Preparation and optical characterization of e-beam deposited cerium oxide films

Optical Materials 34 (2012) 2101–2107

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Preparation and optical characterization of e-beam deposited cerium oxide films Tadeusz Wiktorczyk ⇑, Piotr Biegan´ski, Eunika Zielony ´ skiego 27, 50-370 Wrocław, Poland Institute of Physics, Wrocław University of Technology, Wybrzez_ e Wyspian

a r t i c l e

i n f o

Article history: Available online 22 June 2012 Keywords: Cerium oxide Rare earth oxides Refractive index Optical properties Thin films

a b s t r a c t Cerium oxide films, of 0.3–1 lm thickness, were reactively deposited in the oxygen atmosphere onto quartz plates by the PVD method. An electron gun was used as an evaporation source. Films were characterized with the AFM method, Raman spectroscopy and spectrophotometrically. Optical properties of these films were examined for the wavelength range 0.2–2.5 lm. Films were characterized by high transparency, between 0.38 and 2.5 lm. The complex refractive index, n=n  jk, was evaluated. The dispersion characteristics for n(k) and k(k) were presented. We found that the refractive index strongly depends on the temperature of substrates (300 K 6 Ts 6 673 K) during film deposition. Estimated values of the refractive index (at k = 0.55 lm) were in the range 1.91–2.34. Ó 2012 Elsevier B.V. All rights reserved.

1. Introduction Thin films of rare earth oxides are interesting materials for fabrication of different optical, optoelectronic and nanoelectronic devices. Cerium oxide films from the rare-earth-oxide family have been the subject of extensive examinations [1–30]. The most popular form of cerium oxide is CeO2 (cold Ceria) which melts in the temperature 2873 K. CeO2 has a fluorite structure with a lattice constant a = 0.541 nm [10,14,17]. Its lattice parameter matches very well with that of Si (a = 0.543 nm) making Ceria films interesting for epitaxial growth. CeO2 films have a band gap between 3.2 and 3.6 eV for direct electron transition [23] and a high value dielectric constant (e = 25–42) [3]. These properties are very useful from the point of view of potential applications in microelectronic and nanoelectronic metal-insulator semiconductor (MIS) or metalinsulator-metal (MIM) structures [3,26]. Ceria films are transparent in the visible and near infrared (NIR) spectral range with large changes of refractive index. These properties make them very useful coating material for fabrication of multilayer coatings (i.e. antireflection coatings, waveguide coatings, dielectric mirrors, etc.) [28]. Recently Ceria oxide films have also been intensively examined as attractive electrolyte material for low temperature solid oxide fuel cells (SOFCs) [16] and for fabrication of oxygen sensors [27], carbon monoxide sensors [24] or glucose biosensors [30]. Moreover thin films of CeO2 have been widely investigated for electrochromic applications [8,11,18]. Several methods have been applied for fabrication of these films, such as atomic layer deposition (ALD) [3,15], radio-frequency (RF) sputtering [1,4,7,28], pulsed laser deposition (PLD) [5,14,30], chemical vapor deposition (CVD)

⇑ Corresponding author. E-mail address: [email protected] (T. Wiktorczyk). 0925-3467/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.optmat.2012.05.027

[13], metalloorganic CVD (MOCVD) [10] sol–gel spin coating or dip coating deposition [8,11,29] and colloidal suspension spin coating method [9,16]. E-beam deposition has also been used [2,6,12,18–25] and seems to be well elaborated to grow cerium oxide films. In order to have an influence on different deposition parameters, recently this method has also been combined with ion beam assisted deposition (IBAD) method [17,23]. Polycrystalline films are usually formed during e-beam deposition of Ceria onto substrates at the temperature from room temperature (RT) to 1223 K. The grain size of such films was in the range 5–65 nm [6,19,21–23]. Their refractive index was found to be between 1.6 and 2.56 [2,6,12,18,23,25]. It has been found that many technological factors affect the refractive index. In our work we applied reactive e-beam deposition at relatively high oxygen pressure for preparation of cerium oxide films for multilayer optical coatings. This paper reports the results of optical examinations, Raman scattering and surface morphology studies of cerium oxide films fabricated onto quartz plates. We examined the influence of substrate temperature during film deposition on optical properties of these films.

