Study by Transmission Electron Microscopy of the

Microstructure of La2Zr2O7 (LZO) Thin films grown

on LaAlO3 and Si. LZO are obtained by MOCVD or

MOD deposition.

Long Term Practical work

By :Tesfaye Belete and Jens Q Adolphsen

April 2011

Grenobel France

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Table of Contents Introduction .......................................................................................................................................... 4

LZO Deposition Techniques ................................................................................................................ 5

A. Metal Organic Chemical Vapor Deposition (MOCVD) .............................................................. 5

B. Metal–organic Deposition (MOD) .............................................................................................. 7

Characterization Techniques ................................................................................................................ 8

A. The Transmission Electron Microscope (TEM) .......................................................................... 8

TEM parts and working principles ................................................................................................ 10

Electron gun ............................................................................................................................... 10

Electromagnetic lenses............................................................................................................... 10

Lens Aberrations ........................................................................................................................ 11

Spherical aberration: .................................................................................................................. 11

Chromatic aberration ................................................................................................................. 12

Astigmatism ............................................................................................................................... 12

Imaging and Diffraction in the TEM ............................................................................................. 12

Imaging mode ............................................................................................................................ 13

Diffraction mode ....................................................................................................................... 13

Contrast ...................................................................................................................................... 14

Chemical Analyses by Energy-dispersive X-ray spectroscopy (EDS) .......................................... 14

X-Rays and X-rays Diffraction .......................................................................................................... 15

Continuous and Characteristic X-ray Spectra ................................................................................ 16

X-ray Diffraction and Bragg's Law ................................................................................................ 17

Experimental ...................................................................................................................................... 18

Sample preparation mechanism ..................................................................................................... 19

A. Preparation LZO/LAO samples obtained by MOD&MOCVD for TEM ............................. 19

B. Preparation of LZO/Si sample for TEM ............................................................................... 24

Result ................................................................................................................................................. 24

(i).LZO obtained by MOCVD on LAO single crystal ................................................................... 24

(ii). LZO obtained by MOD on LAO single crystal ...................................................................... 26

The X rays Experiments................................................................................................................. 26

Cross-section observation .............................................................................................................. 28

(iii).LZO obtained by MOD on Si single Crystal .......................................................................... 30

Conclusion ......................................................................................................................................... 33

References .......................................................................................................................................... 34

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Figure 1: Rolling-assisted biaxially textured substrates (RABiTs) stack ............................................ 4

Figure 2: Metal-Organic Chemical Vapor Deposition (MO-CVD reactor setup ................................. 5 Figure 3: Schematic representation of CVD deposition ...................................................................... 6 Figure 4: MOD deep coating techniques ............................................................................................. 8

Figure 5: Different signals generated from sample under incident electron beam (a) and Schematic

diagram of TEM (b) ............................................................................................................................. 9 Figure 6: The geometry of an electron gun. ....................................................................................... 10 Figure 7: Electromagnetic lens .......................................................................................................... 11 Figure 8: spherical aberration ............................................................................................................ 11

Figure 9: Chromatic aberration in Electromagnetic lens ................................................................... 12 Figure 10: Schematic diagram showing image mode and diffraction mode ..................................... 12 Figure 11: Different images obtained under TEM ............................................................................. 13 Figure 12: Diffraction ring of Polycrystalline material ..................................................................... 13

Figure 13: Single crystal (Si) diffraction ........................................................................................... 14 Figure 14: TEM EDS of LZO film and present of Cu from TEM sample holder ............................. 15

Figure 15: X-ray tube5 ....................................................................................................................... 16 Figure 16: Continuous x-rays spectrum ............................................................................................. 17

Figure 17: characteristic x-rays spectrum .......................................................................................... 17 Figure 18: Bragg’s diffraction from crystal lattice plane ................................................................... 18 Figure 19: Bulk sample (a) and Sample cut from the bulk (b). ......................................................... 19

Figure 20: Sandwiched sample with glue (a) and sample compressor (b) ........................................ 20 Figure 21: Diamond grinding disks of different grain sizes (a) and Tripod (b). ................................ 20

Figure 22: Grinding machine ............................................................................................................. 21 Figure 23: Inverted microscope ......................................................................................................... 22 Figure 24: Detaching the sample with acetone from tripod ............................................................... 22

Figure 25: Schematics of sample transfer to the copper holder ....................................................... 22

Figure 26: Precision ion polishing system ......................................................................................... 23 Figure 27: TEM sample holder and sample ....................................................................................... 23 Figure 28: Bright-field image of the LZO/LAO structure with low magnification ........................... 24

