Ni interfaces

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INVERSE AND DIRECT PHOTOEMISSION ( U V R A N G E ) O F T H E Si/Ni I N T E R F A C E S

EXPERIMENTS

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I. Introduction I n\ c! ~e p h o t o e n l l s ' q o n ( l P E ) ~ l n d l pl o b c s u n o c c u p m d st atcs lo~.,lted a b m c the i . m m l level a p p e , l p , to gl~e ~ a l u a b l c u m l p l e m e n t a r ~ m t o r m a t i o n on the e l e c l r o n l t , s t r u c t u r e ol m a t e r i a l s m COlllUllCllOn V~,l[h o t h c l c o m m o n l \ u s e d spcctl o s c o p l c I c c h n i q u e s ( ' h a n g c s o b s e i x cd i11 the s h a p e ol deilSltles ot M,IIeS ( D O S ) d u r i n g t h e grox~th ol slhcidc ph,lscs can l e a d to
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interface chemistry The present paper follows our previous work on polycrystalline and amorphous silicon ( U P S - I P E ) [1] and on the two complementary systems S1/NI(100) [2] and NI/SI(ll 1) [3] studied by XPS

2. Experimental Experiments were carried out in an X P S - U P S apparatus (base pressure 4 x 10 E~mbar) to which an inverse photoemisslon spectrometer equipped with multldetectlon has been connected [41 The IPE spectra have been enregistered with an electronic current density of 300/tA/cm 2 in typical recording times of 1000-3000 s The initial electron energy was 35 eV relative to the Fermi level of the sample In these experiments the electron beam was not focused so that the whole Brlllouln zone was sampled, under these conditions the IPE spectra reflect, in a tlrst approximation, the total three-dimensional DOS broadened by the spectrometer resolution ( A E = 0 6 eV) The SI/NI(100) interfaces were prepared by direct evaporation ot a small SI rod using electron bombardment, the evaporation rate was checked with a quartz mlcrobalance and also controlled with results obtained using the exponential attenuation ol the XPS Mo 3d signals when more and more silicon is deposited on the surface (the S1/Mo interface is unreacted [2]) During the SI evaporation. the vacuum was better than 5 × 10 -m mbar in the preparation chamber with an evaporation rate ot ~ 1 5 A/rain Then, the sample was transferred to the analysis chamber and measured The UPS spectra were enreglstered before and after each IPE spectrum in order to verify the absence of contaminants or to display an eventual modification of the spectrum Indeed, m one case. as we will see later, we could observe a small difference belore and after the IPE measurement This change could be due to a modlficatmn of the surface as a consequence ot electron radiation damage or ol a dltfuslon d m e n by temperature Naturally, the initial clean nickel surtace has been checked by XPS and L E E D before silicon evaporation

3. Results and discussion Figs 1 and 2 show the photoemlsslon spectra (Hel at hv = 21 2 eV and Hell at try = 40 8 eV) betore and after various increasing slhcon deposmons (from ~1 5 to 100 ~ ) The corresponding results for IPE are represented m fig 3 The lower curves in figs 1 and 2 represent the energy distribution curves (EDCs) for a clean NI(J00) sample One observes a large structure with a pronounced shoulder just at the Fermi level This peak has been attributed to a surface emission in directional photoemlsslon and has been found to be very sensitive to sulphur adsorption [5] The IPE spectrum of

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clean m e t a l h c m c k e l r e p r e s e n t s the 3d hole p o r t i o n l o c a t e d just a b o v e E~ a n d which has a D i r a c - f u n c U o n - l l k e s h a p e b r o a d e n e d by the r e s o l u t i o n of the I P E a p p a r a t u s A b o v e it a r e two structures l o c a t e d at I 9 and 3 1 e V , which have b e e n i n t e r p r e t e d mn t e r m s of direct t r a n s i t i o n s i n t e g r a t e d o v e r the w h o l e Brllloum z o n e ot the nickel s a m p l e [6] T h e s e two s t r u c t u r e s b r o a d e n the D O S p e a k a n d g w e r o u g h l y a lull width at half m a x i m u m ( F W H M ) ol 2 7 e V T h e F e r m i level p o s i t i o n is s i t u a t e d at 80% of the rise as d e t e r m i n e d by IPE on a s t a n d a r d gold s a m p l e This u n o c c u p i e d p o r t i o n ol the c o n d u c t i o n b a n d ( C B ) c o r r e s p o n d s to the m e t a l h c conhgurat~on of nickel, a m i x t u r e ol 3d"4s ~ and 3d H' a t o m i c c o n h g u r a t k ) n s or 3d ~ ~4s~'' ,is d e t e r l n m e d by b a n d structures calculations [71 A l t e r 1 m,n o l e v a p o r a t i o n (1 5 A ) . o n e o b s e r v e s on the UPS s p e c t r a hrst the a t t e n u a t i o n of the m e t a l h c m c k e l structures and s e c o n d l y the e m e r g e n c e of a new s t r u c t u r e at 1 e V b e l o w EF T h e larger width of the s p e c t r a c o m p a r e d to the nickel case can be d u e to v o l u m e c o n t n b u U o n I n d e e d . the m e a n tree p a t h being ol the o r d e r o f 5 A 18], we o b s e r v e the c o m p o s i t e s p e c t r a of the r e a c t e d p h a s e s u p e r p o s e d on the nickel s u b s t r a t e T h e CB D O S c h a n g e s to a t r i a n g u l a r s h a p e ( s o m e w h a t e n l a r g e d ) and the F e r m ! level p o s i t i o n is n e a r e r to the inflection p o i n t of the rising curve F o r 2 rain of e v a p o r a t i o n , the U P S cur~es r e v e a l a d o m i n a n t s t r u c t u r e c e n t e r e d at 1 e V b e l o w E~ and also a small b u m p l o c a t e d at - 2 5 e V T h e