2. Preparation of cerium thin films Thin film dielectric coatings were prepared by physical vapor deposition (PVD) of cerium oxide onto quartz plates. All depositions were carried out at the oxygen pressure of 8  105 Torr (1  102 Pa) as a reactive deposition process. Before deposition the apparatus was pumped down to about 107 Torr (1.3  105 Pa). A watercooled, electron gun of the power of 10 kW was used as an evaporation source. CeO2 of 99.9% purity, fabricated by Sigma, was used as a source material. All films, denoted as A, B, C, D and E, were deposited

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Table 1 Deposition parameters of cerium oxide thin films. Sample

Film thickness (nm)

Substrate temperature (K)

Oxygen pressure (Pa)

Deposition rate (nm/s)

A(1) A(2) B C(1) C(2) D E

352.4 ± 1.5 756.1 ± 1.6 1043 ± 1.3 430.1 ± 3.0 805.8 ± 1.0 518.3 ± 1.1 273 ± 2.0

300 300 383 473 473 573 673

1  102 1  102 1  102 1  102 1  102 1  102 1  102

2.2 2.52 0.58 0.36 0.61 0.3 0.27

onto quartz plates at various substrate temperatures, from 300 K to 673 K. Film thickness was monitored during the evaporation process with a crystal (quartz) monitor attached to the vacuum system. Post deposition film thickness measurements were carried out with an optical interference microscope (Tolansky’s method). All film deposition parameters are given in Table 1. 3. Surface morphology 3.1. AFM characterization Surface topography observations and measurements were carried out with XE-70 PARK-System. Fig. 1a–e shows the surface topography of cerium oxide films deposited at various substrate temperatures. On the surface of films deposited at 300 K 6 Ts 6 383 K there appear small grains with porous structure. For temperatures around 473 K the small grains form bigger complexes. For Ts P 573 K AFM topography images show rapid changes in the structure. The well-grained structure is observed in this range. At 673 K the grains are the largest. Fig. 2a–e presents topographic height measurement along a line-cut of the results in Fig. 1a–e, respectively. Films A and B obtained at the lowest deposition temperature (i.e. for 300 K 6 Ts 6 383 K) are relatively smooth. One can see from the z-ranges for films C and D that the surface structure for these films is more complex. These samples consist of smaller irregularities which are imposed on large surface inhomogeneities. Finally, the z-range, for films E, indicates that largest grains are formed. We have performed a quantitative analysis of above results with the root mean square roughness (RMS roughness, rAFM) and surface area ratio (SAR), defined as ((surface area  geometric area)/geometric area)100%. In Fig. 3 we have shown the change in RMS roughness and SAR versus the substrate temperature during film deposition. Presented results show that films deposited at 300 K 6 Ts 6 383 K exhibit the lowest RMS roughness (rAFM = 2.7–4 nm). As we have mentioned, the surface of CeO2 films deposited at these conditions is relatively smooth. For higher temperatures rAFM increases to about 7, after then decreases to 6.1 at 673 K. Estimated values of SAR increases monotonously from 2.7% to 8.8% with temperature increasing up to Ts = 573 K, then decreases down to 1.5%. Both of these parameters indicate that the morphology of the films deposited at Ts = 473–573 K is more complex, however at 673 K the film surface structure becomes smoother. 3.2. Diffuse reflectance spectra in UV To determine independently the surface roughness of cerium oxide films the reflectance measurements in the range of UV were performed. The Jasco type V-570 spectrophotometer with integrating sphere was applied for obtaining the diffuse reflectance spectra.

Fig. 1. AFM surface topography images (2 lm  2 lm) of cerium oxide films deposited at various substrate temperatures: 300 K (a), 383 K (b), 473 K (c), 573 K (d), and 673 K (e).

