Introduction

Among semiconducting oxides, TiO₂  has received much attention because of its chemical and physical properties such as nontoxicity, stability, wide band gap, high refractive index, permittivity, and high transmittance in the visible region [1-2]. TiO₂ films have three different structural phases: anatase, rutile, and brookite [3-6]. Depending on the structural phase, TiO₂ has different applications such as gas sensors [7], optical cells [8], optical filters [9], solar energy conversion [10], antireflection coatings [11], waveguides [12], photo catalysts [13], and ceramic membranes [14]. TiO₂ is one of the most attractive materials for the window layer in solar cells [15]. For solar cell applications, the main requirement of a wide-gap semiconductor is high optical transmission. The optical electronic transport properties of TiO₂ strongly depend on the deposition methodology and the structural phase.

Several deposition techniques such as liquid-phase crystal deposition [16], sol–gel deposition [17,18], hydrothermal deposition [19,20], and electrophoretic deposition [21] have been used to deposit TiO₂ films. Atomic layer deposition (ALD) has significant advantages over the other methods, such as stoichiometric control, low-temperature processing, precision in material thickness, and self-limiting nature [22].

Oxide semiconductors are highly responsive to gamma radiation. The gamma radiation dissipates as energy into the lattice of the material. In general, gamma radiation interacts with the material in two ways: defect creation and defect annihilation  [23]. The number of defects created is more than the number of defects annihilated at higher gamma irradiation doses, and vice versa at lower doses [24,25].  Interaction with radiations can alter the optical, nanostructural, and electrical properties of thin films [26].  In this work, TiO₂ thin films were deposited on a fluorine-doped tin oxide (FTO) glass substrate by the ALD method, and the modification of the structural, optical, and electrical properties caused by gamma irradiation was studied.

Experimental

The TiO₂ thin films were deposited on FTO substrates using the ALD technique. First, the FTO glass was cleaned by washing with (HCl+ H₂ O₂ + 6 DI) solution and drying under nitrogen (N₂ ) flow. The deposition process for the TiO₂ thin films involved alternate cycles. The precursors (TiC1₄ and deionized water) were carried into the reaction chamber alternately using N₂ as the carrier gas under a flow rate of 200 sccm and pressure of 9 hPa. After each precursor cycle, the residual unreacted reactant and the byproduct from the reaction chamber were purged by pure N₂ gas. The pulse width and the growth rate per cycle were measured to be 0.1 s and 0.4 Å, respectively. 100-nm-thick TiO₂ thin films were obtained by performing 2500 cycles.

For irradiation, the TiO₂ films were exposed to a ⁶⁰Co gamma source with a half-life of 5.2714 years, energy of 1.25 MeV, and dose rate of 7.328 kGy/h; the dose was varied between 25, 50, 75, and 100 kGy.

XRD analysis of the as-deposited and gamma-irradiated samples was performed by using an X-ray diffractometer (Bruker, D8 Discover) with Cu-Kα source radiation. UV-Vis reflectance and transmittance measurements were carried out at room temperature using a UV-Vis spectrophotometer (JASCO–V 670) in the wavelength range 200-800 nm. The room-temperature photoluminescence (PL) of the non-irradiated and gamma-irradiated TiO₂ samples was determined upon excitation at 325 nm using a spectrofluorometer (FP-8200). The impedance spectra were measured by a KEITHLEY 4200 SCS system at room temperature, in air, under dark conditions.

Results and discussion

The XRD patterns of the non-irradiated and gamma-irradiated TiO₂ films are depicted in Fig. 1. It can be clearly seen that the XRD patterns of the TiO₂ thin films before and after irradiation have plans analogous to FTO, anatase, and rutile. The diffraction patterns agreed well with those of the anatase and rutile phases after removal of the FTO peaks. As the gamma irradiation dose increased, all the peaks became more distinct and shifted to a larger 2θ value. The increase in the peak intensity indicated that the TiO₂ thin films became highly crystalline as the gamma irradiation dose was increased. The shift in the peaks might be due to the strain produced by gamma irradiation

Figure 1Full-screen View

(Color online) XRD patterns of TiO₂/FTO for different gamma irradiation doses.

The average crystallite size (D), microstrain (μ) and dislocation (δ) of the TiO₂ films are calculated on the basis of the XRD results, using formulae 1, 2, and 3:

where λ (1.5406 Å) is the wavelength of the X-ray, θ (degree) is the diffraction angle, and β is the full width at half maximum of the peak (radian).

The crystallite size of the films decreased (97 nm to 55.7 nm) and the microstrain increased as the gamma irradiation dose was changed from 0 to 100 kGy. The calculated results based on the XRD patterns are given in Table 1.

