MINORITY CARRIER LIFETIME AND POTENTIAL BARRIER HEIGHT IN POLYCRYSTALLINE SILICON : EFFECTS OF LOW TEMPERATURE ANNEALINGS AND NEUTRON IRRADIATION

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MINORITY CARRIER LIFETIME AND POTENTIAL BARRIER HEIGHT IN POLYCRYSTALLINE

SILICON : EFFECTS OF LOW TEMPERATURE ANNEALINGS AND NEUTRON IRRADIATION

D. Bielle-Daspet, M. Roux, J. Farah

To cite this version:

D. Bielle-Daspet, M. Roux, J. Farah. MINORITY CARRIER LIFETIME AND POTENTIAL BAR- RIER HEIGHT IN POLYCRYSTALLINE SILICON : EFFECTS OF LOW TEMPERATURE AN- NEALINGS AND NEUTRON IRRADIATION. Journal de Physique Colloques, 1982, 43 (C1), pp.C1- 95-C1-101. �10.1051/jphyscol:1982114�. �jpa-00221770�

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JOURNAL DE PHYSIQUE

CoZZoque C I , suppzdment au nO1O, Tome 43, o c t o b r e 1982 page C1-95

MINORITY CARRIER LIFETIME AND POTENTIAL BARRIER HEIGHT IN POLYCRYSTALLINE SILICON : EFFECTS OF LOW TEMPERATURE ANNEALINGS AND NEUTRON IRRADIATION+

D. Bielle-Daspet, M. Roux* and J. Farah

Laboratoire d 'Automatique e t d'AnaZyse des s y s t2rne.s du C . N . R. S., 7, Avenue du Colonel Roche, 31400 TouZouse, France

*C. E . R. T . , D . E.R.T.S., 2, Avenue Edouard BeZin, 31055 TouZouse Cedex, France

Rbswnd. - Les effets de recuits 1 tempdrature ~,(450~~, sous argon ou sous hydrogsne, et d'irradiations par neutrons de 14 MeV sont compards en terme de durde de vie volumiqueGdes porteurs minoritaires et de hauteur OB des zones de barrisre de potentiel. Les mesures sont essentiellement rdalisdes par pho- tocourant et photoconductivit6 transitoires, et par caractgristiques I(V,T), dans des rubans RAD 1 gros grains (CZ, dopd B, Ia2.cm).

Les rdsultats montrent que les recuits, qu'ils soient rsalisgs sous argon ou sous hydrogsne, affectent p e u 2 v ( ~ 1 B 0,5 ps). 11 en est de mzme de OB

(0,lO 2 0,20 eV). Par contre, les effets des zones de barrisre OB sur la conduction ne suivent le modsle de doubles barrisres de Schottky que dans les Cchantillons non recuits sous hydrogsne : en passivant les joints prGs de la surface, l'hydrogsne rend les effets des zones de barrisre restantes dquiva- lents 2 ceux de zones dlectriquement isolantes.

La sensibilitg du matdriau aux irradiations slavPre Sgalement liCe 2 la sensi- bilitd des zones de barriPre de potentiel :

- la ddgradation dezvest Gquivalente 2 celle obtenue dans un monocristal ;

- les variations de OB entrainent une modification importante de la conduction pour des doses OII la conductivitg d'un monocristal n'est pas affec- t6e.

A b s t r a c t . - The effects of annealing, at temperatures2<450~C under argon or hydrogen atmosphere, and of 14 MeV neutron irradiation on bulk lifetime FVof minority carriers and height OB of the potential barrier regions are compared.

The measurements essentially involve transient photocurrent and photoconductivity responses, and I(V,T) characteristics, of samples from large grain R.A.D.

ribbons (CZ silicon, boron doped, I9.cm).

The results show that the low temperature annealing with argon or hydrogen scarcely modifies both the < ( I to 0,5p s) and OB ( N 0.10 to 0.20 eV) values.

But the effects of the regions with ~otential barriers OB on the material conductivity only agree with the double-Schottky barrier model when the samples have not been annealed under hydrogen : by passivating grain boundaries near the sample surface, hydrogen makes the effects of the remaining barrier regions analoguous to the effects of insulating zones.

Following neutron irradiation the behaviour of the electrical properties of the material is again strongly related to the behaviour of the potential barrier regions :

- the degradation of?,, appears comparable to the degradation observed in single-cristalline samples ;

- the variations of OB are responsible for the change in sample conduction which is observed at the low irradiation doses where single-cristal conductivity is not affected.

