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Progress and Perspectives of Atomically Engineered Perovskite Oxide Interfaces forElectronics and Electrocatalysts

Chen, Yunzhong; Green, Robert

Published in:Advanced Materials Interfaces

Link to article, DOI:10.1002/admi.201900547

Publication date:2019

Document VersionPeer reviewed version

Link back to DTU Orbit

Citation (APA):Chen, Y., & Green, R. (2019). Progress and Perspectives of Atomically Engineered Perovskite Oxide Interfacesfor Electronics and Electrocatalysts. Advanced Materials Interfaces, 6(15), [1900547].https://doi.org/10.1002/admi.201900547

1

+DOI: 10.1002/ ((please add manuscript number))

Article type: Progress Report

Progress and Perspectives of Atomically Engineered Perovskite Oxide

Interfaces for Electronics and Electrocatalysts

Yunzhong Chen1*, and Robert Green,

2,3*

Yunzhong Chen 1Department of Energy Conversion and Storage, Technical University of Denmark, Risø

Campus, 4000 Roskilde, Denmark

E-mail: yunc@dtu.dk

Robert Green 2Stewart Blusson Quantum Matter Institute, Department of Physics and Astronomy,

University of British Columbia, Vancouver,

British Columbia V6T 1Z4, Canada 3Department of Physics & Engineering Physics, University of Saskatchewan, Saskatoon,

Saskatchewan S7N 5E2, Canada

Email: rgreen@physics.ubc.ca

Abstract:

The two-dimensional electron gases (2DEGs) at the surfaces and interfaces of SrTiO3-based

homo- and hetero-structures provide new opportunities for electronics and spintronics. Herein,

the recent progresses in the creation of advanced material systems of conducting oxide

interfaces with engineered electronic reconstructions and redox reactions, as well as their

characterization by a non-destructive resonant X-ray reflectivity technique, are summarized.

Moreover, the development of modulation-doped high-mobility oxide 2DEGs and the

magnetic-proximity-induced spin-polarized 2DEGs at oxide interfaces are also discussed.

Finally, the perspectives on design of conducting oxide interfaces for a new generation of

quantum devices and mixed electronic and ionic conducting electrocatalysts are addressed.

Atomically-engineered oxide interfaces will represent a unique family of quantum materials

for future information and energy technologies.

2

1. Introduction

Among the technological problems that require attention today are that silicon electronics has

reached fundamental scaling limits and that a secure society transition from fossil fuels to

renewable energy sources is required. Solutions to both of these challenges require

breakthroughs in material science. Thanks to the strong interplay between the charge, spin,

orbital and lattice degrees of freedom, the interfaces and electrochemical interfaces of

transitional metal oxides, particularly perovskite oxides1-3

, present the richest variety of

emergent states, including colossal magnetoresistance, high-temperature superconductivity,

high mobility two-dimensional electron gases (2DEGs) and enhanced oxygen surface

exchange as well as the rate of oxygen redox reactions (ORR), just to name a few. These

extraordinary properties provide new opportunities for electronics and electrocatalysts1-3

.

Notably, the continuous development in thin film epitaxial growth and characterization of

complex oxides with atomic precision over the past 30 years, particularly the monolayer-by-

monolayer design and control of perovskite oxide thin films4,5

and the atomic and element-

specific characterization of the structural and electronic structure of oxide interfaces,6,7

has

significantly improved our ability and understanding of oxide interfaces for electronics. A

particular significant development is the emergence of the electronic conduction at the oxide

interfaces between two band insulators, one of which is the perovskite SrTiO3 (STO),8-10

as

illustrated in Fig.1 for its crystalline and electronic structures11-13

. However, mechanisms

underlying the enhancement of the ionic conductivity and the ORR activity remain topics

which are hotly debated14

. Moreover, the atomic-scale design and synthesis principles of

complex oxide heterostructures developed exclusively in the electronic community have

rarely been applied to the research of ionic conductors and electrocatalysts. In particular, a

bridge between the atomically engineered electronic oxide interfaces and the ionic oxide

interfaces remains missing. Herein, we summarized the recent progress in the creation and

characterization of advanced materials of conducting oxide interfaces for electronics. The

3

perspectives on applying conducting oxide interfaces for quantum devices and on using the

design principles to create future electrocatalysts are also addressed. Note that the two

techniques widely used to grow crystalline complex oxide films are: (i) pulsed laser

deposition (PLD) and (ii) molecular beam epitaxy (MBE). For their physical mechanisms and

the film growth dynamics, the readers can refer to a former review written by Chambers.15

2. Fundamental physics and chemistry of conducting oxide interfaces

One of the central issues concerning the engineered oxide interfaces is to understand and

control the interface interactions which consist of two fundamental aspects: (i) interfacial

charge reconstructions—electronic charge transfer; and (ii) interfacial atom transport—

intermixing and/or chemical reactions.

