PULSED-LASER ATOM-PROBE STUDY OF Si, GaAs AND GaP

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PULSED-LASER ATOM-PROBE STUDY OF Si, GaAs AND GaP

O. Nishikawa, E. Nomura, M. Yanagisawa, M. Nagai

To cite this version:

O. Nishikawa, E. Nomura, M. Yanagisawa, M. Nagai. PULSED-LASER ATOM-PROBE STUDY OF Si, GaAs AND GaP. Journal de Physique Colloques, 1986, 47 (C2), pp.C2-303-C2-308.

�10.1051/jphyscol:1986246�. �jpa-00225680�

JOURNAL DE PHYSIQUE

Colloque C2, suppl6ment au n03, Tome 47, mars 1986 page

PULSED-LASER ATOM-PROBE STUDY OF Si, GaAs AND Gap

0. NISHIKAWA, E. NOMURA, M. YANAGISAWA and M. NAGAI

Department of Materials Science and Engineering, The Graduate School at Nagatsuta, Tokyo Institute of Technology, 4259 Nagatsuta.

Midori-ku, Yokohama 227, Japan

Abstract - Si, GaAs and Gap were mass analyzed by the pulsed- laser energy-forcusing atom-probe. More than 30% of Si atoms were detected as clusters of up to Silo. The number of singly charged clusters is larger than that of doubly charged clusters.

No ~i~~~ and quadruply charged clusters were observed possibly due to the dissociation of densely charged ions by the Coulomb repulsive force. Although the stoichiometric compositions of GaAs and Gap were obtained, the composition of Gap appeared to vary with tip voltage. MostGaandPatomswere detectedas cluster ions and the size of the most abundant cluster shifts to the smaller side as the tip voltage increases. Laser-stimulated diffusion of surface atoms and the preferential field evaporation of Ga might be playing a key roll for the observed results.

I - INTRODUCTION

In the mass analysis of metals using a voltage-pulse atom-probe cluster ions of a few atoms have been hardly observed/l/. However, non-metal- lic elements such as As and P were detected as cluster ions while mass analyzing GaAs and GaP/2,3/. Pulsed-laser stimulated field evaporation led to the detection of cluster ions of metals/4/ and a large number of Si clusters/5,6/. Unexpected finding is the observation of mu1 iply charged large Si clusters such as ~ i ~i~~'. sig4+ and SiljZir and ~ ~ ~ ~ . possibly ~ i ~ ~ ~ + / 6 / . The purpose of the present study is to explore the formation mechanism of clusters by examining the variation of the size and abundance of clusters with tip voltage, tip temperature and laser power,

I1 - Si CLUSTERS

The energy focusing time-of-flight atom-probe with a 3 m l o n g flight

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986246

~ 2 - 3 0 4

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p a t h / 7 / and a n i t r o g e n l a s e r w i t h a peak power o f 2 0 0 k W a n d t h e p u l s e w i d t h o f 0 . 8 n s e c w e r e u s e d . The mass s p e c t r u m o f S i a n a l y z e d a t 3 0 0 K

, F i g . 1, i n d i c a t e s t h a t more t h a n 60 ^% o f 6 1 4 1 i o n s d e t e c t e d a r e si+

a n d s i 2 + . The number o f c l u s t e r s d e c r e a s e d w i t h s i z e a n d t h e l a r g e s t c l u s t e r a p p e a r s t o b e S i l o . Tsong r e p o r t e d t h a t m o s t c l u s t e r s a r e d o u b l y i o n i z e d a n d some a r e q u a d r u p l y c h a r g e d / 6 / . The e x p a n d e d s p e c -

t r u m , F i g . 2 , i n d i c a t e s t h a t t h e m a s s l i n e s a t m/n= 2 8 , 29 a n d 30 a r e c l e a r l y s e p a r a t e d b u t n o m a s s l i n e s a t 28.5 a n d 29.5 c o r r e s p o n d i n g t o 28Si29Si2+ a n d 2 9 S i 3 0 ~ i 2 + a r e s e e n i n d i c a t i n g n o d e t e c t i o n Of S i 2 2+ .

The t o t a l number o f i o n s i n t h e m a s s r a n g e from 56 t o 60 i s 499. The d e t e c t i o n o f 40 i o n s w i t h t h e m a s s 5 6 . 5 a n d 5 i o n s w i t h t h e mass 5 7 . 5

i n d i c a t e s t h e e x i s t e n c e o f ~ i 4 ~ ' . The n a t u r a l a b u n d a n c e s o f t h e i o n s w i t h t h e s e m a s s - t o - c h a r g e r a t i o s a r e 1 4 . 7 % a n d 1 . 5 % o f Al- t h o u g h t h e d e t e c t e d number o f t h e s e i o n s a r e n o t l a r g e e n o u g h t o e s t i - m a t e t h e numbers o f si2+ a n d ~ i i o n s a c c u r a t e l y , t h e number o f ~ ~ + si2+

