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Ultraviolet and visible reflective TiO2/SiO2 thin films on silicon using sol-gel spin coater
S. Saravanan, R. S. Dubey
Advanced Research Laboratory for Nanomaterials & Devices, Department of Nanotechnology, Swarnandhra College of Engineering & Technology, Narsapur-534 280, West Godavari (A.P.), India
DOI 10.17586/2220-8054-2021-12-3-311-316
TiO2/SiO2 alternative thin films (stacks) were deposited on silicon substrates using sol-gel spin-coating techniques. The prepared samples had their corresponding optical properties analyzed by UV-Visible spectrophotometry (UV-Vis), X-ray diffractometry (XRD), a surface profilometer, and Raman spectroscopy. The optical and crystallization properties of thin films were varied and compared by changing the number of stacks. UV-Vis spectrum showed high reflectance and shifting towards the infrared region with effect of increased TiO2/SiO2 stacks. XRD spectra confirmed the existence of anatase TiO2 and SiO2 diffraction peaks. The multilayer film thickness was calculated at 109 and 151 nm at two and four stacks by a surface profilometer. The Raman spectra confirmed the Si-O-Si and TiO2 stretching modes at 2600, 980, and 519 cm-1. This investigation reveals the promising and effective UV-Visible reflective property of alternative TiO2/SiO2 thin films on a silicon substrate. Keywords: Sol-gel, reflectance, multilayer, anatase, thickness. Received: 12 November 2020 Revised: 20 March 2021
1. Introduction
Thin films could find a significant role in various semiconductor industrial applications. Numerous oxide combinations are extensively useful to improve optical (or surface) performance, such as TiO2/SiO2, Al2O3/SiO2, ZnO/SiO2 and ZnO/ZrO2. In solar cells, the highest quantity of light absorption convert is possible by selecting a good combination of anti-reflection coatings materials and synthesis methods. Among the various materials, TiO2/SiO2 has been in demand due to its highest refractive index contrast, good passivity and providing conductive pathway [1,2]. The formations of various metal oxide thin film and their stabilities determined by the selection of the materials [3]. The metallic coatings on multiple substrates are useful to obtain higher ultraviolet (UV), visible, infrared (IR) reflectance like silver (Ag), aluminium (Al), etc. But, the high maintenance and fabrication cost is a major issue with metallic coatings. The ultraviolet spectral region from 100 to 400 nm and reflection of this wavelength is of great importance to human life, causing the aging of skin, sunburn, etc [4]. The UV reflector is fabricated using atmospheric pressure physical vapor depositions (AP-PVD), sol-gel spin-coating, dip-coating, plasma-enhanced chemical vapor deposition (PECVD), and wet-chemical routes and multistep procedures [5-10]. This proposed work focused on UV and Visible wavelength reflective coatings of TiO2/SiO2 multilayers on a silicon substrate using sol-gel spin-coating methods. Sol-gel spin-coating is the easiest way to synthesize and fabricate One-dimensional photonic crystal (1DPC) layers by tuning the synthesis (precursor concentration, temperature) and fabrication (spin speed, calcined temperature, number of layers) parameters. Venkatesh Yepuri et al. (2020) addressed the process optimization of low-cost and rapid fabrication of TiO2/SiO2 reflectors with 100 % reflectivity. They summarized the desired reflection band of a selected wavelength range observed by changing various parameters such as the precursor concentrations, spin speed (rpm), annealing etc. Finally, the reflection window was observed from the visible to the near infrared region with 2.5 stacks [11]. Dubey etal. (2018) fabricated visible and near-IR wavelength TiO2/SiO2 reflectors using a sol-gel spin coater for various light trapping management applications. The homogeneous, uniform film thickness (56 - 94 nm) and cross-section of TiO2/SiO2 stacks were evidenced by field emission scanning electron microscopy (FESEM). Further, the elemental peaks (Ti, Si, O) and reflectance 100 %) properties were studied using energy-dispersive X-ray spectroscopy (EDX) and UV-Visible spectrophotometer [12]. Sedrati et al. (2019) fabricated the TiO2/SiO2 Bragg reflectors using the sol-gel dip-coating process. The Raman spectrum confirmed the anatase phase. The UV-Vis-IR range shifted towards the more extended wavelength region with the effect of doping transition metals (copper, nickel, cobalt, and chromium). Also, doping TiO2 showed enlarged stop-band of TiO2/SiO2 Bragg reflectors and enhanced optical properties [13]. Zhao et al. (2019) investigated the mesoporous SiO2 and SiO2-TiO2 thin films prepared by the sol-gel method. The Raman spectrum confirmed the anatase TiO2 phase, and the UV-vis reflectance spectrum shifted towards the more extended wavelength region with the effects of transition doped metals [14]. Dubey et al. (2017) fabricated TiO2/SiO2 Bragg reflectors for light trapping applications using a cost-effective and straightforward sol-gel spin coating method. UV-Vis spectrum showed 90 % reflectance at 617 nm using seven distributed Bragg reflectors
(DBR) stacks based structure, and the FESEM image confirmed alternative layers of one-dimensional TiO2 and SiO2 thin films. This DBR structure integrated solar cell design significantly enhanced light absorption [15].
