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Effect of thermal annealing on the microstructure and morphology of erbium films
Thin Solid Films 520 (2012) 6196–6200
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Effect of thermal annealing on the microstructure and morphology of erbium films H.H. Shen a, S.M. Peng b, X.G. Long b, X.S. Zhou b, L. Yang a, K. Sun c, X.T. Zu a,⁎ a b c
Department of Applied Physics, University of Electronic Science and Technology of China, Chengdu 610054, China Institute of Nuclear Physics and Chemistry, China Academy of Engineering Physics, Mianyang 621900, China Electron Microbeam Analysis Laboratory, University of Michigan, Ann Arbor, MI 48109-2143, USA
a r t i c l e
i n f o
Article history: Received 10 November 2011 Received in revised form 13 April 2012 Accepted 30 May 2012 Available online 5 June 2012 Keywords: Erbium film Thermal annealing Preferred orientation Columnar grain
a b s t r a c t The effects of thermal annealing on the microstructure and morphology of erbium films were investigated by X-ray diffraction and scanning electron microscopy. All the erbium films were fabricated by electron-beam vapor deposition. The columnar grain sizes of as-received erbium films increased with the substrate temperatures and were enlarged by the coalescence and migration of grains during the high temperature annealing. The intrinsic stresses of erbium films, fabricated at a low substrate temperature (200 °C), were relaxed accompanied with the appearance of cracks on the films surface. The films deposited at 200 °C had (002) preferred orientation, and the film deposited at 450 °C had mixed (100) and (101) texture. The peak positions and the full width at half maximum of (100), (002), and (101) diffraction lines of erbium shift towards higher angles and sharply decrease during the annealing process, indicating that the stress inside the film was relaxed. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Erbium (Er) has been extensively studied in both aspects of theory [1] and experiment [2–7] in recent years. One of the most important applications for erbium in nuclear industry is that its film, with thickness of 3–6 μm [8], can be used as tritium (T) target material which is the key component of neutron generators [8–10]. The major factor for the potential value of erbium is that erbium dihydride film has high thermal stability [11–13] in comparison to Ti dihydride [10,13], i.e. hydrogen isotopes would be decomposed in erbium dihydride film at a higher temperature than that in Ti dihydride film. Again, helium (He), generated by the decay of T with the half-life of 12.32 year [3,7], would be released from the Er–T–He ternary system at a critical rate when the ratio of generated He to Er increases to 0.32 [7,14,15]. The growth mechanism of erbium film has been investigated by many researchers [4,16,17], and the results demonstrated that the microstructure and morphology of erbium film were both sensitive to the deposition parameters, including the substrate temperature [4,16,17], deposition rate [4,16,17], substrate material [4,16], residual gases [4,16,17] and surface contamination [17]. The preferred orientation of erbium was changed with the above deposition conditions [4,18]. It was also found that the grain size increases with substrate temperature and deposition rate, and the dependence of the grain size on substrate temperature is consistent with the activation energy model proposed by Grovenor et al. [19]. The thermodynamic property of erbium film for absorbing tritium, a core parameter for estimating its application value
⁎ Corresponding author. Tel./fax: + 86 28 83202130. E-mail address: [email protected] (X.T. Zu). 0040-6090/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2012.05.084
[20], would be significantly affected by its microstructure. Parish et al. [18] found that ErD2 grain sizes are strongly influenced by the priormetal grain size, with small metal grains leading to large ErD2 grains. One can expect that the critical release of He [14,15] from ErT2 − xHex would be easily conducted by the microstructure of erbium film. Therefore, it is very important to understand the dependence of the microstructure of erbium film on the deposition condition, with the destination to improve the properties of erbium film for absorbing T and stacking He. Generally, the grain size and the intrinsic stress of film would be changed by the heat annealing treatment [21,22]. The vacuum annealed In2O3 thin films display a grain size enlargement and preferential orientation [23]. The effect of the annealing processing on the microstructure and morphology of erbium is not clearly understood up to now. In the present study, the annealing effects on the microstructure and morphology were systematically evaluated, as well as the processing effects, including the substrate temperature and deposition rate, were also discussed.