2.1 Band alignment and charge transfer at oxide interfaces

At the heterointerface between two semiconductors or oxides, the mismatch of the work

functions will result in electronic charge being transferred between the two materials, i.e.

charge transfer occurs across the heterointerface to equalize the chemical potential (i.e. Fermi

level) on both sides. The understanding of the mechanism underlying charge transfer is of the

utmost importance, as it determines the origin of the charge carriers and sets the design rules

for the growth sequence as well as band alignment. While the energy band alignment of

conventional semiconductors is quite well understood16

, systematic experimental studies on

oxides are still missing. During the past several years, extensive experimentation has been

performed to determine the intrinsic band-alignment of most functional oxides (Fig. 2)17

as

well as those of 3d and 5d complex oxide heterostructures. This band alignment offers a

rationale for the estimation of valence band offsets at intimate contacts between different

materials. Notably, the charge transfer at complex oxide interfaces is generally much more

complex than those of conventional semiconductors, as the energy bands in correlated oxides

are dynamic, i.e. they respond dramatically to changes in doping, strain, and the

accompanying changes in long range magnetic and orbital ordering.18

Generally, the energy band alignment at contacts of different materials can be determined by

X-ray photoelectron spectroscopy. For insulating oxides, such as SrTiO3, the band-alignment

and/or charge transfer with another complex oxide can be determined alternatively by making

epitaxial Schottky or p-n junctions consisting of Nb-doped SrTiO3 (Nb:STO) and the target

oxides.19,20

Assuming that the work function is almost the same as the electron affinity in the

4

Nb:STO, the work function of the target oxides can be estimated as the sum of the electron

affinity of STO and the difference of the Fermi levels between the oxide and Nb:STO,

determined by the Schottky barrier height and built-in potential of the junction. Figure 3

shows the relative energy band alignment between STO and the other two typical complex

oxides, manganites (La1-xSrxMnO3) and cuprates.18

As we can see, the Fermi level of most

manganites and cuprates is lower than that of the Nb-doped STO. This nontrival information

indicates that most transitional metal oxides could be used as an electron sink when placed in

proximity to the surface or interface STO 2DEGs as discussed later.21,22

Another remarkable property of charge transfer at oxide interface is its short-range transfer

length on the order of 1-4 nm,23

and due to the strong electron correlation, a large number of

charge carriers in perovskite oxides tend to localize on a specific site and exhibit normally a

heavy effective mass. This implies a rather short electron wavelength as well as a small

Thomas-Fermi screening length on the order of only a few nanometers.24

As a consequence,

for desired properties, it is essential to master the growth of epitaxial oxide heterostructures at

the atomic level.

2.2 Chemical redox reactions

Besides the physical mechanisms, there is plenty of experimental evidence showing that

chemical reactions play an important role in the formation of conducting oxide interfaces.25-27

The chemical interactions involve mass transport over a relatively long lattice distance. The

driving force behind the mass transport in the solids is a gradient in the electrochemical

potential, consisting of a gradient in the electrical potential and/or chemical potential. The

chemical interaction can be generally classified into four different groups: (i) redox reaction;

(ii) alloy formation; (iii) encapsulation, and (iv) interdiffusion.28

Chemical redox reactions

and cation interdiffusion have often been suggested as the origin of interface conduction at the

polar LaAlO3/STO interfaces,25,27

though this view is hotly debated. Here we focus mainly on

the redox reaction on the STO substrate by oxidizing the oxide overlayer and reducing the

STO surface29

. The chemical redox reactions depend strongly on the substrate temperature

and oxygen background pressure during film growth. Therefore, they often show

thermodynamic and kinetic limits as well as extreme sensitivity to to electrochemical reaction

conditions, rendering them difficult to investigate by conventional surface-science techniques.

Moreover, nonequilibrium thermodynamics and reaction kinetics must be also considered.

Consequently, similar conducting oxide interfaces may show dramatically different behavior

under thermodynamic equilibrium heating procedures30

.

5

3 The conducting surface and interfaces of SrTiO3 for electronics

As silicon is the basis of modern microelectronics, the perovskite oxide insulator, STO, is the

foundation of the emerging field of oxide electronics, as it shows reasonably good lattice-

match with most of other functional oxides with perovskite, spinel or fluorite crystalline

structures. This facilitates the possibility to combining various functional oxides into a single

epitaxial heterostructure21-33

. Moreover, unlike the conducting states of conventional

semiconductor heterostructures, which are derived from sp hybrid orbitals having a largely

covalent nature, the conduction in STO originates from strongly correlated Ti 3d-electrons as

illustrated in Fig.1, where the orbitals have a strong coupling with the lattice, charge and spin

degrees of freedom. The strong electronic correlations give rise to a variety of emergent

physical properties.34

Therefore, since the discovery of the 2DEG at LAO/STO interface in

2004,8 significant attention has been focused on the exploration of exotic and unusual

electronic properties, such as ferromagnetism35-37

, superconductivity38-40

, quantum Hall

effect41-43

, tunable spin-orbit coupling44-46

and sensitivity to external light illumintion47

.

Additionally, the search for alternative systems with better properties has resulted in the

discovery of a number of new material systems of conducting oxide interfaces and surfaces.

In the following, the representative 2DEG systems based on STO and the primary

mechanisms underlying the surface/interface conduction are summarized.

3.1 2DEGs at polar/non-polar oxide interfaces

In the study of the metallic conduction created at the interface of LAO/STO heterostructures,

much effort is directed at revealing the origin of the interface conduction. In simple ionic

terms, LAO is composed of charged layers of (La3+

O2-

)+ and (Al

3+O

2-2)

-, whereas the

terminated planes of STO, (Ti4+

O2-

2)0 and (Sr

2+O

2-)0, are formally neutral, as shown in Fig.4.

If the interface is abrupt, the polar discontinuity between (La3+

O2-

)+/(Ti

4+O

2-2)

0 or (Al

3+O

2-2)

-/

(Sr2+

O2-

)0 will create a built-in potential in the LAO film. The accumulation of this potential

to a large enough value, which is comparable to the band-gap of STO, 3.2 eV, will result in an

injection of carriers from the film surface into the interface, thus forming a 2DEG48

(Fig.4b).

