Kinetic model of silicon — Hydrogen network formation

JOURNA L O F

Journal of Non-Crystalline Solids 137&138 (1991) 6494552 North-Holland

NON-CRglKIISOIBS

KINETIC MODEL OF SILICON - HYDROGENNETWORK FORMATION Svetoslav KOYNOV CL SENES

-

Bulgarian Academy of Sciences, Bvd. Lenin 72, 1784 Sofia, Bulgaria

A model is proposed to explain the mechanism of a-Si:H network formation via gas-phase decomposit i o n of silanes. I t is based on a set of thermally independent reactions o f . S i H 3 and:SiH 2 r a d i c a l s with surfaceSi andSi-H bonds. Some s t e r i c l i m i t a t i o n s of the reactions at the interface gas/solid are taken in focus. I t is suggested that such l i m i t a t i o n s r e s u l t i n a growth mechanism consisting of two p a r a l l e l processes: 1)propagation of dense Si network covered with a hydrogen enriched surface; 2)pulsed development of polyhydride chains. The competition between these processes results in a Si-H network containing (SiH2) n bubbles. I t is shown that density and dimensions of bubbles are k i n e t i c a l l y determined by the temperature of the s o l i d phase and by the r a t i o o f . S i H 3 and :SiH 2 concentrations at the growing surface. 1. INTRODUCTION

gas-phase conditions and a subject of

The neutral S i l y l monoradical (.SiH3) and the S i l y l e n e b i r a d i c a l (:SiH2) have been recognized as possible precursors of the a-Si:H

f i l m 1'4'5.

Both of these radicals are found as intermediate

other

studies, remain beyond the scope of the model. The temperature of the s o l i d phase Ts i s taken in consideration as another parameter, r e s t r i c t ed to the range I00 - 350°C.

products of plasma 1, photo2and thermal 3 decompo-

s i t i o n of silanes in gas-phase, used in d i f f e r e n t

3. SURFACE REACTIONS

CVD methods f o r a-Si:H preparation. However, a general discussion s t i l l

exists concerning which

of them is the major p a r t i c i p a n t in the

film

growth 4'5. Meanwhile, the mechanism by which the

The heterogeneous reactions responsible f o r the Si-H network propagation and interconnection are not well understood, but they should have the f o l l o w i n g features:

radicals form the atomic network of a-Si:H is not clear enough. Many experimental studies have

-

in them from the side of the s o l i d phase;

revealed that the preparation conditions determine a Si-H network of complex microstructure,

free Si bonds and Si-H bonds p a r t i c i p a t e

- they are fast and have nearly zero activat i o n energy, because the a-Si:H growth rate is

correlated with compositional inhomogeneitie~'{

l i m i t e d by the a r r i v a l of radicals from the gas

This work proposes a mechanism of Si-H network

phase and i t is independent of Ts.

formation r e s u l t i n g in such a microstructure.

A set of such reactions is l i s t e d below. Most of them have a known prototype, occuring in gas

2. BASIC ASSUMPTIONS

phase. These reactions (except of la) have been

An assumption of the model is that both :SiH 2 and -SiH 3 radicals take part in the Si-H network

suggested by Scott et a l . 9 ' l O t r y i n g to explain a-Si:H growth. Here, they are shown in a way

formation. The r a t i o R = [:SiH2]/[.SiH3]between

i l l u s t r a t i n g some of t h e i r kinetic and

t h e i r concentrations in the gas-phase adjacent

chemical features. The dashes represent satura-

to the growing surface is taken in consideration

ted chemical bonds. The unsaturated (free)bonds

stereo-

as a generalized gas parameter, defining d e t a i l s

are denoted by ( . ) , while the dotted connections

of network formation. The ways of e s t a b l i s h i n g

(..)

certain values of R, related to a v a r i e t y

the chemical act.

of

represent bonds in reconstruction during

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

650

S. Koynov/Kinetic model of sUicon-hydrogen networkformation

/ "SiH3

-Si ~i--+ /SIli (or) ~{_ .-+ /{ (surface) (surface) £surface) (surface)

(1)

Si

./ :SiH2

al l i m i t

HS~':

H2 #

HxSli:

Si -+ ( s ~rlf~ac e ) ~

(2)

~..