+ work performed under collaboration with P.I.R.S.E.N.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1982114

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C1-96 JOURNAL DE PHYSIQUE

G r a i n b o u d a r i e s a r e known t o change t h e c o n d u c t i o n p r o p e r t i e s o f p o l y c r i s t a l l i n e m a t e r i a l s : a s t h e s e b o u d a r i e s a r e s u r r o u n d e d by p o t e n t i a l b a r r i e r s o f h e i g h t OB,

m a t e r i a l c o n d u c t i o n i s then r e g u l a t e d by t h e p o t e n t i a l b a r r i e r r e g i o n s , i . e . , by t h e e f f e c t s of t h e b a r r i e r s on t h e m a t e r i a l f r e e c a r r i e r s . I t i s t h u s o f m a j o r i n t e r e s t t o know what changes i n t h e p o t e n t i a l b a r r i e r a p p e a r due t o t h e t r e a t m e n t s ( t h e r m a l a n n e a l i n g , p a r t i c l e i r r a d i a t i o n , ... ⁾t h e m a t e r i a l h a s been s u b j e c t e d t o , and t o c o r - r e l a t e t h e s e changes w i t h t h o s e i n d u c e d i n t h e c h a r g e t r a n s p o r t p r o p e r t i e s o f t h e sample, e s p e c i a l l y c o n d u c t i v i t y and f r e e c a r r i e r l i f e t i m e .

The c a s e s o f low t e m p e r a t u r e a n n e a l i n g s I T , < 4 5 o 0 c ) and o f room t e m p e r a t u r e i r r a d i a - t i o n by 14 MeV n e u t r o n s up t o f l u x o f 1 0 ~ ~ n . c m - 2 a r e i n v e s t i g a t e d i n t h e p r e s e n t pa- p e r . The low t e m p e r a t u r e a n n e a l i n g s a l l o w t o a n n e a l t h e m a t e r i a l p o i n t d e f e c t s ( v a c a n c y - i m p u r i t y complexes, i n t e r s t i t i a l s ... ¹¹¹⁾a n d , i n t h e p r e s e n c e o f hydrogen, t o s a t u r a t e t h e d a n g l i n g bonds i n t h e g r a i n boundary r e g i o n s / 2 / . On t h e o t h e r hand, i r r a d i a t i o n by p a r t i c l e s s u c h a s 14 MeV n e u t r o n s i s a way f o r i n t r o d u c i n g vacancy- t y p e complexes ~ 2 , V - I . . . : t h e e f f e c t s o f d e f e c t i n t r o d u c t i o n i n p o l y c r i s t a l l i n e m a t e r i a l and i n s i n g l e - c r i s t a l l i n e m a t e r i a l may b e compared.

The work h a s b e e n performed on s i n g l e - l a y e r e d p o l y c r i s t a l l i n e s i l i c o n w i t h l a r g e g r a i n s o f 3 100 x 300 /J m 2 , 60-80 p m t h i c k , e l a b o r a t e d f o r p h o t o v o l t a i c p u r p o s e from CZ b o r o n doped 14cm s i l i c o n , on c a r b o n s u b s t r a t e , by t h e R.A.D. t e c h n i q u e / 3 / . A dozen of s a m p l e s i s s u e d from a same t e c h n o l o g i c a l p r o c e s s h a v e b e e n used i n t h e s t u d y .

I . E L E C T R T C A L R O P E R T T ES : MEASUREMENTS A P T R E H A V I O U R

The e l e c t r i c a l p r o p e r t i e s d e a l t w i t h a r e t h e c o n c e r n o f t h e c h a r g e t r a n s p o r t a l l o n g t h e p u l l i n g a x i s o f t h e p o l y c r i s t a l l i n e r i b b o n .

Two measurement t e c h n i q u e s a r e i n v o l v e d :

- I(V) c h a r a c t e r i s t i c s v e r s u s measurement t e m p e r a t u r e / 4 / ;

- e l e c t r i c a l r e s p o n s e s t o t r a n s i e n t e x c i t a t i o n by 1.06 ym l a s e r p u l s e o f 40 n s d u r a t i o n : ( i ) d e c a y o f t h e p h o t o c o n d u c t i v i t y r e s p o n s e s AV(t) 1 5 1 , ( i i ) ways o f decay o f t h e d i f f u s i o n p h o t o c u r r e n t i D ( t ) a s s o c i a t e d t o t h e f r e e m i n o r i t y c a r - r i e r s / 6 , 7 / .

