Layered phase composition and microstructure of κ-Ga2O3-dominant heteroepitaxial films grown via MOCVD

Abstract

Phase and microstructural evolution of gallium oxide (Ga2O3) films grown on vicinal (0001) sapphire substrates was investigated using a suite of analytical tools. High-resolution transmission electron microscopy and scanning transmission electron microscopy of a film grown at 530 °C revealed the initial pseudomorphic growth of three to four monolayers of α-Ga2O3, a 20–60 nm transition layer that contained both β- and γ-Ga2O3, and a top ∼700 nm-thick layer of phase-pure κ-Ga2O3. Explanations for the occurrence of these phases and their sequence of formation are presented. Additional growths of Ga2O3 films in tandem with x-ray diffraction and scanning electron microscopy investigations revealed that the top layer varied in phase composition between ∼100% κ-Ga2O3 and ∼100% β-Ga2O3; the surface microstructure ranged from poorly coalesced to completely coalesced grains as a function of growth temperature, growth rate, or diluent gas flow rate. In general, it was found that the κ-phase is favored at lower growth temperatures and higher triethylgallium flow rates (low VI/III ratios). The growth of nominally single-phase κ-Ga2O3 within the top layer was observed in a temperature range between 500 and 530 °C. Below 470 °C, only amorphous Ga2O3 was obtained; above 570 °C, only the β-phase was deposited.

Orthorhombic undoped κ-Ga2O3 epitaxial thin films for sensitive, fast, and stable direct X-ray detectors

“…It is worth noting here that often the literature indifferently reports κ-and ε-Ga 2 O 3 ; this ambiguity originates from the six-fold symmetry of early diffraction patterns and the consequent classification of such Ga 2 O 3 phase as "hexagonal", hence the ε-Ga 2 O 3 ascription. A comprehensive TEM study 20 , as well as other works recently published [21][22][23][24] , showed definitely that the so-called κ phase is indeed orthorhombic and not hexagonal. In analogy to the usual nomenclature of other metal-oxides, it is therefore correct to denominate the orthorhombic phase as κ, as it will be done in this paper.…”

High-quality heteroepitaxy of ε-Ga2O3 films on 4H-SiC substrates grown via MOCVD

“…6(a), the ε-Ga 2 O 3 film, with a thickness of approximately 970 nm, has a high density of tilt line-like contrast threading the whole film. The presence of this contrast can be attributed to boundaries between rotational domains, 23,28 and the tilting phenomenon is caused by the lattice tilt in ε-Ga 2 O 3 induced by misoriented substrates. 29 The nucleation layer is not composed solely of the ε-phase but rather consists of a mixed-phase structure that includes the β-phase and ε-phase, as shown in Fig.…”

Two-step growth of κ-Ga2O3 thin films on 4H-SiC substrates with temperature-varied buffer layers using mist chemical vapor deposition

“…31) The occurrence of rotational domains in orthorhombic κ-Ga 2 O 3 has also been an obstacle in achieving high phase purity despite the attempt at realizing various domain-matched substrates. 32,33) In most cases, the misoriented crystal domains cause deterioration of the surface morphology and crystal quality of κ-Ga 2 O 3 thin films. 32,33) Considering these behaviors of κ-Ga 2 O 3 , we conclude that three factors play a key role in growing good-quality κ-Ga 2 O 3 thin films on heterosubstrates: (i) reaction temperature in deposition system for phase stability, (ii) selection of substrates for domainmatched atomic arrangement, and (iii) control of initial nucleation and subsequent evolutionary growth of thin films.…”

“…control of growth temperature and gas flow, adoption of lattice-matched substrates, three-dimensional nucleation, two-step growth, etc. 28,30,[33][34][35][36] In particular, there has been remarkable progress in the epitaxial growth of single-domain κ-Ga 2 O 3 thin films using floating-zone-grown ε-GaFeO 3 substrates having lattice parameters almost similar to those of κ-Ga 2 O 3 . 36) In-plane orientation control of (001) κ-Ga 2 O 3 was also demonstrated by epitaxial lateral overgrowth.…”