2. Experimental procedure The erbium thick films were electron-beam evaporated on the rolled molybdenum (Mo) substrates at substrate temperatures of 200 °C and 450 °C. Evaporation rates detected by IC5 thin film deposition controller (Inficon, USA) were either 0.5 or 10 nm/s. Before deposition, all substrates were ultrasonically cleaned with ethanol and subsequently in acetone. In addition, the molybdenum holder and molybdenum substrates were outgassed by increasing the temperature from room temperature to 700 °C and heating for 2 h at 700 °C. The source material
H.H. Shen et al. / Thin Solid Films 520 (2012) 6196–6200
erbium bulk (99.95% purity, 25 mm in diameter) was outgassed by melting it using electron beam to remove the surface oxide layer and possible residual gases absorbed from air. After outgas of substrates and source material, the molybdenum substrates were cooled to the desired temperature for film deposition and the vacuum pressure was better than 4.0 × 10 − 4 Pa. The erbium flux was then deposited onto the substrates with the direction parallel to the substrate normal. To obtain uniform film, the sample holder was rotated during deposition and the distance between source material and sample holder was ~20 cm. The heat annealing treatment was processed in the same vacuum system as the erbium deposition, and the vacuum pressure was better than 1.0 × 10− 4 Pa prior to the start of heat annealing. The selected specimens were annealed at 600 °C for 30 min, and then naturally cooled to room temperature in the vacuum system. The microstructures of erbium films, before and after the annealing processing, are both preliminarily identified by X-ray diffractometer (XRD) using a X'pert PRO MPD (Panalytical, Holand) with a Cu Kα radiation. Typical scan parameters were 25 to 90° 2θ, 0.04° step size, and 1 s count time. The surface and cross-section morphologies of as-received and heat annealed erbium films were carried out using a field-emission scanning electron microscope (FE-SEM) (Obducat CamScan, UK) at 15 keV. The schematic for simply acquiring the cross-section morphology of erbium film is plotted in Fig. 1. First, a gap in the middle of the direction along the diameter was mechanically cut after the molybdenum substrate was rolled into a wafer, followed by cleaning it with ethanol and subsequently in acetone ultrasonically. Second, the erbium film was deposited onto the above substrate. Erbium film would be found on the surface of the substrate and also in the gap, with the exception of the area in the fringe of the gap. Finally, the substrate was fractured in two pieces and the cross-section morphology of film can be easily observed in the fringe area of the gap. Although the cross-section morphology of film is not flat since the molybdenum substrate is not mechanically polished, the kind of grain and the tendency of grain size to change with substrate temperature are clearly understood by the proposed scheme. Also, the thickness of film cannot be evaluated accurately by the observation of cross-section morphology. 3. Results and discussion The deposition parameters (substrate, substrate temperature and deposition rate) and the thickness of the erbium films, which were selected for heat annealing, are listed in Table 1. As shown, the thickness of film deposited at a higher rate (~ 10 nm/s) is much thicker than that of film deposited at the rate of ~0.5 nm/s. The deposition rate cannot be accurately controlled when the rate is relatively high.
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Based on our experimental result (not shown here), the microstructure and morphology of the film, however, would not be apparently affected by the film thickness when the thickness of film was in the order of magnitude of μm. The three representative specimens were designated as Er200-0.5, Er200-10 and Er450-0.5, deducing from the substrate temperature (200 °C or 450 °C) and deposition rate (~0.5 nm/s or ~10 nm/s). The surface and cross-section morphologies (in the inset) of all three specimens, before and after annealing, were shown in Fig. 2. The morphology of columnar grain [16–19], derived from the crosssection morphologies, can be observed for all specimens before annealing, which is consistent with the structure-zone models (SZMs) suggested by Savaloni et al. [16,17] and Gu et al. [4] for the specific case of erbium-thin-films. SZMs illustrated that the structure of polycrystalline thin-film was determined by processing parameters, which was defined as Ts/Tm (where Ts is the substrate temperature in Kelvin and Tm the melting point of film material in Kelvin). Here, the values of Ts/Tm are 0.26 and 0.40 for the substrate temperatures of 200 °C and 450 °C, which correspond to the behaviors (columnar grain) of “Zone T” (0.2 b Ts/Tm b 0.3) and “Zone 2” (0.3 b Ts/Tm b 0.5) which result from SZMs [4]. The “Zone T” structure was columnar, consisting of tapered units defined by voided growth boundaries of the type. The “Zone 2” structure consisted of columnar grains, which were defined by metallurgical grain boundaries and increased in width with Ts/Tm in accordance with activation energies of surface diffusion. The grain sizes of erbium films increase with the increasing substrate temperatures when the deposition rates are fixed at ~0.5 nm/s. It may be attributed to the different diffusion mechanisms of erbium adatoms, which were surface diffusion and bulk diffusion for “Zone T” and “Zone 2”, respectively [19]. One can conclude from Fig. 2(a) and (c), the film deposited with the rate of ~10 nm/s, having less pores between the grains, is denser than that of the film deposited with ~0.5 nm/s at the substrate temperature of 200 °C. This phenomenon can be explained by the stucking effect of erbium adatoms, which were deposited at the porous sites for the high deposition rate. After the films were heat annealed, the surfaces of the films were smoothened and many cracks appeared on the surfaces as shown in Fig. 2(a) and (b) for Er200-0.5 and Fig. 2(c) and (d) for Er200-10. The coalescence and migration of grains were observed both from the surface and cross-section morphologies. It is also interesting to note that some proportions of the film were separated from the Mo substrate after the annealing treatment (inset of Fig. 2(d)), however, that was not detected for Er200-0.5 (inset of Fig. 2(b)). The reason for this difference is not still clear. The two dominant factors, ascribed to the film separation from the substrate, are as follows: (I) the erbium grains, divided by grain boundaries or pores [16–18], coalesce and migrate by overcoming the energy barrier during the annealing process, while the grains of Mo substrate remain unchanged or change very little. The discrepancy in the degree of change in grain sizes leads to the separation of erbium film from the Mo substrate. Pang et al. [22] found that different layers of film are separated by clearly defined interfaces because of diffusion and crystal growth. Diffusion may take place in every layer and interface, but the energy barrier for interfacial diffusion is much higher than that in layers, so diffusion is much easier in layers and Table 1 Sample specifications. All of the samples were electron-beam deposited on rolled molybdenum (rolled-Mo) substrate. The approximate thicknesses of erbium films were equal to the mass, measured by electronic balance, divided by the product of the density of erbium (9.05 g/cm3) and the surface area of film.
Fig. 1. Schematic for observing the cross-section morphology of erbium film. A gap in the middle of the direction along the diameter was mechanically cut after the molybdenum substrate was rolled into a wafer, followed by the deposition of erbium film. The cross-section morphology of film can be easily observed by SEM, after the substrate is fractured in two pieces, in the fringe area of the gap.