Figure 29: Cross section of LZO/LAO at high magnification ........................................................... 25 Figure 30: LZO Pyrochlore film ........................................................................................................ 25

Figure 31: Pole figure measurement6 ................................................................................................. 26

Figure 32: The 2 Theta scale LZO and LAO. .................................................................................... 27

Figure 33: Theta scale Rocking Curve ............................................................................................... 28 Figure 34: Low magnification of LZO/LAO (a) and high magnification image of LZO layer (b) ... 29 Figure 35 LZO/LAO diffraction and image. ...................................................................................... 29 Figure 36: The nanovoids observed in LZO Fluorite film due to MOD technique. .......................... 30 Figure 37: Low magnification image of LAO/Si sample .................................................................. 31

Figure 38: High magnification image of LZO/Si sample .................................................................. 31 Figure 39: Polycrystalline LZO fluorite diffraction (a) and Si Single Crystal diffraction (b). .......... 32

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Introduction

Superconductor materials are known primarily through their electrical properties at some relatively

low temperature their electrical resistance is exactly zero, and for this reason we use them for

electric power transmission lines cable.

Coated conductors developed from rolling-assisted biaxially textured substrates (RABiTs) are

considered a low-cost architecture: nickel-based textured substrate is used as a template to biaxially

grow the superconductingYBa2Cu3O7 (YBCO) layer. But YBCO layer cannot be deposited directly

on nickel because of the chemical interaction of nickel with YBCO at the usual deposition

temperature. Good results are obtained when using lanthanum zirconate, La2Zr2O7 (LZO), as the

first buffer layer. Metal organic decomposition (MOD) allows the growth of LZO on Ni-based

substrates, which reduces the oxidation of the substrate.1

Figure 1: Rolling-assisted biaxially textured substrates (RABiTs) stack

LZO can crystallize into two different structures, LZO pyrochlore structure (Space Group: Fd3m;

73-0444 with lattice parameter a = 10.808 A˚) and LZO fluorite structure (Space group: Fm-3m; 75-

0346 with lattice parameter a = 5.407 A˚). Pyrochlore LZO behaves as a buffer layer in this

architecture because its lattice parameters match those of YBCO [(𝜀YBCO–𝜀LZO)/ 𝜀LZO =

1.05%], and pyrochlore structure is more stable against O2 diffusion because its cations are well

ordered. The presence of the pyrochlore phase can be determined by the (111) and (331) reflections,

corresponding to spacing values of 6.24 and 2.48 A˚ with an intensity of 3% and 5%, respectively,

which do not exist in the fluorite structure1.

The crystal structure of LZO was grown by MOD on Silicon and LaAlO3 (LAO) single crystal

substrate, and LZO grown by MOCVD on LAO substrate was studied by high-resolution

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transmission electron microscope and LZO grown by MOD on LAO was studied by x-ray

diffraction (XRD). Results obtained in this study can be applied to coated conductors if the growth

mechanisms are independent of substrate. Results on XRD are compared with the electron

diffraction obtained by TEM to determine the fluorite or pyrochlore formation. The microstructure

of these films is related to the synthesis method and a characteristic feature is the presence of

nanovoids in a well-crystallized matrix, which is LZO grown by MOD technique.

LZO Deposition Techniques

A. Metal Organic Chemical Vapor Deposition (MOCVD)

The LZO film was deposited on the LAO substrate with either MOD or MOCVD deposition

technique. The deposition technique used when growing a thin film is crucial to the film quality

obtained. However, several considerations need to be made when determining the method for film

growth. Among these are temperature, cost, time etc.

Metal organic chemical wafer deposition (MOCVD) is a deposition method, which is used to grow

thin films. The process makes use of precursors, which in this case are metalorganic and growth is

chemical.

The CVD system consists of: (i).Gas sources and feed line. The gases are both inert gases and

precursor(s). (ii).Mass flow controllers for metering the gases into the system. (iii). A reaction

chamber where the actual film is grown. (iv). A heating source to heat up the substrate where the

film is grown. (v). Temperature and pressure sensors in order to control the growth of the film.

(vi).An exhaust pump to get rid of by products2.

