Improvement in the boron-doping efficiency of hydrogenated amorphous silicon carbide films using BF3

268

Journal of Non-Crystalline Solids 114 (1989) 268-270 North-Holland

IMPROVEMENT USING BF 3

IN THE BORON-DOPING

EFFICIENCY OF HYDROGENATED

AMORPHOUS

SILICON CARBIDE FILMS

A.ASANO and H.SAEAI Fuji Electric Corporate Research and Development, 2 2 1Nagasaka, Yokosuka City, 240-01 Japan

Ltd.,

Nide o p t i c a l gap ( 2 . 0 e V ) b o r o n - d o p e d a SiC:H f i l m s w e r e p r e p a r e d by t h e p l a s m a - a s s i s t e d c h e m i c a l v a p o r d e p o s i t i o n t e c h n i q u e from a §~Ha+CHa+BFa+H9 g a s m i x t u r e f o r t h e f i r s t t i m e . The f i l m showed a p h o t o c o n d u c t i v i t y o f l x 1 0 v S / c m - u n d ~ r ~ s u n i l l u m i n a t i o n , w h i c h was h i g h e r by a f a c t o r o f 5 t h a n t h e f i l m s doped w i t h B2H6.

6 . 5 mW/cm2,

I. INTRODUCTION The use of boron-doped

a-SiC:H films as a

a gas pressure

CH~, and H2 f l o w r a t e s

of 0.53 Torr,

of 2.0,

wide gap window layer has led to a significant

cm / m i n ,

improvement

in the efficiency of an a-Si based I solar cells. Recently, a hydrogen dilution

BF3 was added w i t h a f l o w r a t e

technique 2 has been developed

was added w i t h a f l o w r a t e

highly photosensitive

for preparing

undoped a SiC:H films.

It

respectively.

and f o r p r e p a r a t i o n

6.5,

For t h e p - t y p e doping, o f 0 8 cm3/min,

of a reference

series,

selection

was expected that by the H 2 dilution technique,

no o x y g e n i m p u r i t i e s

( 02 , H20, C02,SO2,

one could readily improve the photoconductivity

was i m p o r t a n t

in o r d e r t o i n c r e a s e

of p-type a SiC:H films.

of the films.

Under t h e s e c o n d i t i o n s ,

in photoconductivity

of boron doped a SiC:H

films prepared under the H2-dilution condition 3 instead of B2H 6, as well as on the

using BF 3

problem encountered

in the B2H 6 doping.

We

adopted BF 3 because

: (I) fluorine atoms bonded

deposition optical

r a t e was a r o u n d O . l ~ / s

hydrogen atoms,

against

conductivities

the BF 3 is by far less decomposed than B2H 6 by

similar

the glow discharge,

a Si:H f i l m s ,

and we can expect the

absence of disturbance introduction

in plasma condition by

of BF 3.

The a p p a r a t u s

reduction

capacitively discharge

coupled rf

reactor

deposition temperature

optical

(13.56MHz) glow

with a load-lock

conditions o f 230tC,

were

chamber. The

: a substrate

an r f power d e n s i t y

in the film

The c h a n g e s i n b o t h

which is ascribed

in d e f e c t

are

density

to the

i n t h e undoped

In the boron c o n c e n t r a t i o n both the photoconductivity

range and

gap b e g i n t o d e c r e a s e b e f o r e t h e d a r k

conductivity devices,

lOOmW/cm2) and gap p l o t t e d

to t h o s e in the case of boron doping to

a SiC:H f i l m s .

u s e d h e r e was a c o n v e n t i o n a l

the

and t h e

with boron concentration

a b o v e 1019 em- 3 , 2. EXPERIMENTAL DETAILS

(AM1.5;

and o p t i c a l

the boron concentration

doped w i t h B2H6. and (3)

the quality

Doping w i t h B2H6

FIGURE 1 shows p h o t o dark conductivities,

reactivity of boron fluorine radicals,

etc.)

gap was 2.0eV.

to boron may act as a better terminator than (2) the maximum electron

It

o f BF 3 g a s w i t h

3. RESULTS AND DISCUSSION 3.1.

negativity of fluorine may reduce the

B2H6

o f 0 0 . 0 6 cm3/min.

s h o u l d be n o t e d t h a t

In this paper, we report on the improvement

SiH 4,

and 300

becomes s u f f i c i e n t for p-i-n -7 S/cm. The d e c r e a s e i n

i.e.,~wlO

photoconductivity

i s due t o d e f e c t

creation

by

doping. of

0022-3093/89/$03.50 © Elsevier Science Publishers B.V. (North-Holland)

