Chapter 6 Studies on Pure and Co Doped MgTiO 3
6.3 MgTiO 3 thin films
6.3.3 Optical Characterization
The optical packing density (P) of the films is calculated using the relation [15],
− +
+
= −
1 2 2
1
2 2 2
2
b b f
f
n n n
P n (6.2)
The porosity ratio (the volume of pores per volume of film) of the films obtained using the following expression [16]
−
− −
= 1
1 2 1
2
b f
n
P n (6.3)
Where nf and nb are the film and bulk refractive index of MgTiO3 (single crystal refractive index = 2.31) [17], respectively.
The variation in refractive index and packing density of as - deposited and annealed MTO thin films deposited on quartz and amorphous SiO2 substrates as a function of OMP was shown in Figure 6.7(a, b). Porosity ratios as a function of OMP for as - deposited and annealed films were shown in Figure 6.7(c, d), respectively. It was observed that the refractive index of the MTO thin films decreased with an increase in OMP concentration both in as - deposited and annealed films. The decrease in refractive indices with an increase in OMP has been related to the packing density and crystallinity of the MTO films. The refractive index of MTO films increased with annealing, which can be attributed to the increase in the packing density, crystallinity, reduction in porosity ratio, and the improvement in morphology. The refractive index and packing densities of the as - deposited films were in the range of 2.00 - 2.06 @ 600 nm and 0.85 - 0.87, respectively and on annealing it increases to 2.12 - 2.18 and 0.90 - 0.94, respectively for MTO films deposited on quartz substrates. Whereas as-deposited MTO films on amorphous SiO2 substrates exhibited refractive index and packing density in the range of 1.84 - 0.96 @ 600 nm and 0.75 - 0.82, respectively and on annealing increases to 1.98 - 2.14 and 0.83 - 0.92, respectively. Upon annealing, both the refractive index and packing density exhibited a similar behavior as a function OMP. The porosity ratios were found to decrease on annealing, which complements the increase in refractive index and packing density of the films. It is known that the refractive index of a transparent thin film is directly proportional to its electronic
polarization, and the electronic polarization is in turn inversely proportional to the inter atomic separation [18]. Post deposition annealing causes a
spacing due to crystallization that leads to higher densification, improvement in morphology and hence an increase in the refractive index.
due to their amorphous nature
which in turn results in the lower refractive index.
on the energy of the deposited particles at higher OMP [
process is lower at higher OMP due to this one can expect low deposition rate reduction in thickness, increase in porosity and hence decrease in refractive index.
Figure 6.7: (a & b) Variation of refractive index and packing density as a function of OMP and (c & d) porosity ratio as a function of OMP of as
films on quartz and amorphous SiO
The variation in optical bandgap as a annealed MTO films on quartz and amorphous SiO
and the electronic polarization is in turn inversely proportional to the inter ]. Post deposition annealing causes a reduction in the inter
spacing due to crystallization that leads to higher densification, improvement in morphology and hence an increase in the refractive index. The as - deposited films are highly disordered resultant in lower film density and low adatom mobility, which in turn results in the lower refractive index. The packing density of the films depends on the energy of the deposited particles at higher OMP [19]. Since the momentum transfer gher OMP due to this one can expect low deposition rate. This leads to a reduction in thickness, increase in porosity and hence decrease in refractive index.
b) Variation of refractive index and packing density as a function of OMP and (c & d) porosity ratio as a function of OMP of as - deposited and annealed MgTiO films on quartz and amorphous SiO2 substrates.
bandgap as a function of OMP for the as - deposited and films on quartz and amorphous SiO substrates is shown in Figure 6.8(a) and and the electronic polarization is in turn inversely proportional to the inter -
inter - atomic spacing due to crystallization that leads to higher densification, improvement in morphology deposited films are highly disordered in lower film density and low adatom mobility, The packing density of the films depends Since the momentum transfer This leads to a reduction in thickness, increase in porosity and hence decrease in refractive index.
