Infrared Physics & Technology 52 (2009) 204–207

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Infrared Physics & Technology

journal homepage: www.elsevier .com/locate / infrared

Characterization and application of cerium fluoride film in infraredantireflection coating

Wei-tao Su a,*, Bin Li b, Ding-quan Liu b, Feng-shan Zhang b

a Institute of Material Physics, Hangzhou Dianzi University, 310018 Hangzhou, Chinab Shanghai Institute of Technical Physics, Chinese Academy of Sciences, 200083 Shanghai, China

a r t i c l e i n f o

Article history:Received 14 October 2008Available online 24 July 2009

PACS:78.30.Hv78.20.Ci78.20.Bh

Keywords:CeF3 thin filmMorphologyInfrared optical propertiesBroadband antireflection coating

1350-4495/$ - see front matter � 2009 Elsevier B.V. Adoi:10.1016/j.infrared.2009.07.008

* Corresponding author. Tel.: +86 571 86919037; faE-mail address: suweitao622@163.com (W.-t. Su).

a b s t r a c t

Cerium fluoride (CeF3) thin films were evaporated to the germanium substrates at different substratetemperature from 100 �C to 250 �C. Structural and optical properties were characterized by X-ray diffrac-tion (XRD), scan electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR). The mor-phology of samples deposited at different temperature can be closely related to preferred orientation. Theinfrared optical constants were obtained by fitting the transmission spectrum using Lorentz oscillatormodel. A simple example for fabrication of long-wave infrared broadband antireflection coating was alsopresented.

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1. Introduction

High quality broadband antireflection coatings are required toreduce the surface reflectivity for optical components used in thelong-wave infrared up to 15 lm [1]. Some material properties,such as refractive index, weak absorption in long-wave infrared,good physical bonding compatibility and chemical stability, areessential to infrared coating materials. However, the design anddeposition of long-wave broadband antireflection coating haveusually been limited because of the selection of coating materials,especially the low refractive index materials [1,2].

In the past two decades, more and more attentions have beenpaid to the rare earth fluorides, such as LaF3 [3–6], PrF3 [7], YbF3

[5,6], YF3 [3,5,6,8]. CeF3 is an excellent fluoride with a transparentrange from visible to deep ultra violet (DUV) due to its large bandgap [5,9]. CeF3 has a P3C1 crystal structure like LaF3 [7], whichmake it transparent up to 15 lm or even longer in the infrared.Although some physical properties of CeF3 have been investigated[3–6], the structural and infrared optical properties, in particular,the optical constants up to 15 lm have not been reported. In thispaper, the structure and optical properties of CeF3 thin films wereinvestigated. An application of CeF3 into long-wave infrared broad-band antireflection coatings was also present in the experiment.

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2. Experiments

Granular CeF3 with purity of 99.99% was used as source mate-rial for deposition on Ge (1 1 1) substrates. Prior to deposition,the base pressure in chamber was pumped down to 3 � 10�3 Pausing an oil diffusion pump. The deposition rate was kept at about2.5 nm per second, and the substrate temperature was changedfrom 100 �C to 250 �C. All thin films have an identical optical thick-ness of 3.6 lm.

The crystallographic structure of the thin films were identifiedusing D/max 2550 V X-ray diffractometer from 20� to 80� with stepof 0.02� (Cu Ka1, k = 1.5406 Å). The infrared transmission spectrawere measured using Fourier transform infrared spectrometer(Perkin–Elmer GX90) from 5000 cm�1 to 500 cm�1 with spectrumresolution of 4 cm�1. The surface and cross-section morphology ofthin films were measured on Hitachi S-4800 and Sirion 200 scan-ning electron microscope (SEM), respectively. The optical constantsof CeF3 thin films were calculated by fitting the transmission spec-trum using classical Lorentz oscillator model.

3. Results and discussions

The XRD patterns of CeF3 thin films deposited at different tem-perature are shown in Fig. 1. All thin films show crystalline struc-ture with different preferred growth orientation. The films

20 30 40 50 60 70 800

50010001500200025003000

2theta ( )°

100°

C

(211

)

0500

10001500200025003000

150°

C

0500

10001500200025003000

200°

C

0200400600800

100012001400

250°

C

(110

)G

e(11

1)

(221

)

a

b

c

d

Fig. 1. XRD spectrum of CeF3 films deposited at different substrate temperature: (a)100 �C; (b) 150 �C; (c) 200 �C and (d) 250 �C.

Table 1Calculated lattice parameters of CeF3 films deposited at different substratetemperature.

Temperature (�C) a (Å) ea (%) c (Å) ec (%)

100 7.1283 0.2 7.2493 0.32150 7.1188 0.06 7.3592 1.18200 7.1261 0.17 7.3679 1.3250 7.1236 0.13 7.4088 1.86

Standard (JCPDS-#721436) 7.114 7.273

W.-t. Su et al. / Infrared Physics & Technology 52 (2009) 204–207 205

deposited at 100 �C and 150 �C show apparent (2 1 1) preferen-tial orientation. For film deposited at 200 �C, a weak peak

Fig. 2. Surface SEM images of CeF3 films deposited at different subs

corresponding to (1 1 0) emerges, while the intensity for (2 1 1)peak is comparably strong. For the film deposited at 250 �C, itshows apparently (1 1 0) orientation, while the intensity for otherpeaks is comparably very weak. Preferred growth orientation willalso lead to the different surface morphology, which will be dis-cussed in the following.

