Multilayered BiFeO3 (BFO)/LaAlO3 (LAO) thin film samples were fabricated on SrTiO3 (STO) substrates by pulsed laser deposition. In this work, the ferroelectric polarization of a multiferroic BFO ad-layer on top of the quasi-two-dimensional electron gas (2DEG) at the LAO/STO interface is used to manipulate the conductivity of the quasi-2DEG. By microstructuring the conductive area of the LAO/STO-interface, a four-point geometry for the measurement of the resistivity was achieved. Piezo force microscopy allows for imaging and poling the spontaneous ferroelectric polarization of the multiferroic layer. The resistance changes showed a linear dependence on the area scanned and a hysteretic behavior with respect to the voltages applied in the scanning process. This is evidence for the ferroelectric polarization of the multiferroic causing the resistance changes. Coupling the antiferromagnetic BFO layer to another ferromagnetic layer could enable a magnetic field control of the conductance of the quasi-2DEG at the LAO/STO interface.

Research on oxide interface systems is a topic of continuously increasing interest. Improvements in the deposition of perovskite thin films have led to a wide range of heteroepitaxial interface systems.1,2 The most studied system is given by the LaAlO3 (LAO)/SrTiO3 (STO) interface that exhibits a conductive interface between the two insulating materials.3 The quasi-two-dimensional electron gas (2DEG) that arises at the interface between LAO and STO can be explained by electronic reconstruction within the limits of the model of a polar catastrophe.4 While the existence of the 2DEG at TiO2-terminated STO surfaces has been experimentally confirmed by many groups, more investigations need to be done to achieve a complete understanding of its origin.5 Electric fields in direction normal to the interface have shown a strong influence on the transport properties of the quasi-2DEG.6,7 Such phenomenon can be exploited for functional devices based on the LAO/STO-interface as shown by Jany et al.8 

The electric fields employed in Refs. 6–8 are applied by means of an electrode fabricated on top of the LAO/STO interface system or on the back of the substrate. Tra et al. employed a ferroelectric layer of the perovskite Pb(ZrxTi1–x)O3 (Ref. 9) to fabricate a ferroelectric field effect system with two possible states given by the ferroelectric polarization states pointing along the cubic axis. Here, we use a multiferroic material with a more complex crystallographic and ferroelectric order. As bulk material BiFeO3 (BFO) has a rhombohedrally distorted perovskite like crystal structure with the ferroelectric polarization pointing along the pseudocubic [111] directions. This material exhibits both an antiferromagnetic order and a ferroelectric domain structure at room temperature.10–12 The use of BFO in heterostructures has been reviewed recently by Martin et al.13 Combining the ferroelectric polarization of a BFO ad-layer with the quasi-2DEG at the LAO/STO-interface enables the prospect to manipulate the carrier density at the LAO/STO interface additionally by small magnetic fields after exchange coupling the antiferromagnetic BFO to another ferromagnetic layer.

In this work, we report a comprehensive study of the BFO/LAO/STO system. We first show the heteroepitaxial growth by means of pulsed laser deposition (PLD) and the lithographical micro-structuring procedure of the LAO/STO conductive interface system with a multiferroic BFO ad-layer deposited on top, which enables the manipulation of the electrical conductance of the quasi-2DEG. Piezo force microscopy (PFM) is used as a tool to both image and manipulate the ferroelectric polarization of the BFO layer at a microscopic length scale. The sample geometry allows for a precise control of the ferroelectric polarization of the BFO ad-layer on top of a micro-structured line of the quasi-2DEG. This leads to a relative change in resistance that scales with the area of the BFO layer on top of the quasi-2DEG that was poled by the use of PFM.

BFO/LAO thin film samples were heteroepitaxially grown on TiO2-terminated STO substrates (CrysTec GmbH) in (001) orientation by PLD, using a Compex Pro laser at 248 nm with 20 ns laser pulses. A spherical ultra high vacuum chamber with an additional load-lock system was used to ensure a base pressure of 5×108mbar. The laser energy density was set to 1.0–1.2 J/cm2 and the laser repetition rate to 1 and 5 Hz for the LAO and BFO depositions, respectively. During the deposition, substrate temperatures of 580 °C and 780 °C for BFO and LAO were used, respectively. A pyrometer camera (DIAS infrared) was applied for the crucial substrate temperature control. After the deposition of the LAO layer at a deposition pressure of 1.0×103mbar of oxygen, a BFO layer was deposited at a pressure of 1.0×102mbar of oxygen. The growth conditions for the BFO were chosen to obtain single crystalline thin films.14,15

For the experiments presented here, the BFO and LAO layers were grown with thicknesses of 30 nm and 6 unit cells (u.c.), respectively. Deposition rates were determined by X-ray reflectometry (Philips X'Pert Pro) measurements. X-ray diffraction was employed to investigate the crystal structure of the BFO thin films as shown in Fig. 1. Resistance measurements were carried out in four-point geometry utilizing a commercial source meter (Keithley 2400).

