A combined scanning tunneling microscopy and electron energy loss spectroscopy study on the formation of thin, well-ordered β-Ga2O3 films on CoGa(001)

The formation of thin, well-ordered $\mathrm{\beta -}{\mathrm{Ga}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{3}}$ films on CoGa(001) was studied by means of high resolution electron energy loss spectroscopy (EELS), scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), and Auger electron spectroscopy. The crystalline $\mathrm{\beta -}{\mathrm{Ga}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{3}}$ films on CoGa(001) are prepared upon adsorption of ${\mathrm{O}}_{\mathrm{2}}$ at 300 K and subsequent annealing at 700 K or by oxidation of the sample directly at 700 K, respectively. EEL spectra of these films exhibit Fuchs–Kliewer modes at 305, 455, 645, and $\text{785\hspace{0.17em}}{\mathrm{cm}}^{\mathrm{-1}}$ in good agreement with calculated spectra using the IR parameters of ${\mathrm{Ga}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{3}}.$ The band gap was determined to be $\text{4.5±0.2\hspace{0.17em}}\mathrm{eV}.$ In addition, a gap state at 3.3 eV was found. The observed LEED pattern of $\mathrm{\beta -}{\mathrm{Ga}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{3}}\mathrm{/CoGa}$ (001) can be explained by a $\text{(2×1)}$ structure in two perpendicularly oriented domains. STM images exhibit atomically flat and large oxide terraces (up to ${\text{2500×700\hspace{0.17em}Å}}^2$ ) mainly of rectangular shape. STM pictures with atomic resolution confirm the $\text{(2×1)}$ structure. In addition, a square substructure can be observed which is related to the closed-packed oxygen lattice of $\mathrm{\beta -}{\mathrm{Ga}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{3}}.$