Dehydrogenation process of Mg–Ni based switchable mirrors analyzed by in situ spectroscopic ellipsometry

Solar Energy Materials & Solar Cells 99 (2012) 84–87

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Solar Energy Materials & Solar Cells journal homepage: www.elsevier.com/locate/solmat

Dehydrogenation process of Mg–Ni based switchable mirrors analyzed by in situ spectroscopic ellipsometry Y. Yamada a,n, K. Tajima a, M. Okada a, M. Tazawa a, A. Roos b, K. Yoshimura a a b

National Institute of Advanced Industrial Science and Technology (AIST), Anagahora 2266-98, Shimoshidami, Moriyama-ku, Nagoya 463-8560, Japan ˚ Department of Engineering Sciences, The Angstr¨ om Laboratory, Uppsala University, 751 21 Uppsala, Sweden

a r t i c l e i n f o

a b s t r a c t

Available online 22 April 2011

The dehydrogenation process of hydrogenated switchable mirrors using magnesium–nickel alloy thin film was studied in situ using spectroscopic ellipsometry. Ellipsometric angles C and D of the switchable mirrors varied drastically as a result of dehydrogenation, which is a transformation from transparent to reflective states. The process was analyzed by dividing into the following three phases. The first phase was the dehydrogenation process of a thin Mg4Ni layer with several nanometers at a hydrogenated Pd/Mg4Ni interface. The second phase was the dehydrogenation processes of the hydrogenated Mg4Ni layer, which proceeded from the Pd/Mg4Ni interface to the substrate. The final phase was the desorption process of hydrogen, which was absorbed in Mg4Ni as solid solution and the dehydrogenation process was terminated. & 2011 Elsevier B.V. All rights reserved.

Keywords: Switchable mirrors Mg–Ni alloy in situ ellipsometry Dehydrogenation processes Black state

1. Introduction Switchable mirrors based on magnesium–nickel alloys exhibits a curious optical state, the so-called ‘black state’, during hydrogenation and dehydrogenation in addition to the two normal ‘reflective’ and ‘transparent’ states [1,2]. This black state was characterized by the low reflectance from the substrate side over the entire visible range while the transmittance was also very low. It was found from optical and 15N-hydrogen depth profile measurements that the black state observed during hydrogenation process originates from the nucleation of Mg alloy hydride at the interface between substrate and film of the hydrogen solid solution [3–6]. However, the complete switching process, especially dehydrogenation, of the switchable mirrors including a Pd layer remains unclear. In this study, we investigate in detail the dehydrogenation process of the complete Mg–Ni alloy switchable mirrors including a Pd layer from their transparent state by focusing on the drastic variation in the optical constants of the alloy as a result of the dehydrogenation. In our previous study about hydrogenation process, the ellipsometric angles C and D of the switchable mirrors varied continuously with proceeding hydrogenation. Therefore we have also analyzed the dehydrogenation process of hydrogenated switchable mirrors using Mg–Ni alloy thin films by measuring the variation in ellipsometric angles C and D during dehydrogenation in situ, in real time. From the viewpoint of

windows application of switchable mirrors, we have decided on the composition Mg4Ni for the present experiment. 2. Experimental Mg–Ni alloy thin films were deposited on quartz substrates by co-sputtering of Mg (99.99%) and Ni (99.99%) targets. The alloy films were capped subsequently with an ultra-thin Pd film, sputtered with a Pd (99.99%) target without breaking the vacuum. The Pd film works as a catalyst to take in hydrogen gas as well as protection of the alloy layer from oxidation. The base pressure of the deposition chamber was 5  10  5 Pa and the working pressure during deposition was kept at  1.0 Pa using a mass-flow controller of Ar. From our previous research [7], we have decided the nominal thicknesses of Mg–Ni and Pd layers to be about 35 and 7.5 nm, respectively. The composition of the alloys was controlled to be Mg4Ni by adjusting the sputtering power ratio of Mg–Ni targets. The details of the deposition conditions were described elsewhere [7]. Ellipsometric angles C and D were measured in the photon energy (E) range of 0.7–3.3 eV using light with an incidence angles of 501, 601, and 701 to the films with a J.A. Woollam M2000 ellipsometer at room temperature. The angles C(E) and D(E) are defined as the following equations, rp ¼ tan½CðEÞexp½iDðEÞ, rs

n

Corresponding author. E-mail address: [email protected] (Y. Yamada).

