Structural, Optical and Morphological Properties of TiO2/Ag/TiO2 MultilayerFilmsM. M. Hasan, A. S. M. A. Haseeb, R. Saidur, H. H. Masjuki, and M. R. Johan Citation: AIP Conf. Proc. 1136, 229 (2009); doi: 10.1063/1.3160137 View online: http://dx.doi.org/10.1063/1.3160137 View Table of Contents: http://proceedings.aip.org/dbt/dbt.jsp?KEY=APCPCS&Volume=1136&Issue=1 Published by the AIP Publishing LLC. Additional information on AIP Conf. Proc.Journal Homepage: http://proceedings.aip.org/ Journal Information: http://proceedings.aip.org/about/about_the_proceedings Top downloads: http://proceedings.aip.org/dbt/most_downloaded.jsp?KEY=APCPCS Information for Authors: http://proceedings.aip.org/authors/information_for_authors

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Structural, Optical and Morphological Properties of TiO2/Ag/TiO2 Multilayer Films

M.M. Hasan*, A.S.M.A. Haseeb, R. Saidur, H.H. Masjuki, M.R. Johan

Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia *Email:mmhasan92@yahoo.com

Abstract. In this investigation, TiO2/Ag/TiO2 multilayer-films were deposited on microscope glass slides with varying individual layer thicknesses by radio-frequency reactive magnetron sputtering. Prior to multilayer development, single layers of Ag and TiO2 were deposited and characterized. All the films were prepared at a moderately high pressure at room temperature. It was found that single layer of TiO2 showed anatase polycrystalline structure. It also exhibited high visible transmittance of above 80% and higher refractive index of 2.31 at a wavelength of 550 nm. The indirect optical band gap of the TiO2 films was estimated as 3.39 eV. The Ag single layer films were found to be crystalline with a very high reflectance for IR (Infra-red) light. Finally, the multilayers have been deposited and characterized by X-ray diffraction, UV-visible-IR spectrophotometry, scanning electron microscopy and profilometry.

Keywords: Multilayer coatings, dielectric material, structural and optical properties. PACS: 68.55.-a, 78.20.-e, 68.37.Ps

INTRODUCTION As one of the important dielectric layer with wide-band-gap (Eg>3 eV) and

high refractive index, titanium dioxide (TiO2) has been subject to extensive academic and technological research for decades. For its remarkable optical properties, today thin films of TiO2 are frequently employed for many optical devices in optics industry [1] and solar cells [2]. The highly transparent TiO2 films can be used as anti-reflection coatings for increasing the visible transmittance in the photocatalytic heat mirrors [3]. High transmittance in the visible region (X=380-760nm) is one of important optical requirements for the heat mirror. It can be achieved by the optimization of the deposition techniques. TiO2 can exist as an amorphous layer and also in three crystalline phases: anatase (tetragonal), rutile (tetragonal) and brookite (orthorombic). Only rutile phase is thermodynamically stable at high temperature. Antase TiO2 mainly contributes to the photocatalysis for the multilayer. The refractive index at 500 nm for anatase and rutile bulk titania is about 2.5 and 2.7 respectively [4]. There are many deposition methods used to prepare TiO2

thin films, such as electron-beam evaporation [5], ion-beam assisted deposition [6], DC reactive magnetron sputtering [7], RF reactive magnetron sputtering [8], Sol-gel dip coating method [9], sol-gel spin coating method [10], chemical vapor deposition[11] and plasma enhanced chemical vapor deposition [12].

The properties of the titanium dioxide films depend not only on the preparation techniques but also on the deposition conditions. PVD (Physical vapor deposition)

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technology is still a mainstream production tool for functional coatings. Sputter deposition techniques are widely utilized methods in industrial products because the high quality films with better mechanical property can be obtained at low substrate temperature with good uniformity of the film thickness in a large area.

