STO Interface

• Energy loss Ti 2p-3d excitation • Peak intensity decreases when the final state is occupied Information from EELS Ti L2,3 lines 9 2. Experimental EE...

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Origin of the 2DEG at the LAO/STO Interface Umberto Scotti di Uccio

S. Amoruso, C. Aruta, R. Bruzzese, E. Di Gennaro, A. Sambri, X. Wang and F. Miletto Granozio University FEDERICO II & CNR-SPIN, Napoli (Italy)

D. Maccariello, P. Perna IMDEA, Madrid (Spain)

C. Cantoni , J. Gazquez , M. P. Oxley , M. Varela , A.R. Lupini , S.J. Pennycook Oak Ridge National Laboratory

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This presentation regards the still open issue of the origin of the two dimensional electron gas at the LAO/STO interface. I will show some experimental data and make comments on this subject, but I’d like to state that my contribution is not completely general because it is limited to epitaxial samples. I will not speak of amorphous LAO samples, that have very different fabrication procedure and structure.

1. Introduction

2D Electron Gas in semiconductors

donors

Main ingredients: • Quantum well • Donor states

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2deg’s are observed in several different systems, such as for instance semiconducting structures based on gallium compounds. But in any cases they share two main ingredients, that are the existence of a quantum well and of donor states that populate the well with charge carriers. In the case of STO/LAO interfaces everybody agrees that the quantum well is formed within STO, close to the interface. So when I say “origin of the 2deg” I specifically refer to the nature and location of the donor states.

1. Introduction

Two alternative mechanisms for crystalline STO/LAO Electronic reconstruction

Oxygen vacancies

charge AlO2 LaO AlO2 LaO TiO2 SrO TiO2

(-1) (+1) (-1) (+1) (0) (0) (0)

(Sr+2) (Ti+4) (O-2)3 O ×O reduction  →

1 O 2 (g ) + 2 e' + VO•• 2

LaAlO3 - - - - - e-

e-

CB VB VB

STO

V V

O (g)

V

SrTiO3 LAO 3

Grossly speaking, there are two alternative models. The first one is the electronic reconstruction model. Instead, one may consider defects as donors, such as oxygen vacancies, or some kind of cation intermixing at the interface.

1. Introduction

Two alternative mechanisms for crystalline STO/LAO Electronic reconstruction

Oxygen vacancies/ intermixing

+ + + + + + + LaAlO3

-

-

-

-

-

LaAlO3

-

-

LaAlO3

+ + + + + + +

-

-

-

-

-

-

-

-

-

-

-

-

+

+

Donors are on the top of LAO

-

+ +

+

SrTiO3

-

+

+

SrTiO3

SrTiO3

Donors are at the interface

Donors are within STO

Different local electric field expected 4

One interesting difference between the models is the location of donors. In the electronic reconstruction case they are on the top of LAO, in the other cases they are at the interface or within STO. And as a consequence of the different location, different local electric fields are expected.

1. Introduction

An HRTEM+EELS experiment to probe local fields Can we directly determine E ? Not so easy. But we can measure: • the injected charge σ • the polarization of both layers + + + + + + + Polar state

Polar state

-

-

-

-

-

-

-

Injected charge

SrTiO3

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Unfortunately, the direct determination of the local electric field is not that easy. As I will show, we can instead measure the injected charge and the polarization of both layers.

1. Introduction

An HRTEM+EELS experiment to probe local fields Preliminary considerations: Simple electrostatics

(see, e.g., M. Stengel, PRL 2011)

The electric displacement D depends on the injected charge (i.e., the free charge) STO

r Po

+

+

AlO2 LaO AlO2 LaO TiO2 SrO TiO2

High-k dielectric

r r PST O = ε o (k '−1) E

r r PST O ≈ D

LAO High-k dielectric plus topological polar state

r r r PLA O = ε o (k − 1) E + Po

r r r Po PLA O ≈ D + k 6

However, this is enough, because based on the theoretical work by Stengel we can define the microscopic electrical displacement in STO/LAO, and the displacement is directly connected to the injected charge. Then we can write electrostatics equations that directly connect displacement and polarization. The case of STO is simple. STO is a high-k dielectric and we immediately get that the displacement is approximately equal to the polarization. The epitaxial layer of LAO is different, because it possesses a built-in, topological polar state, with polarization Po. Besides, it can also react to the field, and this gives a dielectric polarization. But again it is easy to find out a relation between polarization and displacement. Now we have a theoretical framework and we can look at experiment.