However, as a matter of fact, the hallmark of this microscopic view of a pure electronic

reconstruction scenario—the huge potential drop across the LAO over-layer—has

experimentally remained elusive so far. Alternatively, recent measurements performed by soft

x-ray angle-resolved resonant photoelectron spectroscopy49

, as well as theoretical calculations

of electronic and defect properties of LAO/STO interfaces by first principles50,51

, reveal a

6

promising modified electronic reconstruction scenario: the built-in potential in the LAO is

compensated by positively charged surface defects such as O vacancies or other in-gap states

on the LAO side, which act as charge donors (Fig.4c). Notably, Since there are also extensive

redox reactions on the STO surface when exposure to oxygen-deficient LAO films29

, for a

long time, it has been extremely challenging to isolate the sought-after pure electronic

reconstruction effect from the extrinsic doping effects induced by formation of oxygen

vacancies. As discussed in section 3.6, this could be achieved by selecting a suitable buffer

layer at the interface of LAO/STO21,22,52

.

The conduction of the LAO/STO interface is extremely sensitive to the film growth

temperature and oxygen background pressure.35,53

Generally, when the LAO film is grown at

a temperature above 700 ˚C and at low oxygen pressure less than about 10-6

mbar, the

conduction detected is three dimensional54

and is dominated by oxygen defects, which

extends deeply into or through the whole substrate. It is generally accepted that for samples

grown at higher pressure (over about 10-5

mbar) the conduction is confined to the interface.

Additionally, with the purpose to suppress the formation of oxygen vacancies, most samples

are subject to post annealing at high oxygen pressure, which might have a deleterious effect

on the carrier mobility. Carrier mobility is one of the most sensitive measures of the purity

and cleanliness of a material. The typical 2D electron mobility in LAO/STO polar interfaces

is ~10 cm2V

-1s

-1 at room temperature , and ~1000 cm

2V

-1s

-1 at 2 K

4,5,34 (Figure 5a), with a

sheet carrier density, ns, in the range of 1013

-1014

cm-2

. This mobility is still much lower than

those for 3D oxygen-deficient STO single crystals55

and La-doped STO epitaxial films56

,

amounting to 1.3×104 cm

2V

-1s

-1 and 3.2×10

4 cm

2V

-1s

-1 at 2 K, respectively. Although the

electron mobility remains below 10 cm2V

-1s

-1 at room temperature, there have been reports

that the mobility of STO 2DEGs at low temperatures can be increased to comparable or 4-6

times higher than that of the bulk mobility by surface or interface engineering.21, 57, 58

Although the mechanism underlying the mobility enhancement induced by surface

engineering remains unclear,57,58

the mobility enhancement induced by the interface

engineering with a buffered layer could result from a modulation doping effect21

, as discussed

in Section 3.5. Notably, in comparison with the conventional semiconductor heterostructures,

such as GaAs59

, where the carrier mobility has been increased continuously over the past 40

year (Fig.5b), the mobility enhancement in the oxide 2DEGs is just starting.

3.2 Surface/interfacial 2DEGs due to chemical doping or interface redox reactions

7

Besides the intensively investigated 2DEG induced by the interface polar discontinuity, the

most conventional way to create STO 2DEG is the trilayer homostructure of La or Nb-doped

STO sandwiched between two undoped STO, which can show reasonably high mobility when

the doping level is low43

. Moreover, the recently discovered 2DEGs at the surface of fractured

STO under exposure of the surface to intense ultraviolet light60-63

, and at the interface between

STO and an insulating amorphous overlayer29

, also show similar and nontrival low

temperature physical properties. A notable feature of these three systems is the presence of

localized carriers trapped in the in-gap states, as illustrated in Fig.6. As a consequence, when

the carrier density is high (normally above the order of 1013

cm-2

), the temperature dependent

carrier density often shows carrier freezing out phenomena64

.

The 2DEG in amorphous STO heterostructures (Fig.6c) results from interfacial redox

reaction and is of particular interest because the interface conduction can be enhanced or

suppressed by selecting different capping layers. For the ABO3 perovskite-type interface of

intensive research, where A is an electropositive cation such as an alkali, alkaline-earth or

rare-earth ion, the chemical reaction and electron transfer across the perovskite-type interface

is expected to depend primarily on the electronegativity and coordination environment of the

transition metal ions on the B site. Taking this into account, the perovskite oxide interface

investigated so far can be divided into three families: (1) the metallic interfaces of crystalline

LAO/STO8, crystalline LaTiO3/STO

65 , crystalline CaZrO3/STO

66, amorphous LAO/STO

29,

amorphous STO/STO29

, amorphous YSZ/STO29

, and amorphous CaHfO3/STO67

, which fulfill

all criteria for oxide-oxide interface redox reactions of ΔHfO<-350 kJ/(mol O) and 3.75 eV<φ

<4.4 eV, where ΔHfO and φ is the formation heat of metal oxide and the work function of the

B-site metal, respectively, as illustrated in Fig.7b; (2) the insulating interfaces of

LaFeO3/STO68

and LaMnO3/STO69

which have ΔHfO>-350 kJ/(mol O) and φ >4.0 eV; and (3)

the either metallic interfaces of crystalline LaVO3/STO70

and crystalline LaGaO3/STO71

or the

insulating interface of LaCrO3/STO72

, where the B-site metals reside in an intermediate region

of -350 kJ/(mol O) <ΔHfO<-450 kJ/(mol O) and 4.1 eV<φ <4.5 eV. Therefore, the appearance

or the lack of interface redox reaction provides an inspiring clue on the metallic and insulating

nature of oxide interfaces reported so far. Note that for the manganite/titanate heterointerface,

since the lowest unoccupied state in STO (Ti t2g) is about 1eV higher than that of the

manganite film (Mn eg), the pure electronic reconstruction mechanism can also explain the

insulating behavior as charge transfer or reconstruction occurs predominantly in the MnO2

sublayers in proximity to the interface64,69

.