Si

-+

to the surface plane, can

H •

, -+ i ( SurTace )

.../H

si

si

/IX

, /1~ t s urTace )

HXs~i/H • H

t SiH 4

s,i

si

,s,'i

/1\

(surface)

+

si,, /,/('siS~Ur fa c/el~

si /

/I\

\/Sli x

molecule (or r a d i c a l )

of one

is switched to another.The

rate constant of t h i s r e a c t i o n is lower than t h a t of the i n s e r t i o n reactions in gas phase 4, but i t

bond w i t h o u t propagation of the network. H2 ~

Y Si---~

The gas phase analogon of r e a c t i o n (3) isknown a s " d i s p r o p o r t i o n " i n which the Si-H bond

ing. The r e s u l t of (3) is a c r e a t i o n of a free Si

(sur}ace)

H . .H (4) - S i

due to the screening e f f e c t of the surface.

should not be so s e n s i t i v e to the surface screen-

•

(3)

the

to t h a t of i t s gas phase analog can be expected

H4/HI

,Sl\ H

in

Thus, a s i g n i f i c a n t

reduction of the rate constant of (2) in respect

( Su/rlf~ace )

:SiH2

~/ "SiH3

Si

/IX

Note how-

incidence at a shallow

ensure the r i g h t d i s p o s i t i o n of atoms t r a n s i e n t state of (2).

(la)

a :SiH 2 i n t o a Si-H

w i t h o u t thermal a c t i v a t i o n .

ever, t h a t only a radical angle, n e a r l y p a r a l l e l

N.°N

~/ "Sill3

r e f e r r e d to a s " i n s e r t i o n " o f

bond~ I t occurs w i t h a rate close to the c o l l i s i o n -

Reaction (4) is a " d i s p r o p o r t i o n " w i t h

the same

;sisi{

energy balance l i k e t h a t of ( l a ) .

/

of neighboring SiH 3, SiH 2 or S i H s u r f a c e t e r m i n a -

,sli

7.'\ / X (surface)

tors take part in i t .

The Si-H bonds

The r e s u l t is

an i n t e r -

connection of Si neighbors with a reduction of Reactions (1) are t r i v i a l

" a s s o c i a t i o n s " of

t h e i r hydrogen coverage. Experimental evidence,

• SiH 3 or :SiH 2 r a d i c a l s w i t h unsaturatedSi bonds

t h a t such reactions occur w i t h high rate at Ts

from the growing surface. Such reactions do not

as low as 160 - 200°C are a v a i l a b l e 11. The rate

need thermal a c t i v a t i o n and occur w i t h extremely

constant of reaction (4) should depend on

high rates (close to the c o l l i s i o n a l

flexibility

low Ts. In r e s u l t ,

limit)

at

the atomic network propagates

by SiH 3 or SiH 2 terminators.

With increasing Ts,

of the p a r t i c i p a t i n g

the

groups, because

the r i g h t p o s i t i o n of atoms in i t s t r a n s i e n t state needs bending of some chemical bonds. So

the

the d i s s i p a t i o n of Si-Si bond formation energy

s i n g l e connected SiH 3 groups should i n t e r a c t

becomes d i f f i c u l t .

f a s t e r than the double connectedSiH 2 groups,etc.

