ANALYSIS AND ORIGIN OF POINT DEFECTS IN SILICON AFTER LIQUID PHASE TRANSIENT ANNEALING

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ANALYSIS AND ORIGIN OF POINT DEFECTS IN

SILICON AFTER LIQUID PHASE TRANSIENT

ANNEALING

A. Mesli, J. Muller, P. Siffert

To cite this version:

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JOURNAL DE PHYSIQUE

Colloque C5, suppl6ment au nO1O, Tome 44, octobre 1983 _{page C5-281}

ANALYSIS AND ORIGIN OF POINT DEFECTS I N S I L I C O N AFTER L I Q U I D PHASE TRANSIENT ANNEALING

A. Mesli, J.C. Muller and P. Siffert

Centre de Recherches NucZe'aires, Laboratoire PHASE, 67037 Strasbourg Cedes, France

Resume

-

Nous presentons une revue des travaux consacres ti l l @ t u d e des defauts ponctuels observes p a r des mesures e l e c t r i q u e s s u r du s i l i c i u m i r r a d i e par l a - s e r pulse. Dans l e cas du s i l i c i u m amorphise p a r i m p l a n t a t i o n , il e s t montre que l e s e f f e t s de queue de d i s t r i b u t i o n sont responsables de l a m a j o r i t e des defauts r @ s i d u e l s observes. Par contre, des analyses e f f e c t u e e s s u r du s i l i - cium legerement endommage, o n t montre qu

'

il e s t d i ff i c i l e de r e c u i r e c e r t a i n s d @ f a u t s ponctuels quand l a dur@e d ' i m p u l s i o n diminue. E n f i n , l ' e t u d e des de- f a u t s ponctuels crees p a r l e processus l a s e r dans l e s i l i c i u m v i e r g e met en evidence, essentiellement, des e t a t s donneurs. Ces d e r n i e r s i n t r o d u i s e n t un e f f e t de compensation des materiaux de type

P.

A b s t r a c t

-

Works devoted t o the study of p o i n t d e f e c t s i n pulsed l a s e r annealed s i l i c o n a r e reviewed by means o f e l e c t r i c a l c h a r a c t e r i z a t i o n . T a i l i n g e f f e c t s of the implanted d i s t r i b u t i o n a r e r e s p o n s i b l e f o r the observed r e s i d u a l d e f e c t s i n the case o f a l a y e r amorphized by i o n i m p l a n t a t i o n . However, d i f f i c u l t y i n annealing some kinds of p o i n t d e f e c t s by reducing t h e d u r a t i o n o f l a s e r p u l s e have been shown u s i n g s l i g h t l y disordered s i l i c o n . F i n a l l y , i n t h e case o f i r - r a d i a t e d v i r g i n s i l i c o n , a h i g h l e v e l of p o i n t d e f e c t s a r e created, which a r e e s s e n t i a l l y donor l e v e l s t h a t i n t r o d u c e a compensating e f f e c t i n P-type s i l i c o n

INTRODUCTION

The use o f h i g h power l a s e r annealing has been considered w i t h much i n t e r e s t over t h e p a s t few years since i t o f f e r s c e r t a i n advantages compared t o t h e furnace an- n e a l i n g o r t h e continuous l a s e r process. The t h r e e major p r o p e r t i e s of t h e pulsed l a s e r annealing can be summarized as f o l l o w :

F i r s t , s p a t i a l l y l o c a l i z e d annealing o f f e r s advantages i n m i c r o e l e c t r o n i c s technology. For example, i t can be used t o anneal the implanted m i c r o s t r u c t u r e s w i t h o u t degrading m i n o r i t y c a r r i e r 1 if e t i m e i n t h e bulk.

Secondly, t h e process o f f e r s a h i g h e f f i c i e n c y f o r t h e i n t r o d u c t i o n o f the i m p l a n t dopant i n t o s u b s t i t u t i o n a l p o s i t i o n s /1,

3/

except f o r t h e implanted i m p u r i t y t h a t e x h i b i t s a l o w - e q u i l i b r i u m s o l u b i l i t y i n t h e l a t t i c e t h a t p r e c i p i t a t e s upon annea- l i n g /4/.

Thi rdl3vpu'fsed annealing seems t o produce good qua1 i t y r e c r y s t a l 1 i z a t i o n o f t h e im- plan,&d region. This t o p i c has been l a r g e l y analyzed u s i n g techniques l i k e RBS and TEW /5/. However, more s e n s i t i v e methods i n c l u d i n g photo1 uminescence IR absorption, E P , ~ a n a l y s i s , and e l e c t r i c a l measurements such as DLTS have shown t h a t d e f e c t s Sub- d i s t s i n t h e annealed l a y e r o r a r e created by t h e l a s e r process. The e l e c t r i c a l j o l e o f these d e f e c t s i s c r i t i c a l and can p a r t i a l l y reduce t h e p o t e n t i a l advantages

brought t o semiconductor technology by l o c a l i z e d surface m e l t i n g .

I n t h i s paper we w i l l review 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 pulsed l a s e r i r r a d i a t i o n o f s i l i c o n :

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JOURNAL DE PHYSIQUE

-

For P-N j u n c t i o n prepared by i o n i m p l a n t a t i o n a t a doping l e v e l assuming t h e amor- p h i z a t i o n o f t h e f r o n t l a y e r .

- F o r s l i g h t l y disordered l a y e r s by means o f low dose l i g h t i o n i m p l a n t a t i o n o r

l i g h t p a r t i c l e i r r a d i a t i o n (e-, n, y,

.

. ) .

-

For Schottky s t r u c t u r e s r e a l i z e d on v i r g i n m a t e r i a l processed by pulsed l a s e r .