0927-0248/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2011.03.035

ð1Þ

where rp and rs are the complex Fresnel reflection coefficients r of parallel (p) and perpendicular (s) polarized light, respectively

Y. Yamada et al. / Solar Energy Materials & Solar Cells 99 (2012) 84–87

[8,9]. A sample was set on a stage covered by a quartz window with semicylindrical shape [7–9], which make it possible to control the atmosphere and to measure ellipsometric angles with multiple angles of incidence. First, we have hydrogenated switchable mirrors using 4% hydrogen in Ar and then dehydrogenation processes of hydrogenated switchable mirrors were studied by

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analyzing the variation in ellipsometric angles C and D in flowing dry air adjusted by a mass-flow controller.

3. Results and discussion 3.1. Analytical models for the dehydrogenation process To examine the dehydrogenation process of the hydrogenated switchable mirrors, spectra of the ellipsometric angles C and D in the photon energy range of 0.7–3.3 eV were measured for every 2.75 sec using dry air with a flow rate of 15 sccm for the change from the hydrogenated state to the metallic state. The spectra varied slowly with elapsed time t until 12.5 min, then drastically to 19.5 min, slowly again to 45.0 min, and did not vary after 45 min as shown in Fig. 1. Thus, to analyze the acquired ellipsometric data, we divided the process into the following three phases; (a) Phase I: 0.0rtr12.5, (b) Phase II: 12.5otr19.5, (c) Phase III: 19.5otr50.0. In our previous study of the hydrogenation process, we proposed the formation of following five materials: metal, hydrogen-solid-solution, and hydride of Mg–Ni alloy, and metal and hydride of Pd, which are denoted by m-Mg4Ni, s-Mg4Ni, h-Mg4Ni, m-Pd, and h-Pd, respectively. We have then analyzed the processes using the optical constants of these materials by fitting the parameters in the structural models [7]. In this study, the C and D spectra at fully dehydrogenated state (t ¼50.0 min) corresponded exactly with those of as-deposited initial metallic state switchable mirror before hydrogenation. Therefore we also used these dielectric constants evaluated in the previous study to analyze the dehydrogenation process. From simple analysis, we built the following structural models to analyze the dehydrogenation process. Phase I is the dehydrogenation process of a thin h-Mg4Ni layer with several nanometers at the h-Pd/h-Mg4Ni interface. To represent this process, we supposed a mixture layer of m-Mg4Ni, s-Mg4Ni, and h-Mg4Ni using the Bruggeman EMA at h-Pd/h-Mg4Ni interface. In this phase, the dehydrogenation of h-Pd layer also proceeded. Thus we built a structural model as shown in Fig. 2(a). Phase II is the dehydrogenation processes of h-Mg4Ni, which proceeded from the h-Pd/h-Mg4Ni interface to the substrate, and built a structural model as shown in Fig. 2(b). Phase III is the desorption process of hydrogen, which was absorbed in s-Mg4Ni, in the mixture of s-Mg4Ni and m-Mg4Ni layer. Thus we built a structural model as shown in Fig. 2(c). Although a slight compression in film thickness of the Pd layer is anticipated as a result of dehydrogenation [8,9], we fixed the thickness to be 7.5 nm in all phases to reduce the number of fit parameters in this analysis.

Fig. 1. Experimental variations in spectra of ellipsometric angles C (a) and D (b) during dehydrogenation in the photon energy range of 0.7–3.3 eV as a function of elapsed time. These spectra were measured for every 2.75 sec using dry air with a flow rate of 15 sccm.