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20

16

10

A

9

0

-20

5

0.5

1.0

1.5

2.0

x [μ m]

(b)

6

0.1

3

SAR

σAFM

4

2 2

1 20

B

0.0

0

0 300

10

z [nm]

8

σdr

4 0.0

10

P

6

14 12

0.2

7

-10

CeO 2

P

8

σ [nm]

z [nm]

10

SAR [%]

(a)

400

500

600

700

Ts [K] 0 Fig. 3. A dependence of the evaluated parameters: rAFM, rdr, SAR and P for cerium oxide films upon substrate temperature during film deposition.

-10 -20 0.0

0.5

1.0

1.5

4.0

2.0

x [μm] 20

Diffuse reflectance [%]

(c)

C

z [nm]

10 0 -10 -20 0.0

0.5

1.0

1.5

2.5 2.0 1.5 1.0 0.5 180

200

220

240

260

280

300

320

Wavelength [nm] 20

D

Fig. 4. Diffuse reflectance spectra of CeO2 specimens – experimental data and theoretical curves evaluated at following parameters: (i) rdr = 6.9 nm, R0 = 10% – sample C2, Ts = 473 K; (ii) rdr = 7.6 nm, R0 = 13% – sample D, Ts = 573 K.

10

z [nm]

3.0

2.0

x [μ m]

(d)

sample C(2), experimental data sample C(2), theoretical dep.- Eq.2 sample D, experimental data sample D, theoretical dep.- Eq.2

3.5

0 -10

R ¼ R0  e

4pr 2 dr

k

ð1Þ

-20 0.0

0.5

1.0

1.5

2.0

x [μ m]

(e)

20

where R0 denotes the reflectance of a perfectly smooth surface and rdr is the RMS roughness. The diffuse part of reflectance RD can be derived from the above relation:

 4pr 2  dr RD ¼ R0  1  e k

E

ð2Þ

z [nm]

10 0 -10 -20 0.0

0.5

1.0

1.5

2.0

x [μm] Fig. 2. Topographic height measurement (z-ranges) along a line-cut for AFM images from Fig. 1.

From this equation we can estimate rdr. Fig. 4 illustrates the examples of the diffuse reflectance spectra of CeO2 specimens. The presented results were fitted according to Eq. (2) from which we obtained values of rdr for all specimens. Results are shown in Fig. 3. One can see that rdr increases monotonically from 6.3 nm to 7.6 nm with increasing temperature (300 K 6 Ts 6 573 K), then decreases to 6.5 nm at 673 K. These results are comparable to those obtained from AFM studies, except for the lowest temperatures range (300–383 K). 4. Raman scattering

For monochromatic incidence light the normal reflectance can be expressed in the following way [31]:

The micro-Raman measurements were performed at room temperature with the help of the T64000 Jobin–Yvon spectrometer

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configured in backscattering geometry and the triple subtractive mode of operation equipped with a multichannel CCD camera (illuminated spot 1 lm). The samples were excited by an Ar2+ laser working at a wavelength of 514.5 nm. Fig. 5 presents the Raman spectra of cerium oxide films deposited at substrates of various temperatures (Ts), Ts = 300–673 K. The observed spectra of all examined samples are characterized with a single peak situated at 456–466 cm1. This line is strictly connected with a single allowed F2g Raman active mode, typical for fluorite CeO2 structure [32]. The peak experience remarkable change in frequency position and shape. Fig. 6 shows temperature dependence of the F2g peak position that increases with increasing substrate temperature during film deposition. Moreover, the peak broadens for samples with lower substrate temperature. The change of the foul-width at half-maximum of the peak (FWHM) with respect to substrate temperature is shown in the same figure. The observed results may be explained as a size-inducted effect in the examined films, which correlate with their grain structure.