The highly transparent TiO2 films have been widely used as a dielectric material in a multilayer structure of dielectric/metal or nitride films/dielectric layer stacks [4, 5, 13-16]. Most of those works aim at the development of heat mirror coatings with better optical properties. In the present study, TiO2 and Ag films with various thicknesses have been deposited by RF magnetron sputtering at an elevated deposition pressure. Later the multilayer stacks of TiO2/Ag/TiO2 have been deposited with varying Ag film thickness. Finally, it has been attempted to discuss those films in the perspective of structural, optical and morphological properties.

EXPERIMENTAL DETAILS

In this work, TiO2, Ag and the multilayers were prepared on microscope glass slides by radio-frequency (RF) magnetron sputtering using Ti (99.99%) and Ag (99.99%) targets. TiO2 films were grown by reactive sputtering under a mixture of 46 sccm of Ar (99.999%) and 10 sccm of O2 (99.999%). The sputtering chamber was evacuated down to 5 x 10-4 Pa and the working pressure was kept at 3 Pa. During the depositions, the rf power was 250 W. Ag films were prepared using 65 sccm flow of Ar and 50 W rf power with a Ag target of 4 N purity. For the multilayers, the same conditions were maintained. The crystalline quality of the deposited films were investigated by X-ray diffraction (XRD) measurements (Model-D5000, Siemens) in 8-28 geometry using Cu Ka radiations (k=0.15406 nm). The UV-visible-NIR optical transmission spectra of TiO2 thin films were recorded by a double-beam spectrophotometer (Jasco 570). The NanoTest system by Micro Materials Ltd., UK was used for surface roughness measurement. The surface morphological features were observed by a field emission scanning electron microscope (SEM). The thicknesses of the individual TiO2 and Ag films were measured by SEM and a surface profiler (Tencor Alpha Step 200) respectively.

RESULTS AND DISCUSSION

Structure

It has been shown in our earlier report [18] that the thickness of the TiO2 film measured by SEM was approximately 340 nm. The TiO2 film deposited at room temperature and at an elevated pressure of 3 Pa was found to be polycrystalline having anatase phase only as shown in Figure 1. The reason may be that the kinetic energy of the impinging particle is high enough to initiate crystallization. Sung and Kim et al. [2] also observed anatase phase in their titania films deposited by sputtering at an elevated pressure. The stronger diffraction was found along the anatase (101) crystal plane. From full width at half maximum (FWHM) of the diffraction peaks one may estimate the average grain size of polycrystalline materials by applying Scherrer formula [19].

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Based on the dominant diffraction peak (101), it is estimated that the grain size is about 44 nm. Figure 1 also shows the XRD spectrum of the 330 nm thick Ag film

Cubic (220) Cubic (311)

j lAg fi lm of 340 nrrJl

Anatase (200)

| Anatase (211)

40 50 60 26 (degree)

T i O j f i l m of 330 nm

Figure 1. XRD spectrum of TiO2 and Ag films deposited on microscope slide

where the thickness was measured by a surface profiler. It is observed that as-deposited Ag film at room temperature is found to be crystalline and possesses stable cubic structure as it shows few strong peaks along planes of (111), (200), (220) and (311). The appearance of many peaks indicates the polycrystalline nature of the film. The preferential orientation of the crystals is found to be along the (111) crystal plane. The origin of the preference is the fact that (111) is the lowest energy plane in the fcc (face centered cubic) structure. For a high Ar pressure almost similar to ours, same polycrystallinity has been previously observed for Ag films prepared by rf magnetron sputtering at room temperature [20]. The grain size determined via the Scherrer formula for Ag film is found to be 34 nm which is in good agreement with the mentioned research findings [20].