Phase I

m-Pd + h-Pd m-Mg4 Ni + s-Mg4 Ni + h-Mg4 Ni h-Mg4 Ni Quartz substrate

3.2. Identification of the dehydrogenation process The variations in C and D spectra due to dehydrogenation as shown in Fig. 1(a) and (b) were analyzed by fitting parameters in

Phase III

Phase II

7.5 nm t2 t1

m-Pd + h-Pd m-Mg4 Ni + s-Mg4 Ni s-Mg4 Ni + h-Mg4 Ni h-Mg4 Ni Quartz substrate

7.5 nm t5 t4 t3

m-Pd m-Mg4 Ni + s-Mg4 Ni

7.5 nm t6

Quartz substrate

Fig. 2. Structural models to analyze the dehydrogenation process of the complete switchable mirror using Mg4Ni alloy. The model in Phase I (a) is composed of two layers of the Mg–Ni alloy layer, which are uniform h-Mg4Ni, m-Mg4Ni, s-Mg4Ni and h-Mg4Ni mixture layers stacked from the substrate side, capped with a mixture layer of m-Pd and h-Pd using Bruggeman EMA. The model in Phase II (b) is composed of three layers of the Mg–Ni alloy layer, which are uniform h-Mg4Ni, a mixture of s-Mg4Ni and h-Mg4Ni, and a mixture of m-Mg4Ni and s-Mg4Ni layers stacked from the substrate side, capped with a mixture layer of m-Pd and h-Pd using Bruggeman EMA. The model in Phase III (c) is composed of a single mixture layer of m-Mg4Ni and s-Mg4Ni capped with m-Pd.

Y. Yamada et al. / Solar Energy Materials & Solar Cells 99 (2012) 84–87

structural models as shown in Fig. 2 using the dielectric constants of the five materials evaluated in the previous study. Fig. 3(a) and (b) show the experimental variation in ellipsometric angles C and D as a function of elapsed time at the photon energy of 2.49, 2.01, 1.50, and 1.01 eV, respectively, together with their calculated fit curves. In these figures, the time scales of Phase II are expanded to improve visibility of the variation in C and D angles. We can see good agreement between the experimental data and calculated variation in C and D angles. From the above analysis, we identified the dehydrogenation processes of hydrogenated Mg4Ni and Pd layers as follows. In the initial stage of the Phase I, hydrogenated Pd layer (h-Pd) began to dehydrogenate before dehydrogenation of h-Mg4Ni. When the  10% of h-Pd dehydrogenated, thin h-Mg4Ni layer with several nanometers at h-Pd/h-Mg4Ni interface began to dehydrogenate and the fully dehydrogenated m-Mg4Ni layer was formed at the interface. As the dehydrogenation process moved to Phase II, the dehydrogenation of h-Mg4Ni proceeded from the h-Pd/h-Mg4Ni interface to the substrate and the Mg4Ni layer was composed of the following three layers from the interface side; mixture layer of m-Mg4Ni and s-Mg4Ni, mixture layer of s-Mg4Ni and h-Mg4Ni, and h-Mg4Ni. During this phase the concentrations of s-Mg4Ni in m-Mg4Ni and h-Mg4Ni were 0.1 and  0.7, respectively. The small amount of s-Mg4Ni in m-Mg4Ni may be formed because m-Mg4Ni absorbed hydrogen, which was generated by dehydrogenating h-Mg4Ni. On proceeding dehydrogenation the thickness of mixture layer of m-Mg4Ni and s-Mg4Ni increased sharply and the whole Mg–Ni alloy layer changed to the

Phase I

Phase II

mixture layer. In this Phase, the h-Pd layer was dehydrogenated completely before the full dehydrogenation of h-Mg4Ni. In Phase III, s-Mg4Ni, whose concentration was less than 5% in the mixture layer of m-Mg4Ni and s-Mg4Ni, desorbed hydrogen and the dehydrogenation process was terminated. The thickness of the Mg–Ni alloy layer was constant in Phase I. As the process moved to Phase II, the thickness decreased with proceeding dehydrogenation and became constant in Phase III as shown in Fig. 4. The decrease at Phase II was due to the dehydrogenation of h-Mg4Ni because the lattice constant of the Mg4Ni is smaller than that of the hydride. Thus, the dehydrogenation process of the whole hydrogenated Mg4Ni switchable mirror including the Pd layer could be analyzed in detail by measuring the variation in ellipsometric C and D during dehydrogenation using in situ spectroscopic ellipsometry. Fig. 5 shows a phase diagram summarizing the analytical results of the dehydrogenation process.