Substrate Substrate temperature (°C)
Deposition rate (nm/s)
Thickness of film (μm)
Designation
RolledMo
~ 0.5 ~ 10 ~ 0.5
~ 1.73 ~ 5.88 ~ 1.37
Er200-0.5 Er200-10 Er450-0.5
200 200 450
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Fig. 2. The surface and cross-section (in the inset) morphologies of as-received ((a), (c), (e)) and post-annealed ((b), (d), (f)) erbium films, which are designated as Er200-0.5 ((a), (b)), Er200-10 ((c), (d)) and Er450-0.5 ((e), (f)).
clearer interfaces are seen while annealing at a higher temperature; (II) the coefficient of thermal expansion (CTE) is 4.8 × 10 − 6/K for pure molybdenum, which is significantly smaller than that of 12.2 × 10 − 6/K for pure erbium [24]. The distinction, in the shrinkage of Mo and Er when the annealing process was stopped and the temperature was cooled to room temperature naturally, also led to the separation between the film and substrate [25]. Adams et al. [25] found that a tensile stress develops due to mismatched CTE of the substrate–film couple when samples are cooled to ambient temperature after the scandium reaction with deuterium. The residual stress of the ScD2 film and the propensity for films to crack during cool down depend on the CTE misfit between the substrate and film materials. There is no visible change in the morphology of Er450-0.5, before and after the annealing (as shown in Fig. 2(e) and (f)), indicating that the film deposited at 450 °C is completely crystallized and has less residual stress inside the film. Fig. 3 shows the XRD patterns with 2θ ranging from 28° to 41° of as-received and post-annealed erbium films, Er200-0.5, Er200-10, and Er450-0.5. The relative intensities of diffraction lines of (100), (002), and (101) are listed in Table 2. To prove the preferred orientation of erbium films, the calculated XRD data (PDF card 01-089-3033) of the relative intensity of diffraction peaks was also shown in Table 2. The films deposited at 200 °C had (002) preferred orientation, and the film deposited at 450 °C had mixed (100) and (101) texture. The
regulation of preferred orientation of erbium film changing with substrate temperature is opposite to the result obtained by Parish et al. [18]. The (002) texture of erbium film deposited at 200 °C should be conducted by the theory of surface energy minimization, which
Fig. 3. The XRD patterns of as-received and post-annealed erbium films, which are designated as Er200-0.5, Er200-10 and Er450-0.5.
H.H. Shen et al. / Thin Solid Films 520 (2012) 6196–6200
showed that the basal plane (001) has the minimum surface energy for a hexagonal-close-packed (hcp) metal [26]. So, from the surface energy minimization, the (001) texture should be favored in the hcp films, which is consistent with the experimental results [27]. The mixed (100) and (101) texture, however, may be dominated by the diffusion of adatoms and the recrystallization of the film grains [28]. The annealing process had no effect on the texture of films (Fig. 3 and Table 2). The phase structure of erbium oxide (Er2O3) was revealed in Fig. 3 for post-annealed Er200-0.5 and Er200-10 specimens, whereas it was not evident that the Er2O3 phase was formed after Er450-0.5 film was annealed. It seems reasonable that the film, with small grain size and maximum surface energy, is susceptible to be oxidized [29]. The full width at half maximum (FWHM) of (100), (002), and (101) diffraction lines of three as-received erbium films was sharply decreased with increasing substrate temperature, as seen from Fig. 3 and Table 3. The annealing process has a direct impact on the FWHM of Er200-0.5 and Er200-10. The increase in FWHM originated from the increase in grain size and the relaxation of stress. Although it is difficult to quantify the dominant factor here, the increase of grain size and the relaxation of stress with high temperature annealing were reported by many researchers [21–23]. Based on all the XRD patterns plotted in Fig. 3, the diffraction line of the Mo substrate was always unchanged whenever the films were annealed. Furthermore, after the films were annealed, the peak positions of (100), (002), and (101) diffraction lines of erbium films shift towards higher angles [25], more drastically for Er200-0.5 and Er200-10 and insignificantly for Er450-0.5. The lattice constants calculated from the XRD data were tabulated in the right two columnar of Table 3. The lattice constants of a and c for Er450-0.5 changed a little, whereas a and c were markedly changed for Er200-0.5 and Er200-10, before and after the annealing treatment. All the above results, including the changes of FWHM and the lattice constants, are attributed to the evolution of compressive and tensile stresses inside the films during the deposition and annealing process. Compressive stress in the erbium films was enhanced due to the energetic particle bombardment during deposition [30], while the tensile stress was led by the difference in CTE of erbium and molybdenum materials when the as-received film was cooled down to room temperature. After the deposition, both the compressive stress and tensile stress were relaxed by the heat annealing. A compressive stress in thick films is not surprising considering that electron-beam vapor deposition is used for growth. Recently, research by Chason et al. [31] identifies grain boundaries as an important factor in generating a compressively-stressed, continuous film. The evolution of stress in ScD2/Cr thin films fabricated by evaporation and high temperature reaction was reviewed in detail [25]. Reaction of scandium metal with deuterium at elevated temperatures to form a stoichiometric dideuteride phase leads to a large compressive in-plane film stress, due to an increased atomic density compared with Table 2 The relative intensity of three diffraction lines, (100), (002) and (101), of as-received and post-annealed (annealing at 600 °C for 30 min) erbium films. The normalized relative intensity of (100), (002) and (101) diffraction lines of PDF card (01-089-3033) was also shown in this table, so as to demonstrate the preferential orientation of the samples. Sample list
