Electrical transport properties of polar heterointerface between KTaO3
  and SrTiO3
Electrical transport properties of polar heterointerface between
KTaO3 and SrTiO3
A. Kalabukhov,1, ∗ R. Gunnarsson,1 T. Claeson,1 and D. Winkler1
1Department of Microtechnology and Nanoscience
(MC2), Chalmers University of Technology
Göteborg, Sweden
(Dated: August 10, 2021)
Abstract
Electrical transport of a polar heterointerface between two insulating perovskites, KTaO3 and
SrTiO3, is studied. It is formed between a thin KTaO3 film deposited on a top of TiO2-terminated
(100) SrTiO3 substrate. The resulting (KO)
1−(TiO2)
0 heterointerface is expected to be hole-
doped according to formal valences of K (1+) and Ti (4+). We observed electrical conductivity and
mobility in the KTaO3/SrTiO3 similar to values measured earlier in electron-doped LaAlO3/SrTiO3
heterointerfaces. However, the sign of the charge carriers in KTaO3/SrTiO3 obtained from the
Hall measurements is negative. The result is an important clue to the true origin of the doping at
perovskite oxide hetero-interfaces.
PACS numbers: 73.20.-r,73.21.Ac,73.40.-c
http://arxiv.org/abs/0704.1050v1
The mechanism of doping in hetero-interfaces between two insulating perovskite oxides
has been intensively discussed since the observation of large electrical conductivity in the
hetero-structure between LaAlO3 (LAO) and SrTiO3 (STO).
1,2,3,4,5,6 It was argued that
when a thin LAO film is coherently grown on the TiO2-terminated STO substrate, the
resulting interface (LaO)+(TiO2)
0 is expected to be polar, provided that the bulk formal
valences of Ti and La are maintained at the interface and that the structure of the interface
is atomically perfect. The polar structure at the interface results in an infinitely growing
electrostatic potential in the (001) direction when the thickness of LAO film is increasing.
In order to compensate for the charge discontinuity at the interface, half of an electron per
square unit cell may be released leading to conductivity at the interface.7 The possibility
of this doping mechanism was supported by theoretical works.8,9 However, there are other
possible doping mechanisms in perovskite oxides. It is well known that electrical properties of
STO can be changed from insulating to metallic ones by a small reduction of oxygen from its
stoichiometric composition.10,11 The possibility that the electrical property at the LAO/STO
heterointerface is not due to the oxygen vacancies was presumably ruled out by keeping the
STO substrate at deposition conditions which does not result in bulk conductivity due to
oxygen vacancies. However, it is still possible that the deposition of the LAO film itself
can reduce oxygen from a shallow layer at the STO substrate, as argued by Siemons et al.6
We have previously suggested that oxygen vacancies play an important role in the electrical
properties of the LAO/STO heterointerface.4 However, the true microscopic origin of the
conductivity at the interface between LAO and STO could not be understood.
In this work we treat another polar interface between two insulating perovskite oxides,
KTaO3 (KTO) and STO. KTO is a well known material with a cubic structure and lattice
parameter of 3.99 Å(compare with 3.905 Åin STO). It is incipient ferroelectric at room
temperature with a dielectric constant of about 260.12,13 Tantalum has a formal valence
of 5+, and potassium 1+ in KTO. The KTO film should grow as a sequence of layers on
a single TiO2-terminated STO substrate in the (001) direction and the resulting interface
should have the structure of (KO)−(TiO2)
0. This means that half a hole per square unit
cell should be released. This is opposite to the (LaO)+(TiO2)
0 heterointerface, where half
an electron per unit cell is transferred to the interface.
We have grown thin KTO films on STO substrates and found that the KTO/STO inter-
face is indeed conducting with electrical properties very similar to the LAO/STO interface.
0 1 2 3 4 5 6
Position ( m)
FIG. 1: (Color online) Atomic force microscope image (top) and cross section (bottom) of the
surface of the 6 nm thick KTaO3 film grown on a TiO2-terminated (100) SrTiO3 substrate. Unit
cell steps are seen about every 250 nm along the surface.
However, the charge of electrical carriers deduced from Hall effect measurements is nega-
tive. We discuss possible reasons for this interesting result in view of interface structure and
possible doping mechanisms.
KTO films were prepared by pulsed laser deposition with in-situ reflection high-energy
electron diffraction (RHEED) used to monitor film growth during deposition. The growth
conditions were similar to what we used previously to fabricate LAO/STO hetero-interfaces:4
deposition temperature TD = 750
◦C, oxygen pressure pO2 = 10
−4 mbar, laser energy density
J = 1.5 J/cm2. RHEED oscillations could be observed during the initial part of the film
growth. However the intensity decreased rapidly and after 3 unit cells it was too low to
observe oscillations. The deposition rate estimated from the first RHEED oscillations was 1
unit cell per 10 pulses. The thickness of the KTO films was 13 u.c. layers (approx. 6 nm).
Atomic force microscopy (AFM) showed very smooth step-like surface of the KTO film, see
-2 -1 0 1 2
 6 nm LAO/STO
 6 nm KTO/STO
T = 300 K
H (T)
FIG. 2: (Color online) (a) Experimental configuration for determination of Hall coefficient; (b) Hall
resistance RXY for KTO/STO and LAO/STO heterostructures measured at room temperature and
the same experimental configuration.
