Characteristic luminescence in the high crystalline alpha-Ga ₂O ₃ grown on the thin-wall single crystal Al ₂O ₃

A high-quality alpha-Ga ₂O ₃ thin film was successfully grown on a single-crystal Al ₂O ₃ thin-wall channel structure. The alpha- Ga ₂O ₃ crystals developed a 3-fold symmetry based on the Al ₂O ₃ surface, resulting in domain boundaries due to slight rotations within the mosaic structure. Figure 1(a) illustrates a schematic view of the alpha- Ga ₂O ₃ thin films, with the blue area indicating the sampling zone for transmission electron microscopy observations. Figure 1(b) presents a brightfield image of the transition region between lateral and vertical growth, while Figure 1(c) shows the ADF image of this same transition region.

The boundaries of the Anti-phase domains (APDs) were identified with weak contrast, and changes in the growth direction were observed in the valley region. APDs were confirmed through convergent-beam electron diffraction (CBED), indicating that they formed during the nucleation stage, likely due to unstable or uneven Al-O bonding on the Al ₂O ₃ surface. The in-phase domains and APD boundaries exhibited bright contrast in ADF imaging, attributed to local strain resulting from crystallographic rotation rather than variations in gallium concentration.

Cathodoluminescence (CL) measurements taken within transmission electron microscopy (TEM) indicated that the bandgap energy of alpha- Ga ₂O ₃ is approximately 5.56 eV, corresponding to a near-band-edge transition at around 223 nm. While the heights and positions of the luminescence peaks vary slightly across different regions, four distinct peaks and a broad tail in the wavelength spectrum are consistently observed throughout the crystal. Two-dimensional mapping within a specific wavelength range reveals clear differences between the interior domain, in-phase domain boundary, and APD boundary, with a notable luminescence peak at 350 nm specifically at the APD boundary.

Figure 1

(a) illustrates a schematic representation of the alpha-Ga ₂O ₃ thin film growth on the thin Al ₂O ₃ bridge. (b) shows an ADF image from the transition region of the merged valley, and (c) reveals a high-magnification view of the transition region, highlighting the two distinct growth modes of the anti-phase domains.