R. Ajay Kumar1, R. S. Dubey1, a , V. Ganesan2
1 – Advanced Research Laboratory for Nanomaterials and Devices, Department of Nanotechnology, Swarnandhra College of Engineering and Technology, Seetharampuram, Narsapur (A.P.), India
2 – UGC-DAE Consortium for Scientific Research, Indore, (M.P.), India
Keywords: TiO2, SiO2, TiO2/ SiO2, thin films, sol-gel spin coating method, refractive index, reflection.
ABSTRACT. Multilayer dielectric thin film structure has been demanded for its application in optoelectronic devices such as optical waveguides, vertical cavity surface-emitting devices, biosensors etc. In this paper, we present the fabrication and characterization of bilayer thin films of TiO2/SiO2 using sol-gel spin coating method. Ellipsometer measurement showed refractive index values 1.46, 2.1 corresponding to the SiO2 and TiO2 films respectively. The FTIR transmittance peaks observed at ~970 cm-1, ~1100 cm-1 and ~1400 cm-1 are attributed to the Ti-O-Si, Si-O-Si and Ti-O-Ti bonds respectively. Maximum reflectance is observed from two bilayer film structure which can be further optimized to get the high reflection to a broad wavelength range.
Introduction. Presently, nanomaterials have been demanded due to their several applications in electronics, optoelectronics, sensors and much more. A multilayer structure of dielectric films also known as the one-dimensional photonic crystal is an essential passive component for the optical applications. By tuning the refractive index and thickness of its dielectric layers, the desired band of reflection can be obtained. One-dimensional photonic crystal is composed of distinct dielectric layers of quarter-wavelength thickness which possesses a forbidden band of a specified wavelength within that the propagation of light is completely prohibited. For the fabrication of such multilayer thin film structures, various combination of dielectric materials has been investigated such as TiO2/SiO2, SiN/SiO2, ZrO2/ZnO etc.
During the fabrication of multilayer film structure, the refractive index and thickness of the films can be tuned to get the desired range of reflection/stop band. The fabrication of multilayer thin films has been explored by using the Plasma Chemical Vapor Deposition, Sputtering, Molecular Beam Epitaxy, Metal Organic Chemical Vapour Deposition, Hydrothermal Method, Sol-gel Spin Coating etc. Among these methods, the sol-gel spin coating method is one of the easiest and cost effective method. Several literature have been reported on the fabrication of TiO2/SiO2 bilayer films by using sol-gel spin coating method. The sol-gel method is mainly involved two steps, hydrolysis and condensation .
The quality of the film can be controlled by the solution aging, spin time and spin speed. In this paper, we present the fabrication of TiO2/SiO2 bilayer film structure onto silicon and glass substrates by using sol-gel spin coating method. With two bilayer film structure, a reflection band from 500-670 nm is observed. Section second describes the experimental details and results are discussed in Section third. Finally, Section fourth summarizes the paper.
Experimental Details. For the deposition of TiO2 and SiO2 films, Ethanol (AR 99.9%), Acetic acid (Sd fine), Titanium butoxide (Fluka).and Tetraethyl orthosilicate (Aldrich) were used. The solutions of TiO2 and SiO2 were prepared by the sol-gel method, as described here The solutions for TiO2 and SiO2 were prepared by the using the precursors Titanium butoxide (TBOT) and Tetraethyl orthosilicate (TEOS) respectively. Ethanol was used as the solvent and acetic acid (Sd Fine) as the chelating agent. At first, required amounts of ethanol and acetic acid were stirred with the help of magnetic stirrer up to 15 minutes in a beaker. The precursor was slowly added drop by drop to the above solution under the vigorous stirring condition upto 1 hour in order to get the clear and transparent solutions. With vigorous stirring of TiO2 solution, the light transparent yellow color was observed while SiO2 solution was clear and transparent. Both the solutions were without precipitation and further, solutions were kept for 24 hours aging to get enough viscous solution. Before deposition, silicon substrates were cleaned separately in trichloroethylene, acetone and methanol by boiling upto 10 minutes and dried in nitrogen gas flow.
In a similar way, glass substrates were cleaned using soap solution and ultrasonicated with ethanol and DI water to remove the surface impurities. For the deposition of TiO2 and SiO2 films, spin coating technique was employed with 3000 RPM and 30 seconds spinning time. The annealing temperatures were 300 0C and 500 0C for TiO2 & SiO2 films respectively. After preparation, samples were characterized for transmittance and reflection using UV-1800 (Shimadzu). The thickness and refractive index were measured using Ellipsometer (Phillips 1000) and FTIR transmittance spectrum was recorded by using FTIR spectroscopy (Nicolet 380).
Results & Discussion. Using sol-gel spin coating method four samples were prepared and named as A:SiO2 film,B:TiO2 film,C:1-bilayer(TiO2/SiO2) and D:2-bilayer.
Fig. 1. UV- Vis transmittance spectra of sample A, B, C and D prepared onto glass substrates.
Fig. 1 shows the transmittance spectra of sample A, B, C and D after annealing. As can be seen in the figure, sample A shows the highest transmittance while low transmittance is observable for the sample B. With comparison to the samples A and B, reduced transmittance is observed for the samples C and D with the appearance of few small peaks. As the number of bilayers is increased the transmittance is found to be decreased . We have also prepared some samples onto the silicon substrates to measure the refractive index and thickness of the TiO2 and SiO2 films using Ellipsometer. The measured refractive indices of TiO2 and SiO2 films were 2.1 and 1.46 with thicknesses 161.2 and 361.9 respectively.
Fig. 2. FTIR transmittance spectra of sample C and D.
Fig. 2 depicts the FTIR spectra of samples C and D. The transmittance peak at ~945 cm-1 is corresponding to the TiO2-O-SiO2 band. A strong transmission peak at ~1065 cm-1 can be observed which is attributed to asymmetric vibration of Si-O-Si bonds.
A small peak at 1399 cm-1 is attributed to the Ti-O-Ti bonds while peak 1637 cm-1 is corresponding to the alkoxide OH groups. A broad peak approximately from 3100 to 3500 cm-1 represents the OH stretching vibrations of Si-OH , .
Fig. 3. UV-Vis reflectance spectra of sample C and D.
Figure 3 shows the reflectance of samples C and D prepared onto glass substrates. The reflection curve of sample C shows a small reflection band centered at 440 nm. However, an enhancement in reflection with a shift in a higher wavelength is observed for the sample D. This enhancement in reflection is due to the reflected light after striking onto the four layers of TiO2 and SiO2 respectively. A shifting in the reflection band to a higher wavelength centered at 570 nm shows the tunability of sample D. Accordingly, N-number of bilayers can give high reflectance within a specified wavelength range with the optimization of the fabrication process parameters. In this way, a filter can be tailored and fabricated for the use as a back reflector in solar cells in which unabsorbed light coming from the thin active region can be folded back via reflections.
Summary. Single layer and bilayer films of TiO2/SiO2 have been prepared using Sol-gel spin coating method and characterized for the study of their optical and structural properties. FTIR analysis showed the desired peaks of Ti-O-Ti and Si-O-Si and found good in matching with others reported works. It is found that as the number of bilayers is increased the reflectance is also increased. The maximum reflectance was observed through 2-bilayer film structure.
By doing optimization of fabrication parameters, a specified forbidden/reflection/stop band can be obtained which is demanded for its application in waveguides, vertical cavity surface emitting diodes and solar cells.
Acknowledgment. The financial support provided by UGC-DAE CSR, Indore, INDIA is highly acknowledged.
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