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Transport properties of halogenated amorphous silicon
301
Materials Chemistry and Physics, 9 (1983) 301-305
TRANSPORT
PROPERTIES
V.AUGELLI
and R.MURRI
Dipartimento Universita
OF HALOGENATED
di Fisica and Gruppo di Bari, Via Amendola
AMORPHOUS
Nazionale
SILICON
di Struttura
della Materia,
173, 70126 Bari (Italy)
ABSTRACT Electrical silicon
and photoelectronic
films have been studied
The dark conductivity deposition
properties
of halogenated
as a function
of several
and the photoconductivity
and
hydrogenated
deposition
are strongly
parameters.
dependent
onthese
parameters.
From these trends,
hypotheses
on the structural
model of the film are made.
INTRODUCTION The growing to two reasons. material
interest
The first is that amorphous
for solar energy
ting technological reason
in the study of amorphous
conversion
applications
latively
high semiconductor
bility of doping tes dangling
the physics quality
lowering
the density
cause of this, much attention red by dissociation
of silane.
pacitively silicon
coupled
Moreover,
0254_0584/83/US $3.00
an
due
inexpensive
C21
. The second
gives us the opportunity
that hydrogen
amorphous
to go dee-
semiconductors.
silicon
of states
properties
mixtures
glow-discharge
tetrachloride
etc.)
incorporation
silicon
silicon
C5,61. Our samples
possisatura-
E41.
Be-
films
prena-
films which
were
were deposited
by using as starting
The re-
is due to the
in the pseudo-gap
has been paid to amorphous
also stable and had good electrical SiF4 or Sic14 and hydrogen
represents
of amorphous
of amorphous
~31 and to the discovery
bonds,
is essentially
Lll, as well as for other interes-
(transistor, xerography,
is that the study of its properties
per in to the understandingof
silicon
devices
silicon
from
starting
were deposited material
in a ca-
a mixture
of
and hydrogen,
0 Elsevier Sequoia/P~nted
in The Netherlands
302 Experimental
details
about the deposition
In this paper we will discuss tivity of a-Si:H:Cl
the dependence
films on some deposition
rature of the film, Td; the discharge fined as the percentage gap state distribution
apparatus
can be found in ref.C71.
of the dark- and the photo-conducparameters:
pressure,
the deposition
p, and the feeding
of Sic14 in the total flow, Finally
tempe-
ratio R, de-
some remarks on the
will be made.
EXPERIMENTAL Films of halogenated -discharge thickness equipped
of silicon
Four contacts
measurements.
Photo-conductivity
was measured
using ~nochromatic
by using a constant recorded
current
film
on sapphire,
were
were used for re-
light
the applied
photon flux was 101s-1014
. The
on the outer surface.
chopped
with a lock-in amplifier;
171
for photo-conductivity
In both cases, they are coplanar
Current Source and the voltage Keithley
The films deposited
in the van der Pauw configuration
5x10* V/cm and the incident
by r-f. glow -
and hydrogen
1.5 mm apart,
contacts,
was measured
The current was measured
ductivity
silicon were prepared
tetrachloride
ranged from 0.5 to 1.5 microns. with two Al sputtered
measurements. sistivity
and hydrogenated
from a mixture
20 Hz).
=
field was about
photons/s.
supplied
(V
The dark
con-
by a Keithley
by an high input impedance
225
(>1012 ohm)
616 electrometer.
RESULTS In
this paper we present
phous silicon
deposited
some experimental
under different
infer a trend for the dependence rameters.
A more systematic
paper. First,
a set of samples ture Td ranging
deposited between
of the measured
investigation
let us consider
p=2 torr; Td=320
as a function
of
tempera-
the room temperature
conduc-
of Td. As one can see, d RT increases practically
constant.
of 0 RT on R is displayedby
Figs.lb
The conductivity
with R. In Fig.lb the experimental
at W=70 W; Td=320 OC; R=3%; while
p varies
samples
and lc
increases
points refer to from 0.7 to 4.5
deposited
at W=70 W;
"C and R=3 to 15%.
The same qualitative are reported
pa-
on Td of the dark conductivity
of o RT on p and R, respectively.
torr. The dependence
to
on the discharge
at p-2 torr; R=3%; W=7OW and at deposition
it diminishes
samples deposited
films ofamorallow us
will be the object of a forthcoming
the dependence
rapidly up Td ~300 "C and then it remains
with p whilst
Such results
quantity
175 and 430 "C. In Fig.la,
tivity u RT, is reported
show the behaviour
results concerning
conditions.
behaviour
the experimental
is shown by the photoconductivity.
points of the photo-conductivity*
tion of Td, p and R. As one can see, o ph increases and then it is practically
constant
(Fig.2a).
