INTERACTION BETWEEN DISLOCATIONS AND IMPURITIES IN SILICON

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INTERACTION BETWEEN DISLOCATIONS AND IMPURITIES IN SILICON

K. Sumino

To cite this version:

K. Sumino. INTERACTION BETWEEN DISLOCATIONS AND IMPURITIES IN SILICON. Jour- nal de Physique Colloques, 1983, 44 (C4), pp.C4-195-C4-205. �10.1051/jphyscol:1983424�. �jpa- 00223043�

JOURNAL DE PHYSIQUE

Colloque C4, supplément au n°9, Tome 44, septembre 1983 page C4-195

INTERACTION B E T W E E N D I S L O C A T I O N S A N D IMPURITIES IN S I L I C O N

K. Sumino

The Research Institute for Iron, Steel and Other Metals, Tohoku University, Sendai 980, Japan

Résumé - En se basant sur les travaux de l ' é q u i p e dont l ' a u t e u r f a i t p a r t i e , c e r t a i n s aspects de l ' i n t e r a c t i o n entre les d i s l o c a t i o n s e t les impuretés dans des c r i s t a u x de s i l i c i u m , sont étudiés i c i . Les s u j e t s étudiés sont : les conséquences de l a présence d'impuretés sur la dyna- mique de l a d i s l o c a t i o n , l e blocage des d i s l o c a t i o n s par les impuretés, l a c i n é t i q u e de l ' a g r é g a t i o n d'atomes d'oxygène sur l e coeur, et l a s t r u c - t u r e et l ' a c t i v i t é é l e c t r i q u e des agrégats d'oxygène qui se développent au coeur des d i s l o c a t i o n s .

Abstract - Some aspects on the interaction between dislocations and im- purities in silicon crystals are reviewed on the basis of the works of author's group. Topics taken up are effects of impurities on the dy- namic behaviour of dislocations, locking of dislocations by impurities, kinetics of the aggregation of oxygen atoms on dislocation core, and, the structure and electrical activity, of oxygen aggregates developed at dislocation core.

1. Introduction

Interaction between dislocations and impurities in semiconductors is attracting a great amount of attention from both the fundamental and practical view points. The dislocation core or, at least, some irregularities on a dislocation line such as jogs and kinks are now known to be electrically active. Such segments naturally interact with electrically active impurities in the crystal. The electrostatic interaction is expected to play much more important role in the dislocation-impurity interaction in a semiconductor crystal than the elastic interaction due to the size misfit between an impurity atom and a host atom : the interaction energy of the former at an interatomic distance is much larger than that of the latter. Further, it should also be noted that the atomic arrangement characteristic in the dislocation core is never realized in a bulk crystal region. The impurity atoms located in the core region of dislocation is, thus, under a very special circumstance. The electronic structure of the impurity atoms may be altered from that in the normal bulk state and, also, some chemical reactions of the type which does not take place in the bulk crystal may proceed in this special region. In this paper, some aspects of the dislocation-impurity interactions in silicon will be presented briefly on the basis of the investigations conducted by author's group.

The dislocation-impurity interaction is now widely utilized in device production technology to suppress the occurrence of slip and warping of silicon wafers due to thermal stress at high temperature processings and also to remove harmful impurities from electro-active regions of device elements.

2. Experimental Background

The in situ X-ray topographic technique developed by our group /!/ has

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1983424

C4-196 JOURNAL DE PHYSIQUE

f a c i l i t a t e d the study of dynamic behaviour of dislocations in impure s i l i c o n c r y s t a l s t o a g r e a t extent. In the i n t e r m i t t e n t technique commonly adopted in previous works, dislocations a r e displaced a t elevated temperatures and the resulting e f f e c t s a r e observed a t room temperature. Dislocations a r e kept a t r e s t during the heating and cooling of the specimen. I t i s unavoidable with such intermittent technique t o have the occurrence of dislocation locking due t o the development of impurity atmosphere while the specimen i s kept under zero e f f e c t i v e s t r e s s a t elevated temperatures. This brings about a serious d i f f i c u l t y t o investigate the dislocation behaviour i n impure c r y s t a l s . Usually, the dislocation v e l o c i t i e s i n impure c r y s t a l s have been deduced from the distance travelled by the leading dislocation of a row of dislocations generated from a scratch made on the specimen surface /2-4/. Such a technique naturally involves many origins f o r e r r o r .

In our i n situ X-ray topographic technique, isolated fresh dislocations a r e successfully introduced i n t o the impure specimen without the occurrence of locking : introduced dislocations a r e always kept i n motion by the application of s t r e s s so t h a t the impurity atmosphere can not be developed, and the motion of the dislocations i s followed continuously on a T.V. monitor. This in s i t u technique has been applied t o the investigation of dynamic behaviour of dislocations in both pure and impure s i l i c o n c r y s t a l s /5,6/.

The developments of new techniques f o r impurity controlling have also made i t possible t o study the d e t a i l s of the dislocation-impurity interaction i n s i l i c o n . The technique f o r the controlling of oxygen concentration in s i l i c o n i n the range of 2

-

3. Effect of Impurities on Dislocation Velocity

The dislocation velocity v in high purity s i l i c o n c r y s t a l s measured by the in s i t u X-ray topographic technique i s expressed a s a function of the s t r e s s r and the temperature T by the following simple equation /5/ ;

i n the temperature range 600 - 800°C a d the s t r e s s ra ge 1.2

a

3 -

.

