Proceedings of 6th Thin Film Materials & Devices Meeting
November 2-3, 2009, Kyoto, Japan
http://www.tfmd.jp/

Transistor Characteristics and Thermopower Modulation of KTaO3 Single
Crystal Based Field-Effect Transistors with a-C12A7 Gate Dielectric
Akira Yoshikawa1, Kosuke Uchida1, Kunihito Koumoto1, Takeharu Kato2,
Yuichi Ikuhara2,3, Hiromichi Ohta1,41Graduate School of Engineering, Nagoya University, Chikusa, Nagoya 464-8603
2Japan Fine Ceramics Center, Atsuta, Nagoya 456-8587
3
Institute of Engineering Innovation, The University of Tokyo, Yayoi, Bunkyo113-8656
4PRESTO, Japan Science and Technology Agency, Honcho, Kawaguchi 332-0012
Keywords: KTaO3, amorphous C12A7, field modulation, thermopower

Abstract. We show herein fabrication and field-modulated thermopower for KTaO3
single-crystal based field-effect transistors (FETs). The KTaO3 FET exhibits field effectmobility of ~8 cm2V-1s-1, which is ~4 times larger than that of SrTiO3 FETs. The thermopowerof the KTaO3 FET decreased from 600 to 220 µVK−1 by the application of gate electric field up
to 1.5 MVcm-1, ~400 µVK−1 below that of an SrTiO3 FET, clearly reflecting the smaller carrier
effective mass of KTaO3. The thermoelectric power factor of KTaO3 was estimated to be ~1.5
mWm-1K-2 at room temperature, which is approximately 50 % for that of SrTiO3.

Introduction
Thermoelectric energy conversion technology attracts attention to convert the waste heat into
electricity.[1] Generally, the performance of thermoelectric materials is evaluated in terms of adimensionless figure of merit, ZT = S2·σ·T·κ-1, where Z, T, S, σ and κ are a figure of merit, theabsolute temperature, the thermopower, the electrical conductivity, and the thermal
conductivity, respectively. Today, thermoelectric materials with ZT > 1, which is necessary forpractical applications, are being rigorously explored though the ZT values of several materials
such as Bi2Te3 and PbTe exceed 1 because these materials are not attractive, particularly
operating at high temperatures (T~1,000K). Further, the use of these heavy metals should be
limited to specific environments such as space because they are mostly toxic, low in abundanceas natural resources, and thus not environmentally benign. Based on this background, recently,
metal oxides such as layered cobaltates [2] and electron doped SrTiO3 [3] attract much attention
for thermoelectric power generation material at high temperatures on the basis of their potential
advantages over heavy metallic alloys in chemical and thermal robustness.
Thermoelectric materials are explored mostly using charge carrier-doped semiconductors
with various doping levels because the S2·σ value must be enhanced according to the commonly
observed trade-off relationship between two material parameters in terms of charge carrier
concentration (n): σ increases almost linearly with increasing n until ionized impurity scattering
or electron-electron scattering becomes dominant, while |S| decreases with n. Therefore, a great
deal of material is needed to optimize thermoelectric properties.
Very recently, Sakai et al.[4] reported thermoelectric properties of several Ba-doped KTaO3
single crystals (n = 5.4 × 1018 – 1.4 × 1020 cm-3). They found that heavy (>1020 cm-3) electron
doping in KTaO3 would provide large thermoelectric properties. KTaO3 (S.G.: Pm3m, lattice
parameter a = 3.989 Å) is a typical band insulator with a large band gap of ~3.8 eV.[5] Metallic

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conductivity in KTaO3 can be obtained by the appropriate impurity doping [6] and/or the
introduction of oxygen vacancies.[7-9] KTaO3 exhibits very high Hall mobility of >104
cm2V-1s-1 at 2 K.[10] Since these physical properties of KTaO3 are similar to those of
SrTiO3,[11,12] which exhibits largest ZT among transition metal oxides (n-type), KTaO3 would
be promising candidate for thermoelectric application.
In order to examine the thermoelectric
properties of S and σ for KTaO3, we
fabricated a field effect transistor (FET)
structure on a single crystal KTaO3 becausea FET structure on single-crystalline
material would be a powerful tool in
optimizing
thermoelectric
properties
because it provides the charge carrier
dependence of both S- and σ-values
simultaneously.[13] Here we report the Fig. 1: (a) The schematic device structure andfabrication and thermopower modulation of (b) a photograph of the KTaO3 FET. Ti films
the KTaO3 FET. The resultant KTaO3 FET (20-nm thick) are used as the source, drain andexhibits excellent transistor characteristics: gate electrodes. A 200-nm thick a-C12A7 film
on-off current ratio of ~105, sub-threshold is used as the gate insulator. Channel length (L)swing S-factor of 1.2 Vdecade-1, threshold and channel width (W) are 200 and 400 µm,
gate voltage Vth of +5.2 V, and field effect respectively.
mobility µFE of ~8 cm2V-1s-1. The
thermopower |S| of this KTaO3 FET can be
modulated from 600 to 220 µVK−1 by theapplication of gate electric field up to 1.5
MVcm-1. The thermoelectric power factor
S2·σof KTaO3 was estimated to be ~1.5
mWm-1K-2 at room temperature, which isapproximately 50 % for that of SrTiO3.