RHEED-assisted PLD KrF Excimer laser

λ = 248 nm, 1 Hz, 2 J cm-2

Ts = 800 °C

Buffer Gas: P(O2) = 10-3 mbar

2. Experimental

Thickness 5-10 u.c. LAO

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Conducting samples were fabricated by PLD at 10-3 mbar oxygen pressure at Napoli. I skip the details…

2. Experimental

STEM + EELS Electron Energy-Loss Spectroscopy with atomic-scale resolution in the aberration corrected microscope

5 u.c. LAO – 10-3 P(O2) Conducting sample

C. Cantoni, et al. ADV. MAT. 2012

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…and analyzed by Scanning Tunneling Electron Microscopy and Electron Energy Loss Spectroscopy in an aberration corrected microscope. This slide shows the microstructure of the interface and an EELS scan across the interface to show the sample quality and the capability of EELS to determine the local chemical composition with atomic resolution.

2. Experimental

2DEG DOS: free charge injection Information from EELS Ti L2,3 lines

Ti 3d eg Ti 3d t2g E-∆E

|ψ’>

CB

c E

|ψ>

Core level

a

d

b

Ti 2p 3/2 Ti 2p 1/2 • Energy loss  Ti 2p-3d excitation • Peak intensity decreases when the final state is occupied

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EELS also allows one to determine the occupancy of the conduction band. To this aim we can investigate the Ti L2,3 lines

2. Experimental

2DEG DOS: free charge injection Information from EELS Ti L2,3 lines

Ti 3d eg Ti 3d t2g c a Energy (eV)

d

b

Ti 2p 3/2

LAO

Ti 2p 1/2

STO c/d decreases if the CB is occupied

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…doing that at different distance from the interface

2. Experimental

2DEG DOS: free charge injection Information from EELS O K lines

c a

O 2p + Ti 3d eg

d

O 2p + Ti 3d t2g

b multiple scattering

a

b

LAO

O 1s

a decreases if the CB is occupied STO

∆E decreases if the CB is occupied

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We can also investigate the O K line…

2DEG confinement within STO

2. Experimental

(u.c.)

ρ (e- / u. c.)

depth

LAO

STO Fabrication P(O2) = 10-3 mbar Ts = 800 °C

• depth of confinement: ≈ 1 nm • integral: 0.3 e- / square unit cell 12

…and this is the summary. From the map we extract the plot of the injected charge density vs. distance from the interface. The main results are the depth of confinement, that is about 1 nm, and the total injected charge, amounting to 0.3 electrons per unit square cell.

Polarization measurement

2. Experimental

unit . cell i z

P =

∑q

j

zj

j • Assuming the formal charge of ions • Neglecting the deformation of valence orbitals

AlO2

Al

LaO La

AlO2 LaO

interface

TiO2 SrO TiO2

Ti C. Cantoni, et al. ADV. MAT. 2012

Sr 13

Let’s come now to the measurement of polarization. We determine from STEM the average position of each cation and define the polarization in a classical way, assuming the formal charge of ions and neglecting the deformation of valence orbitals.

2. Experimental

STO Polarization Ti Sr O

LAO

STO

measurement

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Here are the results for STO. We observe the rumpling of each crystal plane and a polarization close to the interface and quickly approaching to zero when moving toward the bulk.

2. Experimental

LAO Polarization La Al O

LAO

STO

measurement

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In LAO, instead, the polarization is uniform.

3. Discussion

Discussion 0.4

3+

• depth of confinement: ≈ 1 nm • integrated charge σo ≈ 0.3 e- / square u.c.

