Antireflective Properties of Nano-Structured CeO 2 and CeO2 SiO 2 Composite Thin Films

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International Journal of Emerging Technology and Advanced Engineering

Website: www.ijetae.com (ISSN 2250-2459,ISO 9001:2008 Certified Journal, Volume 3, Issue 7, July 2013)

131

Antireflective Properties of Nano-Structured CeO

2

and CeO

2

-SiO

2

Composite Thin Films

Naveen C S

1

, Raghu P

2

, Mahesh H M

3

Department of Electronic Science, Bangalore University, Jnanabharathi, Bangalore – 560056

Abstract—Nano-structured CeO2 and CeO2-SiO2 composite

thin films with antireflective properties were prepared by solgel technique on two distinct substrates. Structural studies confirmed the presence of CeO2 with (111) and (200) face centered fluorite orientations. The pure CeO2 thin film exhibits maximum transmittance of 85% whereas CeO2-SiO2 composite (2:1) film exhibits excellent antireflective properties with 94% with reflectance of 1.08%. Pure CeO2 shows superior antireflective properties (0% reflectance) on silicon substrate. The thickness and refractive index of the films were calculated through transmission and reflection measurements. The refractive index of 2.05 is observed for CeO2 films deposited on glass.

Keywords— Antireflection, CeO2, Nano-composites, Nanostructure, SiO2, Solgel, Thin films,

I. INTRODUCTION

Developing Antireflective (AR) thin film coating comprising higher transmittance, low reflectance, greater stability and durability are the crucial concerns for photovoltaic and optoelectronic applications. The optical AR coatings of zero reflectance can be tuned by introducing the composite material by controlling the refractive index or through implementing a coating material of low refractive index. The low refractive index material known so far is MgF2, which is found suitable for AR

coatings, but the thickness dependent composite thin film coating exhibits low spectral reflectance compared with the surface of the uncoated substrate with stability and durability [1]. Oliviera et al and Belleville et al reported the anti-reflective effect using a sol–gel derived coating with tunable refractive indices and improved mechanical performance and also in high damage applications where anti-reflective coatings are necessary to maximize light intensity [2-4]. In these types of AR coatings require thermal curing step or a chemical cure in the presence of NH3 to carry out the necessary condensation reaction.

These procedures consumes much time and cost in the deposition process. The thin films developed by these procedures were lack of abrasion resistance as well. However, demand exists for a simplified coating method yielding good optical efficiency and abrasion resistance.

As a result, we have developed multilayered nanoporous cerium oxide and its composite thin films by solgel dip coating technique as ARCs on glass and silicon substrates.

II. METHODOLOGY

Film Preparation

Cerium nitrate hexahydrate and Tetra ethyl ortho silicate (TEOS) were used as the initial materials for synthesizing of CeO2 and SiO2 solutions respectively. Fig. 1 shows the

flowchart of the steps involved in preparing the solution and thin film deposition by dip coating process. Prior to deposition, substrates were subjected to thorough cleaning process with double distilled water followed by ultrasonic cleaning in acetone and dried in hot air. The cleaned substrates were dip coated with dip speeds and retrieval speeds ranging from 1mm/sec to 4mm/sec followed by heating at 70 oC between successive dips. The thin films are annealed at 200 ºC, 400 ºC and 600 ºC temperatures for 3 hours in the air ambient. All deposited films have visual stability which has confirmed through scotch tape test.

Characterization

The as-deposited and annealed films are characterized for their optical, structural and morphological properties. The transmittance and reflectance of the films were measured using a spectrophotometer (HR-4000, Ocean optics, USA) in the range of 400-1000 nm. The optical constants like thickness and refractive index were estimated using equations (1) and (2) by envelope technique [5].

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max min min max

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

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





(1)

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(2)

International Journal of Emerging Technology and Advanced Engineering

132

Figure 1: Flow chart for the preparation of thin films of CeO2, SiO2

by solgel dip coating technique

III. RESULTS AND DISCUSSION

A. Optical Properties of Multilayered CeO2 thin films on

Glass

The Transmittance and reflectance spectral variations of multilayered thin films of CeO2 on glass substrate annealed

at 200 ºC is shown in Fig.2. The characteristic layer by layer transmittance and reflectance properties are presented. The single layer CeO2 has a maximum and

minimum reflectance at 500 nm and 800 nm respectively. As the number of coatings increase (double, triple, quad), a shift in maxima was observed in the spectral range of 600-700 nm. This shift in maxima and minima can be attributed to the high refractive index of CeO2 (2.05) in comparison to

glass substrate (1.52). Similar observations have been reported for CeO2 thin films prepared by spin coating and

electron beam evaporation techniques [6, 7].

The multiple coatings exhibit high reflection (20%) in 450-750 nm wavelengths. The maximum transmittance of 85% is achieved in case of double layer coating while the minimum of 80% for single layer coating. Notwithstanding that the multilayer deposition enhances the thickness of the thin films; it is interesting to note that, the rate of increase in the thickness was found to be suppressed after the deposition of the third layer inspite of maintaining the unvarying conditions of deposition. This is due to fact that, when the thin film is pre-heated, the remaining residual solvent on the film surface restrain the further deposition of the film due to incomplete evaporation. The thickness and refractive index was determined by Envelope technique and are tabulated in Table I. The refractive index observed in the present work is found to be in close agreement with the experimental studies reported elsewhere [7].

