MASS ANALYSIS OF GaAs AND GaP BY THE COMBINED ATOM-PROBE

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MASS ANALYSIS OF GaAs AND GaP BY THE COMBINED ATOM-PROBE

O. Nishikawa, E. Nomura, H. Kawada, K. Oida

To cite this version:

O. Nishikawa, E. Nomura, H. Kawada, K. Oida. MASS ANALYSIS OF GaAs AND GaP BY

THE COMBINED ATOM-PROBE. Journal de Physique Colloques, 1986, 47 (C2), pp.C2-297-C2-

302. �10.1051/jphyscol:1986245�. �jpa-00225679�

JOURNAL DE PHYSIQUE

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

c2-297

MASS ANALYSIS OF GaAs AND Gap BY THE COMBINED ATOM-PROBE

0. NISHIKAWA, E. NOMURA, H. KAWADA

and

K. OIDA

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

o f

Technology, 4259 Nagatsuta, Midori-ku, Yokoharna 227, Japan

Abstract - A time-of-flight atom-probe with straight and deflect- ed flight paths was constructed. One section of the outer elec- trode of the Poschenrieder type deflector is formed by 3mmapart parallel

W

wires of 0.3mmin diameter to pass straightly flying

ions. The distribution of ion energy examined by the straight path A-P indicates the evaporation of slow ions with low ener- gies. The adjustment of the voltages of the deflector electrodes to let the slow ions pass through the deflector made possible to obtain the stoichiometric composition of GaAs but not of Gap because the energy of Ga ions evaporated from Gap was signifi- cantly lower than that of P. Accordingly the erroneous composi- tion of GaAs and Gap observed

by

the

A-P

analysis is the result of the preferential field evaporation of unstabilized Ga atoms due to the evaporation of nearby surface atoms, possibly As and

P.

I - INTRODUCTION

Immediately after the introduction of atom-probe to the mass analysis of compound semiconductors, it has been realized that the instruments tend to fail to give the stoichiometric composition/lr2/. The mass analysis of GaAs and Gap resulted in the deficiency of Ga/2-4/. The cause of the deficiency was attributed to the preferential field evaporation of Ga at a low field exerted by the dc voltage during the intervals between successively applied superposing voltage pulses/3/

and to the formation of vacancy clusters due to preferential evapora- tion of As/4/. The purpose of the present study is to clarify the evaporation process of GaAs and Gap fully utilizing the capability Of the newly constructed combined atom-probe.

I1

- COMBINED ATOM-PROBE

The comblned atom-probe.has two flight paths, straight and deflected, Fig. 1. The straight flight path is 1.2mlong and terminated

by

a

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986245

JOURNAL DE _PHYSIQUE

c h e v r o n d e t e c t o r o f 7 5 m m i n d i a m e t e r . The o t h e r f l i g h t p a t h is d e f l e c t e d by t h e P o s c h e n r i e d e r t y p e e l e c t r o d e s a n d i s 2 . 5 m l o n g . F o r t h e s t r a i g h t f l i g h t p a t h o n e s e c t i o n o f t h e o u t e r d e f l e c t o r e l e c t r o d e , 70mmX177mml i s c u t o p e n a n d t h e 3 m m a p a r t p a r a l l e l W wires o f 0.3mm i n d i a m e t e r f o r m t h e o p e n s e c t i o n o f t h e e l e c t r o d e . The t i p - t o - s c r e e n d i s t a n c e c a n b e c h a n g e d f r o m 50 t o 125mm. S i n c e t h e maximum a c c e p t a n c e a n g l e t o t h e p r o b e h o l e i s a s w i d e a s 5.7', more t h a n 200 i o n s a r e d e t e c t e d from o n e a t o m i c l a y e r o f t h e ( 0 1 1 ) p l a n e o f t h e W t i p w i t h t h e t i p r a d i u s o f 45 nm.

I n o r d e r t o a c c e p t t h e i o n s d i s t r i b u t e d i n t h e w i d e a c c e p t a n c e a n g l e t h e 1 7 8 m m h i g h o u t e r a n d i n n e r d e f l e c t o r e l e c t r o d e s a r e s e t 100mm a p a r t . The t o p a n d b o t t o m o f t h e d e f l e c t o r s p a c e a r e p a r t i t i o n e d b y t w o s e t s o f 6 c o n c e n t r i c s t a i n l e s s s t e e l w i r e s a t t h e p o t e n t i a l s p r o p o r t i o n a l t o t h e d e f l e c t o r v o l t a g e s t o r e d u c e t h e d i s t o r t i o n o f t h e e l e c t r i c f i e l d i n t h e d e f l e c t o r s p a c e by t h e g r o u n d e d m e t a l p l a t e s e n c a s i n g t h e e l e c t r o d e s .

