Investigation of Alumina Atomic-Scale Structure and Crystallization Behavior Across a β-Ga2O3/Al2O3 Interface

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Interfaces can induce unique material behavior due to strain, electronic band bending, and their potential stabilization of defects. Several prior studies have observed crystallization behavior within the alumina (Al2O3) system, especially when grown on beta gallium oxide (β-Ga2O3) [1-3]. β-Ga2O3 is a promising next generation ultrawide band gap semiconductor, possessing a band gap of 4.8 eV and a tunable carrier concentration via an array of doping candidates [4]. In an amorphous form, Al2O3 has been extensively studied as an insulator in the semiconductor industry and specifically as a gate dielectric in β-Ga2O3 devices [5-7]. Understanding Al2O3/β-Ga2O3 heterointerfaces is vital for developing versatile β-Ga2O3 device applications, since Al2O3’s relation to β-Ga2O3 across an interface is of broad interest in the power electronics industry. The most stable alumina crystalline polymorph, α-Al2O3, is a well understood crystal with the corundum structure (R-3c point group). The exact atomic structures of the other Al2O3 polymorphs, however, are not as well understood [8-10]. Monoclinic θ-Al2O3 has primarily been studied for applications in catalysis and was recently proposed to be a mixture of β-Ga2O3-like monoclinic regions interspersed by perpendicular columns of octahedral Al atoms, creating a monoclinic structure not fully analogous to β-Ga2O3 [11]. In this study, we present some of the first examples of θ-Al2O3 stabilization across a β-Ga2O3 interface. Our prior studies have shown defect-free β-Ga2O3 near a Al2O3 dielectric layer grown at 600°C [12]. In these samples, the Al2O3 layer was observed to be crystalline exactly along the interface and turning almost entirely amorphous stepping a few nanometers away from the β-Ga2O3 interface. Here, we investigate the crystallization behavior of a Al2O3 dielectric layer in an in-situ metal organic chemical vapor deposited β-Ga2O3/Al2O3/Ni/Au capacitor stack grown at 800°C. Via electron diffraction, atomic-resolution scanning/transmission electron microscopy (S/TEM), and electron energy loss spectroscopy (EELS), we observe complete crystallization of the 25 nm thick Al2O3 layer (Figure 1). Within 5 to 10 nm of the β-Ga2O3 interface, well-ordered Al2O3 epitaxial growth occurs in the θ phase, with some regions displaying a [010] zone axis projection while other regions show a rotation to the [001] zone axis. Fast-Fourier Transform patterns and high-resolution TEM imaging indicate a crystalline structure maintained throughout the film with structural disorder and defects prominently in the form of stacking faults (Figure 2). This work further explores the atomic-scale structure of the monoclinic polymorph of Al2O3, and the variations in the bonding through S/TEM-EELS at the interface, while informing its structural relation to β-Ga2O3 in heterostructures [13]. Atomic-resolution annular dark-field S/TEM image of the β-Ga2O3/Al2O3 interface. The boxed region in red is shown in the magnified inset to the right, better demonstrating the atomic structure of the Al2O3 layer. Clearly resolved atoms are visible near the interface, but this contrast decreases moving further away from the interface. The atomic structure motif resolved near the interface does not exactly match that of the β-Ga2O3. (a) Annular bright-field S/TEM image of the heterostructure stack. Some regions are visible with faint inclined contrast streaks within the Al2O3 layer, suggesting the presence of stacking faults. These defects are better visualized in (b), a high-resolution TEM image of the same area shown in (a). Here, the inclined streaks are highlighted in yellow, again demonstrating how they do not form near the β-Ga2O3/Al2O3 interface.