For t h e o p t i c a l

gap n a r r o w i n g ,

three

possible

A. Asano, H. Sakai/ Improvement in the boron-doping efficiency

I

10"5

I

I

I

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~dork

161o

s

o v

10 "11

/

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10 "12

i i

I 3000

i I J I I I 1016 1017 1018 10 TM 1020 1021 boron concentration {cm"3}

FIGURE 1 Photo- and dark conductivities, a n d o p t i c a l gap plotted against the boron concentration in the a - S i C : H f i l m p r e p a r e d f r o m a SiH4+CH4+B2H6+H2 gas mixture.

I I I I 2500 2000 1500 1000 w a v e n u m b e r (cm "1)

c a n be m e n t i o n e d

carbon content,

a n d 3) b o r o n s i l i c o n

boron-carbon

alloying.

2.02eV,

1.85eY,

spectra,

spectra

m o d e s (2890,

concentration.

films,

absorption

verified

of

t h e u n d o p e d f i l m a) t o 0 . 1 5 f o r t h e

Instead,

a catalytic

effect

This is opposite

the effect

o f BH3 r a d i c a l s

g r o w i n g s u r f a c e 4 ; BH3 r a d i c a l s remove the surface

consequently

the sticking

to the

is consistent

with

on t h e f i l m

on t h e g r o w i n g

covering probability

also

hydrogen, of the

causes

(660cm-1),

n in the

therefore

concluded that,

condition,

the narrowing

should

defect in As f o r

m o d e s , SiH ( 2 0 0 0 c m - 1 ) ,

a n d SiH

SiH 2

no s i g n i f i c a n t

IR s p e c t r a .

[t is

under the H2-dilution of the optical

gap o f

by d o p i n g w i t h B2H5 i s p r o b a b l y and/or

boron-silicon

alloying. 3.2.

D o p i n g w i t h BF 3

conductivities, from

It

in

the decrease

FIGURE 3 s h o w s p h o t o -

by t h e ESCA

increases.

with boron doping.

due to boron-carbon

with boron

increased

(2080cm 1 ) ,

a-SiC:H films

in the carbon

The c a r b o n c o n t e n t

boron doped film c).

surface

intensity

increases

The i n c r e a s e

was f u r t h e r

measurement.

case 1).

the

o f t h e CH2 a n d CH3

t h e S i - C mode ( 740 cm - 1 )

factor

consequently

change is observed

a n d c)

2940 cm - 1 ) d i s a p p e a r ,

the boron-doped

0.08 for

In a l l

were

hydrogen under the

the reduction

silicon-hydrogen

The

indicating 2 of a dense network structure. For

formation

content

1.95eV,

respectively.

the absorption

condition, 2'5

photoconductivity,

gaps of the films

b) 0 . 3 4 ~ m ,

covering

creation,

with

w h i c h was r e d u c e d by

H2-dilution

hydrogen-coverage

f o r a) u n d o p e d

1 . 5 x 1 0 2 1 cm - 3 b o r o n s .

and o p t i c a l

a) 0 . 2 6 ~ m ,

and/or

radicals,

the surface

be n o t e d t h a t

b) d o p e d a - S i C : H f i l m s

3xlO 20 c m - 3 , a n d c)

in

in hydrogen

FIGURE 2 c o m p a r e s

transmission

a SiC:H f i l m ,

0.44pm,

: 1) t h e d e c r e a s e

2) t h e d e c r e a s e

I 500

FIGURE 2 Infrared transmission s p e c t r a f o r a) u n d o p e d a - S i ~ n H f i~m, b) d o p e d a ~ C : H _ ~ i l m s w i t h 3x10 - v cm V , a n d c) 1 . 5 x l O - " cm v b o r o n s u s i n g B2H 6 .

carbon related

thicknesses

I

CL 0

10"9

infrared

I

1.8_

10-8

content,

I

2.1 ~

lO-'~

reasons

I

Q

J

IO'E

269

and dark

and optical

the boron concentration

gap p l o t t e d

BF 3. The c h a n g e i n p h o t o c o n d u c t i v i t y doping is similar however, is higher increase

the value

against

in the film doped with

to that

with boron

o f B2HG-doped f i l m s ,

i n t h e r a n g e a b o v e 1019 cm - 3

by a f a c t o r

o f 5.