F o l l o w i n g t h e a n a l y s i s g i v e n i n a p r e v i o u s p a p e r / 8 / , t h e e f f e c t o f t h e g r a i n boun- d a r y r e g i o n s on t h e I ( V ) c h a r a c t e r i s t i c of s u c h a m a t e r i a l may b e s c h e m a t i z e d u s i n g two c u r r e n t components ( f i g u r e 1) :

( 1 I a I 1 + I 2

The component 11 i s t h e e l e c t r i c a l c u r r e n t c r o s s i n g a d o u b l e b a r r i e r of S c h o t t k y d i o d e w i t h a r e a S. T h i s component i s g i v e n , under low p e r t u b a t i o n c o n d i t i o n s , by :

where m i s t h e i d e a l i t y f a c t o r o f t h e S c h o t t k y b a r r i e r , VA t h e a p p l i e d v o l t a g e a c c r o s s t h e d o u b l e b a r r i e r o f S c h o t t k y and J s t h e d e n s i t y o f t h e s a t u r a t i o n c u r r e n t .

The component I? i s t h e e l e c t r i c a l c u r r e n t t h r o u g h t h e p o s s i b l e d i s c o n t i n u i t i e s o f t h e boundary r e p r e s e n t e d by t h e s h u n t r e s i s t a n c e Rsh :

( 3 ) I 2 = "A / R s h

where VA c o r r e s p o n d s t o ( c f . f i g u r e 1 ) : ( 4 ) v A = v m - IRs

i f V i s t h e v o l t a g e a p p l i e d a c c r o s s t h e N modulus " g r a i n - g r a i n boundary" of t h e sam- p l e . Using t h e t h e r m o i n i c e m i s s i o n - d i f f u s i o n t h e o r y / 9 / , t h e c u r r e n t d e n s i t y Js may b e w r i t t e n a s f o l l o w s : ^.²

c+ 2

J~ = A T exp. (- Z Q - ) _kT

EF, i s t h e Fermi l e v e l i n s i d e t h e boundary r e g i o n s u c h a s

1 6 ) EFj = OBo ⁺ (EFv - Ev)

flBo b e i n g t h e g r a i n h u n d a r y p o t e n t i a l b a r r i e r h e i g h t and Epv t h e Fermi l e v e l i n s i d e t h e g r a i n . I n (5). A i s t h e e f f e c t i v e R i c h a r d s o n c o n s t a n t :

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if we note by A the Richardson constant, f~ the tunneling carrier probability, C

fpr the overall probability of the carrier backdiffusion, v~ the effective recombination velocity of the carriers entering into the barrier region and vD the velocity of these carriers inside this region (usually vD>>vR). For large enough values of V so that VA>)mkT/q, the general exvression of the current thus obey the relation :

~2 A*

exp ( - ---

Rsh I + f f v /v kT 2mkT

T p r R D

The plot of Log1 versus V leads to Js at given temperature T. Measurements at various temperatures allow to study the variation of Log (JS/T2) versus I/T (figure 2) and we obtain a value OBo of the potential barriers which is an averaged value for the potential barriers inside the sample.

The values of @Do obtained in the as-grown samples are given in Table 1.

Table I : I(V) behaviour, barrier height OB, bulk minority carrier -- lifetime rv and time constant T~ at the decay beginning of

the photoconductivity response AVO in the as-grown polycristalline samples.

At the lowest temperatures (cf. figure 2), the encrease in Log(Js/T 2 ) may be explai- ned by carrier tunneling through the boundaries /9,10/ due to the encreased effect of fT -see (7)- when T decreases /9/.

Transient diffusion photocurrent iD(t) is due to the excess minority carriers (electrons in our case) created in the sample by uniform bulk photoexcitation and collected by diffusion.

With thin samples, this method allows to study both zv( ^bulk carrier lifetime) at the decay beginning, and tG (due to surface recombination, G at the end of the decay/6/(figure 3).