Sample A

5.1. Transmittance and reflectance measurements Optical properties of cerium oxide thin film coatings were examined in the range of wavelengths from 0.2 lm to 2.5 lm with a Jasco type V-570 two-beam spectrophotometer. The transmittance measurements (Tm) of the specimens were carried out with relation to the transmittance of the same quartz plate without any coating (Tm/q) or with relation to the air (Tm/a). Then, the transmittance (T) of the film (in the system: film-finite substrate) was evaluated from Eqs. (3a), (3b), respectively [33]. The reflectance Rm of the sample was measured with respect to the standard mirror. The reflectance of the film was determined from Eq. (4).

T¼

T m=q ½1  Rs R0  1 þ Rs

ð3aÞ

T¼

T m=a ½1  Rs  R0  1  Rs

ð3bÞ

T 2 Rs 1  R0  Rs

ð4Þ

Intensity [a. u.]

Sample C

R ¼ Rm 

350

400

450

500

Sample D

in which:

Sample E

R0 ¼

550

600

650

700

-1

Raman shift [cm ] Fig. 5. Raman spectra of CeO2 films deposited at various substrate temperatures: 300 K (sample A1), 383 K (sample B), 473 K (sample C2), 573 K (sample D) and 673 K (sample E).

70

468 F2g peak position

60

464

50

462

40

460

30

458

20

-1

466

FWHM [cm ]

FWHM

-1

5. Optical properties

Sample B

300

Raman shift [cm ]

Higher substrate temperature leads to formation of grains of larger diameter than for films formed at room temperature. Presented results are in a good agreement with recent Raman examinations of CeO2 films obtained by e-beam deposition [22] and by PLD method [14].

CeO2

456

300

400

10

500

600

700

Ts [K] Fig. 6. The change of the F2g peak position and its FWHM with respect to substrate temperature.

R0m  Rs ½ð1  Rs Þ2 þ Rs ðR0m  Rs Þ

ð5Þ

Rs is the reflectance of the quartz plate. In above equations Rm (R) denotes front measured (evaluated) reflectance for the film facing the incident light and back measured (evaluated) reflectance R0m (R0 ) for a substrate facing the incident light, respectively. In Fig. 7 we have shown measured transmittance and reflectance spectra: Tm/q(k), Tm/a(k), Rm(k) and R0m (k) for the thickest examined specimen (sample B). In the same figure, the evaluated characteristics of T(k), R(k) and R0 (k), which characterize the properties of the film for this specimen, are presented. These characteristics were evaluated according to Eqs. 3a, 3b, 4, and 5, respectively. One can see a very good agreement between the transmittance characteristics T(Eq. (3a)) and T(Eq. (3b)) determined from Eq. (3a) and (3b) and from Tm/q(k), Tm/a(k) curves, respectively. This also shows that the above procedure evaluation of the transmittance characteristics is correct. 5.2. Determination of optical constants Optical properties of any thin uniform film at a given wavelength k are characterized by the complex refractive index: n ðkÞ ¼ nðkÞ  jkðkÞ, where n is the refractive index of the specimen and k denotes its extinction coefficient. In this paper, the refractive index and extinction coefficient were evaluated by the well-known envelope method [34,35]. For this purpose we drown upper and lower envelopes through the maxima and minima of the transmittance curves. For the case of 2 2 week absorption (i.e. for k << ðn  n0 Þ2 and k << ðn  n2 Þ2 ) n and k were determined from the following expressions [34,35]:

h i1=2 n ¼ N þ ðN2  n20 n22 Þ1=2

ð6Þ

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T. Wiktorczyk et al. / Optical Materials 34 (2012) 2101–2107

100

100

100 573K 473K 300K

Tm/q

Transmittance [%]

80

60

40

40

20

Rm R'm R R'

Reflectance [%]

60

383K

80

Tm/a

Transmittance [%]

T(eq.3a) T(eq.3b)

80

673K

60

60

40

40 673K

20 20

20 383K

0

0 0

500

1000

1500

2000

80

Reflectance [%]

100

473K

2500

300K 573K

0 0

Wavelength [nm]

500

1000

1500

2000

0

2500

Wavelength [nm]

k¼

8 h i1=2 9 > >
k  ln > 4pd :

ðn  1Þ3  ðn  n22 Þ

> ;

Fig. 8. Transmission (T) and reflection (R) spectra for cerium oxide thin films deposited at different substrate temperatures.