Figure 2: Metal-Organic Chemical Vapor Deposition (MO-CVD reactor setup

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The basic CVD process consists of several steps. 1). A specific mix of precursors and inert gases are

mixed and introduced into the reaction chamber. 2). Precursor molecules are transported to the

surface by diffusion. It is noted that the flow of gas through the reaction chamber is laminar. 3).The

precursors are transported to the substrate primarily by diffusion. Under this process the gas

molecules react 4).The precursor molecules adsorb to the surface of the substrate after migration

5).The molecules react with the free bonds at the surface creating the film and leaving one or more

byproducts. 6).The byproducts will desorbs and diffuse away from the surface eventually leaving

the chamber with the gas flow. The amount of reactants converted to film is governed by chemical

reaction thermodynamics and the speed of the chemical process is governed by the chemical

reaction kinetics. At low temperatures the growth rate is typically limited by the lowest reaction rate

in one of the process steps of the chemical reaction know as”surface limited”. At high temperatures

where the reaction rates are typically high the limiting factor is the flux of precursor molecules to

the surface known as “mass transport limited”.

Figure 3: Schematic representation of CVD deposition

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B. Metal–organic Deposition (MOD)

The metal–organic deposition (MOD) method is widely used to prepare oxide films because of its

many advantages, such as low cost and easy operation compared to the physical deposition

technology1. That also makes it a possible choice for large scale production of coated

superconductors.

The MOD deposition method is also chemical but liquid based and is basically a method where the

substrate is dipped in a liquid solution and later pulled up again creating a thin film. An alternative

method is spin coating where the liquid is applied while the substrate is spinning. This method is

often used in the microelectronics industry but will not be covered in this section. To begin with the

liquid solution containing metal organic precursors need to be prepared. The next step is attaching

the substrate to a holder, which is operated at a constant speed. Now the actual process can begin,

which consists of three main steps:

Immersion: the substrate is immersed into the solution at a slow, constant speed preferably without

any shaking.

Start-up: The substrate remains in the solution completely covered and is held still to allow for the

desired molecules to cover it. The substrate is now ready to be pulled-up again.

Deposition: The substrate is pulled up at a constant speed. At this stage a thin layer will deposit on

the substrate as it is pulled up. Excess liquid will drain from the surface and the excess solvent will

evaporate from the surface. Depending on what kind of film is deposited evaporation might start

already when the substrate is pulled up. This is for example the case with solvents containing

alcohol.

The last part after the film has dried is an annealing step. This is done to improve the crystalline

structure and adhesion of the film and to eliminate organic impurities.The thickness of the film is

governed by the following equation:

h = 0.94(𝜂.𝑣)2/3

𝛾16(𝜌.𝑔)1/2

1 M.S. Bhuiyan, M.P. Paranthaman, K. Salama, Supercond. Sci.

Technol. 19, R1 (2006)

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Where

h is the thickness of the film,

𝜂 -is the viscosity of the precursor,

v is the speed the sample is pulled up with,

𝛾 is the surface tension from liquid to vapor,

𝜌 the density of the liquid and

g is gravity.

From the formula it is worth noting that increasing the pull speed will actually increase the film

thickness.

Figure 4: MOD deep coating techniques

Characterization Techniques

A. The Transmission Electron Microscope (TEM) The transmission electron microscope (TEM) is used widely in material science to understand the

physical and chemical properties of materials. It uses beam of electrons that transmitted through

very thin sample of 10-100nm thickness.

TEMs are capable of imaging at a significantly higher resolution than light microscopes, owing to

the small de Broglie wavelength of electrons. This enables the instrument's user to examine fine

detail, even as small as a single column of atoms, which is tens of thousands times smaller than the

smallest resolvable object in a light microscope. TEM forms a major analysis method in a range of

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scientific fields, in both physical and biological sciences. TEMs find application in cancer research,

virology, materials science as well as pollution and semiconductor research3.

Electrons are usually generated in an electron microscope by thermionic emission from hot filament

of tungsten and it is accelerated by an electric potential then and focused by electromagnetic lenses

onto the sample. Unlike that of light microscope which uses glass lens, in TEM we use

electromagnetic lens in which we can manipulate the electron beam. In a TEM, the electrons are

accelerated at high voltage (100-1000 kV) to a velocity approaching the speed of light and the

associated wavelength is five orders of magnitude smaller than the light wavelength (0.04-0.008 Å).

When those electrons are focused on a material, they can scatter or backscatter elastically or

inelastically, or produce many interactions, source of different signals such as X-rays, Auger

electrons and backscattered electrons but we use the transmission electron for TEM. There are

different information obtainable from TEM: image, morphology, crystal structure, crystal defects

atomic structure,(chemical) elemental composition and electronic structure

Figure 5: Different signals generated from sample under incident electron beam (a) and Schematic diagram of TEM (b)

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TEM parts and working principles

Electron gun

It is the first and basic part of the microscopes and the source of electrons. It is usually a V shaped

filament made of LaB6 or W (tungsten) that is wreathed with Wehnelt electrode (Wehnelt Cap). The

two usual types of electron guns are the conventional electron guns and the field emission guns

(FEG).

