APPLIED PHYSICS LETTERS 92, 112902 共2008兲
Thermal stability of dysprosium scandate thin films C. Adelmann,a兲 S. Van Elshocht, A. Franquet, T. Conard, O. Richard, H. Bender, P. Lehnen,b兲 and S. De Gendtc兲 IMEC vzw., Kapeldreef 75, B-3001 Heverlee (Leuven), Belgium
共Received 19 December 2007; accepted 16 February 2008; published online 17 March 2008兲 The thermal stability of DyScO3 thin films in contact with SiO2 or HfO2 during annealing up to 1000 ° C has been studied. It is found that DyScO3 / SiO2 stacks react during annealing and a phase separation into polycrystalline Sc-rich 共and relatively Si-poor兲 DySc silicate on top of an amorphous Dy-rich DySc silicate is observed. In contrast, DyScO3 is found to be thermodynamically stable in contact with HfO2 and to recrystallize upon annealing. These results demonstrate that the previously reported high crystallization temperature of ⬎1000 ° C for DyScO3 is not an intrinsic material property but caused by silicate formation. © 2008 American Institute of Physics. 关DOI: 10.1063/1.2894573兴 Rare earth scandates, such as LaScO3, GdScO3, or DyScO3, have been recently considered as dielectrics in complementary metal-oxide-semiconductor devices.1 These materials appear promising because of their high dielectric permittivity 共 ⬎ 20兲,2–5 large band gap 共⬎5.5 eV兲,6,7 thermodynamic stability on Si,1,8,9 and, in particular, a reported high crystallization temperature of ⬎1000 ° C,2,3,5 which allows keeping thin films amorphous during hightemperature device processing. However, reports on the crystallization behavior of rare earth scandates are conflicting. While thin films of DyScO3 共Refs. 2, 3, and 10兲 and GdScO3 共Refs. 2, 4, and 5兲 grown on Si were reported to remain amorphous up to 1000 ° C, films grown on LaAlO3 substrates were observed to recrystallize at temperatures of around 750– 800 ° C.11 LaScO3 films directly grown on Si 共without interfacial SiO2兲 were also observed to recrystallize at around 800 ° C.9,12 In this letter, we discuss the crystallization and interface formation of DyScO3 thin films grown on SiO2 / Si and HfO2 and show that reactions between DyScO3 and SiO2 are the key to understanding the recrystallization behavior. DyScO3 films were grown on 300 mm Si 共100兲 wafers by atomic-vapor deposition 共AVD兲 in an Aixtron Tricent™ reactor using tris共6-ethyl-2,2-dimethyl-3,5-decanedionato兲 dysprosium 关Dy共EDMDD兲3兴, tris共6-ethyl-2,2-dimethyl-3,5decanedionato兲 scandium 关Sc共EDMDD兲3兴, and molecular O2. The susceptor temperature was 550 ° C. Details of the growth process can be found in Refs. 10 and 13. Prior to DyScO3 deposition, either SiO2 共by rapid thermal oxidation兲 or HfO2 共by AVD, Refs. 10 and 14兲 templates were prepared on the Si wafers. To study recrystallization and interfacial reactions, the layers were postdeposition annealed for 60 s at 1000 ° C in an inert N2 atmosphere and subsequently characterized by grazing-incidence x-ray diffraction 共GIXRD兲 共 = 1 ° 兲, x-ray reflectivity 共XRR兲, time-of-flight secondaryion mass spectroscopy 共ToF-SIMS兲, x-ray photoelectron spectroscopy 共XPS兲 with sputter profiling, and transmission electron microscopy 共TEM兲 including high-angle annular dark field 共HAADF兲 Z-contrast imaging. a兲
Electronic mail:
[email protected]. b兲 IMEC assignee from AIXTRON AG, D-52072 Aachen, Germany. c兲 Also at Department of Chemistry, Katholieke Universiteit Leuven, B-3001 Heverlee, Belgium.
Previous x-ray diffraction studies of DyScO3 thin films on SiO2 / Si substrates have shown a weakly crystalline to amorphous pattern, both as grown and after annealing at temperatures up to ⬎1000 ° C.2,3,10 This behavior is illustrated in Fig. 1共a兲, which shows a GIXRD pattern of a 12 nm thick DyScO3 film on 20 nm SiO2 / Si. Only a weak and broad peak at around 30° is visible, which can be indexed as a superposition of the 共020兲, 共112兲, and 共200兲 reflections corresponding to the orthorhombic structure of DyScO3. Annealing of the layer at 1000 ° C does not lead to any crystallization of the films as shown by a second GIXRD pattern in Fig. 1共a兲; instead, the peak intensity even decreases to the noise level.
