3.1 XRD Structural Analysis
The crystalline quality of the In2S3 films at different molar concentrations was studied using X-ray diffraction in the 2θ range of 10° to 80° and the obtained spectra are presented in Figure 1. The samples deposited using 0.01 M and 0.015 M solutions are found to be amorphous. The XRD spectra reveal that the crystallinity of the sample increases as the molarity of the solution was increased. The amorphous nature of the films is visible for the low molar concentration of 0.01 M and 0.15 M. As the molar concentration was increased, the amorphous nature disappeared and the polycrystalline nature of the films is visible giving out sharp peaks in the XRD spectra. The deposited In2S3 films showed diffraction peaks at a 2θ angle of 14.27°, 28.72°, 33.26°, 43.55° and 59.45°. Usually In2S3 gives rise to peaks at (002), (222), (400), (511) and (444) corresponding to the JCPDS card No. 65-0459 [32][33][34]. In our work, the sample deposited with 0.03 M solution shows high intense peaks at 14.16°, 28.61°, 33.14°, 43.56° and 59.26° reflected from (103), (206), (220), (309) and (4012) planes confirming the formation of β In2S3 as depicted by JCPDS card No. 51-1160 [35] affirming the existence of tetragonal structure. No other peaks corresponding to In2O3 and S were observed in the diffraction spectra confirming the absence of undesirable phases. Nadir F Habubi et al. changed the molarity from 0.05 M to 0.20 M and deposited cubic In2S3 films through the chemical spray pyrolysis technique [18].
The crystallite size Davg in the films and the existing microstrain were calculated and shown in Table 1. The crystallite size was found through the Scherrer relation [36] given by
(1)
where Davg represents the size of the crystallite, λ represents source wavelength, β is the full width of the dominant (103) peak at its half maximum and θ represents Bragg angle. It is seen from Table 1 that the crystallite size value increases as the molar concentration was increased. This enhancement in crystallite size is due to an increase in the nucleation on the substrate surface due to migration of the grain boundaries caused by the decrease in the surface energy of the film [37].
Table: 1 Structural parameters and bandgap of spray pyrolysed In2S3 thin films
Molar
Concentration
(M)
|
Crystallite
Size
(nm)
|
Strain
x 10-3
|
Bandgap
(eV)
|
0.01
|
-
|
-
|
2.58
|
0.015
|
-
|
-
|
2.52
|
0.02
|
25
|
10.8
|
2.44
|
0.025
|
33
|
8.25
|
2.39
|
0.03
|
36
|
7.69
|
2.38
|
The strain in the prepared films can be found using the relation
(2)
where β and θ take the as usual meaning. The calculated microstrain in the films was found to decrease with increment in the molar concentration probably due to the decrease in the cohesion between the In2S3 film and the substrate surface [37].
3.3 Compositional Analysis
The EDX spectra of the In2S3 films at different molar concentrations of the precursor solution with its elemental composition were shown in Figure 3. It was observed that no other element except Indium and sulfur were visible in the spectra confirming the purity of In2S3 in the films. Well-defined Indium and sulfur peaks were observed in all the samples. As the molar value of the precursor solution was raised from 0.01 M to 0.03 M, the ratio of indium to sulfur also increases from 1.02 to 1.16. Initially, the ratio incremented swiftly and became almost meager when the molar concentration was raised from 0.025 M to 0.03 M. The atomic composition of Indium increased and that of sulfur decreased when the precursor concentration was increased from 0.005 M to 0.030 M.
3.4 Optical analysis
The optical absorbance spectra of the nebulizer sprayed β In2S3 thin films in the wavelength range of 300 nm to 900 nm were shown in Figure 4a. The absorption seems to increase when the molar concentration was increased. This might be attributed to the rise in the grain size as evidenced from surface morphology images [17]. The films were found to be transparent in the UV region. When the molarity of the solution is increased, localized energy levels are created within the forbidden energy gap increasing the absorption of light and consequently, a decrease in its transmittance is seen. The absorption coefficient was calculated using the expression [38]
(4)
where d represents the thickness of the films and T is the value of transmittance.
Sample prepared with 0.03 M precursor solution exhibit a higher absorption when compared with the other films. The absorption coefficient of the samples was calculated and found to be around 104 cm−1. From the Tauc’s relation and the calculated absorption coefficient values and the optical transition of β In2S3 which is direct allowed, the band gap values can be found using the relation [39]
(αhγ)2 = A(hγ - Eg)n (3)
where hγ is the incident energy of the photon and A is the proportionality constant and the value of n is 1/2. A graph was plotted between αhγ along Y-axis and hγ along X-axis and the straight line portion of the obtained curves were linearly fit onto the X-axis. The plot was shown in Figure 4b. The band gap energy was found to change non-linearly from 2.38 eV to 2.58 eV as the molar concentration was varied. The variation is due to the improvement in the crystalline quality as evidenced from XRD results. The observed band gaps were similar to the reported values of Thierno Sall et al. [40] and Teny Theresa John et al. [41] but were slightly higher than the value for the pure sample which is 2.10 eV [42]. The increase in the optical band gap energy may be due to the incorporation of oxygen during the fabrication of the films or due to excess sulfur in the lattice [5]. It might also be attributed to deviation in the stoichiometry of the films, preferential orientation in the film, and quantum size effect [43]. Table 2gives the value of the optical band gap at various molar concentrations.
