IONIZATION OF CLUSTERS SPUTTERED FROM A3B5 COMPOUNDS

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IONIZATION OF CLUSTERS SPUTTERED FROM A3B5 COMPOUNDS

Z. Sroubek, Z. Hulek

To cite this version:

Z. Sroubek, Z. Hulek. IONIZATION OF CLUSTERS SPUTTERED FROM A3B5 COMPOUNDS.

Journal de Physique Colloques, 1989, 50 (C2), pp.C2-93-C2-98. �10.1051/jphyscol:1989217�. �jpa-

00229414�

IONIZATION OF CLUSTERS SPUTTERED FROM A ~ COMPOUNDS B ~

Z. SROUBEK and Z. HULEK'

Czechoslovak Academy of Sciences URE Lumumbova 1, 18251 Prague 8 , Czechoslovakia

'~epartment of Electronics, Prague Technical University Brehova 7 , 11519 Prague 1, Czechoslovakia

P,@sum@ : Le processus de formation dc clusters atomiques charges d'unc forme generale A ~ i 2<n, m<l) emis par des alliages ~ E ~ ~ ~ ' (GaAs, 8 ~ XnP, G a p , TnSh) sous hornbardement avec des ions ler~ts ( ( 2 keV) primafres de gaz rares a (?t@ (?tudi@. I.es donnees exp6rirnentales comportent le::

mesures d'emission ionique sous l'impact d e s ions lIet ?t Kr svec ler- t

.~aleurs d'gnergies primaires

E

,comprises entre 0.2 et 2 keV, ainsi quc, D

les resultats des mesures SNMS sur GaAs. L a variation de E fournit ur-:

P

outil pour contrbler le niveau d'excitation electronique du substrat.

Les donnees peuvent Stre interpr&tees d'une fayon consequente si l'on emploie le modele d'emission directe pour la formation des clust-ers et

le modele d'excita.tion du s1~hst:rat pour l'ionisation dcs clusters. L e r informations sur les temperatures ioniques du suhst.rat, les bnerqier d'ionisatior~ de clusters e t sur les confiqurations atomiques dl!

substrat sont rl&riv@es .?I partir d'une analyse des donndes.

Abstract

-

The process of formation of charged atomic clusters of a gene- ral form :B:A (3dn, mdl) emitted from A3B5 compounds (GaRs, InP, Gap, InSb) under the bombardment of slow rare gas primary ions (<2 keV) has been studied. The experimental data encompass the measurements of ion emission under the impact of He and ~ r + i ions with primary energies E P ranging from 0.2 keV to 2 keV and SNMS data on GaAs. The variation of E

P provides means of controlling the level of substrate electronic excita- tions. The data seem to be consistently interpreted using the direct emi- ssion model for the cluster formation and using the substrate excitation model for ionization of clusters. Information on substrate electronic temperatures, cluster ionization energies and the substrate atomic confi- gurations is deduced from the data analysis.

1

-

INTRODUCTION

Emission of cluster ions from solids during ion bombardment is the result of the atomic cluster formation processes in collision cascades and of subsequent ionizing charge transfer at the surface of the solid. It is usually difficult to assess unambiguously whether cluster formation or ionization are responsible for the observed larae variation of relative intensities of various cluster

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1989217

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i o n s s p u t t e r e d from t h e same sample b e c a u s e b o t h p r o c e s s e s a r e n o t y e t w e l l un- d e r s t o o d /I/. I t i s t h e purposd o f t h i s r e p o r t t o s t u d y t h e a p p l i c a t i o n o f t h e c o m p a r a t i v e l y w e l l e s t a b l i s h e d model o f monomer i o n i z a t i o n / 2 / i n A ~ compoun- B ~ d s ( f u r t h e r AB compounds) on c l u s t e r i o n i z a t i o n i n t h e s e m a t e r i a l s . The assum- e d knowledge of t h e i o n i z a t i o n p r o c e s s a l l o w s t o draw c e r t a i n c o n c l u s i o n s re- g a r d i n g t h e c l u s t e r f o r m a t i o n mechanism. The model h a s been t e s t e d u s i n g s e v e -

+ +

r a l AB compounds bombarded w i t h H e and K r p r i m a r y i o n s of v a r i o u s k i n e t i c e n e r g i e s E

