MINORITY CARRIER LIFETIME MEASUREMENTS IN SEMICONDUCTOR DEVICES MONITORED BY A MICROPROCESSOR IN E.B.I.C, MODE

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MINORITY CARRIER LIFETIME MEASUREMENTS IN SEMICONDUCTOR DEVICES MONITORED BY

A MICROPROCESSOR IN E.B.I.C, MODE

J. Bresse, D. Rivière

To cite this version:

J. Bresse, D. Rivière. MINORITY CARRIER LIFETIME MEASUREMENTS IN SEMICONDUC-

TOR DEVICES MONITORED BY A MICROPROCESSOR IN E.B.I.C, MODE. Journal de Physique

Colloques, 1984, 45 (C2), pp.C2-865-C2-868. �10.1051/jphyscol:19842198�. �jpa-00223874�

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JOURNAL DE PHYSIQUE

Colloque C2, suppl6ment au n02, Tome 45, f6vrier 1984 page C2-865

MINORITY CARRIER LIFETIME MEASUREMENTS IN SEMICONDUCTOR DEVICES MONITORED BY A MICROPROCESSOR IN E , B , I , C , MODE

J . F . ~ r e s s e ' and D . RiviSre

Lab. &l<croscopie Electronique, U.S. T . L., Place BatuiZZon,

34060 Montpellier Cedex, France

Resume

.

Les mesures de decroissance du courant i n d u i t (E.B. I.C.) genere par un f a i s - c e a u ' e l e c t r o n s pres d'une jonction peuvent devenir une technique t r s s f l e x i b l e pour l a determination de l a dur6e de vie des porteurs m i n o r i t a i r e s dans un semiconducteur, s i l'ensemble de l ' e x p e r i e n c e e s t p i l o t 6 par un microprocesseur. Cette technique, adaptee dans n o t r e c a s aux semiconducteurs comme l e s i l i c i u m e t l e germanium, e s t un puissant moyen d ' i n v e s t i g a t i o n s i e l l e e s t couplee aux mesures en E.B.I.C. continu comme nous l e montrons dans l e cas de c e l l u l e s s o l a i r e s s i l i c i u m .

Abstract

.

Electron Beam Induced Conductivities (E.B. I .C.) t r a n s i e n t measurements monitored by a microprocessor can be a very f l e x i b l e technique f o r the measurement of minority c a r r i e r s l i f e t i m e in semiconductors. This technique adapted in our case f o r s i l i c o n and germanium becomes a very powerful tool in conjunction with continuous E.B.I.C. measurements, e s p e c i a l l y i n t h e case of s i l i c o n s o l a r c e l l s .

Fundamentals of E.B.I.C. mode

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Diffusion length and l i f e t i m e measurements

When an e l e c t r o n beam impringes on a semiconductor m a t e r i a l , t h i s gives r i s e t o t h e c r e a t i o n of an excess of minority c a r r i e r s , f o r example in N-type semiconductor

q e l e c t r o n charge, Eo beam energy, I B beam i n t e n s i t y , R e l e c t r o n range, Ep hole- e l e c t r o n p a i r energy c r e a t i o n , kn correction f a c t o r f o r backscattered e l e c t r o n s ,

~p hole 1 i f e t i m e .

In low i n j e c t i o n conditions (an

<<

n e l e c t r o n c o n c e n t r a t i o n ) , t h e o r e t i c a l mode- l i z a t i o n of the penetration of e l e c t r o n s , generation of minority c a r r i e r s and t h e c o l l e c t i o n of t h e c a r r i e r s giving r i s e t o t h e Electron Beam Induced Current (E.B.I.C.) by a junction perpendicular t o t h e s u r f a c e o r p a r a l l e l t o t h e s u r f a c e have been well supported in t h e past ten years ( 1 ) ( 2 ) ( 3 ) and more recently f o r t h e t r a n s i e n t a n a l y s i s ( 4 ) ( 5 ) . The continuous E.B. I.C. measurement permits t o d i r e c t l y achieve t h e value of t h e d i f f u s i o n length of t h e minority c a r r i e r s which represents the average value reached by t h e excess of created minority c a r r i e r s before t o be recombined and corresponds t o a global value on t h e volume covered by the scanned a r e a . The same measurement can a l s o give t h e surface recombination v e l o c i t y ( 6 ) .

