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OPTICS LETTERS / Vol. 39, No. 11 / June 1, 2014

Nanomechanical and optical properties of yttrium thin films by magnetron sputtering R. Ramaseshan,1,* S. Tripura Sundari,1 A. K. Balamurugan,1 Sitaram Dash,1 A. K. Tyagi,1 Y. Sato,2 T. Nakayama,2 and H. Suematsu2 1

2

Thin Film and Coatings Section, Surface and Nanoscience Division, Materials Science Group, Indira Gandhi Centre for Atomic Research, Kalpakkam 603102, India

Extreme Energy Density Research Institute, Nagaoka University of Technology, Nagaoka, Japan *Corresponding author: [email protected] Received December 12, 2013; revised February 28, 2014; accepted March 31, 2014; posted April 21, 2014 (Doc. ID 202688); published May 16, 2014

This Letter reports on nanomechanical and optical properties of yttrium thin films deposited on an Si (100) wafer. Elemental depth profiling by a secondary ion mass spectrometer revealed absence of formation of yttrium hydride, both on the surface and beneath. The optical properties were investigated by spectroscopic ellipsometry, and the refractive indices extracted after suitable modeling were found to be 2.51 at 546 nm. Hardness and elastic modulus of these films were found to be 7 and 142 GPa, respectively. These studies indicate that yttrium thin films are suitable for x-ray mirrors, photocathode emitters in e-beam lithography, electron microscopes, and free-electron lasers. © 2014 Optical Society of America OCIS codes: (120.2130) Ellipsometry and polarimetry; (120.4530) Optical constants; (310.6860) Thin films, optical properties; (310.6870) Thin films, other properties. http://dx.doi.org/10.1364/OL.39.003086

Photocathodes for electron-beam applications require high-brilliance electron injectors [1]. Three major parameters defining the efficiency of the photocathode performance are work function of the material, electron escape length, and attenuation length. However, the photocathode geometry and the materials can be optimized to reduce the length that the electrons need to travel to escape (e.g., illuminate at grazing angle) and attenuation length, respectively. Therefore, low work function photocathodes possessing high melting points are preferred in bulk or thin film form, as they are expected to emit more electrons with the best figure of merit. Recently, low work function photocathode emitters based on polymers have emerged [2]. However, their long-term stability, mechanical integrity, and device lifetime are still under investigation. Therefore, metallic photocathodes are still being widely preferred, as they are more robust and easier to handle. In this context, yttrium is a good candidate owing to its low work function [3,4]. For photocathode applications, films of yttrium have been grown by different techniques viz. pulsed laser deposition, magnetron sputtering, ion-beam sputtering, and e-beam evaporation, etc. [5–9]. In the past, optical properties of multilayer of Mo/Y have been investigated in the energy range 113–155 eV owing to its high and relatively stable reflectance [7]. The ellipsometric characterization of Y2 O3 ∕Y multilayered thin films on hydrogenation, synthesized by ion-beam sputtering, have been investigated by Santjojo et al. [8]. They also have been investigated by ellipsometry of palladium-capped Y films deposited on quartz and glass substrates. Optical absorption studies on single-crystal hcp Y have been reported by Weaver and Olson using reflectivity measurements [10]. Optical properties of Y have been studied by ellipsometry in the context of LaYH films, wherein the crystal structure as examined by x-ray diffraction (XRD) is found to be different, and a contextual discussion is presented in this Letter [9]. However, there is no study on the optical and mechanical 0146-9592/14/113086-04$15.00/0

