Solid State Communications, Printed in Great Britain.
Vol.47,No.lO,
pp.773-777,
1983.
0038-1098/83 $3.00 + .00 Pergamon Press Ltd.
RAMAN STUDIES ON LOCAL STRUCTURAL DISORDER IN SILICON-BASED AMORPHOUS SEMICONDUCTOR FILMS A. Morimoto, S. Oozora, M. Kumeda and T. Shimizu Department of Electronics,
Faculty of Technology, Kanazawa University, Kanazawa 920, Japan
(Received 23 May 1983 by H. Kawamura)
Raman scattering and ESR measurements were carried out for amorphous Si and amorphous Si-based alloy systems such as Si-N and Si-C. A pronounced broadening of the Si-Si TO-like band of Raman spectrum is observed with an increase in the N or C content for amorphous Si-N and Si-C alloy systems. The broadening of the TO-like band for amorphous Si system as well as amorphous Si-N and Si-C systems has a good correlation with an increase in the density of dangling bonds derived from ESR measurements. The fact suggests that dangling bonds are produced in order to relax the local strain due to structural constraint and lower the total energy.
i. INTRODUCTION Raman
scattering
films
have
local
strain
present
been
the
in
amorphous
Si
Si-C 8'9 and Si-N I0 and NS~ study on a-Sil_xC x
be
sensitive
to
:H II have already been reported.
spectra
reported
in
Si-N and Si-C systems. ESR studies on both
the
film.
results
to 1-4
of
a
In
this
paper we
systematic
2. EXPERI)~NTAL
Raman
The various deposition methods used for the
scattering study in conjunction with ESR and NMR studies ated
for amorphous
amorphous
amorphous
phous
silicon
silicon
nitrogen
silicon
alloy
(a-Si),
(a-Si:H) ,
(a-Si:F),
alloy
films
fluorinated
amorphous
silicon-
(a-Sil_xNx) , hydrogenated
silicon-nitrogen
film preparation were summarized in Table I. The
hydrogen-
amor-
were
deposited
substrate
and
AI
scattering
and
both
respectively.
(a-Sil_xNx:H),hy-
The
on
solution
(a-Sil_xCx:H)
fluorinated
spectra
(a-Sil_xC x
overlapping of rotational
and
hydrogenated
silicon-carbon
and
alloy
molecules
:(F,H)). So
far
we
have
reported
that
for
dominant
incident
role
dangling
in
bonds. 5'6
reducing We
have
the also
density reported
of
role
in
reducing
the
density
of
by
power
density
to
incident plane, light normal
Ge
the
of
were
determined
the
and Raman
the
intensity
spectra
of
which
the have
HG-2S
double
with
plane.
The
60
mW
of
electric
of scattering
ESR
and NMR measurements
by
The film composi-
electron probe micro-
3. RESULTS AND DISCUSSION
LO-like a close
Figure I shows Raman spectra for SP a-Sil_ x
correlation with the local strain are correlated
Nx:H
with
compositions.
fro,, ESR.
avoid
in the present work, the width of the
band
the
typically
respectively.
analysis (EPMA).
in
to
were carried out at room temperature and liquid
local strain in the amorphous network structure.
band
vacuum
JOBIN-YVON
film
monochrometer.
tions
TO-like
a
Raman
spectra for N 2 and 02
and the component
dangling bonds increases with an increase in the
Therefore,
in
in HCI
The
to the incident plane was detected
N 2 temperature,
the
measurements,
488 nm line of Ar + laser was used
results,
that
a
excitation
by
speculate
and N ~
measurements.
using
dangling bonds in a-Sil Ge :H by softening the ±-x x 7 amorphous network structure. From these we
Raman
vector of the incident laser light was normal to
that
not only H bonded to Ge but also H bonded to Si a
the
the
measured
monochrometer.
randomly
dispersed H atoms bonded to Si in a-Si:H play a
plays
were
glass
for
foil was dissolved
drogenated amorphous silicon-carbon alloy
amorphous
thick
substrate
ESR
AI
before
500 U m
foil
density
of dangling
Es~ecialls~
an
bonds
attention
determined is
paid
and
GD
subtracting
to 773
a-Sil_xCx:H (I) The the
spectra
background.
films with various are For
corrected
by
convenience,
774
SILICON-BASED AMORPHOUS SEMICONDUCTOR FILMS Table
I. Deposition
target
and
rf
sputtering
conditions
glow
using
for
discharge
a planer
various
decomposition,
magnetron
system.
samples.
