Cross contamination due to dopants in single chamber deposition systems

Cross contamination due to dopants in single chamber deposition systems

JOURNAL Journal of Non-Crystalline Solids 137&138 (1991) 761-764 North-Holland CROSS C O N T A M I N A T I O N DUE TO DOPANTS UGUR, Huseyin S e l...

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JOURNAL

Journal of Non-Crystalline Solids 137&138 (1991) 761-764 North-Holland

CROSS C O N T A M I N A T I O N DUE TO DOPANTS

UGUR,

Huseyin

S e l q u k VAROL,

OF

NON,CRYSTALLINE SOLIDS IN SINGLE C H A M B E R D E P O S I T I O N SYSTEMS

S e v i l a y U G U R and I~±k K A R A B A Y x

TUBiTAK, M A M Dept. of Physics, P.O. Box 21, Gebze, Kocaeli, 41470 T U R K E Y

Cross c o n t a m i n a t i o n due to d o p a n t s in a single c h a m b e r d e p o s i t i o n system for a m o r p h o u s silicon is i n v e s t i g a t e d . D e p o s i t i o n of intrinsic a-Si:H, just after the d e p o s i t i o n of a d o p e d layer, yields a c o n t a m i n a t e d material. In order to minimize the e f f e c t s of the dopants in such a d e p o s i t i o n system, an intermediate i n t r i n s i c b u f f e r d e p o s i t i o n has to be made before o b t a i n i n g a device q u a l i t y m a t e r i a l . Since, this extra d e p o s i t i o n is a time and m a t e r i a l c o n s u m i n g step, t h i c k n e s s of this b u f f e r layer must be o p t i m i z e d for various dopants used. In a l o a d - l o c k s i n g l e ' c h a m b e r d e p o s i t i o n system, a series of intrinsic d e p o s i t i o n s are m a d e as i n t e r m e d i a t e b u f f e r layers after N type, and wide b a n d g a p P type d e p o s i t i o n s . To d e t e r m i n e the m i n i m u m t h i c k n e s s e s of these layers w h i c h are r e q u i r e d to e l i m i n a t e the effects of the dopants, optical and electrical measurements are used for characterization. We have also i n v e s t i g a t e d the c o m p e n s a t i o n c a u s e d by d i f f e r e n t types of dopants used in s u c c e s s i v e d e p o s i t i o n s w i t h o u t an i n t e r m e d i a t e buffer layer. Results indicate that cross c o n t a m i n a t i o n p r o d u c e d by P d e p o s i t i o n requires at least 1.8 ~m of intrinsic b u f f e r layer in order to o b t a i n d e v i c e q u a l i t y I layers. Similarly, for N type d e p o s i t i o n , b u f f e r layer must be at least 1.2 ~m thick. D e p o s i t i o n of P layer just after an N layer did not produce a n o t i c e a b l e change in e l e c t r i c a l and optical p r o p e r t i e s of P layer. On the other hand, a c t i v a t i o n energy of N layer d e p o s i t e d just after a P layer increased, i n d i c a t i n g a c o m p e n s a t i o n effect due to cross c o n t a m i n a t i o n .

i. I N T R O D U C T I O N Many

to

deposition

amorphous

systems

silicon

or

systems.

After

subsequent

contain

small

cleaned

system.

faster,

has

doped

doped layers

the In

of

dopants

quality

case

of

of

the

depositing

a

layer after other type of a d o p e d

deposition, occur.

a

intrinsic

amounts

deteriorating m a t e r i a l I-2 .

depositing

a

compensation

Because

deposition, cleaned,

of these,

either

the w h o l e

or a buffer

effect

after

I

layer.

First and

a doped

system

is

intrinsic layer

is

method many

is

very

time

researchers prefer

x Ylld±z University,

layer

in

Second method,

a build

a

newly

although

in u n c e r t a i n t y

due

to the t h i c k n e s s of the r e q u i r e d b u f f e r layer.

In

this

paper,

we

present

the

results of a study aimed at d e t e r m i n i n g the

thicknesses

required

after

compensation

of

buffer

doped

depositions

in d o p e d

layers

layers

caused

and by

cross c o n t a m i n a t i o n .

may

grown before obtaining a device quality

consuming,

buffer

for

p r o d u c t i o n are single chamber d e p o s i t i o n

material,

a

device

used

research

deposit

P h y s i c s Dept.,

2.