The transient photoconductivity response bgt)is measured under constant polarization current. It leads to the variation in conductivity d 5 ( t ) induced by the excess electrons and holes created in the sample. Thus theAVJ6Jresponse takes into account the density-mobility product, p(t)up(t), n(t)vn(t), for these carriers. In the case of a polycristalline ( P type) material, because of the charge conduction mechanism invol- ved, twocomponentshave to be distinguished in a p h o t o c o n d u c t i v i t y r e s p o n s e / 5 / : one is related to the decrease in the potential barrier height AgB induced by the excess minority (electron) carriers which have been trapped inside the potential barrier regions ; the other one comes from the free excess carriers d r o , p ( t ) ^(where

b m ( t ) ( Op(tj) produced inside the grains. Depending on the mechanism which dominates

the charge carrier transport in the polycristalline sample, behaviours of a signal may be summarized as follows :

- When charge conduction is dominated by carrier transport through the potential barrier regions (i.e., I(V) ruled by 11 component in ( 1 ) ) . photoconductivity d e c a y b is controlled by the variation in the majority carrier mobility pp(C)and the return to equilibrium of the potential barrier variation AdB(t)because trapped minority carriers are emitted over the GBO - A 0~(t) barrier. Because the photoconductivlty measurements are performed under low polarization conditions, expression of the majority carrier mobility can be deduced from expression (8) written for VA<(2mkT/q. It gives :

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JOURNAL DE PHYSIQUE

w i t h

A=-?

(10) P P o ' 2 h N v (- 'BOikT)

and E 7 - E,,

p ( t ) = N, exp (- F;T ) = Po + & p ( t )

- On t h e c o n t r a r y , when c h a r g e c o n d u c t i o n i s d e f i n e d by ohmic p r o c e s s i . e . , I ( V ) g i v e n by t h e I 2 component i n ( I ) , p h o t o c o n d u c t i v i t y d e c a y i s c o n t r o l l e d b y t h e r e c o m b i n a t i o n tv of t h e e x c e s s e l e c t r o n s and h o l e s Am,p(t) . End o f t h e r e s p o n s e may t h e n show a t a i l , due t o t h e c o n t r i b u t i o n o f t r a p p e d m i n o r i t y c a r r i e r s .

IT. RESULTS

Thema2 annealing w i t h hydrogen o r argon

The e l e c t r i c a l p r o p e r t i e s o b t a i n e d w i t h samples a n n e a l e d f o r p e r i o d s of 5 m i n u t e s u n d e r hydrogen (T = 4 5 0 ' ~ ) o r a r g o n (T,(40O0C) a r e s u m a r i z e d i n T a b l e 2.

T a b l e 2 : P a r a m e t e r s o f t h e c h a r g e c a r r i e r t r a n s p o r t i n t h e p o l y c r i s t a l - l i n e samples f o l l o w i n g t h e r m a l a n n e a l i n g w i t h hydrogen o r a r g o n a t Tr = 4 5 0 0 r 4 0 0 ° C ( a n n e a l i n g d u r a t i o n a t T r i s f i v e m i n u t e s ) H a n n e a l

2

A a n n e a l

These p r o p e r t i e s a r e deduced from t h e c h a r a c t e r i s t i c s I(V,T) and t h e t r a n s i e n t pho- t o r e s p o n s e s dVr[t), 6 ~ t ) a t room t e m p e r a t u r e a s p r e v i o u s l y d e s c r i b e d i n S e c t i o n I.

By comparison w i t h t h e e l e c t r i c a l p r o p e r t i e s o f t h e as-grown samples (Cf. T a b l e I ) , t h e f i r s t r e s u l t t o n o t e i s t h e e f f e c t o f t h e a n n e a l i n g w i t h argon. Both t h e I(V,T) c h a r a c t e r i s t i c s and t h e p h o t o c o n d u c t i v i t y r e s p o n s e s bVrlt) s t a y t h e n domi- n a t e d by t h e component a s s o c i a t e d w i t h c a r r i e r t r a n s p o r t t h r o u g h a d o u b l e S c h o t t k y b a r r i e r . Moreover, a n i n c r e a s e o f t h e c o r r e s p o n d i n g p o t e n t i a l b a r r i e r h e i g h t dBO

c l e a r l y a p p e a r s . On t h e c o n t r a r y , f o l l o w i n g a n n e a l a t 4 0 0 ° c , t h e f r e e c a r r i e r l i f e - time r< ^isl i t t l e m o d i f i e d ( f i g u r e 5 a ) . But t h e m i n o r i t y c a r r i e r m o b i l i t y