SP and GD represent
respectively. All
Vol. 47, No. IO
cathode
MG
in
rf sputtering of Si
parenthesis
electrodes
in three
means
the
systems
rf
have a
diameter of 90 mm.
sample
method
gas
rf power (W)
substrate temperature
a-Si:F
SP
Ar+SiF 4
160-250
250-350
a-Si
SP
Ar
200
200
a-Sil_xN x
SP(MG)
Ar+N 2
i00
250
a-Sil_xNx:H
SP(NG)
Ar+N2+H 2
200
250
a-Sil_xCx:H
(I)
GD
SiH4+CH 4
20
350
a-Sil_xCx:H
(II)
GD
SiH4+CH 4
I00
350
GD
SiH4÷CF 4
20
350
a-Sil_xCx:(F,H)
'"1
I
'
I
'
I
I
'
reported by several
|
I
GD a-S i,-xCx :
SP a-Si,-xNx : H
authors
for a-Si and a-Si:H
(l)
films
prepared
conditions.
A~(TO)'
by
various
methods
and
various
discussion
we
define
2-4
For X:O
quantitative which corresponds
to full width at half
maximum of TO-like band.
E c
(°C)
0.01
as
b
a
function
of
Figure 2 shows Aw(TO)'
the
N
or
various
films.
A~(TO)'
for
depends
on
preparation
C
content
x=O
x
for
films
largely
condition.
Hydro-
c
0.13
=
genation E
viously.
#
In 0.:~8 i
200
2OO 400 600 Rclman Shift (crn-I)
4O0
600 Rclman Sh~ft (cm-I )
the
a
decreases 1-4
A~(TO)'
as
reported
pre-
A~(TO)' is found to increase with x.
case of SP a-Sil_xNx:H , the increase
very
prominent.
also
increases
The
width
with x.
of the
TA-like
is
band
It should be noted that
A~(TO)' for GD a-Sil_xCx:H
(I) prepared with rf
power of 20 W is smaller than that for GD Figure i. Raman spectra for" SP a-Sin N :H and • . ±--X X GD a-Si~ C :H (I) films. The spectra are l--X X corrected by subtracting the background.
a-Sil_xCx:H (!I)
prepared
with
rf power of 100
W. We have already reported that the former film has a smaller density of dangling bonds than the
phonon
frequencies
corresponding
transverse-acoustic,
to
the
longitudinal-acoustic,
longitudinal-optical
and
phonons
in crystalline
Si are indicated by TA,
LA,
and
TO,
result
from
with
increase the
x,
as
shown in Fig.
in Ae(TO)'
increase
in the
with LO
I,
x might
intensity.
the
obtained
from
an amorphous
the peak to the half-intensity point at the high
a-Sil_xCx:H and
an increase
(i)
this
is enchanced apparent
intensity of LO-like
However,
system,
in
the
the
figure,
broadenea
respectively,
band
films. 12 Since
though it is inappropriate to use these terms in
G3
LO
transverse-optical
latter
in both SP a-Sil_xNx:H and films,
LO-like band
TO-like is
band
enhanced
is with
in x. There is no clear correlation
frequency
side
free
the
from
increases
between x and the relative intensity of TA-like
A~(TO)'
hand to that of TO-like band. The broadening of
Am(TO)
the
band.