Phase I

Phase II

Phase III

30

20

10

t1+t2 t1

40 35

t3+t4+t5 t3+t4 t3

0 0

5

10

30

Ψ (deg.)

Phase III

40

Thickness (nm)

86

14

16

t6

18 20

30

40

50

Elapsed Time (min.)

25

Fig. 4. Variation in film thickness (a) of Mg–Ni alloy layer during dehydrogenation as a function of elapsed time. Fit parameters are shown in Fig. 3. In these figures, the time scales of Phase II are expanded to improve visibility of the variation in thickness and concentrations.

Model fits 2.49 eV 2.01 eV 1.50 eV 1.01 eV

20 15

Phase I

Phase II

Phase III

Pd layer

10 160

m-Pd

Dehydrogenation of Mg 4Ni at near interface

Mg4 Ni layer

Δ (deg.)

150

h-Pd

140

130

h-Mg 4Ni + s-Mg 4Ni (~67%)

h-Mg 4Ni

m-Mg 4Ni + s-Mg 4Ni (< 5%)

120 0

5

10

14

16

18 20

30

40

50

Elapsed Time (min.) Fig. 3. Experimental variations in ellipsometric angles C (a) and D (b) during dehydrogenation at the photon energy of 2.49, 2.01, 1.50, and 1.01 eV as a function of elapsed time together with their fitted curves. In these figures, the time scales of Phase II are expanded to improve visibility of the variation in C and D.

0

5

10

14

16

18

20

30

40

50

Elapsed time (min.) Fig. 5. Schematic phase diagram summarizing the analytical results of the dehydrogenation process. The vertical axis for Pd layer indicates the concentrations of m-Pd in the mixture layer, while that for the Mg4Ni layer indicates the relative thickness of each layer.

Y. Yamada et al. / Solar Energy Materials & Solar Cells 99 (2012) 84–87

Phase I

Phase II

Phase III

T, R, Rb @ 2.75 eV (%)

100

80

60

40

T @ 450 nm R @ 450 nm Rb @ 450 nm

20

0 0

5

10

14

16

18

20

30

40

50

Elapsed time (min.) Fig. 6. The calculated variation in transmittance (T), reflectance from the film surface side (R), and reflectance from the substrate side (Rb) at a photon energy of 2.75 eV (l ¼450 nm) during dehydrogenation as a function of elapse time. These values are calculated using structural models and optical constants for each material evaluated using ellipsometry.

3.3. Variation in transmittance and reflectance during dehydrogenation We also evaluated the variation in transmittance (T), reflectance from the film surface side (R), and reflectance from the substrate side (Rb) of the switchable mirror during dehydrogenation using structural models and optical constants for each material evaluated using ellipsometry. Fig. 6 shows T, R, and Rb at a photon energy of 2.75 eV (l ¼450 nm) as a function of elapsed time. As the dehydrogenation process moved to Phase II, the Rb-values decreases drastically and takes a local minimal value an elapsed time of about 15 min, while T-value decreases and R-values increases gradually. At this time the switchable mirror shows low T and Rb-values, indicating ‘‘the black state’’. This state is attributed to the fact that the bilayer of h-Mg4Ni and mixture layer of s-Mg4Ni and h-Mg4Ni becomes good antireflective to m-Mg4Ni. 4. Summary and conclusions The dehydrogenation process of hydrogenated switchable mirrors using magnesium–nickel alloy thin film was analyzed by measuring the variation in ellipsometric angles C and D using in situ spectroscopic ellipsometry. The spectra of C and D varied slowly with elapsed time t until 12.5 min, then drastically up to 19.5 min, slowly again up to 45.0 min, and did not vary after 45.0 min. Thus, we divided the process in three phases to analyze the acquired ellipsometric data and identified each phase as

