Structure and phase component of ZrO2 thin films studied by Raman spectroscopy and X-ray diffraction

Materials Science and Engineering B104 (2003) 163–168

Structure and phase component of ZrO2 thin films studied by Raman spectroscopy and X-ray diffraction Le Duc Huy a,∗ , P. Laffez a , Ph. Daniel a , A. Jouanneaux a , Nguyen The Khoi b , D. Siméone c a

Laboratoire de Physique de l’Etat Condensé, UPRES A CNRS no. 6087, F-72085 Le Mans cedex, France Faculty of Physics, Hanoi University of Education, 136 Avenue Xuan Thuy-Cau Giay, Hanoi, Viet Nam c SEMI, Lab. d’Etudes Matériaux Absorbents, CEA, CE Saclay, F-91191 Gif sur Yvette cedex, France

b

Abstract Zirconia (ZrO2 ) thin films were deposited by RF magnetron sputtering on zircaloy-4 (Zy-4) substrates directly from the ZrO2 target. These thin films, deposited at different substrate temperatures from 40 to 800 ◦ C and within different times from 10 to 240 min, were investigated by Raman spectroscopy and by X-ray diffraction (XRD). A rather good agreement between the results given by the two techniques was obtained. By comparison between the Raman studies on zirconia thin films and on bulk zirconia, it is possible to conclude the following: (i) films are polycrystalline; (ii) ZrO2 is not completely dissociated during the deposition process; (iii) the structure and phase composition of the films depend on the substrate temperature and on the deposition time, thickness and, therefore, vary as a function of the distance from the film surface. © 2003 Elsevier B.V. All rights reserved. Keywords: Zirconia thin film; Phase component; RF sputtering; Raman spectroscopy; X-ray diffraction

1. Introduction Zirconium alloys such as zircaloy-4 (Zy-4) are used for fuel-elements cladding and in-core structural element in pressurised water reactor (PWR). One of the major problems encountered in these elements of PWR is the oxidation under water effect. This oxidation process creates a zirconia (ZrO2 ) thin layer. This oxidation is a drastic factor limiting the time-life of the fuel-elements cladding and that restrains the productiveness of the PWR. Previous structural investigations [1] have shown that under neutron irradiation the monoclinic stable room temperature phase of zirconia layer can be partially transformed into tetragonal zirconia. The appearance of the ZrO2 layer in tetragonal form seems to be connected to a quicker degradation of the cladding and conversely the development of a stable monoclinic zirconia is considered as a valid way to limit the destructive corrosion of the zircaloy. Thus it appears very important to understand physical mechanism induced by irradiation in the monoclinic-tetragonal transition.

The aim of this work is to study the mechanism of ZrO2 thin films growth in order to bring valuable information to transfer it in the real system PWR. ZrO2 thin films can be obtained by different methods, such as thermal evaporation [2], reactive dc sputtering [3], RF reactive magnetron sputtering [4–8], sol–gel processing [9], laser and electron ablation [10], chemical vapour deposition [11], and electrochemical deposition [12,13]. It is well known that the crystalline structure and the properties of the films depend strongly on the deposition method and deposition conditions. In this work, the RF magnetron sputtering deposition method was used to produce zirconia thin films on Zy-4 substrate, which is the constitutive material of cladding in PWR. We have grown ZrO2 thin films of different thickness and at different temperature on Zy-4 and have characterised their structure and phase component by X-ray diffraction (XRD) and Raman scattering.

2. Experimental procedure

∗

Corresponding author. E-mail address: duc [email protected] (L. Duc Huy).

0921-5107/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0921-5107(03)00190-9

The zirconia thin films were deposited by RF magnetron sputtering directly from the zirconia target. The MP300 equipment manufactured by PLASSYS was used. The target

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L. Duc Huy et al. / Materials Science and Engineering B104 (2003) 163–168

Table 1 Results of the tetragonal phase proportion calculated with expressions (1) and (2) for zirconia thin films with different substrate temperatures (deposition time: 30 min, film thickness: ≈1300 Å) Sample number Substrate temperature Ct (%) (Raman) Ct2 (%) (XRD)

(◦ C)

M1

M2

M3

M4

M5

M6

M7

M8

M9

40 100 100

150 49 67.9

300 63.4 53.5

350 33.6 32.2

400 51.5 52.4

500 45.8 51.6

600 39.1 39.7

700 57.6 58.9

800 55.1 51.7

Table 2 Results of the tetragonal phase proportion calculated with expressions (1) and (2) for zirconia thin films with different deposition times (substrate temperature: 600 ◦ C) Sample number

M10

M11

M7

M12

M13

M14

M15

Deposition time (min) Thickness (Å) Ct (%) (Raman) Ct2 (%) (XRD) RMS strain (%) for t-ZrO2 RMS strain (%) for m-ZrO2