In conventional electron guns (Fig.6) had a positive electrical potential applied to the anode and the

filament (cathode) is heated until a stream of electrons is produced. A field emission gun consists of

a sharply pointed tungsten tip held at several kilovolts negative potential relative to a nearby

electrode, so that there is a very high potential gradient at the surface of the tungsten tip. FEGs

produce much higher source brightness than in conventional guns (electron current > 1000 times),

better monochromaticity, but requires a very good vacuum ~10-7

pa

Figure 6: The geometry of an electron gun.

Electromagnetic lenses It consists of a coil of copper wires inside the iron pole pieces (Fig.7). In TEM we have different

magnetic lenses:-condenser, objective, intermediate and projector which act to illuminate the

specimen and focus/magnify the specimen on the fluorescent screen.

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Condenser lens controls the beam intensity and convergence coherence, objective lens controls the

magnification and used to introduce contrast, intermediate lens is used for further magnification and

diffraction while, objective lens is used for final magnification. The focusing effect of a magnetic

lens therefore increases/decreased with the magnetic field B, which can be controlled via the current

flowing through the coils.

Figure 7: Electromagnetic lens

Lens Aberrations Theoretically all rays emanating from a point in the object

plane come to the same focal point in the image plane. In

reality, all lenses have defects. The defects of most

importance to us here are spherical aberration; chromatic

aberration and astigmatism.

Spherical aberration: It occurs when parallel light rays that pass through the

central region of the lens focus farther away than light rays

that pass through the edges of the lens. The result is many

focal points, which produce a blurry image. So rather than

a clearly defined focal point, we end up with a “disk of

minimum confusion” in each instance as in Fig 8.

Figure 8: spherical aberration

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Chromatic aberration Chromatic aberration occurs when electron beam generated

by the gun having different energies at the same location in

the column will experience different forces. Thus an

electromagnetic lens will “bend” electrons of lower energy

more strongly than those of higher energy. For spherical

aberration, a disk of minimum confusion (dC) is produced

(Fig 9).

Astigmatism The electromagnetic lenses used in the TEM cannot be machined to perfect symmetry. Thus a lack

of symmetry would result in an oblong beam: the narrower diameter due to the stronger focusing

plane; the wider diameter due to the weaker focusing plane. The net effect is the same as that of the

aberrations above a disk of minimum confusion

rather than a well defined point of focus.

Imaging and Diffraction in the TEM

The objective lens forms a diffraction pattern in

the back focal plane with electrons scattered by

the sample and combines them to generate an

image in the image plane (1. intermediate image).

Thus, diffraction pattern and image are

simultaneously present in the TEM. It depends on

the intermediate lens which of them appears in the

plane of the second intermediate image and

magnified by the projective lens on the viewing

screen. Switching from real space (image) to

reciprocal space (diffraction pattern) is easily

achieved by changing the strength of the

Figure 9: Chromatic aberration in Electromagnetic lens

Figure 10: Schematic diagram showing image mode and diffraction

mode

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intermediate lens.

In imaging mode, an objective aperture can be inserted in the back focal plane to select one or more

beams that contribute to the final image (Bright and dark field). For selected area electron

diffraction (SAED), an aperture in the plane of the first intermediate image defines the region of

which the diffraction is obtained4.

Imaging mode In image mode the scatter is focused by the objective lens to the first intermediate image plane

which subsequently acts as the object plane for the magnifying lenses. In image mode we can see

the microstructure, morphology, defects, dislocation, grains, interfaces can be observed as in Fig.11

Diffraction mode Rays emanated from the specimen that is parallel to

one another come to focus in the back focal plane of

the objective lens. The selected area diaphragm is

used to select only one part of the imaged sample for

example a particle or a precipitate. This mode is

called selected area diffraction SAED. The spherical

aberrations of the objective lens limit the area of the

selected object to few hundred nanometers.

Nevertheless, it is possible to obtain diffraction

patterns of a smaller object by focusing the electron beam with the projector lenses to obtain a small

spot size on the object surface (2-10 nm).

Figure 11: Different images obtained under TEM

Figure 12: Diffraction ring of Polycrystalline material

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The Diffraction image we obtain is either

dotes for single crystal(Fig.13) sample or

ringes for polycrystaline samples(Fig.12).

Contrast

A high resolution signal is worthless if adequate contrast is absent. We have amplitude contrast and

phase contrast. Differences in intensity due to amplitude contrast result from the scattered electrons

being intercepted by the objective aperture and thus not contributing to image formation. While,

differences in intensity due to phase contrast arise from interference effects between scattered and

un-scattered electrons which pass through the objective aperture.