Figure 2: Spectral characteristics of pure CeO2 films deposited on

glass annealed at 200 oC/3hrs in air ambient TABLEI

REFRACTIVE INDEX AND THICKNESS OF CERIUM OXIDE THIN FILMS BY

ENVELOPE TECHNIQUE

CeO2

Dips Thickness

(nm)

Refractive index

1 185 1.98

2 224 2.01

3 247 2.02

4 316 2.05

The three and four multiple deposited CeO2 thin film

showed minimum reflectance less than substrate reflectance from 800–1000 nm.

B. Optical Properties of Multilayered CeO2 thin films on

Silicon substrate

The reflectance spectra recorded for triple layer deposited CeO2 films on silicon substrates and annealed at

different temperatures is shown in Fig. 3. It can be observed that, the minimum reflectance of 1% has been observed for triple layer as-deposited CeO2 films in the

visible region. The deposited films were annealed for 200, 400 and 600 ºC.

Increase in the annealing temperature causes better antireflection property, correspondingly decrease in the thickness and also decrease of reflectance (%) towards zero, with shift in the minimum reflectance towards shorter wavelengths. This decreased reflectance is due to total internal reflection of the porous thin film [8]. The similar shift in minimum reflectance with annealing has been observed for TiO2 films deposited by solgel technique on

silicon substrate [9].

Cerium Nitrate Hexahydrate Tetra ethyl orthosilicate

2-methoxy ethanol (solvent)

Stabilizing Agent

Acid Catalyst

24 Hrs Stirring

Stock Solutions

Dip Coating 4mm/sec 1mm/sec

Silicon

Glass Quartz

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International Journal of Emerging Technology and Advanced Engineering

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Figure 3: Spectral characteristics of pure CeO2 on silicon, annealed at

200, 400 and 600 o_{C/3 hrs in air ambient}

By surface morphology, it is pragmatic that contrast to as-deposited CeO2 thin films, as the temperature increases

the porosity of the film increases which is depicted in Fig.7.

C. Optical Properties of Multilayered CeO2 - SiO2 thin

films on glass substrate

The transmittance and reflectance spectra for CeO2 -

SiO2 composite thin films of different compositions with

ratios of 2:1, 1:1, 1:2, 1:3 and 1:4 on glass substrate are presented in Figure 4.

Figure 4: Transmittance and reflectance spectra for CeO2 - SiO2

composite thin films of different compositions

From the spectra, it can be clearly inferred that, amongst all other volume ratios, the thin film volume ratio of 1:2 showed minimum reflectance of only 3% in visible wavelength region, much lower than the reflectance of the substrate and exhibits a high transmittance of 94%. Hence, this composite has been further optimized with respect to thickness so as to get a minimum reflectance of 1.08% at a wavelength of 600 nm as shown in Figure 5.

Figure 5: Spectral characteristics of thickness dependent CeO2 - SiO2

(1:2) Nanocomposite on glass annealed at 200oC/3h in air ambient

The refractive index and thickness is calculated by considering the maxima and minima in the transparent regime for different compositions as is shown in Table II. The refractive index of the thin films can be controlled via intermediate composite films by controlling the thickness and concentration of the mixture. These intermediate composite films are also significant for their better durability and stability over single or multilayer’s.

TABLEIII

REFRACTIVE INDEX AND THICKNESS OF COMPOSITE THIN FILMS BY

ENVELOPE TECHNIQUE

Material Ratio/Dips Thickness

(nm) Refractive Index

CeO2-SiO2

2:1 85.5 1.72

1:1 97 1.56

1:2 106 1.51

1:3 78.8 1.57

1:4 123 1.67

CeO2-SiO2 (1:2)

1 dip 205 1.55 2 dip 302 1.54 4 dip 349 1.52 6 dip 340 1.52

D. Structural and Morphological properties of CeO2 thin

films

The X-ray diffraction patterns of the films triple layer CeO2 thin films on silicon substrates annealed at 200 °C,

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International Journal of Emerging Technology and Advanced Engineering

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Figure 6: X-Ray diffraction pattern of pure CeO2 deposited on silicon

and annealed at 200oC, 400oC and 600oC/3h in air ambient

For 200 – 400 ºC annealed films, the (111) orientation is found to emerge with increase in (200) peak intensity. However, (111) orientation has become broad due to annealing at 600 ºC. At higher temperatures, the packing density is high; reaching almost the bulk density. This possibility might have delayed the grain growth. Similar behavior has been noticed incase of ion bombarded and e-beam evaporated films [7].

Figure 7: Surface morphology of pure CeO2 on silicon, annealed at

200, 400 and 600o_{C/3 hrs in air ambient, with resolution of 200nm} Fig. 7 shows the SEM micrographs of the CeO2

deposited on silicon substrate and annealed at different elevated temperatures. CeO2 film annealed at 200 ºC

showed smooth and crack free surface. As the annealing temperature is increased, the homogeneous films were changed to porous thin films with a porosity of 20-25 nm diameter which grounds the decrease in reflectance (almost to 0%) on silicon substrates at 600 ºC and the same is depicted by the SEM images.

IV. CONCLUSIONS

Highly transparent and efficient thin films of CeO2 are

prepared by the cost effective solgel derived dip coating technique. The optical properties show that the films can be tailored to exhibit excellent antireflection properties on, glass and silicon substrates. The refractive index as high as 2.05 observed for CeO2 films deposited on glass. Increase

in the annealing temperature causes decrease of reflectance (%) towards zero, exhibiting better antireflection property on silicon substrates. The CeO2 films reveal face centered

fluorite structure has a strong (111) and (200) peaks at 400 ºC. These CeO2 thin films find their applications in the

fabrication of efficient Photovoltaic and optoelectronic devices.

Acknowledgements

Authors are thankful to Department of Instrumentation and Applied Physics, IISc, Bangalore for providing the characterization facility.

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