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【Knowledge Discover】Panoramic Insight into Gallium Oxide (Part II): One Material, Multiple Phases — Understanding the Five Crystal Structures of Ga₂O₃
日期：2026-05-22阅读：14
Introduction
In the previous issue, we systematically introduced the importance of gallium oxide in the field of ultra-wide-bandgap semiconductors. As a new-generation semiconductor material that has attracted significant attention in recent years, gallium oxide features an ultra-wide bandgap of approximately 4.9 eV and a critical breakdown field strength of up to about 8 MV/cm. With these outstanding material parameters, Ga₂O₃ demonstrates broad application prospects in high-power electronics and solar-blind deep ultraviolet optoelectronic devices.
However, what makes gallium oxide particularly fascinating is not only its excellent electrical properties, but also its rich and unique crystallographic diversity. Unlike many semiconductor materials, Ga₂O₃ does not exist in a single stable phase; instead, it can form multiple polymorphs, including the α, β, γ, δ, and ε/κ phases. These different crystal structures arise from distinct atomic arrangements, leading to significant variations in bandgap, thermodynamic stability, polarity, defect behavior, and epitaxial growth characteristics.
This polymorphism endows gallium oxide with deeper scientific richness. Just as carbon can exist as layered graphite or as a three-dimensional covalent network in diamond, the different phases of Ga₂O₃ similarly reveal a profound relationship between crystal structure and material properties. Understanding these polymorphs is not only essential for grasping the fundamental physical and chemical nature of Ga₂O₃, but also provides important guidance for future device design, epitaxial engineering, and the expansion of application scenarios.
Part II: One Material, Five Polymorphs — A Complete Overview of Gallium Oxide Crystal Structures
The Complex Polymorphic Family of Gallium Oxide
If the bandgap and critical breakdown field determine the “performance ceiling” of gallium oxide, then its complex and diverse crystal structures define the material’s most distinctive character. From the thermodynamically stable β-phase to a variety of structurally distinct metastable polymorphs, gallium oxide exhibits a uniquely rich polymorphic nature.
Comparison of Typical Gallium Oxide Crystal Structures [1].
Gallium oxide exhibits rich polymorphic characteristics, with the most common phases including α-, β-, γ-, δ-, and ε(κ)-Ga₂O₃. These correspond respectively to the trigonal/rhombohedral corundum structure, monoclinic β-gallia structure, cubic defect spinel structure, cubic bixbyite-like structure, and orthorhombic structure. Significant differences exist among these polymorphs in terms of lattice symmetry, atomic arrangement, and Ga–O coordination environments, forming the structural basis for the remarkable polymorphism of gallium oxide.
The polymorphic behavior of Ga₂O₃ is closely related to the flexible coordination chemistry of Ga³⁺ ions. Gallium ions can adopt both tetrahedral and octahedral coordination configurations, and these coordination polyhedra can interconnect through corner-sharing or edge-sharing arrangements, thereby constructing crystal frameworks with substantially different structures [2]. Among these polymorphs, the β-phase is the thermodynamically stable phase under ambient conditions and can be grown as bulk single crystals using melt-growth methods, making it the most important phase for substrate and power device research. In contrast, the α-, γ-, δ-, and ε/κ-phases are generally metastable and are typically obtained through thin-film epitaxy, low-temperature synthesis, or specific temperature-pressure conditions. These metastable phases may subsequently transform into the β-phase during thermal processing.
Schematic Illustration of Ga₂O₃ Polymorphs and Their Phase Transformation Relationships [3].
Among the metastable polymorphs, α-Ga₂O₃ is widely regarded as an important phase for heteroepitaxial research because it shares the same corundum structure as sapphire α-Al₂O₃, providing favorable lattice compatibility for epitaxial growth [4]. γ-Ga₂O₃ possesses a defect spinel structure with a high concentration of cation vacancies, attracting significant interest in catalysis, gas sensing, and defect engineering studies [5]. δ-Ga₂O₃ is generally considered to exhibit a cubic bixbyite-like structure. Although experimental investigations remain relatively limited, its high-symmetry crystal structure gives it certain value in theoretical calculations and phase stability studies [6].
In recent years, ε/κ-Ga₂O₃ has emerged as a major focus in thin-film epitaxy research. In particular, the κ-phase is commonly regarded as an orthorhombic structure, and its polar, non-centrosymmetric nature suggests the potential for spontaneous polarization and ferroelectric-related properties. This opens new possibilities for polarization engineering, ferroelectric oxide heterostructures, and next-generation optoelectronic device design [7].
β-Ga₂O₃: The Thermodynamically Stable Phase and Mainstream Device Platform
β-Ga₂O₃ belongs to the monoclinic crystal system with the space group C2/m and is the most thermodynamically stable polymorph of gallium oxide under ambient conditions. Within its crystal structure, there are two inequivalent gallium atomic sites and three inequivalent oxygen atomic sites.