8

The redox-reaction mechanism, which occurs even at room temperature, can also explain

the other forms of conducting STO-based and TiO2-based oxide interfaces when Al-or Ti

containing oxides are involved during the fabrication by pulsed laser deposition, atomic layer

deposition , electron beam evaporation and even chemical spin coating.73-76

. Furthermore, it

provides an additional approach to tune the interface conduction by controlling the interfacial

redox reaction during deposition. For example, by improving the chemical reactivity of the

plasma plume through deposition the LAO film in argon rather than in oxygen, the amorphous

LAO/STO conduction can be enhanced.77

Moreover, the conductivity of amorphous-

LAO/STO interfaces is also found to be modified by an external electric field provided in-situ

through a biased, truncated cone electrode (-10 V ≤ Vbias ≤ 20 V) during film deposition.78,79

By modulating the charge balance of the plasma plume, a substantial surface charge is found

to transiently build up during the initial phase of the film growth when the truncated cone

electrode is biased. The presence of this huge surface potential is expected to greatly influence

the surface/plasma interaction during film deposition. More specifically, experimental

observations seem consistent with a lowering of the incoming Al-ion flux by the applied bias,

which shifts the interfacial conduction of d-LAO/STO from metallic over semiconducting to

insulating transport mode.78

This remarkable behaviour further indicates the importance of

Al-ion flux on the amount of near-interface oxygen vacancies formed at the STO surface and

therefore the interface conduction, which is also tubable by external electric field80

.

3.3 2DEGs at non-polar oxide interfaces induced by strain-induced polarization

Besides interfacial polar discontinuity, spontaneous and/or piezoelectric polarizations have

been found to result in high-mobility 2DEGs in both traditional semiconductor materials such

as III-V semiconductor AlGaN/GaN compound heterostructures81

and the binary oxide

heterostructures of ZnMgO/ZnO82,83

. Recently, strain induced polarization has also been

found to result in a high mobility 2DEG in STO heterostructures at an otherwise nonpolar

perovskite-type interface of (001) CaZrO3/STO (CZO/STO).66

In these systems, the 2DEG

carriers normally originate from the ionized surface donor state. The spontaneous and/or

piezoelectric polarization in the top film provides the driving force to collect the electrons in

the quantum well.66,81

Notably, as illustrated in Fig.8, the CZO/STO heterostructure shows

both atomically sharp interfaces and extremely high electron mobility exceeding 60000 cm2V

-

1s

-1 at 2 K, which is nearly independent of the film thickness

66. There is also a critical

thickness of tc=6 uc for the presence of interface conduction (Fig.8c). This is consistent with

the model based on the polarization induced charge transfer 66

, for ultrathin CZO films with a

9

thickness below tc, the ionized donor surface states, which are sufficiently deep and below the

conduction band of CZO with an energy ED, lie far below the conduction band of STO

(Fig.8d). In this case, no 2DEG is formed. Increasing the CZO film thickness normally lifts

the energy of the surface donor states. At the critical thickness, the surface donor energy

reaches the bottom of the conduction band of STO, as demonstrated in Fig. 8e. Electrons, at

the same time, are transferred to the empty conduction band of STO at the interface, creating

the 2DEG. Until all the surface states are empty, in the ideal case, the Fermi level remains at

the donor energy while more and more electrons are transferred with increasing the film

thickness. Note that first-principles electronic structure calculations predict that the

polarization direction and tc of the CZO could depend on the surface termination of CZO

film.84

More recently, using conductive atomic force microscopy lithography, extremely

narrow nanowires between 1.2 nm to 2.8 nm and various quantum devices were realized at the

CZO/STO interface. Notably, the widths of the nanowires remain narrow at most writing

voltages85

, in contrast to nanowires formed at the LAO/STO interface, where the wire width

depends sensitively on the writing voltage86

. Therefore, much more exotic properties and

functionality are awaiting to be explored for the sharp and high mobility CZO/STO interface.

3.4 Modulation-doped complex oxide interfaces

Most 2DEGs in STO heterostructures show a high carrier density above 1013

cm-2

. These

electrons (t2g electrons) come generally in two species11

, dxy and dxz/dyz (where z is

perpendicular to the 2D layers), as illustrated in Fig.1. The dxy component accounts for the

majority of the 2DEG charge. The dxy electrons reside at or in the immediate proximity of the

interface, but exhibit a rather low mobility. In contrast, dxz or dyz electrons extend further away

from the interface and amount to only a small fraction of the 2DEG population. Yet, their

mobilities reach much higher values than that of the dxy states11

. This intrinsic electronic

structure (the coexistence of both high and low mobility carriers on the same side of the

interface) prevents the application of the conventional modulation-doping technique to

enhance the electron mobility where a spacer layer is introduced which separates spatially the

electron gas from its charged donors on the sample surface87,88

. Recently, by inserting a buffer

layer of LaMnO3 (LMO)-based manganite as thin as one single unit cell (uc), e.g. La1-

xSrxMnO3 (LSMO, x=0, 1/8, and 1/3), at the interface as shown in Fig.9a, a charge-transfer-

induced modulation doping is achieved for the 2DEG in amorphous LAO/STO with a huge

mobility enhancement of more than two orders of magnitude in addition to a remarkable

decrease in the carrier density (Fig.9 b and c).21,22

In contrast to the spacer layer used in

10

conventional semiconductors as illustrated in Fig. 9d, the LMO-based spacer at oxide

interfaces can act as an additional electron sink because it has an empty or partially filled

subband lower than the conduction bands of STO (Fig.9e and also Fig.3). Furthermore, in-

depth X-ray reflectivity64

and oxygen isotope tracing89

measurements reveal that the

manganite can also suppress strongly the formation of oxygen vacancies on the STO side. It is

further revealed that the carriers of the interface 2DEGs show negligible dependence on the

