This document describes an investigation of the LaAlO3-SrTiO3 (LAO-STO) heterointerface using transmission electron microscopy (TEM). The sample was prepared using pulsed laser deposition to grow a thin film of LAO on a STO substrate, followed by ion slicing to produce a wedge-shaped cross-section for TEM analysis. The TEM results revealed a high-density two-dimensional electron gas formed at the LAO-STO interface, which has potential applications in next-generation electronic devices and holds promise for novel electronic properties.

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TEM INVESTIGATION OF LAO-STO
HETEROINTERFACE
Submitted by:
Aryan Sinha (2012UMT1770)
Department of Metallurgy and Materials Engineering
Malaviya National Institute of Technology, Jaipur, India
Submitted to:
Dr. A.K Shrivastava
Dr. N.C Upadhyay
Dr. V.K Sharma
Supervised by:
Prof. Stephen Pennycook
Professor of Materials Science and Engineering
National University of Singapore
Singapore

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CERTIFICATE
This is to certify that the project titled TEM investigation of LAO-STO
heterointerface is a bona fide record of the work carried out by Aryan Sinha
under my supervision and guidance, during his internship at National University
of Singapore, Singapore for the partial fulfillment of the requirements for the
degree of Bachelor of Technology in Metallurgy and Materials Engineering at
Malaviya National Institute of Technology Jaipur, India.
PROF. STEPHEN PENNYCOOK
Dept. of Materials Science and Engineering
National University of Singapore

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ACKNOWLEDGEMENT
I feel immense pleasure and privileged to express my heartfelt gratitude to
Prof. Stephen Pennycook, Professor of Materials Science and Engineering,
National University of Singapore, Singapore as it is because of his belief,
inspiration, guidance and sincere support that I have been able to dedicate myself
to this project with zeal and enthusiasm.
I would like to thanks Prof. T.Venky Venkatesan and the entire team of NUSNNI
for their help in my work. They provided me with all the essential knowledge and
expert assistance required, clearing all my doubts and curious queries with
commendable patience.
I would also like to thanks Jeol Asia for their kind support. I not only enjoyed this
project but also learned a lot of new things by the time I was doing this project.
Without all of them, the successful accomplishment of this project would have
been a distant reality.
ARYAN SINHA
Dept. of Metallurgy and Materials Engineering
Malaviya National Institute of Technology, Jaipur
July 31, 2015

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CONTENTS
1) MOTIVATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2) SAMPLE PREPARATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
3) PULSED LASER DEPOSITION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
4) ION SLICING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
5) RESULTS AND ANALYSIS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
6) CONCLUSION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
7) BIBLIOGRAPHY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

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MOTIVATION: Why are we interested in oxide
interfaces?
The next generation of electronic devices faces the challenge of adequately containing
and controlling extremely high charge densities within structures of nanometer
dimensions. Silicon-based electronics has been extremely successful. Silicon metal–
oxide–semiconductor field effect transistors (MOSFETs) are based on a two-dimensional
electron gas (2DEG) that is formed at the interface between Si and a dielectric, and
electrostatically controlled by a gate voltage. As devices are scaled to atomic dimensions
the capacitance density and the switched current density must both increase. Atomic-
scale transistors must be thin and be able to control extremely high charge densities
(> 1013
cm−2
). Silicon devices typically have 2DEG densities around 1012
cm−2
.To go
beyond that, novel materials that can support extremely high charge densities and high
electric fields need to be explored.
Oxide heterostructures have been shown to exhibit unusual physics and hold the
promise of novel electronic applications. The formation of a two – dimensional electron
gas (2DEG) at the interface between a polar and non – polar perovskite oxide has
received widespread attention [1]. Such high density electron gases could be employed
in next generation electronic devices.
Complex oxides have recently emerged as an attractive materials system to support
these developments. The demonstration of a 2DEG at the LaAlO3 /SrTiO3 (LAO/STO)
interface has triggered an avalanche of research. 2DEG densities up to 3 × 1013
cm−2
can
be achieved in LAO/STO in spite of the large band gaps of its bulk constituents. In
addition, the interfaces have been reported to display unique behavior such
as superconductivity, ferromagnetism, large negative in-plane magneto resistance, and
giant persistent photoconductivity. The study of how these properties emerge at the
LaAlO3/SrTiO3 interface is a growing area of research in condensed matter physics.
So, the motivation behind working on this project is the presence of high mobility two
dimensional electron gas (2DEG) at the interface of LAO-STO which holds the promise
for novel electronic applications and which could be efficiently employed for next
generation electronic devices.