TO-like
Raman
spectrum
like
this
has been
width
the
TO-like
the
value measured
with
can as
of
doubling
be
well
of
the
TO-like
intensity x.
attributed as
in the
band,
of LO-like
Therefore, to
band
Ae(TO) from
which
band,
is
also
the
increase
in
the
increase
in
intensity
of LO-like
Vol. 47, No. IO
775
S I L I C O N - B A S E D A M O R P H O U S S E M I C O N D U C T O R FILMS
200
I
I
a-Sil_xNx:H
I
implies makes
A
and
that
the
GD a-Sil_xCx:H ,
the incorporation
local
therefore,
of N or C atoms
strain around Si atoms
More specifically,
larger.
bond angle and/or bond length
A distortions
150i •
g
in
these
100
o
systems
larger with x.
that an increase
results
[]
o O
four which
O
tion
SP ct-Sil_x Nx:H SP a-Sil_xN x GDa-Sil_xCx:H (I) GDa-Sil_xCx:H (II) GDa-Sit_xCx:(F,H ) SPa-Si:F I i I , 0.2 0.4 Content X
• O • [3 •
0 o.o
amorphous
in an increase
semiconductors
c o n s t r a i n e d because
a
5O
atoms become
already reported
bonded
O0
A
0i -
have
Si
in x
in the
d e n s i t y of d a n g l i n g bonds measured by ESR. 8-IO 13 According to Phillips, tetrahedrally
A
f
We
for
is larger
number
rather
large
in
large
a
are
over-
their c o o r d i n a t i o n number is
for local
than the optimum coordina-
glass
formation.
Therefore
strain
is present,
resulting
amount
of
dangling
bonds.
The
addition of onefold coordianated H atoms reduces the
average
coordination
number,
and,
as
a
result, reduces the local strain and the density of
dangling
addition density
0.6
of
N
bond
much
in
C
dangling of
radius
produces
On
or
introduction
local
structure.
Figure 2. Full w i d t h at half m a x i m u m of TO-like band of R a m a n spectrum, Am (TO)', as a f u n c t i o n of x for various films.
bonds.
of
or
C,
p r o b a b l y because
the
has
than
a covalent
that
of
the amorphous
we expect
increases
the
the the
which
in
hand,
increases
smaller
strain
other
a-Si:H
bonds
N
Therefore
Am(TO)'
the
to
Si,
network
that an increase
density
of
dangling
bonds. Figure
3
shows
the
spin
density
N
for
s
d a n g l i n g bond ESR signal as a function of
The has
been
increase so
in Am(TO)
or Am(TO)'
for a-Si
far interpreted by several authors 2-4 (i)Ishidate et al. s p e c u l a t -
of
increase
increase
in
reported
that the
in several ways: ed
that
the
in bond
length d i s t o r t i o n
play a d o m i n a n t role for the increase in Am (TO )2.
( ii )L a n n i n
increase
in
changes bond
in
Am(TO)
et
Am(TO) ' network
angle
(iii)Tsu
and
et
Am(TO)'. N for x=O films largely increases from 1016 cm -3 s to 1020 cm -3 with an increase in
al.
in
of
short-range
dihedral
al.
interpreted
terms
suggested
order
as 3 distortion.
angle that
the
intrinsic
the
such
increase
in
is related to the bond angle distortion. 4
All of these authors,
however,
interpreted
Am(TO)'. This finding indicates that the density dangling
has
a
bonds
for
local
strain.
close
substrate
increase
with
an
increase
relation
In in
this
in
far
with
it has
an
been
in
paper
Am(TO)'
increase
increases So
Am(TO)' or Am(TO) 2 with the H content, 3 optical gap and dark
temperature,
conductivity, a the
a-Si
it
is
found
has a good
the
density
that
correlation of
dangling
bonds.
the increase in A~(TO)' or Am(TO) by an increase
N for x > 0 films also increases with an s increase in Am(TO)' caused by the incorporation
in
of N or C atom. The i n c o r p o r a t i o n of N or C atom
local
alloy
strain.
systems,
We
Si-N
extend
and Si-C,
the
same
idea
to
a l t h o u g h in these
with
atomic
radius
and
bonding
configuration
systems the p r e s e n c e of N or C atoms n e i g h b o r i n g
largely different from those of Si atom seems to
Si
cause
atoms
may
affect
spectrum.