P' 2

-

EXPERIMENTAL

The SIMS e x p e r i m e n t s w e r e c a r r i e d o u t u s i n g samples o f v a r i o u s AB compounds a n d t h e i r a l l o y s (A=Ga,In and B=P,As,Sb). I n t h i s p a p e r o n l y t h e measurements on b i n a r y s i n g l e c r y s t a l s Gap, GaAs, InP and InSb a r e d i s c u s s e d . The s a m p l e s

+ +

were bombarded u n d e r UHV c o n d i t i o n s w i t h He and K r i o n s w i t h e n e r g i e s E from 200 t o 2000 e V . The s e c o n d a r y i o n s were d e t e c t e d w i t h a u s u a l SIMS set- P -up e q u i p p e d w i t h e n e r g y and q u a d r u p o l e mass f i l t e r s . The e n e r g y r e s o l u t i o n o f t h e e n e r g y f i l t e r was a r o u n d 3 e V . To o b t a i n t o t a l i o n i n t e n s i t i e s t h e measur- e d e n e r g y d e s t r i b u t i o n was i n t e g r a t e d o v e r t h e whole e n e r g y r a n g e . The i n t e n - s i t i e s were a l s o c o r r e c t e d f o r t h e mass s e n s i v i t y dependence o f t h e qua- d r u p o l e s p e c t r o m e t e r . T y p i c a l r e s u l t s o b t a i n e d on Gap and InSb a r e shown i n F i g s . 1 and 2 where t h e i n t e n s i t i e s of s i n g l e and c l u s t e r i o n s a r e p l o t t e d a s a f u n c t i o n o f E They a r e n o r m a l i z e d i n s u c h a way t h a t t h e i n t e n s i t y o f A

+

P'

i o n s i s k e p t c o n s t a n t and e q u a l t o

l o 6

c / s . By f u l l and d o t t e d l i n e s a r e shown

+ +

measurements w i t h p r i m a r y K r and He i o n s , r e s p e c t i v e l y .

500 &lev)+ 1250 2000

F i g . 1

-

R e l a t i v e i n t e n s i t i e s J o f p o s i t i v e i o n s s p u t t e r e d from GaAs by K r

+

and He

+

i o n s o f v a r i o u s k i n e t i c e n e r g i e s E The i n t e n s i t i e s a r e n o r m a l i z e d i n P '

s u c h a way t h a t J o f ~ a + i s k e p t c o n s t a n t and e q u a l t o

lo6

c / s . F u l l and dash- e d l i n e s c o r r e s p c n d t o ~ r + and H e bombardment, r e s p e c t i v e l y .

+

and He

+

i o n s of v a r i o u s k i n e t i c e n e r g i e s E The i n t e n s i t i e s a r e normalized i n

+

^P'

such a way t h a t J o f I n i s k e p t c o n s t a n t and e q u a l t o

lo6

c / s . F u l l and

+ +

dashed l i n e s c o r r e s p o n d t o K r and He bombardment, r e s p e c t i v e l y .

There a r e s e v e r a l c h a r a c t e r i s t i c f e a t u r e s which a r e t y p i c a l f o r a l l measured samples o f AB compounds: a

-

The i n t e n s i t y o f B

+

i o n s ( d e n o t e d by J ( B + ) ) i s

+ + +

much s m a l l e r t h a n t h e i n t e n s i t y o f A i o n s J ( A ) ; b

-

The i n t e n s i t y J ( B ) i n c r e - a s e s w i t h E f o r K r

+

bombardment and d e c r e a s e s ( o r remains a l m o s t c o n s t a n t ) w i t h i n c r e a s i n g P E f o r ~ e + bombardment. T h i s f e a t u r e h a s been f i r s t observed

P

+ +

and a n a l y s e d i n e x p e r i m e n t s on GaAs / 3 / ; c

-

The i n t e n s i t i e s J ( A 2 ) and J ( A B )

d i f f e r by s e v e r a l o r d e r s o f magnitude and t h e i r r a t i o a s a f u n c t i o n o f E beha-

+ + +

p

+

v e s s i m i l a r l y a s t h e r a t i o J ( A ) / J ( B ) ; d

-

The i n t e n s i t i e s J ( A 3 ) and J ( A 2 B )

+ +

a r e n e a r l y t h e same;e

-

The i n t e n s i t i e s J ( A ) and J ( A B ) depend s t r o n g l y on E

+

^{2 P}

when He i o n s a r e used a s primary p a r t i c l e s . No d i r e c t i n d i c a t i o n o f c l u s t e r decomposition o u t s i d e t h e t a r g e t h a s been n o t i c e d i n t h e s e e x p e r i m e n t s and t h u s t h e decomposition p r o c e s s e s have been n e g l e c t e d i n t h e i n t e r p r e t a t i o n .