Relative intensity of diffraction lines (%)
PDF 01-089-3033 As-received
Post-annealed
Er200-0.5 Er200-10 Er450-0.5 Er200-0.5 Er200-10 Er450-0.5
(100)
(002)
(101)
24.9
24.8
100
12.3 11.7 21.8 9.6 5.8 19.8
100 100 8.3 100 100 5.9
64.5 33.2 100 53.0 15.6 100
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Table 3 The FWHM of three diffraction lines, (100), (002) and (101), of as-received and postannealed (annealing at 600 °C for 30 min) erbium films. The lattice constants of the above samples were also shown in the right column. Sample list
As-received
Post-annealed
Er200-0.5 Er200-10 Er450-0.5 Er200-0.5 Er200-10 Er450-0.5
FWHM of erbium diffraction lines (degree)
Lattice constants (nm)
(100)
(002)
(101)
a
c
0.363 0.306 0.125 0.157 0.185 0.099
0.364 0.360 0.184 0.184 0.139 0.169
0.361 0.368 0.136 0.185 0.136 0.116
0.3598 0.3574 0.3560 0.3566 0.3565 0.3557
0.5662 0.5618 0.5600 0.5637 0.5600 0.5594
the as-deposited metal film. A tensile stress develops due to mismatched CTE of the substrate-film couple [25]. Yuan et al. [23] demonstrated that the lattice constant decreases and is close to the bulk one, which indicates that the compressive strain in the In2O3 films are gradually relaxed as the annealing temperature increases. Zhou et al. [21] found that with increasing annealing temperature, the compressive stress decreased due to the elimination of defects and the grain growth caused the lateral shrinkage of film and produced a relative tensile stress. It is instructive to note that, after the films were annealed, the larger shift in the peak position for Er200-0.5 was observed, indicating that the film growth at a low temperature (200 °C) maintains a larger compressive stress (Fig. 2). This is in accordance with the observation of SEM images where significantly more cracks were detected for Er200-0.5 after the film was annealed. 4. Conclusions In summary, the columnar grain sizes of as-received erbium films increased with the substrate temperatures and were enlarged by the coalescence and migration of grains during the high temperature annealing. The intrinsic stresses of erbium films, fabricated at a low substrate temperature (200 °C), were relaxed accompanied with the appearance of cracks on the films surface. The film, with small grain size and maximum surface energy, is susceptible to be oxidized. The films deposited at 200 °C had (002) preferred orientation, and the film deposited at 450 °C had mixed (100) and (101) texture. The peak positions and the FWHM of (100), (002), and (101) diffraction lines of erbium shift towards higher angles and sharply decrease with annealing process, indicating that the stress inside the film was relaxed. Acknowledgment L. Yang and X. T. Zu are grateful for the support by the National Natural Science Foundation of China — NSAF (Grant No: 10976007) and the Fundamental Research Funds for the Central Universities (Grant No: ZYGX2009J040). S.M. Peng and X.G. Long are grateful for the support by the Science and Technology Foundation of CAEP (Grant No: 2009A0301015) and the Major Program of the National Natural Science Foundation of China (Grant No: 91126001). References [1] L. Yang, S.M. Peng, X.G. Long, F. Gao, H.L. Heinisch, R.J. Kurtz, X.T. Zu, J. Appl. Phys. 107 (2010) 054903. [2] M.T. Brumbach, J.A. Ohlhausen, K.R. Zavadil, C.S. Snow, J.C. Woicik, J. Appl. Phys. 109 (2011) 114911. [3] J.A. Knapp, J.F. Browning, G.M. Bond, Nucl. Instrum. Methods B 268 (2010) 2141. [4] E.D. Gu, H. Savaloni, M.A. Player, G.V. Marr, J. Phys. Chem. Solids 53 (1992) 127. [5] G.M. Bond, J.F. Browning, C.S. Snow, J. Appl. Phys. 107 (2010) 083514. [6] R.M. Ferrizz, Erbium Hydride Decomposition Kinetics, Sandia Report: SAND2006-7014, 2006. [7] J.A. Knapp, J.F. Browning, G.M. Bond, J. Appl. Phys. 105 (2009) 053501. [8] P.A. Dow, G.W. Briers, M.A.P. Dewey, D.S. Stark, Nucl. Instrum. Methods 60 (1968) 293.
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[9] E.R. Graves, A.A. Rodrigues, M. Goldblatt, D.I. Meyer, Rev. Sci. Instrum. 20 (1949) 579. [10] R. Redstone, M.C. Rowland, Nature 201 (1964) 1115. [11] I. Gabis, E. Evard, A. Voyt, I. Chernov, Y. Zaika, J. Alloys Compd. 356–357 (2003) 353. [12] R.M. Ferrizz, Erbium Hydride Thermal Desorption: Controlling Kinetics, Sandia Report: SAND2007-2659, 2007. [13] P.M.S. Jones, P. Ellis, T. Aslett, Nature 223 (1969) 829. [14] T. Venhaus, J. Poths, 7th International Conference on Tritium Science and TechnologyFusion Sci. Technol. 48 (2005) 601. [15] L.C. Beavis, W.J. Kass, J. Vac. Sci. Technol. 14 (1977) 509. [16] H. Savaloni, M.A. Player, E. Gu, G.V. Marr, Vacuum 43 (1992) 965. [17] H. Savaloni, M.A. Player, Vacuum 46 (1995) 167. [18] C.M. Parish, C.S. Snow, D.R. Kammler, L.N. Brewer, J. Nucl. Mater. 403 (2010) 191. [19] C.R.M. Grovenor, H.T.G. Hentzell, D.A. Smith, Acta Metall. 32 (1984) 773. [20] C.E. Lundin, Trans. Met. Soc. AIME 242 (1968) 1161. [21] M. Zhou, M. Nose, Y. Makino, K. Nogi, Thin Solid Films 359 (2000) 165.
