LATTICE DEFECT EFFECTS IN INDUCED CURRENT SCANNING MICROSCOPY

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LATTICE DEFECT EFFECTS IN INDUCED CURRENT SCANNING MICROSCOPY

A. Castaldini, A. Cavallini, P. Gondi

To cite this version:

A. Castaldini, A. Cavallini, P. Gondi. LATTICE DEFECT EFFECTS IN INDUCED CURRENT SCANNING MICROSCOPY. Journal de Physique Colloques, 1983, 44 (C4), pp.C4-305-C4-311.

�10.1051/jphyscol:1983436�. �jpa-00223055�

LATTICE DEFECT EFFECTS IN INDUCED CURRENT SCANNING MICROSCOPY A. C a s t a l d i n i , A. Cavallini and P. Gondi

Istituto di Fi.si.ea, Universita di Bologna, Unita G.N.S.M. del C.N.R., Italy

+Istituto di Metallurgia, Universita di Roma, Istituto di Fisica, Universita di Bologna, Unita G.N.S.M. del C.N.R., Italy

Résumé - On considère, dans les semiconducteurs élémentaires, les carac- téristiques des images EBIC et IRBIC et des effets électriques en relation avec les défauts ponctuels et les dislocations introduites par déformation.

Abstract - Characteristics of EBIC and IRBIC images as well as electrical effects are considered on their dependence on both point defects and dis- locations introduced by deformation in elemental semiconductors.

INTRODUCTION

The electrical effects consequent to the introduction of dislocations in semiconduc- tors have been in prevalence interpreted in terms of energy levels of bonds of dis- sociated or not dissociated dislocations /1,2,3/. In particular the effects on car- rier density have been related to space charge cylinders neutralizing the charged dangling bonds in the theory of Read / 4 / and further to the screening cloud of major- ity carriers in the theory of Labusch-Schrbter / 5 / . Concerning the dislocation effects on the recombination phenomena theories have been put forward by Figielsky / 6 / and by Schroter / 7 / , always based on the dislocation band occupation statistics men- tioned.

In alternative we have considered in previous papers /8,9/ that some of the effects considered so far may be related to phenomena involving point defects and/or point defect associates coming about with the dislocations introduced by deformation. Evi- dence of large densities of point defects with the dislocations has been found by EPR measurements/10,11/ and by TEM observations /12/.

In the present paper results of recent EBIC and IRBIC observations are reviewed to- gether'with others°firevious works to emphasize the necessity of considering the point defect effects for the interpretation of the phenomena consequent to deformation and accompanying the introduction of dislocations in semiconductors. Experimental and results are discussed together for each item.

EBIC PROFILES OF DISLOCATIONS INTRODUCED AT RELATIVELY HIGH TEMPERATURES

Trying to put into evidence space charge cylinders or screening clouds around dislo- cations EBIC profiles have been examined of dislocations introduced in as grown silicon , on the basis of theoretical analyses of. various authors /13,14,15 ,16/.

According to Read theory / 4 / the space charge cylinder radius is R=fi s"i(NQ-N/\)"^iT"i f being the occupied fraction of the dangling bonds with distance s, Np, N/\ density of chemical donors.and acceptors.

In the Labusch-Schrb'ter theory /5/ the screening cloud decays exponentially with a characteristic Debye length A = (kTe/eJlp-nl)^ p, n being carrier densities, e electron

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1983436

C4-306 JOURNAL DE PHYSIQUE

charge, o t h e r s as usual. Values o f R and A f o r some s u i t a b l e d e n s i t i e s ND-NA, p, n - ..

and occupation f a c t o r f 3 f a r e g i v e n i n t h e f o l l o w i n g

p-n (cm-3) l 0 I 3 5-10 13 ,014 N0

-

N (cm -3) 5-10

f A 4 0.01 0.05 0.10

R (cm.10 )

0.34

0.76 '1.08 (cm-lo4) 1.29 0.58 0.41

The distances which can be r e s o l v e d i n EBIC images o f d i s l o c a t i o n s a r e q u i t e l a r g e , o f t h e o r d e r o f 10-km; t h e r e f o r e observations t o p u t i n t o evidence R o r A e f f e c t s on t h e EBIC p r o f i l e s were c a r r i e d o u t on h i g h r e s i s t i v i t y Si (100 ohm cm) w i t h r e l a - t i v e l y s h o r t d i f f u s i o n l e n g t h , u s i n g surface b a r r i e r diodes: f u r t h e r averaging o f successive p r o f i l e scans was needed t o c l e a n t h e p r o f i l e from background noise. The e f f e c t s o f averaging a r e i l l u s t r a t e d by t h e s e r i e s i n Fig. 1: i n Fig. 3 a p r o f i l e a f t e r averaging i s compared w i t h t h e o r e t i c a l ones c a l c u l a t e d f o r t h e b u l k d i f f u s i o n l e n g t h L = 7 ~ 1 0 - ~ c m , as measured by t h e method o f Hackett 1171 a t t h e boundaries o f t h e s u r f a c e b a r r i e r , and f o r another L = 2xIOm3 cm as s u i t a b l e f o r a b e t t e r f i t t i n g .