10 160 43 32

25 1115 40 34

30 1340 39.1 39.7 0.40 0.47

60 5000 40.8 30 0.33 0.42

120 10 000 28.7 27.2 0.29 0.35

180 15 000 34 30 0.12 0.36

240 20 000 4.1 13.3 0.11 0.32

made of 99%-pure monoclinic ZrO2 had a diameter of 33 mm. This target was prepared in a mould of 33 mm of diameter. A mixture of m-ZrO2 and polyvinilic alcohol (rhodoviol) 5% was pressed at 320 kg cm−2 then annealed by conforming to the following temperature program: 2 ◦ C min−1

2 ◦ C min−1

2h

1 ◦ C min−1

20 ◦ C −−−−−→ 450 ◦ C − → 450 ◦ C −−−−−→ 1000 ◦ C −−−−−→ 2 ◦ C min−1

10 h

1 ◦ C min−1

Temper

2 ◦ C min−1

1300 ◦ C −−−−−→ 1500 ◦ C − → 1500 ◦ C −−−−−→

a 50 mW laser power, during a total of 6000 s in ten acquisition periods, with a slit width set to 120 ␮m, and at room temperature. The laser beam was focused in a 1 ␮m spot on the sample surface. The zirconia thin films were also investigated by XRD with a Philips X’pert diffractometer in Bragg-Brentano geometry using Cu K␣ radiation. Data were collected in steps of 0.04◦ 2␪ over the angular range 20–85◦ 2␪ with a counting time of 5 s per point.

1300 ◦ C −−−−−→ 1000 ◦ C −−−→ 20 ◦ C 3. Results and discussion After the annealing, the target maintains yet the monoclinic structure. The substrates made of Zy-4 had a dimension of 10 × 20 mm2 . The distance between the target and the substrate was 40 mm. The target and substrate were polarised with a bias of 350 V and a frequency of 13.56 MHz. Before deposition, the substrates were polished mechanically and cleaned by ultrasonic vibrations during 7 min in a bath of acetone and ethanol. The vacuum chamber was evacuated by using a turbo molecular pump down to 5 × 10−6 mmbar. The sputtering gas Ar with a purity of 99.99% was introduced and controlled by a mass flow controller. The flow rate of Ar gas was 60 cm3 min−1 . Before deposition the target was cleaned by sputtering with a RF power of 50 W during 5 min in pure Ar atmosphere, while the substrate was covered with a shield. 16 zirconia thin films were deposited at different substrate temperatures from 40 up to 800 ◦ C and in different deposition times from 10 up to 240 min. The sample parameters are summarised in Tables 1 and 2. The Raman spectra of the films were recorded by using a T64000 model Jobin-Yvon multichannel spectrometer, in back-scattering geometry, adjusted in triple subtractive configuration. The samples were excited with the 514.5 nm line of an Argon–Krypton laser. Typical spectra were taken with

3.1. Influence of the substrate temperature Among all the deposited samples, nine zirconia thin films were studied with a common time deposition of 30 min at different substrate temperatures ranging from 40 to 800 ◦ C and by fixing all others technologic parameters. The thickness of all these films was estimated by using X-rays reflectivity in the range 130–1400 Å. The Raman spectra of these thin films compared with reference spectra of tetragonal nanometric powder zirconia (t-ZrO2 ) and monoclinic powder zirconia (m-ZrO2 ) are shown in Fig. 1. It can be concluded: (i) all the films spectra show characteristic peaks of t-ZrO2 or/and m-ZrO2 , which is the first evidence that the deposited film is effectively ZrO2 and not other compounds; (ii) at low substrate temperature, 40 ◦ C, the obtained zirconia thin films are single-phased, only the characteristic peaks of t-ZrO2 are present in the spectrum; (iii) with increasing the substrate temperature, the tetragonal phase component strongly decreases. The spectra of the films deposited at substrate temperatures from 150 to 800 ◦ C show that all these films are polyphased. For the higher substrate temperature, the characteristic peaks of t-ZrO2 and m-ZrO2 are visible on the spectra.

L. Duc Huy et al. / Materials Science and Engineering B104 (2003) 163–168

Fig. 1. Raman spectra of zirconia thin films deposited at different substrate temperatures in comparison with that of monoclinic and tetragonal powder zirconia.

