EPITAXIAL GROWTH AND SOME PROPERTIES OF SAMARIUM CRYSTALS ON TUNGSTEN

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1 EPITAXIAL GROWTH AND SOME PROPERTIES OF SAMARIUM CRYSTALS ON TUNGSTEN A. Ciszewski, A. Melmed To cite this version: A. Ciszewski, A. Melmed. EPITAXIAL GROWTH AND SOME PROPERTIES OF SAMAR- IUM CRYSTALS ON TUNGSTEN. Journal de Physique Colloques, 1984, 45 (C9), pp.c9-39-c9-42. < /jphyscol: >. <jpa > HAL Id: jpa Submitted on 1 Jan 1984 HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

2 JOURNAL DE PHYSIQUE Colloque C9, supplkment au n012, Tome 45, dccembre 1984 page C9-39 EPITAXIAL GROWTH AND SOME PROPERTIES OF SAMARIUM CRYSTALS ON TUNGSTEN + + A. ~iszewski* and A.J. Melmed NationaZ Bureau of Standards, Gaithersburg, MD 20899, U.S.A. R6sumB: La croissance cristalline de couches de samarium a Bt6 r6alis6e par deposition de vapeur sur des pointes de tungsten pour une etude par microscopie 5 emission de champ Blectronique et sur des monocristaux orient& (011) de tungstenepour une Btude par diffraction des electron lents. Les conditions optimales de croissance ont 6t6 r6alis6es pour une temperature du substrat comprise entre 650 et 750 K. La relation d16pitaxie la plus couramment observee Btait (0001)Sm // (011)W avec [11.20]Sm // [001]W. Le parametre cristallin du Sm dans le plan (0001) est de quelques pour cents sup6rieur B la valeur correspondant au volume. Abstract: Samarium epitaxial crystalline layers have been grown by vapor deposition onto either tungsten field-electron emitters or a single macro-cyrstal, (011)-oriented tungsten low-energy-electron diffraction specimen. Optimum growth occurred for substrate temperatures in the range of K. The epitaxial relationship most commonly observed was (0001)Sm // (011)W with [1120]Sm // C0011W. The surface lattice constant of Sm(0001) appears to be a few per cent larger than the bulk value. Samarium metal has been found to possess mixed valence /I/; the bulk atoms are trivalent and the surface atoms divalent /2/. This has led to the suggestion that the surface lattice parameters of Sm should be larger than the corresponding bulk values /3/. Direct experimental testing of this idea, however, requires a well-defined clean Sm surface, which is non-trivial to produce. We have learned how to grow clean crystals of Sm in ultrahigh vacuum by epitaxy from the vapor phase onto 1) tungsten in a field electron microscope (FEM, base pressure < 10-l2 Torr) and 2) a (011)W macro-crystal in a low-energy-electron-diffraction (LEED) chamber (base pressure about 1 x10-i Torr). Due to the high vapor pressure of Sm at temperatures sufficient to enable adequate surface diffusion for crystal growth, the conditions for epitaxial growth of large crystals were relatively difficult to find. Compared to other metals grown by this technique, much higher vapor fluxes were found to be necessary. From the FEM experiments, we learned that K was the optimal substrate temperature range for growth of large crystals, with the crystal shape sensitive to the temperature. At temperatures below 992 $, Sm crystallizes in a rhomohedral structure with a lattice constant a = A, and a = 23O 13'. Described in terms of a hexagonal unit cell, the stacking sequence is given by ABABCBCAC. This complexity caused difficulty in crystallographic indexing of micpographs, but ultimately the FEM patterns from Sm were understandable with the aid of ball models. Figure 1 shows photographs of the crystallographically indexed ball models and figure 2 shows some field-electron micrographs typical of our FEM +~uest worker at NBS, supported mainly by the Welch Foundation IUVST Scholarship. Permanent address: Institute for Experimental Physics, University of Wroclaw Wroclaw, Poland. +*Visiting Scientist at the Fritz-Haber-Institut der Max-Planck- Gesellschaft, W. Berlin, Federal Republic of Germany. Article published online by EDP Sciences and available at

3 C9-40 JOURNAL DE PHYSIQUE Fig. 1 Plastic ball models of various crystal planes of Sm, as indicated by labels.

4 results. Based on vapor deposition times, we estimate that the crystal layer5 were generally a few tens of nanometers thick. The most commonly occurring epitaxial relationship was (0001)Sm // (0ll)W with CCll?OISm // [OOIIW. Similar conditions to those found in the FEM experiments were successfully used to grow crystalline films of (0001)Sm on a (011)-oriented W macro-crystal. For this purpose, a conical-coil W wire basket containi-ng pieces of 99.9 % pure Sm, as used in the FEM experiments, was welded to Fig. 2 Field electron micrographs for Sm crystals (at 78 K) epitaxially grown on W. a) Clean W, (011) in center, b) Sm crystal grown, on W shown in a, at 650 K, c) After additional Sm deposition at 700 K, d) Aftor further Sm deposition at 750 I:.

5 C9-42 JOURNAL DE PHYSIQUE Fig. 3. LEED patterns for (0001)Sm/(011)W. a) Low coverage, outer spots due to W substrate. 84 ev, b) high coverage. 141 ev. degassable W wire leads and attached to a moveable assembly inside the LEED chamber. During vapor deposition, the W specimen was rotated go0 to face the Sm source, which was brought to a distance of about 1 cm from the specimen. The epitaxial relationship most commonly observed in the FEM experiments was also the relationship found in the LEED experiments. Interesting lattice parameter determinations resulted from the LEED pattern geometry measurements; however, these should be considered tentative, as the work is still in progress. Assuming the same inner potentials for Sm and W, we found that the surface lattice constant for (0001)Sm is indeed larger than the bulk value. The enlargement varied from 15 $ to 4 $ as the Sm film thickness increased. Figure 3 shows a LEED pattern typical of the results for relatively small doses of Sm. Diffraction spots from both the (Ol1)W substrate and (0001)Sm are evident. Further experiments are planned to determine whether the Sm spots are due to a very thin continuous layer or to a patchy layer. For much larger deposits of Sm, the W spots were not visible, as expected, and then only a simple hexagonal-symmetry LEED pattern occurred, as also seen in figure 3. Extra spots were clearly evident after one or two hours exposure of the Sm crystal layers to the residual gas of the LEED chamber. We conclude that epitaxial growth by vapor deposition on a W surface is a relatively easy way to produce Sm crystals which are suitable for investigation by FEM and LEED. References 1, G. K. Wertheim and M. Campagna, Chem. Phys. Lett. 3 (1977) See, for example E. Bertel, G. Strasser, F. P. Netzer, and J. A. D. Matthew, Phys. Rev. B25 (1982) A. Rosengren and B. Johansson, Phys. Rev. B26 (1982) Acknowledgment This work was motivated by, and we fully appreciate some stimulating remarks made by Dr. A. Zangwill to the authors in a conversation atop a mountain ridge in New'Mexico in June 1983.