r&

^o⁽ ^?&'

s h o u l d b e i n c r e a s e d by a f a c t o r 2 ( f i g u r e 5b) : t h i s w e l l a g r e e s w i t h t h e o p p o s i t e e f f e c t s t h a t a p o t e n t i a l b a r r i e r h a s on t h e m a j o r i t y and m i n o r i t y c a r r i e r m o b i l i t i e s . E f f e c t of thermal a n n e a l i n g w i t h hydrogen i s s t r o n g l y d i f f e r e n t s i n c e c a r r i e r conduc-

t i o n w i t h ohmic c h a r a c t e r t h e n becomes dominant i n t h e I ( V ) c h a r a c t e r i s t i c s

b e e I 2 i n (1)). S i m u l t a n e o u s l y , t h e a p p e a r a n c y o f a s t r o n g component from f r e e c a r - r i e r s A,%\,p[t) i s o b s e r v e d i n t h e p h o t o c o n d u c t i v i t y r e s p o n s e s .

Neutron i r r a d i a t i o n e f f e c t s 1 (V)

-

^I2

I I

The v a r i a t i o n s which have been measured v e r s u s n e u t r o n f l u x a r e i l l u s t r a t e d i n t h e f i g u r e s 4 b , 5 and 6 f o r , r e s p e c t i v e l y : ( i ) OBo, S j and - , i n t h e c a s e s o f a n as-grown sample and of a sample previously a n n e a l e d up t o 4 0 0 ' ~ w i t h a r g o n , and ( i i ) f o r r V i n t h e c a s e o f P-type m o n o c r i s t a l l i n e and p o l y c r i s t a l l i n e samples i r r a d i a t e d s i m u l t a n e o u s l y .

General b e h a v i o u r o f t h e r e s u l t s o b t a i n e d w i t h t h e p o l y c r i s t a l l i n e samplesshows t h a t t h e d e g r a d a t i o n r a t e of t h e m i n o r i t y c a r r i e r l i f e t i m e s t a y s a n a l o g o u s t o t h e d e g r a d a t i o n r a t e measured i n m o n o c r i s t a l l i n e m a t e r i a l s o f comparable r e s i s t i v i t y .

'BO (ev)

-

0. 18

?-" ( i n ) ( p s )

0 . 8 - ^1.0

0.5

s

⁽^a~

( p s )

( r v ] I a O

( 20

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But, due t o v a r i a t i o n s i n t h e v a l u e s o f OB, n o t a b l e change i n c a r r i e r c o n d u c t i v i t y may a p p e a r i n t h e p o l y c r i s t a l l i n e m a t e r i a l s w h e n t h e y a r e i r r a d i a t e d by n e u t r o n f l u x e s a t which t h e c o n d u c t i v i t y o f r a o n o c r i s t a l l i n e samples i s n o t a f f e c t e d ( s e e r e s u l t s a t D = 1 0 ~ ~ n . c m - 2 , f o r i n s t a n c e ) . *

F u r t h e r m o r e , i t i s t o n o t i c e t h a t , u n d e r i r r a d i a t i o n a n as-grown sample shows a.

h i g h e r d e g r a d a t i o n r a t e o f 2 ' y a n d a lower m o d i f i c a t i o n o f c o n d u c t i o n t h a n a sam- p l e p r e v i o u s l y a n n e a l e d up t o 400'~ w i t h argon.

There a r e t h e p o t e n t i a l b a r r i e r z o n e s which a r e s e n s i t i v e t o t h e c o n s t r a i n t s ; and, t h u s , t h e y r u l e t h e e l e c t r i c a l b e h a v i o u r of a l a r g e g r a i n p o l y c r i s t a l .

I n d c c d . t h r r o n d u c t i v i t y v a r i a t i o n s a r e a l w a y s a s s o c i a t e d t o t h e p o t e n t i a l b a r r i e r s zones a f t e r t h e a n n e a l i n g u n d e r hydrogen o r a r g o n a t m o s p h e r e .

I t i s t h e same f o r t h e n e u t r o n i r r a d i a t i o n s i n t h e low f l u x r a n g e : t h e y a f f e c t n o t rin1y t h e c a r r i e r l i f e t i m e ( a s p i n m o n o c r i s t a l l i n e sample c a s e ) , b u t a l s o t h e sam- p l e e l e c t r i c a l c o n d u c t i v i t y due t o t h e impact of t h e i r r a d i a t i o n o n t h e p o t e n t i a l b a r r i e r h e i g h t s .