b) Variation of refractive index and packing density as a function of OMP deposited and annealed MgTiO3 thin
deposited and substrates is shown in Figure 6.8(a) and
6.9(a), respectively. The optical band gap ( Tauc equation [20], which wa
where C is a constant, α is an absorption coefficient,
γ = 0.5, 1.5, 2 or 3 for allowed direct, forbidden direct, allowed indirect and forbidden indirect electronic transitions
estimated by assuming γ =
extrapolating the linear portion of the plot of (
that the disordered titanates are characterized by an indirect allowed electronic transition [21]. The obtained bandgap values for as
3.72 - 3.77 eV and 4.11 - 4.19 eV on quartz substrates and 3.72
on amorphous SiO2 substrates. It was observed that the bandgap values were found to increase for the annealed films, which
al. [13].
Figure 6.8: (a) Variation in bandgap as a function of OMP for as MTOthin films deposited on quartz substrates. (b) A plot of ( thin films deposited at 70% OMP.
The increase in bandgap values associated to decrease of intermediary energy levels within the optical bandgap and was confirmed from optical transmittance spectra (Figure 6.6(a) and 6.6(b)). The shif
increase in the bandgap upon annealing. Figures 6.8(b) and 6.9(b) shows (
respectively. The optical band gap (Eg) for all the thin films was calculated using the Tauc equation [20], which was given by
(
αhv)
=C(
hv−Eg)
γ α is an absorption coefficient, hυ is the incident photon energy and= 0.5, 1.5, 2 or 3 for allowed direct, forbidden direct, allowed indirect and forbidden indirect electronic transitions, respectively. In the present case, the band gap energy has been
= 2. The bandgap energy (Eg) of the films was extrapolating the linear portion of the plot of (αhυ)1/2 against hυ to (αhυ)1/2
that the disordered titanates are characterized by an indirect allowed electronic transition [21]. The obtained bandgap values for as - deposited and annealed films were in the range of
4.19 eV on quartz substrates and 3.72 - 3.76 eV and 4.07
substrates. It was observed that the bandgap values were found to increase for the annealed films, which is in good agreement with the earlier report by Ferri
(a) Variation in bandgap as a function of OMP for as - deposited and annealed on quartz substrates. (b) A plot of (αhυ)1/2 versus h
thin films deposited at 70% OMP.
The increase in bandgap values associated to decrease of intermediary energy levels within the optical bandgap and was confirmed from optical transmittance spectra (Figure 6.6(a) and 6.6(b)). The shift in the absorption edge to a lower wavelength indicates an increase in the bandgap upon annealing. Figures 6.8(b) and 6.9(b) shows (
) for all the thin films was calculated using the
(6.4) is the incident photon energy and
= 0.5, 1.5, 2 or 3 for allowed direct, forbidden direct, allowed indirect and forbidden the band gap energy has been ) of the films was obtained by = 0. It is known that the disordered titanates are characterized by an indirect allowed electronic transition and annealed films were in the range of 3.76 eV and 4.07 - 4.23 eV substrates. It was observed that the bandgap values were found to is in good agreement with the earlier report by Ferri et
deposited and annealed versus hυ for the MTO
The increase in bandgap values associated to decrease of intermediary energy levels within the optical bandgap and was confirmed from optical transmittance spectra (Figure t in the absorption edge to a lower wavelength indicates an αhυ)1/2 versus hυ
substrates, respectively. A slight increase in the bandgap is observed with the increase in OMP. Although the bandgap energy is a constant for a material in the bulk form, it is known to vary in thin films. The dependence of optical bandgap on crystallite size MTO thin films on various substrates is shown in Figure 6.10. It was observed that the films with smaller crystallite exhibit larger bandgap energy than those with larger crystallite size. It is demonstrated in the number of other oxide films that the bandgap decreases with an increase in crystallite size exhibiting approximately 1/d2 dependence where d is the crystallite diameter [22].
Figure 6.9: (a) Variation in bandgap as a function of OMP for as - deposited and annealed MTOthin films deposited on quartz substrates. (b) A plot of (αhυ)1/2 versus hυ for the MTO thin films deposited at 60% OMP.
Figure 6.10: The variation in bandgap as a function of average crystallite size for MTO thin