CT
In
Fig.2
as a func-
ph rapidly up to Td 2300 "C
a ph increases
with pressure
in
303
0
1
2
3
4
Pftorr)
Fig.1
Room temperature
perature
conductivity
(6) discharge
, Td;
as a function
pressure,
of:
(a) depositiontem-
p and (c) feeding
ratio, R.
b m 5
0
/
=
0)
0
/
0 \ \
4 9
\
& ?0-2
./
I’
!
0-
0
/
/
\
I 2
1
I 3
I 4
Pltorrl
1 -
I I
-I
IO
i I
150 Fig.2
Photo-conductivity
(b) discharge
pressure,
4, as a function
shown by oph &
R (Fig.2c)
the range 3 to 10% and little variation A more extensive films is reported
of:
p and (c) feeding
the range 0.8 to 3.0 torr with a maximum The behaviour
5
0
I
350
250
analysis
in ref.C81.
RI%1
(a) deposition
10
15
temperature,Td;
ratio R.
at p=3 torr and then decreases (FQ..2b). shows a quite clear
lowering
of
uph in
in the range 10 to 15%.
of the photo-conductive
properties
of a-Si:H:Cl
304
DISCUSSION
Though the experimental results reported here concern the dependence on the deposition parameters of the dark-and the photo-conductivity,we would like to point out some characteristicfeatures of the electronic transport in our a-Si: :H:Cl films. Measurements of the dark conductivity as a function of the temperature have clearly shown that three different temperature ranges exist in which od has a definite activation energy C91 . Approximately in the range 90 to250 k all samples show an activation energy of ud of about 0.01 eV. This seems a characteristic feature of all samples, independent of the deposition conditions.For higher temperature, up to about 550 K, the activation energy depends on the
de-
position conditions and takes values in the range 0.3 to 0.5 eV. Since theoptical gap of our undoped samples is about 2 eV ClOl, the previous activationenergies are associated to hopping transport processes. When the temperature is raised (550
the
films. Let us assume a two-phase structural model Clll, in which islands of
mi-
crocrystallinematerial are interconnectedby connective tissue, with high density of defects. Probably~, the volume occupied by the tissue diminishes, increasins Td and the conductivitv increases. The structural rearranaement is comoetitive with that related to the hvdroaen and chlorine contents in the filmnetwork, as Td increases. This could explain the practically constant behaviour of Ud for Td > 300 "C. As far as the dependence of od on R or p is concerned, it could be argued that both parameters contribute to a worsening of the film quality.
Ac-
tually, we found that ud decreases as R increases, but, at least in the investigated pressure range, we did not find a similar behaviour for Ud w
p. Now,
let
us discuss the photo-conductiveproperties of our a-Si:H:Cl films. As one cansee in Fig.2a, the photo-conductivityvs Td curve shows a step rise up to 300 'C. This behaviour confirms other experimental results C12, 131, and is determinedby the decrease of the recombination centre density and, as a consequence, by
the
increase of the recombination lifetime. After Td 2:300 "C, oph is practically constant. As far as the pressure dependence of aph is concerned, it must bepointed out that as increasing p, two mechanisms take place ClO, 141: the first concerns the increase of the hydrogen and chlorine content in the film network; the second concerns nucleation mechanisms producing a deterioration in the filmstruc ture. The last mechanism seems to become important for p > 2 torr. In the range 0.8 to 3.0 torr, oph increases, since the more important process is the compen sation of the dangling bonds due to hydrogen and chlorine. For greater pressure,
305
the
increasing
Fig.2c
density
of defects
shows the dependence
of (SiC14tH2).
Although
causes a lowering
of oph on the percentange
the experimental
280 < Td < 430 "C, we can equally pendent
less chemical
establish
and physical
ted out, an increasein
of Sic14
in the total flow
points refer to films deposited
at
a trend that dph is practicallyinde-
of Tdin that range. We found that samples
displayed
of the photoconductivity.
stability
deposited
at high flow(R>lO%)
[lo]. In any case, as alreadypoin
R gives rise to an increase
of defects
and then to a
mi-
nor photo-conductivity.
ACKNOWLEDGEMENT The authors wish to thank the researchesrs Chimica
dei Plasmi"
lo for typewriting to Finalizzato
of the "Centro
of C.N.R. of Bari for preparing the manuscript.