I11

-

EXPERIMENTAL

P u l s e v o l t a g e s a p p l i e d t o a s e m i c o n d u c t o r t i p a t t e n u a t e s i g n i f i c a n t l y w h i l e i t t r a v e l s t h r o u g h t h e t i p s h a n k and t h e e f f e c t i v e p u l s e v o l t a g e a t t h e t i p a p e x is l o w e r t h a n t h e a p p l i e d v o l t a g e . T h u s t h e o b s e r v e d

F i g . 1 - S c h e m a t i c o f t h e combined a t o m - p r o b e .

m a s s e s o f i o n s s h i f t t o t h e h e a v i e r s i d e . The a d v a n t a g e o f t h e

s t r a i g h t t y p e A-P is t h a t i t a l l o w s u s t o m e a s u r e t h e mass s h i f t , t h a t i s , t h e e f f e c t i v e p u l s e v o l t a g e . The e f f e c t i v e p u l s e v o l t a g e s f o r Zn- doped GaAs w i t h t h e r e s i s t i v i t y o f 3 X 1 0 - ~ Q . c m a n d f o r S-doped Gap w i t h 3~ 1 0 - ~ ~ . c m a r e 9 5 % a n d 7 5 % o f t h e a p p l i e d v o l t a g e s , r e s p e c t i v e l y . F i g u r e 2 shows t h e v a r i a t i o n o f m a s s s p e c t r u m o f GaAs w i t h p u l s e f r a c - t i o n f , t h e r a t i o o f t h e e f f e c t i v e p u l s e v o l t a g e Vp t o t h e t o t a l . t i p v o l t a g e , VDC+Vp. The h o r i z o n t a l s o l i d l i n e s w i t h a s h o r t v e r t i c a l b a r a t e n d s i n d i c a t e t h e m a s s r a n g e s o f 6 9 ~ a + a n d 7 1 ~ a + e v a p o r a t e d a t t h e v o l t a g e s b e t w e e n VDC+Vp a n d VDC a n d c o v e r n e a r l y a l l Ga i o n s d e t e c t e d . Then t h e n u m b e r s o f Ga a n d A s i o n s w e r e c o u n t e d a c c u r a t e l y u t i l i z i n g t h e h i g h m a s s r e s o l u t i o n o f t h e d e f l e c t o r t y p e A-P. S i n c e t h e d e f l e c t o r s p a c e o f t h i s A-P i s w i d e , i o n s c a n p a s s t h r o u g h t h e s p a c e e v e n i f t h e i o n e n e r g y v a r i e d a b o u t f 7 % . Thus t h e d e f l e c t o r v o l t a g e s w e r e s e t f o r t h e i o n s w i t h t h e e n e r g y 7% l o w e r t h a n f u l l y

55 30

1 0 5

Mass to charge ratio

m/n

F i g . 2

-

Mass s p e c t r a o f GaAs f o r v a r i o u s p u l s e f r a c t i o n f , t h e r a t i o o f t h e p u l s e v o l t a g e V p t o t h e t o t a l t i p v o l t a g e V t = V D C + V p . H o r i z o n - t a l l i n e s w i t h a v e r t i c a l b a r a t e n d s i n d i c a t e t h e r a n g e s o f a p p a r e n t m a s s e s o f Ga e v a p o r a t e d a t t h e v o l t a g e b e t w e e n V t a n d VDC.

C2-300 JOURNAL DE PHYSIQUE

accelerated ions. Then GaAs was analyzed applying the pulse voltage of f=13.5%, that is, the ions evaporated even at VDC can pass through the deflected space. Accordingly the obtained mass spectrum exhibited the stoichiometric composition, Fig. 3.

Fig the

Mass to charge ratio m/n

.

³

- Mass spectrum of GaAs analyzed by the atom-probe operated as deflector type.

Similar analysis of Gap by the straight flight path atom-probe

resulted in broad mass peaks of Ga extending beyond the expected mass ranges indicated by the horizontal lines, Fig. 4. Therefore, even if the deflector voltages were adjusted to pass the maximum number of Ga ions through the deflector, the ratio of the number of Ga ions to As ions was found to be

0.68,

Fig.