I

-

LASER PULSE ANNEALING OF AMORPHIZED LAYERS BY ION BOMBARDMENT

1.1

-

EXISTENCE OF A LASER POWER THRESHOLD FOR MINIMUM DEFECTS

Processing by pulse annealing o f i o n implanted l a y e r s must ensure a h i g h e l e c t r i c a l a c t i v a t i o n o f t h e introduced dopant and as w e l l as a minimum r e s i d u a l d z f e c t l e v e l . The l a s e r power which s a t i s f i e s b o t h o f these c o n d i t i o n s i s c a l l e d t h e t h r e s h o l d energy t o have a minimum d e f e c t d e n s i t y " lower power leaves a h i g h c o n c e n t r a t i o n of p o i n t d e f e c t s near t h e j u n c t i o n , w h i l e h i g h e r power may i n t r o d u c e l a s e r process r e - 1 a t e d defects.

K i m e r l i n g and a l . /6/ have shown ( F i g u r e 1 ) t h a t the l a s e r power t h r e s h o l d f o r m i n i - mum defects i s independent on t h e type o f pulsed>er used. I n t h e case of LAG-

(1.06 pm) t h e y have shown a decreasing minimum p o i n t d e f e c t w i t h i n c r e a s i n g dopant implanted dose due t o t h e enhanced f r e e c a r r i e r s c o u p l i n g

.

EI (om HI o f t h e l i g h t . On the o t h e r hand f o r the doubled .- - -

E~ ( o n u ) frequency Nd-YAG i n which band t o band absorption

C E~ 10 4 5 H) occurs, t h e l i m i t i s lower and independent o f

a" dose.

l o i 4 ~ si-0-LZ

These r e s u l t s a r e s i m i l a r t o those obtained f o r a ruby l a s e r (Figure 2)

-

7'

lPb a8 F i g . 2

- T o t e l defect s t a t e c o n c e n t r a t i o n o f t h e

LASER R ~ W E R ( J / ~ & I t h r e e e l e c t r o n t r a p s a f t e r various ruby l a s e r

p u l s e energies ../7/. p.!015crn-3

IMPLANT: 5 0 kevAs F i g . 1 - T o t a l d e f e c t s t a t e

PULSE WIDTH: i 5 o n s e c c o n c e n t r a t i o n observed by

I

capacitance t r a n s i e n t spec-

-

z troscopy i n a r s e n i c i m p h n - ted, l a s e r processed n'p Z j u n c t i o n s . Low defect w o t h r e s h o l d c o n d i t i o n s a r e

8

\ 1 ~ 1 4 denoted as (a) A = 0.53 pm 1E15 l i g h t f o r a l l i m p l a n t doses: ( b ) A = 1.06 pm l i g h t . 1E 16 W As (50 keV) = 1016 Y o ( c ) A = 1.06 pm. AS (50 (a) (b) ( c ) keV) = 1014 cm-'. /6/ I I 1 .

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1.2

-

ORIGIN OF THE DEFECT ANNEALING LIMIT

I f t h e r e i s general agreement / 6 - 1 3 / t h a t f o r device p r e p a r a t i o n t h e l a s e r proces- s i n g must m e l t b o t h t h e implanted dopant d i s t r i b u t i o n as w e l l as any d e f e c t b u r i e d beyond the f r o n t amorphized l a y e r i n a r e g i o n c a l l e d the " I m p l a n t a t i o n t a i l " , i t remains i m p o r t a n t t o determine i f l a s e r annealing c o n d i t i o n s can be found t o assure o r 'not the c o n d i t i o n "depth o f m e l t i n g equals t h e depth o f i m p l a n t a t i o n damage>> 16, 8 1 -

Two concepts can be p o s t u l a t e d . F i r s t : a) m e l t depth never equals the i m p l a n t damage depth and second : b) the molten l a y e r covers the damage zone and a t higher l a - s e r power t h e l a s e r r e l a t e d d e f e c t s are dominant.

a) Concept o f i m p l a n t r e s i d u a l d e f e c t s 0 0 . 2 0.4 0 . 6 0.8 D e ~ t h microns of silicon According t o t h i s concept, t h e d e f e c t annealing l i m i t i s c o r r e l a t e d w i t h r e - s i d u a l d e f e c t s b u r i e d too deeply t o be touched by t h e m e l t depth, so t h a t h i g h e r l a s e r energies, b u t s t i l l lower than t h e l i m i t o f v i s i b l e degradation o f t h e surface, can n o t guarantee the c o n d i t i o n "depth o f m e l t i n g equals t h e depth o f imp1 a n t a t i o n damage".

Blood and a l . /14/ have shown t h a t the t a i l i n g e f f e c t i s i m p o r t a n t even more if

t h e i m p l a n t a t i o n i s performed a t room temperature and o f f - a x i s as i l l u s t r a t e d on F i g u r e 3. The t a i l has an exponential slope which r e s u l t s i n a g r e a t penetra- t i o n (over 1 pm) f o r c o n c e n t r a t i o n four

o r f i v e orders o f magnitude lower than t h e maximum.

F i g . 3

-

The dose dependence o f p r o f i l e s 32

o f 40 keV p i o n s implanted i n t o m i s a l i - gned <110> c r y s t a l s a t room temperature 1141.

The t a i l i n g e f f e c t i s due t o a pure d i f f u s i o n mechanism /15/ which can be enhanced by the presence o f p o i n t defects, by t h e c r y s t a l temperature o r by channeling pheno- mena i f t h e i m p l a n t a t i o n i s performed on-axis.

I n t h e l a t t e r case the r e c r y s t a l l i z a t i o n w i l l be i n i t i a t e d i n a r e g i o n c o n t a i n i n g defects and the q u a l i t y o f regrowth w i l l be i n f l u e n c e d by the number o f r e s i d u a l t r a p s a t the p o i n t o f regrowth.

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C5-284 JOURNAL DE PHYSIQUE

I n t h e same way Brower and Peercy /13/ nave found by EPR measurements and ruby l a s e r annealing o f phosphorus implanted i n s i l i c o n simultaneously amorphized by s i l i c o n ions t h a t the e f f i c i e n c y o f t h e l a s e r annealing increases when the energy of the i n c i d e n t i o n s i s reduced.