Fig. 2 shows the UV-Vis spectra of the non-irradiated and gamma-irradiated TiO₂ samples on the FTO substrate. It can be seen that the as-deposited sample shows high transmittance in the visible region, but there is a sudden decrease in transmittance in the ultraviolet region due to the deep absorption. The surface roughness of the thin films had a significant effect, especially in the transparent zone. After gamma irradiation, the roughness of the TiO₂ thin films led to a decrease in the transmission as a function of the dose.

Figure 2Full-screen View

(Color online) Transmission spectra of non-irradiated and gamma-irradiated TiO₂ thin films.

For crystalline materials, direct transitions are valid according to Tauc’s relation [30]. Subsequently, for the present TiO₂ thin films, only the direct band gap energy was determined by the Tauc linearization (Fig. 3). The optical band gap was determined to be 3.31 eV for the as-deposited sample, and it increased to 3.38 eV with an increase in the gamma irradiation dose. This difference may be attributed to the decrease in the particle size or oxygen defect band states formed in the band gap.

Figure 3Full-screen View

(Color online) Tauc’s plot for non-irradiated and gamma-irradiated TiO₂ thin films.

Figure 4 depicts the PL spectra of the non-irradiated and gamma-irradiated TiO₂ thin films for different irradiation doses. It is clear that the gamma-irradiated TiO₂ films at three different excitation wavelengths (367, 378, and 550 nm) and the as-deposited sample show only one excitation (367 nm) [inset of Fig. 4]. The main excitation at 367 nm was due to the energy band gap emission for the non-irradiated and gamma-irradiated films. Since, the band gaps for the non-irradiated and irradiated samples were calculated from 3.31 to 3.38 eV. The emissions at 378 nm and 550 nm were lower than the band gaps of the films, implying that the gamma irradiation resulted in additional defect states within the band gap of the TiO₂ films. The intensity of the band gap and defect emission peaks clearly increased with an increase in the gamma irradiation dose, and the peaks shifted to the longer-wavelength side. The increase in the intensity of the emission peaks of the TiO₂ film strongly depended on the nanocrystalline structure. The traps in the band gap were probably associated with the intrinsic defects in the TiO₂ films. The XRD results for the TiO₂ thin films are favorable for the band gap emission (PL peak).

Figure 4Full-screen View

(Color online) Photoluminescence spectra of non-irradiated and gamma-irradiated TiO₂ thin films.

Frequency-dependent electrical conductivity measurements were conducted for the as-deposited and gamma-irradiated TiO₂ /FTO samples for the charge transfer and recombination processes. As per the Nyquist plots shown in Fig. 5 (a, b, c, and d), each sample exhibits a semicircle in the low-frequency region, which is associated with the electron transport mechanism at the interface between TiO₂ and FTO. The electron transport resistance decreased with an increase in the gamma irradiation dose.

Figure 5Full-screen View

(Color online) Nyquist plots for non-irradiated and gamma-irradiated TiO₂ thin films.

The semicircle fitting of the non-irradiated and gamma-irradiated TiO₂ thin films represent a single charge transportation process. The equivalent circuit obtained by the simulation of the experimental data can be seen in Fig. 6 (a), where R_s indicates the contact resistance, R_TiO2lFTO indicates the bulk resistance of the films, and Cp denotes the interface capacitance. The simulated values of all these parameters are obtained using Z-View software and listed in Table 2.

Figure 6Full-screen View

(Color online) (a) Equivalent circuit and (b) Bode plot of non-irradiated and gamma-irradiated TiO₂ thin films.

The Bode and phase angle diagrams of the non-irradiated and gamma-irradiated TiO₂ thin films are shown in Figure 6 (b). As the peak frequency of the middle semicircle has an inverse relationship with the electronlifetime at the TiO₂/FTO interface, the peak frequency increases, indicating a decrease in the lifetime with increased gamma irradiation doses.

Conclusion

We investigated the gamma irradiation-induced modification effects on the structure optical and electrical properties of TiO₂ films deposited by ALD. The XRD results for the irradiated TiO₂ films revealed an enhancement of the crystallinity. The micro strain and crystallite size increased upon increasing the gamma irradiation dose. The energy band gap and refractive index of the TiO₂ films, as determined by UV-Vis spectroscopy, increased for the initial dose and then decreased with increasing doses. The two extra peaks after irradiation could be due to the electron captured by the surface defects and the traps produced by the oxygen deficiencies. In the Nyquist plots for each sample, a semicircle appeared in the low-frequency region, which was associated with the charge-transfer process at the TiO₂ /FTO interface. The charge transfer resistance decreased with increasing gamma irradiation doses. The electron lifetime of the samples, which was calculated from the Bode phase plot, decreased upon increasing the gamma irradiation doses.