Preparation of Si thin films by spontaneous chemical deposition

172

Journal of Non-Crystalline Solids 114 (1989) 172-174 North-Holland

P R E P A R A T I O N

J.Hanna*,

OF

A.Kamo,

Si

T H I N

T.Komiya,

F I L M S

BY

I.Shimizu

S P O N T A N E O U S

C H E M I C A L

D E P O S I T I O N

and H . K o k a d o *

T o k y o I n s t i t u t e of T e c h n o l o g y , I m a g i n g S c i e n c e a n d E n g i n e e r i n g Lab.* a n d G r a d u a t e S c h o o l at N a g a t s u t a , N a g a t s u t a , M i d o r i - k u Y o k o h a m a 227 J A P A N

We

have

proposed

a

novel

p r e p a r a t i o n t e c h n i q u e f o r Si t h i n f i l m s t e r m e d f e a t u r i n g the s p o n t a n e o u s r e a c t i o n s of s i l a n e w i t h f l u o r i n e at the r e d u c e d p r e s s u r e . Fluorine contributed not o n l y i n t o the gas p h a s e r e a c t i o n s so as to a f f o r d f l u o r i n a t e d p r e c u r s o r s , SiHnF, but also into the chemical processes on the growing surface, resulting in promoting the propagation of S i - n e t w o r k s in the films. T h u s t h i s t e c h n i q u e p r o v i d e d us a v a r i e t y of the f i l m s f r o m "amorphous" to '~ingle c r y s t a l l i n e " u n d e r a w i d e c h o i c e of the r e a c t i o n c o n d i t i o n s . The films exhibited rather high photoconductivity in spite of high d e p o s i t i o n r a t e s o v e r i0 A / s e c .

"Spontaneous Chemical Deposition",

I.INTRODUCTION

Little progress, however, has been achieved in

Nowadays thin films of hydrogenated a-Si

by

the o t h e r

problems.

It w o u l d

seem

to be

rf-glow discharge of SiH~ have been established

attributed to a lack of the external parameters

as a large-area electronic material, applied to

for controlling the chemical processes on the

solar cells, electrophotographic drums, TFT

growing surface in the SiH4 plasma process,

arrays for liquid crystal displays and so on.

except the substrate temperature.

Some of the properties in the films, have remained

to be i m p r o v e d

advanced performances in these

however,

for f u r t h e r devices. These

Our attention to a chemical process in the film growth from fluorinated materials,

where

the propagation of Si-network

via a

include a stability to light illumination, hole

chemical

transport characteristics and conductivity

fluorine,

the doped films,

in addition

in

interaction lead

us

proceeds

between to

hydrogen

a new

and

preparation

to a dilemma

technique for Si thin films termed Hydrogen

between the film quality and deposition rates.

Radical Enhanced CVD (HR-CVD). 4) This technique

Much attention has been paid to an enhancement

is featured by "an intentional control" of the

of a rigidity of Si-network in the films for

chemical process in the film growth with an aid

the improvements, because these poor properties

of atomic hydrogen.

are

deeply

concerned

structure of Si-network

with

the

inferior

in the films.

Under the same concept of

"the intentional

control'~ we have developed a novel method for

Great efforts have been made toward the

the films, featured by a non-plasma and purely

improvements in the approaches focused mainly

chemical process based on the

on understanding the chemical processes in the

of SiH4 in the gas phase.

We will describe the

film

general

new

g r o w t h I),2)

and

establishing

a new

preparation technique. 3),4) Indeed, it has been

features of the

fluoro-oxidation

method

termed

"Spontaneous Chemical Deposition'~

demonstrated that application of the magnetic field and the low pressure helps a consistency between

high

deposition

rates

and

the

quality.5), 6)

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

2.EXPERIMENT The e x p e r i m e n t a l elsewhere. 7 )

setup

is i l l u s t r a t e d

173

J. Hanna et al. / Preparation of Si thin films by spontaneous chemical deposition

The reactor consisted of a pyrex glass tube,

substrate

distance,

a substrate holder and a gas nozzle for mixing

condition

of

SiH4 and F2, as is illustrated in Fig.l,

dsub.=40mm, and Press.=550mtorr the deposition

and

was hacked by mechanical booster and rotary

dsub..

In a t y p i c a l

SiH4(30sccm),

F2(30sccm),

rate was 12 A/sec.

pumps. Pure silane and fluorine diluted with He

The chemical structure of the films were

down to 10 vol% were used as the raw materials,

easily controlled by the external parameters in

Which

were

introduced

separately.

The

into

substrate

the

monitored by a thermocouple a t t a c h e d substrate

directly.