X COORDINATE

(mm)

S k o l n i c k e t a l .

/12/

have s t u d i e d by photoluminescence the d e f e c t s i n s i l i c o n amorphized by s i l i c o n i o n s . A f t e r ruby l a s e r annealing, they found a photoluminescent l i n e n o t removed even a t an energy o f 2.5 J/cmZ. They conclude t h a t t h i s d e f e c t i s produced d u r i n g t h e i m p l a n t a t i o n i n the r e g i o n o f lower damage beyond t h e amorphous r e g i o n i n t h e channeling t a i l o f the i m - p l a n t even i f t h e i m p l a n t a t i o n i s performed o f f - a x i s .

F i g . 4

-

Depth p r o f i l e o f the t h r e e d e f e c t s a f -

2

t e r 1 J/cm ruby l a s e r annealing. The l i m i t o f measurements a t zero b i a s voltage i s given by l1 the dashed l i n e . /7/. loU

-

", '

?

.a12 E ;,lon 0

."

SAMPLE 630 (I001 - -

toa

r e

x

10" "A; a-' ..-

AT 170 KeV

-

O E a 2 . 3 J c i '

-

la'_

A E - 2 . 7 0 E.3.2 J J cm" cma -

- ' 0 6

P

- -

-

! 16'

I l

i l

b

i l

_s

lb

_1'1

II ~i

_I, *

X

COORDINATE

(nn)

DEPTH l~vrnl 5, P R 4 1 7 , 1-5 nm

1

V ~ = o V lrrpl 3 1 ~ * 15 kcv 1 0 ~ ~ ~ ~ - ~ 1 Rub, loser onrrolorq

I\

.

El ( 0 30 ev1 v E~ I 0 3 7 eV1 r E~ ( 0 4 5 .VI - I I I 01 as 12

F i g . 5 ( a )

-

Diode leakage c u r r e n t d e n s i t y F i g . 5 ( b )

-

Diode leakage c u r r e n t den- f o r samples implanted a t 90 keV w i t h an As s i t y f o r samples implanted a t 170 keV.

fluence of 8 1015 ,,-2. ~h~ laser annea- Other parameters are t h e same as those Tin9 energy d e n s i t y was 2.3 J-cm-2 (O), i n ( a ) . /ll/.

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This i s also confirmed by the diode leakage current measurements performed by Wang e t a l .

/11/

(Fig. 5 a and b ) . Complementary c a r r i e r collection efficiency (coming from EBIC data) suggest t h a t the defects d i s t r i b u t i o n has a long t a i l w i t h a signi- f i c a n t defect concentration.

By double X-ray diffractometry Servidori e t a l . /17/ have shown t h a t the observed disorder which decreases the l a t t i c e parameters of s i l i c o n extend t o depths much grea t h e r than the amorphi zed region.

To confirm t h a t the observed defects a r e due to some "long t a i l " . Mesli e t a l . /7/ have removed the defects in the l i g h t l y damaged region by a low temperature annealing (400" C ) not s u f f i c i e n t t o change, the absorption condition in the surface amorphized layer so t h a t the melt depth will not be changed.

In t h i s case DLTS spectra have shown t h a t defects are no longer present (Fig. 6 ) af- t e r l a s e r i r r a d i a t i o n i n the space harge region of the junction. I t i s a l s o t o be

5

noted t h a t the l a s e r energy (1 J/cm ) i s too low t o c r e a t e l a s e r related defects i n t h i s deep region.

S i p FZ ~ 1 1 1 5 I - 5 n c m 1mpl.l' P+ 15 keV 1cq6 ims /on2 en,

.

143 5-I C2V,+O.5V)

E3 (0.45 cV1 (1) Loser onneoling 1 ~ / c r n ~

Fig. 6

-

DLTS signal when

a

conventional thermal annealing i s performed before the

1 aser treatment. / 7/.

(2) thermol treatment !.mar

'

20 min)

15[ M o r e Loser onnealirg 1 ~ c m 2

b) Laser introduced defects a f t e r high l a s e r power exposure

This concept i s i l l u s t r a t e d i n Figure

1

by the f a c t t h a t an increase of the concentration of point defects i s observed f o r l a s e r energies over the minimum defect threshold. Several possible sources of defect introduction during l a s e r treatment can be enumerated : surface contamination, ambiant processing i n a i r , crystal regrowth velocity, high temperature gradients, quenching, e t c

.

The most important question i s t o determine the importance of l a s e r related defects present deep in the crystal in the charge space region of the junction.

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JOURNAL DE PHYSIQUE

1.3

-

ROLE OF THE FRONT AL4ORPHIZED LAYER

The question here i s i f a f r o n t amorphized l a y e r can a c t as a mash f o r good annea- l i n g o f deep defects, o r i n c o n t r a r y , can be b e t t e r as t h e absorption o f l i g h t and m e l t depth i s greather.

a) The amorphized l a y e r as a mash

Brower /13/ has shown by EPR a n a l y s i s o f the paramagnetic l a t t i c e damage recovery t h a t the r e c r y s t a l l i z a t i o n o f low dose phosphorus i m p l a n t a t i o n i s b e t t e r i f t h e f r o n t l a y e r i s n o t amorphized.

Moreover t h e argument t h a t t h e amorphized l a y e r can p l a y a r o l e o f b a r r i e r has been used by Sasaki e t a l . /18/ i n t h e discussion o f the two k i n e t i c models "melting" o r "plasma" annealing. This group has performed double i m p l a n t a t i o n o f 3 1 ~ , low dose t o dope and h i g h dose t o forman amorphized f r o n t l a y e r . I n agreement w i t h precedent works /6,

7 1 ,

they found t r a p s underneath the amorphized l a y e r .