Chemical Analyses by Energy-dispersive X-ray spectroscopy (EDS) As inelastic interactions between electrons and matter give different kinds of signals: secondary

electrons, Auger electrons, X-rays, light and lattice vibrations (Fig5). The X-ray energy corresponds

to a difference between K and L energy levels of the electron cloud of an atom can be detected. The

X-rays emitted are detected by a semi-conductor and processed by a detector protected by an

ultrathin window and cooled at liquid nitrogen temperature to avoid the thermal noise and the

diffusion of the dopant in the semi-conductor. The EDS spectrum is constituted by a background

produced by the Bremsstrahlung X-rays and by peaks characteristic to the chemical elements of the

material, as shown in Fig.14.The identification is quite straightforward for elements beyond carbon

when the peaks do not overlap. The drawback of EDs is light atoms are not easily detected.

Figure 13: Single crystal (Si) diffraction

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Figure 14: TEM EDS of LZO film and present of Cu from TEM sample holder

X-Rays and X-rays Diffraction X-rays are electromagnetic radiation with wavelengths between about 0.02 Å and 100 Å,

corresponding to frequencies in the range of 3×1016

Hz to 3×1019

Hz. X-rays have wavelengths

similar to the size of atoms, they are useful to explore crystals. The energy of X-rays, like all

electromagnetic radiation, is inversely proportional to their wavelength as given by the Einstein

equation:

E = hν = hc/λ

Where E = energy

h = Planck's constant, 6.62517 x 10-27

erge.se

ν = frequency

c = velocity of light = 2.99793 x 1010

cm/sec

λ = wavelength

Thus, since X-rays have a smaller wavelength than visible light, they have higher energy. With

their higher energy, X-rays can penetrate matter more easily than can visible light. Their ability to

penetrate matter depends on the density of the matter.

X-rays are produced whenever high-speed electrons collide with a metal target. A source of

electrons – hot W filament, a high accelerating voltage between the cathode (W) and the anode and

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a metal target, Cu, Al, Mo, Mg. The anode is a water-cooled block of Cu containing desired target

metal.

X-rays are produced in a device called an X-ray tube. Such a tube is illustrated in Fig15.

It consists of an evacuated chamber with a tungsten filament at one end of the tube, called the

cathode, and a metal target at the other end, called an anode. Electrical current is run through the

tungsten filament, causing it to glow and emit electrons. A large voltage difference (measured in

kilovolts) is placed between the cathode and the anode, causing the electrons to move at high

velocity from the filament to the anode target. Upon striking the atoms in the target, the electrons

dislodge inner shell electrons resulting in outer shell electrons having to jump to a lower energy

shell to replace the dislodged electrons. These electronic transitions results in the generation of X-

rays.

Continuous and Characteristic X-ray Spectra When the target material of the X-ray tube is bombarded with electrons accelerated from the

cathode filament, two types of X-ray spectra are produced, continuous spectra and characteristic

spectra.

Figure 15: X-ray tube5

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The continuous spectrum consists of a range of wavelengths of X-rays with minimum wavelength

and intensity (Fig.16) (measured in counts per second) dependent on the target material and the

voltage across the X-ray tube.

The characteristic spectrum is produced at high

voltage as a result of specific electronic transitions

that take place within individual atoms of the

target material (Fig.17).

X-ray Diffraction and Bragg's Law The atomic planes of a crystal cause an incident beam of X-rays to interfere with one another as

they leave the crystal. The phenomenon is called X-ray diffraction. The atoms in crystals interact

with X-ray waves in such a way as to produce interference. The interaction can be thought of as if

the atoms in a crystal structure reflect the waves. But, because a crystal structure consists of an

orderly arrangement of atoms, the reflections occur from what appears to be planes of atoms.

For beam of X-rays entering a crystal with one of these planes of atoms oriented at an angle of θ to

the incoming beam of monochromatic X-rays as shown below:

Figure 16: Continuous x-rays spectrum

Figure 17: characteristic x-rays spectrum

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We have Bragg's Law for X-ray diffraction. There are many families of planes of various

inclinations which can be drawn through the scattering centers. Each family of planes consists of an

almost infinite set of members, all parallel and all equally spaced. Each family of planes is

characterized by its Miller indices (h, k, l). Bragg's law, as stated above, can be used to obtain

the lattice spacing of a particular cubic system through the following relation:

Diffraction from different planes of atoms produces a diffraction pattern, which contains

information about the atomic arrangement within the crystal.XRD is a nondestructive technique

used to identify crystalline phases and orientation, structural properties, lattice parameters, strain,

phase composition, preferred orientation (Laue) order-disorder transformation, and to determine

atomic arrangement.