3

-

DISCUSSION

To e x p l a i n q u a n t i t a t i v e l y t h e v a l u e and t h e secondary i o n e n e r g y dependence of i o n i z a t i o n o f monomers s p u t t e r e d from AB compounds t h e c o n c e p t of high e l e c t r o - n i c t e m p e r a t u r e h a s been invoked /2/. T h i s h i g h t e m p e r a t u r e i s t r a n s i e n t l y pro- duced i n t h e r e g i o n of c o l l i s i o n c a s c a d e and i s r e s p o n s i b l e f o r t h e c h a r g e f o r m a t i o n o f m a j o r i t y o f s p u t t e r e d p a r t i c l e s . The n u m e r i c a l e s t i m a t i o n o f e l e - c t r o n i c e x c i t a t i o n s i n c o l l i s i o n c a s c a d e s and t h e d e s c r i p t i o n of time-space e v o l u t i o n of t h e e x c i t a t i o n s i n t e r m s o f s i m p l e t r a n s p o r t d i f f u s i o n e q u a t i o n y i e l d v a l u e s o f Te which a r e i n agreement w i t h e x p e r i m e n t a l l y determined Te.

The agreement between t h e o r y and e x p e r i m e n t s g i v e s c r e d i b i l i t y t o t h i s model,-

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o f i o n i z a t i o n though d i r e c t e x p e r i m e n t a l p r o o f o f t h e e x i s t e n c e o f h i g h T i s

+

^e

s t i l l l a c k i n g . W i t h i n t h e model t h e i o n i z a t i o n c o e f f i c i e n t P ( v ) depends upon t h e v e l o c i t y v of t h e a n a l y z e d s e c o n d a r y p a r t i c l e / 2 / and i s g i v e n by

+

^{Z + I ( v ' -@)}

P ( v ) =

-

e x p ( -

-

z0

kTe

where I ( v ) i s t h e v e l o c i t y dependent e f f e c t i v e i o n i z a t i o n e n e r g y of t h e s p u t t - e r e d p a r t i c l e and $3 i s t h e work f u n c t i o n o f t h e s u b s t r a t e . Z+ and Z O a r e t h e p a r t i t i o n f u n c t i o n s o f i o n i z e d and n e u t r a l s p u t t e r e d p a r t i c l e s , r e s p e c t i v e l y . I ( v ) depends upon t h e v e l o c i t y of t h e s p u t t e r e d p a r t i c l e i n a c o m p l i c a t e d way t h r o u g h t h e dependence o f t h e e f f e c t i v e i o n i z a t i o n e n e r g y on t h e d i s t a n c e of t h e p a r t i c l e from t h e s u r f a c e . However t h e e x p e r i m e n t s i n d i c a t e t h a t P+ i n AB compounds i s o n l y weakly v-dependent and t h u s f o r t h e p r e s e n t a n a l y s i s we may s i m p l i f y (1) by s e t t i n g I ( v ) = I , where I i s t h e i o n i z a t i o n e n e r g y of t h e p a r - t i c l e i n vacuum.

The t e m p e r a t u r e s Te of v a r i o u s substrates AB have been d e t e r m i n e d from t h e ex-

+ +

p e r i m e n t a l r a t i o s J ( A ) / J ( B ) and p l o t t e d a s a f u n c t i o n o f E i n F i g . 3 . The

+ +

^P

ratios J(A )/J(B ) can be advantageously used for obtaining values of T be- e c a u s e t h e number o f s p u t t e r e d p a r t i c l e s A i s known t o b e e q u a l t o t h e number of s p u t t e r e d p a r t i c l e s B. Thus J (A') /J (B') = ( P + ( A ) /P+ ( B ) ) ( J ( A ) /J ( B ) =P+ ( A P

+

(B)

,

+ +

where we assume P ( A ) < < l , P ( B ) < < l . The e q u a l i t y of n e u t r a l f l u x e s J ( A ) and J ( B ) h a s been proved e x p e r i m e n t a l l y by SNMS / 4 / i n t h e c a s e o f GaAs. The u s e o f r a t i o s of P

+

h a s a l s o t h e a d v a n t a g e t h a t p o s s i b l e e r r o r s due t o t h e n e g l e c - t e d v-dependence of P

+

a r e reduced. I t i s c l e a r from F i g . 3 t h a t t h e c h a r a c t e - r i s t i c dependence of T on E i s s i m i l a r i n t h e s t u d i e d f o u r compounds and

e P

seems t o be t y p i c a l f o r a l l AB compounds and t h e i r a l l o y s .