Table: 2 Photosensing parameters of spray pyrolysed In2S3 thin films
Molar
Concentration
(M)
|
Idark (A)
|
Iphoto (A)
|
R(AW-1)
|
EQE (%)
|
D*
(Jones)
|
Rise time
(s)
|
Decay time
(s)
|
0.01
|
9.96E-08
|
2.64E-07
|
3.28E-02
|
0.77
|
5.81E+08
|
1.3
|
4.2
|
0.015
|
1.00E-07
|
6.68E-07
|
1.14E-01
|
2.65
|
2.01E+9
|
1.3
|
4.2
|
0.02
|
1.13E-07
|
4.45E-06
|
8.68E-01
|
20.3
|
1.45E+10
|
1.2
|
4.1
|
0.025
|
1.40E-07
|
5.82E-06
|
1.14E+00
|
41.5
|
1.70E+10
|
1.1
|
3.7
|
0.03
|
3.34E-07
|
1.40E-05
|
2.74E+00
|
63.9
|
2.65E+10
|
1.1
|
3.5
|
3.5 Photoluminescence analysis
The defect levels in the spray deposited In2S3 thin films at different molar concentrations were studied using photoluminescence spectroscopic analysis at an excitation wavelength of 450 nm and the obtained spectra are shown in Figure 5. Two emission bands were seen in the spectra, a green emission, and a red emission. Commonly bulk In2S3 at room temperature is not expected to be luminescent. The peak at 523 nm corresponds to the transition between donors created by sulfur vacancies (Vs) and acceptors due to indium vacancies (VIn)[44]. This broad peak was already reported by Zhao et al in In2S3 nanoflakes [45]. The second peak located at 680nm was assigned to transition from indium interstitial (Ini) donors to oxygen in the vacancy of sulfur (OVs) acceptors [44]. The appearance of oxygen vacancy in In2S3 thin films can be due to the higher coating temperature of 350°C. It is worthy to mention that the formation of oxygen vacancy defects will always open a new door for the improvement of photo conducting properties.
3.6 I-V Characteristic analysis
In2S3 films under different molar concentrations were taken in the size of 1 cm × 1 cm with silver paste as the contact. The transverse I-V characteristic of the films is studied under dark and light mode with the external source bias voltage within the range of ± 5 V is shown in Figure 6. It is observed that a higher current is realized for all the samples especially the sample prepared with 0.03 M precursor solution displayed a higher photocurrent of 1.4 × 10−5 A compared to dark condition for the same bias voltage from the source meter. This ascertains the capability of the deposited films to absorb light photons and create electron-hole pair whose separation and collection leads to the consequent increase in the current. Under dark condition oxygen molecules will get adsorbed on the surface of the photodetector which captures all the free electrons in its conduction band and hence a low current density. When light is allowed to fall on the photodetector, its energy is absorbed resulting in the transfer of an electron to the conduction band from the valence band. The holes thus produced in the valence band drift towards the surface and desorb the oxygen atom over there. The more number of free electrons in the conduction band then gives rise to an increase in the photocurrent. The phenomenon is depicted in Figure 7. As the precursor concentration was increased, the photocurrent value also increases to as high as 14 µA for a 0.03 M sample.
3.7 Photoresponse analysis
An efficient photodetector is desirable to have a high response time and high sensitivity which enables it to detect light of very feeble intensity apart from its cost and power consumption.
The responsivity, quantum efficiency, and detectivity of the prepared In2S3 films at various molar concentrations were determined and given in Table 2. Responsivity can be found using the relation [46]
(4)
Substituting the value of photocurrent Iph, power of the incident light P, and the area of the film surface A, the responsivity of the photodetector was calculated and found to vary from 3.28 × 10-2 A/W to 2.74 A/W as molar concentration was increased from 0.01 M to 0.03 M. the responsivity certainly seems to vary with molar concentration. A large responsivity of 2.74 A/W was seen for the film prepared at a molar concentration of 0.03 M. The value of EQE gives the amount of light that is converted to useful current which can be calculated from the expression [47]
(5)
Substituting R, the responsivity of the In2S3 film, Planck’s constant h, the velocity of light c, the wavelength of the incident light λ, and charge of electron e, EQE can be calculated. The EQE is found to vary from 0.77 % to as high as 63.9% when the molar concentration of the precursor solution was increased from 0.01 M to 0.03 M. The highest efficiency was observed for the film with the highest molarity. The obtained responsivity is close to the value reported by Hemanth Kumar et al. which was 5.360 mA/W for In2S3 metal semiconductor metal photodetector produced by co-evaporation technique [15].
The detectivity of the films can be determined by substituting the values of responsivity R, Area of the film A, and dark current Id in the following equation
(6)
The detectivity D* defines the sensitivity of the photodetector. The detectivity for the 0.01 M film was found to be 5.81 × 108 Jones. As the molar concentration of the solution was increased, the detectivity of the film also increased. The 0.03 M film showed a higher detectivity of 2.65 × 1010 Jones. The obtained values of detectivity are much higher than early reported values [15]. The performance of the In2S3 photodetector can be well analyzed by studying the transient current characteristics. The transient photoresponse of the nebulizer sprayed In2S3 thin films are shown in Figure 8. The intensity of the light incident on the In2S3 thin film photodetector was varied from 1 to 5 mW. The rise time was found to be a minimum of 1.1 S for the 0.03 M and 0.025 M films and a maximum rise time of 1.3 S was seen for the 0.01 M and 0.015 M films. The 0.03M film recorded the lowest fall time of 3.5 S and 0.01 M and 0.015 M films showed a fall time of 4.2 S. The prepared films can also be used for gas sensing applications [48].