Sr-doping level of the spacer.22

In contrast, a large tunability of the interface conduction was

achieved recently by doping LaMnO3 into the LAO top layers, i.e. via the formation of a

diluted oxide interface as demonstrated in Fig. 10.90

By increasing the Mn-doping level of

LaAl1-xMnxO3(LAMO)/STO (0 ≤ x ≤ 1), the interface undergoes a Lifshitz transition at a

critical carrier density of nc = 2.8×1013

cm-2

, where a peak TSC ≈ 255 mK of the

superconducting transition temperature is observed. Moreover, at x = 0.3, where the metallic

interface is populated by only dxy electrons and the LAMO becomes ferromagnetic, a possible

coexistence of superconductivity and ferromagnetism is achieved. This provides a unique and

effective way to tailor oxide interfaces for designing on-demand electronic and spintronic

devices.

3.5 Magnetic 2DEGs at STO-based heterointerfaces

The possibility to obtain a spin-polarized 2DEG based on STO 2DEGs provides also new

opportunities for spintronics. Although there are reports on the possible magnetic ordering in

LAO/STO heterostructures, these samples tend to be insulating or show signatures related to

the interaction between itinerant electrons and localized magnetic moments.35-37

To

introduce strong/unambiguous magnetism into the highly conductive LAO/STO, three ways

have turned out to be effective so far. The first is the magnetic proximity effect induced by a

EuTiO3-ferromagnetic buffer layer at the LAO/STO interface.91

The spin polarization

emerges below the ferromagnetic transition temperature of the EuTiO3 epitaxial layer (T=6–8

K) and is due to the exchange interaction between the magnetic moments of Eu-4f and of Ti-

3d electrons as confirmed by X-ray magnetic circular dichroism (XMCD) measurements. A

second effective way is the introduction of the ferromagnetism into the top layer of LAO or

the introduction of a ferromagnetic capping layer (mostly metal) directly on STO as

demonstrated in the cases of the magnetic diluted oxide interface of LAMO/STO90

as well as

EuO/STO92

, and MAl2O4 (M=Ni, Co)/STO93

. Notably, both the LAO/EuTiO3/STO and

LAMO/STO systems show signatures of the coexistence of ferromagnetism and

superconductivity, although their carriers come from two-band and single band conduction

11

respectively. The final and the direct approach towards magnetic 2DEGs is the conventional

doping with magnetic ions.94-96

For example, a robust anomalous Hall effect95

and large

negative magnetoresistance96

were observed when Fe-doped STO (Fe-STO) was used as the

substrate for the fabrication of the conducting interface of LAO/Fe-STO. The anomalous Hall

effect has been observed up to room temperature.95

On the other hand, since the magnetic Fe

dopant is localized within the STO substrate, the mobility of this type of 2DEG is generally

very low (below 100 cm2V

-1s

-1 at low temperatures).

4. Atomic and elemental characterization of conducting oxide interfaces

To understand the origin and detailed nature of conducting interfaces in correlated oxides,

many experimental probes are often employed, particularly high-angle annular dark-field

(HAADF) scanning transmission electron microscopy (STEM ) imaging with electron energy

loss spectroscopy (EELS) and energy dispersive X-ray (EDX) spectroscopy measurements.6,97

To study buried interfaces in a non-destructive manner, a particularly useful technique is that

of X-ray spectroscopy.7 In particular, for interfaces where 3d transition metals are active

elements (e.g. for STO-based interfaces), X-ray absorption spectroscopy (XAS) is often used

to probe the L2,3 core level resonances of the transition metal element. The resulting spectra

carry detailed information about the local electronic and magnetic structure of the element

under study98

. Figure 11 shows the sensitivity of Ti L2,3 scattering factors to the valence state

of Ti, and thus the presence or absence of a 2DEG. As shown in the figure, XAS can be made

more surface- or bulk-sensitive via the detection method used (electron yield or fluorescence

yield, respectively).

Salluzzo et al. used XAS to reveal orbital reconstruction at the (001) LaAlO3/SrTiO3

interface99

, where a tetragonal symmetry breaking lifts the degeneracy of the t2g Ti 3d orbitals,

shifting the dxy orbital lower in energy compared to the dxz and dyz near the interface. This

lowering of the dxy orbital energy indicates that it will be preferentially occupied by the 2DEG

near the interface, and will thus determine the 2DEG transport properties. Later XAS studies

12

revealed orbital polarization with a different symmetry at (110) LAO/STO interfaces100

, as

well as magnetism at oxygen deficient (001) LAO/STO interfaces101

.