properties of films, except for a maiden reference pertaining to single crystal [11]. As far as mechanical properties are concerned, compounds of yttrium rather than yttrium as such have been used for switchable mirrors and investigated [12–14]. In this Letter, we report optical and mechanical properties of yttrium thin films grown by DC sputtering. Deposition of yttrium thin films of thickness around 340 nm were grown on Si (100) by DC magnetron sputtering equipment (MECA—2000, France). The power used was 200 W. A 4 N pure yttrium target with a diameter of 50 mm and Ar gas (20 sccm) were used. The base pressure of the chamber was less than 9.6 × 10−6 mbar, while the sputtering pressure was maintained at 4.1 × 10−3 mbar. The substrates were kept at room temperature (25°C) with a continuous rotation for uniform coating. The target to substrate distance (TSD) was varied between 10.5 and 12.25 cm to understand the growth of the thin films and change in the film properties. To study the crystalline quality, the grazing incident x-ray diffraction (GIXRD) (Bruker D8, Germany) technique was used. In order to examine yttrium hydride formation, a highly sensitive and high-resolution SIMS (IMS 7f Cameca, France) was employed. A 20 nA primary ion beam of 10 keV Cs was rastered over a square area of 250 μm width, and the sputtered secondary ions complexes of CsY, CsCH, and CsO were monitored from the central region of 62 μm diameter. The specimen was kept at a potential of 5 kV, so that the resulting primary ions reach the specimen with an effective energy of 5 keV at an angle of incidence of 45°. The SIMS crater depth was measured using a DEKTAK 6M (Veeco, USA) profilometer. A rotating polarizer-type spectroscopic ellipsometer (SOPRA, ESVG, France) was used to measure the refractive index and extinction coefficient in the energy range 1.4–5 eV at an incident angle of 75°. The indentation experiments were preformed using Nano-Indenter G200 (Agilent, USA). A three-sided pyramidal diamond Berkovich tip with an end radius of 100 nm was used in this © 2014 Optical Society of America

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study. In the present case, a peak load P max of 0.06 mN was applied to the indenter. The hardness H and elastic modulus E values reported in this manuscript were evaluated by averaging the number of indentations of the same depth (12 nm). The Oliver–Pharr method was used to extract H and E values [15]. It is a practice to cap Y films by Pd in order to avoid any surface oxide or hydride formation. However, in the present experiment, we have not capped the films with Pd. In Fig. 1, the GIXRD patterns of yttrium thin films deposited at different TSDs are shown, which are in good agreement with JCPDS data 33–1458, with lattice parameters a  0.36471 nm and c  0.57285 nm. While the most intense peak in the JCPDS data card pertains to yttrium 101, the films deposited in the present experiment show orientation toward (002). Intensity of the 002 peak diminishes with the increase of TSD. The decrease in intensity of the 002 peaks with the increase of TSD is considered to be associated closely with the kinetic energy of the sputtered atoms or particles when arriving at the substrate. A decrease in TSD reduces the collision probability among the sputtered species, thereby enabling them to reach the substrate with higher energy. This energetically favors the sputtered species to contribute toward larger grain size. In GIXRD, no crystallized oxide layers were observed, as detection of such thin native oxide is below the detection limit of GIXRD. Although the interest in the present Letter is on Y films and not on Y-H, we have monitored the presence of hydrogen in the film by SIMS depth profiling, as this technique is ideally suitable (sensitivity: right from hydrogen) for detection of hydrogen with high resolution. In the present experiment, complexes of Cs were monitored during depth profiling, as it offered better scope for quantitative evaluation of the secondary ion complexes [16]. Figure 2 shows the normalized intensity of Y, O, and CH complexes of Cs plotted against calibrated depth as obtained from SIMS depth profiling of Y film at a TSD of 10.5 cm. The SIMS depth profiling clearly shows the presence of Y (with no other impurity), negligible amount of O, CH complexes of Cs, and the absence of formation of YH, which are contrary to the report by Wildes et al. [17]. The oxygen profile seen on the film surface is due to the surface oxidation, which is less intense

Fig. 1. GIXRD patterns of yttrium thin films deposited at different TSD.

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Fig. 2. SIMS depth profile of yttrium thin film on glass substrate (film thickness ≈340 nm).

than that on the substrate. The contribution to the width of the oxygen profile is from three sources, namely, the inherent broadening due to ion-beam mixing, roughness of the sputtered surface, and the thickness of the intrinsic oxide layer [18]. The ion-beam mixing, as simulated by SRIM [19] code for the above SIMS primary ion-beam conditions, is given in Fig. 3, where the values of the Y recoil profile is shown on the right-hand side. In SRIM, 3 Å oxygen layer (one monolayer) was defined over Y to get the recoil distribution of Y and O. The FWHM of this profile is approximately 6 nm and is taken as the typical depth resolution of SIMS profile (ignoring the sputter induced roughness). Considering the surface roughness as 6 nm (by surface profiler), the oxide layer is estimated to be 9 nm, since the profiles match a Gaussian shape. Figure 4 shows the experimentally measured ellipsometric parameters (tan ψ and cos Δ) [20] of as deposited yttrium film. For the purpose of quantitative analysis, the experimentally measured ellipsometric data was fitted using a four-layer (c-Si/Y film/surface roughness/ambient) model. It is to be noted that the yttrium film was modeled based on Drude–Lorentz theory, as has been adopted by van Gogh et al. [9]. The surface roughness was modeled with a mixture of Y2 O3 and voids, as it is well known that yttrium has a strong tendency to oxidize with heat of formation of Y2 O3 being 1.9 MJ∕mole [21].

