- Email: [email protected]
Reversible dangling-bond generation in amorphous silicon thin-film transistors
Journal of Non-CrystallineSolids97&98 (1987) 1339-1342 North-Holland,Amsterdam
REVERSIBLE
DD~NGLING-BOND
1339
GENERATION
IN
AMORPHOUS
SILICON
THIN-FILM
TR~SISTORS*
R.E.I. SCHROPP, A.J. BOONSTRA and T.M. KLAPWIJK Department of Applied Physics, University of Groningen, Nijenborgh 18, 9747 AG Groningen, The Netherlands.
The field-induced degradation and thermal recovery of amorphous silicon thin-film transistors have been studied. Experimental evidence is given for the single carrier trapping-induced creation of dangling bonds in the amorphous silicon layer. It is suggested that the recovery is due to the re-formation of weak-bonds after a rate-limiting step of thermal electronemission out of these defects with an activation energy of 0.77 eV and an attempt frequency of 4x106 Hz.
i. INTRODUCTION Thin-film
transistors (TFTs)
(a-Si:H) generally show a operation
under
gradual
accumulation
made with ON-current
conditions.
hydrogenated
amorphous silicon
degradation
during continuous
This type of degradation shows up
experimentally as a threshold voltage shift, which can be reversed by temperature annealing.
The degradation has therefore been attributed to tunneling of
electrons in the gate dielectric I. However, the film resulting out 2. Recently,
effect of from the
an increased
dangling-bond density
applied transverse
we reported 3
on slow
electric field
in the a-Si:H
can not be ruled
field-effect transients
in TFTs which
were fabricated on a thermally oxidized crystalline silicon substrate. The use of the high-quality SiO 2 gate insulator, forming a electrons from
the silicon
conduction band
barrier of
over 3
and having much fewer traps than
silicon nitride, allows an unambiguous interpretation of the data in degradation of
eV for
terms of
the a-Si:H itself. Based on these experiments a model has been
developed 3 for field-induced degradation,
which consists
of reversible dang-
ling-bond generation by trapping of charge carriers at weak Si-Si bonds, whose energy levels are in
the band
tails, followed
by a
structural metastabili-
zation. The degradation was shown to be thermally activated with an activation energy of 0.4 eV, which differs from what would be expected in
case of direct
tunneling into the insulator. The purpose of this paper is to present supporting evidence for the dangling bond
generation model
as the
origin of field-
*This work was supported by The Foundation for Fundamental Research on Matter (Stichting F.O.M., Utrecht).
0022-3093/87/$03.50 ©Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
R.E.L Schropp et al. / Reversible dangling-bond generation
1340
effect (FE)
current deterioration.
The recovery of the TFTs has been studied
to extract the annealing activation energy.
2. DEGRADATION To confirm that the
observed degradation
originates from
the presence of
only one type of carrier, we prepared undoped amorphous silicon TFTs which had strictly n- or p-channel operation. This was achieved by source and
drain contact
regions 4, so
implantation at the
that both types of TFTs could be made
using amorphous silicon from the same run. There are a number of arguments in favour model. First,
the degradation
of the
dangling bond generation
of a TFT over a 6x104 s application period, is
independent of gate voltage above saturation and at least up to 20
V. Second,
the degraded state can not be reserved by applying a gate bias of the opposite sign. On the contrary, a bias stress in an n-channel threshold voltage
shift irrespective
that in structures magnitude of
with
a
nitride
of the gate
TFT produces
sign of
insulator
a positive
the stress field. Note the
relatively smaller
negative threshold voltage shift at negative bias as compared to
the effect at positive bias I defect generation
can
effect. Third,
obviously
be
the current
explained
by
this opposing
decay in an n-channel TFT at a
negative bias (-i0 V) and monitored at a positive gate voltage (+i0 V), though saturating towards the same level, proceeds at a faster rate than if permanently kept at a positive bias of +i0 the
present
breaking weak
model,
considering
bonds because
tail-states. The
V (fig.l). that
of a
hole
This can
be understood within
trapping
is
stronger localization
more efficient in of the valence-band
difference in relaxation times of two orders of magnitude on
these stretched exponential functional forms compares well with the difference in equilibration times observed upon thermal quenching of p- and n-type !
?
~
10-5 ~o {0-~
n-ICHFINN['L V
Initin{ state
L. ~ k~_
0.5 N
stress
%
monitor ÷]0 V
-10 V
u
\~.
E I 101
I 102
~
R f t e r +40 V stress
10-0
10-9 10-I0
.... stress
c 01 I00
doped
I 103
I {04
I
I05 F{e}d application time (s)
lOG
FIGURE 1 Decay of the ON-current in n- and pchannel TFTs at various conditions.
