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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.
�10.1051/jphyscol:1984907�. �jpa-00224383�
JOURNAL DE PHYSIQUE
Colloque C9, supplkment au n012, Tome 45, dCcembre 1984 page C9-39
E P I T A X I A L 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 c r o i s s a n c e c r i s t a l l i n e de couches de samarium a Bt6 r 6 a l i s 6 e p a r d e p o s i t i o n de vapeur s u r d e s p o i n t e s de t u n g s t e n pour une e t u d e par microscopie 5 emission de champ Blectronique et s u r des monocristaux o r i e n t & (011) de tungstenepour une Btude par d i f f r a c t i o n des e l e c t r o n l e n t s . Les c o n d i t i o n s optimales de c r o i s s a n c e o n t 6 t 6 r 6 a l i s 6 e s pour une temperature du s u b s t r a t comprise e n t r e 650 e t 750 K. La r e l a t i o n d 1 6 p i t a x i e l a p l u s couramment observee B t a i t (0001)Sm / / (011)W avec [11.20]Sm / / [001]W. Le parametre c r i s t a l l i n du Sm dans l e plan (0001 ) e s t de quelques pour c e n t s s u p 6 r i e u r B l a v a l e u r correspondant au volume.
A b s t r a c t : Samarium e p i t a x i a l c r y s t a l l i n e l a y e r s have been grown by vapor d e p o s i t i o n o n t o e i t h e r t u n g s t e n f i e l d - e l e c t r o n e m i t t e r s o r a s i n g l e macro-cyrstal, (011)-oriented t u n g s t e n low-energy-electron d i f f r a c t i o n specimen. Optimum growth occurred f o r s u b s t r a t e temperatures i n t h e r a n g e of 650
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750 K. The e p i t a x i a l r e l a t i o n s h i p most commonly observed was (0001)Sm / / (01 1 )W w i t h [1120]Sm / / C0011W. The s u r f a c e l a t t i c e c o n s t a n t of Sm(0001) appears t o be a few per c e n t l a r g e r t h a n t h e bulk value.Samarium metal has been found t o possess mixed valence / I / ; t h e bulk atoms a r e t r i v a l e n t and t h e s u r f a c e atoms d i v a l e n t /2/. This has l e d t o t h e s u g g e s t i o n t h a t t h e s u r f a c e l a t t i c e parameters of Sm should be l a r g e r t h a n t h e corresponding bulk v a l u e s /3/. D i r e c t experimental t e s t i n g of t h i s i d e a , however, r e q u i r e s a well-defined c l e a n Sm s u r f a c e , which is n o n - t r i v i a l t o produce.
We have l e a r n e d how t o grow c l e a n c r y s t a l s o f Sm i n u l t r a h i g h vacuum by e p i t a x y from t h e vapor phase o n t o 1 ) t u n g s t e n i n a f i e l d e l e c t r o n microscope (FEM, base p r e s s u r e < 1 0-l2 T o r r ) and 2) a (01 1 )W macro-crystal i n a
low-energy-electron-diffraction (LEED) chamber ( b a s e p r e s s u r e about 1 x10-I T o r r ) . Due t o t h e h i g h vapor p r e s s u r e of Sm a t temperatures s u f f i c i e n t t o e n a b l e adequate s u r f a c e d i f f u s i o n f o r c r y s t a l growth, t h e c o n d i t i o n s f o r e p i t a x i a l growth of l a r g e c r y s t a l s were r e l a t i v e l y d i f f i c u l t t o f i n d . Compared t o o t h e r m e t a l s grown by t h i s t e c h n i q u e , much higher vapor f l u x e s were found t o be necessary. From t h e FEM experiments, w e l e a r n e d t h a t 650
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750 K was t h e o p t i m a l s u b s t r a t e temperature range f o r growth o f l a r g e c r y s t a l s , w i t h t h e c r y s t a l shape s e n s i t i v e t o t h e temperature.A t temperatures below 992 $, Sm c r y s t a l l i z e s i n a rhomohedral s t r u c t u r e w i t h a l a t t i c e c o n s t a n t a = 8.996 A , and a = 2 3 O 1 3 ' . Described i n terms of a hexagonal u n i t c e l l , t h e s t a c k i n g sequence is given by ABABCBCAC. This complexity caused d i f f i c u l t y i n c r y s t a l l o g r a p h i c indexing of micPographs, but u l t i m a t e l y t h e FEM p a t t e r n s from Sm were understandable w i t h t h e a i d of b a l l models. F i g u r e 1 shows photographs of t h e c r y s t a l l o g r a p h i c a l l y indexed b a l l models and f i g u r e 2 shows some f i e l d - e l e c t r o n micrographs t y p i c a l of our FEM
+ ~ u e s t worker a t NBS, supported mainly by t h e 1982-1983 Welch Foundation IUVST S c h o l a r s h i p . Permanent address: I n s t i t u t e f o r Experimental Physics, U n i v e r s i t y of Wroclaw. 50-205 Wroclaw, Poland.
+ * V i s i t i n g S c i e n t i s t a t t h e F r i t z - H a b e r - I n s t i t u t d e r Max-Planck- G e s e l l s c h a f t , W. B e r l i n , Federal Republic of Germany.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984907
C9-40 JOURNAL DE PHYSIQUE
F i g . 1 P l a s t i c b a l l models o f v a r i o u s c r y s t a l p l a n e s o f Sm, a s i n d i c a t e d by l a b e l s .
r e s u l t s . Based on vapor d e p o s i t i o n t i m e s , we e s t i m a t e t h a t t h e c r y s t a l l a y e r 5 were g e n e r a l l y a few t e n s o f nanometers t h i c k . The most commonly o c c u r r i n g e p i t a x i a l r e l a t i o n s h i p was (0001)Sm / / ( 0 l l ) W w i t h CCll?OISm / / [OOIIW.
S i m i l a r c o n d i t i o n s t o t h o s e f o u n d i n t h e FEM e x p e r i m e n t s were
s u c c e s s f u l l y used t o grow c r y s t a l l i n e f i l m s o f (0001)Sm on a ( 0 1 1 ) - o r i e n t e d W m a c r o - c r y s t a l . For t h i s p u r p o s e , a c o n i c a l - c o i l W w i r e b a s k e t c o n t a i n i - n g p i e c e s of 99.9 % p u r e Sm, a s used i n t h e FEM e x p e r i m e n t s , was welded t o
F i g . 2 F i e l d e l e c t r o n m i c r o g r a p h s f o r Sm c r y s t a l s ( a t 78 K) e p i t a x i a l l y grown on W. a ) Clean W, (011) i n c e n t e r , b) Sm c r y s t a l grown, on W shown i n a , a t 650 K, c ) A f t e r a d d i t i o n a l Sm d e p o s i t i o n a t 700 K , d ) A f t o r f u r t h e r Sm d e p o s i t i o n a t 750 I:.
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 hexago- nal-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) 182.2. See, for example E. Bertel, G. Strasser, F. P. Netzer, and J. A. D.
Matthew, Phys. Rev.
B25
(1982) 3374.3. A. Rosengren and B. Johansson, Phys. Rev.
B26
(1982) 3068.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.