The D. R. Clarke and F. Adar empirical expression [14]: Ct =

0.97[It (148) + It (272)] , 0.97[It (148) + It (272)] + Im (179) + Im (189)

(1)

based on the relation between some most characteristic peaks of m-ZrO2 and t-ZrO2 was used to estimate the tetragonal phase ratio for our zirconia thin films. The calculation results are listed in Table 1 and illustrated in Fig. 3. From this Figure, it is clear that the tetragonal phase ratio decreases rapidly with increasing substrate temperature and becomes stable at substrate temperatures higher than 400 ◦ C. It is known that in normal conditions of pressure and at temperatures under 1300 ◦ C, the stable phase of bulk ZrO2

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is monoclinic. The tetragonal symmetry is encountered only between 1300 and 2700 ◦ C. Note that the tetragonal phase can be stabilised at room temperature, thanks to an adapted chemical process [15], with crystalline grains of nanometric size [16]. In our case of low substrate temperature (40 ◦ C), during the deposition process the ZrO2 species and groups of zirconia species gather and probably form nanometric grains on the substrate. The surface tension of these grains may be large enough to prevent the penetration of other zirconia species, and then new nanometric zirconia grains are formed. At higher substrate temperature, the ZrO2 molecules energy is large enough to allow them to penetrate into the ZrO2 nanometric grains, hence the zirconia grains can grow and the monoclinic zirconia structure is established. In addition, at high temperature, the nanometric grains themselves might combine due to their thermal energy and migrate on the film surface. These activations occur more strongly on the film surface than in the depth of the film. Yet a question arises: why is the tetragonal fraction still considerable in films deposited at high temperature? This may be explained by the fact that the first layers of nanometric grains are fixed immediately on Zy-4 substrate. The film structure is predominantly tetragonal at the interface between the film and the substrate and highly monoclinic near the surface of the film. This is proved by studying the dependence of the tetragonal phase component as a function of the distance from the film surface. Fig. 2 shows the XRD patterns of zirconia thin films deposited at different substrate temperatures compared with three references: monoclinic zirconia, tetragonal zirconia and Zy-4. The t-ZrO2 and m-ZrO2 phases can be identified and monitored unambiguously below 31.5◦ 2␪, where no severe peak overlap exists, by using the three most charac¯ t(101) teristic Bragg reflections for t and m phases: m(111),

Fig. 2. XRD patterns of zirconia thin films deposited at different substrate compared with those of monoclinic and tetragonal zirconia powder and of Zy-4 substrate.

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Fig. 3. Comparative results of Raman (䊉) and XRD (䊏) studies of the tetragonal phase proportion as a function of the substrate temperature (estimated according to the [14,17], respectively). The solid line is only a guide eye.

and m(111), respectively at about 28.2, 30.2 and 31.4◦ 2␪. As shown in Fig. 2, the higher the substrate temperature is, the more the film is polyphased; this confirms the conclusions obtained with Raman scattering results. After extraction of integrated intensities of these three reflections, the tetragonal phase proportion was calculated by using the Garvie and Nicholson empirical expression [17]: Ct =

I(101)t ¯ m + I(111)m I(101)t + I(111)

(2)

The results are gathered in Table 1 and shown in Fig. 3, together with those calculated from Raman expression (1). The agreement between the results from two investigating methods, Raman spectroscopy and XRD, is rather satisfactory, considering the numerous possible errors, either in the models used or in the calculations intensities.

Fig. 4. Influence of the time deposition on Raman spectra of the zirconia thin films.

strate contain tetragonal zirconia, whereas the layers near the surface contain monoclinic zirconia. The XRD study provides the same features: as shown in Fig. 5, the ratio between the intensity of t(101) peak and that ¯ peak decreases with the deposition times and, of m(111) therefore, with the film thickness. The results obtained for the amount of tetragonal phase in the grown films, calculated on the basis of expression (2), are listed in Table 2 and shown in Fig. 6. A size-strain X-ray study has been also undertaken, in a first step only as a function of deposition time from 30 min (sample M7) to 240 min (sample M15). XRD patterns were analysed by whole pattern profile refinement (WPPR) with the Rietveld software WinMProf [18], whose multiphase option was used because of the simultaneous presence in the patterns of t-ZrO2 , m-ZrO2 and Zy-4 components. Le Bail decomposition method [19] was applied

3.2. Influence of the thickness of the films In order to check the crystal state of the thin films versus thickness, seven zirconia thin films were deposited at the same substrate temperature of 600 ◦ C, but with different deposition times in the range from 10 to 240 min, according to the assumption that the thickness is directly proportional to the time deposition. In this framework the Raman spectra of zirconia films were recorded as shown in Fig. 4. The tetragonal proportions calculated with expression (1) are listed in Table 2 and shown in Fig. 6 as a function of film thickness. These two figures demonstrate that the tetragonal ratio decreases with increasing of the film thickness. As illustrated in Fig. 4, the intensity of characteristic peaks in the monoclinic phase increases with the deposition time (and then the film thickness) while the intensity of the characteristic tetragonal peaks decreases. It means that the first layers grown near the sub-

Fig. 5. XRD patterns of the zirconia thin films with different thickness (different deposition times).