F i g . 3

-

E l e c t r o n i c t e m p e r a t u r e s o f v a r i o u s compounds a s a f u n c t i o n of t h e k i n e t i c e n e r g y of bombarding p a r t i c l e s . Bombarding p a r t i c l e s a r e e i t h e r K r

+

( f u l l l i n e s ) o r He

+

( d a s h e d l i n e s ) . The t e m p e r a t u r e s were deduced from (1) u s i n g t h e d a t a on monomers.

The a p p l i c a t i o n of (1) on c l u s t e r s i s hampered by t h e l a c k o f knowledge

p r e c i s e knowledge o f t h e s e p a r a m e t e r s . The l a r g e d i f f e r e n c e s i n a m p l i t u d e s of

+ +

J ( A 2 ) and J ( A B ) prove t h a t t h e s e dimers a r e s p u t t e r e d d i r e c t l y o r formed by a r e c o m b i n a t i o n mechanism v e r y c l o s e t o t h e s u r f a c e (<2

2).

The argument a g a i n s t t h e f o r m a t i o n o f AB molecules f a r away from t h e s u r f a c e by recombina- t i o n i s based on t h e o b s e r v a t i o n t h a t t h e m o l e c u l e s AB

+

would b e most p r o b a b l y formed by t h e recombination p r o c e s s A+-B and t h e molecules A2

+

by A+-A. As t h e numbers of B and A atoms a r e t h e same, such p r o c e s s e s would l e a d , c o n t r a r y t o

+ +

e x p e r i m e n t s , t o J ( A B ) = J ( A 2 ) . T h e r e f o r e i t i s c l e a r t h a t t h e d e g r e e o f i o n i z a - t i o n i s determined by t h e i o n i z a t i o n e n e r g i e s o f t h e m o l e c u l e s and n o t by t h e i o n i z a t i o n e n e r g i e s of t h e i r atomic c o n s t i t u e n t s . For example, t h e i o n i z a t i o n e n e r g y I ( G a 2 ) i s c e r t a i n l y much s m a l l e r t h a n I(GaAs) and t h u s t h e experimenta-

+ +

l l y o b s e r v e d r e l a t i o n J(GaAs ) < < J ( G a 2 ) can b e r e a d i l y e x p l a i n e d i n t e r m s o f ( 1 ) .

Using ~ e + primary p a r t i c l e s w i t h E =1 keV it h a s been shown by SNMS t h a t t h e P

numbers of s p u t t e r e d d i m e r s A 2 , AB and 3 a r e n e a r l y t h e same ( l a b . of p r o f . 2

Benninghoven, Munster, BRD, p r i v a t e communication). S i n c e we a r e u n a b l e t o s e e B2

+

i n o u r e x p e r i m e n t s we conclude t h a t t h e i o n i z a t i o n e n e r g y of B2 i s r a t h e r l a r g e . I f we assume t h e same t e m p e r a t u r e a s f o r monomers and suppose

+ +

Z (AB) / z O ( A B ) =Z ( A ~ ) / z O ( A ~ ) we can deduce from (1) t h e d i f f e r e n c e between t h e i o n i z a t i o n e n e r g i e s I (AB) and I ( A 2 )

.

From d a t a t a k e n w i t h E ( K r

+

) =2 keV t h e

P v a l u e s o f t h e e n e r g y d i f f e r e n c e s ( i n eV) a r e t h e f o l l o w i n g :

+ + +

The r a t i o J ( A ) / J ( A B ) s h o u l d i n c r e a s e w i t h t h e d e c r e a s e o f E ( K r ) b e c a u s e o f P

t h e d e c r e a s e o f Te (see F i g . 3 ) w i t h E However t h e o b s e r v e d change o f

P'

+

J (A') /J (AB') i s s l i g h t l y s m a l l e r t h a n e x p e c t e d . F o r t h e He bombardment t h e r e

+ +

i s a marked dependence of J ( A 2 ) and J ( A B ) on E ( s e e F i g s . 1 and 2 ) which i s c e r t a i n l y due t o t h e i n c r e a s e of t h e dimer f o r m a t i o n p r o b a b i l i t y w i t h t h e P

+ + + +

i n c r e a s i n g E (He )

.

