The real structure of ε-Ga₂O₃ and its relation to κ-phase

Abstract

A comprehensive study by high-resolution transmission electron microscopy (TEM) and X-ray diffraction (XRD) was carried out on Ga 2 O 3 epilayers grown at low temperature (650°C) by vapor phase epitaxy in order to investigate the real structure at the nanoscale. Initial XRD measurements showed that the films were of the so-called ε phase; i.e. they exhibited hexagonal P6 3 mc space group symmetry, characterized by disordered and partial occupation of the Ga sites. This work clarifies the crystal structure of Ga 2 O 3 layers deposited at low temperature at the nanoscale: TEM investigation demonstrates that the Ga atoms and vacancies are not randomly distributed, but actually possess ordering, with (110)-twinned domains of 5-10 nm size. Each domain has orthorhombic structure with Pna2 1 space group symmetry, referred to as κ-Ga 2 O 3. Further XRD analysis carried out on thicker samples (9-10 μm) confirmed this finding and provided refined structural parameters. The six (110)-type twinned ordered domains togetherif the domain size falls below the actual resolution of the probing techniquescan be misinterpreted as the disordered structure with its P6 3 mc space group symmetry usually referred to as ε-Ga 2 O 3 in the current literature. The crystal structure of these Ga 2 O 3 layers consists of an ABAC oxygen close-packed stacking, where Ga atoms occupy octahedral and tetrahedral sites in between, forming two types of polyhedral layers parallel to (001). The edge-sharing octahedra and the corner-sharing tetrahedra form zigzag ribbons along the [100] direction. Anti-phase boundaries are common inside the domains. The polar character of the structure is confirmed, in agreement with the characteristics of the Pna2 1 space group and previous observations.

α-Ga2O3 grown by low temperature atomic layer deposition on sapphire

“…Bandgap engineering is required to construct various optoelectronic devices, and polymorphic control is required to achieve the necessary variation in composition in ternary and quaternary group III-oxides. Ga2O3 adopts numerous polymorphs and complete understanding of polymorphism in this system remains lacking [2,3]. High-quality structures are known for rhombohedral  [4], monoclinic  [5], cubic γ [6], and hexagonal  [2] polymorphs, all of which are based on approximately hexagonally close-packed layers of oxygen ions with gallium ions occupying the octahedral and tetrahedral sites differently [2].…”

“…High-quality structures are known for rhombohedral  [4], monoclinic  [5], cubic γ [6], and hexagonal  [2] polymorphs, all of which are based on approximately hexagonally close-packed layers of oxygen ions with gallium ions occupying the octahedral and tetrahedral sites differently [2]. Two further polymorphs have been suggested, namely δ [6] and orthorhombic κ [2,3], the latter being related to the  phase as an ordered variant.…”

Ordinary dielectric function of corundumlike α−Ga2O3 from 40 meV to 20 eV

“…Additionally, other metastable polytypes of Ga 2 O 3 are known, and their precise physical properties and even their crystal structures are still partly under debate [16,17]. These polytypes have not been studied yet in sufficient detail to * martin.feneberg@ovgu.de unleash their obviously high potential.…”

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

“…In each designated Ga 2 O 3 polymorphs, oxygen atoms form the same close packing environment, whereas Ga atoms occupy octahedral and tetrahedral positions with different proportions. [ 22 ] β‐Ga 2 O 3 polymorph contains equal proportions of Ga 3+ (with 1:1 ratio) in octahedral and tetrahedral sites. [ 23 ] It is the most stable phase amid the reported Ga 2 O 3 polymorphs.…”

“…The structure is found similar to ε‐Fe 2 O 3 and κ‐Al 2 O 3 . [ 22,26–30 ] However, TEM measurement shows Ga and their vacancies retain ordering with (110)‐twined domains. The domain possesses an orthorhombic phase with space group Pna 2 1 referred to as κ‐Ga 2 O 3 .…”