Experimental
The schematic structure and photograph of
the KTaO3 FET are shown in Figs. 1(a) and
1(b), respectively. First, we treated (001)
KTaO3 single crystal plates (10 × 10 × 0.5
mm, SHINKOSHA Co.) with buffered
NH4F-HF solution (BHF, pH = 4.5) toobtain an atomically smooth surface [14]because atomically smooth heterointerfaceof the gate insulator/oxide may be necessary
for FET fabrication.[15] The NH4F
concentration was kept at 10 mol/l. After the
BHF treatment, we obtained a relatively
smooth surface with steps (~0.4 nm) andterraces [Fig. 2(b)] as compared to an
untreated surface [Fig. 2(a)]. Second,
20-nm-thick metallic Ti films, which would
serve as source and drain electrodes, were

Fig.2: Topographic AFM images of (a)untreated and (b) BHF-treated KTaO3
single-crystal surface. A smooth surface withsteps and terraces is observed. (c)Cross-sectional HRTEM image of the
200-nm-thick a-C12A7/KTaO3 heterointerface.
An abrupt interface is visible. Selected area
electron diffraction patterns of a-C12A7 and100228090-2
KTaO3 are also shown.

deposited onto the stepped KTaO3 surface by electron beam (EB, no substrate heating, basepressure ~10-4 Pa) evaporation through a stencil mask. Third, 200-nm-thick amorphous
12CaO·7Al2O3 (a-C12A7, permittivity εr = 12) glass film was deposited by a pulsed laser
deposition (PLD, ~3 Jcm-2pulse-1, oxygen pressure ~0.1 Pa). It should be noted that a-C12A7
glass would be an appropriate gate insulator for SrTiO3 and KTaO3 as compared toa-Al2O3.[15,16] Finally, a 30-nm Ti film was deposited by EB evaporation as described above.
In order to reduce the off current, the FETs were annealed at 150 ºC in air. Transistor
characteristics of the resultant KTaO3 FETs were measured by using a semiconductor device
analyzer (B1500A, Agilent Technologies) at room temperature.

Results
Figure 2(c) shows a cross-sectional
high-resolution transmission electron
microscope image of the a-C12A7/KTaO3
interface region (HRTEM, TOPCON
EM-002B, acceleration voltage 200 kV,
TOPCON). The featureless a-C12A7(upper part) is observed, although the
KTaO3 layer exhibits a lattice (lower part).
A broad halo is seen in the selected area
electron diffraction patterns of a-C12A7,
indicating that amorphous a-C12A7 film
was deposited on the KTaO3 layer.
Figure 3 summarizes typical transistor
characteristics,
suchastransfercharacteristics, field-effect mobility, sheet
charge concentration, and the output ofthese FETs. The drain current (Id) of the
KTaO3 FET increased markedly as thegate voltage (Vg) increased, hence thechannel was n-type, and electron carriers
were accumulated by positive Vg [Fig.
3(a)]. Relatively large hysteresis (~1 V) in
Id was also observed, most likely due totraps (~1012 cm-2) at the a-C12A7/KTaO3
interface. The on-off current ratio and theS-factor were >105 and ~1.2 Vdecade−1,
respectively. The threshold gate voltage(Vth), obtained from a linear fit of the
Id0.5-Vg plot [Fig. 3(b)], was +5.2 V.
Using the above measured values, we
calculated the sheet charge concentration(nxx) and the field-effect mobility (µ FE) of
the KTaO3 FETs. The nxx values wereobtained from nxx = Ci (Vg−Vth), where Ci
was the capacitance per unit area (51nFcm-2). The µ FE values were obtained
from µ FE = gm[(W/L)Ci •Vd]-1, where gm