The r a t i o of J ( A ) / J ( A B 2 ) i s a l m o s t i n d e p e n d e n t o f E (He )

P

+ + +

P

and i s g e n e r a l l y s m a l l e r t h a n J ( A 2 ) / J ( A B ) observed d u r i n g K r bombardment.

From t h e s e o b s e r v a t i o n s we deduce t h a t t h e number of s p u t t e r e d AB molecules i s p r o b a b l y l a r g e r t h a n t h e number o f A 2 molecules when E ( K r

+

) i s reduced o r

+ +

^P

when He i s used i n s t e a d of K r a s t h e primary p a r t i c l e . T h i s assumption i s n o t u n r e a s o n a b l e b e c a u s e t h e atomic c o n f i g u r a t i o n s i n u n p e r t u r b e d c r y s t a l s c e r t a i n l y f a v o u r AB e m i s s i o n o v e r A2 e m i s s i o n . I n more e n e r g e t i c c o l l i s i o n c a s c a d e s t h e dynamic mixing smears t h e l o c a l o r d e r i n g and l e a d s t o J ( A 2 ) = J ( A B ) a s proved by d i r e c t SNllS measurements ( l a b . o f . p r o f . Benninghoven, Miinster, BRD, p r i v a t e communication). S i m i l a r SNPlS e x p e r i m e n t s ' w o u l d be n e c e s s a r y t o prove t h a t t h e i n t e r e s t i n g r e l a t i o n J ( A B ) > J ( A 2 ) i s i n d e e d v a l i d d u r i n g t h e low e n e r g y He bombardment a t l e a s t i n some AB c r y s t a l s ( I n S b ) .

+ +

The i n t e n s i t i e s o f l a r g e r c l u s t e r s , J ( A 2 B ) and J ( A 3 ) , a r e o n l y weakly depend- e n t on E ( K r

+

) and t h e i r i n t e n s i t i e s a r e n o t much d i f f e r e n t . T h i s i s e x p e c t e d b e c a u s e t h e i o n i z a t i o n e n e r g y P I ( A 2 B ) c a n n o t d i f f e r much from I ( A 3 ) . Using t h e

-t

+

r e l a t i o n (1) and u s i n g t h e assumptions t h a t Z (A2B) = Z ( A 3 )

,

^{Z O ( A} 2 B ) = z O ( A ~ ) and

C2-98 JOURNAL DE PHYSIQUE

Te e q u a l s t o Te o f monomers we o b t a i n t h e f o l l o w i n g d i f f e r e n c e s of c l u s t e r i o - n i z a t i o n e n e r g i e s ( i n eV):

4

-

CONCLUSIONS

+ +

The c l u s t e r i o n s s p u t t e r e d from Gap, GaAs, InP and InSb b y K r and He i o n s have been a n a l y z e d u s i n g t h e s u b s t r a t e e x c i t a t i o n model of i o n i z a t i o n . The e f f e c t i v e e l e c t r o n i c t e m p e r a t u r e s a r e found t o be d i f f e r e n t f o r d i f f e r e n t ma- t e r i a l s b u t a r e t a k e n t o be t h e same f o r a l l monomers and c l u s t e r s s p u t t e r e d from t h e same compound. Within t h i s assum]?tion r e l a t i v e v a l u e s o f i o n i z a t i o n e n e r g i e s o f dimers and t r i m e r s have been e s t i m a t e d . To e x p l a i n c o n s i s t e n t l y t h e secondary i o n s p e c t r a an i n c r e a s e o f J ( A B ) / J ( A 2 ) i s assumed t o t a k e p l a c e

+ +

i n some AB c r y s t a l s when He i o n s a r e used i n s t e a d o f K r

.

T h i s a s p e c t o f c l u - s t e r e m i s s i o n from AB compounds needs t o b e confirmed by independent SMMS e x p e r i m e n t s .

REFERENCES

/1/ Winograd, N . , Prog. S o l i d S t a t e Chem. Vol. 1 3 , pp. 344, 1982 Pergamon P r e s s Ltd.

/ 2 / Groubek, Z . , Proc. o f t h e S i x t h I n t e r n a t i o n a l Conference on SIMS (SIMS V I ) , pp. 1 7 , 1988, John Wiley and Sons, and r e f e r e n c e s t h e r e i n .

/3/ Sroubek, Z., Appl. Phys. L e t t .

45

(1984)849.

/4/ Oechsner, H . , Thin f i l m and d e p t h p r o f i l e a n a l y s i s , S p r i n g e r V e r l a g 1384, B e r l i n , H e i l d e l b e r g , B e r l i n , Tokyo#, p.71.