Unravelling the surface structure of β-Ga2O3 (100)

Abstract

The present work is on a comprehensive surface atomic structure investigation of β-Ga₂O₃ (100). The β-Ga₂O₃ single crystal was studied by a structural model system in the simulations and in situ characterization via X-ray photoelectron spectroscopy (XPS), low-energy electron diffraction (LEED) and X-ray photoelectron diffraction (XPD) allowed for probing the outermost layers' properties. In situ XPD characterization allows for the collection of valuable element-specific short-range information from the β-Ga₂O₃ surface, and the results were compared to a systematic and precise multiple scattering simulation approach. The experiments, characterizations, and simulations indicated strong evidence of considerable structural variations in the interatomic layer's distances. Such atomic displacement could clarify the electronic phenomena observed in theoretical studies. The comparison between experimental and theoretical XPD results involving multiple scattering calculations indicated that the β-Ga₂O₃ surface has two possible terminations. The best fits to the photoelectron diffraction curves are used to calculate the interplanar relaxation in the first five atomic layers. The results show good agreement with previous density functional theory calculations, establishing XPD as a useful tool for probing the atomic structure of oxide surfaces.

Fig. 1. Conventional unit cell of monoclinic (C2/m) β-Ga₂O₃. The two different gallium (large red spheres) positions and the three inequivalent oxygen (little blue spheres) positions are illustrated. The possible stable surface termination (100) A and (100) B.

Fig. 2. (a) LEED patterns (188 eV) and (b) XPS survey spectra for the clean β-Ga₂O₃ (100) sample surface.

Fig. 3. HRXPS for (a) Ga 2p_3/2 and (b) O 1s XPS regions from β-Ga₂O₃(100) sample surface.

Fig. 4. (a) Experimental XPD pattern obtained from the β-Ga₂O₃(100) sample surface for O 1s and (b) R_a factor values as a function of termination A and B in the β-Ga₂O₃ (100) surface, considering O as emitter atoms.

Fig. 5. Contour map in the search of lowest R_a factor for (a) d₁₂versus d₂₃ and (b) d₃₄versus d₄₅ for the A termination.

Fig. 6. Contour map in the search of lowest R_a factor for (a) d₁₂versus d₂₃ and (b) d₃₄versus d₄₅ for the B termination.

Fig. 7. Simulated (gray lines) and experimental (open points) azimuthal scans for selected polar angles (for θ = 24°, 33°, 39°, 45°, and 51°) for (a) termination A (blue open points), and (b) termination B (green open points) model structure surface for O as emitter atom from β-Ga₂O₃(100) surface.

Fig. 8. Theoretical XPD patterns obtained from the β-Ga₂O₃(100) surface models, for termination (a) A and (b) B.

Fig. 9. Interlayer distances calculated for the models with termination (a) A and (b) B proposed to describe the β-Ga₂O₃(100) surfaces.