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DIRECT IMAGING OF RECONSTRUCTED IRIDIUM SURFACES IN THE FIELD ION MICROSCOPE
J. Witt, K. Müller
To cite this version:
J. Witt, K. Müller. DIRECT IMAGING OF RECONSTRUCTED IRIDIUM SURFACES IN THE FIELD ION MICROSCOPE. Journal de Physique Colloques, 1986, 47 (C7), pp.C7-29-C7-34.
�10.1051/jphyscol:1986706�. �jpa-00225895�
JOURNAL DE PHYSIQUE
C o l l o q u e C7, suppl&ment au no 11, T o m e 47, N o v e m b r e 1986
DIRECT IMAGING OF RECONSTRUCTED IRIDIUM SURFACES IN THE FIELD ION MICROSCOPE
J. WITT a n d K. M ~ ~ L L E R
I n s t i t u t fur a n g e w a n d t e P h y s i k , L e h r s t u h l fur Festkb'rperphysik, U n i v e r s i t d t Erlangen-Niirnberg, Erwin-Rommel-Str. 1,
0-8520 Erlangen, F.R.G.
Abstract - E x p e r i m e n t s a r e described during w h i c h both t h e Ir(llO)lx2 and the Ir(1OO)lxS reconstructed s u r f a c e s could be imaged in t h e FIM. F u r t h e r m o r e evidence for a possible 1 x 2 re- construction o f the Ir(113) s u r f a c e w a s found.
Introduction
It i s well k h o w n t h a t o n many semiconductor and s o m e metal s u r f a c e s the topmost layer o f a t o m s c a n rearrange to form s t r u c t u r e s different from those o f the underlying bulk net planes. These reconstructed sur- faces have been studied mainly by diffraction and scattering tech- niques. Except for two s t u d i e s /1, 2/ FIM h a s c o n t r i b u t e d rather l i t t l e t o this s u b j e c t u p to now. With i t s unique capability o f imaging s i n g l e surface a t o m s , however, i t should be a b l e t o reveal valuable atomic details o f t h e reconstruction process inaccessible t o other techniques.
In the following a s h o r t account o f e x p e r i m e n t s will be given during which both t h e reconstructed Ir(llO)lx2 and the Ir(100)1x5 s u r f a c e s a s well a s their emergence from the respective 1 x 1 s t r u c t u r e s were o b - served i n the FIM. The experiments are described i n m o r e detail else- where / 3 / .
Experimental
The experiments were performed i n a s t a i n l e s s s t e e l FIM providing a base pressure o f lo-'' mbar and a minimum t i p temperature o f 2 0 K.
Immediately before the e x p e r i m e n t s the Ir-tips were cleaned by heating t o 1 5 0 0 K for several minutes. Subsequent Ne-sputtering produced s h a r p t i p s o f a r a d i u s r 2 7 0 1. F o r a precise determination o f faint s p o t locations the i n t e n s i t i e s o f the FIM patterns were recorded by means o f a s e n s i t i v e video measuring system / 4 / . Noise i n the measured pro- files w a s reduced by signal averaging.
The Ir(llO)lx2 Surface
F o r the 1 x 2 reconstruction o f t h e Ir(ll0) s u r f a c e various s t r u c t u r e s h a v e been proposed. Most recently the missing row model in which every second row o f s u r f a c e a t o m s i s missing could be confirmed by a LEED
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986706
JOURNAL DE.PHYSIQUE
Fiq. 1 (a)-(e):
The lx1+1x2 s t r u c t u r e transition on Ir(ll0) a s observed in the FIM:
(a) Ir(ll0) prepared b y low temperature field e v a p o r a t i o n ;
(b)-(e) s a m e s u r f a c e after successive 2 0 s heating periods t o T2750K (el. field switched off);
(f) Ir-tip after heating and slight field evaporation: 1 x 2 missing row s t r u c t u r e s on the (110) and (113) terraces;
structure determination /5/. Two earlier FIM s t u d i e s addressing I r (110) s u r f a c e reconstruction r e p o r t a missing row s t r u c t u r e obtained either by l o w temperature /6/ or high temperature /7/ field evapo- ration.