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follows. In the initial stage of the process, Pd layer (h-Pd) dehydrogenated. When the 10% of h-Pd dehydrogenated, thin h-Mg4Ni layer with several nanometers at h-Pd/h-Mg4Ni interface began to dehydrogenate and the fully dehydrogenated m-Mg4Ni layer was formed at the interface. As the dehydrogenation process moved to Phase II, the dehydrogenation of h-Mg4Ni proceeded from the h-Pd/h-Mg4Ni interface to the substrate and the Mg4Ni layer was composed of three layers of mixture layer of m-Mg4Ni and s-Mg4Ni, mixture layer of s-Mg4Ni and h-Mg4Ni, and h-Mg4Ni. On proceeding the dehydrogenation, the thickness of mixture layer of m-Mg4Ni and s-Mg4Ni increased sharply and the whole Mg–Ni alloy layer changed to the mixture layer. In this Phase, the h-Pd layer was dehydrogenated completely before the dehydrogenation of h-Mg4Ni was terminated. In Phase III, s-Mg4Ni, whose concentration was less than 5%, in the mixture layer of m-Mg4Ni and s-Mg4Ni desorbed hydrogen and the dehydrogenation process was terminated. Thus, we have evaluated the dehydrogenation processes of hydrogenated switchable mirrors ‘‘in situ’’, ‘‘in real time’’, ‘‘microscopically’’, and ‘‘nondestructively’’.

Acknowledgments This work was supported by the Grant for Industrial Technology Research Program in 2008 (Project ID: 08E51506d) of the New Energy and Industrial Technology Development Organization (NEDO) of Japan. References [1] J. Isidorsson, I.A.M.E. Giebels, R. Griessen, M. Di Vece, Tunable reflectance Mg–Ni–H films, Appl. Phys. Lett. 80 (2002) 2305–2307. [2] J.L.M. van Mechelen, B. Noheda, W. Lohstroh, R.J. Westerwaal, J.H. Rector, B. Dam, R. Griessen, Mg–Ni–H films as selective coatings: tunable reflectance by layered hydrogenation, Appl. Phys. Lett. 84 (2004) 3651–3653. [3] W. Lohstroh, R.J. Westerwaal, B. Noheda, S. Enache, I.A.M.E. Giebels, B. Dam, R. Griessen, Self-organized layered hydrogenation in black Mg2NiHx switchable mirrors, Phys. Rev. Lett. 93 (2004) 197404-1–197404-4. [4] W. Lohstroh, R.J. Westerwaal, J.L.M. van Mechelen, C. Chacon, E. Johansson, B. Dam, R. Griessen, Structural and optical properties of Mg2NiHx switchable mirrors upon hydrogen loading, Phys. Rev. B 70 (2004) 165411-1–165411-11. [5] R.J. Westerwaal, A. Borgschulte, W. Lohstroh, B. Dam, R. Griessen, Microstructural origin of the optical black state in Mg2NiHx thin films, J. Alloys Compd. 404-406 (2005) 481–484. [6] R.J. Westerwaal, A. Borgschulte, W. Lohstroh, B. Dam, B. Kooi, G. ten Brink, M.J.P. Hopstaken, P.H.L. Notten, The growth-induced microstructural origin of the optical black state of Mg2NiHx thin films, J. Alloys Compd. 416 (2006) 2–10. [7] Y. Yamada, S. Bao, K. Tajima, M. Okada, M. Tazawa, A. Roos, K. Yoshimura, In situ spectroscopic ellipsometry study of the hydrogenation process of switchable mirrors based on magnesium–nickel alloy thin films, J. Appl. Phys. 107 (2010) 43517-1–43517-8. [8] Y. Yamada, K. Tajima, S. Bao, M. Okada, A. Roos, K. Yoshimura, Real time characterization of hydrogenation mechanism of palladium thin films by in situ spectroscopic ellipsometry, J. Appl. Phys. 106 (2009) 13523-1–13523-5. [9] Y. Yamada, K. Tajima, S. Bao, M. Okada, K. Yoshimura, Hydrogenation and dehydrogenation processes of palladium thin films measured in situ by spectroscopic ellipsometry, Sol. Energy Mater. Sol. Cells 93 (2009) 2143–2147.