10_11
~- 10-12 . . . . . . . . . . . . . . . . . ¢~ 0 -5 0 5 Gate
Voltac3e
V9
(V)
FIGURE 2 Transfer characteristics of ambipolarTFTs before and after a positive and a negative stress period.
IO
R.E.L Schropp et al. / Reversible dangling-bond generation
1
VQ (V)
U_L_U
1341
,
!
+
7
IL
a)
t. i_
T ('C)
$,
'657 ,'
!
97
hi
t
0 E
o
O.
? Rnneal
FIGURE 3 Procedure for the study of the FE current recovery after degradation.
a-Si:H 5. This
suggests that
i
J
103
104
Time
84 105
(s)
FIGURE 4 Recovery of the FE current at temperatures indicated. Symbols are measured data, curves are calculated.
a suddenly imposed space charge makes the struc-
ture relax in a similar way towards an
equilibrium of increased dangling bond
density. A p-channel TFT, whose decay can be monitored during application of a negative stress, degrades at approximately the same rate under the
same conditions
either sign applied to appear to
have moved
(fig. i).
TFTs
with
results in
ambipolar
as an
a bias
n-channel TFT
stress period of
characteristics
the
FE slopes
apart (fig. 2). Moreover, a negative field has a larger
effect than a positive field of with the
Fourth, after
the same
unipolar TFTs.
magnitude and
The deep
duration, consistent
defect density
stressing, as shown in fig. 2 by the increase in the distance
has risen upon between thresh-
old voltages for n- and p-channel conduction 6.
3. RECOVERY The recovery of the structure was studied by annealing a device in the dark at temperatures T A between 84 and 127°C, maintaining the gate and taking
snapshots of
the current
identically stressed
defects. Before
condition was
achieved by
TFT at 165°C for 3 hours and subsequently stressing it (i0 V,
65°C, 6x104
normalized with figure also
0 V
at +i0 V at time intervals sufficiently
large to avoid simultaneous re-introduction of experiment an
voltage at
each recovery annealing the
under fixed conditions
s) (fig. 3). Fig. 4 contains the recovery data which were
the currents
measured at
+i0 V
in the
annealed state. The
contains calculated recovery curves assuming first order kinetics
with a single activation energy EA and attempt-to-anneal is the simplest possible mechanism. Then,
frequency
~o, which
P.E.L Schropp et al. / Reversible dangling-bond generation
1342
Nrec(t ) = Nrec(0) exp [-Po exp (-EA/kTA)t ]
in
which
Nre c
is
the
density
of
generated in addition to the stable state. We
recoverable
dangling bonds, which were
defects already
present in
the annealed
let the current be determined by the interface band-bending result-
ing from a uniform mid-gap density of states.
Although the
fit was
not very
sensitive as long as the attempt-to-anneal frequency and the activation energy were taken correlated in a way identical to the Meyer-Neldel exp(AEA) with
A =
rule ( V o =
Voo
30.3 ev-l), a simplex minimization method indicated values
of 4x106 s -I and 0.77 eV, respectively. Our EA result
is comparable
with the
value of 0.85 eV found by Hepburn et al. 2 for the energy level of bias-induced dangling bonds. Our interpretation is that creates
charged
dangling
the
bonds
positive below
bias
the
stress
period essentially
Fermi-level.
The
low attempt-
frequency is consistent with the metastability of the threshold voltage shifts observed. The
recovery mechanism
dangling bonds into a emission of
weak Si-Si
these electrons
energy is determined by impurity-rich
samples 7 ,
which implies recombination of neighbouring bond is
therefore rate-limited
by thermal
out of the dangling bonds. The anneal activation
their energy suggesting
level. Both that
the
Vo
origin
and E A of
are typical of
the field-induced
dangling bonds is different from intrinsic dangling bonds. Perhaps this is due to the
different structural
and chemical
composition of a-Si:H close to the
interface.
ACKNOWLEDGEMENT We wish to thank P.S. van Dijk for performing part of the measurements.
REFERENCES i) M. J. Powell, Appl. Phys. Lett. 43 (1983) 587. 2) A. R. Hepburn, J. M. Marshall, C. Main, M. J. Powell and C. van Berkel, Phys. Rev. Lett. 56 (1986) 2215. 3) R. E. I. Schropp and J. F. Verwey, Appl. Phys. Lett. 50 (1987) 185. 4) R. E. I. Schropp and J. F. Verwey, to be publ. in Mat. Res. Symp. Proc., Anaheim, 1987. 5) R. A. Street, J. Kakalios, C. C. Tsai and T. M. Hayes, Phys. Rev. B35 (!987) 1316. 6) M. J. Powell, C. van Berkel and I. D. French, this volume. 7) M. Stutzmann, W. B. Jackson and C. C. Tsai, J. Non-Cryst. solids. 77&78 (1985) 363.
