University of Groningen
A ferroelectric transparent thin-film transistor
Prins, M. W. J.; Grosse-Holz, K. O.; Müller, G.; Cillessen, J. F. M.; Giesbers, J. B.; Weening,
R. P.; Wolf, RM
Published in:
Applied Physics Letters
DOI:
10.1063/1.115759
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Publication date:
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Citation for published version (APA):
Prins, M. W. J., Grosse-Holz, K. O., Müller, G., Cillessen, J. F. M., Giesbers, J. B., Weening, R. P., & Wolf,
RM. (1996). A ferroelectric transparent thin-film transistor.
Applied Physics Letters
,
68
(25), 3650-3652.
https://doi.org/10.1063/1.115759
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A ferroelectric transparent thin-film transistor
M. W. J. Prins, K.-O. Grosse-Holz, G. Müller, J. F. M. Cillessen, J. B. Giesbers, R. P. Weening, and R. M. Wolf
Citation: Appl. Phys. Lett. 68, 3650 (1996); doi: 10.1063/1.115759
View online: https://doi.org/10.1063/1.115759
View Table of Contents: http://aip.scitation.org/toc/apl/68/25
Published by the American Institute of Physics
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A ferroelectric transparent thin-film transistor
M. W. J. Prins,
a)
K.-O. Grosse-Holz,
b)
G. Mu
¨
ller, J. F. M. Cillessen, J. B. Giesbers,
R. P. Weening,
c)
and R. M. Wolf
d)
Philips Research Laboratories, Prof. Holstlaan 4, 5656 AA Eindhoven, The Netherlands
~Received 3 January 1996; accepted for publication 12 April 1996!
Operation is demonstrated of a field-effect transistor made of transparant oxidic thin films, showing
an intrinsic memory function due to the usage of a ferroelectric insulator. The device consists of a
high mobility Sb-doped n-type SnO
2
semiconductor layer, PbZr
0.2
Ti
0.8
O
3
as a ferroelectric
insulator, and SrRuO
3
as a gate electrode, each layer prepared by pulsed laser deposition. The
hysteresis behavior of the channel conductance is studied. Using gate voltage pulses of 100
m
s
duration and a pulse height of 6 3 V, a change of a factor of two in the remnant conductance is
achieved. The dependence of the conductance on the polarity of the gate pulse proves that the
memory effect is driven by the ferroelectric polarization. The influence of charge trapping is also
observed and discussed. © 1996 American Institute of Physics. @S0003-6951~96!01025-X#
Field-effect transistors with a ferroelectric insulator have
been the subject of numerous studies because of their
memory function and non-destructive readout properties.
Early works used a thin film of semiconductor material de-
posited on a bulk ferroelectic substrate.
1
Since the seventies
the research has been focused on integration with IC
technology,
2,3
where a ferroelectric thin film is deposited on
a semiconductor substrate. Only very recently the fabrication
of a ferroelectric thin-film transistor has become feasible,
where the ferroelectric as well as the semiconductor material
are deposited as thin films.
4–7
Although this approach is
technologically more demanding, it offers the advantage of
flexibility with respect to the choice of substrate. In particu-
lar, the usage of an optically transparent substrate in combi-
nation with wide bandgap materials for the thin-film transis-
tor gives interesting opportunities for optical applications,
e.g. as on-screen electronic devices in displays, projectors
and cameras.
In this letter, we report on the operation of a ferroelectric
transparent field-effect transistor realized completely in ox-
idic thin films. The gate insulator is composed of a ferroelec-
tric Pb~Zr,Ti!O
3
thin film, while the semiconductor channel
consists of an antimony doped n-type SnO
2
film. Measure-
ments are performed at ambient temperature and pressure.
The device construction is illustrated in Fig. 1. The layers
were deposited on a SrTiO
3
~100! substrate by pulsed laser
deposition with the off-axis technique.
6,8
First, a 140-nm
SrRuO
3
layer was grown and subsequently patterned by re-
active ion etching in order to form the gate structures. There-
after, in a single run 160 nm PbZr
0.2
Ti
0.8
O
3
and 110 nm
SnO
2
:Sb were deposited. By reactive ion etching, the semi-
conductor channels were defined and contact holes were
made to the buried gates. Ferroelectric Pb~Zr,Ti!O
3
finds
wide application ~e.g. Refs. 9 and 10! due to its large rem-
nant polarization ~of the order of 10
m
Ccm
22
) and low
coercive field strength ~between 10 and 10
2
kV/cm!. The
semiconductor material is doped with 220
m
g antimony per
gram of SnO
2
~equivalent to a dopant concentration of 8
3 10
18
cm
2 3
) and shows a textured ~111! structure with a
grain size of 30–60 nm. SnO
2
is a transparent semiconductor
with an electron mobility as high as 240 cm
2
V
2 1
s
2 1
for
bulk material.