Fig.1.
Electrical measurements were made in a four point van der Pauw configuration14 in the
temperature range 2 K – 300 K and in magnetic field up to 5 T. First we proved that the
KTO film itself is not conducting by using ”soft” contacts: silver wires glued on the film
surface using silver epoxy. In order to reach the interface, we used Ti/Au contact pads
fabricated by sputtering through metal mask. The resistance between Ti/Au contacts and
contacts glued by silver epoxy was above 10 MΩ, indicating an absence of pinholes in the
KTO film.
The electrical properties of KTO films may be compared to those of 15 u.c. thick LAO
films on TiO2-terminated STO substrates prepared in the same conditions. Both hetero-
structures show metallic conductivity with relatively high mobilities and charge carrier con-
centrations. Fig.2 shows Hall resistances measured at room temperature under the same
experimental configuration (i.e. magnetic field and current direction, see Fig.2a). Both
KTO/STO and LAO/STO heterointerfaces had the same sign of Hall coefficient. The sign
of the charge carriers is negative according to the magnetic field and bias current directions.
The values of the sheet resistance RS, the Hall mobility µH and the charge carrier density
nS of the KTO/STO heterointerfaces are very similar to those of LAO/STO, see Fig.3. We
1 10 100
T (K)
 6 nm KTO/STO
 6 nm LAO/STO
1 10 100
15 (b)
T (K)
1 10 100
T (K)
FIG. 3: (Color online) Temperature dependence of sheet resistivity RS (a), charge carriers density
nS (b) and Hall mobility µH for LAO/STO and KTO/STO heterointerfaces prepared at 10
−4 mbar
oxygen pressure.
measured three KTO films prepared in similar deposition conditions and they all showed
similar transport properties.
It is known that potassium deficiency is a significant problem in growth of KTO films
due to the high vapor pressure of potassium at high temperatures.13 If this were the case
here, the actual heterointerface between KTO/STO may have different reconstruction from
the one described above. This possibility needs to be ruled out in a future determination of
the microstructure of the hetero-structure by electron microscopy.
Independent of the KTO/STO heterointerface microstructure being perfect or not, it
is quite remarkable that the electrical properties are very similar to those of LAO/STO
heterointerface. This could suggests that there is a common doping mechanism where the
type and concentration of charge carriers do not directly depend on the sign of the polar
interface deduced from the formal bulk valences. We have previously argued that the high
conductivity, mobility, and charge carrier density found in hetero-junctions of LAO/STO
prepared at low oxygen pressure mainly are due to oxygen vacancies residing in STO close
to the interface. That conclusion is further strengthened by the present findings.
The work was supported by the Swedish KAW, SSF, and VR foundations, the EU
NANOXIDE, and ESF THIOX programs.
∗ Electronic address: alexei.kalaboukhov@mc2.chalmers.se
1 A. Ohtomo, and H.Y. Hwang, Nature 427, 423 (2004); 441, 120 (2006).
mailto:alexei.kalaboukhov@mc2.chalmers.se
2 M. Huijben, G. Rijnders,D. H. A. Blank, S. Bals, S. van Aert, J. Verbeeck, G. van Tendeloo,
A. Brinkman and H. Hilgenkamp, Nature Materials 5, 556 (2006).
3 S. Thiel, G. Hammerl, A. Schmehl, C. W.Schneider and J. Mannhart, Science 313, 1935 (2006).
4 A. Kalabukhov, R. Gunnarsson, J. Börjesson, E. Olsson, T. Claeson and D. Winkler, Phys.
Rev. B 75, 121404(R) (2007).
5 G. Herranz, M. Basletic, M. Bibes, R. Ranchal, A. Hamzic, E. Tafra, K. Bouzehouane, E.
Jacquet, J. P. Contour, A. Barthelemy and A. Fert, Phys. Rev. B 73, 064403 (2006).
6 W. Siemons, G. Koster, H. Yamamoto, W. A. Harrison, G. Lukovsky, T. H. Geballe, D. H. A.
Blank and M. R. Beasley, cond-mat/0612223 (2006).
7 N. Nakagawa, H. Y. Hwang and D. A. Muller, Nature Mat., 5, 204 (2006).
8 R. Pentcheva and W. E. Pickett, Phys. Rev. B 74, 035112 (2006)
9 M. S. Park, S. H. Rhim and A. J. Freeman, Phys. Rev. B 74, 205416 (2006)
10 O. N. Tufte and P. W. Chapman, Phys. Rev. 155, 796 (1967).
11 C. S. Koonce, M. L. Cohen, J. F. Schooley, W. R. Hosler and E. R. Pfeiffer, Phys. Rev. 163,
380 (1967).
12 D. Z. Bozinis, J. P. Hurrel, Phys. Rev. B 13, 3109 (1976).
13 H.-J. Bae, J. Sigman, S.-J. Park, Y.-H. Heo, L. A. Boatner and D. P. Norton, Solid State
Electronics 48, 51 (2004).
14 J. L. van der Pauw Philips Res. Rep. 13, 1 (1958).
http://arxiv.org/abs/cond-mat/0612223
	References