REVERSIBLE
DD~NGLING-BOND
1339
GENERATION
IN
AMORPHOUS
SILICON
THIN-FILM
TR~SISTORS*
R.E.I. SCHROPP, A.J. BOONSTRA and T.M. KLAPWIJK Department of Applied Physics, University of Groningen, Nijenborgh 18, 9747 AG Groningen, The Netherlands.
The field-induced degradation and thermal recovery of amorphous silicon thin-film transistors have been studied. Experimental evidence is given for the single carrier trapping-induced creation of dangling bonds in the amorphous silicon layer. It is suggested that the recovery is due to the re-formation of weak-bonds after a rate-limiting step of thermal electronemission out of these defects with an activation energy of 0.77 eV and an attempt frequency of 4x106 Hz.
i. INTRODUCTION Thin-film
transistors (TFTs)
(a-Si:H) generally show a operation
under
gradual
accumulation
made with ON-current
conditions.
hydrogenated
amorphous silicon
degradation
during continuous
This type of degradation shows up
experimentally as a threshold voltage shift, which can be reversed by temperature annealing.
The degradation has therefore been attributed to tunneling of
electrons in the gate dielectric I. However, the film resulting out 2. Recently,
effect of from the
an increased
dangling-bond density
applied transverse
we reported 3
on slow
electric field
in the a-Si:H
can not be ruled
field-effect transients
in TFTs which
were fabricated on a thermally oxidized crystalline silicon substrate. The use of the high-quality SiO 2 gate insulator, forming a electrons from
the silicon
conduction band
barrier of
over 3
and having much fewer traps than
silicon nitride, allows an unambiguous interpretation of the data in degradation of
eV for
terms of
the a-Si:H itself. Based on these experiments a model has been
developed 3 for field-induced degradation,
which consists
of reversible dang-
ling-bond generation by trapping of charge carriers at weak Si-Si bonds, whose energy levels are in
the band
tails, followed
by a
structural metastabili-
zation. The degradation was shown to be thermally activated with an activation energy of 0.4 eV, which differs from what would be expected in
case of direct
tunneling into the insulator. The purpose of this paper is to present supporting evidence for the dangling bond
generation model
as the
origin of field-
*This work was supported by The Foundation for Fundamental Research on Matter (Stichting F.O.M., Utrecht).
0022-3093/87/$03.50 ©Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
R.E.L Schropp et al. / Reversible dangling-bond generation
1340
effect (FE)
current deterioration.
The recovery of the TFTs has been studied
to extract the annealing activation energy.
2. DEGRADATION To confirm that the
observed degradation
originates from
the presence of
only one type of carrier, we prepared undoped amorphous silicon TFTs which had strictly n- or p-channel operation. This was achieved by source and
drain contact
regions 4, so
implantation at the
that both types of TFTs could be made
using amorphous silicon from the same run. There are a number of arguments in favour model. First,
the degradation
of the
dangling bond generation
of a TFT over a 6x104 s application period, is
independent of gate voltage above saturation and at least up to 20
V. Second,
the degraded state can not be reserved by applying a gate bias of the opposite sign. On the contrary, a bias stress in an n-channel threshold voltage
shift irrespective
that in structures magnitude of
with
a
nitride
of the gate
TFT produces
sign of
insulator
a positive
the stress field. Note the
relatively smaller
negative threshold voltage shift at negative bias as compared to
the effect at positive bias I defect generation
can
effect. Third,
obviously
be
the current
explained
by
this opposing
decay in an n-channel TFT at a
negative bias (-i0 V) and monitored at a positive gate voltage (+i0 V), though saturating towards the same level, proceeds at a faster rate than if permanently kept at a positive bias of +i0 the
present
breaking weak
model,
considering
bonds because
tail-states. The
V (fig.l). that
of a
hole
This can
be understood within
trapping
is
stronger localization
more efficient in of the valence-band
difference in relaxation times of two orders of magnitude on
these stretched exponential functional forms compares well with the difference in equilibration times observed upon thermal quenching of p- and n-type !
?
~
10-5 ~o {0-~
n-ICHFINN['L V
Initin{ state
L. ~ k~_
0.5 N
stress
%
monitor ÷]0 V
-10 V
u
\~.
E I 101
I 102
~
R f t e r +40 V stress
10-0
10-9 10-I0
.... stress
c 01 I00
doped
I 103
I {04
I
I05 F{e}d application time (s)
lOG
FIGURE 1 Decay of the ON-current in n- and pchannel TFTs at various conditions.