In addition,

in dark conductivity

t h a n i n t h e B2H6 d o p i n g .

the

is more rapid

T h e s e show i m p r o v e m e n t

in the boron doping efficiency

: formation

of

A. Asano, H. Sakai / Improvement in the borou-dopizlg efliciency

270

1

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16 5

g

..]'~

E

10e

g

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u-~. 1 lo-?

==

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o

o. o

E tO

10"

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u

I

1

I

3000

i

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J;

2500 2000 1500 1000 w o v e n u m b e r (crn "1)

16'~z

500

I I I I I 10i6 1017 10 ~6 10 ~9 1020 10 21

boron concentration (cm "31 FIGURE 3 P h o t o - and d a r k c o n d u c t i v i t i e s , and o p t i c a l gap plotted against the boron concentration in the a - S i C : H f i l m p r e p a r e d from a SiH4+CH4+BF3+H2 gas mixture.

FIGURE 4 I n f r a r e d t r a n s m i s s i o n s p e c t r a f o r t h e BF3-doped a - S i C : H f i l m s . Th~9borg~ c o u c e n t r a t ~ B n s ]~ t h e f i l m s were 9 . 4 x 1 0 cm and 5 . 1 x 1 0 - - cm - f o r t h e f i l m s d) and e ) , r e s p e c t i v e l y .

4. CONCLUSIONS defects

is less

fourfold

and/or the concentration

coordinated

of

boron is higher than in

photoconductivity

FIGURE 4 shows i n f r a r e d

transmission

f o r t h e BF3-doped a - S i C : H f i l m s .

spectra

The b o r o n

i n t h e f i l m s were 9 . 4 x 1 0 1 9 cm-3

a factor

photoconductivity gap o f 2.0eV.

respectively.

that

].91eV,

respectively.

o f B2H6 d o p i n g ,

and o p t i c a l

2.00eV and e) 0.38~m, In contrast

the absorption

of the

in t h e f i l m s were measured

by ESCA t o be i d e n t i c a l film.

These i n d i c a te

to that

that

i n t h e undoped

the surface

covering

h y d r o g e n was n o t removed o w i n g t o t h e c a t a l y t i c effect

of boron related

the defect

creation

boron source gas is to a lower reactivity radicals,

Consequently,

the surface-covering

indicating

h y d r o g e n was n o t

removed by t h e b o r o n - r e l a t e d

radicals.

ACKNONLEDGMENTS The a u t h o r s useful

are grateful

discussion.

t o M.Ohsawa f o r

T h i s work was s u p p o r t e d by

t h e New E n e r g y and I n d u s t r i a l Development O r g a n i z a t i o n

Technology

under a contract

with

the Sunshine Project. REFERENCES 1. Y.Tawada e t a l . , A p p l . P h y s . L e t t . 237.

39 (1981)

of boron fluorine still

2. A.Matsuda and E . T a n a k a , 97&98 (1987) ]367.

J.

Non-Cryst.

Solids

t h e i m p r o v e m e n t may be due t o t h e

in c o n c e ntration

of fourfold

coordinated

boron, or to the absence of

disturbance

in plasma condition

o f BF3 .

in the f i l m s

w i t h BF3 f l o w r a t e ,

of

T h i s i s p r o b a b l y due

however, the possibilities

remain that increase

radicals.

with introduction less.

o f l x l 0 5 S/cm a t an o p t i c a l

with the case

intensity

S i - C mode ( 740 cm 1) i s u n c h a n g e d w i t h d o p i n g . The c a r b o n c o n t e n t s

was i m p r o v e d by

The c a r b o n c o n t e n t

and 5 . 1 x 1 0 2 0 cm-3 f o r t h e f i l m s d) and e ) , The f i l m t h i c k n e s s e s

condition

o f 5. The a - S i C : H f i l m showed a

did not i n c r e a s e

g a p s w e r e d) 0.40~m,

by d o p i n g w i t h BF 3, t h e

of a-SiC:H films prepared

under the H2-dilution

t h e B2H6-doped f i l m s .

concentrations

We h a v e shown t h a t

by i n t r o d u c t i o n

3. A.H.Mahan e t a l . , (1983) 1033. 4. 3 . P e r r i n

et a l.,

5. A . h s a n o e t a l . ,

J.

Electron.

Surf. Sci.

Mater.

210 (1989)

3.Appl.Phys.

6

114.

65 (1989) 2439.