FIG. 1.

X-ray diffractogram of a BFO (30 nm)/LAO (6 u.c.)/STO sample grown in (001) orientation. (001)- and (002)-reflexes of the substrate and the BFO thin film are marked by S and F, respectively. The peaks marked S′ result from insufficient suppression of Cu Kβ radiation for the very intense substrate peaks.

FIG. 1.

X-ray diffractogram of a BFO (30 nm)/LAO (6 u.c.)/STO sample grown in (001) orientation. (001)- and (002)-reflexes of the substrate and the BFO thin film are marked by S and F, respectively. The peaks marked S′ result from insufficient suppression of Cu Kβ radiation for the very intense substrate peaks.

Close modal

The sample was micro-structured by means of electron beam lithography (Raith Pioneer) followed by a lift-off step. In this fabrication step, the lithographical mask was written on the positive poly(methyl methacrylate) (PMMA, MicroChem GmbH) resist, employing a 10 keV focused electron beam with a deposited area dose of ca. 120 μC/cm2. The resist was developed by immersion for 20 s in a 1:3 solution of methyl isobutyl ketone (Sigma-Aldrich, reagent grade) and isopropanol, followed by immersion in pure isopropanol for 40 s. An amorphous 10 nm thick layer of LAO where no quasi-2DEG arises at the LAO/STO interface was deposited by PLD at room temperature, with an oxygen pressure of 1.0×103mbar. After the deposition, the amorphous LAO deposited on top of the PMMA lithographical mask was lifted-off by immersion in pure acetone, leaving a clean substrate surface on the areas where the PMMA mask was present. On these substrate areas, an epitaxial 6 u.c.-thick LAO layer was grown with the deposition parameters given above, resulting in an epitaxial growth of the LAO and thus the formation of the quasi-2DEG only in the areas not covered by the amorphous LAO mask. Substrate areas where the amorphous LAO layer was not removed did not recrystallize epitaxially during this process, thus keeping the conductive 2DEG confined to the patterned geometries.16,17

X-ray diffraction was used to investigate the lattice constant and orientation of the BFO thin films on the LAO/STO-system. Fig. 1 shows an X-ray diffractogram of a BFO (30 nm)/LAO (6 u.c.)/STO sample. The substrate and BFO film peaks are marked by S and F, respectively. Due to the low thickness of the LAO layer, no film peaks which could be linked to this material were observed on the X-ray diffractogram. The angles of the BFO thin film reflexes in the 2θ/θ-scan result in an out-of-plane lattice constant of 4.07 Å. This value is equal to the lattice constant of BFO layers of the same thickness grown on (001) oriented STO substrates without a LAO buffer layer.15 The ω-scan of the (001)-reflex shows a full width at half maximum (FWHM) of 0.1°, very close to the values of single crystalline films deposited without a LAO buffer layer.

Fig. 2 shows an atomic force microscopy (AFM) topography of a micro-structured LAO (6 u.c.)/STO sample before the deposition of the BFO top layer. The AFM topography was acquired at the border of the heteroepitaxially grown LAO and the amorphous LAO of the mask structure. The heteroepitaxially grown area shows the steps of the TiO2-terminated miscut STO substrate surface, typically, observed for thin films growing in step-flow or layer-by-layer growth mode.

FIG. 2.

AFM topography of a micro-structured LAO (6 u.c.)/STO sample. The steps in the epitaxially grown region show a step-flow growth of the LAO layer. The step height to the amorphously grown area of the LAO layer at the line scan marked by 1 is 15 nm.

FIG. 2.

AFM topography of a micro-structured LAO (6 u.c.)/STO sample. The steps in the epitaxially grown region show a step-flow growth of the LAO layer. The step height to the amorphously grown area of the LAO layer at the line scan marked by 1 is 15 nm.

Close modal

The AFM topography in Fig. 3(a) shows the heteroepitaxially grown area of a BFO (30 nm)/LAO (6 u.c.)/STO sample. The root mean square roughness deduced from this image is below 1 nm, confirming the high quality of the BFO layer. The variation of the thickness of the BFO ad-layer in an interval of 5–120 nm shows a layer-by-layer growth mode at very low thicknesses, a mixed growth mode at intermediate thicknesses and an island growth mode at higher thicknesses. As island growth resulted in a high roughness an intermediate thickness of 30 nm was chosen for the BFO ad-layer.

FIG. 3.