The
increase
the in
Si-Si
vibrational
Am (TO) '
increase in the N or C c o n t e n t x for SP
with
an
the
structural
network
leading
in
case
the
of
disorder
to an increase C
atom
of
amorphous
in local strain.
structural
flexibility
776
SILICON-BASED AMORPHOUS SEMICONDUCTOR FILMS I
a large amount of charged N defects without spin
A SP a-Sil_xNx:H
is
•
because
SP cl-Sil_xN x
O GDa-Sil_xCx:H
(I)
•
We
v
Z
has
the
lone
ESR
defect detected by ESR pair
electrons. 14
measurement
might
The
clarify
and the s content of randomly dispersed H atoms contribut-
SPa-Si:F ~:X=O
'Eu
N
this.
n GD a - Sil_xCx: (F, H )
1021
present besides
photo-induced
• GDa-Sil_xCx:H (~)
A"
Vol. 47, No. i0
~
1020
reported
that
N
ing to the narrow component of NMR specrum [ H]n, a good correlation in a-Si: H .5,6 NMR
AA
have
g
>,
previously
measurements
for
GD
a-Sil_xCx:H (II) II
and
SP
a-Sil_xNx:Hl5 show that [ HI n in these films has c
1019
a tendency
t--
likely
'~.
#
1018
cn
not
Z~
%.o []r l 1017
A
only
around
the
incorporation of N
directly
Si
atoms
decreasing [ H]n, Z~
A
increases but
the
also
resulting
or C atoms
local
strain
increases in the
it
increase
by in
the density of dangling bonds. The present view that the presence of the large local strain due
~A 1016 50
to decrease with x. Therefore it is
that
I 100
to structural constraint produces dangling bonds
I 150
200
in order to relax the local strain and lower the total
AW(TO)' (cm-I )
energy
is
also
cons_Jtent
with
our
previous finding that not only H atoms bonded to Figure 3. Am(TO)'
Spin
density
N
as
s
a
function
of
Ge
but also those bonded to Si have an ability
to
reduce
the
density
of
dangling
bonds
by
softening the amorphous network structure. 7 In conclusion, the pronounced broadening of
smaller than that of Si atom 12 is also expected the to
contribute
to the
increase
in local
Si-Si
TO-like
band
for
amorphous Si-N and
strain Si-C
alloy systems
is observed with increasing
around Si atoms. As can be seen from Fig.3, the the relation between Am(TO)'
and N
N
or
C
content.
The
broadening
of
the
is similar for s
TO-like band for amorphous Si system as well as
all films including GD a-Sil_xCx:H except for SP amorphous Si-N and Si-C alloy systems has a good a-Sil_xNx:H.
Therefore
it
is
reasonable
to correlation with
assume
that
the
an
increase in the density of
broadening of the TO-like band dangling bonds. The fact suggests that dangling
is
brought
about
mainly
by
the
local
strain bonds
around
Si
atoms
and
the
change
of
the
are produced in order to relax the local
Si-Si strain
due
to
structural
constraint and
lower
vibrational spectrunl by the presence of C atoms the total energy. neighboring Si atoms does not largely influence Am(TO)'.
In
case
of
SP
a-Sil_xNx:H,
however,
Am(TO)' incre3ses very rapidly with x, although
Acknowledgements-The authors thank T. Miura f o r
an
his
increase
in I~
with x
is not so prominent.
s
help
in
was
larger than Si
Japan.
Project
partly
Ministry of International T~ade and Industry in
electronegativity far
Sunshine
work
which
has
the
This
supported
influences Am(TO)'. Another possibility is that
by
experiment.
Therefore it is possible that the presence of N
of
the
Vol. 47, No. IO
SILICON-BASED AMORPHOUS SEMICONDUCTOR FILMS
777
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