11,12
However, in thin films the reported mo-
bilities are about an order of magnitude lower,
11,13
a phe-
nomenon associated with the presence of grain boundaries.
The semiconductor material used for the transistor channel
has a resistivity of the order of 1 V cm. We verified that the
resistivity of undoped SnO
2
films is more than three orders
of magnitude higher; hence, the free carrier density in our
films is entirely supplied by the antimony dopant atoms. The
negative temperature dependence of the resistivity confirmed
the semiconductive character of the SnO
2
. Hall measure-
ments showed n-type conductivity, a carrier concentration of
a!
Electronic mail: prins@natlab.research.philips.com
b!
Also: Institut fu
¨
r Werkstoffe der Elektrotechnik, RWTH, Aachen, Ger-
many.
c!
Also: Department of Applied Physics, University of Groningen, The Neth-
erlands.
d!
Present address: Philips Laboratories, Briarcliff Manor, NY.
FIG. 1. Picture and sketch of the transistor ~top view and cross sectional
view!. The picture shows the text ‘solid state physics’ that is read through
the device, so seen through the thin-film device and the SrTiO
3
substrate
with a thickness of 1 mm. The picture was taken with interference contrast,
in order to visualize the edges of the thin films. The picture size is 1.4 mm
3 1.8 mm.
3650 Appl. Phys. Lett. 68 (25), 17 June 1996 0003-6951/96/68(25)/3650/3/$10.00 © 1996 American Institute of Physics
about 10
18
cm
2 3
, and a mobility of 5 cm
2
V
2 1
s
2 1
. The
latter value is still fairly high for a thin-film transistor. In
another paper
14
we will present a detailed study on the elec-
tronic properties of SnO
2
as a function of doping concentra-
tion. Figure 1 shows a sketch of the device construction,
while the picture illustrates the optical transparency of the
transistor. In the channel area of the present structure, the
optical absorption ~tens of percents! is due to the gate elec-
trode layer of SrRuO
3
. By fabricating an all-oxide thin-film
transistor with heavily doped SnO
2
for the gate electrode, we
verified that the realization of a fully transparent thin-film
transistor is possible.
Figure 2 shows the measured drain current (I
D
) and the
gate current (I
G
) as a function of the gate voltage. Note that
the current leaking through the ferroelectric insulator is more
than two orders of magnitude smaller than the drain current,
demonstrative of a proper transistor operation. By changing
the band bending in the semiconductor, the gate voltage
causes the drain-source channel conductance to be larger at a
positive gate voltage ~forward bias, channel enrichment! than
at a negative gate voltage ~reverse bias, channel depletion!.
As Fig. 2 shows, a change of the channel conductance is
achieved of a factor of I
max
/I
min
536
m
A/0.6
m
A560. This
can be understood as follows. For a constant density of space
charge eN
d
, the width of the depletion region in the semi-
conductor is given by
@
2
e
0
e
r
V
bb
/eN
d
#
1/2
,
15
where V
bb
is the
band bending potential. If we assume that the full gate volt-
age sweep induces a change of the band bending V
bb
. 4V
and using a carrier concentration N
d
510
186 0.5
cm
2 3
, we find
a change of the depletion width of 806 40 nm (
e
r
. 10!.In
other words, the large change of channel conductance is due
to the fact that the change of the depletion width is compa-
rable to the thickness of the semiconductor channel.
The memory function of this thin-film transistor is dem-
onstrated by the hysteresis behavior of the drain current.
16
The remnant channel conductance, i.e. the conductance at
zero gate voltage, depends on the history of the gate voltage:
the remnant conductance is large when a positive gate volt-
age has been applied ~on-state! and small after a negative
voltage ~off-state!. This proves that the memory effect is
driven by the ferroelectric polarization, and is not a result of
the injection of charge into the insulator layer ~this issue is
discussed in detail in Refs. 2 and 4!. The voltage shift be-
tween the up-sweep and down-sweep is 1.6 V. This should
be compared with 2d
f
E
c
, where d
f
is the thickness of the
ferroelectric layer and E
c
its coercive field. Using d
f
5160
nm, we find a coercive field of 50 kV/cm. In order to verify
that the hysteresis is due to the field effect, we measured the
charge collected on the gate electrode as a function of the
gate voltage ~not shown! in a Sawyer-Tower measurement
17
;
this revealed a hysteresis behavior with a remnant charge
density of 10
m
Ccm
22
.