L. Duc Huy et al. / Materials Science and Engineering B104 (2003) 163–168

Fig. 6. Tetragonal phase proportion calculated with expressions (1) (䊉) and (2) (䊏) for ZrO2 thin films with different thickness. Two solid lines are a guide eye.

in order to avoid texture effects in intensity calculations. Peak profiles were described by the TCH [20] peakshape function, which yields good quality of fit and enables the Size and Strain parameters to be easily determined. The first results indicate that: (i) the sample broadening is isotropic; (ii) size and strain broadenings are respectively of lorentzian and gaussian types, as frequently experimentally observed; (iii) for both t-ZrO2 and m-ZrO2 phases, the volume-weighted domain size is constant with the deposition time: Dv = 160 (30) Å and Dv =290 (40) Å, respectively; (iv) the root-mean-square (RMS) strain parameter tends to decrease with increasing deposition time (Table 2). This size-strain X-ray analysis is still in progress. Further calculations will be carried out, and comparisons will be made between WPPR and results obtained directly ¯ and t(101). from the single Bragg peaks m(111) 3.3. Thin films growth model deduced from Raman scattering In order to confirm the previous results concerning a preferred growth first with tetragonal phase, followed by monoclinic component, we choose to record six Raman spectra on the same sample (M15) at six different points of the edge of the film (see Fig. 8). In this case we benefited by the tilt of the edge of the film and the measurements can be performed with a 0.4 ␮m step in the 2 ␮m depth of the films. In Fig. 7 are shown six different ZrO2 thin film Raman spectra at different positions of the edge and, therefore, at different depths of the film. The evolution of tetragonal amount as a function of the depth of M15 is confirmed by the evolution of the peak at about 275 cm−1 . Fig. 7 allows us to approach again the assumption that the film structure is predominantly tetragonal at the interface between the film and the substrate and highly monoclinic near the thin film surface.

167

Fig. 7. Raman spectra at six point of the edge of M15.

Fig. 8. Six focus points at the edge of M15 thin film where six Raman spectra (see Fig. 7) were recorded.

Moreover, to confirm the independence of the t-ZrO2 component on the annealing time during sputtering process, M16 thin film has been grown with the same experimental sputtering conditions of M10 (time: 10 min.; temperature: 600 ◦ C). After deposition, the sample was annealed for 3 h in the same conditions of temperature and argon pressure as during sputtering process. The result of the tetragonal phase proportion calculated for M16 by using expression (2) is about 33%. This result is very close to that of M10 (32%). Thus we can conclude that the phase proportion does not depend on the annealing time. 3.4. Evidence of impurity contamination by Raman scattering The Raman and XRD results appear very consistent. However, we have to mention some minor points in vibrational spectra, which contribute to some difficulties of interpretation of the Raman data: i) Raman spectrum of m-ZrO2 contains 14 peaks in the range from 100 to 700 cm−1 . These peaks superimpose on six peaks of t-ZrO2 . It appears then difficult to determine the integrated intensity of the m-ZrO2 and t-ZrO2 characteristic peaks. Additionally the expression (1) is originally empiric, hence it could involved some error.

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ii) The Raman peaks of ZrO2 thin films are clearly broader than those of bulk ZrO2 . This broadening of Raman peaks may be related to a static disorder or, at least, to some distortions in the crystalline structure. iii) Figs. 1 and 4 show the existence of one additional peak at about 410 cm−1 in Raman spectra. This peak was first supposed to be related to the luminescence of impurity ions, which are naturally present in ZrO2 chemical products. To check this point, a new Raman experiment with a different wavelength (blue line 488 nm) was performed, which has shown that the line did not move in absolute frequency. This demonstrates that the additional peak can be attributed to impurity luminescence. Moreover the shape of the peak indicates that this impurity is probably a rare earth element. With the help of the free ion lanthanum energy schema of the 4fn level, we found three possible types of rare earth ion which could be present in very low proportions: Neodymium (Nd), Europium (Eu) or Erbium (Er). Note that Raman technique is able to detect very low amount of rare earth impurity (<1 ppm), which does not affect the physical properties of the samples.

4. Conclusion In summary, we studied the structural properties of zirconia thin films deposited by RF magnetron sputtering directly from the ZrO2 target on Zy-4 substrate by means of Raman scattering and XRD. At low substrate temperatures, the obtained film contains only t-ZrO2 . The tetragonal proportion decreases rapidly with increasing of the deposition temperature and then keeps stable in the temperature range from 400 to 800 ◦ C. At substrate temperature of about 600 ◦ C, we have shown monoclinic zirconia layers are formed beyond those of tetragonal phase. Near the film surface, the monoclinic phase is dominant.

Acknowledgements We wish to thank Dr. Christian Launay (IUT Le Mans, France) who helped us to polish substrates. We acknowledge

M. Gérard Niesseron who helped us to produce, at the Condensed Matter Physics Laboratory (LPEC Le Mans, France), the films studied in this work.

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