Atomic Layer Epitaxy of (002) κ-Ga ₂ O ₃ on <i>c</i> -Plane Sapphire

Atomic layer epitaxy has been a powerful technique for several decades to grow epitaxial thin films of III–V and II–VI semiconducting compounds. However, the polycrystalline nature of thin films usually prevails for conducting and semiconducting binary oxides, which strongly limits their crystalline and structural quality. Here, we demonstrate the epitaxial growth of (002)-oriented κ-Ga2O3 thin films with a subnm surface roughness on c-plane sapphire by atomic layer deposition at 350 °C and using the relatively unexplored combination of triethylgallium and O3 as chemical precursors. The 16 nm-thick κ-Ga2O3 thin films exhibit an in-plane epitaxial relationship given by (060) κ-Ga2O3||(300) α-Al2O3 and (200) κ-Ga2O3||(110) α-Al2O3. The pure epitaxial κ-Ga2O3 phase is further revealed using X-ray absorption near-edge structure and X-ray linear dichroism with synchrotron radiation along with transmission electron microscopy. In particular, the (004) X-ray diffraction peak for the κ-Ga2O3 thin film grown at 350 °C is shown to have a remarkable full-width-at-half-maximum value of 0.1° following rocking curve measurements, which is comparable to the typical lowest values reported for κ-Ga2O3 thick films grown by more sophisticated epitaxial chemical and physical vapor deposition techniques. The optical bandgap energy is eventually evaluated at 4.8 eV from optical transmittance spectra. These findings show the high potential of atomic layer deposition for growing epitaxial Ga2O3 thin films at relatively low temperatures in the framework of atomic layer epitaxy, strongly challenging the well-known epitaxial physical and chemical deposition techniques.