Study of structural and optical properties of InSb-doped SiO2 thin films

The non-linear absorption and the structure of rf-cosputtered SiO2:InSb films were investigated after different heat-treatments at temperatures ranging from 200 to 900 °C. The heat-treatment was 1 min or 4 h in an atmosphere of nitrogen or under vacuum, respectively. After annealing at a temperature higher than 200 °C, the presence of nanocrystallites in the layers was detected by transmission electron microscopy (TEM) and Raman spectroscopy. The growth of either Sb or InSb nanocrystals essentially depends on both the annealing temperature and the heat-treatment time. Particularly, it was shown that a rapid thermal annealing (RTA) is necessary to obtain crystals of InSb with size estimated to be about 15 nm. The largest positive non-linear coefficient β, as measured by the Z-scan method at 1064 nm in a nanosecond regime, was found to be equal to 1.8 × 10−3 cm/W when the films contain InSb particles of larger size (up to 100 nm). However, when increasing the incident irradiance on the sample, the non-linear effect does not increase and seems to saturate, which can explain the discrepancies observed with previous works on similar materials. This disagreement may also be due to different response times of the non-linearities related to thermal effects in the continuous wave (CW) open-aperture Z-scan configuration.