The switching characteristics of the transistor are de-
picted in Fig. 3, showing the time evolution of the remnant
conductance of the source-drain channel. Gate voltage pulses
with a duration of 100
m
s and a pulse height of 6 3 V were
used. A switching is observed with an on/off ratio close to a
factor of 2.
18
This can be compared with previous achieve-
ments in ferroelectric thin-film transistors: Seager et al.
4
ob-
served a memory effect that was dominated by charge injec-
tion and opposite to the ferroelectric polarization, while
Watanabe
5
showed a polarization-type memory effect of 5%,
with gate voltage pulses of 7-V amplitude and 10-ms dura-
tion. We observed a substantial decrease of the on/off ratio
with pulses shorter than 10
m
s. This is not due to the ferro-
electric material, because in Pb~Zr,Ti!O
3
the switching of
polarization takes place on a nanosecond time scale.
19
A
speed limitation is given by the RC switching time constant,
that is the product of the source-drain channel resistance
(; 10 kV) and the gate capacitance (; 0.3 nF!. The theo-
retical limit to the device speed is given by L
2
/V
m
,
15
where
L is the channel length, V is the applied voltage, and
m
is the
carrier mobility. As an example, using one volt, a mobility of
10 cm
2
V
2 1
s
2 1
, and a channel length of 1
m
m, a switching
speed of 1 ns should be possible.
In addition to the switching behavior, a relaxation of the
on-state and the off-state is observed in Fig. 3. The relax-
ation is of equal direction for the two states ~toward a higher
conductance! and the relaxation is of similar magnitude. It is
well-known that the polarization of ferroelectric materials is
partially volatile.
10
However, a ferroelectric relaxation would
lead to a reduction of the on-state conductance and an in-
crease of the off-state conductance with time, in constrast to
what is recorded in Fig. 3; in addition, we also observed the
FIG. 2. Drain current (I
D
) and gate current (I
G
) vs gate voltage (V
G
). The
gate voltage was swept at a rate of 2 V/s. Note the difference in the scales.
The drain was at 100 mV with respect to the source. In this device, the
distance between the source and drain contacts (L)is5
m
m and the channel
width (W) is 300
m
m.
FIG. 3. Switching levels of the drain current, upon the application of gate
voltage pulses with a pulse height of 6 3V and a duration of 100
m
s. V
D
5100 mV. For a device with L510
m
m and W5300
m
m.
3651Appl. Phys. Lett., Vol. 68, No. 25, 17 June 1996 Prins
et al.
relaxation for low voltages, when the ferroelectric material is
hardly switched ~low on/off ratio!. Our observations point to
the presence of electron trap states, that are filled by the gate
current pulse. During the relaxation process, the liberation of
trapped electrons leads to an increase of the free carrier den-
sity in the n-type channel. We verified experimentally that
gate pulses of longer duration give stronger relaxation ef-
fects, as is expected for an increased amount of trapped
charge. Also, the relaxation time decreased upon illuminat-
ing the sample with photons of sub-bandgap energy, which is
another indication that gap states are involved. The trap
states may be related to the inactive fraction of dopant atoms,
or to structural defects at the grain boundaries in the semi-
conductor thin film.
20
Summarizing, we have demonstrated
the operation of an all-oxide transistor with the following
features: ~i! an all-thin-film design, ~ii! the incorporation of a
high mobility semiconductor like SnO
2
, ~iii! an inherent
memory function due to a ferroelectric insulator, ~iv! a low
switching voltage, and ~v! optical transparency.
The authors thank P.W.M. Blom and L.F. Feiner for
helpful discussions. E. Pastoor and M.H.J. Slangen are ac-
knowledged for assisting with the measurements.
1
J.C. Crawford and F.L. English, IEEE Trans. Electron Devices ED-16,
525 ~1969!, and references therein.
2
S.-Y. Wu, IEEE Trans. Electron Devices 21, 499 ~1974!; Ferroelectrics
11, 379 ~1976!; K. Sugibuchi, Y. Kurogi, and N. Endo, J. Appl. Phys. 46,
2877 ~1975!.