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SAMPLE PREPARATION
The sample preparation consists of 2 stages –
i) Epitaxial growth of LAO thin film on STO substrate using Pulsed Laser
Deposition (PLD)
ii) TEM sample preparation of this thin film using Ion Slicing.
The following subsections elucidate each of these materials in brief.
Materials used:
Lanthanum Aluminate (LAO):
LaAlO3 is a polar perovskite based insulator with a high band gap of 5.6 eV. It has been
used as the insulator sandwiched between two electrodes. LAO is used due to the
possibility of fabricating high crystalline quality thin films. Though LAO single crystals are
reddish brown in color, thin films of LAO about 100 nms thick were found to be
transparent. LAO’s crystal structure is a rhombohedral distorted perovskite with a
pseudocubic lattice parameter of 3.863 angstroms.
Strontium Titanate (STO):
SrTiO3 is a non – polar perovskite based insulator with a band gap of 3.2 eV. It is an
excellent substrate for epitaxial growth of high temperature superconductors and many
oxides based thin films. Doping STO with niobium makes it electrically conductive, being
one of the only conductive commercially available single crystal substrates for the
growth of perovskite oxides. Its bulk lattice parameter is 3.905 angstrom makes it
suitable as a substrate for the growth of many other oxides, including the rare-earth
manganites, titanates, LAO and many others.
Oxygen vacancies are fairly common in SrTiO3 crystals and thin films. Oxygen vacancies
induce free electrons in the conduction band of the material, making it more conductive
and opaque. These vacancies can be caused by exposure to reducing conditions, such as
high vacuum at elevated temperatures.
High quality, epitaxial SrTiO3 layers can also be grown on Silicon without forming SiO2,
thereby making it an alternative gate dielectric material. This also enables the
integration of other thin film perovskite oxides onto Silicon.

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Fig 1: SrTiO3 and LaAlO3: Wide band gap oxides
The above explained materials were deposited using Pulsed Laser Deposition
(PLD) technique. An overview of this revolutionary technique has been given in
the following section.

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PULSED LASER DEPOSITION
Thin films have always been of great importance in the electronics manufacturing industry.
Depositions of thin films are mainly carried out by various chemical and physical methods.
Pulsed Laser Deposition is a versatile technique for growing thin films and can be applied to
wide range of materials.
Growth of Epitaxial thin films
Fig 1: Schematic diagram of a PLD chamber
In this method a high powered pulsed laser beam inside a vacuum chamber is made to strike a
target of the material to be deposited. The material or target vaporizes in the form of a plasma
plume and it deposits as a thin film on a substrate facing the target. The 20 to 30 nanosecond
wide laser pulse is focused to an energy density of 1 to 5 J/cm2
to vaporize a few hundred
angstroms of surface material. The vapor contains neutral atoms, positive and negative ions,
electrons, molecules and molecular ions, free radicals of the target material. These particles
emit radiation and acquire kinetic energy of 1 to 5 eV and move in the direction perpendicular
to the target. The sum total of all the particles corresponds to the chemical composition of the
target. They deposit on a substrate generally heated to a temperature to grow crystalline film.

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Generally, KrF (248 nm) and XeCl (308 nm) UV excimer lasers are used in PLD. The system
consists of a vacuum chamber which is evacuated by a pump to 10-6
Torr. The laser beam
incident at an angle of 45 degrees to the target surface is focused on to the rotating target,
which is a single crystalline pellet of the material whose film is to be deposited. Laser beams of
energy 180 mJ to 400 mJ are focused to a size such that energy density is maintained between 1
to 5 J/sq.cm. The target holder is designed to have six targets so that multilayer film can be
grown in situ. The substrate on which the film is to be grown is placed opposite to the target.
Substrates can be heated to 800o
C using a heater attached to it. Gas inlet facility is provided so
that films can be grown in oxygen or any other gaseous environment. Generally, 300 to 400
mTorr gas pressure is employed during the growth of the film; however the optimum pressure
requirement varies for each substrate target combination. For growing films other than oxides,
base vacuum needed is 10-9
Torr. The structural layout of a PLD chamber can be understood
from fig 1 and fig 2.
Fig 2: Image of Laser Plume inside a PLD chamber
Deposition Parameters
The five main deposition parameters for the growth of thin films using PLD are distance
between target and substrate, partial pressure of oxygen, energy inside the chamber,
repetition rate of laser pulse and substrate temperature.
Most of the thin films deposited by PLD technique are perovskite oxides.

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Deposition of LAO on STO single crystal substrate:
A single crystal LAO target was used to deposit LAO thin film of 5 unit cells thickness on STO
single crystal substrate. While deposition it should be noted that the laser pulse is incident on
the centre of the target so as to ensure efficient utilization of the single crystal target. The
deposition parameters were as follow –
Target – Substrate distance: 4.5 cm
Partial Pressure of oxygen: 0.3 millitorr
Energy inside the chamber: 90 mJ
Repetition rate of laser pulse: 2 Hz
Substrate temperature: 760o
C
The single crystal STO substrate was pre – etched by Hydrofluoric acid and annealed at 950o
C
for 2 hours. After deposition the sample was cooled in the PLD chamber to room
temperature at a ramp rate of 20o
C / min to prevent the cracking of the thin film.