Transient E.B.I.C. can give using t h e slope of the curve measured in l o g a r i t h - mic s c a l e , t h e minority c a r r i e r l i f e t i m e , a f t e r a c o r r e c t i o n f a c t o r which takes i n t o account of t h e s u r f a c e recombination and t h e distance-spot junction. (The beam i s fixed in t h i s expbriment). The l i f e t i m e value corresponds t o a more punctual volume which takes i n t o account of t h e presence of punctual d e f e c t s which a c t a s t r a p s f o r t h e c a r r i e r s . Following SHOCHLEYSIALL- READ(^) t h e 1 ifetime can be expressed a s :

T P = ( a p N t vth)-1

o capture c r o s s section of t r a p s , N t number of t r a p s , vth thermal velocity of ePectrons.

Diffusion length and l i f e t i m e a r e r e l a t e d by Lp2 = Dp ^T~ (LIP d i f f u s i o n cons- t a n t of holes)

.

' p r e s e n t address : C . N . E . T . , PMM/SPD, 196 rue de P a r i s , 92220 Bagneux, France

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19842198

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C2-866 JOURNAL DE PHYSIQUE

The presence o f e l e c t r i c a l l y a c t i v e d e f e c t s ( p o i n t - l i n e o r l i n e - s h a p e 9 d e f e c t s ) can be i n t e r p r e t e d as a p e r t u r b a t i o n w i t h a l i f e t i m e equal t o ^{T D}= (ND ^OD v t h 1 - l where ND i s t h e d e n s i t y o f r e c o m b i n a t i o n c e n t r e s i n s i d e t h e d e f e c t and T D t x e d e f e c t c a p t u r e c r o s s - s e c t i o n . The measured l i f e t i m e i s i n t h i s case a c o m b i n a t i o n o f two l i f e t i m e s so ^T ^- ^~= T ~ - ' TD-'.

I n i n d i r e c t bandgap semiconductors l i k e s i l i c o n and germanium t h e d i f f u s i o n l e n g t h v a l u e can r e a c h s e v e r a l t e n s o r hundreds o f microns and i n t h a t case t h e e l e c - t r o n p e n e t r a t i o n volume can be approximate by a sphere o r a p o i n t - s o u r c e . The l i m i - t a t i o n s and accuracy on t h e d e t e r m i n a t i o n o f t h e parameters l i f e t i m e and d i f f u s i o n l e n g t h a r e a c t u a l l y c l e a r l y d e f i n e d t a k i n g i n t o account o f t h e v a l u e o f t h e s u r f a c e r e c o m b i n a t i o n ( 8 ) .

M i n o r i t y c a r r i e r l i f e t i m e measurement by EBIC m o n i t o r e d by a m i c r o p r o c e s s o r F o r a range o f l i f e t i m e v a r i p i n g f r o m 100 nsec t o s e v e r a l ysec a system where a m i c r o p r o c e s s o r p i l o t s t h e o b t a i n i n g o f t h e t r a n s i e n t decay o f t h e EBIC f o r a beam f i x e d a t a c e r t a i n d i s t a n c e f r o m t h e j u n c t i o n and g i v e s t h e d a t a p r o c e s s i n g has been e s p e c i a l l y designed f o r t h i s purpose ( 9 ) .

(, )

ELECTRON GUN BEAM BLANKING

-I___

. - - - . - -

I

CONVERTER MICdOPROCESSOR r 7---I-:

CHOICE OF PROGRAM

DURATION OF THE BEAM BLANKING

I "

FREQUENCY OF THE CYCLE

F i g . 1 : System u s i n g a m i c r o p r o c e s s o r f o r t h e l i f e t i m e measurement.

