The route to superconducting Sr2RuO4

Bachelor Thesis

The Route to Superconducting Sr 2 RuO 4 by

J.R.A. Smit (s0201731) & C.J. Walhout (s1004581) As presented on August 6, 2012

Daily Supervisor: Rik Groenen

Supervisor: Gertjan Koster

Preface

This Bachelor Thesis has been our devotion for the past 10 weeks, and it has been a huge learning experience. We have seen ourselves and our daily supervisor from a side we had not yet experienced, and we greatly enjoyed this. We learned first hand how science is put into practice at the Inorganic Materials Science group, and how certain ambitious results draw great interest from lots of other people inside and outside this group.

We would like to thank our daily supervisor, Rik Groenen, for his time, guidance and great entertaining values during the past weeks. We would like to thank Gert-Jan Koster for his input and sharing his knowledge about this subject. We would also like to thank Peter Brinks and Ruud Steenwelle for having us in their office and sharing their knowledge. Last but not least, we would like to thank the entire IMS group for the fun, and the lunches and colloquia we had the honour of attending.

We can only hope the coming experiences will be as entertaining and challenging as this last one.

Have fun reading this report.

Jasper Smit & Caspar Walhout

1 Summary

As a result of previous research on Sr

RuO

, and the possibility to reach the temperatures required to synthesize this material by laser heating, a study towards the the influence of the pulsed laser deposition parameters on the properties of Sr

RuO

thin films and possibility of superconductivity has been started. This report focuses on determining the influence of two parameters, namely the substrate temperature and the background pressure.

For the first set of samples, the substrate temperature was increased from 900

C with increments of 50

C to a maximum achievable temperature of 1080

C while the oxygen pressure was maintained at 10

mbar. Then layers were grown at increased background oxygen pressures, at 10

mbar and 10

mbar, with a substrate temperature of 1000

C. The same was done for argon pressures of 10

mbar and 10

mbar, while maintaining an oxygen pressure of 10

mbar. All films were grown on SrTiO

substrates which were etched and annealed beforehand.

Analyses of the resulting films were performed using X-Ray Diffraction (XRD), Atomic Force Microscopy (AFM), Reflective High-Energy Electron Diffraction (RHEED) and by performing a four point resistivity measurement over a temperature ranging from 293K to a lowest value of 2K (PPMS) to calculate Residual Resitivity Ratio (RRR) values.

Our findings for the substrate temperature dependence of the crystallinity are as follows. The crystallinity seems to be decreased due to volatile RuO

and RuO

at temperatures above 1050

C.

Epitaxial growth can be achieved for at least some circumstances, and the temperature seems to influence the crystallinity more than the background pressure.

For the growth kinetics it was concluded that layer-by-layer growth will be maintained the longest somewhere around 1000

C, and the roughness increases as the temperature does.

It appears that samples grown below 1000

C show low RRR values. Better values are obtained above this temperature.

The findings for the background pressure dependence of the crystallinity are as follows. At higher oxygen pressure smaller differences in lattice parameters from bulk SRO were seen. The most constant unit cell heights, however, were obtained at the lower pressures, most notably 10

mbar oxygen.

For the growth kinetics, a higher background pressure will result in a rougher film surface.

The transport properties appear to be best at lower pressures, with an optimal value around 10

argon, combined a 10

oxygen pressure.

Futhermore, although no exact results where obtained, an anisotropy in the resistance values

and the RRR measurements of the thin films seems to be present. This is expected to be a result

of phase boundaries in the thin films due to the step-edges of the substrate.