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ION SLICING
Ion Slicing is a low-energy broad-ion-beam shadowing technique which produces a
wedge of which the tip is electron transparent. A copper belt located precisely above
the narrow edge (30–100 um thick) of the sample shields a lamellar sample portion from
the beam. The sample portion directly beneath the belt is protected from the ion beam.
The argon ion beam slices off the protruding sample parts on both sides of the belt and
creates a large elongated wedge. So the wedge is created by letting the incident beam
hit the protruding sample parts almost at a right angle. The copper belt itself deflects
the beam slightly contributing to the before mentioned inclination of the incident beam,
which alternates between front and backside during the slicing procedure. The
inclination of the incident angle can be varied from parallel 0o
to 6o
with respect to the
plane of the sample surface and Cu-belt. Since the developing thin film is located almost
parallel to the beam propagation direction, it is almost unaffected from any irradiation
damage and a phase dependent preferred thinning is not observed.
Fig1: Schematic diagram of the Ion Slicer
An acceleration voltage between 0.5 and 8 kV can be applied. The entire sample stage is
rocked from side to side while the slicing is in process, this would reduce the effect of
preferential slicing. A camera located in the slicing chamber enables observation during
the thinning procedure. This is extremely important because treatment times can vary

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greatly depending on sample thickness and material properties and the slicing process
has to be interrupted by the operator manually.
Ion Slicing can prepare thin film specimens without solvents or chemicals and requires
no prior treatment of the specimen other than rectangular slicing (no disc grinding or
dimple grinding). The ion slicer prepares thin – film specimens faster and easier than
conventional preparation tools. It can efficiently prepare thin films from specimens
having different compositions, even those having porous composites. The advantages of
using ion slicing method are minimal surface damage, fast preparation and no
complicated pre - treatments.
Fig 2: Images showing the positions of the belt and specimen inside the Ion Slicer
TEM sample preparation of cross-sectional LAO-STO
A board like sample with dimensions 3mm X 1 mm was cut from LAO-STO sample using
diamond cutter. Silicon with approximately same dimensions was stick on the top of the
thin film LAO as a protective layer using G2 glue. The specimen was then mounted on
the spacer with mounting wax and is smoothened on the sides using diamond film to
rub off the extra silicon. The thickness of the top protective Si layer was reduced until it
becomes very thin. The sample thickness was then reduced to dimensions less than 100
um by hand lapping it on 30 um and 9 um diamond film consecutively.
The layout of the cross–section sample can be seen in the Fig 1.

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Fig 3: Image showing the layout of the cross- section LAO-STO sample
The sample was mounted onto the specimen holder with the help of alignment tool
using wax. The sample was then loaded into the Ion Slicing machine.
Fig 4: Images showing the sample placed on the spacer and specimen holder
There are two steps involved in order to make a cross-sectional sample by Ion Slicing
method.

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Step 1:
The step 1 consists of milling the sample at low tilt angle (0 – 0.5o
). The controlling
parameters were as follow:
Pressure: 3.1E-3 Pa
Voltage: 6 kV
Argon flow rate: 7.2
Tilt angle: 0.5o
Milling time: 11 hours
Fig 5: Images showing the sample at the start and at the end of the Step 1
Step 2:
The step 2 consists of milling the sample at high tilt angle (3.5o
-5o
). This step is done
without using masking belt. The controlling parameters were as follow:
Pressure: 1.6E-3
Voltage: 6 kV
Argon flow rate: 7.1
Tilt angle: 4o
Milling time: 2 hours
Fig 6: Images showing the sample at the start and at the end of the Step 2

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RESULTS AND ANALYSIS
Measurement of Conductivity
The conductivity of LAO/STO was measured using the following concept. Two
wires were connected across the LAO/STO sample and hence the resistances due
to LAO, the interface and STO were assumed to be connected in parallel. The
image shows the layout of the experiment conducted to measure the conductivity
of the sample.
Fig 1: Image showing the simple circuit diagram to measure the conductivity
of the LAO/STO sample.
Since LAO and STO are large band gap insulators, so the resultant conductivity
was because of the interface only. (1/RLAO = 1/RSTO = 0)
REffective = RInterface
The high conductivity shown by LAO/STO interface was due to the formation of
two-dimensional electron gas (2DEG). Two dimensional does not mean that
conductivity has zero thickness but rather the electrons are confined to move
only in two directions.