FIG. 1. 共a兲 GIXRD spectra of 12 nm DyScO3 / 20 nm SiO2 as grown and after annealing at 1000 ° C. 共b兲 XRR spectra of the same sample 共symbols兲 with best fits to the data 共solid lines兲. The data are offset for clarity. The inset shows the electron density profile corresponding to the best fit to the XRR data of the annealed sample.
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FIG. 2. 共a兲 High-resolution TEM image of 12 nm DyScO3 / 20 nm SiO2 after annealing at 1000 ° C. 共b兲 HAADF image of the same sample. 共c兲 High-resolution TEM image of 12 nm DyScO3 / 13 nm HfO2 after annealing at 1000 ° C. 共d兲 HAADF image of the same sample.
However, the cross-sectional TEM image of the same 12 nm DyScO3 / 20 nm SiO2 stack after annealing at 1000 ° C in Fig. 2共a兲 paints a more complicated picture. The image shows that the stack now consists of three distinct layers. Amorphous SiO2 remains at the bottom of the stack, although at a reduced thickness 共12 nm instead of 20 nm兲. On top, two layers are visible with a combined thickness larger than that of the original DyScO3 film. Although the high-resolution image and the corresponding HAADF image 关Fig. 2共b兲兴 show little contrast between the two layers, they clearly differ by their crystallinity. While the lower 12– 13 nm are amorphous, the upper ⬃5 nm are textured polycrystalline. The small size of crystallites and the texture of the polycrystalline films explain the discrepancy between the TEM image and the GIXRD pattern. A reduced SiO2 thickness together with an increased apparent thickness of the DyScO3 layer suggest silicate formation during annealing. Silicate formation of binary lanthanide oxides in contact with SiO2 is a well-established phenomenon.15 The formation of a polycrystalline lanthanide oxide layer on top of an amorphous lanthanide silicate layer during growth or postgrowth annealing has been observed for, e.g., Pr2O3 共Ref. 16兲 or La2O3.17 The formation of crystallized DyScO3 on top of amorphous DySc silicate is, however, inconsistent with the weak contrast in the HAADF image 关Fig. 2共b兲兴 and the XRR data in Fig. 1共b兲. While XRR data of as grown DyScO3 films indicate bulk density,10 best fits to the XRR data of the annealed stack are obtained by a reacted layer with a density significantly below that of bulk DyScO3 关insert in Fig. 1共b兲兴. The above data seem to point to the formation of a nearly homogeneous partially crystallized DySc silicate layer. However, both the normalized ToF-SIMS profiles 关Fig. 3共a兲兴 as well as the XPS composition profile 关Fig. 3共b兲兴 of the annealed DyScO3 / SiO2 stack show a different picture.
Appl. Phys. Lett. 92, 112902 共2008兲
FIG. 3. 共Color online兲 共a兲 ToF-SIMS profiles of 28Si, 45Sc, and 164Dy of 12 nm DyScO3 / 20 nm SiO2 after annealing at 1000 ° C. The inset shows the Sc/ Dy ToF-SIMS yield ratio for annealed DyScO3 layers on SiO2 共main figure兲 and HfO2. All data have been normalized to a common scale. 共b兲 Sputter-XPS cation composition profile of the same stack. In addition, the calculated electron density profile 共open blue diamonds兲 is shown 共taking the measured O content into account and assuming the atomic density of DyScO3 throughout兲.
