Fig. 1: Air-induced surface evolution on engineered sapphire.
a Schematic illustrating the surface structural changes of c-plane sapphire (α-Al2O3) during air exposure and its impact on domain orientation of epitaxy. b Schematic of the formation of aluminum (oxy)hydroxide on sapphire surface during the air exposure. Blue spheres represent Al atoms, red spheres represent O atoms, white spheres represent H atoms. c The atomic force microscopy (AFM) image of freshly annealed sapphire with atomic steps towards m \( < 10\bar{1}0 > \) direction. d AFM image of sapphire, air exposed for 6 weeks, displaying additional cluster formation along atomic steps, which indicates preferential hydrolysis at these locations. e–g Cross-section scanning transmission electron microscope (STEM) images of sapphire surfaces (on the terraces) after freshly annealing and aging in air for 4 and 6 weeks, viewed from the \( < 11\bar{2}0 > \) direction, revealing a crystalline atomic resolved surface on freshly annealed sapphire and the development of amorphous layers during air exposure. h, i, High-resolution Al 2p and O 1 s X-ray photoelectron spectroscopy (XPS) spectra of freshly annealed and 1-week-aged sapphire samples under high low-energy electron flux settings, identifying the characteristic Al-O bonding spectra of α-Al2O3 in freshly annealed sapphire, and spectral shape change becoming detectable by XPS evidenced by additional Al-OH bonding peaks of aluminum (oxy)hydroxide likely arising from surface hydrolysis, as elucidated through XPS peak deconvolution for the 1-week aged sapphire sample.