As shown in Figure 11, X-ray absorption is a technique which averages over the probing

depth studied (either a few nm or several tens of nm, depending on the type of detection), but

a strongly enhanced interface sensitivity can be achieved with the technique of resonant x-ray

reflectometry (RXR)7. With RXR, one probes the same fundamental scattering tensors as in

XAS, but via the specular reflection rather than absorption of X-rays, allowing for naturally

quantitative measurement and analysis having inherent interface sensitivity offered by the

reflection process. RXR is sensitive to elemental concentrations as well as detailed local

electronic and magnetic structures with sub-angstrom depth resolution. Macke et al.

demonstrated the high sensitivity of RXR, by detecting a single monolayer of

La0.005Sr0.995TiO3 buried within a homoepitaxially grown STO film on a STO substrate7.

Using RXR, the authors characterized the concentration of the La dopant as well as the

interdiffusion of La into adjacent monolayers, highlighting the strong elemental sensitivity.

Later, RXR was demonstrated to be valence sensitive, where polarity driven electronic

reconstruction was detected at a polar surface of a LaCoO3 film102

. The authors observed a

valence change of the Co atoms near the polar surface, in accordance with a polarity driven

electronic reconstruction. Additionally, an RXR study detected orbital polarizations near

buried interfaces in a LaNiO3 heterostructure103

while investigating the possibility of

engineering a cuprate-like Fermi surface in a nickelate material. Finally, it was shown that

RXR can be used to determine substrate terminations with atomic plane resolution104

.

When applied particularly to conducting STO-based interfaces, the relatively new technique

of RXR has already provided important insights.21,64

A reduction in 2DEG density was

detected via RXR in high-mobility modulation-doped amorphous LAO/STO interfaces21

. This

finding provided important insight to reveal the mechanism behind the buffer layer induced

high mobility 2DEG, which was discussed previously in section 3.3. Besides electronic

13

reconstruction, the RXR techniques also provides valuable information on the interfacial

redox reactions.64

Figure 12 provides an overview of the application of RXR to study

manganite buffered interfaces between disordered yttria-stabilized zirconia (d-YSZ) and STO,

which show clear competition between pure electronic reconstruction and interfacial redox

reactions64

. Panel (a) depicts schematically (top) and quantitatively (bottom) the elemental

and valence concentrations extracted from RXR analysis of a conducting d-YSZ/STO

heterostructure. A large fraction of Ti3+

is detected in the STO near the interface, and a

corresponding dip in the oxygen density is present in this region, providing evidence for

carriers supplied by redox-induced oxygen vacancies. Panel (b) shows results for an epitaxial,

insulating LSMO buffer layer on STO, where electronic reconstruction yields Mn2+

at the

interface. Some Mn2+

is also observed at the surface due to slight degradation, common in

LSMO films. Finally, panel (c) shows results for a complete d-YSZ/LSMO/STO

heterostructure. The RXR analysis reveals polarity induced electronic reconstruction to Mn2+

at the lower interface, and redox induced Mn2+

at the upper interface. At the upper interface,

distinct Mn sites having octahedral and tetrahedral symmetry are observed, indicative of the

brownmillerite structure in the LSMO present due to oxygen vacancies. The interpretation of

the results is that the uneven potential due to the brownmillerite layer causes some charge

transfer from the Mn2+

at the lower interface into the STO, yielding the exceptionally high

mobility 2DEG. Panels (d) and (e) provide some representative RXR spectra and fits, with

complete data sets available in Ref.64.

The analysis of RXR data is generally rather complex, but important recent advances have

been made. Macke et al. have developed the ReMagX program, which is now widely used for

modelling of the RXR process105

. Additionally, the program QUAD can be used to efficiently

simulate both resonant diffraction and reflection within the same model with full dynamic

effects106,107

, allowing for example the simultaneous modelling of reflectivity and magnetic

14

scattering in heterostructures108

. An additional recent development is a theory-restricted

modelling of the RXR signal during analysis109

. It is noteworthy that resonant X-ray

reflectometry is a complementary technique to STEM-EELS97

, which also provides access to

spatially resolved atomic scattering tensors at core level resonance energies. Both techniques

have distinct advantages: RXR is non-destructive, has a strong signal, straight-forward

polarization dependence, and flexible sample environment, whereas STEM-EELS has the

advantage of operating directly in real space and also providing lateral information (i.e. in the

direction parallel to interfaces). The combination of both techniques can yield a very

comprehensive view of interface properties, as demonstrated in recent studies.21

5. Concluding remarks and outlook

The materials systems for the 3d oxide 2DEGs have gone much beyond the LAO/STO polar

interfaces and we have also learned various ways to improve the quality of the interface

conduction by controlling the electronic and/or chemical interactions, taking into account the

insights provided by the advanced characterization techniques.

The samples with extreme cleanness are of particular interest as they provide unforeseen

opportunities to discover new quantum phenomena at low temperatures. Moreover, the

engineered charge transfer and redox reactions at oxide interfaces are not only important for

electronics but also for the search of electrocatalysts of mixed electronic and ionic oxide

interfaces. The rich landscape of quantum transport phenomena in STO-based

nanostructures110,111

and field-effect transistors (FETs) as building blocks of integrated

circuits of complex oxides112,113

have been demonstrated, and unprecedented discoveries are

expected in the following two research areas:

(a) Quantum transport and quantum devices based on STO 2DEGs.