The profiles consistently indicate that, during annealing, the DyScO3 / SiO2 stack has transformed into a DySc silicate layer in contact with residual SiO2 and a Sc-rich silicate layer at the surface. The electron density profile, which can be calculated from the XPS data, is also shown in Fig. 3共b兲 共open symbols兲. It is qualitatively and quantitatively very similar to the electron density profile deduced from the XRR data 关Fig. 1共b兲兴. It is worth noting that the electron 共and, thus, also mass兲 density varies rather little throughout the reacted film, which is consistent with the weak contrast in the HAADF image in Fig. 2共b兲. These results indicate that the silicate formation is complemented by a phase separation of the alloy. Assuming the sputter rate to be independent of composition in the silicate bilayer, the thickness ratio of the two films is approximately 2:1, which is consistent with the thickness ratio of the crystallized/amorphous reacted films in the TEM image. This suggests that the Sc-rich 共and relatively Si-poor兲 DySc silicate layer has crystallized and the more Dy- and Si-rich 共and relatively Sc-poor兲 DySc silicate is amorphous. The observed phase separation raises the question whether the root cause of this effect lies in the silicate formation or in an intrinsic instability of ternary DyScO3. An intrinsic instability toward phase separation has, e.g., been observed for HfSiOx,18 where the ternary alloys separate in crystalline Hf-rich grains in a SiO2 matrix at high temperature. An alternative cause of phase separation could originate from the experimental observation that Sc2O3 and Dy2O3 behave very differently in terms of silicate formation: while Dy2O3 shows a strong tendency to form silicates, very similar to that of DyScO3 described above, the solubility of SiO2
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in Sc2O3 is found to be negligible and Sc2O3 can be considered thermodynamically stable in contact with SiO2.15,19 This insinuates that the phase separation could also be due to silicate formation because of the different reactivities of Dy2O3 / SiO2 and Sc2O3 / SiO2. In order to study their intrinsic thermal stability, DyScO3 thin films have been grown on HfO2 pseudosubstrates. A TEM image of a 12 nm thick DyScO3 film on 13 nm HfO2 after annealing is shown in Fig. 2共c兲. The corresponding HAADF image 关Fig. 2共d兲兴 shows that the interface between HfO2 and DyScO3 is rough 共due to the crystallinity of the HfO2 layer兲 but abrupt. This suggests that DyScO3 is thermally stable on HfO2. The TEM image shows that the DyScO3 layer is fully polycrystalline, which means that the intrinsic crystallization temperature of DyScO3 thin films is Ⰶ1000 ° C and the apparent amorphization as suggested by the GIXRD data in Fig. 1共a兲 for the film on SiO2 is caused by silicate formation. The inset in Fig. 3共a兲 shows the normalized ratio of the 45 Sc and 164Dy ToF-SIMS ion yields for both annealed DyScO3 / SiO2 and DyScO3 / HfO2. For DyScO3 / HfO2, Si and Hf are only found at background levels in the DyScO3 film 共data not shown兲. The profiles show that the Dy/ Sc ratio is constant throughout the film for DyScO3 / HfO2, which indicates that the observed layered phase separation of DyScO3 / SiO2 stems from the reaction with the underlying SiO2 layer. Also, the HAADF image in Fig. 2共d兲 shows little contrast in the DyScO3 layer; so in contrast to, e.g., HfSiOx, DyScO3 seems to be, thus, intrinsically stable against phase separation. In conclusion, we have studied the thermal stability of thin DyScO3 films during annealing at 1000 ° C in a N2 ambient. The thermal stability was found to strongly depend on the underlying layer. While DyScO3 recrystallizes without phase separation on nonreactive substrates, such as HfO2, contact with SiO2 leads to a reaction with the DyScO3 layer during annealing. As a result, a thin polycrystalline Sc-rich 共and relatively Si-poor兲 DySc silicate was formed on top of an amorphous more Dy- and Si-rich 共and relatively Sc-poor兲 DySc silicate in contact with residual SiO2. These results reconcile the apparently conflicting reports on the recrystallization behavior of DyScO3 films. DyScO3 on top of nonreactive substrates, such as LaAlO3, recrystallizes well below 1000 ° C.11 