Optical study of Ga_2-xSn_xO₃ (0 ≤ x ≤ 0.7) thin films using spectroscopic ellipsometry and cathodoluminescence

Diagnosis of the semiconductor defect levels is essential for their device applications. The optical and electrical properties of Ga_2-xSn_xO₃ (GTO, 0 ≤ x ≤ 0.7) films were investigated to look into the defect levels. Amorphous GTO thin films were grown on Si substrates with various Sn contents via radio frequency magnetron sputtering deposition. Thermal annealing at 900 °C for two hours was performed after the deposition of films at 500 °C to achieve the polycrystalline phase. We examined the physical properties of amorphous β-Ga₂O₃ thin films grown using various O₂ gas flows during growth, and of the post-annealed GTO films. Using spectroscopic ellipsometry, the optical constants in the spectral range between 1.0 and 6.0 eV were determined. We estimated the optical gap energy of the GTO layers using the Tauc method. The optical gap energy was approximately 4.9 eV for amorphous β-Ga₂O₃ films. Optical structures owing to defects were found below optical gap energy in the spectra of the complex refractive index. Using cathodoluminescence (CL) spectroscopy, there were several peaks discovered in the range between 1.5 and 3.0 eV. The CL peaks correspond to transitions between valence (conduction band) and defect-induced gap states. We identified the 1.5 − 1.6 eV peak as the transition between defect states and the conduction band. The extinction coefficient (k) peak became sharper at 5.3 eV suggesting a strong excitonic behavior at the fundamental gap for amorphous Ga₂O₃ films.