5.

The observed wide mass range for Gap can be explained as follows.

Surface Ga atoms might become unstable by the evaporation of nearby

P

atoms. These unstable Ga atoms may field evaporate even at VDc shortly after the evaporation of the nearby atoms, mostly in a few hundred nanoseconds. Highly unstable atoms may evaporate immediately after the evaporation of the nearby atoms, possibly in a few nano- seconds while the pulse voltage is undershooting. Thus the tip

voltage at the time of evaporation is below VDC. The delayed evapora- tion time of more than a few ten nanoseconds can be noticed as a shift of Ga mass peaks to the heavier side when these ions are analyzed through the deflector. Thus the deflector voltages were lowered to pass the delayed low energy ions through the deflector. While the number of As ions decreased with the lowering deflector voltages, low energy Ga ions were detected but no mass shift was noticed, Fig.

6.

This result indicates that Ga atoms of Gap are highly unstabilized by the evaporation of the nearby atoms and evaporate even at the voltages below VDC .

IV

- CONCLUSIONS

The present study clarified that the erroneous compositions of GaAs

and

Gap

obtained by the voltage-pulse atom-probe analysis is the

result of the preferential field evaporation of Ga at the voltages

significantly lower than Vt. The observed result also indicates that

the evaporation of As and P atoms makes thenearby Ga atoms unstable

and induces the preferential field evaporation of Ga.

30 F i g . 4 - Mass s p e c t r a

25 o f G a p f o r v a r i o u s

p u l s e f r a c t i o n s .

20 a ) f = 1 0 . 6 % .

15 b) f = 1 6 . 7 %

10 C ) f = 2 7 . 1 %

d ) f = 3 8 . 7 %

5 0

0

30 25 20 15 10 5

0 0

Mass to charge ratio

m/n

Mass to charge ratio m/n

30

-

F i g . 5

-

Mass s p e c t r u m o f G a p a n a l y z e d b y t h e d e f l e c t o r t y p e A-P.

V ) .

I = .

.Q 25 U .

g

^{2 0 '}

Gn/P=O. 68

s"Go+ V1 =B. 66kV

f =26.1%

7 ' ~ a +

p: '0

%

15

0

l

^{I l l}

I ^{/ l I}

^l I / l l. l 0 ' I I II

20 40 60 80 100 120 140 160

c. 0 1 0 .

$ : n

S

^{5 .}

2 .

P+

P+ _P$+

Pz'

c2-302 JOURNAL

DE

PHYSIQUE

Ie) E n e r g y of t o n - 1 - 0 0 60Ga+

! b l E n e r g y o f L o n = 0 . 9 4

F i g . 6

-

V a r i z t i o n o f m a s s s p e c t r a o f Gap w i t h t h e l o w e r i n g d e f l e c t o r v o l t a g e s . a ) The d e f l e c t o r v o l t a g e s a r e s e t f o r t h e i o n s w i t h t h e f u l l e n e r g y a c c e l e r a t e d by Vt' b ) F o r t h e i o n s w i t h

t h e e n e r g y a c c e l e r a t e d by t h e t i p v o l t a g e ' 0.94 t i m e s Vt.

c ) 0 . 9 1 t i m e s Vt.

d ) 0 . 8 4 t i m e s Vti.

f = 2 6 % .

( a 1 E n e r g y o f lon=0.91

(d1 E n e r g y of l c n = 0 . 8 4

Mass

to

charge ratio

m/n

REFERENCES

/l/ T s o n g , T. T . , N g , Y . S. and Melmed, A . J . , S u r f . S c i .

77

( 1 9 7 8 ) ~ 1 8 7 . / 2 / Yamamoto,M., S e i d m a n , D . N. a n d N a k a m u r a , S . , S u r f . S c i . 1 1 8 ( 1 9 8 2 ) 5 5 5 .

/3/ N i s h i k a w a , O . , Kawada, H . , N a g a i , Y. a n d Nomura, E . , J . d e P h y s i q u e 45-C9 ( 1 9 8 4 ) 465.

/4/ S a k u r a i , T . , Hashizume, T . , J i m b o , A. a n d S a k a t a , T . , J d e P h y s i q u e 45-C9 ( 1 9 8 4 ) 453.