Therefore, we suppose

that

reactions ( I ) are replaced b y " a s s o c i a t i o n s w i t h

Note however, t h a t interconnected SiH2units ( i n

molecular detachment of hydrogen" ( l a ) at higher

a polyhydride chain f o r instance) can not i n t e r -

Ts. This r e a c t i o n passes through an a c t i v a t e d

act by r e a c t i o n (4) because double Si=$i bonds

complex, in which one Si-Si bond (2.3 eV)

are impossible.

and

one H-H bond (4.5 eV) are formed at the expense of breaking two Si-H bonds (3.3 eV each one). Thus two unsaturated Si bonds appear on the pro-

4. FORMATION OF A "PRIMARY" SI-H NETWORK

We c a l l " p r i m a r y " n e t w o r k

t h a t one, which is a

pagating surface. The same r e a c t i o n should occur

d i r e c t r e s u l t of the heterogeneous r e a c t i o n s . l t s

w i t h :SiH 2 r a d i c a l s (not shown). Reaction (2) has a known gas phase analogon,

r e c o n s t r u c t i o n in the bulk of the s o l i d phase is considered l a t e r on.

S. Koynov / Kinetic model of silicon-hydrogen network formation

651

4.1. Growth of"basic Si network"

4.2. Surface Polyhydride Cycle

Reactions (1-4) explain the growth of an inter-

The "basic network" propagates l o c a l l y in two

connected atomic network, referred l a t e r on to as

ways - by one step reactions ( I ) , ( l a ) , ( 2 ) ,

"basic network",in the following way. The

well as by two step consequence (3) --+ ( 1 ) , ( l a ) .

net-

as

work propagate l o c a l l y by one step due to the

Due to the temporary delays in the l a t t e r case,

reactions ( 1 ) , ( l a ) or (2).Two e f f e c t i v e c o l l i +

the propagation surface is a t o m i c a l l y rough. As

sions with radicals are necessary f o r one step

noted above, the reaction (2) should be very

propagation by the sequence (3) ~

sensitive to surface screening effects - i . e . t o

( I ) or ( l a ) .

A l l these propagation reactions r e s u l t

in

a

surface roughness. Therefore, i t can be assumed

termination of the surface by SiH3,SiH 2 and SiH

that (2) w i l l occur with a reduced rate constant

groups. Although some of these terminators have

within one"screening l a y e r " a s thick as

aver-

unsaturated Si bonds, the population of the newly

age roughness. Because of the random character

developed surface with Si-H bonds is very high,

of local propagations, some Si-H terminators

because reactions ( i ) and (2) are the f a s t e s t .

w i l l cross the"screening l a y e r " , thus appearing

So, the reaction (4) occurs on the surface with

in the gas above i t ,

a high rate. Thereby, the f l e x i b l e SiH3surface

ations are not v a l i d . The reaction (2) becomes

terminators are at f i r s t

dominant at the top of such places, so poly-

converted to double con-

where the screening l i m i t -

nected SiH2 terminators, transformed f u r t h e r to

hydride chains s t a r t to develop f a s t l y from them.

Si-H groups bonded with three Si neighbors - i . e .

The displacement of a chain top from i t s s t a r t

to a dense monolayer covered by hydrogen. Accord-

plane follows the well known s t a t i s t i c a l law of

ing to Wagner et a l . l ~ the transformation SiH 2+

Flory - d = C.t~; where d is the mean diameter

+ S i H i s o v e r a f t e r a minute on c-Si surfaces at

of the sphere in which the chain can be enclosed,

Ts= 200°C. Because of the higher c o n f i g ur at ional

t is the time i n t e r v a l from the s t a r t and ~ is

flexibility

a power f a c t o r of 1/2 or 3/5 f o r 2D- or 3D-case

of a-Si:H surface, the characteris-

t i c time might be assumed lower f o r this case.

r e s pec t iv ely . The p r o p o r t i o n a l i t y constant C

The time between two local propagations can be

depends on Ts. At low Ts t h e " t r a n s " i s o m e r con-

o

estimated at as l s e c . f o r growth rate of IA/s.

f i g u r a t i o n is repeated preferably along the chain

Comparing these time scales, one concludes that

while with increasing Ts the chain can propagate

the f l e x i b l e SiH 3 terminators are completely converted into SiH2 groups before the next effect i v e radical a r r i v a l , while SiH 2 groups ( i f remained on any surface) are transformed during the growth of few monolayers. Thus, r e p e t i t i o n of reactions (1-4) results in propagation of a hydrogen rich surface,

below which remains a

crosslinked network of Si atoms. The same is the r e s u l t from the more d e t a i l e d models of Scott et al. 9 ' I 0 . We believe that such a"basic Sinetwork" is formed i n i t i a l l y

on the most

places at the

surface. However, the mechanism of i t s growth

~ime

can not explain adequately the d e t a i l s of hydrogen incorporation in a-Si. Therefore, we a t t r i bute hydrogen incorporation to another process.

