Effect of substrates on the structural, surface chemical state, and optical properties of Ga-doped Al2O3 thin films

We report the stabilization of Ga-doped Al2O3 2 O 3 (GaAlO) thin films into rhombohedral structure (R 3 c space group). We used the target material of composition A1 1.5 Ga 0.5 O 3 (bulk) to deposit the films on Alumina (0001) and p-type Si (100) substrates by using the electron beam evaporation technique. X-ray diffraction pattern confirmed the formation of polycrystalline rhombohedral structure for the as-grown films on Alumina substrate, whereas the films grown on Si substrate showed amorphous structure. The post-heat treatment of as-grown films at 550 degrees C stabilized rhombohedral structure in all the films, irrespective of the substrates. X-ray photoelectron spectra confirmed modification in surface chemical state of the films depending on nature of the substrates. Optical bandgap of the GaAlO films was stabilized in the range of 3.44 eV-4.04 eV, which is substantially reduced in comparison to corundum structured alpha-Al2O3 2 O 3 having optical band gap in the range of 8-9 eV and alpha-Ga2O3 2 O 3 having optical band gap of 4.5-5.5 eV. The defect-induced electronic states and substrate induced interfacial diffusion contributed additional energy gap in the range of 1.50-2.92 eV for GaAlO films grown on Si substrate. The development of highly crystalline corundum structured alpha-(Al, Ga)2O3 2 O 3 alloyed thin films and engineering their band gap energy in a wide range (1.5-4 eV) is useful for applications of semiconductor metal oxides in high power and medium frequency opto-electronic devices.