[22] X.L. Pang, H.S. Yang, X.L. Liu, K.W. Gao, Y.B. Wang, A.A. Volinsky, A.A. Levin, Thin Solid Films 519 (2011) 5831. [23] Z.J. Yuan, X.M. Zhu, X. Wang, X.K. Cai, B.P. Zhang, D.J. Qiu, H.Z. Wu, Thin Solid Films 519 (2011) 3254. [24] J.G. Speight, Lange's Handbook of Chemistry, 16th edition McGraw-Hill, 2005. [25] D.P. Adams, J.A. Romero, M.A. Rodriguez, J.A. Floro, P.G. Kotula, Microstructure, Phase Formation, and Stress of Reactively-Deposited Metal Hydride Thin Films, Sandia Report: SAND2002-1466, 2002. [26] J.M. Zhang, D.D. Wang, K.W. Xu, Appl. Surf. Sci. 253 (2006) 2018. [27] S.S. Dhesi, R.G. White, A.J. Patchett, M.H. Lee, R.I.R. Blyth, F.M. Leibsle, S.D. Barrett, Phys. Rev. B 51 (1995) 17946. [28] D.N. Lee, Int. J. Mech. Sci. 42 (2000) 1645. [29] H.H. Shen, S.M. Peng, X.G. Long, X.S. Zhou, L. Yang, X.T. Zu, Vacuum 86 (2012) 1097. [30] F.M. D'Heurle, Metall. Trans. 1 (1970) 725. [31] E. Chason, B.W. Sheldon, L.B. Freund, J.A. Floro, S.J. Hearne, Phys. Rev. Lett. 88 (2002) 6013.
Contents lists available at SciVerse ScienceDirect
Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
Effect of thermal annealing on the microstructure and morphology of erbium films H.H. Shen a, S.M. Peng b, X.G. Long b, X.S. Zhou b, L. Yang a, K. Sun c, X.T. Zu a,⁎ a b c
Department of Applied Physics, University of Electronic Science and Technology of China, Chengdu 610054, China Institute of Nuclear Physics and Chemistry, China Academy of Engineering Physics, Mianyang 621900, China Electron Microbeam Analysis Laboratory, University of Michigan, Ann Arbor, MI 48109-2143, USA
a r t i c l e
i n f o
Article history: Received 10 November 2011 Received in revised form 13 April 2012 Accepted 30 May 2012 Available online 5 June 2012 Keywords: Erbium film Thermal annealing Preferred orientation Columnar grain
a b s t r a c t The effects of thermal annealing on the microstructure and morphology of erbium films were investigated by X-ray diffraction and scanning electron microscopy. All the erbium films were fabricated by electron-beam vapor deposition. The columnar grain sizes of as-received erbium films increased with the substrate temperatures and were enlarged by the coalescence and migration of grains during the high temperature annealing. The intrinsic stresses of erbium films, fabricated at a low substrate temperature (200 °C), were relaxed accompanied with the appearance of cracks on the films surface. The films deposited at 200 °C had (002) preferred orientation, and the film deposited at 450 °C had mixed (100) and (101) texture. The peak positions and the full width at half maximum of (100), (002), and (101) diffraction lines of erbium shift towards higher angles and sharply decrease during the annealing process, indicating that the stress inside the film was relaxed. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Erbium (Er) has been extensively studied in both aspects of theory [1] and experiment [2–7] in recent years. One of the most important applications for erbium in nuclear industry is that its film, with thickness of 3–6 μm [8], can be used as tritium (T) target material which is the key component of neutron generators [8–10]. The major factor for the potential value of erbium is that erbium dihydride film has high thermal stability [11–13] in comparison to Ti dihydride [10,13], i.e. hydrogen isotopes would be decomposed in erbium dihydride film at a higher temperature than that in Ti dihydride film. Again, helium (He), generated by the decay of T with the half-life of 12.32 year [3,7], would be released from the Er–T–He ternary system at a critical rate when the ratio of generated He to Er increases to 0.32 [7,14,15]. The growth mechanism of erbium film has been investigated by many researchers [4,16,17], and the results demonstrated that the microstructure and morphology of erbium film were both sensitive to the deposition parameters, including the substrate temperature [4,16,17], deposition rate [4,16,17], substrate material [4,16], residual gases [4,16,17] and surface contamination [17]. The preferred orientation of erbium was changed with the above deposition conditions [4,18]. It was also found that the grain size increases with substrate temperature and deposition rate, and the dependence of the grain size on substrate temperature is consistent with the activation energy model proposed by Grovenor et al. [19]. The thermodynamic property of erbium film for absorbing tritium, a core parameter for estimating its application value
⁎ Corresponding author. Tel./fax: + 86 28 83202130. E-mail address: [email protected] (X.T. Zu). 0040-6090/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2012.05.084
[20], would be significantly affected by its microstructure. Parish et al. [18] found that ErD2 grain sizes are strongly influenced by the priormetal grain size, with small metal grains leading to large ErD2 grains. One can expect that the critical release of He [14,15] from ErT2 − xHex would be easily conducted by the microstructure of erbium film. Therefore, it is very important to understand the dependence of the microstructure of erbium film on the deposition condition, with the destination to improve the properties of erbium film for absorbing T and stacking He. Generally, the grain size and the intrinsic stress of film would be changed by the heat annealing treatment [21,22]. The vacuum annealed In2O3 thin films display a grain size enlargement and preferential orientation [23]. The effect of the annealing processing on the microstructure and morphology of erbium is not clearly understood up to now. In the present study, the annealing effects on the microstructure and morphology were systematically evaluated, as well as the processing effects, including the substrate temperature and deposition rate, were also discussed.