(a) AFM topography of a BFO (30 nm)/LAO (6 u.c.)/STO sample. (b) Image of the out-of-plane PFM phase of the same sample at the same position. Red and green marked areas were previously scanned by PFM with a constant tip voltage of ±8 V, respectively.

FIG. 3.

(a) AFM topography of a BFO (30 nm)/LAO (6 u.c.)/STO sample. (b) Image of the out-of-plane PFM phase of the same sample at the same position. Red and green marked areas were previously scanned by PFM with a constant tip voltage of ±8 V, respectively.

Close modal

The ferroelectric polarization was imaged by PFM in Fig. 3(b). In order to verify the ferroelectric properties of the BFO layer, the red marked area in Fig. 3(b) was scanned with a DC voltage of +8 V in between the conductive tip and the grounded quasi-2DEG. Afterwards, the inner green marked area was scanned with a DC tip voltage of −8 V leading to the PFM contrast shown in Fig. 3(b). The contrast visible in the PFM phase images is related to the out-of-plane polarization component. Evidently, it is possible to switch the ferroelectric BFO polarization using the quasi-2DEG as the bottom electrode.

The switchable polarization of the BFO ad-layer was employed to manipulate the resistance of the quasi-2DEG at the LAO/STO-interface. The inset in Fig. 4 shows the 200 μm long and 50 μm wide micro-structured line that allowed for four-point-measurement of its resistance. Before the poling of different areas of the polarization of BFO layer on top of the line, the polarization of the BFO layer was homogeneously poled to the P state. This resulted in a low resistance state for the whole line. The BFO was then poled to the P state by scanning the red marked areas in the inset of Fig. 4 with the PFM tip at a voltage of –8 V. Fig. 4 shows the relative change in resistivity of the line for the area poled to the P state. Poling of the whole BFO layer on top of the line resulted in a change of the resistivity of ca. 21% with respect to the P state. This change is in agreement with the assumption of an electron depletion of the quasi-2DEG for the P state and an electron accumulation for the P state leading to a lower resistance.9 

FIG. 4.

Relative change in the resistance of a micro-structured line fabricated on a BFO (30 nm)/LAO (6 u.c.)/STO sample as function of the area where the BFO exhibits an opposite polarization. The polarization of the BFO layer on top of the quasi-2DEG system was poled to the polarization down state before the measurement resulting in R0 = 2.74 MΩ. The inset shows the micro-structured line of the quasi-2DEG with a length of 200 μm and a width of 50 μm on top of the BFO (orange)/LAO (green)/STO (blue) sample. Red marked are the areas poled for this measurement. Epitaxial areas are marked by ep and amorphous areas by am, respectively.

FIG. 4.

Relative change in the resistance of a micro-structured line fabricated on a BFO (30 nm)/LAO (6 u.c.)/STO sample as function of the area where the BFO exhibits an opposite polarization. The polarization of the BFO layer on top of the quasi-2DEG system was poled to the polarization down state before the measurement resulting in R0 = 2.74 MΩ. The inset shows the micro-structured line of the quasi-2DEG with a length of 200 μm and a width of 50 μm on top of the BFO (orange)/LAO (green)/STO (blue) sample. Red marked are the areas poled for this measurement. Epitaxial areas are marked by ep and amorphous areas by am, respectively.

Close modal

In order to determine the switching characteristics of the BFO/LAO/STO system, the BFO layer was poled with different voltages. The ferroelectric polarization of the line was homogeneously poled to the P state. Fig. 5 shows the measured resistance for different tip voltages used to scan an identical area of 50 × 50 μm2. This resulted in a hysteretic cycle starting at data point 1 to 9 for the sweep from positive to negative voltage and ending at data point 15. The observed hysteresis is expected due to the ferroelectric properties of the BFO layer.

FIG. 5.

Resistance of a micro-structured line of a BFO (30 nm)/LAO (6 u.c.)/STO sample as a function of the voltage used to pole the polarization of the BFO layer. The polarization of the BFO layer on top of the quasi-2DEG system was poled to the P state before the measurement.

FIG. 5.

Resistance of a micro-structured line of a BFO (30 nm)/LAO (6 u.c.)/STO sample as a function of the voltage used to pole the polarization of the BFO layer. The polarization of the BFO layer on top of the quasi-2DEG system was poled to the P state before the measurement.