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I P E s p e c t r u m shows a w i d e r t r i a n g u l a r s h a p e , f o l l o w e d by a d,p at 4 2 e V and a large structure c e n t e r e d at ~-7 5 e V a b o v e E~ T h e F e r m i level is l o c a t e d at the m f l e c n o n p o i n t P e a k p o s m o n s m the valence b a n d ( V B ) and the shape ot the CB D O S m d m a t e that we have r e a c h e d a "'N12SF" t y p e p h a s e W e use q u o t a n o n m a r k s b e c a u s e it the U P S s p e c t r u m is similar to those r e p o r t e d m p r e v i o u s h t e r a t u r e [9] the CB D O S (sensitive to l o n g - r a n g e o r d e r ) ts s o m e w h a t o b s c u r e d c o m p a r e d to its c o u n t e r p a r t m a w e l l - d e f i n e d think NI~S! layer we have p r e p a r e d by a n n e a h n g a ll)0 A Si d e p o s i t on NI(100) at 400°C 110] A l t e r a 7 5 * c o v e r a g e , the e v o l u t i o n of the VB a n d CB shows that we have r e a c h e d the N1SI p h a s e with the principal p e a k at - 1 7 e V and a w e a k b r o a d s t r u c t u r e at - 2 5 eV [9, l(11, h o w e v e r , by I P E , vestiges ot thc NbS! s p e c t r u m are a l r e a d y p r e s e n t F o r d e p o s m o n g r e a t e l than l(i A the s e c o n d p e a k l o c a t e d at - 2 5 cV grows g r a d u a l l y c o m p a r e d to the principal one F o r high c m e r a g e , the a-Si 3p states c o n t r i b u t e to the intensity ot this structure T h e c h a n g c o b s e r v e d by U P S b e f o r e a n d a l t e r I P E and m e n t i o n e d a b o v e is r e l a t e d to this s t r u c t u r e ~ h l c h 1%a t t e n u a t e d a t l e r a long IPE r e c o r d i n g n m c W c think that this c o n c ,,ponds to a t r a n q o r m a U o n ot the a-S1 3p states ot the t e t r a h e d r a l sp ~ b o n d i n g into states r e p r c s e n t , m ~ e of the N1SI b o n d i n g T h e slight d | t t e r e n c e s m the s h a p e s and p e a k p o s m o n s o b s e r v e d m the s p e c t r a excited w,~th the H e l or H e l I hnes are first a t t r i b u t e d to ~ a r l a n o n m the e l e c t r o n m e a n tree p a t h s (6 and 4 A respecn~ ely [8]) allov~mg obser~ a n o n ot h e t e r o g e n e o u s o r h o m o g e n e o u s u p p e r regions ot the s a m p l e and s c c o n d l y to c r o s s - s e c n o n effects, the raUo ot the S1 3p to N1 3d cross secUon being h~c times less at hv = 40 8 e V than at hv = 21 2 c V T h e p n n c l p a l p e a k l o c a t e d at - I 7 eV m the VB still arising from NI 3d o r b l t a l s as m the case ot N1SI c o o r d i n a t i o n ~.,m bc o b s e r v e d until a c m e r a g c ot ~2() A is r e a c h e d T h e I P E s p e c t l a look hke the NISI one [1()], xvlth a m e t a l h c edge at the F e r m i level tor c o v e r a g e b e t w e e n l() and 20 A O n N lol d e p o s i t s > 2(1 A d o e s tins mctalh~, edgc d i s a p p e a r and do we get thc a n l o l p h o u s sdlcon ( ' B D O S , that is to s a \ , a s m o o t h e d s t e p itln(.tlon shllted a b o x c thc F c r m l Icvcl and v~lth s o m e r c m a n e n t u n d u l a t i o n s [1, 11] T h e clear o b s e r v a t i o n ot Ihc VB ,rod CB D O S a l t e r d e p o s m o n ol 1 to l()() A of silicon on t o p ol a N1(1(10) cr.,,,qal e n a b l e s us to i d e n t | t y the n a t u r e ot the slhcldes t o r m e d at the i n t e r l a c e with the mckel s u b s t r a t e The hrst stage indicates that the local o r d e r is ol the N1,S! t y p e This is not surprising i1 v~e c o n s i d e r that the NI,SI p h a s e is the p h a s c the most rich m mctalh~, a t o m s , a n d which reduces the smallest d e l o r m a t u m ol the m c k e l lattice I n d e e d , the c o o r d m a n o n radius Is only d e c r e a s e d b~ 5 % l o r N~SI relatlxc to m e t a l h c nickel [12] F u r t h e r , ~ c ha~c to n o n c e that no e n e r g e t l c c o n s l d c r a l u m prevails b e c a u s e the c o h e s i o n energy ot silicon (4 4 e V / a t o m ) is high e n o u g h to reduce the t o r m a t l o n ot any one ol the t h r e e sfllcldcs NI,SI NISI and NISI~ T h e existence ol a N1SI phase tor h i g h e r c o v e r a g e s could bc duc to thc