A

nozzle

temperature,Ts

typical

the gas phase reaction such as the pressure and

was

the ratio of the reactants: the SiHn stretching

to the

mode at 2000cm -I of the it-spectra tended to be

preparation

dominant when either

SiHd/F~

ratio

or

the

condition was similar to that in the rf-glow

pressure

discharge of SiHd.

The substrate temperature

3.RESULTS AND DISCUSSIONS

affect deeply npt only the chemical structures

was

decreased. found

to be more

effective parameter for the control

SiH,

reacted

with

Fz

spontaneously,

but also Si-network structures

and

to

in the films.

when these gasses were mixed through the nozzle

We found a very interesting

at the

reduced pressure similar to that in the

temperature on deposition rates and structures

plasma CVD. The reaction was e x o t h e r m i c and

of the films. Fig.2 shows deposition rates as a

accompanied

function of the substrate temperature.

by

an

intense

blue

chemi-

effects

of the

luminescence mainly from SiF*(420-480nm), as is shown in the photograph of Fig.l.

Substrate holder Nozzle

15

Press. = 5 2 0 - 5 5 0 r e t o r t Substrate Position =

40mm

SiH4:F2 =

S i H4

1:1

ov

r

-0l

,

150

200

L

, _l.

,

300

J

400

500

SubstrateTemp. (o C ) FIGURE 2 Deposition rate as a function temperature FIGURE 1 Schematic diagram of the reactor and photograph of the chemi-luminescence

As

the

temperature

was

crystallization took place The deposition rate was increased as the

of substrate

increased,

the

(filled circles in

Fig.2) and then the deposition rates went down

and d e c r e a s e d

rapidly. Both of the critical temperatures for

exponentially with an increase in the nozzle-

the crystallization and the film d e p o s i t i o n

SiH4 flow rate was increased,

J. Hanna et M./Preparation of Si thin films by spontaneous chemical deposition

174

tended to be shifted down as the gas flow ratio

phase reaction such as a gas flow ratio of SiH4

was d e c r e a s e d .

to F2 and the pressure the Si-network structure

These

facts

suggest

that

fluorine takes part in the chemical processes

in the films can be controlled intentionally.

on the growing

Thus this technique provides us a variety of

surface

so as

to promote

the

The films show high photoconductivity, ~ / r product of 10-Tcm2/V,

films from '~morphous" to "single crystalline" in e p i t a x i a l m a n n e r at low

the

propagation of Si-networks.

not only in the intrinsic

substrate temperature.

a-Si films but also in the ~c-Si films in spite

The experimental observations suggest that fluorine contributes into the chemical process

of high deposition rates over 10 A/sec. We observed the epitaxial growth of Si thin

on

the

growing

surface

directly

so

as

to

were

promote the propagation of Si-network and lead

applied as a substrate at the growth condition

to a long range relaxation of the Si-network at

for pc-Si films on the glass substrate,

the low temperature.

films at 300-400°C

when

e-Si

clearly seen in the photographs

wafers

as is

of the RHEED REFERENCES

patterns from the films. Ts=300°C

i.A.Matsuda and 2 (1987) 14~

Ts=370oC

K.Tanaka,

Mat.Sci.Rep.,

2. R.Robertson and A.Gallagher, J.Chem.Phys., 85 (1986) 3623. 3. T.L.Chu, S.S.Chu, E.G.Bylander, and S.T.Ang, Appl.Phys.Lett., 52 (1988) 807.

i

4. J.Hanna,

FIGURE 3 RHEED patterns from Si thin films prepared at 300°C and 3700C on c-Si(100) 4. CONCLUSIONS We have

established

a new

preparation

technique for Si thin films termed "Spontaneous

Chemical Deposition", which purely

chemical

reactions

process

of s i l a n e

is

of the

with

based

on

a

spontaneous

fluorine

at the

reduced pressure. With

the external

parameters

in the gas

N.Shibata, K.Fukuda, H.Ohtoshi, S.0da, and I.Shimizu, Disordered Semiconductors (Plenum Press, New York, 1987) p435.

5. T.Watanabe, M.Tanaka, K.Azuma, M.Nakatani, T.Sonobe, T.Shimada, Jpn.J.Appl.Phys., 26 (1987) 1215. 6. M.Ohnishi, H.Nishikawa, K.Uchihashi, K.Yoshida, M.Tanaka, K.Ninomiya, M.Nishikuni, N.Nakamura, S.Tsuda, S.Nakano, T.Yazaki, and Y.Kuwano, Jpn.J.Appl.Phys., 27 (1988) 40. 7. J.Hanna, A.Kamo, T.Komiya, H.D.Nguyen, I.Shimizu and H.Kokado, Mat.Res.Soc.Symp. Proc., (1989) in press.