RESULTS AND DISCUSSION
4.9 Optical Properties
Optical measurement was performed to study of optical properties of the sample. The optical measurements of (a) 0 at.% (b) 2 at.% (c) 4 at.% (d) 6 at.% and (e) 8 at.%
Ba1-xCaxTiO3 thin films were performed by using a UVβvis spectrophotometer in the wavelength range of 190-1000 nm at the room temperature. Various types of optical properties such as optical transmittance, absorption coefficient, refractive index, extinction coefficient, optical conductivity, and optical bandgap of Ba1-xCaxTiO3 thin films were studied by using UV data.
4.9.1 Transmittance
Fig. 4.8 shows the optical transmittance spectra of the Ba1-xCaxTiO3 thin films in the wavelength range of 190-1000 nm. In the beginning, the curves of the transmittance spectra of deposited samples have increased within the wavelength 350 nm and after that, the curves move toward the higher wavelength sides with nearly constant values. It can be noted that all the films are transparent in the visible range.
The transmittance of Ba1-xCaxTiO3 is 19% at x = 0. Maneeshya et al. [68] found similar nature of transmittance for BTO thin films. As doping concentration increases, the transmittance of the doped film enhances up to 47%. The increase in the transmittance is attributed to the well crystallization of films. In the visible region, transmittance (T) increases with a consecutive increase in Ca doping concentration.
The films are transparent in the NIR region, and the 2 at% Ca doped film exhibits the value of transmittance, which is about 47%. This may be due to the existence of a huge amount of oxygen in the deposited films (Table 4.1) [65].
The values of transmittance of the deposited samples slightly decreased after 2 at.%
concentration of Ca. The lower transmittance is found for Ba1-xCaxTiO3 thin film at 6 at.%
Ca. This may be presence of impurity in the films. The transmission of the films decreased sharply when the wavelength was reduced to around 350 nm, which could be due to the fundamental absorption of light in the deposited thin films and occurrence of inter-band transitions [62, 69].
Fig. 4.8 Transmittance of Ba1-xCaxTiO3 thin films for various Ca substitution.
The transmissions of the deposited samples slightly increased after 6 at.% Ca doping concentration. This result may be because of the reduction of the porous or voids in the sample and the improvement of the homogeneous structure with uniformly distributed particles, thereby increasing the optical scattering.
4.9.2 Absorption coefficient
The optical absorption coefficient (Ξ±) has been evaluated from the transmittance spectra in the wavelength range of 190-1000 nm by using the following relation [70]
πΌ =2.303π΄
π‘ (4.12) where A is the absorbance and t is the thickness.
Fig. 4.9 Absorption coefficient of Ba1-xCaxTiO3 thin films for various Ca substitution.
Fig. 4.9 shows the dependence of Ξ± for all the sprayed thin films on the wavelength. The shift in the absorption edge indicates the decrease in the band gap (red shift) of the deposited thin films. The absorption edge has been obtained at a shorter wavelength [71].
4.9.3 Refractive index
The refractive index at the different wavelength of Ba1-xCaxTiO3 thin film formed at substrate temperature 350 ΒΊC can be evaluated according to the formula [72]
π = (1 + π
1 β π ) + β( 4π
(1 β π )2β π2) (4.13)
Where, Ξ· is the refractive index, R is the reflectance and k is the extinction coefficient.
The Ξ· is related to the atomβs density, lattice parameter, and to the crystal structure [73].
Fig.4.10 shows the variation of Ξ· plotted against wavelength. Reduction in the Ξ· is observed with the increase in wavelength and similar trend that at higher wavelength and to end with reaches to nearly constant level [82].
It is evident from the Fig. 4.10; the values of Ξ· are varied in the range of 1.5 β2.6 for the
Fig. 4.10 Variation of refractive index (Ξ·) of Ba1-xCaxTiO3 thin films for various Ca substitution.
deposited sample of Ba1-xCaxTiO3 thin films. The reported value of the refractive index (Ξ·) of Ba1-xCaxTiO3 for x=0 is 1.5 which is nearly similar with the reported literature [74].
From the Fig. 4.10 it is seen that the value of Ξ· is increasing with doping concentration of Ca and the highest value of Ξ· is found 2.6 for 2 at.% Ca doping concentration which may be due to increase in transmittance and decrease in absorption coefficient with wavelength within the deposited films [75]. Also, Ξ· is found low for higher doping concentration which may be due to the presence of impurities and defects within the crystal lattice of the Ba1-xCaxTiO3 thin films. [76].
4.9.4 Extinction coefficient
The extinction coefficient (also called molar absorptivity) is a parameter defining how strongly a substance absorbs light at a given wavelength per mass density or per molar concentration respectively. The following equation has been used for calculation of the values of extinction coefficient (k) at different wavelengths.
π =πΌπ
4π (4.14)
The k is the imaginary part of the complex index of refraction, which also relates to light absorption [77, 78].