Optical Properties

In this work, from Figure 2a, it is found that average visible transmittance of

luu

90

80

70

60

20

10

(a)

^^-~~

'Y/C /l^^^^

f/ / J

TiO; film of 32 nm

- - — ^ ^ ^ Ti02/Ag/Ti02 film

TiO;/Ag film

~ - ~ ~ - _ _ _ _ A g film of 20 nm

90

80

70

60

30

20

10

0

300 350 400 450 500 550 600 650 700 750

Wavelength (nm)

300 350 400 450 500 550 600 650 700 750

Wavelength (nm)

Figure 2. Transmittance curves of (a) different single and multilayer films and (b) TiO2/Ag/TiO2 films at different Ag thickness

as-deposited TiO2 films of 32 nm thick is about 80% with respect of blank glass substrate. For Ag film of 20 nm thick shows a high infra red (IR) reflectance of about 80%. In our earlier work [18], optical properties of as-deposited TiO2 films have been discussed in details. The refractive index at 550 nm for TiO2 film

iOO Cubic (111)

250

200

Cubic (200) 50

00

Anatase (101) 50

20 30 70 30

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was reported to be 2.31 and the indirect optical band gap of the TiO2 films was estimated as 3.39 eV. From Figure 2a, it is also observed that TiO2/Ag film exhibits very low transmittance of about 35% but TiO2/Ag/TiO2 multilayer film exhibit a better visible transmission of about 65% and an IR reflectivity of 50% (at 800nm). It is well known that Ag films thinner than 15 nm tend to be inhomogeneous and granular and more realistic values for the silver layer thickness are considered to be those of 16-24 nm [16]. From Figure 2b, it is observed that the variation of Ag thickness affects IR reflectivity of the D/M/D films. IR reflectivity is found to decrease with the increase of Ag film thickness.

Morphological Properties

The morphology of the studied samples of TiO2, Ag, TiO2/Ag and TiO2/Ag/ TiO2

films were examined using scanning electron microscopy (SEM) and propilomety. Figure 3 shows the selected plane-view SEM images for the different individual and

Figure 3. Surface SEM images obtained for (a) TiO2 thin film of 32 nm, (b) Ag film of 20 nm, (c) TiO2/Ag film stack and (d) TiO2/Ag/ TiO2 multilayer film

multilayer films. Generally speaking, the TiO2 thin films that were grown by RF sputtering shows quite smooth surfaces with no cracks. From the figure, it is observable that Ag film is slightly different from TiO2 film as Ag film surface shows some white spots with probable higher surface roughness. It has been verified with the surface roughness measurement by the Nano-Test system. The mean and root mean square (rms) roughness of those films are shown in Table 1. From Table 1, it is observed that Ag film shows higher roughness when deposited on glass slides. But Ag film shows lower roughness values when it is deposited on TiO2 thin film. TiO2 films deposited on glass or Ag film do not cause much variation in roughness values.

Table 1. Roughness values of different films deposited on glass slides.

Samples TiO2 film Ag film TiO2/Ag film stack TiO2/Ag/ TiO2 film stack

Rms roughness (nm) 21.32 27.79 18.74 20.32

Mean roughness (nm) 25.81 33.85 22.83 24.78

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CONCLUSION In this work, different combinations of TiO2 and Ag films with their multilayer

stacks were deposited on microscope glass slides by radio-frequency reactive magnetron sputtering. It was found that single layer of TiO2 showed anatase polycrystalline structure having crystallite size of about 44 nm. It also exhibited high visible transmittance of above 80% and higher refractive index of 2.31 at a wavelength of 550 nm. The Ag single layer films were also found to be polycrystalline having a fcc structure. The grain size was found to be about 34 nm. It was found that TiO2/Ag film exhibited very low transmittance of about 35% but TiO2/Ag/TiO2 multilayer film exhibited a better visible transmission of about 65% and an IR reflectivity of 50% (at 800nm). Meanwhile, the dependence of transmittance on the thickness of Ag layer has been discussed.

ACKNOWLEDGMENTS

The authors gratefully acknowledge the financial support given for this work by the Ministry of Science, Technology and Innovation Malaysia (MOSTI) under the ScienceFund by Grant No: 13-02-03-3033 and the Institute of Research Management and Consultancy, University of Malaya (UM) by PPP Fund Project No. : PS 051/2007B.

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