Chimica
This work was partially
Fine e Secondaria"
di Studio
the specimens
per
la
and R.M.Cannil
supportedby
"Proget-
of C.N.R.
REFERENCES
1
D.E.Carlson,
2
W.E.Spear,
Solar Energy Materials P.G. Le Comber,
2 (1980) 503
A.J.Snell
and R.A.Gibson,
Thin Solid Films,
-90
(1982) 359 3
W.E.Spear
4
A.J.Lewis,
and P.G.Le Comber, G.A.N.Connell,
Bonded Amorphous
Phil. Mag.33 (1976) 935 J.R.Pawlik and R.J.Temkin,
W.Paul,
Semiconductors
, ed by M.Brodsky,
inTetrahedrally
S.Kirkpatrick,
D.Weaire
(AIP, New York 1974) pp 27-32 5
A.Madan,
6
R.D.Plattner, W.W.Kruhler,
S.R.Ovshinsky
E.C. Photovoltaic W.Palz.
(Reidel,
and E.Benn,
Phil. Flag. -40 (1979) 259
B.Rauscher,
W.Stetter
Solar Energy Conferenze
and J.G.Grabmaier,
Proc. 2nd
ed. by R.Van Overstraeten
and
1979) pag.860
7
G.Bruno,
P.Capezzuto,
F.Cramarossa
and D'Agostino,
8
V.Augelli,
R.Murri
and N.Alba,
9
V.Augelli,
R.Murri
and L.Schiavulli,
10
V.Augelli,
R.Murri,
Thin Solid Films67 -
(1980)
103
gelisti
L.Schiavulli,
and G.Fortunato, and D.A.Anderson,
W.Paul
12
W.E.Spear
and P.G.Le Comber,
by J.Mort
and D.M.Pai
P.J.Zanzucchi,
14
E.C.Freeman
G.Bruno,
P.Capezzuto,
Solar Energy Materials,
F.Cramarossa,
F.Evan
C.R.Wronski
2 (1981) 229
in Photo-conductivity
(Elsevier,
and W.Paul,
Phys. 54 (1983) 248
Thin Solid Films -90 (1982)
Thin Solid Films, 86 (1981) 359
11
13
J.Appl.
and Related
Phenomena,
New York 1976)
and D.E.Carlson,
Phys. Rev. -20 B
J.Appl.PHys.
(1979) 716
-48 (1977) 5227
ed.
Materials Chemistry and Physics, 9 (1983) 301-305
TRANSPORT
PROPERTIES
V.AUGELLI
and R.MURRI
Dipartimento Universita
OF HALOGENATED
di Fisica and Gruppo di Bari, Via Amendola
AMORPHOUS
Nazionale
SILICON
di Struttura
della Materia,
173, 70126 Bari (Italy)
ABSTRACT Electrical silicon
and photoelectronic
films have been studied
The dark conductivity deposition
properties
of halogenated
as a function
of several
and the photoconductivity
and
hydrogenated
deposition
are strongly
parameters.
dependent
onthese
parameters.
From these trends,
hypotheses
on the structural
model of the film are made.
INTRODUCTION The growing to two reasons. material
interest
The first is that amorphous
for solar energy
ting technological reason
in the study of amorphous
conversion
applications
latively
high semiconductor
bility of doping tes dangling
the physics quality
lowering
the density
cause of this, much attention red by dissociation
of silane.
pacitively silicon
coupled
Moreover,
0254_0584/83/US $3.00
an
due
inexpensive
C21
. The second
gives us the opportunity
that hydrogen
amorphous
to go dee-
semiconductors.
silicon
of states
properties
mixtures
glow-discharge
tetrachloride
etc.)
incorporation
silicon
silicon
C5,61. Our samples
possisatura-
E41.
Be-
films
prena-
films which
were
were deposited
by using as starting
The re-
is due to the
in the pseudo-gap
has been paid to amorphous
also stable and had good electrical SiF4 or Sic14 and hydrogen
represents
of amorphous
of amorphous
~31 and to the discovery
bonds,
is essentially
Lll, as well as for other interes-
(transistor, xerography,
is that the study of its properties
per in to the understandingof
silicon
devices
silicon
from
starting
were deposited material
in a ca-
a mixture
of
and hydrogen,
0 Elsevier Sequoia/P~nted
in The Netherlands
302 Experimental
details
about the deposition
In this paper we will discuss tivity of a-Si:H:Cl
the dependence
films on some deposition
rature of the film, Td; the discharge fined as the percentage gap state distribution
apparatus
can be found in ref.C71.
of the dark- and the photo-conducparameters:
pressure,
the deposition
p, and the feeding
of Sic14 in the total flow, Finally
tempe-
ratio R, de-
some remarks on the
will be made.