I n c l a s s i c a l thermal model c a r r i e r recovery may have occured e a s i e r i n t h e sample which have t h e surface amorphized l a y e r , i n t h i s case m e l t depth i s l a r g e r than i n c r y s t a l l i n e s i l i c o n f o r the same energy as p r e d i c t e d by t h e models /19/. As t h i s assumption c o n t r a d i c t e s t h e previous r e s u l t s , t h e authors conclude t h a t t h e process o f c a r r i e r recovery i n t h e deep r e g i o n i s n o t mel.ting and r e c r y s t a l l i z a t i o n b u t i s r e l a t e d t o the deep p e n e t r a t i o n o f l i g h t i n t h e c r y s t a l so t h a t i t i s t h e plasma mechanism t h a t occurs.

18:

By u s i n g double l a s e r i r r a d i a t i o n (Figure 7 )

However, i t must be remembered t h a t annealing of some p o i n t defects can occur by s o l i d phase e p i t a x y i n t h e h o t r e g i o n adjacent t o the m e l t as w i l l seen i n the sec- t i o n 11.

b ) The p o s i t i v e r o l e of t h e amorphized l a y e r

Nakashima e t a1

.

/20/ r e p o r t e d by photoluminescence o f l a s e r annealing of p h o s p h o r ~ s implanted l a y e r s t h a t the regrowth i s b e t t e r f o r h i g h doses o f 3 1 ~ + than f o r low doses, due t o a h i g h e r absorption r e g i o n which enhances t h e temperature r i s e .

h e r e t h e f i r s t removes t h e amorphized l a y e r and t h e second anneals t h e deep defects, they have a b e t t e r annealing t h a t using a

- .

I t has been a l s o shown by Young /21/ t h a t t h e r e c r y s t a l l i z a t i o n v e l o c i t y V can p l a y a r o l e . The decrease o f V has been c o r r e l a t e d t o b e t t e r e l e c t r i c a l c h a r a c t e r i s t i c s of t h e j u n c t i o n ( b y measurements o f s o l a r c e l l parameters under i l l u m i n a t i o n ) . Expe-

.

P'

mo

K ~ V 1 = 10'~ cvi2

.

2s ~~y 5.101~ cm-2 LSS;?,

1

0 - s w Ruby # ,

..

I # I'

::

I I s s i n g l e l a s e r shoot through the amorphized

I

::

I $ 1 ayer.

These authors suggest t h a t a f t e r the f i r s t l a s e r p u l s e t h e absorption l e n g t h o f s u r f a - ce r e g i o n becomes l a r g e r so t h a t t h e second l a s e r beam can be t r a n s m i t t e d deeper i n t o t h e c r y s t a l and can anneal the deep l y i n g defects. I a .

.-

L

2

1 L -

u10

10 u) 3.0

Distance from Junction X(pm)

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r i m e n t a l l y t h e regrowth v e l o c i t y has been diminished by i n c r e a s i n g t h e temperature of t h e s u b s t r a t e d u r i n g t h e l a s e r annealing, b u t the suggest t h a t the q u a l i t y of the regrowth l a y e r i s dependent on V cannot be confirmed by t h i s experiment w h i l e t h e melt-depth increases so t h a t the c o n c e n t r a t i o n sprawl o u t and the j u n c t i o n i s deeper. For t h e f i r s t concept, i n which the r e c r y s t a l l i z a t i o n i s i n i t i a t e d i n the exponential t a i l o f the implanted damage, the q u a l i t y o f regrowth w i l l be b e t t e r i f we s t a r t i n a more deeper r e g i o n / 7 / .

1.4

- ASSIGNMENT AND ANNEALING OF POINT DEFECTS

The f i r s t t e n t a t i v e assignment o f t h e l e v e l s was made by K i m e r l i n g e t a l . /6/ and Wang e t a l . /11/. They tended t o separate those observed i n i o n i m p l a n t a t i o n from those present i n i r r a d i a t e d bare s i l i c o n . Some o f them are c l e a r l y r e l a t e d t o r e s i - dual i o n damage l i k e E (0.18 eV) which i s observed a l s o a f t e r 1 MeV e l e c t r o n bombardment, t h i s can then be assigned t o

I V

- 01 d e f e c t s a l s o r e p o r t e d by /7/ w i t h an an- n e a l i n g temperature o f 400' C . This i s d i f f e r e n t from E (0.19 eV) r e p o r t e d by K i m e r l i n g /6/ on bare l i q u i d phase processed s i l i c o n t h a t disappears o n l y a t 550°C. I n t h e range o f 0.27

-

0.37 eV, e l e c t r o n t r a p s a r e found c o r r e l a t e d t o defects which increase i n c o n c e n t r a t i o n by i n c r e a s i n g t h e mass p a r t i c u l e s . They are assigned t o multicomplex defects E (0.27 eV) o r multivacancies centers.

A s i m i l a r assignment i s given f o r t h e photoluminescent l i n e and f o r d e f e c t s observed by X-ray d i f f r a c t o m e t r y /17/.

The H (0.36 eV) has been assigned t o carbon r e l a t e d d e f e c t s /8/ I V

-

Ci

- 0

I

which disappears a t 400

-

500° C.

Deeper d e f e c t s E (0.45 eV

-

0.53 eV) have been associated w i t h divacancy and penta- vacancy /7/ w i t h a h i g h e r annealing temperature.

The most i m p o r t a n t p o i n t i s t h a t a l l these defects disappear a t l e a s t f o r t h e most complex d e f e c t s ( ~ 5 ) , a t a temperature around 600" C so t h a t a p o s t l a s e r treatment performed a t t h i s temperature i s necessary and s u f f i c i e n t t o a t t a i n good j u n c t i o n c h a r a c t e r i s t i c s as r e p o r t e d by /22, 23/.