2. Experimental procedure The erbium thick films were electron-beam evaporated on the rolled molybdenum (Mo) substrates at substrate temperatures of 200 °C and 450 °C. Evaporation rates detected by IC5 thin film deposition controller (Inficon, USA) were either 0.5 or 10 nm/s. Before deposition, all substrates were ultrasonically cleaned with ethanol and subsequently in acetone. In addition, the molybdenum holder and molybdenum substrates were outgassed by increasing the temperature from room temperature to 700 °C and heating for 2 h at 700 °C. The source material
H.H. Shen et al. / Thin Solid Films 520 (2012) 6196–6200
erbium bulk (99.95% purity, 25 mm in diameter) was outgassed by melting it using electron beam to remove the surface oxide layer and possible residual gases absorbed from air. After outgas of substrates and source material, the molybdenum substrates were cooled to the desired temperature for film deposition and the vacuum pressure was better than 4.0 × 10 − 4 Pa. The erbium flux was then deposited onto the substrates with the direction parallel to the substrate normal. To obtain uniform film, the sample holder was rotated during deposition and the distance between source material and sample holder was ~20 cm. The heat annealing treatment was processed in the same vacuum system as the erbium deposition, and the vacuum pressure was better than 1.0 × 10− 4 Pa prior to the start of heat annealing. The selected specimens were annealed at 600 °C for 30 min, and then naturally cooled to room temperature in the vacuum system. The microstructures of erbium films, before and after the annealing processing, are both preliminarily identified by X-ray diffractometer (XRD) using a X'pert PRO MPD (Panalytical, Holand) with a Cu Kα radiation. Typical scan parameters were 25 to 90° 2θ, 0.04° step size, and 1 s count time. The surface and cross-section morphologies of as-received and heat annealed erbium films were carried out using a field-emission scanning electron microscope (FE-SEM) (Obducat CamScan, UK) at 15 keV. The schematic for simply acquiring the cross-section morphology of erbium film is plotted in Fig. 1. First, a gap in the middle of the direction along the diameter was mechanically cut after the molybdenum substrate was rolled into a wafer, followed by cleaning it with ethanol and subsequently in acetone ultrasonically. Second, the erbium film was deposited onto the above substrate. Erbium film would be found on the surface of the substrate and also in the gap, with the exception of the area in the fringe of the gap. Finally, the substrate was fractured in two pieces and the cross-section morphology of film can be easily observed in the fringe area of the gap. Although the cross-section morphology of film is not flat since the molybdenum substrate is not mechanically polished, the kind of grain and the tendency of grain size to change with substrate temperature are clearly understood by the proposed scheme. Also, the thickness of film cannot be evaluated accurately by the observation of cross-section morphology. 3. Results and discussion The deposition parameters (substrate, substrate temperature and deposition rate) and the thickness of the erbium films, which were selected for heat annealing, are listed in Table 1. As shown, the thickness of film deposited at a higher rate (~ 10 nm/s) is much thicker than that of film deposited at the rate of ~0.5 nm/s. The deposition rate cannot be accurately controlled when the rate is relatively high.
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Based on our experimental result (not shown here), the microstructure and morphology of the film, however, would not be apparently affected by the film thickness when the thickness of film was in the order of magnitude of μm. The three representative specimens were designated as Er200-0.5, Er200-10 and Er450-0.5, deducing from the substrate temperature (200 °C or 450 °C) and deposition rate (~0.5 nm/s or ~10 nm/s). The surface and cross-section morphologies (in the inset) of all three specimens, before and after annealing, were shown in Fig. 2. The morphology of columnar grain [16–19], derived from the crosssection morphologies, can be observed for all specimens before annealing, which is consistent with the structure-zone models (SZMs) suggested by Savaloni et al. [16,17] and Gu et al. [4] for the specific case of erbium-thin-films. SZMs illustrated that the structure of polycrystalline thin-film was determined by processing parameters, which was defined as Ts/Tm (where Ts is the substrate temperature in Kelvin and Tm the melting point of film material in Kelvin). Here, the values of Ts/Tm are 0.26 and 0.40 for the substrate temperatures of 200 °C and 450 °C, which correspond to the behaviors (columnar grain) of “Zone T” (0.2 b Ts/Tm b 0.3) and “Zone 2” (0.3 b Ts/Tm b 0.5) which result from SZMs [4]. The “Zone T” structure was columnar, consisting of tapered units defined by voided growth boundaries of the type. The “Zone 2” structure consisted of columnar grains, which were defined by metallurgical grain boundaries and increased in width with Ts/Tm in accordance with activation energies of surface diffusion. The grain sizes of erbium films increase with the increasing substrate temperatures when the deposition rates are fixed at ~0.5 nm/s. It may be attributed to the different diffusion mechanisms of erbium adatoms, which were surface diffusion and bulk diffusion for “Zone T” and “Zone 2”, respectively [19]. One can conclude from Fig. 2(a) and (c), the film deposited with the rate of ~10 nm/s, having less pores between the grains, is denser than that of the film deposited with ~0.5 nm/s at the substrate temperature of 200 °C. This phenomenon can be explained by the stucking effect of erbium adatoms, which were deposited at the porous sites for the high deposition rate. After the films were heat annealed, the surfaces of the films were smoothened and many cracks appeared on the surfaces as shown in Fig. 2(a) and (b) for Er200-0.5 and Fig. 2(c) and (d) for Er200-10. The coalescence and migration of grains were observed both from the surface and cross-section morphologies. It is also interesting to note that some proportions of the film were separated from the Mo substrate after the annealing treatment (inset of Fig. 2(d)), however, that was not detected for Er200-0.5 (inset of Fig. 2(b)). The reason for this difference is not still clear. The two dominant factors, ascribed to the film separation from the substrate, are as follows: (I) the erbium grains, divided by grain boundaries or pores [16–18], coalesce and migrate by overcoming the energy barrier during the annealing process, while the grains of Mo substrate remain unchanged or change very little. The discrepancy in the degree of change in grain sizes leads to the separation of erbium film from the Mo substrate. Pang et al. [22] found that different layers of film are separated by clearly defined interfaces because of diffusion and crystal growth. Diffusion may take place in every layer and interface, but the energy barrier for interfacial diffusion is much higher than that in layers, so diffusion is much easier in layers and Table 1 Sample specifications. All of the samples were electron-beam deposited on rolled molybdenum (rolled-Mo) substrate. The approximate thicknesses of erbium films were equal to the mass, measured by electronic balance, divided by the product of the density of erbium (9.05 g/cm3) and the surface area of film.