EXPERIMENTAL Films of halogenated -discharge thickness equipped
of silicon
Four contacts
measurements.
Photo-conductivity
was measured
using ~nochromatic
by using a constant recorded
current
film
on sapphire,
were
were used for re-
light
the applied
photon flux was 101s-1014
. The
on the outer surface.
chopped
with a lock-in amplifier;
171
for photo-conductivity
In both cases, they are coplanar
Current Source and the voltage Keithley
The films deposited
in the van der Pauw configuration
5x10* V/cm and the incident
by r-f. glow -
and hydrogen
1.5 mm apart,
contacts,
was measured
The current was measured
ductivity
silicon were prepared
tetrachloride
ranged from 0.5 to 1.5 microns. with two Al sputtered
measurements. sistivity
and hydrogenated
from a mixture
20 Hz).
=
field was about
photons/s.
supplied
(V
The dark
con-
by a Keithley
by an high input impedance
225
(>1012 ohm)
616 electrometer.
RESULTS In
this paper we present
phous silicon
deposited
some experimental
under different
infer a trend for the dependence rameters.
A more systematic
paper. First,
a set of samples ture Td ranging
deposited between
of the measured
investigation
let us consider
p=2 torr; Td=320
as a function
of
tempera-
the room temperature
conduc-
of Td. As one can see, d RT increases practically
constant.
of 0 RT on R is displayedby
Figs.lb
The conductivity
with R. In Fig.lb the experimental
at W=70 W; Td=320 OC; R=3%; while
p varies
samples
and lc
increases
points refer to from 0.7 to 4.5
deposited
at W=70 W;
"C and R=3 to 15%.
The same qualitative are reported
pa-
on Td of the dark conductivity
of o RT on p and R, respectively.
torr. The dependence
to
on the discharge
at p-2 torr; R=3%; W=7OW and at deposition
it diminishes
samples deposited
films ofamorallow us
will be the object of a forthcoming
the dependence
rapidly up Td ~300 "C and then it remains
with p whilst
Such results
quantity
175 and 430 "C. In Fig.la,
tivity u RT, is reported
show the behaviour
results concerning
conditions.
behaviour
the experimental
is shown by the photoconductivity.
points of the photo-conductivity*
tion of Td, p and R. As one can see, o ph increases and then it is practically
constant
(Fig.2a).
CT
In
Fig.2
as a func-
ph rapidly up to Td 2300 "C
a ph increases
with pressure
in
303
0
1
2
3
4
Pftorr)
Fig.1
Room temperature
perature
conductivity
(6) discharge
, Td;
as a function
pressure,
of:
(a) depositiontem-
p and (c) feeding
ratio, R.
b m 5
0
/
=
0)
0
/
0 \ \
4 9
\
& ?0-2
./
I’
!
0-
0
/
/
\
I 2
1
I 3
I 4
Pltorrl
1 -
I I
-I
IO
i I
150 Fig.2
Photo-conductivity
(b) discharge
pressure,
4, as a function
shown by oph &
R (Fig.2c)
the range 3 to 10% and little variation A more extensive films is reported
of:
p and (c) feeding
the range 0.8 to 3.0 torr with a maximum The behaviour
5
0
I
350
250
analysis
in ref.C81.
RI%1
(a) deposition
10
15
temperature,Td;
ratio R.
at p=3 torr and then decreases (FQ..2b). shows a quite clear
lowering
of
uph in
in the range 10 to 15%.
of the photo-conductive
properties
of a-Si:H:Cl
304
DISCUSSION
Though the experimental results reported here concern the dependence on the deposition parameters of the dark-and the photo-conductivity,we would like to point out some characteristicfeatures of the electronic transport in our a-Si: :H:Cl films. Measurements of the dark conductivity as a function of the temperature have clearly shown that three different temperature ranges exist in which od has a definite activation energy C91 . Approximately in the range 90 to250 k all samples show an activation energy of ud of about 0.01 eV. This seems a characteristic feature of all samples, independent of the deposition conditions.For higher temperature, up to about 550 K, the activation energy depends on the
de-
position conditions and takes values in the range 0.3 to 0.5 eV. Since theoptical gap of our undoped samples is about 2 eV ClOl, the previous activationenergies are associated to hopping transport processes. When the temperature is raised (550
the
films. Let us assume a two-phase structural model Clll, in which islands of
mi-
crocrystallinematerial are interconnectedby connective tissue, with high density of defects. Probably~, the volume occupied by the tissue diminishes, increasins Td and the conductivitv increases. The structural rearranaement is comoetitive with that related to the hvdroaen and chlorine contents in the filmnetwork, as Td increases. This could explain the practically constant behaviour of Ud for Td > 300 "C. As far as the dependence of od on R or p is concerned, it could be argued that both parameters contribute to a worsening of the film quality.