F i g u r e 1 shows t h e p r i n c i p l e o f t h e system. The c l o c k o f t h e m i c r o p r o c e s s o r generates t h e p u l s e s f o r t h e beam b l a n k i n g where t h e o f f p e r i o d can be v a r i e d f r o m 10 ysec t o 1 msec. The r i s e t i m e o f t h e v o l t a g e p u l s e s a p p l i e d t o t h e e l e c t r o - s t a t i c p l a t e s i s l i m i t e d by t h e r i s e t i m e o f t h e power a m p l i f i e r which t r a n s m i t s t h e v o l t a g e t o t h e p l a t e s .

I n o u r system t h e r i s e t i m e i s a b o u t 50 nsec, l o w e r t h a n t h e range o f l i f e t i m e measured. The a m p l i f i c a t i o n o f t h e s i g n a l i s made by a wideband a m p l i f i e r and t h e beam c u r r e n t i s a d j u s t e d t o r e a c h 5 v o l t s peak t o peak f o r t h e o u t p u t o f t h e amp1 i f ie r ( u s u a l l y i n l o w i n j e c t i o n c o n d i t i o n ) . A sampler-holder g i v e s t h e decay c u r v e a s f u n c t i o n o f t i m e p o i n t by p o i n t u s i n g a sampling i n t e r v a l o f 50 nsec d u r i n g a t i m e o f 5 ysec. Data a c q u i s i t i o n and a v e r a g i n g ( u p t o 16 t i m e s ) i s made by a m i c r o - p r o c e s s o r w h i c h c a n p l o t t h e decay c u r v e on a screen o r on a X-Y p l o t t e r o r p r i n t t h e

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n u m e r i c a l d a t a .

Examples o f use o f t h e system on s i l i c o n and germanium de- a ) $ j l j c " - s o l ~ - c g ~ 1 ~ ( p o l y c r i s t a l l i n e N-type l a r g e g r a i n s )

The c o n t i n u o u s EBIC measurement g i v e s a d i f f u s i o n l e n g t h v a l u e o f a b o u t 24 pm and a s u r f a c e r e c o m b i n a t i o n v e l o c i t y on t h e s e c t i o n s o f l o 4 cm/s. The v a l u e s o b t a i n e d a t d i f f e r e n t d i s t a n c e s p o t - j u n c t i o n a r e r e p o r t e d i n t h e Table I g i v i n g t h e l i f e t i m e

~ p and t h e c a l c u l a t e d c o r r e s p o n d i n g Lp v a l u e ( w i t h Dp = 12.5 cm"

-'

i n good agrea- ment w i t h t h e measured v a l u e i n c o n t i n u o u s mode.

TABLE 1 D i s t a n c e (ym)

s p o t - j u n c t i on

The d i f f e r e n t values c o n f i r m t h e good homogeneity o f t h e doping i n t h e p o l y c r i s t a l - 1 i n e m a t e r i a l .

. -

I I I

b ) S j l j c m - S o l g r - c g l l s ( p o l y c r i s t a l l i n e P-type s m a l l g r a i n s )

Continuous EBIC measurements g i v e d i f f u s i o n l e n g t h v a l u e s v a r y i n g f r o m 4.8 Mm t o 10.4 pm depending on t h e p o s i t i o n t h e beam a l o n g t h e j u n c t i o n p l a n e and a very h i g h s u r f a c e r e c o m b i n a t i o n v e l o c i t y .

0.53

EBIC t r a n s i e n t measurements t a k e n a t d i f f e r e n t d i s t a n c e s s p o t - j u n c t i o n g i v e u s i n g a s e l f c o n s i s t a n t approach t a k i n q i n t o a c c o u n t o f t h e i m p o r t a n t o f t h e c o r - r e c t i o n f a c t o r g i v e s l i f e t i m e values of 0.45 psec which does n o t shows g r e a t v a r i a - t i o n s w i t h t h e p o s i t i o n o f t h e s p o t . The e q u i v a l e n t d i f f u s i o n l e n g t h i s a b o u t 40 Mm ( w i t h Dn = 36 cm2/S). The l o w d o p i n g o f t h e m a t e r i a l does n o t i n d i c a t e a v a r i a t i o n o f t h e d i f f u s i o n c o n s t a n t b u t t h e g r e a t d i s c r e p a n c y o f t h e d i f f u s i o n l e n g t h values can o n l y be e x p l a i n e d by t h e presence o f t h e c o l u m n a r s t r u c t u r e o f t h e grains which can l i m i t t h e c o l l e c t i o n of c a r r i e r s i n t h e case o f t h e c o n t i n u o u s EBIC measurement as shown i n F i g . 2.