M Aztzan et al / L/Vphotoemtsslon of St/Nt ~urJa¢es

239

fact that a d~ffUSlOn barrier to S1 and N~ atoms is set up by a NhS~ fdm In this context, it would be interesting to estimate the thickness of th~s NI,S~ hlm by energy-dependent photoem~sslon m order to study its eventual stagnation The occurrence of the N~Sl-hke phase for mtermedmte coverage is supported by the N~ 2pv2 core level evolutmn study [2], which shows a gradual sh~ft when sd~con is deposited on a mckel substrate until a stabfl~zahon of th~s level (AE = - 1 eV) occurs for a thickness of ~10 A Finally, by comparison with standard UPS [9] and IPE [10] spectra of NxSL,, it can be deduced that such a phase does not appear fo~ any sflmon cox e rage

4. Conclusion Using UPS and IPE we have studmd the electromc structure below and above the Ferm~ level for low coverages of silicon deposited at room temperature on the N~(100) crystal face The model we propose assumes the mmal growth oi a N~eS~-hke phase, followed by the growth of a "'N~SF" sd~cMe and finally appearance of amorphous sflmon at the top of the substrate for thmknesses greater than 30 A No evidence of a N~S~2phase has been found

References [1] R Baptp, t G ( h a u ~ c l a n d ] A Ngm, L.nFan ",EMMe~tmg La,,Vega,, USA Apnllg8'~ SLanmng Electron Micro~cop~ to h¢ pubh~hcd 121 T A Ngm~n Tan, M AzJzan R ( (-mt] (~ ( h a m e t and R Bapt]q 3mt,..c S~_] 162 (lggS) (~SI [~,1 F A Ngu~en ] , m and R C ( m t J Ph'~slca %t.NptaT4(198z;) 17~'1 ]4] (, ( h a u x e t ,rod R Bapt],,t, J klt.ctron %pt.ctro,,~_ Rclat,~d Phenomen,i to be subnnttcd ['~] F I A Ngu~en and R ( ( ' m u , Surla~_c ",t.i 68 (It~77) 566 [6] D P Woodrull N V %mHh P D John'~on,md W A B o ; e r Phi,,, Rex B26(19,'42)294:~ [7] W A Harl],,on, Elcctmmc Structurt_ ,rod th~ Propeltlcs ot %ohd,, (Fret.man San Fram_]s~_o 19frO) p 49g [,'41 | Rmchardt I. Le', and R I Johnson 1 Non-Ct'~,,tal Sohds ~9/6(1(198g)g29 [~1] ~k lq-,mtm)~,l J H ~ c a ~ e r and F A ",chmldt Ph'~,, R¢_~ B2(~ (lqX2)546 [l()] NI *kTI/dI], P, BapUq (, ( hau,,t.t dlld T A Ngu~en Fan ",ohd ~,tatc ( ommtli1 to be pubhsh~_d [11] W B l,u_k',on,% M KLI~,o ( ( T,,,u 1\~ A I I L n a n d S J ()h Ph'~,~ t~,~ B~,l(19g'z,)~l~7 [12] \ l~r,mcU~Sl J H Wc_ax¢_~ 1) (, ()Nelrl "~ ( b a h a i I [= Ro\~. J M l'oat~_ () I~l'q,md ( (,d,md~a I \'d,,_tluln %c~ T e d m o l 21 (1¢1h2)¢~24