I1

-

PULSED LASER ANNEALING I N SLIGHTLY DISORDERED SILICON

The p r i n c i p a l conclusion o f the f o r e g o i n g s e c t i o n i s t h a t pulsed l a s e r annealing o f h i g h l y doped l a y e r s by i o n i m p l a n t a t i o n i s much more e f f i c i e n t i n t h e amorphized r e - gion than i n the s l i g h t l y disordered adjacent t a i l region.

I n t h i s s e c t i o n we w i l l discuss more p r e c i s e l y t h e annealing o f s l i g h t l y disordered c r y s t a l l i n e s i l i c o n by means o f low dose i o n i m p l a n t a t i o n o r l i g h t p a r t i c u l e s i r r a - d i a t i o n (em, n, y . . . ) .

Much l e s s a t t e n t i o n has been devoted t o t h i s s i t u a t i o n i n which t h e mechanism of an- n e a l i n g o f each i n d i v i d u a l p o i n t d e f e c t can be s t u d i e d as opposed t o the o v e r a l l an- n e a l i n g described i n t h e f i r s t s e c t i o n i n which amorphization takes place.

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C5-288 JOURNAL DE PHYSIQUE

the case o f thermal annealing (700" C, 15 mn, dashed curve) we n o t i c e t h e w e l l know d e c l i n e i n the u t i l i z a t i o n c o e f f i c i e n t w i t h i m p l a n t a t i o n dose due t o t h e dopant pre- c i p i t a t i o n process.

However, f o r pulsed l a s e r annealing, the c o e f f i c i e n t decreases w i t h de-

-

----

+-- _{creasing dose, and t h e e f f i c i e n c y}

o f the doping increases w i t h l a s e r energy. This shows t h a t a t low dose i m p l a n t a t i o n , i t i s d i f f i c u l t t o have a t o t a l a c t i v i t y o f the dopant even i f the s i l i c o n i s o n l y s l i g h t l y disordered.

*

\

- I 10"

1

10

"

10 l3 10 \ l4

'

\ 0 ' I 1) 1.6; 2) 2.5. Thermal annealing

F i g . 8

-

Dependence o f t h e i m p u r i -

9

_{phosphorus i o n dose a f t e r l a s e r}t y u t i l i z a t i o n c o e f f i c i e n t on t h e

(1, 2) and thermal ( 3 ) annealing. 2 Laser p u l s e energy (J/cm ) :

D,

cm-' a t 700" C f o r 15 min. /25/.

I t seems t h a t i t i s more d i f f i c u l t t o anneal completly t h e r e s i d u a l damage even w i t h h i g h l a s e r power.

These observations a r e i n c o n t r a s t t o c l a s s i c a l annealing k i n e t i c s when most r e s i d u - a l d e f e c t s l i k e P

-

V complex ( E c e n t e r ) o r V

-

0 ( A c e n t e r ) and vacancy complexes anneal o u t i n t h e range o f 150

-

500" C.

I n o r d e r t o understand the pulsed l a s e r annealing k i n e t i c s o f p o i n t d e f e c t s , a u n i - form f o r m a t i o n o f damage by bombardment w i t h l i g h t p a r t i c u l e s associated w i t h ana- l y s i s techniques such as DLTS o r I R absorption would appear t o be u s e f u l l . These kinds o f methods o f f e r t h e p o s s i b i l i t y o f c o n t r o l l i n g the n a t u r e o f p o i n t d e f e c t s and o f f o l l o w i n g t h e i r c o n c e n t r a t i o n and p r o f i l e s i n t h e b u l k w i t h t h e parameters o f t h e annealing process.

Using these techniques, Kachurin e t a l . /26/ and K i m e r l i n g e t a l . /6/ have shown t h a t , a f t e r f a s t e l e c t r o n i r r a d i a t i o n o f s i l i c o n and Q switched ruby o r Nd YAG l a s e r annealing,the various types o f p o i n t d e f e c t s created a r e n o t a l l removed a t t h e same time. C e r t a i n s d e f e c t s need h i g h e r power as i l l u s t r a t e d i n Figures 9 and 10.

F i g . 9

-

Peak amplitude v a r i a - t i o n s i n DLTS spectra of r a d i a - t i o n d e f e c t s formed by e l e c t r o n i r r a d i a t i o n , a f t e r t h e a c t i o n o f l a s e r pulses w i t h various

energies. Laser pulse energy

, ,

(10)

F i g . 10

-

Defect s t a t e spectrum o f 1-MeV e l e c t r o n i r r a d i a t e d , phosphorus doped, f l o a t - z o n e d s i l i c o n . @ = bombardment dose. s i - P - F Z T = d e f e c t s t a t e emission time

+

= SX(O'' e-/cme constant a t temperature o f peak

n = z ~ i o ~ % m - ~ maximum. S o l i d l i n e : as-bom-

r = 6mS barded and Q-switch l a s e r anneal:

dashed curves : I s . 10s. 30s cw I l a s e r anneal, consecutively. E(O.44) A ( l a s e r ) = 1.06 pm. / 6 / .

_----

I 5 0 200 250 TEMPERATURE ( O K )

I n F i g . 9, i t i s seen t h a t t h e E c e n t e r and l e v e l a t Ec

- 0.30 eV disappear when t h e

2

p u l s e energy i s E 5 1 J/cm f o r t h e ruby l a s e r . However, the A c e n t e r and t h e diva-

2

cancy needs h i g h e r energies ( E 1.5 J/cm ) . The same authors have measured t h e r a - 2 d i a t i o n d e f e c t c o n c e n t r a t i o n as f u n c t i o n s o f depth f o r a pulse energy o f 2.5 J/cm

.

The r e s u l t s r e p o r t e d i n F i g . 11 show t h a t , f o r a s u f f i c i e n t l y l a r g e pulse energy, t h e r a d i a t i o n d e f e c t s disappear o n l y from a t h i n n e r l a y e r (0.2

-

0.3

urn)

where the tem- p e r a t u r e exceeds t h a t needed t o m e l t t h e surface.