Determining the atom p o s i t i o n s from FIM intensity profiles s h o w s a 1 x 1 structure for the low temperature field evaporated Ir(ll0) s u r - face /3/. We found t h a t reconstruction c a n be induced o n i t by mere thermal activation w i t h o u t t h e influence o f a n electric field similar to the c a s e o f platinc~t /2/. An Ir(ll0) terrace o f 2 5 a i n diameter reconstructs a t T > 7 5 0 K a s s h o w n in fig. la-e. The 1 x 1 + 1 x 2 transi- tion s t a r t s with t h e disappearance o f one outer r o w (lb). Subsequently the inner r o w o f t h e remaining c l u s t e r i s broken u p (lc) and emptied by diffusion o f s i n g l e a t o m s o r - eventually - s m a l l c l u s t e r s o f ato:as a s proposed by Gao and Tsong / 8 / . The final s t r u c t u r e a g r e e s with the missing r o w model. Fig. I f s h o w s the s u r f a c e o f an Ir-tip after heat treatment and slight field evaporation. In a d d i t i o n t o the a p p a r e n t reconstruction of the (110) s u r f a c e a partially developed 1 x 2 missing r o w s t r u c t u r e i s a l s o encountered o n the (113) terrace. A r e c o n s t r u c - tion of t h e Ir(113) s u r f a c e should not be surprising a s i t s s t r u c t u r e i s very similar to t h a t o f t h e (110) plane.
The Ir(100)1x5 S u r f a c e
Heating the field evaporated Ir(l00) s u r f a c e t o t e m p e r a t u r e s a b o v e 1 3 0 0 K for s o m e s e c o n d s i n the absence o f t h e imaging field r e s u l t s i n the appearance o f ordered a t o m i c r o w s oriented along the direction o f nearest neighbour d i s t a n c e within the plane (fig. 2). Measuring intensity profiles along an a c r o s s the r o w s a l l o w s t o determine their relative distance i n the FIM pattern (fig. 3). In t h i s way a r a t i o o f mean row t o nearest neighbour distance within the r o w s o f r / a = 1.6 t 0.1 i s obtained suggesting a s t r u c t u r e o f t h e first layer in- commensurable to t h a t o f the substrate.
Recently LEED s t r u c t u r e analyses confirmed t h e corrugated quasihexa- gonal s t r u c t u r e m o d e l developed earlier for t h e reconstructed Ir(100) 1 x 5 s u r f a c e ( / 9 / and r e f e r e n c e s therein). It i s o b v i o u s from fig. 4 t h a t its t o p m o s t layer c o n t a i n s c l o s e packed atom r o w s o f different elevation. The protruding r o w s - indicated by s h a d i n g - h a v e a d i s - tance o f 1.67 a. T h e s u p e r s t r u c t u r e observed i n t h e FIM on Ir(100) i s i n good agreement with the s t r u c t u r e model for the Ir(100)1x5 recon- struction i f o n e a s s u m e s the elevated r o w s t o be imaged only. Under c e r t a i n c i r c u m s t a n c e s a t o m s o f the lower 1 y i n g . r o w s c a n be o b s e r v e d in the F I M pattern near the e d g e s o f the s u p e r s t r u c t u r e possibly due to a lattice deformation in the top layer (fig. 5b).
As the superstructure rests on a s u b s t r a t e with fourfold symmetry two possible d o m a i n o r i e n t a t i o n s rotated by 9 0 " with r e s p e c t to each other are possible. The LEED pattern o f Ir(100)1x5 i n d i c a t e s their e q u a l presence on macroscopic (100) surfaces. D u e t o the s m a l l s i z e o f the
(100) terrace o n t h e FIM tip (40 1) only one domain c a n be observed on it. Field desorption o f the topmost layer and s u b s e q u e n t h e a t treatment, however, results in random e m e r g e n c e o f either orientation (fig. 5).