Here, the magnitudes of vo a r e 1 .Ox 10 and 3.5x 10 m 1MN.s and those of E are 2.20 and 2.35 eV f o r 60' and screw dislocations, respectively, and k i s the Boltzmann constant. In contrast t o a previously published work done Bwith the intermi t e n t technique

t

Equation (1) should be regarded as the appropriate one i n the limiting case of low applied s t r e s s r e l a t i v e t o the Peierls s t r e s s . Probably, the model based on the linear-response theory will give the correct description f o r the velocity r e l a t i o n under low s t r e s s e s . The Hirth-Lothe theory /15/ based on such a model gives the dependences of v on T and T given by E q . ( 1 ) . However, the magnitude of v o predicted by t h e i r theory turns out t o be three t o four orders of magnitude lower than the experimental one. This large discrepancy can be removed i f t h e activation energy f o r the dislocation motion has a l i n e a r temperature dependence with a temperature-coefficient of (Ishioka, S. : private communication).

-

-

Light element impurities show some peculiar influence on t h e velocity vs.

s t r e s s r e l a t i o n /5/. Carbon atoms up t o a concentration of 2 at.ppm have no appreciable influence on the dislocation mobility. Oxygen atoms even of high concentrations a l s o have no influence on the dislocation velocity in the high s t r e s s range. This i s t r u e down t o a very low applied s t r e s s f o r oxygen atoms

F i g . 1 The r e l a t i o n between v e l o c i t y v and s t r e s s f o r 60° d i s l o c a t i o n s a t 647°C i n s i l i c o n c r y s t a l s doped w i t h various concentrations o f oxygen shown i n t h e f i g u r e . Open marks a r e f o r a h i g h p u r i t y f l o a t i n g - z o n e c r y s t a l /5/.

a t a c o n c e n t r a t i o n o f 3 at.ppm. How- ever, i n c r y s t a l s doped w i t h h i g h con- c e n t r a t i o n s o f oxygen, t h e d i s l o c a t i o n v e l o c i t y becomes lower than t h a t i n t h e h i g h p u r i t y c r y s t a l w i t h t h e decrease i n t h e stress, thus, b r i n g i n g

-

about t h e d e v i a t i o n from t h e l i n e a r v

-

The d e v i a t i o n becomes more remarkable ,

w i t h t h e increase o f oxygen concen- t r a t i o n and w i t h t h e decreases i n t h e s t r e s s and t h e temperature. F u r t h e r r e d u c t i o n o f t h e a p p l i e d s t r e s s r e s u l t s i n t h e cessation o f d i s l o c a t i o n mo- t i o n . The v e r t i c a l broken l i n e i n Fig. 1 shows t h e c r i t i c a l s t r e s s below which d i s l o c a t i o n s o r i g i n a l l y moving under h i g h stresses become immobile.

This c r i t i c a l s t r e s s f o r t h e cessa- t i o n o f d i s l o c a t i o n motion increases w i t h the increase i n t h e oxygen con- c e n t r a t i o n . N i t r o g e n atoms a t a con- c e n t r a t i o n o f 0.11 at.ppm show no ap- p r e c i a b l e i n f l u e n c e on t h e d i s l o c a t i o n

v e l o c i t y f o r t h e s t r e s s h i g h e r than 1 10 1 I about 4 M N / ~ ~ . However, d i s l o c a t i o n s

which have been movinq under h i g h T: ( M N I ~ ~ )

stresses a r e found t o cease

t o move and t o become im- Table I The magnitudes o f t h e c r i t i c a l mobile when they are brought s t r e s s rc f o r t h e cessation o f d i s l o c a t i o n under s t r e s s s lower

5

about 4 MW/m

.

n i t u d e s o f t h e mean c r i t i c a l

s t r e s s f o r t h e cessation o f M a t e r i a l s I m p u r i t y ( a t . ~ ~ r n ) rc ( M N / ~ ~ ) d i s l o c a t i o n motion a r e shown -

i n Table I f o r c r y s t a l s doped

w i t h v a r i o u s types o f im- FZ S i B : 0.00004 < 1,2

p u r i t y . C : 2.0 ~ 1 . 2

The d i s l o c a t i o n m o b i l i t y N : 0.11 4.2

i s increased by t h e doping P : 240 8.5

o f donor i m p u r i t i e s 151.

The increase i n t h e d i s l o c a - MCZ Si 0 : 3.0 <1.2

t i o n v e l o c i t y i s c o n t r o l l e d 0 : 5.0 1.8

by t h e c o n c e n t r a t i o n and n o t

by t h e species o f t h e dop- CZ S i 0 : 15 3.0

ants. The h i g h d i s l o c a t i o n 0 : 18 8.0

m o b i l i t y i s r e l a t e d t o t h e P : 120 (0 : 5 1 2 ) 4.5 r e d u c t i o n i n t h e magnitude o f P : 300 ( 0 : $ 1 2 ) 11.5

E i n Eq. (1). These r e - 6 : 280 ( 0 : $14) 5.0

s u l t s a r e i n agreement w i t h previous works /2-4,16,17/.

The d i s l o c a t i o n v e l o c i t y i n

t h e c r y s t a l s doped w i t h acceptor i m p u r i t i e s i s measured t o be o n l y s l i g h t l y lower than i n t h e h i g h p u r i t y c r y s t a l throughout t h e whole temperature range,

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and t h e magnitude o f t h e a c t i v a t i o n energy a l s o t o be o n l y s l i g h t l y s m a l l e r than i n t h e h i g h p u r i t y c r y s t a l . These r e s u l t s a r e i n serious disagreement w i t h p r e v i o u s l y published data. The v - ^T r e l a t i o n s i n t h e c r y s t a l s doped w i t h donor o r acceptor i m p u r i t i e s a r e both l i n e a r i n t h e h i g h s t r e s s range, being given by t h e equations o f t h e type o f Eq. (1 ) . However, t h e d e v i a t i o n from t h e l i n e a r i t y as observed i n t h e oxygen-doped c r y s t a l s i s observed i n t h e low s t r e s s range. The c r i t i c a l s t r e s s f o r t h e cessation o f d i s l o c a t i o n motion e x i s t s a l s o i n these types o f c r y s t a l . The above disagreements on p-type c r y s t a l s between o u r data and t h e previous ones may be a t t r i b u t e d t o the e r r o r s i n v o l v e d i n t h e previous measurements associated w i t h t h e d i s l o c a t i o n l o c k i n g by i m p u r i t i e s .