FIGURE 1 Competition between the "basic network" growth and the (SiH2) n chain propagation.

652

S. Koynov ~Kinetic model of silicon-hydrogen network formation

in any direction by a random consequence of d i f f e r e n t isomer bonding configurations. Meanwhile the network surface propagates also, but with a constant v e l o c i t y . Figure i i l l u s t r a t e s the interaction of a polyhydride chain with the "basic Si network"during growth. The s t a t i s t i c a l dependences of propagation for the network surface ( l i n e a r ) and for the chain top (sublinear) are shown on the l e f t of the figure. The i n i t i a l advance of the chain (marked by arrow) vanishes with time t i l l

the chain top is reached by the

"screening layer"(the intercept of dependences). Thus, the chain finishes i t s development and remains burried within the basic network. This is i l l u s t r a t e d on the r i g h t of Fig. 1 for an a r b i t r a r y Ts=Tsl(here and thereafter O, ° and + represent Si atom, H atom and a dangling bond respectively). The dimensions of the burried (SiH2) n clusters are determined by Tsasshown by the second sublinear dependence (dashed l i n e )

FIGURE 3 Fragments of " f i n a l Si-H network"resulting from the corresponding fragments, shown in Figure 1. 5. FORMATION OF "FINAL" SI-H NETWORK

The excess of defects and hydrogen in the v i s i n i t y of the (SiH2) n clusters

should r e s u l t

in a reconstruction of the "primary network",

corresponding to Ts2 < Tsl(higher C). Figure 2 i l l u s t r a t e s the microstructure of the resulting

conditioned by Ts. That is i l l u s t r a t e d in Fig.3.

"primary Si-H network" for two extremal cases.

Detailed explanation i s a s u b j e c t o f a n e x t paper.

The dependence of the density of (SiH2)nClUsters dispersed in the"primary Si-H network"on the gas parameter R w i l l be demonstrated in a more detailed version of the model.

REFERENCES 1. K.R. Ryan, I.C. Plumb, CRC Crit.Rev. in Sol. St. and Mat.Sc., 15 Issue 2 (1988) 153. 2. G.G.A. Perkins, E.R. Austin, F.W. Lampe, J. Amer.Chem.Soc., 101 (1979) 1109. 3. B.A. Scott, R.D. Estes, J.M. Jasinski, J.Chem. Phys., 89 (1988) 2544. 4. A. Gallaghner, J.Appl.Phys., 63 (1988) 2406. 5. S. Veprek, M.G.J. Veprek-Heijman, Appl.Phys. L e t t . , 56 (1990) 1766. 6. J.A.Reimer, R.W.Vaughan, J. Knights, Phys. Rev. L e t t . , 44 (1980) 193, 7. D.L. Williamson, A.H. Mahan, B.P. Nelson, R.S.Crandall, J.Non-Cryst.Sol., 114 (1989)226. 8. J.M. Jasinski, J.O. Chu, J.Chem.Phys., 88 (1988) 1678.

FIGURE 2 Fragments of"primary Si-H networks" grown at: a) low Ts; b) high Ts.

9. B.A. Scott, R.M.Plecenik; E.E. Simonyi, Appl. Phys. LettT, 39 (1981) 73. 10. B.A.Scott, J.A.Re~mer, P.A.Longeway, J.Appl. Phys., 54 (1983) 6853. i i . H. Wagner, H. Ibach, Adv. in Sol.St. Phys., XXlII (1983) 165.