The Ga(I) sites typically exhibit tetrahedral coordination, located at the center of a tetrahedron formed by four neighboring oxygen atoms, constituting [GaO₄] tetrahedra. In contrast, the Ga(II) sites exhibit octahedral coordination, positioned at the center of oxygen octahedra to form [GaO₆] octahedra. The three inequivalent oxygen sites possess distinct local coordination environments, collectively giving rise to the characteristic low-symmetry crystal framework of β-Ga₂O₃.
Schematic crystal structure of monoclinic β-Ga₂O₃. The green and red spheres represent Ga and O atoms, respectively [8].
This structure, formed by the interconnected arrangement of [GaO₄] tetrahedra and [GaO₆] octahedra, is one of the defining characteristics that distinguishes β-Ga₂O₃ from many other oxide semiconductors. The tetrahedral and octahedral units are linked through corner-sharing and edge-sharing configurations, creating an anisotropic crystal network.
Because the conduction band minimum is primarily derived from the Ga s orbitals, β-Ga₂O₃ exhibits a relatively small electron effective mass and favorable electron transport capability. At the same time, its low-symmetry lattice leads to pronounced anisotropy in electron transport, thermal transport, and defect behavior. Together, these structural characteristics form the important microscopic foundation underlying the excellent semiconductor properties of β-Ga₂O₃.
The widespread attention attracted by β-Ga₂O₃ largely stems from its unique combination of advantages, including high breakdown capability, deep-ultraviolet response, and scalability for large-area crystal growth and device fabrication.
Ultraviolet Spectral Regions and Their Band Classification [9].
Among these advantages, the most critical strength of β-Ga₂O₃ lies in its outstanding industrialization potential. Its melting point is approximately 1800 °C, and the β-phase remains stable even near the melting temperature, enabling the growth of large-size substrates through melt-growth techniques such as the edge-defined film-fed growth (EFG) method, Czochralski method, and Bridgman method.
Compared with SiC and GaN, which generally rely on more complex and expensive substrate fabrication routes, β-Ga₂O₃ demonstrates more significant potential advantages in substrate scaling and manufacturing cost reduction.
Typical Melt-Growth Methods for Bulk β-Ga₂O₃ Single Crystals [10].
α-Ga₂O₃: Epitaxial Advantages Enabled by the Corundum Structure
From the perspective of atomic arrangement, the crystal structure of α-Ga₂O₃ is relatively well ordered. Within its lattice, Ga atoms are primarily located in sixfold coordination environments, surrounded by oxygen atoms to form [GaO₆] octahedra. The oxygen atoms are bonded to multiple Ga atoms, collectively constructing the dense crystal framework characteristic of corundum-type oxides.
Compared with the more complex structure of β-Ga₂O₃, where tetrahedrally coordinated and octahedrally coordinated Ga atoms coexist, α-Ga₂O₃ possesses a more uniform local coordination environment and higher structural symmetry.
Schematic crystal structure of corundum-type α-Ga₂O₃.
The green spheres represent Ga atoms, while the red spheres represent O atoms [11].
It is precisely this crystal structure, which closely resembles that of commercially mature sapphire substrates, that gives α-Ga₂O₃ inherent advantages in heteroepitaxial growth. When α-Ga₂O₃ thin films are grown on sapphire substrates, the substrate can serve as a structural template, reducing the difficulty of phase formation during epitaxy. As a result, α-Ga₂O₃ is widely regarded as one of the most representative metastable polymorphs in gallium oxide heteroepitaxy research.
Atomic-Scale Structural Characterization of the α-Ga₂O₃/α-Al₂O₃ Heterointerface [12].
γ/δ-Ga₂O₃: The Dual Faces of Cubic Phases
In addition to the α-, β-, and ε/κ-phases, γ-Ga₂O₃ and δ-Ga₂O₃ are also two notable metastable polymorphs within the gallium oxide family. Both are generally classified within the cubic crystal system, yet they differ significantly in atomic arrangement, coordination environment, and research focus, representing two distinct structural manifestations of cubic-phase gallium oxide.
Defect Spinel Structure and Local Cation Occupancy Disorder Characteristics of γ-Ga₂O₃ [5].
The space group of γ-Ga₂O₃ is commonly approximated as Fd-3m. Its structure is closely related to the typical spinel structure of MgAl₂O₄. However, because the stoichiometry of Ga₂O₃ does not perfectly match that of an ideal spinel lattice, a certain concentration of cation vacancies exists within the crystal, which is why γ-Ga₂O₃ is often referred to as a “defect spinel phase.”
This vacancy-rich structure gives γ-Ga₂O₃ unique characteristics in defect chemistry, surface reactions, and ion migration. In contrast to β-Ga₂O₃, which is primarily studied for power electronics applications, γ-Ga₂O₃ is more commonly investigated in catalysis, gas sensing, nanomaterials, and surface chemistry research. However, γ-Ga₂O₃ exhibits relatively poor thermal stability and can readily transform into the β-phase after moderate thermal treatment, resulting in a comparatively limited high-temperature processing window.