Close modal

In conclusion, we have demonstrated the possibility to gate the quasi-2DEG at the LAO/STO interface by the ferroelectric polarization of a multiferroic BFO ad-layer at room temperature. PFM allowed for both the manipulation and imaging of the ferroelectric polarization of the BFO layer using the quasi-2DEG as the bottom electrode. The electrical field created by the charged BFO/LAO-interface enabled a non-volatile direct control of the resistance of the quasi-2DEG. This allowed for a precise control of the resistance of the quasi-2DEG by poling certain areas of the BFO layer by PFM. A linear dependence of the resistance of a micro-structured line with respect to the area of the BFO layer poled in opposite polarization was found. In addition, the use of different voltages to scan the area of the BFO layer above the quasi-2DEG led to a hysteretic behaviour of the resistance of the line in dependence of the poling voltage. The addition of a micro-structured electrode on top of the line could easily lead to a ferroelectric field effect transistor and open a path to a functionality in oxide-based microelectronics. This functionality could in future be further enhanced using additional ferromagnetic layers showing exchange coupling to the antiferromagnetism of the BFO.

The authors acknowledge the financial support from the Stiftung Rheinland-Pfalz für Innovation (Grant No. 961-386261/944), the Graduate School of Excellence “Materials Science in Mainz” (Grant No. GSC 266), and the DFG.

1.
P.
Yu
,
Y.-H.
Chu
, and
R.
Ramesh
,
Mater. Today
15
,
320
(
2012
).
2.
S. M.
Wu
,
S. A.
Cybart
,
P.
Yu
,
M. D.
Rossell
,
J. X.
Zhang
,
R.
Ramesh
, and
R. C.
Dynes
,
Nat. Mater.
9
,
756
(
2010
).
3.
A.
Ohtomo
and
H. Y.
Hwang
,
Nature
427
,
423
(
2004
).
4.
N.
Nakagawa
,
H. Y.
Hwang
, and
D. A.
Müller
,
Nat. Mater.
5
,
204
(
2006
).
5.
H.
Chen
,
A. M.
Kolpak
, and
S.
Ismail-Beigi
,
Adv. Mater.
22
,
2881
(
2010
).
6.
S.
Thiel
,
G.
Hammerl
,
A.
Schmehl
,
C. W.
Schneider
, and
J.
Mannhart
,
Science
313
,
1942
(
2006
).
7.
B.
Förg
,
C.
Richter
, and
J.
Mannhart
,
Appl. Phys. Lett.
100
,
053506
(
2012
).
8.
R.
Jany
,
C.
Richter
,
C.
Woltmann
,
G.
Pfanzelt
,
B.
Förg
,
M.
Rommel
,
T.
Reindl
,
U.
Waizmann
,
J.
Weis
,
J. A.
Mundy
,
D. A.
Muller
,
H.
Boschker
, and
J.
Mannhart
, “
Monolithically integrated circuits from functional oxides
,”
Adv. Mater. Interfaces
1
,
1300031
(
2014
).
9.
V. T.
Tra
,
J.-W.
Chen
,
P.-C.
Huang
,
B.-C.
Huang
,
Y.
Cao
,
C.-H.
Yeh
,
H.-J.
Liu
,
E. A.
Eliseev
,
A. N.
Morozovska
,
J.-Y.
Lin
,
Y.-C.
Chen
,
M.-W.
Chu
,
P.-W.
Chiu
,
Y.-P.
Chiu
,
L.-Q.
Chen
,
C.-L.
Wu
, and
Y.-H.
Chu
, “
Ferroelectric control of the conduction at the LaAlO3/SrTiO3 heterointerface
,”
Adv. Mater.
25
,
3357
(
2013
).
10.
Y.-H.
Chu
,
L. W.
Martin
,
M. B.
Holcomb
, and
R.
Ramesh
,
Mater. Today
10
,
16
(
2007
).
11.
J. R.
Teague
,
R.
Gerson
, and
W. J.
James
,
Solid State Commun.
8
,
1073
(
1970
).
12.
S. V.
Kiselev
,
R. P.
Ozerov
, and
G. S.
Zhdanov
,
Sov. Phys. Dokl.
7
,
742
(
1963
).
13.
L. W.
Martin
and
R.
Ramesh
,
Acta Mater.
60
,
2449
(
2012
).
14.
H.
Béa
,
M.
Bibes
,
A.
Barthélémy
,
K.
Bouzehouane
,
E.
Jacquet
,
A.
Khodan
,
J.-P.
Contour
,
S.
Fusil
,
F.
Wyczisk
,
A.
Forget
,
D.
Lebeugle
,
D.
Colson
, and
M.
Viret
,
Appl. Phys. Lett.
87
,
072508
(
2005
).
15.
C.
Mix
and
G.
Jakob
,
J. Appl. Phys.
113
,
17D907
(
2013
).
16.
C. W.
Schneider
,
S.
Thiel
,
G.
Hammerl
,
C.
Richter
, and
J.
Mannhart
,
Appl. Phys. Lett.
89
,
122101
(
2006
).
17.
N.
Banerjee
,
M.
Huijben
,
G.
Koster
, and
G.
Rijnders
,
Appl. Phys. Lett.
100
,
041601
(
2012
).