Ac-
tually, we found that ud decreases as R increases, but, at least in the investigated pressure range, we did not find a similar behaviour for Ud w
p. Now,
let
us discuss the photo-conductiveproperties of our a-Si:H:Cl films. As one cansee in Fig.2a, the photo-conductivityvs Td curve shows a step rise up to 300 'C. This behaviour confirms other experimental results C12, 131, and is determinedby the decrease of the recombination centre density and, as a consequence, by
the
increase of the recombination lifetime. After Td 2:300 "C, oph is practically constant. As far as the pressure dependence of aph is concerned, it must bepointed out that as increasing p, two mechanisms take place ClO, 141: the first concerns the increase of the hydrogen and chlorine content in the film network; the second concerns nucleation mechanisms producing a deterioration in the filmstruc ture. The last mechanism seems to become important for p > 2 torr. In the range 0.8 to 3.0 torr, oph increases, since the more important process is the compen sation of the dangling bonds due to hydrogen and chlorine. For greater pressure,
305
the
increasing
Fig.2c
density
of defects
shows the dependence
of (SiC14tH2).
Although
causes a lowering
of oph on the percentange
the experimental
280 < Td < 430 "C, we can equally pendent
less chemical
establish
and physical
ted out, an increasein
of Sic14
in the total flow
points refer to films deposited
at
a trend that dph is practicallyinde-
of Tdin that range. We found that samples
displayed
of the photoconductivity.
stability
deposited
at high flow(R>lO%)
[lo]. In any case, as alreadypoin
R gives rise to an increase
of defects
and then to a
mi-
nor photo-conductivity.
ACKNOWLEDGEMENT The authors wish to thank the researchesrs Chimica
dei Plasmi"
lo for typewriting to Finalizzato
of the "Centro
of C.N.R. of Bari for preparing the manuscript.
Chimica
This work was partially
Fine e Secondaria"
di Studio
the specimens
per
la
and R.M.Cannil
supportedby
"Proget-
of C.N.R.
REFERENCES
1
D.E.Carlson,
2
W.E.Spear,
Solar Energy Materials P.G. Le Comber,
2 (1980) 503
A.J.Snell
and R.A.Gibson,
Thin Solid Films,
-90
(1982) 359 3
W.E.Spear
4
A.J.Lewis,
and P.G.Le Comber, G.A.N.Connell,
Bonded Amorphous
Phil. Mag.33 (1976) 935 J.R.Pawlik and R.J.Temkin,
W.Paul,
Semiconductors
, ed by M.Brodsky,
inTetrahedrally
S.Kirkpatrick,
D.Weaire
(AIP, New York 1974) pp 27-32 5
A.Madan,
6
R.D.Plattner, W.W.Kruhler,
S.R.Ovshinsky
E.C. Photovoltaic W.Palz.
(Reidel,
and E.Benn,
Phil. Flag. -40 (1979) 259
B.Rauscher,
W.Stetter
Solar Energy Conferenze
and J.G.Grabmaier,
Proc. 2nd
ed. by R.Van Overstraeten
and
1979) pag.860
7
G.Bruno,
P.Capezzuto,
F.Cramarossa
and D'Agostino,
8
V.Augelli,
R.Murri
and N.Alba,
9
V.Augelli,
R.Murri
and L.Schiavulli,
10
V.Augelli,
R.Murri,
Thin Solid Films67 -
(1980)
103
gelisti
L.Schiavulli,
and G.Fortunato, and D.A.Anderson,
W.Paul
12
W.E.Spear
and P.G.Le Comber,
by J.Mort
and D.M.Pai
P.J.Zanzucchi,
14
E.C.Freeman
G.Bruno,
P.Capezzuto,
Solar Energy Materials,
F.Cramarossa,
F.Evan
C.R.Wronski
2 (1981) 229
in Photo-conductivity
(Elsevier,
and W.Paul,
Phys. 54 (1983) 248
Thin Solid Films -90 (1982)
Thin Solid Films, 86 (1981) 359
11
13
J.Appl.
and Related
Phenomena,
New York 1976)
and D.E.Carlson,
Phys. Rev. -20 B
J.Appl.PHys.
(1979) 716
-48 (1977) 5227
ed.