6. Third order Non-linear optical properties of PLD Si, SiO x and SiC thin films
6.5 Conclusion
Chapter 6: Third order Nonlinear optical properties of PLD Si, SiOx and a-SiC thin films.
156 | P a g e stoichiometry and crystallinity of the films, later two of which were dependent on the deposition parameters of PLD technique.
Figure 6.11 χ(3) values for the different band gap engineered PLD Si-based thin films deposited under different deposition parameters.
Chapter 6: Third order Nonlinear optical properties of PLD Si, SiOx and a-SiC thin films.
157 | P a g e order of 10-4 cm2/W. A large third order NLO susceptibility of the order of 10-1 esu was observed in these PLD nc-Si films, which is 109 times higher as compared to that of a bulk Si. The optical limiting was observed in all these nc-Si films where limiting threshold power was observed to be increasing with growth temperature due to decrease in values of ß.
The SiOx (~0.03 – 2.14) films fabricated as a function of background O2 pressure, from 10-4 to 0.5 mbar also exhibited third order NLO. The value of β was observed to be decreasing from 21.4 - 2.0 cm/W, with increase in the O2 pressure, changing x from 0.01 to 2.14. The value of n2 was found to be decreasing from 44.2 × 10-5 - 1.9 × 10-5 cm2/W, with similar increase in O2 pressure. The significantly large NLO susceptibility of the order of 10-1-10-3 esu was observed in these films. The SiOx films except the one containing maximum oxygen content exhibited OL. The OL effect in these films were due to RSA and optical limiting threshold was found to increase with increasing oxygen content. Thus these results indicate that a tunable NLO and OL films can be fabricated by controlling the stoichiometry of SiOx films which in case of PLD technique can be performed simply by controlling the background oxygen pressure.
The values of ß and n2 for PLD a-SiC thin films were found to be decreasing from 0.75 - 0.34 cm/W and 7.68 - 3.14 × 10-5 cm2/W respectively with increasing Ts from RT to 750 °C, following similar decreasing trend of α. This is due to the change in stoichiometry from Si-rich to nearly stoichiometric a-SiC with increasing Ts from RT to 700° C during deposition. The NLO susceptibility of the order of 10-2 esu was observed in these films.
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158 | P a g e
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Chapter 7
Conclusions
The present research work was aimed towards the fabrication and characterization of nc-Si, SiOx and SiC semiconductor thin films via PLD technique in order to study the effect of substrate temperature and background gas pressure on their stoichiometry and crystallinity, which in turn control structural as well as optical properties. In addition a- Si:SiO2 nanostructures were also fabricated by direct laser irradiation of Si wafer as a function of laser fluence. The room temperature (RT) photoluminescence properties of these films and nanostructures along with their possible origins were investigated. The effect of film stoichiometry on the third order NLO properties and optical limiting (OL) properties were studied as well.
Initially, the a-Si:SiO2 nanostructures were fabricated directly on crystalline Si (c- Si) substrate by irradiating it with nearly focused beam of a second harmonic (532 nm) of a Q switched high power Nd: YAG laser (pulse duration of 8 ns and repetition rate of 10 Hz). The laser ablation was performed in air at RT as a function of laser fluence approximately from 0.35 to 2.67 Jcm-2. This resulted in the formation of crater-like central crystalline region surrounded by the micron sized cauliflower-like clusters of a-Si embedded within SiO2 matrix. The amorphous nature of the clusters, which were composed of a-Si as well as amorphous SiO2,was confirmed by Raman spectra, FTIR spectra and SAED patterns. The EDX spectroscopy results showed increase in oxidation of Si with increasing laser fluence which was also confirmed by infrared absorption of Si- O-Si stretching vibration mode. TEM images showed that with increasing laser fluence, the size of the nanoparticles decreases while their SAED pattern indicated the amorphous structure. These nanostructures exhibited an intense RT broad band PL ranging from 1.6 to 2.2 eV with twin peaks around red region. The origin of luminescence in these
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