EXPERIMENTAL METHODS A series of samples are p r e p a r e d

17

different

depositions

producing

in 153

samples on C o r n i n g 7059 glass substrates in

a

load-locked

Process

[email protected],

chamber

istanbul,

0022-3093/91/$03.50 © 1991 - Elsevier Science Publishers B.V. All rights reserved.

deposition is

always

TURKEY

system.

kept under

762

11. U~ur et al. / Cross contamination due to dopants

vacuum

and

at

deposition

This

is necessary

the

effects

temperatures.

in order to eliminate

of

contamination

due

to

distribution of device quality intrinsic samples

prepared

laboratory.

substrate loading. Samples used for this

results

study

deviation

were

prepared

under

conditions

producing

device

materials.

Optical

measurements

performed

on

two

samples

the

quality are

for

each

and

For

were

measured

doped

in

layers,

similar.

The

our the

largest

(4.0%) was again in deposition

rate and the sm~llest in optical bandgap (1.0%).

Fig.2

shows

the

results

of

optical bandgap and slope of absorption

deposition, yielding film thickness (t),

edge

deposition rate (r), index of refraction

the cumulative thickness of all I layers

(n),

deposited

optical

bandgap

a b s o r p t i o n tail activation measurements

Samples

(Ea)

factor are

for

deposition

the

electrical on

are

used

'R'.

samples

reference

indicated

by

are made

after

a

points

doped

letter

,

>-

850 -

©

~

800 --I





,



750-

--

not

see

measurements. showed

All

very

deposition with

samples.

The

effect

of

the

of

1.8-I



1.7 0.0

u

I

0.5

1.0

cross

samples in

refraction,

largest

to

the

reference

variation

was

in

smallest

was in optical bandgap (1.1%). All these are

After n







I

FIGURE

I

1.5 d (urn)

2.0

2

Optical bandgap and slope of absorption gap for intrinsic samples prepared after N type (square points) and P type (round points) depositions.

and slope of absorption

respect

deposition rate (4.9%) and the values



the are

deviation

index

After p

optical

intrinsic

small

rate,

optical bandgap, edge

any on



R

1

3. RESULTS AND DISCUSSION did

I



of wide

Deposition sequence used for measurements. Reference samples indicated by letter 'R'

We

intrinsic

points after P type runs.

1.9

contamination

deposition.

represent

CH 4 mixture.

FIGURE

is

samples deposited after N type and round

(1.9 eV) material using SiH 4 and

IINNIIIIPPIIIIPNP R R

axis

The

in Fig.l.

for

Square

Horizontal

two

deposition. is given

are

bandgap

(o0) ,

each

measurements P type

and

performed

sequence

which

slope of

In order to obtain

energy

preexponential samples

(Eg) and

(B).

measurements.

within

the

statistical

For electrical measurements, we have observed

the

contamination.

effects Results

energy

measurements

Fig.3.

Square

points

are

of of

cross

activation

summarized

show

in

activation

H. U~ur et al. / Cross contamination due to dopants

1.2 " P' m



7

1.0..............................................

0.8

763

" i " ................ i " . . . . . . . . . . .

O

6

- . : ..........................•

i

...........................................................................................

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

>

I

O

©

n

,--4

0.6 m

P 0.4

bo 4 • After p

0.2-

I

I

0.5

i

0.0

2.0

for

I

I

0.5

1.0

I

I ].5

2.0

d(um) FIGURE

3

intrinsic

4

Preexponential factor of intrinsic samples deposited after N type layers (square points) and after P type layers (round points). Intrinsic region is shown by dashed lines.

Activation energy of intrinsic samples deposited after N type (square points) and after P type (round points) depositions. Intrinsic region is shown by dashed lines. values

• After n

2

I

1.0 1.5 d (urn)

FIGURE

energy

• After p 3 _N P

• After n

o.o 0.0

i

~0 O

"N

samples

to

y

axis

is

very

close

to

P'

This

deposited after an N type deposition and

shows a very severe cross contamination.

round

Only

points

deposition. the dashed device and

samples Intrinsic

lines,

quality

measured

conditions

after

a

region,

P

shown

is determined

I type in

our

similar

samples the

by

by all

prepared

laboratory

to

type

under

reference

I

the

after type

For

type

N

contamination shows

a

the

contamination.

sample. after

energy

conduction Intrinsic

P

type

between

band samples

deposition

for

Fermi P

type

deposited have

large

is

deposited

recovery faster

from

(around

and extrapolation of data to y

activation

and

sequence, totalling

samples

deposition,

are also shown on the y axis. P' denotes approximate

the

after

axis

the

depositions

cross

I, N, and P samples,

in

could be considered a good I

sample.