This paper deals with the optical and structural properties of TiO2 (anatase and rutile) and SiO2 multilayers fabricated on silicon substrates using a sol-gel spin-coater. In the second section we describe the synthetic process and fabrication of thin-film TiO2/SiO2 structures. UV-Vis, XRD, Raman spectroscopy results are discussed in section third and summarized the work in the fourth section.
2. Experimental approach
Synthesis and fabrication procedural steps of TiO2/SiO2 thin films are shown in Fig. 1. Titanium butoxide (TBOT, Ti(OBu)4), tetraethyl orthosilicate (TEOS, Si(OC2H5)4), methanol (CH3OH), acetic acid (CH3COOH), and deionized water (DI) are the starting chemicals used without any additional purification. For the preparation of TiO2 and SiO2 solutions, Ti(OBu)4: CH3OH: CH3COOH: DI and Si(OC2H5)4:CH3OH: CH3COOH in the ratios of 1.2:20:1.7 and 1.5:20:2.3 respectively.
Fig. 1. The sol-gel synthesis of TiO2 and SiO2 thin film processing step
Initially, 1.2 ml acetic acid added in methanol (20 ml) and stirred for 5 minutes using a magnetic stirrer at room temperature. Titanium butoxide solution added drop-wise and stirring maintained for 90 minutes at room temperature (28 °C). Finally, TiO2 solution (solution 'A') appears transparent color and was aged for 24 hrs. Next, 1.5 ml acetic acid was mixed with 20 ml methanol and stirred for five minutes with the continued dropwise addition of tetraethyl orthosilicate (2.3 ml) at 2 minutes intervals. The prepared SiO2 solution was continuously stirred at 90 minutes (solution 'B') and aged 24 hrs. Before the deposition process, substrates were cleaned at room temperature by ultrasonication process using DI water, ethanol, and aqueous hydrofluoric (HF) acid. Using spin-coater, the alternative coating of TiO2 and SiO2 solution deposition coated on the glass substrates at 3000 rpm/30 sec. After the coating, thin film samples were calcined at 650 °C (TiO2) and 450 °C (SiO2) for 1 hour. Further, analyses were performed using the following instruments: X-ray diffractometer (XRD, Bruker D8 Advance), UV-visible spectrophotometer (UV-Vis, 1800 Shimadzu), and Surface Profilometer (SJ-301 Mitutoyo). Raman spectroscopy (micro Raman) was also used to study their optical properties.
3. Results and discussion
Figure 2 shows the reflectance spectra of 2-TiO2/SiO2 stacks (S1) and 4-TiO2/SiO2 stacks (S2) by UV-Visible spectrophotometry. Here, one alternating layer of TiO2 and SiO2 thin film is known as a stack. The obtained results show the high reflectance and shifted towards the longer wavelength region due to the increment of stacks. Similarly, Dubey et al. (2017) reported improved reflectance with the effect of an increased number of coating layers [15].
The enhanced reflectance could be possible in the whole UV-Visible spectrum due to the large refractive index contrast between TiO2 and SiO2 layers, as reported by Zhang et al. (2006) [16]. Significantly, the TiO2 thin films could increase the reflectance in the visible and infra-red region as reported by Dalapati et al. (2015) [17]. Fig. 3 depicts
(a)
(b)
Fig. 2. Reflectance spectra of two (S1) and four (S2) stacks of TiO2/SiO2 thin films prepared on Si substrates
the reflectance (UV-visible) fullwidth half maximum (FWHM) of samples S1 and S2 corresponding enhancement 103 and 267 nm. Here, the FWHM enhancement ~ 267 nm achieved by the highest deposition layers of silicon substrate. Saravanan et al. (2019) reported work revealed the enhanced FWHM concerning the increment of stacks, and this reflectance is mainly dependent on the thickness and surface of the structure [18]. Consequently, surface profilometer techniques revealed the sample thickness of 120 nm (S1) and 160 nm (S2).