Fig. 1 Fig.2 Fig.3

Figs. 1, 2, 3

-

I n F i g . 1 e f f e c t s o f s i g n a l averaging i n successive sweeps, on de- t a i l s o f t h e EBIC p r o f i l e o f t h e d i s l o c a t i o n imaged i n F i g . 2. I n Fig. 3 comparison between averaged p r o f i l e w i t h p r o f i l e s c a l c u l a t e d f o r t h e d i f f u s i o n l e n g t h measured i n t h e b u l k (L = 7 ~ 1 0 - ~ c m ) and another(L = 2 ~ l O - ~ c m ) c h o s e n f o r f i t t i n g .

Agreement i s found o n l y f o r t h i s second l e n g t h , s m a l l e r than t h e b u l k one. This i s taken as an i n d i c a t i o n o f a r e d u c t i o n o f t h e l i f e - t i m e around t h e d i s l o c a t i o n , as a consequence o f p o i n t d e f e c t s concentrated here,probably i n C o t t r e l o r Suzuki atmo- spheres.

Due t o t h i s p o i n t d e f e c t doping l o c a l r e s i s t i v i t y decreases occur near t o t h e d i s l o - c a t i o n s , thus e x p l a i n i n g t h e absence o f t h a t broadening on t h e EBIC p r o f i l e bottom which was expected f o r R o r A c o n s i s t e n t w i t h t h e . b u l k r e s i s t i v i t y .

More d e t a i l s on these observations are g i v e n i n a paper presented now f o r plblication.

DISLOCATIONS INTRODUCED AT RELATIVELY LOW TEMPERATURES

For comparable d i s l o c a t i o n d e n s i t i e s t h e e f f e c t s o f deformation a r e d i f f e r e n t accord- i n g t o t h a t t h e deformation temperatures a r e h i g h e r o r s m a l l e r than c r i t i c a l tempera- t u r e s Tc: f o r Ge TC- 750 K, f o r S i

-

^{1000 K} /18,19,20,21/. The s a i d d i f f e r e n c e i s

a t 900

K.

Application of the Labusch-Schrijter theory leads t o agreement with the

Fig.

4

Carrier density vs 1/T f o r Ge before deformation ((u) n-type) and a f t e r deformation a t 620 K

(p-type) (etch p i t density

1 O 7

cmm2) and a t 900

K

(p-type)(etch p i t density lo7 cm").

experimental trend a f t e r the lower

T

deformation f o r dislocation s t a t e s a t 10.02 eV from the valence band /22/: s t a r t i n g from the i n i t i a l n-type i t i s necessary t o as- sume t h a t previous donors a r e compensated, i n part o r on the whole, by acceptors re- l a t e d t o point defects introduced during t h e deformation.

A

half f i l l e d band a t

0.09eV

from the valence band has been related,on t h e other hand, t o the dislocations intro- duced by deformation a t the higher temperatures

/5/.

In EBIC scanning microscopy atypical images r e s u l t f o r the dislocations introduced a t the lower

T ,

i . e . broad recombination zones around each dislocation instead of the usual l i n e image /16/. The phenomenon i s i l l u s t r a t e d in Figs.

5a)-b) ,

referring t o dislocations emitted from a microhardness indentation r o s e t t e i n S i , as put i n t o mutual evidence by SEM of the etch p i t s and by EBIC.

Fig. 5

EBIC images (5a) i n correspondence of dislocations emitted a t 1000

K

from a microhardness indentation, a s localized i n the SEM image of Fig. 5b).

Observations on t h e behaviour i n creep of Ge have shown /19/ t h a t the said tempera- tures a r e a l s o c r i t i c a l in respect t o deformation modalities, a s shown by the diagram of Fig. 6 giving average dislocation mobility vs

1/T.

Diffraction contrast analyses i n TEM showed t h a t dislocations introduced in Ge below Tc a r e in prevalence of screw character

/19/:

f u r t h e r these dislocations appear i n general widely dissociated /23/

(Fig.

7 ) .