Fig. 6 s h o w s typical s i t u a t i o n s during the transition o f the field evaporated (100)Ixl s u r f a c e i n t o i t s reconstructed form. After heating to T I = 1 0 0 0 K for 2 0 s with t h e irnaqing field switched o f f one r o w o f a t o m s a p p e a r s o n the (100) t e r r a c e (6b). A second heatlng period a t T Z = 1 0 5 0 K r e s u l t s in the formation o f a double row from which
C7-32 JOURNAL DE PHYSIQUE
Fiq. 2: (top)
FIM i m a q e o f an Ir-tip after heating t o T t 1 3 0 0 K ; a r o w s t r u c - ture has appeared on the (100) terrace
Fiq. 3: (right)
FIM intensity profiles along (a) and a c r o s s (b) t h e r o w s o f the Ir(100) superstructure
-
C 6 = 4 2i0 2d~stance [arb un.1-
distance larb. un I--+
Fiq. 4:
Structure model for the recon- structed IrflOO)lx5 s u r f a c e
o.sA
Fiq. 5:
Emergence o f two reconstructed domain orientations on Ir(100);
flashing the tip to 1500K re- sulted in pattern (a); after field desorption o f the top- most layer and heating to 1200K the 90° rotated d o m a ~ n w a s obtained ( b )
a b
Fiq. 6:
The 1 x 1 + 1x5 transition on Ir(100) a s ob- served in the FIM
(a) Ir(100) prepared by field evaporation (b)-(d) s a m e t e r r a c e after 2 0 s heating t o lOOOK (b), 1050K (c) and 1200K (d)
Table 1: Comparison o f FIM r e s u l t s on Ir s u r f a c e reconstruction to those obtained on macroscopic s i n g l e c r y s t a l s u r f a c e s
Macroscopic s i n g l e c r y s t a l s u r f a c e s
(110): 1 x 2 missing row
(100): c o r r u g a t e d , quasihexa- gonal overlayer, ele- vated rows: r/a=1.6
(110): 6 9 0 K < T < 8 9 0 K /11/
(100): 8 0 0 K < T < 1 3 0 0 K
1-D order during transition
(11°): 1x2T=7tj0K O 2 -1xl 02/C0 (100): 1x5T=750K -1xl structure
s t r u c t u r e transition
influence o f adsor- bates
FIM
(110): 1 x 2 missing row (100): atom r o w s ,
r/a = 1.6 + 0.1
(110): T > 7 5 0 K (100): 8 0 0 K < T < 1 3 0 0 K appearance o f r o w s during transition
O 2
(110): 1 x 2 T = 8 0 0 K - 1 ~ 1 0 /res.gas (100) : 1 x 5 -1~1
C7-34 JOURNAL D E PHYSIQUE
after final heating to T j = 1 2 0 0 K the superstructure emerges. A com- pletely ordered r o w s t r u c t u r e a s i n figs. 2 and 5 i s obtained only after flashing t o T > 1 2 0 0 K. The temperature interval for t h e s t r u c - ture transition observed o n t h e FIM tip i s i n good agreement with the results o f a L E E D study /lo/. The appearance o f r o w s in the FIM i m a g e during t h e transition c o r r e s p o n d s w e l l to the L E E D o b s e r v a t i o n o f one- dimensional o r d e r i n the transition region (800 K - 1 2 0 0 K).
Heating t h e s u p e r s t r u c t u r e s o f the Ir(ll0) and (100) s u r f a c e s on the FIM t i p i n oxygen c a u s e s their transformation i n t o the 1 x 1 p h a s e s in accordance with t h e LEED experiment.
Conclusion
Table 1 s u m m a r i z e s the F I M o b s e r v a t i o n s concerning t h e s u p e r s t r u c t u r e s on Ir(ll0) and (100) in comparison t o t h e r e s u l t s obtained by other techniques. T h e good agreement between t h e present F I M s t u d y and earlier r e s u l t s j u s t i f i e s the assumption that the superstructures on s m a l l t e r r a c e s o f t h e FIM t i p are t h e s a m e a s t h o s e o n macroscopic surfaces. More FIM e x p e r i m e n t s are reqiured t o study atomic details o f t h e reconstruction process and i t s driving forces.
References
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