I

11

I n excess o f t h i s depth, t h e annealing

L >

T,

OC k i n e t i c s i s s p e c i f i c t o each d e f e c t . These observations, agree w i t h what

I

was s a i d above about the d i f f i c u l t i e s i n annealing t h e t a i l p a r t o f t h e p o i n t d e f e c t d i s t r i b u t i o n since they are s i -

3000 t u a t e d i n a r e g i o n adjacent t o t h e m e l t

depth.

m To e x p l a i n t h i s r e s u l t we must consider

8 t h e d u r a t i o n o f annealing which plays

E

-

2000

an i m p o r t a n t r o l e . U

*-

F i g . 11

-

Dependence o f t h e r a d i a t i o n

1000

d e f e c t c o n c e n t r a t i o n on t h e depth f o r 2

1 aser p u l s e energy 2.5 J/cm

.

Chain curves : c a l c u l a t e d maximum temperature p r o f i l e .

I )

M e l t i n g region. 11) SU- perheated region. / 2 6 / .

0

0.5

7.0 x,

P

I n the c l a s s i c a l model o f t h e thermal annealing k i n e t i c s , t h e temperature T and the annealing time t are r e l a t e d by t h e f o l l o w i n g expressions :

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C5-290 JOURNAL DE PHYSIQUE

where NTT i s t h e c o n c e n t r a t i o n o f p o i n t defects remaining a f t e r the l a s e r process, NTo i s t h e i r i n i t i a l c o n c e n t r a t i o n and T t h e r a t e o f d e f e c t annealing. This l a t e r

parameter depends on t h e temperature and a c t i v a t i o n energy U o f the process.

4

Elsewhere f o r p u l s e times used (20

-

50 ns), a l l c l a s s i c a l thermal models i n d i c a t e t h a t the m e l t i n g time i s a few hundred nanoseconds and t h e t o t a l h e a t i n g time i s one o t two orders o f magnitude g r e a t e r . This ensure the annealing o f V

-

P and V

-

V centers i n the molten r e g i o n o f course, b u t a l s o i n t h e heated r e g i o n adjacent t o t h e melted zone where the temperature i s s u f f i c i e n t l y h i g h t o anneal w i t h i n the dura- t i o n o f t h e process. Whereas t h i s time i s too s h o r t f o r any recovery o f the V

-

0 a s s o c i a t i o n i n a r e g i o n deeper than t h e m e l t , t h i s e x p l a i n s the abrupt v a r i a t i o n seen by Kachurin /26/ f o r t h i s l e v e l a t the boundary o f the m e l t f r o n t .

-

v

-

P

2000

".

0 x o m CI

:

0 C

The p o s s i b i l i t y o f annealing some k i n d s o f p o i n t defects, l i k e t h e P

-

V and V - V a s s o c i a t i o n s , i n t h e h o t r e g i o n near the molten l a y e r can e x p l a i n ( i n terms o f the thermal model) t h e r e s u l t s o f Sasaki /18/. The use o f a double l a s e r i r r a d i a t i o n m u l t i p l i e s by two t h e t o t a l d u r a t i o n o f t h e annealing. U 1 - exp

- -

( 2 ) (- i s cons- To KT N,-N,fxp I-t/r) 0 t a n t )

-

\

\ ik-i/%exp (-u/~T) Using f i r s t o r d e r k i n e t i c s and t h e

\

\ data published /27, 29/, and pro-

% \ '\ v i d e d t h a t e x t r a p o l a t i o n t o s h o r t

i \ time i s j u s t i f i e d , t h e removal

?ti

?

temperature o f V

-

P , V

-

V and

t \ Fusion _V

-

_{0 centers are given i n F i g . 12}

S t e i n /30/ has s t u d i e d t h e divacancy annealing i n boron implanted s i l i c o n a t a dose

C 0 0 L) I n t h i s model, we assume t h a t l a -

g

1000, s e r i r r a d i a t i o n does n o t i t s e l f u L c r e a t e p o i n t d e f e c t s i n a concen-

X

t r a t i o n comparable t o t h a t produ-

5

I- ced by bombardment o r i m p l a n t a t i o n

500- laser and e a t low doses, and t h a t the annea- pulse anneal.

-

l e d d e f e c t s do n o t reform t o pro-

duce new centers.

CW laser.

o 1 I I I F i g . 12

- Annealing k i n e t i c s ex-

10-8 10-3 . 100 103 t r a p o l a t e d t o t h e s h o r t times.

Time o f annealing Is)

F i g . 12 shows t h a t i n t h e c l a s s i c a l annealing ;=ion the temperature needed t o anneal p o i n t s defects i s s l i g h t l y g r e a t h e r than t h a t given i n the l i t e r a t u r e /27, 29/. This i s due t o t h e r a t i o NTo

/

NTT = 10 considered i n our c a l c u l a t i o n . The temperature needed t o anneal 90% o f the p o i n t d e f e c t s increases when t h e h e a t i n g time i s reduced, so t h a t a temperature above t h e m e l t i n g c o n d i t i o n seems t o be necessary t o remove t h e

V

-

0 a s s o c i a t i o n f o r annealing times l e s s than 10-3 sec. This i s i n agreement w i t h t h e r e s u l t s r e p o r t e d by Kachurin e t a l . i n F i g . 9, since t h i s a s s o c i a t i o n i s o n l y r e - moved i n the l i q u i d phase extension.

o f

1014

~ m - ~ . He f i n d s a l s o t h a t the V

-

V annealing begins a t l a s e r energies l o - wer,that those expected f o r m e l t i n g i n c r y s t a l l i n e s i l i c o n as r e p o r t e d i n F i g . 13. However,for t h e i m p l a n t a t i o n dose used, t h e damage can enhance photon absorption i n the implanted l a y e r and induce a decrease o f t h e thermal c o n d u c t i v i t y so t h a t the m e l t temperature i s achieved a t a lower l a s e r energy t h a t i s s i t u a t e d between l i m i - t i n g c o n d i t i o n s d e f i n e d f o r amorphous and undamaged s i l i c o n .