When Si substrates are used, interfacial SiO2 is commonly used to provide a stable starting surface or can be generated during growth. Also, if oxygen is present in the annealing ambient 共even as a nonintenional impurity兲, interfacial SiO2 can regrow during annealing since DyScO3 does not provide an efficient diffusion barrier for oxygen.19,20 The presence of SiO2 on Si wafers will lead to the above thin film reactions and DyScO3 films can appear to remain amorphous due to silicate formation. Finally, it should be mentioned that since the phase separation is driven by the different reactivities of Sc2O3 and Dy2O3 with SiO2, the above effects show a pronounced com-
positional dependence and become considerably weaker for Sc-rich alloys.19 Because of the strong similarity of the chemical behavior of lanthanide oxides,15 it is likely that the above mechanisms are universal for other rare earth scandates and rare earth–containing ternary oxides also. This work was supported in part by the European Commission as a contribution to the Project Pullnano under Contract No. IST-026828 from the Information Society Technologies Programme within the European Union’s Sixth RTD Framework Programme. Bede Ltd. is aknowledged for the support of the x-ray measurements. D. G. Schlom and J. H. Haeni, MRS Bull. 27, 198 共2002兲. C. Zhao, T. Witters, B. Brijs, H. Bender, O. Richard, M. Caymax, T. Heeg, J. Schubert, V. V. Afanas’ev, and A. Stesmans, Appl. Phys. Lett. 86, 132903 共2005兲. 3 R. Thomas, P. Ehrhart, M. Luysberg, M. Boese, R. Waser, M. Roeckerath, E. Rije, J. Schubert, S. Van Elshocht, and M. Caymax, Appl. Phys. Lett. 89, 232902 共2006兲. 4 M. Wagner, T. Heeg, J. Schubert, S. Lenk, S. Mantl, C. Zhao, M. Caymax, and S. De Gendt, Appl. Phys. Lett. 88, 172901 共2006兲. 5 K. H. Kim, D. B. Farmer, J.-S. M. Lehn, P. Venkateswara Rao, and R. G. Gordon, Appl. Phys. Lett. 89, 133512 共2006兲. 6 S.-G. Lim, S. Kriventsov, T. N. Jackson, J. H. Haeni, D. G. Schlom, A. M. Balbashov, R. Uecker, P. Reiche, J. L. Freeouf, and G. Lucovsky, J. Appl. Phys. 91, 4500 共2002兲. 7 V. V. Afanas’ev, A. Stesmans, C. Zhao, M. Caymax, T. Heeg, J. Schubert, Y. Jia, D. G. Schlom, and G. Lucovsky, Appl. Phys. Lett. 85, 5917 共2004兲. 8 K. J. Hubbard and D. G. Schlom, J. Mater. Res. 11, 2757 共1996兲. 9 P. Sivasubramani, T. H. Lee, M. J. Kim, J. Kim, B. E. Gnade, R. M. Wallace, L. F. Edge, D. G. Schlom, F. A. Stevie, R. Garcia, Z. Zhu, and D. P. Griffis, Appl. Phys. Lett. 89, 242907 共2006兲. 10 C. Adelmann, P. Lehnen, S. Van Elshocht, C. Zhao, B. Brijs, A. Franquet, T. Conard, M. Roeckerath, J. Schubert, O. Boissière, C. Lohe, and S. De Gendt, Chem. Vap. Deposition 13, 567 共2007兲. 11 H. M. Christen, G. E. JellisonJr., I. Ohkubo, S. Huang, M. E. Reeves, E. Cicerrella, J. L. Freeouf, Y. Jia, and D. G. Schlom, Appl. Phys. Lett. 88, 262906 共2006兲. 12 L. F. Edge, D. G. Schlom, S. Rivillon, Y. J. Chabal, M. P. Augustin, S. Stemmer, T. Lee, M. J. Kim, H. S. Craft, J.-P. Maria, M. E. Hawley, B. Holländer, J. Schubert, and K. Eisenbeiser, Appl. Phys. Lett. 89, 062902 共2006兲. 13 R. Thomas, P. Ehrhart, M. Roeckerath, S. Van Elshocht, E. Rije, M. Luysberg, M. Boese, J. Schubert, M. Caymax, and R. Waser, J. Electrochem. Soc. 154, G147 共2007兲. 14 S. Van Elshocht, U. Weber, T. Conard, V. Kaushik, M. Houssa, S. Hyun, B. Seitzinger, P. Lehnen, M. Schumacher, J. Lindner, M. Caymax, S. De Gendt, and M. Heyns, J. Electrochem. Soc. 152, F185 共2005兲. 15 H. Ono and T. Katsumata, Appl. Phys. Lett. 78, 1832 共2001兲. 16 A. Sakai, S. Sakashita, M. Sakashita, Y. Yasuda, S. Zaima, and S. Miyazaki, Appl. Phys. Lett. 85, 5322 共2004兲. 17 X. Wu, D. Landheer, G. I. Sproule, T. Quance, and G. A. Botton, J. Vac. Sci. Technol. A 20, 1141 共2002兲. 18 S. Stemmer, Y. Li, B. Foran, P. S. Lysaght, S. K. Streiffer, P. Fuoss, and S. Seifert, Appl. Phys. Lett. 83, 3141 共2003兲. 19 S. Van Elshocht, C. Adelmann, T. Conard, A. Delabie, A. Franquet, P. Lehnen, L. Nyns, O. Richard, J. Swerts, and S. De Gendt, “Silicate formation and thermal stability of ternary rare earth oxides as high-k dielectrics,” J. Vac. Sci. Technol. A 共in press兲. 20 J. M. J. Lopes, U. Littmark, M. Roeckerath, S. Lenk, J. Schubert, and A. Besmehn, J. Appl. Phys. 101, 104109 共2007兲. 1 2
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