is n e a r l y e q u a l t o o r more t h a n t h a t o f S i 2 + . S i m i l a r l y t h e number o f

si3+ c l u s t e r s i s n o t s m a l l e r t h a n t h a t o f !!is2+. The i n t e r g r a l mass

l-

+ ^{2 c} 2+

l i n e s o f S i , S i 5 and si6+ s u g g e s t t h a t t h e numbers o f S i 8 , S i l o a n d ~ i i r e t o o s m a l l t o b e d e t e c t e d o r t h e s e c l u s t e r i o n s a r e n o t ~ ~ ~ + f o r m e d . T h e r e f o r e t h e number o f s i n g l y c h a r g e d i o n s i s l a r g e r t h a n t h e number o f d o u b l y c h a r g e d c l u s t e r s .

T h e m o s t a b u n d a n t c l u s t e r i s S i 4 . S i 6 , S i 3 , S i 2 a n d S i 5 f o l l o w i n t h e o r d e r Of a b u n d a n c e . Q u a d r u p l y c h a r g e d c l u s t e r s , ~ i s i q 4 + ~ a n d ~ i ~ ~ ~ + / 6 / ~ + were ~

2+ -

n o t f o u n d . D e n s e l y c h a r g e d i o n s , s u c h a s S i 2 a n d m i g h t be d i s s o c i a t e d s h o r t l y a f t e r f i e l d e v a p o r a t i o n by t h e Coulomb r e p u l s i v e f o r c e a n d t h e d i s s o c i a t e d f r a g m e n t s c a n n o t p a s s t h e d e f l e c t e d p a t h .

I 1 1 - AS CLUSTERS AND OBSERVED COMPOSITION OF GaAs

I n t h e v o l t a g e - p u l s e a t o m - p r o b e m a s s a n a l y s i s t h e s t o i c h i o m e t r i c com- p o s i t i o n o f GaAs was h a r d l y o b t a i n e d u n l e s s t h e v o l t a g e s o f d e f l e c t o r e l e c t r o d e s a r e a d j u s t e d t o p a s s t h e s l o w Ga i o n s t h r o u g h t h e d e f l e c t o r 3 P u l s e d - l a s e r s t i m u l a t e d f i e l d e v a p o r a t i o n made p o s s i b l e t o o b t a i n t h e e x p e c t e d c o m p o s i t i o n . However, m o s t A s i o n s w e r e d e t e c t e d a s

~ i ;

ond

U t 3 0 0 / Q)

l l ^l

Silicon , ^{1.8 kV}

300 K

F i g . 1 - Mass s p e c t r u m o f S i t a k e n by p u l s e d - l a s e r s t i m u l a t e d f i e l d

e v a p o r a t i o n . B i n w i d t h is 0 . 1 amu.

'l0270

Si , 1.8 kV,300K si+

Sif

and

I

Mass to charge ratio m/n

Fig. 2 - Expanded mass spectrum of Si. Bin width is 0.01 amu.

clusters such as AS^+ ^{and As5} + . The abundance of large clusters decreases with the increase of tip voltage and the decrease of laser power. Figure 3 shows that the shift of the most abundant cluster from AS^' to AS^+ by the increase of the tip voltage from 4.0 kV to 7.0 kV. The observed composition is close to the stoichiometric compositon and no significant change with voltage and temperature was noticed.

IV - P CLUSTERS AND OBSERVED COMPOSITION OF Gap

The apparent composition of Gap and the distribution of clusters vary significantly with tip voltages. When the tip voltage V is 4 2 % 0 f the hydrogen imaging voltage VH2, the ratio of the number of detected P atoms to that of Ga is only 0.18, Fig. 4(a). As the tip voltage rises, the abundance 0f.P increases and the stoichiometric composition of Gap is obtained at V/VH2= 0.93, Fig. 4(c). The most abundant P cluster also shifts from P4 at V/VH2 = 0.63, Fig. 4(b), to P2 at V/VH2= 1.20, Fig. 5(b). The mass spectrum obtained by the voltage-pulse atom-probe

at V/VEI =1.46, Fig. 5(c), is quite similar to the spectrum shown by

Fig. 5(8) except the broader mass lines of Fig. 5(b) possibly due to the photoexcitation effect/8/.

The shift of the most abundant cluster with tip voltage can be explain- ed as follows. The formation of clusters is caused by the promoted diffusion of surface atoms by the thermal effect of laser irradiation and the bigger the cluster size is, the lower the evaporation field is.

Thus at a low field clusters can grow up but at a high field clusters

are field evaporated before they grow.

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Moss to charge ratio m/n

40..

30-

Fig. 3 - Mass spectrum of GaAs taken at room temperature by stimulating field evaporation with a pulsed laser. (a) Tip voltage V = 4.0 kV and the ratio of the number of detected As atoms NAs to that 86 ^{Ga atoms}

N G ~ is 0.91. (b) VDC

7.0 kV, NAs/NGa = 1.07.