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Fig 2: Image showing the Sheet resistance (Rs) versus Temperature (T) of 5
unit cells of LAO on STO prepared at 850 0C under different oxygen partial
pressures (P O2) of 10-5, 10-4, 10-3 and 10-2 mbar.
Various theories have been proposed explaining the mechanism for the formation
of two dimensional electron gas (2DEG) at the interface. Some theories suggest
that the metallic channel is formed due to the polar discontinuity or by the
mechanism of the electronic reconstruction whereas others suggest that the
electronic properties are because of the oxygen vacancies.
The conductivity is higher at lower temperature because the electrons are
localized at this temperature. However, an increase in temperature increases the
vibrations of the atoms which results in the scattering of the electrons and hence
the conductivity decreases with increase in temperature.

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TEM EXAMINATION
In TEM examination, we observed crystalline LAO and STO layer. The thickness of
LAO thin film was around 2 nm. However, there was some inter diffusion layer
between LAO and STO which was quite surprising and exciting.
Fig 1: Cross-sectional bright-field TEM images of 2 nm thick LAO thin film grown on a
single crystal STO substrate at different resolutions. There is a inter diffusion layer
between LAO and STO. Arrow indicates the thickness of LAO layer.
The reasons for the formation of inter diffusion layer could be some dislocations,
defects, oxygen vacancies introduced during annealing or may be due to the
deposition method used for growing thin film. The chemical characteristics of this
inter diffusion layer can be studied by Electron energy loss spectroscopy (EELS)
which can give us exact reason for their formation.

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CONCLUSION
High quality LAO thin film was grown epitaxially on a single crystal STO substrate
using Pulsed Laser Deposition. Then we prepared cross-sectional LAO-STO sample
for TEM examination using Ion Slicing method. The TEM examination showed
crystalline LAO and STO layer with some inter diffusion layer at the interface
which can be quite interesting and exciting to study. The reasons for the
formation of inter diffusion could be some dislocations, defects, oxygen vacancies
introduced during annealing or may be due to the deposition method used for
growing thin film. The high conductivity shown by LAO-STO in spite of the large
band gap of its bulk constituents due to the formation of 2D electron gas at the
interface was quite stimulating. This conductivity certainly could be the pathway
for the next generation electronic devices!!
The elemental analysis or the chemical characteristics of this inter diffusion layer
can be studied by Electron energy loss spectroscopy (EELS) and Energy-dispersive
X-ray spectroscopy (EDX). This study can give us the reasons for the formation of
inter diffusion layer which can be quite confounding!!! Also the study of LAO-STO
under aberration corrected scanning transmission electron microscope can give
us the exact arrangement of the atoms at the interface and hence the mechanism
for the formation of 2D electron gas. I hope to return to NUS later in the year to
pursue this research further with their aberration-corrected microscope which
will be operational by that time.

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BIBLIOGRAPHY
1. Ohtomo, A.; Hwang (29 Jan 2004). A high-mobility electron gas at the
LaAlO3/SrTiO3 heterointerface. Nature 427 (6973): 423–426.
2. Gariglio, S; Reyren, Caviglia, Triscone (31 March 2009). Superconductivity at
the LaAlO3/SrTiO3 interface. Journal of Physics: Condensed Matter 21 (16):
164213.
3. Cantoni; Gazquez, Granozio; Oxley, Varela; Lupini, Pennycook; Aruta, Uccio;
Perna, Maccariello (19 June 2009). Electron Transfer and Ionic Displacements
at the Origin of the 2D Electron Gas at the LAO/STO Interface: Direct
Measurements with Atomic-Column Spatial Resolution. Advanced
Materials 24 (29): 3952.
4. Perna, P.; Maccariello, Radovic, Scott di Uccio, Pallecchi, Codda, Marre,
Cantoni, Gazquez, Varela, Pennycook, Granozio (14 October 2010).
Conducting interfaces between band insulating oxides: The
LaGaO3/SrTiO3 heterostructure. Applied Physics Letters 97 (15): 152111
5. Z. Q. Liu; C. J. Li, W. M. Lu, Z. Huang, S. W. Zeng, X. P. Qiu, L. S. Huang, A.
Annadi, J. S. Chen, J. M. D. Coey, T. Venkatesan, Ariando (30 May 2013). Origin
of the two-dimensional electron gas at LaAlO3/SrTiO3 interfaces - The role of
oxygen vacancies and electronic reconstruction. Physical Review X 3 (2):
021010
6. Ariando; X. Wang, G. Baskaran, Z. Q. Liu, J. Huijben, J. B. Yi, A. Annadi, A. Roy
Barman, A. Rusydi, S. Dhar, Y. P. Feng, J. Ding, H. Hilgenkamp & T. Venkatesan
(8 February 2011). Electronic phase separation at the
LaAlO3/SrTiO3 interface. Nature Communications 2: 188.
7. scholarbank.nus.edu.sg