4. E f f e c t o f I m p u r i t i e s on t h e Morphology o f Moving D i s l o c a t i o n s

The shape o f d i s l o c a t i o n s i n motion i n h i g h p u r i t y s i l i c o n c r y s t a l s observed by t h e i n s i t u X-ray topography i s a r e g u l a r hexagon o r a half-hexagon t h e segments o f which a r e s t r a j g h t along <110> over t h e whole range o f t h e a p p l i e d s t r e s s 1.2

-

.

Whenever d i s l o c a t i o n s move a t t h e v e l o c i t i e s d e v i a t e d from those obeying t h e l i n e a r v - ^T r e l a t i o n s h i p , t h e segments o f a moving l o o p o r a moving h a l f - l o o p a r e observed t o be perturbed from <110> s t r a i g h t l i n e s : o r i g i n a l - l y s t r a i g h t segments i n motion under a h i g h s t r e s s become t o be perturbed from <110> s t r a i g h t l i n e s d u r i n g motion when the a p p l i e d s t r e s s i s reduced 5 Such p e r t u r b a t i o n i n t h e shape o f d i s l o c a t i o n s i n motion i s r e v e r s i b l e . I f moving segments perturbed i n t h e shape a r e brought under a h i g h s t r e s s , t h e segments r e s t o r e the s t r a i g h t n e s s .

As mentioned i n 3 t h e cessation o f d i s l o c a t i o n motion takes p l a c e i n t h e lower s t r e s s region. Once o r i g i n a l l y movinp d i s l o c a t i o n s cease t o move i n impure c r y s t a l s , t h e s t r e s s necessary t o r e s t a r t t h e d i s l o c a t i o n s increases w i t h increases i n t h e d u r a t i o n and temperature where t h e d i s l o c a t i o n s have been k e p t a t r e s t , depending on t h e species and c o n c e n t r a t i o n o f t h e i m p u r i t i e s i n v o l v e d

.

A l l the above e f f e c t s a r e observed commonly f o r both 60° and screw segments /5/.

The p e r t u r b a t i o n i n t h e shape o f moving d i s l o c a t i o n s and t h e cessation o f d i s l o c a t i o n motion are c h a r a c t e r i s t i c o f t h e impure c r y s t a l s . Thus, these phenomena a r e a t t r i b u t e d t o the i n t e r a c t i o n between d i s l o c a t i o n s and i m p u r i t y atoms. Probably, t h e shape p e r t u r b a t i o n i s r e l a t e d t o t h e l o c a l l o c k i n g o f d i s l o c a t i o n s by i m p u r i t y atoms and t h e cessation o f t h e motion t o t h e c l o s e l o c k i n g along t h e d i s l o c a t i o n 1 ine. The maximum o f t h e d i s l o c a t i o n v e l o c i t i e s a t which the p e r t u r b a t i o n i n t h e shape o f moving d i s l o c a t i o n s i s observed i s one o r two orders o f magnitude h i g h e r than t h e t h e o r e t i c a l v e l o c i t y a t which t h e atmosphere s t a r t s t o develop /18,19/. This discrepancy may be a t t r i b u t e d t o t h e f a c t t h a t i m p u r i t y atoms a r e a c t u a l l y d i s t r i b u t e d d i s c r e t e l y i n t h e l a t t i c e w h i l e t h e theory t r e a t s them as t o be d i s t r i b u t e d on each l a t t i c e s i t e a t t h e equal p r o b a b i l i t y .

5. Locking o f D i s l o c a t i o n s by I m p u r i t i e s

The i n s i t u X-ray topography has been a p p l i e d t o study t h e r e l a t i o n s h i p between the l o c k i n g o f d i s l o c a t i o n s and t h e development o f i m p u r i t y atmosphere 6 O r i g i n a l l y f r e s h d i s l o c a t i o n s i n s i l i c o n c r y s t a l s have been aged a t e l e v a t e d temperatures under no a p p l i e d s t r e s s and t h e s t r e s s necessary t o s t a r t such aged d i s l o c a t i o n s has been measured as f u n c t i o n s o f t h e d u r a t i o n and temperatures o f aging, and o f t h e soecies and c o n c e n t r a t i o n o f i m p u r i t i e s involved. This s t a r t i n g s t r e s s i s d e f i n e d as t h e l o c k i n g s t r e s s , being known t o be s e n s i t i v e t o t h e temperature a t which t h e d i s l o c a t i o n s a r e released and

t o be i n s e n s i t i v e t o t h e r e l e a s i n g r a t e o f t h e d i s l o c a t i o n s .

The l o c k i n g s t r e s s a t a c e r t a i n temperature i s measured t o increase w i t h the increase i n t h e d u r a t i o n o f ageing f o r t h e c r y s t a l s doped w i t h oxygen, n i t r o g e n o r phosphorus. No appreciable l o c k i n g o f d i s l o c a t i o n s i s detected t o occur i n t h e h i g h p u r i t y c r y s t a l s and t h e c r y s t a l s doped w i t h carbon up t o a c o n c e n t r a t i o n o f 2 at-ppm.