Figures 4 and 5 also show the variation of observed composition of Gap with tip voltage. The deficiency of Ga at high voltages can be attri- buted to the preferential field evaporation of Ga/3/ and the overabun- dance of Ga at low voltages to the diffusion of Ga from the tip shank to the tip apex by the.therma1 effect of laser irradiation.

to 300

V - CONCLUSION t

The present study with the pulsed-laser atom-probe indicated that the most abundant Si cluster is Si4. The number of singly charged cluster

ions is larger than that of doubly charged clusters. No s i 2 2 + nor quadruply charged ions were observed possibly due to the dissociation by the Coulomb repulsive force. The stoichiometric composition of GaAs and Gap are obtained. Observed compositions and mass spectra of GaAs and Gap do not vary noticeably with tip temperature nor with laser power but with tip voltage. The formation of cluster is explained tentatively as the result of promoted diffusion of surface atoms by the thermal effect of laser beam and the shift of the most abundant cluster with tip voltage is attributed to the decrease of the evapora- tion field of cluster with the increase of cluster size.

to 170

(a)

6 9 ~ ~ + 71&

GaAs, Laser Pulse

Room Temp. Lbc24.0kV

NAs /NGaa0.91

( a ) Gap , 2 0 K

Laser Pulse v/vH2=o.42 Np/NGa=0.18

60 40 20

0 , : : ^{, ,} : . " .

.

20 40 60 80 100 I20 l40

V)

V/VH,=0.63

b, Np/NGa-1.10

Q) 40 U

C

l!\

1 : : : : : : : :

0 2 0

ki

: : : ; :

fi 0

. .,

I b : !

I : : :

I00

I :

Z V/V,,= 0.93

c )

Np/NGa=l .02

ass to charge ratio m/n

F t g . 4 - V a r i a t i o n o f m a s s s p e c t r u m and o b s e r v e d c o m p o s i t i o n o f Gap w l t h t i p v o l t a g e . The s p e c t r a w e r e o b t a i n e d by t h e p u l s e d - l a s e r atom- p r o b e . The r a t i o o f t h e t i p v o l t a g e V t o t h e h y d r o g e n i m a g i n g v o l t a g e V H ~ and t h e r a t i o o f t h e number o f d e t e c t e d P a t o m s Np t o t h a t o f Ga a t o m s NGa a r e i n d i c a t e d i n e a c h f i g u r e , F i g . 4 ( a ) , ( b ) a n d ( c ) .

REFERENCES

/l/ ~ c l l e r , E . W. a n d T s o n g , T. T . , P r o g . S u r f . S c i . 4 ( 1 9 7 3 ) P a r t 1.

/2/ N i s h i k a w a ,

Kaneda, O., S h i b a t a , . M . a n d Nomura, E . , Phys. Rev.

~ e t t . 53 ( 1 9 8 4 ) 1 2 5 2 .

/3/ ~ i x i k a w a , O., Nomura, E., Kawada, H and O i d a , K . , P r o c . 32nd I n t e r n a t i o n a l F i e l d E m i s s i o n Symposium ( 1 9 8 5 ) .

/4/ D r a c h s e l , W . , J e n t s c h , Th. a n d B l o c k , J. H . , P r o c . 2 9 t h I n t e r - n a t i o n a l F i e l d E m i s s i o n Symposium, A l m q v i s t a n d W i k s e l l ~ n t e r n a t i o n a l , S t o c k h o l m , ( 1 9 8 2 ) p. 299.

/5/ K e l l o g g , G. L . , A p p l . S u r f . S c i . 11/12 ( 1 9 8 2 ) 1 8 6 . / 6 / T s o n g , T. T . , Appf. Phys. L e t t . 45 ( 1 9 8 4 ) 1 1 4 9 .

/7-/Nishikawa, O., K u r l h a r a , K . , N a c h i , M - , K o n i s h i , M. a n d Wada, M . 1 R e v . S c i . I n s t r u m . 52 ( 1 9 8 1 ) 810.

/8/ T s o n g , T. T . , ~ Y d e P h y s i q u e 45-C9 ( 1 9 8 4 ) 83.

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6 0

(a)

Gap, 2 0 K V/VH2 =!.l0 Np/N~a10.79

20 10

a

I : . : : /

^Pi+

20 40 60 80 100 120 140

In

(b) V/Vn2= 1.20

C

Np/NGa*0.89

Q

L

a 1

E m

(c)

69Ga+ Voltage Pulse

P+ Np/NGa=1.20

P2+ 71Ga+

40 20

20 40 60 80 100 120 140

Mass to charge ratio m/n

Fig. 5 - Variation o f m a s s spectrum and observed composition o f G a p with t i p voltage. (a) and (b) a r e taken by pulsed-laser field evapo- ration. (c) taken by voltage-pulse atom-probe at V/VH =1.46.

2