C4-308 JOURNAL DE PHYSIQUE

a )

Fig. 6 Fig. 7

e=l@ D m / d

Temperature depend- TEM o f d i s s o c i -

ence o f d i s l o c a t i o n a t e d d i s l o c a - m o b i l i t y i n station-

.

^{OZ52 '} t i o n s (a) i n Ge

a r y creep under a f t e r deformation

55 p l o a d i n G e I l l l } t 620 K and ( b )

specimens w i t h h i g h n S i a f t e r de-

d e n s i t y o f d i s l o c a - ormation a t 840

K.

t i o n sources on t h e surface.

I O O W T ~ K - ~ I

-

These r e s u l t s a r e c o n s i s t e n t w i t h observations on d i s l o c a t i o n m o b i l i t y o f Schaum- burg /24/:for d i s l o c a t i o n d i s s o c i a t i o n r e f e r e n c e i s made t o /25/ as w e l l as t o others, e.g. /26/.

TRANSITIONS

With s u i t a b l e thermal treatments t h e e f f e c t s o f l a t t i c e d e f e c t s i n t r o d u c e d a t r e l a - t i v e l y low temperature changes, t e n d i n g t o those encountered a f t e r deformation a t t h e h i g h e r temperatures /27/. The phenomenon i s i l l u s t r a t e d i n Fig. 8: i t i s n o t i c e d t h a t t h e transformation occurs by h e a t i n g a t temperatures j u s t above t h e c r i t i c a l temperature range.

lo"

-

ro" - 7

I

ldJ-

3

9

rd' -

10" -

Fig. 8

E f f e c t s o f h e a t i n g a t 750 K f o r 30, 60, 120 mins and a t 850 K f o r 30 mins a f t e r deformation a t 6 2 0 K l e a d i n g t o curve

@.

For t h e f i t t i n g of these experimental data complementary v a r i a t i o n s i n t h e d e n s i t i e s o f t h e s t a t e s r e l a t e d t o t h e d i s l o c a t i o n s a t t h e d i f f e r e n t l e v e l s o f energy a r e as- sumed; f u r t h e r , i t appears necessary t o assume t h a t specimen i n i t i a l l y n-type t r a n s - form i n t o p-type, i n t h e b u l k around d i s l o c a t i o n s , phenomenon which i s again r e l a t e d t o p o i n t defects, e i t h e r coming as i m p u r i t i e s , by f a s t d i f f u s i o n from t h e surface, o r c ~ m i n g about as a consequence o f r e a c t i o n s o f a s s o c i a t i o n i n v o l v i n g a l s o p o i n t d e f e c t s i n t r o d u c e d t o g e t h e r w i t h the d i s l o c a t i o n s .

Also t h e e f f e c t s of deformation on Ge i n i t i a l l y p-type /5/ can be i n t e r p r e t e d as a consequence o f a s s o c i a t i o n between acceptors and p o i n t d e f e c t s accompanying t h e d i s l w c a t i o n s /8/. F i g u r e 9 shows t h e f i t t i n g o f t h e experimental data which i s obtained on

Fig.9 F '

i tting of experimental data

0

of Ge initially p-type deformed at 920 K(curve d - ^{2x10 7}

dislocations per cm2)assuming point defect associates in zones with dif- ferent densities

p,

and

p,

.Volu- me fraction of heavily doped zone A2=0.Ol= 1 - ^A,

The same experimental results have been interpreted by Labusch-SchrBter/5/ by attri- buting to each neutral dislocation a half-filled band.0n the other hand it is noti- ced that previous results of Fuller-Wolfstirn/28/on the effects of quenching from high temperatures are in agreement with our interpretation based on the determining action of point defects.

DEFORMATION AND E.IINORITY CARRIER LIFE-TIME

Scanning microscopy with Infrared Beam Induced Currents has been used for analyses on the effects of deformation on recombination rates.The experimental set-up used for these observations is illustrated elsewhere/9/.

First the results are recall ed concerning 1 ocal ized measurements on Ge

{

1 1 1

]

sl ices initial 1

y

n-type,30 ohm.cm,deformed at 620

K

up to average etch-pi t densities of

10 C ~ - ~ . A

set of observations concern the dependence on temperature (down to

Fig.

10

- Dependence on T of the IR(Ge fil ter)induced photocurrents by broad area i l - lumination(l)and by a beam44 ym,far from dislocations,after deformation at 620 K up to etch-pit density 10 c m 2 . ^{Fig.1 I} - .IRBIC profiles at different T of dislo=

with EBIC image in

(b).