Fig. 1. Schematic for observing the cross-section morphology of erbium film. A gap in the middle of the direction along the diameter was mechanically cut after the molybdenum substrate was rolled into a wafer, followed by the deposition of erbium film. The cross-section morphology of film can be easily observed by SEM, after the substrate is fractured in two pieces, in the fringe area of the gap.
Substrate Substrate temperature (°C)
Deposition rate (nm/s)
Thickness of film (μm)
Designation
RolledMo
~ 0.5 ~ 10 ~ 0.5
~ 1.73 ~ 5.88 ~ 1.37
Er200-0.5 Er200-10 Er450-0.5
200 200 450
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Fig. 2. The surface and cross-section (in the inset) morphologies of as-received ((a), (c), (e)) and post-annealed ((b), (d), (f)) erbium films, which are designated as Er200-0.5 ((a), (b)), Er200-10 ((c), (d)) and Er450-0.5 ((e), (f)).
clearer interfaces are seen while annealing at a higher temperature; (II) the coefficient of thermal expansion (CTE) is 4.8 × 10 − 6/K for pure molybdenum, which is significantly smaller than that of 12.2 × 10 − 6/K for pure erbium [24]. The distinction, in the shrinkage of Mo and Er when the annealing process was stopped and the temperature was cooled to room temperature naturally, also led to the separation between the film and substrate [25]. Adams et al. [25] found that a tensile stress develops due to mismatched CTE of the substrate–film couple when samples are cooled to ambient temperature after the scandium reaction with deuterium. The residual stress of the ScD2 film and the propensity for films to crack during cool down depend on the CTE misfit between the substrate and film materials. There is no visible change in the morphology of Er450-0.5, before and after the annealing (as shown in Fig. 2(e) and (f)), indicating that the film deposited at 450 °C is completely crystallized and has less residual stress inside the film. Fig. 3 shows the XRD patterns with 2θ ranging from 28° to 41° of as-received and post-annealed erbium films, Er200-0.5, Er200-10, and Er450-0.5. The relative intensities of diffraction lines of (100), (002), and (101) are listed in Table 2. To prove the preferred orientation of erbium films, the calculated XRD data (PDF card 01-089-3033) of the relative intensity of diffraction peaks was also shown in Table 2. The films deposited at 200 °C had (002) preferred orientation, and the film deposited at 450 °C had mixed (100) and (101) texture. The
regulation of preferred orientation of erbium film changing with substrate temperature is opposite to the result obtained by Parish et al. [18]. The (002) texture of erbium film deposited at 200 °C should be conducted by the theory of surface energy minimization, which
Fig. 3. The XRD patterns of as-received and post-annealed erbium films, which are designated as Er200-0.5, Er200-10 and Er450-0.5.
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showed that the basal plane (001) has the minimum surface energy for a hexagonal-close-packed (hcp) metal [26]. So, from the surface energy minimization, the (001) texture should be favored in the hcp films, which is consistent with the experimental results [27]. The mixed (100) and (101) texture, however, may be dominated by the diffusion of adatoms and the recrystallization of the film grains [28]. The annealing process had no effect on the texture of films (Fig. 3 and Table 2). The phase structure of erbium oxide (Er2O3) was revealed in Fig. 3 for post-annealed Er200-0.5 and Er200-10 specimens, whereas it was not evident that the Er2O3 phase was formed after Er450-0.5 film was annealed. It seems reasonable that the film, with small grain size and maximum surface energy, is susceptible to be oxidized [29]. The full width at half maximum (FWHM) of (100), (002), and (101) diffraction lines of three as-received erbium films was sharply decreased with increasing substrate temperature, as seen from Fig. 3 and Table 3. The annealing process has a direct impact on the FWHM of Er200-0.5 and Er200-10. The increase in FWHM originated from the increase in grain size and the relaxation of stress. Although it is difficult to quantify the dominant factor here, the increase of grain size and the relaxation of stress with high temperature annealing were reported by many researchers [21–23]. Based on all the XRD patterns plotted in Fig. 3, the diffraction line of the Mo substrate was always unchanged whenever the films were annealed. Furthermore, after the films were annealed, the peak positions of (100), (002), and (101) diffraction lines of erbium films shift towards higher angles [25], more drastically for Er200-0.5 and Er200-10 and insignificantly for Er450-0.5. The lattice constants calculated from the XRD data were tabulated in the right two columnar of Table 3. The lattice constants of a and c for Er450-0.5 changed a little, whereas a and c were markedly changed for Er200-0.5 and Er200-10, before and after the annealing treatment. All the above results, including the changes of FWHM and the lattice constants, are attributed to the evolution of compressive and tensile stresses inside the films during the deposition and annealing process. Compressive stress in the erbium films was enhanced due