C o l l e c t i o n p l a n e

Scanned 1 i n e

Fig.2 : Colomnar s t r u c t u r e o f a s i l i c o n s o l a r c e l l and p o s i t i o n o f t h e beam f o r t h e EBIc measurements ( c o n t i n u o u s and t r a n s i e n t )

0.34

Slope (psec) 0.29

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C2-868 J O U R N A L D E PHYSIQUE

N e v e r t h e l e s s when t h e beam i s f i x e d , i n t h e t r a n s i e n t case, t h e c o l l e c t i o n o f c a r - r i e r s can be made b y a p a t h i n t h e columnar s t r u c t u r e and t h e s t r u c t u r e has p r a c t i - c a l l y no i n f l u e n c e on t h e EBIC decay. As p r e v i o u s l y shown i n o t h e r m a t e r i a l s , l i k e GaAs, t h e d i f f u s i o n l e n g t h o f t h e m i n o r i t y c a r r i e r s can be l i m i t e d t o t h e average s p a c i n g o f t h e d i s l o c a t i o n s ( 1 0 ) .

The measurements show g r e a t v a r i a t i o n s o f t h e l i f e t i m e f o r measurements made a t 100 pm f r o m t h e j u n c t i o n ( s e e t a b l e 11)

Table I 1 : L i f e t i m e measurements i n s i l i c o n power mesa t r a n s i s t o r .

Continuous EBIC measurements g i v e d i f f u s i o n l e n g t h values v a r y i n g f r o m 34 pm t o 60 pm. I n t h a t c a s e t h e a m p f i t u d e o f v a r i a t i o n s f o r Lp i s g r e a t e r f o r t h e EBIC t r a n s i e n t w h i c h g i v e s a more p u n c t u a l e s t i m a t i o n s o f t h e c a r r i e r s r e c o m b i n a t i o n .

Slope (clsec)

~ p (wet) Lp (~.lm)

Conclusion : EBIC t r a n s i e n t measurement a r e v e r y i m p o r t a n t t o t e s t and e v e n t u a l l y t o map t h e e l e c t r o n i c q u a l i t y of t h e semiconductor m a t e r i a l . The use o f a m i c r o - p r o c e s s o r has p e r m i t t o o b t a i n t h i s r e s u l t s i n a more f l e x i b l e way i n t h e case o f s i l i c o n and germanium d e v i c e s .

2

2.9 6 0

References :

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1.2 39

( 1 ) BRESSE J.F., IITRi/SEM Om J o h a r i Ed. (1972), 105 ( 2 ) HACKETT W.H., J.A.P. 3 ( 4 ) (1972) 1642

( 3 ) BERZ F., KUIKEN H.K., S o l i d S t a t e E l e c t r o n . 19 (1976) 437 ( 4 ) KUIKEN H.K., Sol i d S t a t e E l e c t r o n . (1976) 446

( 5 ) JAKUBOWICZ A., Sol i d S t a t e E l e c t r o n .

1

(1980) 635 ( 6 ) BRESSE J.F., SEM/1977/IOm J o h a r i Ed. (1977) 683 ( 7 ) SHOCKLEY W., READ W.T., Phys. Rev. 8Z (1952) 835 ( 8 ) BRESSE J.F., SEM/1982/IV Om J o h a r i Ed. (1982) 1487 ( 9 ) RIVIERE D., These C.N.A.M. M o n t p e l l i e r , France (1983) [ l o ) ETTENBERG M., J.A.P. 45 ( 2 ) (1974) 901.

1

1.45 42

3.28

4.7 7 7

9.52

13.8 131