Fig. 3. SRIM simulation of Y and O mixing by 5 keV Cs ions in SIMS.

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Fig. 4.

OPTICS LETTERS / Vol. 39, No. 11 / June 1, 2014

Ellipsometric parameters along with the fit results.

Fig. 6. Typical indentation profile of yttrium film with TSD 10.5 cm.

The reference for the refractive indices (n, k) of Y2 O3 and c-Si was taken from a database provided in Winelli 4.08 software supplied by SOPRA. The parameters of the Drude–Lorentz model were varied until the mean-square variation between the experimental and fitted data (χ 2 ) was less than 10−3 . A nonlinear least-squares fit employing Levenberg–Marqaudt approximation was used for fitting (Fig. 4). Unlike van Gogh’s analysis, which employed three Lorentz oscillators to fit the experimental data, we could fit our data using a single Lorentz oscillator [21]. The refractive indices (n, k) of the yttrium film (subtracting the contribution from Y2 O3 ) extracted from the above four-layer analysis is shown in Fig. 5. The n for Y2 O3 is also shown for comparison. Since Y2 O3 is an insulator, its k value is zero, so it is not shown in Fig. 5. As far as the Drude–Lorentz parameters for the extracted Y film is concerned, a large interband Lorentz transition was observed with transition amplitude of 8.85 and centered at 1.99 eV with a broadening of 607 meV, while the plasma frequency was estimated to be 8.556 eV. The interband absorption due to the Lorentz oscillator is comparable with that obtained by Santjojo and von Gogh, except for small differences, while the plasma frequency is particularly large in our case [8,9]. The difference in the values of the parameters and plasma frequency is attributed to two reasons. One, a perusal of the XRD data in [6,9] reveal substantial differences

in the XRD pattern as compared with XRD in the present experiment. As is well known, the optical properties are a manifestation of the interaction of electromagnetic radiation with the underlying microstructure, which, in turn, is intricately dependent on the deposition parameters and method. This is also the reason why we could fit the ellipsometric data with one Lorentz oscillator rather than three as stated in the previous paragraph. Two, a comparison of the refractive indices with literature [8,9] reveal that films in the present experiment exhibit higher refractive index and therefore higher density. The higher density in turn implies higher electron density, thereby leading to a higher plasma frequency. As yttrium films could be employed in device fabrication as electron emitters owing to their low work function (3.0 eV) [3] and high melting point (1796 K), it is worthwhile to examine their mechanical properties. Hardness and elastic modulus are two important mechanical properties, which are highly essential from a device point of view. In this context, a typical indentation profile showing a continuous loading curve is shown in Fig. 6. This is indirect evidence of a continuous integrated structure depth-wise. The absence of any displacement bursts during the nano-indentation indicates the absence of native oxide. Generally, the presence of such bursts are due to brittle fracture under the indenter, which triggers a sudden sink-in of the indenter into

Fig. 5. n and k values of yttrium and yttria [22] thin film for the energy range 1.4–5 eV.

Fig. 7. Nanomechanical properties of yttrium thin films deposited at different TSD.

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the film. This phenomenon is usually caused by a sudden dislocation density difference between the two dissimilar entities [23]. The variation of H and E with different TSDs was measured using nanoindentation as is shown in Fig. 7. The average elastic modulus values are around 142 GPa, while a small increase in the hardness (7 GPa) has been observed with the TSD. This is attributed to the structural transition of the grains from a predominantly crystalline to less crystalline with more defects, with the increase in TSD [24]. With increase in TSD, the longrange periodicity decreases, while the number of grain boundary increases, which impedes the dislocation movement considerably, thereby leading to increase in resistance to plastic deformation. The effect of strengthening due to the decrease in crystallite size is well explained by the Hall–Petch relationship [25,26]. In summary, we have studied the effect of TSD on the crystallinity of Y films and shortest TSD (10.5 cm), which is taken further for optical and mechanical properties. A SIMS depth profile clearly shows the absence of YH and other impurities. The refractive index, as inferred from ellipsometric measurements at 546 nm, is 2.51. The films are found to have a good elastic modulus 142 GPa and hardness of 7 GPa. These characteristics make them ideal candidates for application as electron emitters. The authors thank Dr. R. Rajaraman for his support and encouragement. References 1. W. A. Clay, J. R. Maldonado, P. Pianetta, J. E. P. Dahl, R. M. K. Carlson, P. R. Schreiner, A. A. Fokin, B. A. Tkachenko, N. A. Melosh, and Z.-X. Shen, Appl. Phys. Lett. 101, 241605 (2012). 2. M. G. Helander, Science 336, 302 (2012). 3. A. Lorusso, F. Gontad, A. Perrone, and N. Stankova, Phys. Rev. Spec. Top. Accel. Beams 14, 090401 (2011). 4. G. Caretto, P. Miglietta, V. Nassisi, A. Perrone, and M. V. Siciliano, Radiat. Eff. Defects Solids 163, 365 (2008).