Fig. 4.11 Variation of extinction coefficient (k) of Ba1-xCaxTiO3 thin films for various Ca substitution.
Fig. 4.11 shows the deviation of the k with wavelength for deposited Ba1-xCaxTiO3 thin film. The k value measured as a function of the wavelength (Ξ») was significantly influenced by the doping. It is found that the k values of Ba1-xCaxTiO3 thin film decrease for 2 at.% with consecutive increase in Ca doping concentration while k value increases for 4 at.% of Ca doping, which keeps up a correspondence to the optical band gap of BTO [79].
The falling and rising behavior of k of BTO thin films deposited with different Ca doping concentration levels are observed in Fig. 4.11 and are associated with the absorption of light. The fall in k may be due to the absorption of light within the grain boundaries [74, 80].
The values of k are varied from 0.17 to 1.30 M-1cm-1. This low value reflects the qualitative indication of homogeneity of the deposited BTO thin films, and the high value of k indicates the high surface roughness and the probability of raising the electron
transfers across the mobility gap with the wavelength of the deposited samples at substrate temperature of 350 ΒΊC [79].
4.9.5 Optical conductivity
Optical conductivity refers to the fact that when the light is falling in the material surface, it shows conduction or transport of photons through a material. To evaluate the optical conductivity (Οopt) of Ba1-xCaxTiO3 thin films, the following formula was used (Eq. 4.15)
ππππ‘ = πΌππ
4π (4.15) Where, c is the velocity of light [80]. Fig. 4.14 shows the variation of Οopt with incident photon energy (hΞ½) for all the sprayed thin films [81].
In the Fig. 4.12, increase in Οopt value at high photon energies is observed. This may be due to the high absorbance in that region [82]. The film having a larger band gap (Eg) is expected to be lower Οopt. In the case of the thin film deposited at 2 at.%, Οopt has increased exponentially with Ca concentration at the higher photon energy. At 4 to 8 at.%
Fig. 4.12 Variation of optical conductivity (Ο) of Ba1-xCaxTiO3 thin films for various Ca substitution.
concentration, Οopt has increased exponentially at relatively lower photon energy and for 6 at.%, the sample has required relatively least energy for the increment of Οopt.
The optical conductivity of the prepared thin films demonstrates photon energy independent behavior because of the presence of free charge carriers followed by dispersion above a certain photon energy region where conductivity is sensitive to energy [82, 83].
4.9.6 Optical bandgap
The optical band gap determines what portion of the solar spectrum absorbs. Tauc plot has been used to determine the energy bandgap. The optical energy bandgap of Ba1-xCaxTiO3 thin films was calculated based on optical absorption by using the Tauc relationship [84, 85].
πΌβπ = π΄(βπ β πΈπ)π (4.16) Where Ξ± is the absorption coefficient, hΞ½ is the photon energy, n is the parameter connected to the distribution of the density of states, A is a constant or Tauc parameter [81]. Eg is the optical band gap. Here, for the allowed direct band gap n=1/2 and n = 2 for allowed indirect transitions.
The graphs of (Ξ±hΞ½)2 vs E for direct band gap of Ba1-xCaxTiO3 thin film are shown in the Fig. 4.13. The extrapolation of the linear portion of the (Ξ±hΞ½)2 vs E plot to Ξ±=0 is used to obtain the band gap value of the film samples of Ba1-xCaxTiO3 thin films.
Table 4.4 shows the values of direct band gap for Ba1-xCaxTiO3 thin films. The direct band gap value for Ba1-xCaxTiO3 at x=0 was found to be 3.90 eV from the straight-line portion, which is in good agreement with the previously reported values [77, 78, 84].
The band gap of the BTO thin film increases from the range of 3.90 to 3.98 eV while the Ca doping concentration increases from 0 to 4 at.%. From the Tauc plots, the initial increase of optical band gap is due to the decrease in grain size of the Ba1-xCaxTiO3 thin films with the Ca ions up to 4 at.% Ca doping.
Fig. 4.13 Band gap of Ba1-xCaxTiO3 thin films for various Ca substitution.
Further, increase in the doping concentration of Ca from 6 to 8 at%, the band gap of the sample gradually decreases from the range of 3.98 to 3.80 eV. This is because the differences in the optical band gap might be related to quantum-size effect in BTO thin film [86].
The quantum-size effect resulted in an increase in the band gap energy if the crystallite dimensions became very small. As it was suggested before, the large band gap energies (>3 eV) of the thin films might find applications in integrated optical devices etc. [87].
Table 4.4 The values of indirect band gap for Ba1-xCaxTiO3 thin films.
Ca concentration (at.%)
Band Gap, Eg
(eV)
0 3.90
2 3.95
4 3.98
6 3.86
8 3.80
The observed values of Eg of the Ba1-xCaxTiO3 thin films are slightly greater than band gap (3.75 eV) of BTO thin film which indicates that the formation of grains of the crystal thin films [88]. This changing nature is responsible for the occurrence of a red shift.