Transformation of photoluminescence and Raman scattering spectra of Si-rich Al₂O₃ films at thermal annealing

The effect of thermal annealing on optical properties of Al₂O₃ films with the different Si contents was investigated using the photoluminescence and Raman scattering methods. Si-rich Al₂O₃ films were prepared by RF magnetron co-sputtering of Si and Al₂O₃ targets on long quartz glass substrates. Photoluminescence (PL) spectra of as grown Si-rich Al₂O₃ films are characterized by four PL bands with the peak positions at 2.90, 2.70, 2.30 and 1.45 eV. The small intensity Raman peaks related to the scattering in the amorphous Si phase has been detected in as grown films as well. Thermal annealing at 1150°C for 90 min stimulates the formation of Si nanocrystals (NCs) in the film area with the Si content exceeded 50%. The Raman peak related to the scattering on optic phonons in Si NCs has been detected for this area. After thermal annealing the PL intensity of all mentioned PL bands decreases in the film area with smaller Si content (≤50%) and increases in the film area with higher Si content (≥50%). Simultaneously the new PL band with the peak position at 1.65 eV appears in the film area with higher Si content (≥50%). The new PL band (1.65 eV) is attributed to the exciton recombination inside of small size Si NCs (2.5-2.7 nm). In bigger size Si NCs (3.5-5.0 nm) the PL band at 1.65 eV has been not detected due to the impact, apparently, of elastic strain appeared at the Si/Al₂O₃ interface. Temperature dependences of PL spectra for the Si-rich Al₂O₃ films have been studied in the range of 10-300 K with the aim to reveal the mechanism of recombination transitions for the mentioned above PL bands 2.90, 2.70, 2.30 and 1.45 eV in as grown films. The thermal activation of PL intensity and permanent PL peak positions in the temperature range 10-300 K permit to assign these PL bands to defect related emission in Al₂O₃ matrix.