I n oxygen-doped c r y s t a l s , t h e increase i n t h e l o c k i n g s t r e s s a g a i n s t t h e ageing d u r a t i o n i s found t o be f a s t e r f o r t h e c r y s t a l s w i t h h i g h e r concentrations o f oxygen atoms. The mean d i f f u s i o n d i s t a n c e o f i n d i v i d u a l oxygen atoms d u r i n g t h e ageing which r e s u l t s i n a c e r t a i n magnitude o f l o c k i n g s t r e s s i s almost independent o f t h e ageing temperature provided t h a t t h e concentrations o f oxygen atoms i n t h e c r y s t a l s a r e i d e n t i c a l . This seems t o show t h a t t h e l o c k i n g s t r e s s i s determined by t h e number o f t h e i m p u r i t y atoms accumulated i n a u n i t l e n g t h o f d i s l o c a t i o n s d u r i n g ageing i f t h e species and c o n c e n t r a t i o n o f i m p u r i t y atoms i n t h e c r y s t a l a r e s p e c i f i e d .

The number o f i m p u r i t y atoms accumulated i n a u n i t l e n g t h o f d i s l o c a t i o n s d u r i n g ageing can be evaluated by u s i n g t h e d i f f u s i o n equation f o r i m p u r i t i e s i n t h e s t r e s s f i e l d o f a d i s l o c a t i o n and a l s o by assuming t h a t t h e core r e g i o n o f t h e d i s l o c a t i o n a c t s as a s i n k f o r t h e i m p u r i t y atoms a r r i v e d there. The c a l c u l a t i o n has been done i n a numerical way w i t h a computer. The c o n c e n t r a t i o n o f t h e i m p u r i t y atoms a t t h e d i s l o c a t i o n core has been obtained w i t h t h i s technique as a f u n c t i o n o f t h e ageing d u r a t i o n , t h e d i f f u s i o n c o e f f i c i e n t and t h e i n t e r a c t i o n energy o f t h e i m p u r i t y atom w i t h t h e d i s l o c a t i o n . Thus, t h e e x p e r i m e n t a l l y measured l o c k i n g s t r e s s i s r e l a t e d t o t h e c o n c e n t r a t i o n o f t h e i m p u r i t y atoms along t h e d i s l o c a t i o n l i n e . F i g u r e 2 shows such r e l a t i o n s o b t a i n e d f o r c r y s t a l s doped w i t h various concentrations of oxygen atoms. The l o c k i n g s t r e s s i n a l l t h e c r y s t a l s i s seen t o increase 1 in e a r l y w i t h t h e oxygen c o n c e n t r a t i o n along t h e d i s l o c a t i o n core. Almost i d e n t i c a l r e l a t i o n s a r e obtained f o r t h e c r y s t a l s doped w i t h r a t h e r low concentrations o f oxygen atoms. I n t h e c r y s t a l w i t h a h i g h c o n c e n t r a t i o n o f oxygen, however, a h i g h value o f t h e l o c k i n g s t r e s s i s achieved w i t h t h e same oxygen c o n c e n t r a t i o n along t h e d i s l o c a t i o n core compared w i t h t h e c r y s t a l w i t h a low oxygen c o n c e n t r a t i o n . This phenomenon i s thought t o be r e l a t e d t o t h e inhomogeneous d i s t r i b u t i o n o f oxygen atoms i n h e r e n t t o a h i g h l y concentrated c r y s t a l .

Some i n t e r e s t i n g i n f o r m a t i o n on t h e dislocation-oxygen i n t e r a c t i o n i n s i l i c o n i s deduced i f t h e data i n F i g . 2 a r e analysed w i t h t h e theory on t h e r m a l l y a c t i v a t e d r e l e a s e o f a d i s l o c a t i o n from i t s i m p u r i t y atmosphere.

L e t N* be t h e c o n c e n t r a t i o n o f t h e oxygen atoms accumulated on t h e d i s l o c a t i o n core. C l u s t e r s o f oxygen atoms, each c o n s i s t i n g o f n oxygen atoms, a r e assumed t o be d i s t r i b u t e d a t an equal i n t e r v a l o f n/N* along t h e d i s l o c a t i o n l i n e . The energy o f i n t e r a c t i o n between such a c l u s t e r and t h e d i s l o c a t i o n i s taken t o be En. I f n i s p u t t o be one, t h e c l u s t e r s t u r n t o i n d i v i d u a l oxygen atoms. I t i s conceivable t h a t t h e oxygen atoms accumulated on t h e

Fig. 2 The r e l a t i o n between l o c k i n g s t r e s s TL a t 647'C and t h e number N* o f oxygen atoms accumulated i n a u n i t l e n g t h o f d i s l o c a t i o n s f o r s i l i c o n c r y s t a l s doped w i t h various concentrations o f oxygen o f which magnitudes a r e shown i n t h e f i g u r e 161.

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d i s l o c a t i o n 1 in e d i f f u s e r a p i d l y along t h e d i s l o c a t i o n core t o form small c l u s t e r s .

The l o c k i n g s t r e s s rR i s g i v e n by

3 = P [ En - kg T I n ( L t P v / n l ) ] / nb2 , ⁽²⁾

where L i s t h e l e n g t h o f t h e d i s l o c a t i o n , v t h e frequency o f d i s l o c a t i o n v i b r a t i o n , r t h e r e l e a s i n g r a t e o f t h e d i s l o c a t i o n from i t s atmosphere, and b t h e magnitude o f t h e Burgers v e c t o r o f t h e d i s l o c a t i o n . Since t h e term ln(LN*v/nI') depends weakly on N*, ~2 may be regarded t o be l i n e a r t o N* i n accordance t o t h e r e s u l t i n F i g . 2.

