ga2O3. The systemic structure searches also establish a structural relationship between epsilon-Ga2O3 and kappa-Ga2O3 and how the electronic properties vary with polymorphic phase change.