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5. L. Cultrera, G. Gatti, and A. Lorusso, Radiat. Eff. Defects Solids 165, 609 (2010). 6. A. Lorusso, V. Fasano, and A. Perrone, J. Vac. Sci. Technol. A 29, 031502 (2011). 7. B. Sae-Lao and R. Soufli, Appl. Opt. 41, 7309 (2002). 8. D. J. Santjojo, T. Aizawa, and S. Muraishi, Mater. Trans. 48, 1380 (2007). 9. A. T. M. van Gogh, D. G. Nagengast, E. S. Kooij, N. J. Koeman, J. H. Rector, R. Griessen, C. F. J. Flipse, and R. J. J. G. A. M. Smeets, Phys. Rev. B 63, 195105 (2001). 10. J. H. Weaver and C. G. Olson, Phys. Rev. B 15, 590 (1977). 11. I. I. Papirov, V. V. Vorobev, A. I. Pikalov, and N. I. Moreva, Phys. Stat. Sol. A 65, 141 (1981). 12. J. N. Huiberts, R. Griessen, J. H. Rector, R. J. Wijngaarden, J. P. Dekker, D. G. de Groot, and N. J. Koeman, Nature 380, 231 (1996). 13. D. Setoyama, M. Ito, J. Matsunaga, H. Mura, M. Uno, and S. Yamanaka, J. Alloys Compd. 394, 207 (2005). 14. M. Dornheim, A. Pundt, R. Kirchheim, A. J. v. d. Molen, E. S. Kooij, J. Kerssemakers, and R. Griessen, J. Appl. Phys. 93, 8958 (2003). 15. W. C. Oliver and G. M. Pharr, J. Mater. Res. 19, 3 (2004). 16. A. Wucher, Appl. Surf. Sci. 252, 6482 (2006). 17. A. R. Wildes, R. C. C. Ward, M. R. Wells, and B. Hjorvasson, J. Alloys Compd. 242, 49 (1996). 18. S. Jung, H. Choi, Y. Ju, M. Chang, M. Jo, J. Lee, J. Yoon, C. Lee, and H. Ywang, Appl. Phys. Lett. 95, 242112 (2009). 19. J. F. Ziegler, M. D. Ziegler, and J. P. Biersack, Nucl. Instrum. Methods Phys. Res., Sect. B 268, 1818 (2010). 20. F. Jose, R. Ramaseshan, S. Dash, S. Tripura Sundari, D. Jain, V. Ganesan, P. Chandramohan, M. P. Srinivasan, A. K. Tyagi, and B. Raj, Mater. Chem. Phys. 130, 1033 (2011). 21. S. J. van der Molen, J. W. J. Kerssemakers, J. H. Rector, N. J. Koeman, B. Dam, and R. Griessen, J. Appl. Phys. 86, 6107 (1999). 22. D. F. Bezuidenhout and R. Pretorius, Thin Solid Films 139, 121 (1986). 23. H. Bei, E. P. George, J. L. Hay, and G. M. Pharr, Phys. Rev. Lett. 95, 045501 (2005). 24. J. A. Thornton, J. Vac. Sci. Technol. 11, 666 (1974). 25. E. O. Hall, Proc. Phys. Soc. B 64, 747 (1951). 26. N. J. Petch, J. Iron Steel Inst. 174, 25 (1953).

Nanomechanical and optical properties of yttrium thin films by magnetron sputtering.

This Letter reports on nanomechanical and optical properties of yttrium thin films deposited on an Si (100) wafer. Elemental depth profiling by a seco...
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