Equation ( 2 ) i s f i t t e d t o t h e experimental TQ-N* r e l a t i o n f o r t h e c r y s t a l s w i t h low oxygen concentrations by s u b s t i t u t i n g s u i t a b l e values t o L, v , r ^and

b. Taking n = l , we o b t a i n t h e magnitude o f El t o be 3.0 eV which i s about s i x times as l a r g e as t h a t o f t h e e l a s t i c i n t e r a c t i o n due t o t h e s i z e m i s f i t between an oxygen atom and an i n t e r s t i t i a l s i t e . I t i s n o t known whether t h e r e i s any p o s s i b i l i t y f o r t h e occurrence o f t h e change i n t h e s t a t e of i n d i v i d u a l oxygen atoms when t h e y a r e l o c a t e d a t t h e d i s l o c a t i o n core.

Small c l u s t e r s o f oxygen atoms o r some small complexes i n c l u d i n g oxygen atoms a r e known t o a c t as donors i n s i l i c o n . They a r e o f t e n observed t o develop d u r i n g ageing o f Czochralski s i l i c o n a t r e l a t i v e l y low temperatures.

I n t h e case where each c l u s t e r on t h e d i s l o c a t i o n l i n e i s assumed t o i n c l u d e f o u r oxygen atoms (n=4), t h e f i t t i n g leads t o Eq o f 3.5 eV i n t h e magnitude.

I f such a c l u s t e r a c t s as a donor, i t may i n t e r a c t w i t h an acceptor s i t e on t h e d i s l o c a t i o n e l e c t r o s t a t i c a l l y . On t h e assumption t h a t t h e donor and t h e acceptor a r e b o t h s i n g l y charged, t h e energy o f e l e c t r o s t a t i c i n t e r a c t i o n amounts t o about 2 eV a t one i n t e r a t o m i c distance. The remaining p a r t o f t h e i n t e r a c t i o n energy, 1.5 eV i n t h e magnitude, may be a s c r i b e d t o t h e e l a s t i c i n t e r a c t i o n o f t h e c l u s t e r w i t h t h e d i s l o c a t i o n , which seems t o be o f a reasonable magnitude. F u r t h e r works seem t o be necessary t o c l a r i f y t h e d e t a i l s o f t h e s t a t e o f oxygen l o c a t e d a t t h e d i s l o c a t i o n core.

The r e l a t i v e s t r e n g t h o f an i m p u r i t y atom o f v a r i o u s types i n t h e i n t e r a c t i o n w i t h a d i s l o c a t i o n can be evaluated by comparing t h e mean d i f f u s i o n distances o f v a r i o u s i m p u r i t y atoms d u r i n g t h e ageings which r e s u l t i n t h e same magnitude o f t h e l o c k i n g s t r e s s . It i s known t h a t t h e i n t e r a c t i o n s of i n d i v i d u a l atoms o f phosphorus and n i t r o g e n w i t h a d i s l o c a t i o n a r e much s t r o n g e r than t h a t o f i n d i v i d u a l oxygen atoms. However, u s u a l l y oxygen atoms m a n i f e s t t h e i r i n t e r a c t i o n w i t h d i s l o c a t i o n s most remarkably i n s i l i c o n because o f t h e i r h i g h d i f f u s i o n r a t e and t h e i r r a t h e r h i g h s o l u b i l i t y . The s t r o n g i n t e r a c t i o n o f a phosphorus atom w i t h a d i s l o c a t i o n may be a t t r i b u t e d t o i t s h i g h e l e c t r i c a l a c t i v i t y as a donor and a l s o t o i t s h i g h chemical r e a c t i v i t y w i t h o t h e r k i n d o f atoms. I n d i v i d u a l phosphorus atoms i n t e r a c t w i t h acceptor s i t e s on d i s l o c a t i o n s e l e c t r o s t a t i c a l l y and phosphorus atoms trapped a t t h e d i s l o c a t i o n c o r e w i l l capture f a s t d i f f u s i n g i m p u r i t i e s i n t h e c r y s t a l , l e a d i n g t o t h e f o r m a t i o n o f t h e complexes t h a t have a h i g h i n t e r a c t i o n energy.

L i t t l e i s known about t h e n a t u r e o f n i t r o g e n i n s i l i c o n . Most n i t r o g e n d i s s o l v e d i n a s i l i c o n c r y s t a l a r e e l e c t r i c a l l y i n a c t i v e i n s p i t e o f t h e f a c t t h a t they belong t o t h e group V elements. They a r e b e l i e v e d t o occupy t h e i n t e r s t i t i a l s i t e s . The v e r y small s o l u b i l i t y o f n i t r o g e n i n s i l i c o n seems t o mean t h e energy o f a n i t r o g e n atom d i s s o l v e d i n s i l i c o n t o be high. The very s t r o n g i n t e r a c t i o n o f n i t r o g e n atoms w i t h d i s l o c a t i o n s i n s i l i c o n may be understood w e l l i f some r e a c t i o n which leads t o a v e r y s t a b l e s t a t e o f n i t r o g e n atoms proceeds a t t h e d i s l o c a t i o n core.