Fig.3: Typical transistor characteristics of a
KTaO3 FET with 200-nm thick a-C12A7 (εr =12)gate insulator at room temperature [(a) Transfer
characteristic (Id –Vg plot), (b) Id0.5-Vg plot, (c)field-effect mobility (µFE)-sheet charge density(nxx) - Vg plots, (d) Output characteristic (Id –Vdplot)]. The FET exhibits on-off current ratio ~105,
sub-threshold swing S-factor ~1.2 V·decade-1 andthreshold voltage Vth ~+5.2 V. The value of µFE
increases drastically with Vg and reaches ~8
cm2V-1s-1 or ~25% of the room temperature Hall
mobility of electron-doped KTaO3 (µHall ~30
cm2V-1s-1).

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was the transconductance ∂Id/∂Vg. As
shown in Fig. 3(c), µ FE of the FET[13]
increased drastically with Vg and reached~8 cm2V-1s-1, which is ~25% of the room
temperatureHall
mobilityofelectron-doped
KTaO3(µHall~30
cm2V-1s-1). We also note that µ FE of the
KTaO3 FETs were a factor of 4 greaterthan those of SrTiO3 FETs, [13] most
likely due to the difference in effective
mass of the charge carrier me* (KTaO3:
0.13 m0, SrTiO3: 1.16 m0). Furthermore,
we observed a clear pinch-off and current
saturation in Id [Fig. 3(d)], indicating thatthe operation of this FET conformed tostandard FET theory.
Then, we measured field-modulated Fig. 4: Field-modulated thermopower (S) for thethermopower (SFE) of the KTaO3 FET. KTaO3 FET channel. S for the SrTiO3 FET channelFirst, a temperature difference (∆T = (Ref. 13) is also plotted in the inset for comparison.
0.2–1.5 K) was introduced between the Thermopower |S| of the KTaO3 FET is roughly ~400
−1source and drain electrodes by using two µVK smaller than that of SrTiO3 FET and can be−1
Peltier
devices.
Then, tuned from 600 to 220 µVK .
thermo-electromotive force (VTEMF) was
measured during the Vg-sweeping. The
values of S were obtained from the slope of VTEMF–∆T plots. Figure 4 shows SFE–Vg plots for
the KTaO3 FETs. The SFE-values are negative, confirming that the channel is n-type. |S|FE
gradually decreases from 600 to 220 µVK−1 by the application of gate electric field up to 1.5
MVcm-1, due to the fact that nxx increases with the Vg increases. These |S|FE values areapproximately 400 µVK−1 lower than those for a SrTiO3 FET as shown in the inset.[13] Since
the value of |S|FE strongly depends on me*, this result reflects the fact that me* of KTaO3 (0.13m0) is lower than that of SrTiO3 (1.16 m0).
Here we roughly estimate the maximum thermoelectric power factor (PF = S·σ) of KTaO3
at room temperature. The sheet conductance σxx of KTaO3 was a factor of ~4 greater than thatof SrTiO3. The |S|FE-value of KTaO3 was 220 µVK−1, while that of SrTiO3 was 600 µVK−1 by
the application of gate electric field up to 1.5 MVcm-1. Thus, S2FE·σxx KTaO3 / S2FE·σxx SrTiO3
is ~0.5. Since the maximum PF300 K of SrTiO3 is ~3 mWm-1K-2,[17] KTaO3 would exhibit
maximum PF300 K of ~1.5 mWm-1K-2, which corresponds well with the reported PF300 K value(1.4 mWm-1K-2[4]).

Summary
We have fabricated single crystal KTaO3-based field-effect transistors using amorphous
12CaO·7Al2O3 glass gate insulator. The resultant FET exhibit excellent characteristics: on-off
current ratio of ~105, sub-threshold swing S-factor of 1.2 Vdecade-1, threshold gate voltage Vthof +5.2 V, and field effect mobility µFE of ~8 cm2V-1s-1 (a factor of 4 greater than for SrTiO3
FETs). The observed values of thermopower for the KTaO3 FETs were ~400 µVK−1 below
those of SrTiO3 FETs, clearly demonstrating the difference of carrier effective massme*(KTaO3: 0.13 m0, SrTiO3: 1.16 m0). The thermoelectric power factor S2·σ of KTaO3 wasestimated to be ~1.5 mWm-1K-2 at room temperature, which is approximately 50 % for that of

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SrTiO3, demonstrating the effectiveness of FET structure for the rapid exploration ofthermoelectric materials.

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

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