(12)

However, t h e p u l s e h e a t i n g has a serious drawback i n t h e annealing o f p o i n t defects i n r e g i o n deeper than t h a t reach by t h e m e l t i n g . Then from t h e p r a c t i c a l p o i n t o f view, nanosecond 1 aser annealing o f s l i g h t l y disordered s i l i c o n o r the r e g i o n beyond t h e amorphized l a y e r should be used i n c o n ~ b i n a t i o n w i t h m i l l i s e c o n d o r continuous low temperature treatment as shown i n F i g . 10.

2

The f a c t t h a t Kachurin /26/ f i n d s o n l y 0.2 pm o f m e l t i n g a t 2.5 J/cm i s s u r p r i s s i n g i f we compare t h i s r e s u l t t o t h a t expected i n t h e c l a s s i c a l thermal model i n which a m e l t depth g r e a t e r than 0.5 i s g e n e r a l l y expected. This may mean t h a t t h e m e l t i n g t h r e s h o l d and temperature p r o f i l e i s c r y s t a l l i n e s i l i c o n can be d i f f e r e n t from those expected /33/. Recently some authors /34, 351 have i n c l u d e d i n t h e i r t h e r - mal model the c r y s t a l l i n e q u a l i t y represented by t h e p r o b a b i l i t y o f d i f f u s i o n o f t h e

photogenerated c a r r i e r s before t h e i r t h e r m a l i z a t i o n . So t h a t due t o the increase of the d i f f u s i o n l e n g t h as a f u n c t i o n o f $he r e s i s t i v i t y o f t h e m a t e r i a l , the m e l t i n g

.

t h r e s h o l d can be h i g h e r t h a t those r e p o r t e d by

I 1 1

-

PULSED LASER EFFECT I N CRYSTALLINE SILICON

I I I I 1 1.0

-

_{LASER ANNEALING} OF OIVACANCIES -

=

0 8 -

-

0 o f 0.6- F - f

2

0.4 -

-

o 0.2 -

-

t.5 0.6 0.7

h

,

The f i r s t observations o f damaged r o l e o f t h e pulsed l a s e r have been made on photo- diode, p h o t o t r a n s i s t o r and s o l a r c e l l s /36, 38/.

the c l a s s i c a l thermal model. However t h i s va- r i a t i o n of t h e m e l t i n g t h r e s h o l d i s n o t s u f f i - c i e n t l y i m p o r t a n t t o e x p l a i n the v e r y t h i n me1 t r e g i o n found by Kachurin /26/.

As a conclusion t o these f i r s t two sections, we can say t h a t the s h o r t d u r a t i o n of h e a t i n g i s h i g h l y advantageous t o t h e e l e c t r i c a l a c t i - v a t i o n o f t h e implanted dopant, o f f e r i n g a t t h e same time a complete absence o f macroscopic defects such as d i s l o c a t i o n s o r s t a c k i n g f a u l t s . F i g . 13

-

Unannealed divacancy f r a c t i o n s versus

25

ruby l a s e r energies. Each p o i n t was

I n order t o analyse t h e d e f e c t s created by t h e l a s e r process alone, we consider i n t h i s s e c t i o n t h e v i r g i n c r y s t a l l i n e s i l i c o n .

ENERGY l ~ / c r n ~ ) taken on a separate sample. /30/.

I n such m a t e r i a l , the primary e f f e c t d i r e c t l y observed i s the degradation o f I

-

V

c h a r a c t e r i s t i c s /39, 40/ as shown i n F i g . 14 a) and b ) .

These r e s u l t s obtained w i t h ruby l a s e r (20 ns) on N-type s i l i c o n demonstrate the r e s p o n s a b i l i t y o f t h e l a s e r i n t h e c r e a t i o n o f t r a p p i n g and recombination l e v e l s near t h e surface. These recombination centers determine t h e reverse c u r r e n t o f the s c h o t t k y diode /41, 42/. The experiment o f f i g u r e 14 ( b ) shows moreover t h a t t h e m a j o r i t y o f d e f e c t s are l o c a t e d w i t h i n t h e molten region, so t h a t we can c o n f i r m t h a t i n t h e case o f a P - N j u n c t i o n ( i n which t h e space charge r e g i o n begins near t h e m e l t i n g l i m i t ) t h e observed d e f e c t s are m o s t l y c o r r e l a t e d t o r e s i d u a l implanted damage. Many d e f e c t s a n a l y s i s r e s u l t s have been given i n the l i t e r a t u r e . The e l e c - t r i c a l parameters o f these defects, determined e s s e n t i a l l y by t h e DLTS method, a r e summarized i n Table I f o r N-type s i l i c o n i r r a d i a t e d w i t h ruby and Nd : YAG l a s e r . Concerning t h e energy l e v e l s o f t h e t r a p s detected, the r e s u l t s are q u i t e d i f f e r e n t from author t o author. However,all references agree about t h e thermal annealing temperature. The h i g h values obtained i n d i c a t e the s t a b i l i t y o f the p o i n t defects created by t h e l a s e r process.