6. K i n e t i c s o f Aggregation of I m p u r i t i e s on Deformation-Induced Defects During t h e deformation of a Czochralski s i l i c o n c r y s t a l a t a temperature lower than about l l O O ° C , t h e c o n c e n t r a t i o n o f d i s s o l v e d oxygen i n a supersaturated s t a t e decreases v e r y r a p i d l y /20/. Usually, t h e decreasing r a t e shows t h e maximum a t a shear s t r a i n of 1 0 ~ ~ 2 0 % and then diminishes w i t h t h e i n c r e a s e i n t h e s t r a i n due t o t h e exhaustion o f oxygen atoms i n t h e m a t r i x . The decrease i n t h e c o n c e n t r a t i o n o f d i s s o l v e d oxygen atoms i s so r a p i d t h a t

solved oxygen atoms (Yonenaga and Sumino / t o be published/ ).

almost a l l t h e supersaturated oxygen atoms are removed w i t h i n 1 h r d u r i n g t h e deformation a t 900°C and a shear s t r a i n r a t e o f 10-4 s-1

.

Deformation induces v a r i o u s types o f d e f e c t i n t o a c r y s t a l . I t i s i n t e r e s t i n g t o know how these d e f e c t s p l a y t h e i r r o l e s i n t h e removal o f supersaturated i m p u r i t i e s i n t h e c r y s t a l . This has been i n v e s t i g a t e d by Yonenaga and Sumino / t o be published/ w i t h s i l i c o n c r y s t a l s doped w i t h v a r i o u s concentrations o f oxygen.

On annealing a deformed c r y s t a l a t t h e temperature same as one a t which t h e deformation has been done, the decreasing i n t h e c o n c e n t r a t i o n o f super- s a t u r a t e d oxygen i s found t o proceed much more r a p i d l y than i n a d i s l o c a t i o n - f r e e c r y s t a l , provided t h a t t h e deformation p r i o r t o t h e annealing i s n o t so l a r g e as t o a l l o w t h e removal o f a l l t h e supersaturated oxygen atoms.

The decreasing r a t e i s t h e h i g h e s t a t t h e beginning o f t h e annealing and diminishes m o n o t o n i c a l l y w i t h t h e annealing d u r a t i o n . T h i s seems t o show t h a t t h e d e f e c t s i n t r o d u c e d by deformation a c t as e f f e c t i v e n u c l e a t i o n centers f o r oxygen aggregates. The d u r a t i o n of t h e annealing t o complete t h e removal o f supersaturated oxygen i s s h o r t e r i n t h e c r y s t a l s w i t h l a r g e r amounts of deformation p r i o r t o t h e annealing. The c o n c e n t r a t i o n o f supersaturated oxygen i n t h e c r y s t a l seems n o t t o have any l a r a e i n f l u e n c e on t h e k i n e t i c s o f oxygen removal.

Dissolved oxygen atoms removed from the m a t r i x o f a c r y s t a l by deformation a t around 900°C a r e r e s t o r e d upon annealing t h e deformed c r y s t a l a t tempera- t u r e s h i g h e r than 1100°C. The c o n c e n t r a t i o n o f d i s s o l v e d oxygen recovers t o t h a t p r i o r t o deformation by annealing a t temperatures h i g h e r than 1150 " C f o r s h o r t durations. The deformed c r y s t a l s which a r e subsequently subjected t o a stepwise annealing a t 1100, 1150 and '1200°C, each f o r 15 min, a r e h e r e a f t e r c a l l e d t h e d i s s o l u t i o n - t r e a t e d c r y s t a l s . By such d i s s o l u t i o n - t r e a t m e n t , t h e c o n c e n t r a t i o n o f d i s s o l v e d oxygen p r i o r t o t h e deformation i s completely r e - covered and t h e d i s l o c a t i o n d e n s i t y as determined by e t c h p i t counting i s decreased by about 70 %. Dis-

l o c a t i o n e t c h p i t s observed on

t h e surface o f t h e d i s s o l u t i o n 4 - t r e a t e d c r y s t a l are d i s t r i b -

u t e d u n i f o r m l y w i t h o u t forming polygon boundaries. The shape o f d i s l o c a t i o n s s u r v i v e d a f t e r t h e d i s s o l u t i o n - t r e a t m e n t i s observed t o be much more smooth than t h a t p r i o r t o t h e disso- l u t i o n - t r e a t m e n t by t r a n s - :,

mission e l e c t r o n microscopy. 'r

Furthermore, t h e s o - c a l l e d

k

d e b r i s c h a r a c t e r i s t i c i n t h e

-

deformed s t a t e are removed 2

completely by t h e d i s s o l u t i o n hL -treatment. Thus, t h e r o l e o f

Fig. 3 The decreasing r a t e Vr

o f d i s s o l v e d oxygen atoms a t 1 900°C i n t h e c r y s t a l s w i t h t h

same d i s l o c a t i o n d e n s i t y 5 x 10

7

cm-2 a t deformation, a t an- n e a l i n g a f t e r deformation, and a t annealing a f t e r t h e d i s s o l u - t i o n - t r e a t m e n t p l o t t e d a g a i n s t

t h e c o n c e n t r a t i o n c* o f d i s - 0'

N ~ : 5x10 7cm-2

0

3-

-

with dissolution

- ,a, treatment

7

m

4 for Nd=O

I I I I I I 1

*

1 5

C4-202 JOURNAL DE PHYSIQUE

o n l y d i s l o c a t i o n s i n the aggregation behaviour o f supersaturated oxygen can be i n v e s t i g a t e d w i t h t h e use o f t h e d i s s o l u t i o n - t r e a t e d c r y s t a l s .