0.8 pm),

level

sample

three

1.287 Nm,

sample. Activation energies of reference used in this study,

last

evident

considerably

in

The same behavior is also

the

preexponential

measurements

which

Conventions

used

activation energies indicating an N type

identical

conduction. Extrapolation of data points

observe

higher

energy indicating a moderate

are in

shown this

factor

in Fig.4.

figure

are

to those of Fig.3. We did not

any

compensation

effect

due

to

764

H. U~ur et al. / Cross contamination due to dopants

cross

contamination

deposited hand,

after

N

activation

samples

15%

P

type

after

which

samples

On the

energies

deposited

increased

on

layers.

of

N

P type is

other type

samples

considerably

of

different

the

intrinsic

bandgap

P

for

intrinsic

uncontaminated

some

cross

samples.

This

contamination

compensation

in

N

type

order

layers

previously

d e p o s i t e d just after P runs. One

point

difference

Between kept

and

a

under

is

1.7

~m

at

and

additional

is

Because this

from of

the

this,

study

the

contamination

are

to

1.2

used

for

samples

deposition,

thickness

is

grown

~m

for

in

this

the

study.

after

P

type

we have seen the e v i d e n c e of

due

to

activation

in

type samples d e p o s i t e d after the N type

found

required of

N

system

the

just after an N type

required

layer

type

of

For

contamination,

deposition.

effects due

effects

in

compensation

thicknesses

which

eliminate

the

dopants.

at

buffer

these,

results

previous

measurements,

eliminate

buffer

N

and

deposition

l o w e r i n g the amount of r e s i d u a l d o p a n t s remained

For

the

energy

required

the

deposition

wide

is

deposition, the

a on

deposition

used

is

to

pumping

This,

runs

system

addition

loading.

~m

thickness.

vacuum In

the

separate

1.7

the

an

substrate

of

depositions,

temperatures. there

single

four

total

the

a

is

after

intermediate

intrinsic material

consider

between

deposition yielding

to

to

factor of

case

Based

activation

1.8~m

causes

worst

deposition

preexponential least

The

deposition.

electrical

larger than the 5% statistical d e v i a t i o n

indicates

types.

P

do

energy.

not

seem

to

cross

contamination.

ments

could

to

cross

type

not

be

cross

Further

effected

Optical

detect

contamination

samples.

by

On the other hand,

in

cross

and

runs

caused

indicated by 15% rise in

the

on

by

measure-

effects

any

study

P

of

type

using

of more

s e n s i t i v e t e c h n i q u e s is r e q u i r e d for the

d e p o s i t i o n s are the lower limits. If the

estimation

buffer

p r o f i l e of r e s i d u a l dopants in i n t r i n s i c

run,

layer

is

deposited

in

instead of separate runs,

required

thickness

will

be

a

single

then the

larger.

We

buffer

of

layers

reduction

and the

factor

effect

and

the

of N type

d e p o s i t i o n on s u b s e q u e n t P layers.

e s t i m a t e the d i f f e r e n c e to be around 15% due

to

for

various

produced

our

experience

on

measurements

in

the

samples and

single

used

devices chamber

d e p o s i t i o n system.

4. C O N C L U S I O N S In single chamber d e p o s i t i o n systems, using

dopants

produces

cross

c o n t a m i n a t i o n for the s u b s e q u e n t layers

REFERENCES i. O. Tsuji and T. Tatsuda, in: P r o c e e d i n g s of the 6 th I n t e r n a t i o n a l S y m p o s i u m on Plasma Chemistry, Vol 3, eds. M.I. Boulons and R.J. Munz (1983) pp.782-786. 2. C.C. Tsai, J.C. Knights, Thompson, J. Non-Cryst. (1984) 45.

and M.J. Solids 66