3
Y. Higuma, Y. Matsui, M. Okuyama, T. Nakagawa, and Y. Hamakawa,
Jpn. J. Appl. Phys. 17 Suppl. 17-1, 209 ~1977!; T.A. Rost, H. Lin, and
T.A. Rabson, Appl. Phys. Lett. 59, 3654 ~1991!; D.R. Lampe, D.A. Ad-
ams, M. Austin, M. Polinsky, J. Dzimianski, S. Sinharoy, H. Buhay, P.
Brabant, and Y.M. Liu, Ferroelectrics 133,61~1992!; S. Sinharoy, H.
Buhay, M.H. Francombe, and D.R. Lampe, Integrated Ferroelectrics 3,
217 ~1993!; T.S. Kalkur, ibid. 4, 357 ~1994!; T.A. Rabson, T.A. Rost, and
H. Lin, ibid. 6,15~1995!; Y. Nakao, T. Nakamura, A. Kamisawa, and H.
Takasu, ibid. 6,23~1995!; T. Nakamura, Y. Nakao, A. Kamisawa, and H.
Takasu, ibid. 9, 179 ~1995!.
4
C.H. Seager, D. McIntyre, B.A. Tuttle, and J. Evans, Integrated Ferroelec-
trics 6,47~1995!.
5
Y. Watanabe, Appl. Phys. Lett. 66, 1770 ~1995!.
6
J.F.M. Cillessen, R.M. Wolf, J.B. Giesbers, P.W.M. Blom, K.-O. Grosse-
Holz, and E. Pastoor, Appl. Surf. Sci. ~to be published!.
7
R.M. Wolf, J.F.M. Cillessen, J.B. Giesbers, E. Pastoor, G. Mu
¨
ller, K.-O,
Grosse-Holz, and P.W.M. Blom, in Epitaxial Oxide Thin Films II, Mater.
Soc. Symp. Proc. 401, 163 ~1996!.
8
N.J. Ianno and K.B. Erington, Rev. Sci. Instrum. 63, 3525 ~1992!.
9
Ferroelectric Thin Films III, Mater. Res. Symp. Proc. 310, edited by E.R.
Myers, B.A. Tuttle, S.B. Desu, and P.K. Larsen ~MRS, Pittsburgh, 1993!.
10
R. Moazzami, Semicond. Sci. Technol. 10, 375 ~1995!.
11
K.L. Chopra, S. Major, and D.K. Pandya, Thin Solid Films 102,1~1983!.
12
N. Tsuda, K. Nasu, A. Yanase, and K. Siratori, Electronic Conduction in
Oxides, Springer Series in Solid State Sci. Vol. 94 ~Springer, Berlin,
1991!.
13
H. Viirola and L. Niinisto, Thin Solid Films 251, 127 ~1994!.
14
K.-O. Grosse-Holz, J.F.M. Cillessen, M.W. J. Prins, P.W.M. Blom, R.M.
Wolf,. L.F. Feiner, and R. Waser, in Epitaxial Oxide Thin Films II, Mater.
Soc. Symp. Proc. 401,67~1996!.
15
S.M. Sze, Physics of Semiconductor Devices ~Wiley, New York, 1981!.
16
Also the gate current exhibits hysteresis, due to the ferroelectric polariza-
tion and charging effects; for a discussion on polarization-dependent con-
duction through a ferroelectric system, see for example P.W.M. Blom,
R.M. Wolf, J.F.M. Cillessen, and M.P.C.M. Krijn, Phys. Rev. Lett. 73,
2107 ~1994!; Y. Watanabe, Appl. Phys. Lett. 66,28~1995!.
17
C.B. Sawyer and C.H. Tower, Phys. Rev. 35, 269 ~1930!.
18
Due to the time-dependence of the on- and the off-state ~i.e. the relaxation
effects due to charge trapping; see the following paragraph!, the on/off
ratio is smaller in Fig. 3 ~time scale of minutes! than in Fig. 2 ~taken on a
time scale of seconds!.
19
P.K. Larsen, G.L.M. Kampscho
¨
er, M.J.E. Ulenaers, G.A.C.M. Spierings,
and R. Cuppens, Appl. Phys. Lett. 59, 611 ~1991!.
20
The presence of traps in high mobility oxidic semiconductor thin films
~e.g. In
2
O
3
or ZnO! is for example discussed in Ref. 4 and: V. Srikant, V.
Sergo, and D.R. Clarke, J. Am. Ceram. Soc. 78, 1931 ~1995!; 78, 1935
~1995!.
3652 Appl. Phys. Lett., Vol. 68, No. 25, 17 June 1996 Prins
et al.