Ti fraction

1. The STO side

electrostatics •

0.3 0.2 0.1 0.0 -0.1

PSTO ≈ D

0.4

PSTO

0.3 0.2 0.1 0.0 -0.1

0

2 4 6 distance (u.c.)

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Impossible, the polarization has the wrong sign 16

So we can now discuss the results. Let’s first consider the STO side. Here we roughly see the same length-scale for both charge and polarization. And we also see that the displacement calculated by the integrated injected charge corresponds to the measured polarization at the surface. This confirms the correctness of the approach based on classical electrostatics. There is a second consequence. We can exclude that the donors are deep within STO, because in that case we would have found a different orientation of polarization.

3. Discussion

Discussion 1. The STO side

After charge injection, the lattice is deformed

Second harmonic generation Requirement Breaking of the centro-symmetry

r E

Conducting LAO/STO has efficient SHG

A. Rubano, et al. PRB 2011

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Since we have a strong polarization, we have a strong deformation of unit cells. But if the cubic environment is distorted, also the 3d orbitals of Ti atoms are deformed. This breaking of symmetry explains why STO/LAO interfaces emit so strongly in second harmonic, as we observed for our samples.

Discussion 2. The LAO side

3. Discussion

r r r Po PLA O ≈ D + k

No electric displacement the dielectric response almost conceals the topological polarization Po

r r r Po PLA O ≈ << Po k

Add electric displacement the dielectric response decreases

r r r Po PLA O ≈ D + k

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Now let’s come to the LAO side. Let’s start from the characteristic equation in the green square. If we have no electric displacement, the equation foresees a strong suppression of the topological polarization, due to the dielectric polarization. But if we have electric displacement, the dielectric polarization decreases and the net polarization increases.

Discussion

3. Discussion

2. The LAO side

…but we do observe a large polarization! So, there is a finite displacement

Add electric displacement the dielectric response decreases

r r r Po PLA O ≈ D + k

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In fact, we do observe a large net polarization. Then we conclude that there is a displacement in LAO…

3. Discussion

Discussion 2. The LAO side

…but we do observe a large polarization! So, there is a finite displacement

Consistent explanation of LAO state

D ≈ 0.3 e-/u.c. k ≈ 20 Po ≈ 0.5 e-/u.c.

P ≈ 0.32 e-/u.c.

PLAO ≈ 0.35 e-/u.c.

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and based on a few simple assumptions we deduce that it is about 0.3 electronic charges per square unit cell.

3. Discussion

Discussion 3. Where are the donors?

Experiment + simple electrostatics The electric displacement is continuous at the interface Most donors are on the top of LAO

Consistent explanation of LAO state

D ≈ 0.3 e-/u.c. k ≈ 20 Po ≈ 0.5 e-/u.c.

P ≈ 0.32 e-/u.c.

PLAO ≈ 0.35 e-/u.c.

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So, the estimated displacement in LAO is the same as in STO. In other terms, D is continuous. This brings us back to the question: Where the donors are? Well, they can’t accumulate at the interface, or we would not observe the displacement continuity. They must be on the top of LAO. This is consistent with the electronic reconstruction. Can we exclude at all that some donors are at the interface? Well we can’t, the experimental errors are large enough to allow for a fraction of donors to be there. But most of them must be far away in LAO.

Conclusions 1. LAO/STO Interfaces “with a lot of” V(O) do conduct 2. LAO/STO Interfaces “with negligible” V(O) content do conduct

3. Different types of donors bring to essentially the same 2DEG a) In amorphous samples, V(O) are the donor states b) In samples with negligible V(O), the polarization state of LAO is compatible with the electronic reconstruction

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Conclusions 1. LAO/STO Interfaces “with a lot of” V(O) do conduct 2. LAO/STO Interfaces “with negligible” V(O) content do conduct

3. Different types of donors bring to essentially the same 2DEG a) In amorphous samples, V(O) are the donor states b) In samples with negligible V(O), the polarization state of LAO is compatible with the electronic reconstruction

Thank you for your attention!

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