I t i s found t h a t t h e removal o f t h e supersaturated oxygen proceeds much more s l o w l y i n t h e d i s s o l u t i o n - t r e a t e d c r y s t a l due t o annealing i n t h e temperature range 600 - 900°C conpared w i t h t h e deforned c r y s t a l which i s n o t d i s s o l u t i o n - t r e a t e d and has t h e d i s l o c a t i o n d e n s i t y same as t h a t o f t h e former c r y s t a l . F i g u r e 3 compares t h e decreasing r a t e s o f d i s s o l v e d oxyg5n i n t h e c r y s t a l s w i t h t h e same d e n s i t i e s o f d i s l o c a t i o n s o f 5 x l o 7 cm- f o r the deformation a t 900°C, t h e 900°C annealing o f deformed c r y s t a l s , and t h e 900 " C annealing o f d i s s o l u t i o n - t r e a t e d c r y s t a l s f o r various concentrations of d i s s o l v e d oxygen. The o r d i n a t e shows t h e decreasing r a t e o f d i s s o l v e d oxygen atoms d i v i d e d by t h e t o t a l c o n c e n t r a t i o n o f t h e oxyqen atoms t h a t can be p r e c i p i t a t e d a t 900°C and t h e abscissa shows t h e c o n c e n t r a t i o n o f d i s s o l v e d oxygen atoms i n t h e c r y s t a l a t t h e time o f the measurement o f t h e decreasing r a t e . The decreasing r a t e s f o r deformed c r y s t a l s and f o r d i s s o l u t i o n - t r e a t e d c r y s t a l s are f o r t h e beginning o f annealing a t which t h e decreasing r a t e s a r e t h e h i g h e s t . Comparing t h e decreasing r a t e durincj deformation w i t h t h a t a t annealing o f deformed c r y s t a l s , one may imagine t h a t some processes f a c i 1 i t a t i n g t h e aggregation o f oxygen atoms a r e o p e r a t i n g d u r i n g deformation.

The d i f f u s i o n o f oxygen m i g h t be enhanced d u r i n g deformation t o a g r e a t e x t e n t . The d i f f e r e n c e o f t h e decreasing behaviours i n t h e deformed c r y s t a l s and t h e d i s s o l u t i o n - t r e a t e d c r y s t a l s c l e a r l y shows t h a t d e b r i s and/or some i r r e g u l a r i t i e s on d i s l o c a t i o n s such as jogs p l a y i m p o r t a n t r o l e s i n t h e aggregation o f supersaturated oxygen. Possibly, they can a c t as e f f e c t i v e n u c l e a t i o n centers f o r t h e aggregates. The decreasing r a t e s measured f o r t h e d i s s o l u t i o n - t r e a t e d c r y s t a l s agree approximately w i t h those c a l c u l a t e d w i t h t h e d i f f u s i o n equation f o r oxygen atoms i n t h e s t r e s s f i e l d o f a d i s l o c a t i o n . These r a t e s a r e much h i g h e r than those i n d i s l o c a t i o n - f r e e c r y s t a l s .

7. I n f r a r e d Absorption and E l e c t r i c a l E f f e c t Associated w i t h D i s l o c a t i o n -1mpuri t y I n t e r a c t i o n

I n t e r s t i t i a l l y d i s s o l v e d oxygen atoms i a s i l i c o n c r y s t a l g i v e r i s e t o an o p t i c a l a b s o r p t i o n peak centered a t 1106 cm-' which i s associated w i t h a l o c a l phonon mode o f l a t t i c e v i b r a t i o n . The c o n c e n t r a t i o n o f d i s s o l v e d oxygen atoms i n s i l i c o n i s u s u a l l y determined from t h e h e i g h t o f t h i s a b s o r p t i o n peak.

During deformation o f a Czochralski s i l i c o n c r y s t a l a t 900 "C, t h e h e i g h t o f t h i s a b s o r p t i o n peak decreases and, a t t h e same time, a b s o r p t i o n a t s m a l l e r wave numbers becomes remarkable w i t h t h e increase i n t h e s t r a i n . A broad peak develops i n t h e wave number range 980 - 1080 cm-I f o r l a r g e r s t r a i n s /20/.

On annealing a deformed Czochralski s i l i c o n c r y s t a l a t 900°C, t h i s broad peak becomes d i s t i n c t more and more as t h e annealing d u r a t i o n increases i f t h e supersaturated oxygen atoms s t i l l remain b e f o r e annealing. Yonenaga and Sumino / t o be published/ have found t h a t the d i s s o l u t i o n - t r e a t m e n t o f a deformed Czochralski s i l i c o n c r y s t a l 1 ads t o t h e c o m ~ l e t e disappearance o f t h e broad peak i n t h e r n g e 980 - 1080 cm-' and a l s o t o t h e recovery of t h e peak h e i g h t a t 1106 cm- t o t h a t b e f o r e deformation. On annealing t h e d i s s o l u t i o n - t r e a t e d Czochralski c r y s t a l a t 900 O C , t h e h e i a h t o f t h e 1106 cm-l peak decreases w i t h t h e increase i n t h e annealing d u r a t i o n w i t h o u t accompanying development o f any appreciable peaks i n t h e wave number range 900 - 1300 cm-

jh:

Annealing o f a d i s l o c a t i o n - f r e e c r y s t a l o f Czochralski s i l i c o n a t 900 " C leads t o t h e development o f a d i s t i n c t abso p t i o n peak centered a t 1225 cm-I a t t h e expense o f t h e h e i g h t o f t h e 1106 cm-

7

From the above observations, i t may be known t h a t c l u s t e r s o r complexes o f oxygen atoms w i t h some s p e c i a l atomic arrangements o f which frequencies o f v i b r a t i o n are lower than t h a t o f i n t e r s t i t i a l oxygen atoms develop mainly on d e b r i s i n t r o d u c e d by deformation. It has been confirmed t h a t these atomic arrangements a r e very s t a b l e and do n o t t r a n s f o r m t o t h a t o f c r i s t o b a l i t e even by a prolonged annealing a t 900°C /20/. A considerable f r a c t i o n o f oxygen

atoms removed from t h e matrix c r y s t a l during deformation o r by annealing a f t e r deformation, both a t 900°C, seen i n t h e previous s e c t i o n a r e thought t o be i n t h i s s t a t e . I t i s known t h a t c l u s t e r s o r complexes which develop upon annealing d i s l o c a t i o n - f r e e c r y s t a l s of Czochralski s i l i c o n a t 650 " C show t h e above type of absorption peak /20/. Thus, they may be s i m i l a r i n t h e atomic arrangement t o those developed around t h e d e b r i s . Anyway, t h e s e c l u s t e r s o r complexes a r e not observable with usual transmission e l e c t r o n microscopy possibly because of t h e smallness i n t h e s i z e .