Experimental Lanthanum zirconate was produced by MOD according to a procedure previously described.

Lanthanum (III) 2, 4-pentadionate and zirconium (IV) 2, 4-pentadionate were dissolved in

propionic acid (CH3–CH2–COOH) to form lanthanum and zirconium propionates. Propionic acid

was added to get total cation concentration of 0.6 mol/L. LZO layers were grown on LaAlO3(LAO)

single crystals and Si single crystal, To compare the effect of the deposition method on the LZO

Figure 18: Bragg’s diffraction from crystal lattice plane

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microstructure, some LZO layers were also deposited by PI MOCVD on LAO. A single liquid

source of La(tmhd)3 and Zr(tmhd)4 precursors dissolved in monoglyme at La/Zr = 1 molar ratio

was used for these experiments. i.e we have three samples to study in this practical work (i) LZO

obtained by MOCVD on LAO single crystal ,(ii) LZO obtained by MOD on LAO single crystal and

,(iii) LZO obtained by MOD on Si single crystal . The detail of the deposition techniques will not

be discussed as it is not the aim of this practical work.

To study the three samples by TEM and electron diffraction at 200kV with a 0.19nm point-by-point

resolution, the cross-sections of the samples have been prepared by Tripod method for sample (i)

and (ii) mentioned above and, for sample (iii) it was scratched with diamond pen and the scratched

powder of the sample was placed on copper that is used for holding the sample and analyzed by

TEM.

Sample preparation mechanism

A. Preparation LZO/LAO samples obtained by MOD&MOCVD for TEM It is crucial for any microscopist to know firstly the properties of the material to be analyzed and

secondly the appropriate technique and the artifacts induced by the preparation, in order to be able

to recognize these artifacts during TEM observation Therefore, the mains preparation techniques

(mechanical, ionic, electrolytic, mechanical physical) and their applications are discussed. The

sample should be electrically conductive, stable under vacuum free of hydrocarbons contamination,

should not contain artifacts that could conduct to wrong analyses.

Samples need to be prepared prior to analyzing them with TEM microscopes because they are too

thick. Initially the sample had a size of 10 mm by 10 mm and a height of 0.5-1.0mm as shown in

Fig.19.The thickness of the final prepared sample has to be less than 100nm

Figure 19: Bulk sample (a) and Sample cut from the bulk (b).

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To begin with, samples were cut into strips of 2 mm by10 mm (the height unchanged.)Fig.19. Two

such stripes were glued together in a sandwich structure with the films facing each other and placed

on a glass slides and pressed together to minimize the thickness of glue to 1µm. This was achieved

after heating them at 150 C° for 1 hour and afterwards pressing the sandwich structure together.

The samples were now ready to be reduced in thickness. The sample was attached to a tripod

(Fig.21 b) so both sample and film thickness could be reduced in transverse section. The samples

were now grinded in steps with diamond grinding disks of different grain sizes as shown below in

Fig 21.a.

Figure 20: Sandwiched sample with glue (a) and sample compressor (b)

Figure 21: Diamond grinding disks of different grain sizes (a) and Tripod (b).

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It was important that the tripod was parallel to the grinder in order to obtain an equal thickness.

Polishing the first side until 500-300 µm, then the sample was removed from the holder using

acetone (acetone dissolves glue) and the same procedure was repeated on the opposite side of the

sample. Here we use different diamond grinding disks with different revolution per minutes in order

to get the required thickness with each disk as in Table .1.

Figure 22: Grinding machine

Table 1: Grain size, rotation speed and thickness obtained in each step

Grain size(µm) Rotation speed (rev/min) Thickness of sample (µm)

30 >45 80

15 40 40

6 20-25 20

3 10-20 15-20

1 5-10 Until good quality of the

surface

0.5 <5 To no more scratches

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Therefore with the above procedures the thickness was now supposed to be reduced to 15-20 µm.

To determine the actual thickness of the samples after the diamond grinding process, an inverted

microscope (Fig.23.) with a graduation of 1 µm was used to measure the thickness of the sample.

The sample now had to be removed from the tripod holder and moved to another holder; the sample

was put in acetone on a filter paper to dissolve the glue and detach it from the first holder (Fig 24).

Figure 24: Detaching the sample with acetone from tripod

Figure 25: Schematics of sample transfer to the copper holder

Figure 23: Inverted microscope

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Since the sample was already very thin and fragile it had to be carefully put onto the new holder, a

Cu ring (Fig.25) which then attach to a thin glass plate. Second glass plate was put on top of the

sample, and now the sample is on the copper holder that we can handle easily.