to the energetic particle bombardment during deposition [30], while the tensile stress was led by the difference in CTE of erbium and molybdenum materials when the as-received film was cooled down to room temperature. After the deposition, both the compressive stress and tensile stress were relaxed by the heat annealing. A compressive stress in thick films is not surprising considering that electron-beam vapor deposition is used for growth. Recently, research by Chason et al. [31] identifies grain boundaries as an important factor in generating a compressively-stressed, continuous film. The evolution of stress in ScD2/Cr thin films fabricated by evaporation and high temperature reaction was reviewed in detail [25]. Reaction of scandium metal with deuterium at elevated temperatures to form a stoichiometric dideuteride phase leads to a large compressive in-plane film stress, due to an increased atomic density compared with Table 2 The relative intensity of three diffraction lines, (100), (002) and (101), of as-received and post-annealed (annealing at 600 °C for 30 min) erbium films. The normalized relative intensity of (100), (002) and (101) diffraction lines of PDF card (01-089-3033) was also shown in this table, so as to demonstrate the preferential orientation of the samples. Sample list
Relative intensity of diffraction lines (%)
PDF 01-089-3033 As-received
Post-annealed
Er200-0.5 Er200-10 Er450-0.5 Er200-0.5 Er200-10 Er450-0.5
(100)
(002)
(101)
24.9
24.8
100
12.3 11.7 21.8 9.6 5.8 19.8
100 100 8.3 100 100 5.9
64.5 33.2 100 53.0 15.6 100
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Table 3 The FWHM of three diffraction lines, (100), (002) and (101), of as-received and postannealed (annealing at 600 °C for 30 min) erbium films. The lattice constants of the above samples were also shown in the right column. Sample list
As-received
Post-annealed
Er200-0.5 Er200-10 Er450-0.5 Er200-0.5 Er200-10 Er450-0.5
FWHM of erbium diffraction lines (degree)
Lattice constants (nm)
(100)
(002)
(101)
a
c
0.363 0.306 0.125 0.157 0.185 0.099
0.364 0.360 0.184 0.184 0.139 0.169
0.361 0.368 0.136 0.185 0.136 0.116
0.3598 0.3574 0.3560 0.3566 0.3565 0.3557
0.5662 0.5618 0.5600 0.5637 0.5600 0.5594
the as-deposited metal film. A tensile stress develops due to mismatched CTE of the substrate-film couple [25]. Yuan et al. [23] demonstrated that the lattice constant decreases and is close to the bulk one, which indicates that the compressive strain in the In2O3 films are gradually relaxed as the annealing temperature increases. Zhou et al. [21] found that with increasing annealing temperature, the compressive stress decreased due to the elimination of defects and the grain growth caused the lateral shrinkage of film and produced a relative tensile stress. It is instructive to note that, after the films were annealed, the larger shift in the peak position for Er200-0.5 was observed, indicating that the film growth at a low temperature (200 °C) maintains a larger compressive stress (Fig. 2). This is in accordance with the observation of SEM images where significantly more cracks were detected for Er200-0.5 after the film was annealed. 4. Conclusions In summary, the columnar grain sizes of as-received erbium films increased with the substrate temperatures and were enlarged by the coalescence and migration of grains during the high temperature annealing. The intrinsic stresses of erbium films, fabricated at a low substrate temperature (200 °C), were relaxed accompanied with the appearance of cracks on the films surface. The film, with small grain size and maximum surface energy, is susceptible to be oxidized. The films deposited at 200 °C had (002) preferred orientation, and the film deposited at 450 °C had mixed (100) and (101) texture. The peak positions and the FWHM of (100), (002), and (101) diffraction lines of erbium shift towards higher angles and sharply decrease with annealing process, indicating that the stress inside the film was relaxed. Acknowledgment L. Yang and X. T. Zu are grateful for the support by the National Natural Science Foundation of China — NSAF (Grant No: 10976007) and the Fundamental Research Funds for the Central Universities (Grant No: ZYGX2009J040). S.M. Peng and X.G. Long are grateful for the support by the Science and Technology Foundation of CAEP (Grant No: 2009A0301015) and the Major Program of the National Natural Science Foundation of China (Grant No: 91126001). References [1] L. Yang, S.M. Peng, X.G. Long, F. Gao, H.L. Heinisch, R.J. Kurtz, X.T. Zu, J. Appl. Phys. 107 (2010) 054903. [2] M.T. Brumbach, J.A. Ohlhausen, K.R. Zavadil, C.S. Snow, J.C. Woicik, J. Appl. Phys. 109 (2011) 114911. [3] J.A. Knapp, J.F. Browning, G.M. Bond, Nucl. Instrum. Methods B 268 (2010) 2141. [4] E.D. Gu, H. Savaloni, M.A. Player, G.V. Marr, J. Phys. Chem. Solids 53 (1992) 127. [5] G.M. Bond, J.F. Browning, C.S. Snow, J. Appl. Phys. 107 (2010) 083514. [6] R.M. Ferrizz, Erbium Hydride Decomposition Kinetics, Sandia Report: SAND2006-7014, 2006. [7] J.A. Knapp, J.F. Browning, G.M. Bond, J. Appl. Phys. 105 (2009) 053501. [8] P.A. Dow, G.W. Briers, M.A.P. Dewey, D.S. Stark, Nucl. Instrum. Methods 60 (1968) 293.
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