FIG. 3. Full-width half maximum (FWHM) of S1 (2 stacks) and S2 (4 stacks)
The XRD pattern can provide the structural parameters for TiO2/SiO2 thin films by varying the number of stacks (2-S1 and 4-S2), as shown in Fig. 4(a,b). At room temperature, thin films were fabricated which were attributed to the anatase and rutile mixed phase. The XRD patterns of the samples were noticed with the same nature. The obtained Bragg's diffraction peaks at 26 = 25.36° (A), 28.8° (R), 37.52° (A), 41.9° (A), 47.48° (A) and 53.99° (R) which is corresponding planes (101), (111), (004), (111), (200) and (211) respectively [11,19]. Here, A and R are denoted as anatase and rutile peaks for our convenience. From both diffraction spectra, the intense diffraction peak noticed and confirmed the presence of anatase TiO2 and well-matched with JCPDS#21-1272 [20-22]. The rutile diffraction can be assigned to the ICDD card #00-001-1292 and 01-072-4813. The highest TiO2 thin film calcination temperature (650 °C) provides the rutile phase formation. Both XRD pattern, the broad and low intense peak envelope appeared
FIG. 4. XRD spectra of TiO2/SiO2 thin films (a) 2-stacks, S1 and (b) 4-stacks, S2
at 23°, which is attributed to the existence of amorphous silica in the prepared samples [19,23]. Finally, the impurities were not presented.
Using the Debye-Sherrer formula, the crystalline size can be calculated, (D = kA/ft cos 6), where D is the crystalline size, k is the numerical factor (0.9) referred to as the crystallite shape, A is the X-ray incident wavelength (1.5406 A), ft is the full-width at half maximum (FWHM), and 6 is the diffraction angles [24,25]. Table 1 shows the calculated crystallite size of the samples.
Table 1. The crystallite size of TiO2/SiO2 multilayers on silicon
S.N. Sample (TiO2/SiO2) Peak positions (20) degree FWHM (20) Crystallite Size (nm)
1 S1 (Anatase-TiO2) 61.75 0.1476 62.71
2 S2 (Anatase-TiO2) 61.74 0.1476 62.70
Raman scattering is useful to identify (phase) the structural modification in 650 and 450 °C thermally treated thin film structures. Fig. 5 shows the Raman spectrum of the TiO2/SiO2 thin film with highest and broader intensity peaks within the range 0 to 3000 cm-1. The micro Raman spectroscopy consists of the excitation wavelength, A = 785 nm. Both spectral (S1 & S2) results showed the same nature of the spectrum by varying intensity of the peaks. First, the sharp peak at 519 cm-1 indicates the originating from the silicon (Si) substrate. For our convenience, Anatase, rutile, and silicon dioxide Raman modes were denoted as A, R, and S respectively. The vibrational bands of anatase TiO2 material has centered at 137, 207, 414, 519 and 644.8 cm-1, respectively, corresponding to the Raman active modes of Eg, Bg, B1g, A1g, and Eg as showed in inserted Fig. 5 [26,27]. It can be concluded that strong anatase peak at 137 cm-1. Accordingly, the normal vibrational mode of anatase bands is TA = 2Eg + 2Bg + B1g + A1g. The rutile TiO2 phase identified at 238 (Eg), 450, 612. Similarly, silicon dioxide presented at 233, 333, 488 and 600 cm-1. The Raman spectrum of SiO2 had a weak peak ~ 980 cm-1 which was attributed to the bending of Si-O-Si symmetric bond stretching [28,29]. Finally, the broad peak at 2600 cm-1 indicates the presence of SiO2. This existence of broader peaks in the Raman spectrum attribute to the electronic Raman scattering mechanism [30-32].
4. Conclusions
TiO2/SiO2 thin films were deposited on silicon substrates using sol-gel and spin-coating techniques. The optical properties of thin films have been studied by UV-Vis, XRD and Raman spectroscopy. UV-Vis spectroscopy showed reflectance enhancement as an effect when increasing the number of stacks. XRD and Raman spectra revealed the presence of anatase TiO2 and amorphous silica phase in both samples. Using a surface profilometer, the thin-film
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Fig. 5. Raman spectra of TiO2/SiO2 multilayer two stacks (S1) and four stacks (S2)
thickness of 120 nm (2 stacks) and 160 nm (4 stacks) calculated. Further, changing various parameters with optimization results will be useful for various novel applications, including solar cells, light-emitting diodes, the bandpass filters and lasers.
Acknowledgements
The authors are thankful to Mr. P. Srinivas Rao, School of Nanotechnology, Jawaharlal Nehru Technological University (JNTU), Kakinada, for availed Raman characterization. Dr. G. Ramalingam, Department of Nanoscience and Nanotechnology, Alagappa University, is acknowledged for assistance with the XRD and Surface Profilometer measurements.
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