ð7Þ

2.5 2.4

in which:

n20 þ n22 T max  T min þ 2n0 n2 2 T max  T min 2

EM ¼

8n n2 þ ðn2  1Þ  ðn2  n22 Þ T max

ð9Þ

where d is the film thickness, n2 is the refractive index of the substrate, n0 is the refractive index of the first medium (the air), Tmax and Tmin are the transmittance for a given wavelength k (from the envelopes curves). Independently, the extinction coefficient was also calculated from the well-known formula:

k¼

k T ln 4p  d 1  R

k(λ)

2.3

ð8Þ

Refractive index

N¼

1.0 E D C(2) B 0.8 A(2)

n(λ)

Ts=673K

2.2

0.6

2.1 Ts=573K Ts=383K Ts=300K

1.9 Ts=473K

1.8 1.7 200

400

600

In Fig. 8 the examples of the transmittance and reflectance characteristics of cerium oxide films deposited at different substrate temperatures are shown. All films exhibited relatively ‘‘good transparency’’ from about 380 nm (where onset absorption occurs) up to NIR. Moreover, the transmittance and reflectance of these films exhibit the interference effects in the examined wavelength range. One can see that the oscillation amplitude of these curves depends on substrate temperature during film deposition. The highest substrate temperature (i.e. for Ts = 573 K and 673 K) leads to the highest amplitude of the interference effects and indicates that these films exhibit the highest refractive index. The amplitude of the interference effects for the samples A, B, and C, deposited at lower substrate temperature, is on a similar level. However, for films deposited at Ts = 473 K this amplitude reaches the minimum value and for these films, the minimum value of the refractive index should be observed. Fig. 9 shows the dispersion characteristics of the real and imaginary part of the refractive index. All n(k) curves exhibit normal dispersion and the values of the refractive index are strongly modified by the substrate temperature during depositions of the films. The lowest values of the refractive index were obtained for the

800

1000

1200

1400

1600

0.2

0.0 1800

Wavelength [nm]

ð10Þ

5.3. Optical characteristics of cerium oxide films

0.4

2.0

Extinction coefficient

Fig. 7. Transmission and reflection spectra for cerium oxide films deposited onto quartz substrates at Ts = 383 K.

Fig. 9. Dispersion curves of the refractive index and the extinction coefficient for cerium oxide films deposited at different substrate temperatures. n(k) and k(k) characteristics show results for A2, B, C2, D and E specimens.

films C deposited at 473 K (n = 1.85–1.93), while the highest ones (n = 2.27–2.45) were obtained for the films E deposited at the substrate temperature of 673 K. All films exhibited the relatively low absorption for a visible range and for NIR (see curve k(k) in Fig. 9). The extinction coefficient did not exceed the values of 0.05 for k > 0.38 lm and 0.025 for k > 0.4 lm, respectively. The dependence of the refractive index on the temperature is displayed in Fig. 10. One can see that for k = 0.55 lm, at room temperature, the refractive index is about 2. Then, n decreases to a value of about 1.91 at Ts = 473 K and afterwards it increases up to about 2.34 at Ts = 673 K. Such large changes of the refractive index versus the substrate temperature during film preparations makes them very useful in different optical applications. Values of refractive index of any medium are connected with its packing density and porosity. Assuming the refractive index of bulk Ceria as nb (nb = 2, 4 [23]) we can connect the refractive index of the film (n) by the Lorenz–Lorenz expression [16]

n2  1 n2b  1 ð1  PÞ ¼ n2 þ 2 n2b þ 2

ð11Þ

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of CeO2 films deposited on various substrate temperatures differ significantly, even if we take into account e-beam deposition technique only (see Fig. 11). The reason for such a large discrepancies of results of different works, even for films deposited at similar substrate temperature, indicate that many various factors (such as: deposition rate, e-beam power, residual pressure, and oxygen content) may influence the obtained values of the refractive index of this material. All mentioned factors may have an effect on the structure, density and porosity of cerium oxide films.