When the charge carriers are confined to a two-dimensional system, the Hall resistance can

become exactly quantized under external magnetic field, i.e. the quantum Hall effect can be

observed. Such research performed on the modulation-doped amorphous LAO/STO interface

with extremely high electron mobility and low carrier density reveals that the single interface

consists of multiple quantum wells.41

This could be due to the strong electron correlations that

constrain a number of electrons at a given lattice site, particularly the TiO6 octahedra, intrinsic

to STO interfaces.114

Importantly, when this unique property is coupled to the large Rashba

15

spin-orbit interaction, what will be the characteristic features of quantum devices based on

complex oxide interface?115-117

Moreover, as the conducting oxide interface turns ferromagnetic which leads to anomalous

Hall effects in the normal state, what will be the quantized version of this anomalous Hall

effect? Besides the above quantum phenomena, quantum coherent devices, such as tunneling

junctions or superconducting qubits, important in magnetic sensors as well as information

processing technology, can also be designed.

(b) Atomically engineered oxide interfaces for electrocatalysts

At relatively high temperatures, most perovskite oxides also show mixed electronic and ionic

conductivity, important for oxygen redox reactions.118,119

The conventional way to tune the

intrinsic activity of perovskite oxides is by chemical doping, which often introduces

unintentional impurities. Without introducing unintentional defects, interfacial electronic

reconstruction and redox reactions can also be used to tune these chemical activities. Notably,

the recent development of sophisticated tools for growing complex oxides as thin films with

atomic-layer sensitivity and control has rarely been applied to the search for new oxide

electrocatalysts.

In this work, we have summarized the recent progress in the material science of conducting

oxide interfaces for electronics. The physical and chemical mechanisms and the technical

progress of these atomically engineered oxide interfaces could provide an intriguing

opportunity for the conception and design of mixed electronic and ionic conducting oxide

interfaces that go beyond the limits of state-of-the-art perovskite electrocatalysts.

Acknowledgements

YZC acknowledges the financial support from the European Union’s H2020-FETPROACT

Grant No. 824072. RJG is supported by the Natural Sciences and Engineering Research

Council of Canada.

Received: ((will be filled in by the editorial staff))

Revised: ((will be filled in by the editorial staff))

Published online: ((will be filled in by the editorial staff))

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21

Figures

Figure 1. The perovskite structure of SrTiO3 (a) and the electronic structure of SrTiO3-

based 2DEGs (c). The 2DEG electrons (t2g electrons) come in two species, dxy and dxz/dyz (as

demonstrated in b, z is perpendicular to the 2D layers) with different spatial dispersion. The

dxy electrons dominate the carrier population but exhibit a relatively lower carrier mobility.

Reproduced with permission from ref. [11-13]. Copyright 2013, Nature Publishing Group and

American Physical Society.

22

Figure 2 Energy band alignment at contacts of different oxides determined from a large set of

interface experiments with photoelectron spectroscopy.17

The valence band maximum energy

of SnO2 is represented by the dashed line and is set as a reference of the energy scale as a

guide to the eye. Vertical numbers refer to the band gaps of the materials in eV. Horizontal

numbers refer to the offset of the valence band maximum for each material with respect to the

valence band maximum of SnO2 in eV. The valence band discontinuity between any

combinations can be obtained assuming transitivity by subtracting the respective offsets to

SnO2.

Fig. 3 Energy band alignment between two typical complex oxides, manganites (La1-

xSrxMnO3) and cuprates (LCO: La2CuO4, NCO: Nd2CuO4, SCO: Sm2CuO4, and YBCO:

YBa2Cu3Oy ) with respect to Nb-doped SrTiO3 (Nb:STO). Reproduced with permission from

ref. [18]. Copyright 2007, American Physical Society.

23

Fig.4 (a) The pure electronic polar catastrophe scenario: a building-up of electric potential

upon increasing the layers of polar LAO is essential. (b) Schematic band diagram for the

standard electronic reconstruction scenario in LAO/STO. Electronic reconstruction occurs

when the valence band maximum is lifted to the level of the conduction band bottom of STO.

(c) Tentative band situation with the inclusion of O vacancies at the LAO surface or other in-

gap states as the charge donors for electronic reconstruction.

Fig. 5 Landmark electron mobilities in the history of (a) complex oxide 2DEGs based on

LAO/STOin comparison with that of (b) GaAs. Reproduced with permission from ref. [59].

Copyright 2003, Elsevier. The mobility of 2DEG at the interface of crystalline LAO/STO

shows a limit around 5,000 cm2V

-1s

-1, which is lower than the bulk value (approximately

20,000 cm2V

-1s

-1 at 2 K

55). But the spinel/perovskite interface of γ-Al2O3/STO

10, nonpolar

oxide interface CaZrO3/STO66

and buffered d-LAO/STO21

all show electron mobility higher

than that of the bulk.

24

Figure 6. Schematic picture of device structure (a-c) and electronic band structure (d-f) of

surface 2DEG , homointerfacial 2DEG, and the heterointerfacial 2DEG, respectively, based

on STO.

Fig. 7 (a) Sketch of redox reaction at oxide interfaces; Reproduced with permission from

ref. [29]. Copyright 2011, American Chemical Society. (b) Formation heat of metal oxides

versus the work function of B-site metals for the ABO3/STO (A=La) perovskite-type interface.

For metallic interfaces, the B site metals locate mainly in the region of ΔHfO<-350 kJ/(mol O)

and 3.75 eV<φ <4.4 eV (Square and Diamond). The dot line indicates the border of the redox

reaction for metals on the STO surface28

.

((For Essays, Feature Articles, Progress Reports, and Reviews, please insert up to three

author biographies and photographs here, max. 100 words each))

Author Photograph(s) ((40 mm broad, 50 mm high, gray scale))

25

The table of contents entry should be 50–60 words long and should be written in the present

tense and impersonal style (i.e., avoid we). The text should be different from the abstract text.