Finally the copper holder with the sample on was detached from the glass plate after exposing it to

light for 2 hours. The holder and substrate could now be transferred to the ion milling machine

(Fig.26) using a tweezers.

Figure 26: Precision ion polishing system

In the ion milling process at low voltage of around 1-3keV were used to bombard with Argon ions

(Ar+) at low angle of 1° to 6° towards the two substrate surfaces and thereby sputtering atoms from

the surface for 2hours and at the same time the substrate was rotated. The ion milling machine

eventually reduced the substrate thickness to10-100 nm thickness and minimize contamination.

Finally we put the sample on the sample holder of Transmission Electron Microscope as shown in

(Fig.27) and analyze the sample.

Figure 27: TEM sample holder and sample

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B. Preparation of LZO/Si sample for TEM In this case the sample is prepared unlike that of LZO/LAO; here we scratched the sample LZO/Si

in which Si is the substrate and LZO is film with the diamond pen and we take the powder of the

scratched sample in the copper holder. Then we put the sample in TEM sample holder as show in

above (Fig31) and analyze it.

Result

(i).LZO obtained by MOCVD on LAO single crystal LZO layers of few nm obtained by MOCVD on LAO single crystal was analyzed by TEM. The

sample cross-section was prepared as explained in sample preparation mechanism section; a low

magnification image of LZO/LAO cross-section was observed (Fig.28.a) which we had different

structure. When it is observed at higher magnification (Fig.28.b) the film (LZO) shows columnar

growth structure and the structure is compact near interface and completely separated at the surface.

At low magnification and high magnification the view of interface shows good epitaxy.

Figure 28: Bright-field image of the LZO/LAO structure with low magnification

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Figure 29: Cross section of LZO/LAO at high magnification

The diffraction pattern was indexed and since the (111) diffraction with a spacing between planes of

0.624nm and (331) diffraction with a spacing between planes of 0.248nm were present the

crystallization was interpreted as the pyrochlore structure of LZO when the MOCVD technique was

used for deposition (see Fig 35).

Figure 30: LZO Pyrochlore film

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(ii). LZO obtained by MOD on LAO single crystal

In this section the X-ray diffraction of La2Zr2O7 will be discussed. (LZO) grown by MOD on

LaAlO3 (LAO) was performed in order to compare the structure of LZO obtained from high

resolution transmission electron microscope. The substrate of the sample is LAO which has a

rhomhedral crystal structure. The planes parallel to the surface of this structure are the (012)-planes,

(024)-planes and (312)-planes. This lattice structure is hard to visualize so as an approximation a

pseudo-cubic structure (Pm-3m) is used. Comparing the data sheets for the two lattice structures

one finds that the corresponding planes in the cubic approximation is the (400) family of parallel

planes. The cubic lattice approximation is therefore valid.

Figure 31: Pole figure measurement6

In the following three different angles will be used. Theta is the angle between the sample and the

beam, Chi is the angle by which the sample is tilted and phi is the angle by which the sample is

turned around the chi vector (Fig.31)

The X rays Experiments

The first experiment a theta to two-theta analysis was used to determine the surface orientation of

the deposited LZO film. The theta-2theta scan was made with the following parameters:

Two-theta: 24° to 46° (to capture refractions from the planes (100) and (200)) .Phi =0, Chi =0.

A peak was found for two-theta=33. That peak corresponds to (hkl) = (400), which is a plane of

LZO. A second analysis was made with the following parameters: Two-theta: 48°-80° Phi =0, Chi

=0. Two peaks were found for two-theta =70° and 2theta =76°. 2theta =76° corresponds to the (300)

plane of LAO(Fig.32). The other peak 2theta =70° corresponds to a (800) plane of LZO.

27

From the theta-2theta analysis it can be concluded that the LZO film’s orientation is in the (100)

direction and as expected the LAO substrate’s orientation is in the (100) orientation as well. The

diffraction peaks obtained from the LZO substrate; however, are not sufficient to tell whether the

film is Pyrochlore or fluorite because both diffraction angles are present in both structures.

Figure 32: The 2 Theta scale LZO and LAO.

To determine the uniformity of the LZO film in the out-of-plane orientation a rocking curve scan

was performed. From this a FWHM-value was determined. The FWHM value is a measure of the

width of a peak. A low FWHM-value (narrow peak) thus express that the diffraction is very

uniform, which again can be interpreted as very good out-of-plane alignment of the grains

The RC (rocking curve) scan for the (400)-plane was made around the peak at theta =16.5°. The

other parameters were: Chi =0 and phi = 0°. The following value was obtained: FWHM =0.412°.