In Table 1, the crystal cell parameters are obtained based on theresults from XRD. One can find that all obtained value of a and c arelarger than those of bulk material, which indicates there are tensilestress in all the samples. If the depositing temperature increasesfrom 100 �C to 250 �C, the strain of a do not change apparently,while the strain of c increases from 0.32% to 1.86%, which indicatesthe tensile stress in thin films also increases with the increase ofdeposition temperature. Moreover, fine cracks in tens of nanome-ter could be found in all of the samples, which are not shown inthis paper. The shape and density of micro-cracks are dependenton depositing temperature. Further understanding of the micro-crack and tensile stress is difficult for lacking of mechanicalparameters.

The surface morphology of CeF3 thin films is shown in Fig. 2.The films below 200 �C are composed of irregular granular in thesize of about 100 nm. These granular are indeed stacking up bymuch tinier layered particulates. All of the irregular granular and

trate temperature: (a) 100 �C; (b) 150 �C; (c) 200 �C; (d) 250 �C.

Fig. 3. Cross-section SEM image of CeF3 films deposited at different substrate temperature: (a) 150 �C; (b) 250 �C.

2 4 6 8 10 12 14 16 18 2020

30

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50

60

70

Tran

smis

sion

(%)

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100°C 150°C 200°C 250°C

Fig. 4. Infrared transmission spectrum of CeF3 films deposited at different substratetemperature.

206 W.-t. Su et al. / Infrared Physics & Technology 52 (2009) 204–207

layered particulates grow up with the increase of deposition tem-perature. The films deposited at 250 �C, as shown in Fig. 2d, arecomposed by completely square granular in the size of about50 nm, and it is much denser than those deposited at lower sub-strate temperature. The difference in surface morphology for thefilms deposited at different temperature could be connected withpreferred orientation. Thin films deposited 100 �C, 150 �C and200 �C have (2 1 1) preferred orientation, while those depositedat 250 �C keep (1 1 0) orientation. Different orientations may leadto different surface morphology.

The cross-section images for the thin films deposited at 150 �Cand 250 �C are shown in Fig. 3. There are few apparent voids, whichindicate the evaporated films are much denser. Further study finds

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1.1

1.2

1.3

1.4

1.5

1.6

Ref

ract

ive

inde

x

Wavelength (µm)

100°C 150°C 200°C 250°C

a

Fig. 5. Infrared optical constants of CeF3 films deposited at different sub

the column size of thin films deposited at 250 �C is much smaller,which is compatible with the result from surface morphology.From (Figs. 2 and 3), it can be found that CeF3 thin films depositedat 250 �C have a better crystallographic structure and are muchdenser.

The infrared transmission spectra of CeF3 film deposited at dif-ferent temperature are measured and shown in Fig. 4. The waterabsorption peaks located approximately at 3 lm and 6 lm are pre-sented in all of the spectra, which are linked to the water incorpo-rated in the voids of the film because CeF3 does not dissolve inwater. With the increasing of depositing temperature, the densityof the films is enhanced and the voids shrink, which lead to an alle-viation of water absorption. For the films deposited at 250 �C, thewater absorption peaks apparently dwindle owning to theimprovement of the structural quality. The absorption beyondthe spectral range of 10 lm can be due to the infrared lattice vibra-tion absorption.

In order to investigate its absorption in long-wave infrared re-gion, the optical constants of CeF3 thin films are extracted by fittingthe infrared transmission spectra using classical Lorentz oscillatormodel [7,10,11]. The refractive indices and extinction coefficientsare shown in Fig. 5a and b, respectively. The enhanced density inthin films leads to the increase of refractive index, which is con-firmed by the results of the films deposited at different tempera-ture. The refractive index of CeF3 thin film is about 1.2 at 15 lmwhen 5% calculation error is considered. The 0.5% measure errorfor the transmission will give 10–20% error for the calculation ofextinction coefficient at 15 lm. The extinction coefficient of CeF3

films is about 0.05 at 15 lm while the error is 0.01.An example that CeF3 thin films are applied into the manufac-

ture of long-wave infrared broadband antireflection is presented.CeF3 was used as low index materials (L) and a SUB/M H M L M/AIR system was used as the initial design. Germanium and zinc sul-phide were used as high (H) and middle (M) index material,

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0.02

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ctio

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effic

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strate temperature: (a) refractive index; (b) extinction coefficients.

6 7 8 9 10 11 12 13 14 1530

40

50

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80

90

100Tr

ansm

issi

on (%

)

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As designed As measured

Fig. 6. Designed and measured transmission spectrum of the 6.5–15 lm broadbandantireflection coating composed by CeF3 films.

W.-t. Su et al. / Infrared Physics & Technology 52 (2009) 204–207 207

respectively. The designed transmission curve is shown in Fig. 6.The absorption is also considered for all the coating materials inthe design procedure. The transmission of this simple antireflec-tion coating is about 88% at 13 lm and 78% at 15 lm, which is alsoshown in Fig. 6. The deterioration of measured transmission can bedue to the unstable deposition parameters and the absorption ofgermanium substrate.

4. Conclusions

CeF3 thin films were deposited on germanium substrate at dif-ferent substrate temperature. It is found that the deposition tem-perature could change structural and optical properties notably.The thin film deposited at 250 �C shows (1 1 0) preferred growthorientation and denser microstructure, which differs from thosedeposited at lower substrate temperature. The infrared optical con-

stants, which were obtained by fitting the transmission spectrumusing Lorentz oscillator model, indicated that the refractive indexand extinction coefficient at 15 lm are about 1.2 and 0.05, respec-tively. An example of application of CeF3 into fabrication of long-wave infrared broadband antireflection coating has also beenpresented.

Acknowledgements

This work is supported by the National Natural Science Founda-tion of China (NSFC) (Grant No. 60678058) and Science Foundationof Zhejiang Education Committee (Grant No. Y200804801).

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