Fig.12 - EBIC profiles calculated for different bulk diffusion lengths L and different capture efficiencies Y on dislocation. I, cur-

"rent for

L = m

in absence of dislocations,I current far from dislocation for same L.

b

JOURNAL DE PHYSIQUE

l i q u i d ~ 2 of t h e l o c a l l i f e - t i m e a f t e r deformation i n zones f a r from t h e d i s l o c a - )

t i o n s . L i f e - t i m e i n these zones present t h e behaviour shown i n Fig.10 by diagram 2;

diagram 1 i n Fig.10 shows f o r comparison t h e dependence on T o f t h e average l i f e - time as corresponding t o photocurrents from broad area i l l u m i n a t i o n . S i m i l a r beha- v i o u r was observed t i m e ago by Jastrzebska-Figielski/6/in b u l k experiments and i t was i n t e r p r e t e d i n terms of recombination above t h e v o l t a g e b a r r i e r due t o t h e spa- ce-charge around t h e d i s l o c a t i o n s , t h u s l e a d i n j t o d i s l o c a t i o n energy l e v e l s a t -

-

^0-3

eV from t h e valence band.

Given t h a t t h e same behaviour i s encountered i n zones f a r from t h e d i s l o c a t i o n s a f - t e r deformation we conclude i n s t e a d t h a t t h i s l i f e - t i m e phenomena do n o t depend d i r e c t l y on t h e d i s l o c a t i o n s b u t on p o i n t d e f e c t s i n t r o d u c e d t o g e t h e r w i t h t h e d i s l o c a t i o n s themselves.

As a check o f t h e e f f e c t s t h a t changes i n t h e d i s l o c a t i o n recombination e f f i c i e n - c i e s may have f a r from t h e d i s l o c a t i o n s , F i g . l 2 shows some p r o f i l e s c a l c u l a t e d f o r d i f f e r e n t capture e f f i c i e n c i e s y and f o r d i f f e r e n t d i f f u s i o n l e n g t h s L,i.e. d i f f e = r e n t b u l k l i f e - t i m e s . T h e d i f f e r e n t recombination e f f i c i e n c i e s have n e g l i g i b l e e f = f e c t s on t h e background c u r r e n t s a t distances2L:the background c u r r e n t i n s t e a d de=

pends s t r o n g l y on t h e b u l k l i f e - t i m e , i . e . t h e d i f f u s i o n l e n g t h i n t h e bulk.The o r - d i n a t e s i n one case a r e normalized t o I, ,the photocurrent i n absence o f d i s l o c a - t i o n s and f o r L=a,in t h e o t h e r case i n s t e a d t h e y a r e normalized t o I ,the back- zyound c u r r e n t f a r from t h e d i s l o c a t i o n s f o r t h e same L.These r e s u l t s agree w i t h b the c o n s i d e r a t i o n s above.These c a l c u l a t e d p r o f i l e s r e f e r t o EBIC images w i t h sur- face b a r r i e r diodes: p r o f i le s c a l c u l a t e d f o r e x c i t a t i o n s from an IR beam c r o s s i n g t h e specimens a r e now being ana1yzed;however t h e r e i s agreemnt between EBIC and IRBIC p r o f i l e s as f a r as these d i f f u s i o n l e n g t h and recombination s t r e n g t h e f f e c t s are concerned.

Changes i n t h e d i s l o c a t i o n c o n t r a s t occurr,too,as a f u n c t i o n o f temperature,but f o r l i m i t e d amounts:Fig. 11 shows p r o f i l e s i n IRBIC a t various temperatures,together w i t h the EBIC room temperature image o f t h e corresponding d i s l o c a t i o n i n Ge.Contrast changes o f comparable amounts were observed i n S i by Ourmazd e t a1/29/.

Fig. 13

-

EBIC cumulative p r o f i l e s f o r a r r a y s o f para1 l e l d i s l o c a t i o n s w i t h d i f f e r e n t distances A

.

F i g . 14

-

I n t e g r a t e d photocurrent versus d i s l o c a t i o n d e n s i t y ( c ) f o r cu- m u l a t i v e p r o f i l e s as c a l c u l a t e d i n Fig. l 3 , f o r comparison w i t h experimental data(e) from b i b 1 iography/30/.

Photocurrents w i t h broad area i l l u m i n a t i o n correspond t o i n t e g r a t i o n o v e r t h e s a i d area o f t h e E B l C o r IRBIC,etc. , p r o f i l e s , w i t h due account f o r t h e beam radius.So

sities.These cumulative profiles are obtained by assuming additional and independent contrast for each neighbour dislocation.For comparison in Fig.14 experimental data are also shown by diagram (e),reported from the bibliography/30/.There is agreement in trend,with a shift in the dislocation densities by one order of magnitude appro- ximately.This can be related again to point defects introduced during deformation, in parallel with the dislocations.

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