73 1 R 1 T OGRAPHY

1 CORBETT, J.W. , BOURGOIN, J . C . , CHENG, L . J . , CORELLI, J . C . , LEE, Y.H., MOONEY, P.M. and WEIGEL, C . , I n s t . Phys. Conf. S e r . , 31 (1977) 1 2 GINLEY, D.S. and HAALANG, D.M., A p p Z . Phys. L e t t . , 39 (1.981) 271

3 REI.OUET, C. BELIN, C., SCHNEIDER, J . , PAULIN, J . , 38 P.?'. SoZar En. Conf,, Cannns (7.480)

4 TATWG, M.L., J . A p p l . Phys., 49 1'1978) 4069

5 ROUX, M. and BELLE-DASPET, D . , Rev. Phye. Appz., 16 ( 1 9 8 1 ) 497

6 BIELLE-MSPET, D. and GASSET, G., SoZid. S t a t e Electron., (19781 1219

7 RIE1,X.E-DASPET, D.

.

JOIIAN, A. and ESPIOUSSAS, F. , Rev. Phys. Appz., 15 (19801 945 8 JOHAN. A. AYERZA, J. BIELLE-DASPET, D. ROCHER, A. and FONTAINE, C . , 20 P.V. Sozar

En. Conf., Berlin 11979)

9 See f o r i n s t a n c e SZE, S.M., Physics o f semiconductors devices, IJ. NiZey and 5012 N . r. 1, 1970, pp. SRZ-390

10 SEAGER, C.H. and PIKE, G.E., A p p l . Pfiys. L e t t . , 90 ( 7 9 8 2 ) 471

V I N

de grain

t 8

R S i m u l t a n e o u s l y , t h e DLTS s p e c t r a a l s o d i f f e r from t h e r e s p o n s e of s i n g l e - c r i s t a l l i n e specimen : t h e dominant energy l e v e l s a r e l o c a t e d a t Ev + 0.40 and 0 . 1 6 e V i n t h e s i n g l e - c r i s t a l l i n e and p o l y c r i s t a l l i n e m a t e r i a l s , r e s p e c t i v e l y , a f t e r i r r a d i a t i o n .

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JOURNAL DE PHYSIQUE

F i v 2 ?he Liog (J~/T~) for v>) g, plotted ve- the

-~IRI I/T f o r born A l e s -1 and G S 2 at two M a t i o n fluxes ^D⁼101%.cm -2and D = 1013 n.cnr2

-3 Shape of a diffusion p h o t o c m t

r ~ s p w s e created by brief penetrating excitation of -tim &' / 6 /

.a * ( v = 3VOLTS) F-4 Conductivity <7> (+,-) and

potential barrier heights DB ( (4. , <-^ ) plotted versus the annealing, - r a m Tr

irradiation dcse D.

1s d e d u c e d f r o m t h e .,1

, r a t i o m e a s u r e d u n d e r l o w v a l u e s

4 x o f t h e p o l a r i s a t i o n - c f . ( 1 ) t o ( 3 ) -

U n a n n e a l e d a n d a r g o n - a n n e a l e d 0 S a m p l e s G 9 8 - 1 a n d G 9 8 - 2 .

r m b 3m.C U D ' C 0 A n n e a l i n g d u r a t i o n a t T, i s

T- RECUIT D ( n

.

^em-2) f i v e m i n u t e s .

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2 0 0 ^\'c--

- I h

amb 300 350 400 0

T,PCl dZ

0 ( n . d 2 )

* _amb^I ₃₀₀ ₃₅₀^I _{4 0 0}I I ₁₀₁₂ _ld3

-

5 C C I o (n . cm-')

F i m m 5 Evoluticn of (a) 7 _{b u l k}

carrier lifetime, and (b) ~~,"'lifetim due to surface recabinaticn, ver& annealing Wqxmture Tr, and iradiatim h e D.

Figure 6 Degradation rate

( -1 - ) versus irradiation dose D fo? polycristallines -le? -1 and C982 (G98-2 w l e which is m e l a e d at 4C0°C before irradiation) and 1 2 . m and 10a.cm rmccristalline w l e s .