5. Fabrication and characterization of PLD amorphous SiC thin films
5.2 Effect of substrate temperature in properties of SiC films
5.2.4 Linear optical properties of PLD a-SiC thin films
Chapter 5: Fabrication and characterization of PLD amorphous SiC films
128 | P a g e
Chapter 5: Fabrication and characterization of PLD amorphous SiC films
129 | P a g e Figure 5.13 UV-Vis-NIR transmission spectrum of PLD a-SiC thin film deposited at 750
°C substrate temperature along with interpolated Tmax and Tmin curves.
Table 5.1 Static refractive indices (n0), film thicknesses estimated by envelop method (d) and measured by step profilometer (dstep) for PLD a-SiC thin films as a function of Ts.
The static refractive index (n0) and film thickness were estimated using equation 2.1 and 2.4, respectively as discussed in Chapter 2. The thickness of films dstep was also measured directly using step profilometer. The n0, d and dstep are listed in the Table 5.1. The variation of n as a function of wavelength in the spectral range of 250 nm-3000 nm is shown in Figure 5.14. The refractive index decreases with the increasing wavelength indicating normal dispersion behavior. The value of n0 is found to be decreasing from 2.99 to 2.55, with the increase in the Ts from RT to 750 °C. As the growth temperature
Ts (ºC) n0 d (nm) dstep (nm)
RT 2.99 255 215-245
250 2.85 360 322-330
500 2.63 300 288-297
750 2.55 330 320-336
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Chapter 5: Fabrication and characterization of PLD amorphous SiC films
130 | P a g e increases the film composition is found to be changing from Si-rich to near stoichiometric SiC, hence refractive index decreases.
Figure 5.14 Variation of refractive index as a function of wavelength for PLD a-SiC films deposited at different Ts.
The absorption coefficient estimated from equation (2.6) for the a-SiC thin films was plotted as a function of photon energy in Figure 5.15 and the inset shows the ln(α) vs. hʋ plots for all the samples. A gradual shift of absorption edge toward higher photon energy was observed with increasing Ts. The plot has a linear region within 1.5 to 3 eV which corresponds to Urbach tail of absorption coefficient. The slope of this linear region was calculated by least square fitting and corresponding Urbach Energy (EU) was estimated for all the films. The α for Tauc region was estimated using equation 2.7 and (αhʋ)1/2 vs. hʋ was plotted for all the films as depicted in Figure 5.16 (a). The intercept estimated by extrapolating the linear region of the (αhʋ)1/2 vs. hʋ plot on the hʋ axis gave the optical band gap, Eg for the SiC thin film. A gradual blue shift in Eg from 1.58 (±0.02)
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Chapter 5: Fabrication and characterization of PLD amorphous SiC films
131 | P a g e Figure 5.15 Plot of absorption coefficient as a function of photon energy for PLD a-SiC
films deposited at different Ts. Inset shows the plot of ln α vs hʋ at different Ts.
to 2.32(±0.03) eV was observed in the films with increasing substrate temperature from RT to 750 °C as shown in the Figure 5.16 (b).The observed blue shift of band gap can be explained on basis of ratio of C/Si content of films. With increasing growth temperature, the film composition changes from Si- rich to near stoichiometric a-SiC, as confirmed by Raman studies (Figure 5.4) as well as EDX studies (Figure 5.11d) and hence the optical band gap increases. The maximum optical band gap reported for sp3 rich a-C films is around 3 eV and that of stoichiometric a-SiC is around 2.3 eV. These are much higher than that of a-Si (1.5-1.6 eV at RT). The thin film deposited at RT which was Si-rich, though had more sp3 fraction of carbon in it, but Si content was much larger than C as evident from Raman spectrum (Figure 5.4a) as well as EDX results, Figure 5.11. The valence band maxima and conduction band minima was dominated by Si–Si bonding and antibonding levels, respectively hence it reflects Si like low optical band gap. In the film deposited at 750 °C, where ratio of Ic-c to ISi-Si increased, the conduction band minima was
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132 | P a g e determined by Si–C and C-C antibonding levels. The energy of the Si–C and sp3 rich C-C antibonding level is higher than that of Si–Si antibonding level (indicated by the large optical band gaps of c- SiC as well as sp3 rich C) resulting in higher optical band gap of the a-SiC film deposited at 750 °C.
Figure 5.16 (a) Tauc plots for determining optical band gap (Eg) of a-SiC films deposited at different Ts and (b) variation of Eg of PLD a-SiC thin films with Ts.
Figure 5.17 shows the changes in EU and B parameter (equation 2.9) with increasing Ts. It has been reported that the EU and the B parameter are related to the local and overall structural disorder in the films, respectively [20]. The EU of the a-SiC thin films decreases gradually from 760 to 600 meV with the increase in Ts. The decrease in EU at higher Ts reflects the steep fall of absorption curve resulting smaller band tails. A material with higher structural disorder has a larger tail in absorption edge. Hence smaller band tail in a-SiC, deposited at higher Ts,reflect relatively low structural disorder. The B parameter first slightly decreased from 502 (eVcm)-1/2 for film deposited at RT to 480 (eVcm)-1/2 for that of deposited at 250 °C and then gradually increased to 540 (eVcm)-1/2 for Ts = 750 °C. In most of the earlier published results, it has been reported that B parameter of a-SiC films reduces as C content increases beyond stoichiometric point of
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133 | P a g e Figure 5.17 Variations of EU and B parameter of PLD a-SiC thin films fabricated with
increasing Ts from RT to 750oC.
SiC (C-rich SiC film). Sussman et al. [21] had reported that a decrease in the B parameter is due to a pronounced widening of the localized tail states as the carbon concentration is increased. This widening of the localized band tail states, in turn, is indicative of the increased disorder in the film [20, 22, 23]. In present case, the B parameter increased with increasing Ts indicating definite reduction in disorder. This could be due to the fact that the films fabricated at lower Ts were Si-rich a-SiC thin films which have higher structural disorder due to lack of stoichiometry. With increasing Ts there was a transition of a-SiC films from being Si-rich to nearly stoichiometric a-SiC, but not C-rich and hence less disordered as indicated by higher value of B parameter, which is in confirmation with XRD, Raman spectra as well as EDX spectra.