(13)

JOURNAL DE PHYSIQUE ld

,-

LASER ENERGY 0.9 ~ . a n - ~ t

-

0 0.1 0.2 0.3 ETCHING DEPTH(pml

F i g . 14 a )

-

I n f l u e n c e o f l a s e r energy F i g . 14 b )

-

Change o f t h e average l e a - on t h e I - V c h a r a c t e r i s t i c s o f Au-Si kage c u r r e n t o f Schottky diodes on a S i diodes r e a l i z e d on i r r a d i a t e d surfaces sample i r r a d i a t e d w i t h a s i n g l e pulsed

/39/. l a s e r w i t h e t c h i n g depth. The damaged

l a y e r has a depth comparable t o t h e m e l t i n g depth o f about 0.2 pm. /40/. t h e n a t u r e o f the d e f e c t s depends s t r o n g l y on t h e i n i t i a l i m p u r i t y c o n t e n t /45/ such as oxygen, carbon, dopant

...,

on t h e degree o f s t r u c t u r a l i m p e r f e c t i o n o f the mate- r i a l t h a t determines the t h r e s h o l d m e l t i n g as we have seen i n t h e previous s e c t i o n . This p r o b a b l y e x p l a i n s t h e d i s p e r s i o n observed i n the energies l e v e l s o f t r a p s o f Table

I .

Laser t y p e 1rrad.Energy Energies l e v e l s Capture c r o s s sect. Thermal anneal. REF.

(14)

Other mechanisms should be taken i n t o account i n t h e formation o f l a t t i c e defects. The h i g h s t r e s s g r a d i e n t (10 BOc/cm) and t h e presence o f shock waves has been shown by some authors /46, 471 t o be a c t i v e .

I n P-type s i l i c o n , fewer p o i n t d e f e c t analyses have been made. OLTS analyses p e r f o r - med by Mooney e t a l . 1481 has shown a l e v e l s i t u a t e d a t Ev

+

0.24 eV. Besides K i m e r l i n g e t a l . /6/ has shown t h a t t h e i r r a d i a t i o n o f P-type s i l i c o n creates an exponential d i s t r i b u t i o n o f t h e f r e e c a r r i e r s near t h e surface as shown i n F i g . 15.

l a r g e capture cross s e c t i o n s ( F i g . 16). One o f them (Er = 0.30 eV) i s probably a

F

5

10" L

8

1 0 ~ a I I

s

10'6-

.,

vacancy-oxygen complex i n view o f t h e v a r i a t i o n o f t h e i r c o n c e n t r a t i o n i n CZ and FZ m a t e r i a l . However the deep l e v e l (Ec

-

0.41 eV) i s absent i n t h e CZ s u b s t r a t e . This suggests a c o m p e t i t i o n between Ec

-

0.41 eV and Ec

-

0.30 eV r e l a t e d t o oxygen den- s i t y .

F i g . 15

-

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

.- a-B-co of bare, P-type substrates processed i n

X=l.Wr _{t h e l i q u i d phase regime (pulse w i d t h =}

150 ns). The exponential d i s t r i b u t i o n

-pimZ

suggests a d i f f u s i o n process. / 6 / .

87 MWIC~' T h i s i l l u s t r a t e s t h e compensating e f f e c t

9s nwicm2 created by the presence o f m i n o r i t y c a r -

I

,

I r i e r t r a p s . Using sup-band gap l i g h t

-

F i g . 16

-

M i n o r i t y d e f e c t s s t a t e s

LL

8 E ( . ~ o ~ v ) _Rubysi P _{laser[20ns) iJ/cm2}type i-5flom spectrum i n P-type Schottky diode. The capture cross-sections are res-

-

Fz I fph) -25 $

x

_:

_n

---

cz eM-5.4BE-3s-1 p e c t i v e l y on= 2 10-14, 5.7 10-15

E(.lBeVl I 1 E (.41eVI and 3.5 10-15 cm 2

.

1.5

::

_rn

rn ?2 \\

I- 1.0- \ I Analysis o f N and P-type s i l i c o n

d

\a' has shown t h a t t h e major observed

/

'---

1 d e f e c t s a c t as e l e c t r o n t r a p s . The

0 . 5 - _{r o l e o f oxygen i n t h e formation o f}

d e f e c t s seems be ambiguous. I n

1 I I I N-type m a t e r i a l no v a r i a t i o n i n t h e

100 i 4 5 i ~ o 235 DLTS peak h e i g h t versus oxygen den-

TemperatoPe (.KI s i t y has been observed /6, 43/. B u t

t h i s i m p u r i t y seems be a c t i v e i n P-type s i l i c o n . Moreover these d e f e c t s exhi b i t h i g h s t a b i l i t y . If we consider t h e quenching model where the vacancies generated a t h i g h temperature could p r e c i p i t a t e d u r i n g t h e f a s t c o o l i n g , we suggest the vacancy-oxygen complex and c l u s t e r s of vacancy l i k e surface d e f e c t s (Ec = 0.53 eV), divacancies (Ec = 0.41 eV),

5 2

pentavacancies V (Ec = 0.34 eV) o r divacancy-oxygen

v2

-

0 (Ec

-

0.27 eV) as a s s i - gnments f o r t h e observed defects. A l l these assignments a r e coherent w i t h t h e f a c t t h a t a r e l a t i v e h i g h temperature ( = 500' C) i s needed t o remove t h e observed defects.

0.6 1.0 1.6 z o as an e x c i t a t i o n source/ /49/. M e s l i

DEPTH f p m ) e t a l . have found t h r e e l e v e l s c o r r e s - ponding t o m i n o r i t y c a r r i e r t r a p s w i t h

CONCLUSION

(15)

C5-294 JOURNAL DE PHYSIQUE

I n s l i g h t l y disordered m a t e r i a l s , t h e recovery o f d e f e c t s depends i n t h e i r nature, so t h a t t h e annealing o f deep l y i n g d e f e c t s i n the c r y s t a l i s b e t t e r achieved by longer annealing time (msec t o sec) o r otherwise needs a p o s t l a s e r thermal treatment. F i n a l l y i n v i r g i n s i l i c o n , s t a b l e d e f e c t s found i n the molten l a y e r are mostly c o r r e - l a t e d t o an a s s o c i a t i o n of vacancies, which anneals i n t h e range o f 500

-

600° C. They a r e m a i n l y e l e c t r o n t r a p s which can p l a y a compensating r o l e i n P-type m a t e r i a l s .

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