The same scan was made for the (800)-plane around the peak theta =70° a similar result was

obtained: FWHM=0.491°. Further the same RC-scan was performed with the same parameters

except for phi which was set to 90°. This was done to determine whether the out-of-plane alignment

of grains was similar in another orientation parallel to the surface. For the (400)-plane the FWHM =

0.390° was obtained, which is quite similar. Generally the FWHM values obtained are considered

low, which means that the grains are orientated very similarly in the out-of-plane direction (Fig 33).

28

To investigate the uniformity of the LZO film in the in-plane orientation a phi-scan was performed.

The phi-scan was performed in the phi-direction from -180° to 180°. As for the rocking curve scan

the FWHM value was used to determine the quality of the film in the in-plane direction. Since phi

was varied 360° four peaks were obtained from the same family. The phi-scan measured the

diffraction from a family of planes in the bulk so the most intense diffraction plane in the bulk was

chosen. This is the (222)-plane. In order to measure diffraction from this plane the angle between

the surface plane and the (222)-plane was calculated: Chi =54.7°. The phi-scan was performed and

the mean FWHM value obtained was: FWHM = 0.48°, which is in the same order of magnitude as

for the RC scans. The in-plane orientation of the grains is thus very similar in the two directions.

Figure 33: Theta scale Rocking Curve

Cross-section observation The LZO sample grown by MOD on single crystal of LAO was prepared for TEM cross-sectional

observation with low and high magnification. Both low and high magnification images of

LZO/LAO shows heterostructure the LZO layer shows great contrast (Fig 34).

29

Figure 34: Low magnification of LZO/LAO (a) and high magnification image of LZO layer (b)

Figure 35 LZO/LAO diffraction and image.

30

The diffraction pattern obtained in the LZO/LAO interface (Fig. 35), was interpreted as fluorite

LZO structure in [000] LZO zone axis since the (111) and (311) planes of the pyrochlore structure

were not present in the diffraction image. The LZO film has variation in contrast that could be due

to density which is related to nanovoid that appears white in the LZO part as in Fig .36.

If we compare the LZO obtained by MOCVD with the one obtained by MOD technique, the

nanovoides (Fig.36.a) are only observed in LZO obtained by MOD. And the sample of LZO

obtained by MOCVD was observed by HTEM and there is no void found. This show the voids in

LZO appear due to the synthesis process of LZO1.

Figure 36: The nanovoids observed in LZO Fluorite film due to MOD technique.

(iii).LZO obtained by MOD on Si single Crystal

The other sample analyzed with the TEM was LZO obtained by MOD on Si single crystal. Here

the sample is prepared as states in sample preparation section, and different part of the sample is

observed in image mode and diffraction mode too.

In this sample voids were obtained (Fig.37 andFig.38) low magnification and high magnification

images respectively.

31

Figure 37: Low magnification image of LAO/Si sample

Figure 38: High magnification image of LZO/Si sample

The diffraction pattern of selected part of the sample is obtained with TEM (Fig.39.a) for LZO and

(Fig.39.b) for silicon. The diffraction image of LZO gives ring which implies polycrystalline

fluorite phase.

32

Figure 39: Polycrystalline LZO fluorite diffraction (a) and Si Single Crystal diffraction (b).

The EDS analysis (Fig14) of the sample is obtained in which the type of atom in the sample is seen,

and the observed atoms are Zr, La, Cu and O. The intensity of oxygen is very low because the EDS

analysis is not very sensitive to light elements (atoms), while the copper is from the sample holder.

33

Conclusion

In this report LZO layers deposited on LAO by MOD and MOCVD have been analyzed using XRD

and TEM microscopy. Moreover an LZO layer deposited on a Si substrate was studied for

comparison. From the XRD study of the MOD deposited layer it was shown using a theta-2theta

scan that the surface orientation of the LZO film was [100]. It could not be concluded whether it

was the pyrochlore or fluorite structure though. Moreover the FWHM values of app. 0.5 degrees

obtained from the RC-scan and the phi-scan showed a good texture of the film. For the TEM

images the samples were prepared using the tripod-method. TEM images were obtained from both

image and diffraction mode. Diffraction images of the MOD deposited film showed that the film

had crystallized in the fluorite structure and the real images indicated that nanovoids were present.

For the MOCVD deposited LZO film the (331) and (111) diffractions were present, which was

interpreted as the pyrochlore structure. The LZO-film deposited on silicon crystallized in the

polycrystalline fluorite structure. This was evident due to the ring formation of the diffracted planes

obtained in diffraction imaging. The results show that the microstructure of LZO does not depend

on the substrate used but it depends on deposition technique.

34

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