Fig.8 High mobility 2DEG at the nominally nonpolar CZO/STO interface. (a) A typical

HAADF-STEM image across the heterointerface. (b) Temperature dependence of the

interface sheet resistance, Rs, at different CZO film thickness (c) sheet carrier density as a

function of the film thickness, (d) and (e) Sketch of the band alignment before and after the

formation of 2DEG, respectively, at the CZO/STO heterointerface. Reproduced with

permission from ref. [66]. Copyright 2015, American Chemical Society.

26

Fig.9 Modulation doping of STO-based heterostructures. (a) TEM structure of a LSMO-

buffered d-LAO/STO interface; (b) and (c), The temperature dependence of carrier density, ns,

and mobility, μ, respectively for the buffered and unbuffered d-LAO/STO interfaces. The

unbuffered sample always shows a carrier freeze-out at around 100 K, while, the buffered

samples exhibit remarkable temperature independent ns and a large μ often exceeding

approximately 10000 cm2V

-1s

-1 at 2 K. Reproduced with permission from ref. [21]. Copyright

2015, Nature Publishing Group. (d) and (e) The principle of modulation-doping at

semiconductor and complex oxide interfaces, respectively. In modulation-doped

semiconductors, the electrons are spatially separated from their donors by a spacer, and

ionized impurity scattering is thus reduced. However, in modulation-doping oxide interfaces,

the spacer should be also able to trap the low mobility carriers by the presence of empty or

partially filled bands below the Fermi level of STO 2DEG. EC and EV denote the conduction

band edge and valence band edge, respectively, and EF denotes the Fermi level.

27

Fig.10. 2DEG in the diluted LAMO/STO heterostructures. (a) Sketch of the structure of

Mn-dopants on the polar LAO side; (b) Temperature dependence of the sheet resistance (Rs)

at different Mn doping concentrations; (c) Phase diagram of LAMO/STO heterostructures as a

function of carrier density. At x = 0 (LAO/STO), the samples are metallic conducting with

itinerant electrons occupying both dxy and dxz/dyz bands and becoming superconducting below

34 mK. Here, ns denotes the total carrier density measured at 2 K. At a lower carrier density

of nc = 2.8×1013

cm-2

(x = 0.225), the superconducting transition temperatures (TSC) peaks at a

value of 255 mK and here a Liftshitz transition point is observed. Below nc, the TSC decreases

and transport occurs only in the dxy band until the samples turn insulating (ns < 2.9×1012

cm-2

).

For 2.9×1012

cm-2

≤ ns ≤ 1.1×1013

cm-2

, the interface shows an anomalous Hall effect with

occupation of only the dxy band. In addition, a magnetically ordered state appears in the

LAMO top film at T ≤ 21 K for x ≥ 0.25, whereas no signature of magnetism is observed

below this doping level. Finally, the sample in a van der Pauw geometry (ns=8.9×1012

cm-2

)

with full superconducting transition where Rs = 0 is reached at TSC ~ 160 mK that also shows

AHE is marked as filled square in the figure.

Figure 11 – X-ray spectroscopy for thin film interfaces. (a) The Ti resonant scattering factor

changes strongly with the Ti valence, and can be used to probe the presence of a 2DEG in

STO. XAS probes directly the imaginary component of the scattering factor F. (b) Whether

probed using electron yield (EY) or fluorescence yield (FY), the exponential probing depth

profiles of XAS offer little interface sensitivity. RXR on the other hand involves the reflection

of x-rays, which allows for significantly enhanced interface sensitivity.

28

Figure 12 - Resonant X-ray reflectometry of SrTiO3-based 2DEG systems. (a−c, upper) Schematic depictions of

the Ti and Mn valence changes at the interfaces for (a) d-YSZ/STO, (b) LSMO/STO, and (c) d-YSZ/LSMO/STO

heterostructures. (a−c, lower) Element and valence concentration profiles for the three samples, offset into

anions, A-site cations, and B-site cations for clarity. The interface valence changes are labeled for the B-site

densities. (d,e) RXR experimental data and fits exhibiting the sensitivity to the interface valence changes for (d)

d-YSZ/STO and (e) LSMO/STO and d-YSZ/LSMO/STO. In each case, a model spectrum with no interface

valence change is shown for comparison to the best fit containing the valence change. Reproduced with

permission from ref. [64]. Copyright 2017, American Chemical Society

Yunzhong Chen received his Ph.D. in Condensed Matter Physics in 2009 from Institute of

Physics, Chinese Academy of Sciences, and the classic Doctor Technices Degree from Technical

University of Denmark (DTU) in 2016. From 2009-2011, He was a post-doc at Risø National

Laboratory for Sustainable Energy, Denmark and became a scientist in Department of Energy

Conversion and Storage at DTU in 2011. Since 2013, he has been an associate professor at

DTU Energy. His research focuses on the creation and understanding of interface phenomena

in atomically engineered oxide heterostructures for high electronic and ionic conductivity,

ferromagnetism, and enhanced catalytic activity.

(bio and photographs of Robert Green will be added later)

The recent progress in material science of conducting oxide interfaces based on SrTiO3 is

submmarized. The priniciple mechanisms and perspectives on quantum devices and

atomically engineered perovskite electrocatalysts are also discussed.

Keyword

Oxide interfaces, Charge transfer, Electronic reconstructions, Two-dimensional electron gases,

Oxygen redox reactions

29

Yunzhong Chen1*, and Robert Green,

2,3*Corresponding Author*

Title: Progress and Perspectives of Atomically Engineered Perovskite Oxide Interfaces

for Electronics and Electrocatalysts

TOC figure: