Proceedings of 6th Thin Film Materials & Devices Meeting
November 2-3, 2009, Kyoto, Japan
http://www.tfmd.jp/Thin-Film Transistor Fabricated on the SrTiO3 Epitaxial Film
Annealed in an Oxygen Atmosphere
Kosuke Uchida1, Akira Yoshikawa1, Kunihito Koumoto1,Takeharu Kato2, Yuichi Ikuhara2,3 and Hiromichi Ohta1,4,
1

Graduate School of Engineering, Nagoya University, Chikusa, Nagoya 464-8603, Japan
2
Japan Fine Ceramics Center, Atsuta, Nagoya 456-8587, Japan
3
Institute of Engineering Innovation, The University of Tokyo, Bunkyo, Tokyo 113-8656, Japan
4

PRESTO, Japan Science and Technology Agency, Honcho, Kawaguchi 332-0012, Japan

Keywords: SrTiO3, Epitaxial Film, Oxide TFT, Pulsed Laser Deposition, Thermal annealing

Abstract. We report herein fabrication and characterization of a thin-film transistor (TFT)
using a SrTiO3 epitaxial film grown on (001) face of LSAT [(LaAlO3)0.3(Sr2AlTaO6)0.7]
substrate by a pulsed laser deposition technique. Abrupt stepped-and-terraced surface of SrTiO3film, which can be obtained by the thermal annealing of the layer-by-layer grown SrTiO3 film at900ºC in an oxygen pressure of ~1 Pa, is found to be a key material to obtain excellent TFT
characteristics. In the present case, the SrTiO3-TFT exhibits following characteristics at room
temperature: on-to-off current ratio >105, threshold gate voltage Vth = +6.5 V, sub-thresholdswing S-factor ~2.1 Vdecade-1, and field effect mobility µFE ~0.8 cm2V-1s-1.

Introduction
Strontium titanate (SrTiO3, cubic perovskite, Pm3m, lattice constant a =3.905 Å) is known
as a band insulator with a wide bandgap of ~3.2 eV. SrTiO3 has attracted growing attention for
the next generation of oxide electronics because SrTiO3 exhibits several unique properties:
Charge carrier concentration of SrTiO3 can be easily controlled from insulator to metal (n3D~1021 cm-3) by appropriate substitutional doping such as Nb or La [1,2]. SrTiO3 exhibits
extremely high Hall mobility of >104 cm2V-1s-1 at 2 K [3]. High quality single crystals of
SrTiO3, which is commercially available, are widely applied for the heteroepitaxial film growthof several perovskite oxides. Recent finding of high-density two-dimensional electron gas(2DEG) by Ohtomo and Hwang [4], which is confined within extremely thin layer at the100228048-1

LaAlO3/SrTiO3 heterointerface, accelerates the motivation toward the realization ofSrTiO3-based electronic devices.
In order to realize the SrTiO3-based electronic devices, SrTiO3-based field effect transistor(FET) is essentially important because charge carrier density in SrTiO3 can be modulated
electrostatically. A number of SrTiO3-based FETs have been reported to date using high quality
single crystals of SrTiO3 with stepped and terraced surface [5-10]. Very recently, Ueno andco-workers observed superconducting transition (Tc ~0.4 K) of electrostatically accumulated
two-dimensional electron channel (sheet charge concentration, n2D =1−10 × 1013 cm−2) in
SrTiO3 single crystal using electric double layer gating technique [11]. They modulated themean depth of carrier distribution from 16 to 3 nm, which is far thinner than the thickness of
single crystal plate (~100 µm). Thus, one considers that SrTiO3-based thin film transistor (TFT)
with good transistor characteristics is appropriate to further clarify the condensed-matterphysics of SrTiO3. However, SrTiO3-based thin TFT has not been reported so far.
In our preliminary study, we fabricated SrTiO3-TFTs using as-deposited SrTiO3 epitaxial
films, which were composed of several grains. The resultant TFT was normally-on typetransistor and the values of the on－to－off current ratio and S-factor are <102 and ~20
Vdecade−1, indicating that oxygen vacancies and/or carrier traps were generated in the SrTiO3
film (data not shown). In order to improve the TFT characteristics, the as-deposited SrTiO3films were annealed in an oxygen atmosphere.
Here we report fabrication and characterization of SrTiO3 TFTs using high quality epitaxial
films of SrTiO3, which was obtained by the thermal annealing of the layer-by-layer grown
SrTiO3 film at 900ºC in an oxygen pressure ~1 Pa. We selected amorphous 12CaO·7Al2O3(a-C12A7, permittivity εr = 12) as the gate insulator for the fabrication of TFT because
a-C12A7 gated SrTiO3 FETs exhibit excellent transistor characteristics [10]. The resultant
SrTiO3-TFT exhibits following characteristics at room temperature: on-to-off current ratio >105,
sub-threshold swing ~2.1 Vdecade-1, and field effect mobility ~0.8 cm2V-1s-1.
Experimental
SrTiO3 epitaxial films (thickness: ~60 nm) were fabricated on (001) face of LSAT[(LaAlO3)0.3(Sr2AlTaO6)0.7] substrates by a pulsed laser deposition (PLD) technique (KrF
excimer laser, ~0.5 Jcm-1pulse-1, 20 ns, 5 Hz). During the film deposition, we monitored
specular spot intensity of reflection high energy electron diffraction (RHEED) to control thefilm thickness. Substrate temperature was kept at 900ºC during the film deposition. After thefilm deposition, pure O2 gas was additionally introduced into the PLD chamber to fill up
oxygen deficiency of the SrTiO3 film. Then, the film was cooled down to room temperature.
Crystallographic orientation and thickness of the films were evaluated by high resolution x-ray
diffraction (HRXRD, ATX-G, Rigaku Co.) using monochrometed Cu Kα1 beam.
Then, we fabricated a top-gate-type TFT structure on the SrTiO3 single crystal films as100228048-2

schematically shown in Fig. 1. First,
20-nm-thick metallic Ti films, used as thesource and drain electrodes, were deposited
through a stencil mask by electron beam (EB)evaporation (base pressure ~10−4 Pa, nosubstrate heating). Then, 160 nm-thickamorphous-12CaO·7Al2O3 (a-C12A7, εr=12)
film was deposited through a stencil mask by
PLD (~3 Jcm−2pulse−1, oxygen pressure ~0.1Pa) using dense polycrystalline C12A7
ceramic as target. Finally, gate electrode,which is 20-nm-thick metallic Ti film, wasdeposited through a stencil mask by EBevaporation. The resultant TFTs wereannealed at 200ºC in air atmosphere to reduce
off current.

Fig. 1: Schematic structure of theSrTiO3-TFT. Ti films (20 nm thick) are usedas the source, drain and gate electrodes. A 160
nm-thick a-C12A7 film is used as the gateinsulator. Channel length (L) and channelwidth (W) are 200 and 400 µm, respectively.

Results
In the out of plane XRD pattern of the resultant SrTiO3 film, Pendellösung fringes wereclearly observed at around 002 diffraction peak of SrTiO3 (data not shown). Thus, weconfirmed the film thickness (60 nm) using the Pendellösung fringes. Surface morphology wasobserved by an atomic force microscope (AFM, NanoScope E, D.I.)[Fig. 2(a)]. Stepped andFig. 2: (a)Topographic AFM image of the SrTiO3 epitaxial film annealed in an oxygen
atmosphere [Oxygen pressure: 1 Pa ]. (b)Cross-sectional high-resolution transmission electron
microscope (HRTEM) images of the 160-nm-thick a-C12A7/SrTiO3/LSAT heterointerface.

100228048-3

terraced surface are seen. Figure 2(b)showsthecrosssectional
high-resolution transmission electron
microscope
imageofthea-C12A7/SrTiO3/LSATinterface
region (HRTEM, TOPCON EM-002B,
acceleration voltage of 200 kV,
TOPCON). Featureless structure ofa-C12A7 is seen though lattice image
is clearly seen in the SrTiO3 film. A
broad halo pattern is seen in theselected area electron diffraction
patterns o f a-C12A7, indicating thata-C12A7 glass film was deposited on
the SrTiO3 film.
Transistor characteristics of theresultant SrTiO3-TFTs were measured
by using a semiconductor device
analyzer(B1500A,AgilentTechnologies) at room temperature.
The channel width (W) and the channel
length (L) of the TFT are 400 and 200µm, respectively. Figure 3 shows
typical (a) transfer and (b) output

Fig. 3. (a) Typical transfer and (b) output
characteristics of the TFT using the high quality
SrTiO3 epitaxial film, which was annealed in anoxygen atmosphere. The inset of (a) shows Id0.5-Vg

characteristics of the resultant TFT.
Drain current (Id) of the FET increased
as the gate voltage (Vg) increased,
plot of the TFT.
hence the channel was n-type, andelectron carriers were accumulated bypositive Vg [Fig. 3(a)]. Rather large threshold gate voltage (Vgth) of +6.5 V, obtained from alinear fit of an Id0.5-Vg plot [inset of (a)], is observed, which corresponds electron trapping state
density of ~5 × 1012 cm-2. We observed a clear pinch-off and current saturation in Id [Fig. 3(b)],indicating that the operation of this FET conformed to standard FET theory. The on－to－off
current ratio, S-factor and threshold voltage, are >105, ~2.1 Vdecade−1 and +6.5 V, respectively.
We then calculated the sheet charge concentration (nxx), and the field effect mobility (µ FE) of theSrTiO3-TFTs. The nxx values were obtained from nxx = Ci (Vg−Vth), where Ci was the capacitance
per unit area (67 nFcm-2). The µ FE values were obtained from µ FE = gm[(W/L)Ci·Vd]-1, where gm
is transconductance ∂Id/∂Vg. The maximum µ FE of the TFT was ~0.8 cm2V-1s-1 (Vg = 47 V, nxx =2.0 × 1013 cm-2).

100228048-4

Summary
We have demonstrated fabrication and device characteristics of a thin-film transistor (TFT)
fabricated in single-crystalline SrTiO3 thin film grown on (001) face of(LaAlO3)0.3(Sr2AlTaO6)0.7 substrate by a pulsed laser deposition technique. Abrupt
stepped-and-terraced surface of SrTiO3 film, which can be obtained by the thermal annealing ofthe layer-by-layer grown SrTiO3 film at 900ºC in an oxygen pressure of ~1 Pa, is found to be a
key material to obtain excellent TFT characteristics. In the present case, the SrTiO3-TFT
exhibits following characteristics at room temperature: on-to-off current ratio >105,
sub-threshold swing ~2.1 Vdecade-1, and field effect mobility ~0.8 cm2V-1s-1.

Acknowledgements
A part of this work was financially supported by MEXT (Nano Materials Science forAtomic-scale Modification, 20047007).

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