10_11
~- 10-12 . . . . . . . . . . . . . . . . . ¢~ 0 -5 0 5 Gate
Voltac3e
V9
(V)
FIGURE 2 Transfer characteristics of ambipolarTFTs before and after a positive and a negative stress period.
IO
R.E.L Schropp et al. / Reversible dangling-bond generation
1
VQ (V)
U_L_U
1341
,
!
+
7
IL
a)
t. i_
T ('C)
$,
'657 ,'
!
97
hi
t
0 E
o
O.
? Rnneal
FIGURE 3 Procedure for the study of the FE current recovery after degradation.
a-Si:H 5. This
suggests that
i
J
103
104
Time
84 105
(s)
FIGURE 4 Recovery of the FE current at temperatures indicated. Symbols are measured data, curves are calculated.
a suddenly imposed space charge makes the struc-
ture relax in a similar way towards an
equilibrium of increased dangling bond
density. A p-channel TFT, whose decay can be monitored during application of a negative stress, degrades at approximately the same rate under the
same conditions
either sign applied to appear to
have moved
(fig. i).
TFTs
with
results in
ambipolar
as an
a bias
n-channel TFT
stress period of
characteristics
the
FE slopes
apart (fig. 2). Moreover, a negative field has a larger
effect than a positive field of with the
Fourth, after
the same
unipolar TFTs.
magnitude and
The deep
duration, consistent
defect density
stressing, as shown in fig. 2 by the increase in the distance
has risen upon between thresh-
old voltages for n- and p-channel conduction 6.
3. RECOVERY The recovery of the structure was studied by annealing a device in the dark at temperatures T A between 84 and 127°C, maintaining the gate and taking
snapshots of
the current
identically stressed
defects. Before
condition was
achieved by
TFT at 165°C for 3 hours and subsequently stressing it (i0 V,
65°C, 6x104
normalized with figure also
0 V
at +i0 V at time intervals sufficiently
large to avoid simultaneous re-introduction of experiment an
voltage at
each recovery annealing the
under fixed conditions
s) (fig. 3). Fig. 4 contains the recovery data which were
the currents
measured at
+i0 V
in the
annealed state. The
contains calculated recovery curves assuming first order kinetics
with a single activation energy EA and attempt-to-anneal is the simplest possible mechanism. Then,
frequency
~o, which
P.E.L Schropp et al. / Reversible dangling-bond generation
1342
Nrec(t ) = Nrec(0) exp [-Po exp (-EA/kTA)t ]
in
which
Nre c
is
the
density
of
generated in addition to the stable state. We
recoverable
dangling bonds, which were
defects already
present in
the annealed
let the current be determined by the interface band-bending result-
ing from a uniform mid-gap density of states.
Although the
fit was
not very
sensitive as long as the attempt-to-anneal frequency and the activation energy were taken correlated in a way identical to the Meyer-Neldel exp(AEA) with
A =
rule ( V o =
Voo
30.3 ev-l), a simplex minimization method indicated values
of 4x106 s -I and 0.77 eV, respectively. Our EA result
is comparable
with the
value of 0.85 eV found by Hepburn et al. 2 for the energy level of bias-induced dangling bonds. Our interpretation is that creates
charged
dangling
the
bonds
positive below
bias
the
stress
period essentially
Fermi-level.
The
low attempt-
frequency is consistent with the metastability of the threshold voltage shifts observed. The
recovery mechanism
dangling bonds into a emission of
weak Si-Si
these electrons
energy is determined by impurity-rich
samples 7 ,
which implies recombination of neighbouring bond is
therefore rate-limited
by thermal
out of the dangling bonds. The anneal activation
their energy suggesting
level. Both that
the
Vo
origin
and E A of
are typical of
the field-induced
dangling bonds is different from intrinsic dangling bonds. Perhaps this is due to the
different structural
and chemical
composition of a-Si:H close to the
interface.
ACKNOWLEDGEMENT We wish to thank P.S. van Dijk for performing part of the measurements.
REFERENCES i) M. J. Powell, Appl. Phys. Lett. 43 (1983) 587. 2) A. R. Hepburn, J. M. Marshall, C. Main, M. J. Powell and C. van Berkel, Phys. Rev. Lett. 56 (1986) 2215. 3) R. E. I. Schropp and J. F. Verwey, Appl. Phys. Lett. 50 (1987) 185. 4) R. E. I. Schropp and J. F. Verwey, to be publ. in Mat. Res. Symp. Proc., Anaheim, 1987. 5) R. A. Street, J. Kakalios, C. C. Tsai and T. M. Hayes, Phys. Rev. B35 (!987) 1316. 6) M. J. Powell, C. van Berkel and I. D. French, this volume. 7) M. Stutzmann, W. B. Jackson and C. C. Tsai, J. Non-Cryst. solids. 77&78 (1985) 363.