2.6 2.5

λ = 450 nm λ = 550 nm λ = 630 nm λ = 1000 nm

Refractive index

2.4 2.3 2.2 2.1 2.0

6. Summary and conclusions

1.9 1.8 1.7 250

300

350

400

450

500

550

600

650

700

Ts [K] Fig. 10. A dependence of the refractive index for cerium oxide thin films upon the substrate temperature during film deposition. Experimental data show results for samples B, D, E and average data for samples A1, A2 and C1, C2, respectively.

this work

3.6

Refractive index

3.2

2.8

Ref.[2], e-b dep. Ref.[23], e-b dep. Ref.[23], e-b dep. at O2 Ref.[22], e-b dep. Ref.[6,18], e-b dep. Ref.[6,18], e-b dep. at O2 Ref.[12], e-b dep. at 630nm Ref.[21], e-b dep. Ref. [20], e-b dep. Ref.[25],e-b dep. Ref.[7], R-F sput. Ref.[28], R-F sput. Ref.[8], sol-gel dep. Ref.[29], sol-gel dep. Ref.[16], colloid. susp. Ref.[16], coll. susp., annealled at 1273K Ref.[14] PLD

2.4

2.0

1.6 400

600

800

1000

The PVD method was applied for fabrication of cerium oxide thin film coatings of 0.273–1.043 lm thickness on quartz substrates. An electron gun was used as a deposition source. The influence of substrate temperature on film properties was examined. Films were characterized with the AFM method, Raman spectroscopy and with UV–Vis–NIR spectrophotometry. The results can be summarized as follows: (1) Cerium oxide thin film coatings have a good transparency over a wide spectral range, i.e. in the visible range and for NIR. In this wavelength range, the values of the extinction coefficient were below 0.025, while the values of the refractive index of the films were in the range of 1.85–2.45. (2) We observed that the values of the refractive index changes with the change in the substrate temperature during deposition of the films (300 K 6 Ts 6 673 K). We found that for temperatures from 300 K to 473 K the values of n were between 1.91 and 2. For higher temperatures n increased strongly up to n = 2.34 at Ts = 673 K (for k = 0.55 lm). (3) Some changes in the refractive index (decreasing of n) around Ts = 473 K may arise from structural changes (such as grain structure, packing density and modification of the structure) as well as from increasing of the film porosity in this range. (4) Relatively large increase in the refractive index with an increase in the substrate temperature (above 573 K) is connected with a rapid change in the microstructure of the films (well-grained structure is formed) and due to their higher packing density.

1200

Ts [K] Acknowledgements Fig. 11. The refractive index of CeO2 films (at 550 nm) fabricated at various substrate temperatures – the literature data.

In which P denotes the porosity of the medium. Fig. 3 illustrates the calculated porosity of our specimens. Initial value of P for 300 K 6 Ts 6 383 K is about 0.2–0.21 and increases to 0.25 at 473 K, where the maximum porosity is observed. For higher substrate temperature P fall down to 0.024 at 623 K and indicates formation of very dense structure. If we take into account results of our Raman studies we conclude that the granularity of the film structure rises continuously with rising substrate temperature. Such a behavior would suggest a continuous rise of refractive index, similar to that observed by Rao et al. [2] in e-beam deposited CeO2 films. Obtained by us refractive index generally increases with temperature. In fact, a little lowering of n(Ts) curve is seen around 473 K. This effect may be due to rebuilding of the internal film structure. This problem cannot be solved univocally from the presented studies. It seems to us, however, that the film porosity is the most important factor which influences the refractive index value. We would like to note that the literature data on the refractive index

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