I t i s q u i t e i n t e r e s t i n g t o note t h a t oxygen aggregates developed on d i s l o c a t i o n s a t 900°C a r e d i f f e r e n t i n t h e s t r u c t u r e from those developed on d e b r i s . They show no char c t e r i s t i c absorption peak, a t l e a s t , i n t h e wave number range 900 - 1300 cm-

B

Again, they a r e not observable with transmission e l e c t r o n microscopy.

I t i s known t h a t deep a c c e p t o r s a r e introduced i n t o s i l i c o n c r y s t a l s by deformation, of which energy l e v e l i s i n t h e range 0 . 3 - 0 . 4 eV above t h e top o f t h e valence band /21,22/. These a c c e p t o r s a r e a s c r i b e d t o d e b r i s o r some i r r e g u l a r i t i e s on d i s l o c a t i o n l i n e s such a s jogs and/or kinks s i n c e t h e i r d e n s i t y i s found t o be 1 - 2 o r d e r s of magnitude lower than t h e d e n s i t y of dangling bonds geometrically p o s s i b l e a t t h e d i s l o c a t i o n c o r e . The r e l a t i o n between t h e c a r r i e r concentration and t h e temperature measured f o r a deformed n-type specimen of floating-zone s i l i c o n i s f i t t e d t o a t h e o r e t i c a l curve s a t i s f a c t o r i l y by simply adding deformation-induced deep acceptors t o t h e

Table I 1 The values of t h e parameters f o r t h e donors and acceptors i n n-type specimens of floating-zone s i 1 icon (FZ-Si ) and Czochral s k i s i 1 icon ( C Z - ~ i ) induced by deformation a t 900°C /20/.

S t r a i n

Crystal NDEP

( % I (cm-2)

N D E p : Density of d i s l o c a t i o n e t c h p i t s .

- -.

NDD : Concentration o f deformation-induced donors.

E D D ^: Energy l e v e l of deformation-induced donors measured from t h e bottom of conduction band.

NDA : Concentration of deformation-induced deep a c c e p t o r s .

C4-204 JOURNAL DE PHYSIQUE

chemical donors and acceptors present i n t h e c o n t r o l specimen. However, i n t h e case o f Czochralski s i l i c o n w i t h supersaturated oxygen, i t i s impossible t o d e s c r i b e the data on deformed n-type specimens on t h e assumption t h a t o n l y deep acceptors a r e i n t r o d u c e d by deformation ; t h e f i t t i n g needs t h e i n t r o - d u c t i o n o f r e l a t i v e l y shallow donors i n a d d i t i o n t o t h e deformation-induced deep acceptors. These donors a r e thought t o be associated w i t h oxygen atoms removed from t h e m a t r i x d u r i n g deformation. Table I 1 shows t h e values o f f i t t i n g parameters f o r deformation-induced acceptors and donors f o r n-type specimens o f b o t h f l o a t i n g - z o n e and Czochralski s i l i c o n . The f i t t i n g o f t h e data on t h e Czochralski specimen a f t e r a small amount o f deformation needs o n l y one k i n d o f deformation-induced donors, w h i l e those on t h e Czochralski specimens a f t e r l a r g e deformation need two kinds. The energy l e v e l o f these donors becomes deeper as t h e s t r a i n increases. It i s t o be noted t h a t t h e donors a r e i n t r o d u c e d by deformation a t a temperature o f 900 C where t h e generation o f oxygen donors o n l y by annealing i s scarce i n d i s l o c a t i o n - f r e e c r y s t a l s o f Czochralski s i l i c o n .

Yonenaga and Sumino / t o be published/ have found t h a t donors are a l s o developed i n t h e d i s s o l u t i o n - t r e a t e d specimens by t h e annealing a t 900°C. The d e n s i t y o f such donors c o i n c i d e s approximately w i t h t h a t developed d u r i n g deformation i f t h e d i s l o c a t i o n d e n s i t y i n t h e d i s s o l u t i o n - t r e a t e d specimen i s approximately equal t o t h a t i n t h e specimen d u r i n g deformation. I t may thus be concluded t h a t oxygen atoms aggregated on d i s l o c a t i o n s a t 900 OC a r e e l e c t r i c a l l y a c t i v e as donors.

Acknowledgements

The a u t h o r i s g r a t e f u l t o t h e members o f h i s research group, e s p e c i a l l y t o Drs. M. Imai and I. Yonenaga, f o r t h e i r cooperation i n t h e i n v e s t i g a t i o n s on which t h i s review i s based. He a l s o wishes t o express h i s g r a t i t u d e s t o H i t a c h i Ltd., Komatsu E l e c t r o n i c Metals Co., Osaka Titanium Co., Shin-Etsu Handotai Co., and Sony Corp. f o r p r o v i d i n g t h e s i l i c o n c r y s t a l s used i n t h e i n v e s t i g a t i o n . This work was supported p a r t i a l l y by Grant-in-Aid f o r S c i e n t i f i c Research from t h e M i n i s t r y o f Education, Science and C u l t u r e i n Japan.

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