Contents

1 Summary iii

2 Introduction 1

2.1 Properties of Sr

RuO

. . . . 1

2.1.1 Structural properties . . . . 1

2.1.2 Superconducting properties . . . . 1

2.2 Research Goals . . . . 2

3 Theoretical Background 4 3.1 Pulsed Laser Deposition . . . . 4

3.1.1 Influence of the substrate temperature . . . . 4

3.1.2 Influence of background pressure . . . . 5

3.1.3 Target . . . . 5

3.1.4 Substrate . . . . 6

3.2 Phase diagram . . . . 6

3.3 Characterization . . . . 7

3.3.1 Atomic Force Microscopy (AFM) . . . . 7

3.3.2 Reflection High-Energy Electron Diffraction (RHEED) . . . . 7

3.3.3 X-Ray Diffraction (XRD) . . . . 8

3.3.4 Resistivity Measurement (PPMS) . . . . 8

4 Research Questions 10 5 Experiments and Results 11 5.1 Experiments . . . . 11

5.2 Crystallinity . . . . 11

5.2.1 XRD Zoom Analysis . . . . 12

5.2.2 XRD Peak Shifts . . . . 14

5.2.3 Epitaxial Phi-Scan . . . . 15

5.3 Growth Kinetics . . . . 16

5.4 Transport Properties . . . . 17

5.4.1 Residual Resistance Ratio Analysis . . . . 17

5.4.2 Residual Resistance Ratio Anisotropy . . . . 18

6 Conclusions 20 7 Recommendations 22 7.1 Growth Optimalisation . . . . 22

7.2 Other Recommendations . . . . 22

A Appendix 26 A.1 AFM Pre-deposition . . . . 26

A.2 Nelson Riley Analysis . . . . 28

A.3 RHEED Data . . . . 33

A.4 RHEED Snapshots and AFM Post-deposition . . . . 37

A.5 PPMS Curves . . . . 42

A.6 RRR Anisotropy . . . . 47

2 INTRODUCTION

2 Introduction

It was first discovered by Maeno et al. in 1994 that layered perovskites without copper atoms can have superconducting properties[1]. The discovered perovskite is part of the strontium ruthen- ate family. Sr

RuO

is a low-T

superconductor (T

= 0.93 K), and one of the few supercon- ducting perovskites without copper known today. Sr

RuO

is an interesting material because it shows unconventional superconductivity in the form of spin triplet pairing, which might be use- ful in ferroelectric superconductors[2] and for future use in Josephson junctions with odd-paired superconductors[3].

The attention of this material has been raised here at the IMS group since the possibility for reaching the temperatures required to synthesize Sr

RuO

was obtained recently. This possibility is in the form of a laser heating unit, which is capable of heating substrates to a temperature around 1100

C.

2.1 Properties of Sr

RuO

The structural and superconducting properties of bulk Sr

RuO

are described in the following section.

2.1.1 Structural properties

The crystal of our interest, Sr

RuO

(SRO), has the structure of tetragonal perovskite. Having a tetragonal structure means that lattice constants a and b are the same (henceforth called a), differing from c, resulting in a rectangular cuboid. The in-plane and out-of-plane lattice constants of bulk SRO at room temperature were found respectively as a = 3.8730(3) ˚ A, and c = 12.7323(9)

˚ A[4]. The structure, as it is known, can be found in figure 1 which is from the article by Maeno et al.[1].

Figure 1: The crystal structure of Sr

RuO

is exactly the same as the perovskite structure of La

Ba

CuO

, a low T

superconductor with copper. The directions of the tetragonal principal axes are shown[1].

2.1.2 Superconducting properties

Resistivity measurements on Sr

RuO

were done by Maeno et al.[1], showing that in both the

ab-plane and c-plane (see figure 2) the resistivity nearly goes to 0 at around 1 K.

2.2 Research Goals 2 INTRODUCTION

length (ξ

) of Sr

RuO

is 66 nm, this means the in-plane distance between dislocations has to be at least 66 nm to be able to achieve superconductivity[7].

Figure 2: The resistivity ρ of Sr

RuO

below 4 K is shown in two directions. The transition to supercon- ductivity is at T

=0.93 K. In the insets can be seen how the electrodes are attached[1].

2.2 Research Goals

Thin films are interesting because they can be used in a wide variety of technologies using small structures, from coatings to microelectomechanical systems (MEMS). Each material can provide a different set of properties with which new technologies can be developed.

The research goal of this report is the synthesis of of Sr

RuO

thin films using Pulsed Laser Deposition (PLD). Here PLD is an interesting method because is has some advantages over other techniques which can be used to synthesize this material, like the floating zone method and molec- ular beam epitaxy (MBE).

The stoichiometry of PLD grown films is very close or even identical to the stoichiometry of the target. This allows for complex materials to be grown from one single bulk target[8][9].

The pulsed nature of PLD allows for the target to be changed in between pulses. It even allows for the laser parameters to be adjusted with the different targets, allowing optimal laser conditions for each target. This makes it possible to grow high quality superlattices quite easily[8][9].

Group T

(

C) P

(mbar) Substrate Krockenberger et al. 920 0.53*10

O

& 2.61*10

Ar LSAT

Ohnishi et al. 1100 10

O

STO

Zurbuchen et al. 1000 1.07*10

–6.67*10

O

LAO

Group Target Fluency (J/cm

) Superconducting

Krockenberger et al. SrCO

& RuO

4 Yes

Ohnishi et al. SrRuO

0.43–2.08 No

Zurbuchen et al. Sr

RuO

2.7 No

Table 1: Previously published articles on growth of Sr

RuO

films using PLD[3][10][11]

As seen in table 1, thin films of this material have previously been grown with pulsed laser

deposition using SrRuO

targets[10], using SrCO

and RuO

targets combined[3] and using a single

2.2 Research Goals 2 INTRODUCTION

Sr

RuO

target[11]. However, the films grown from the single Sr

RuO

target often contained a

lot of impurities[11] or dislocations[5].

3 THEORETICAL BACKGROUND

3 Theoretical Background

Pulsed Laser Deposition (PLD) will be used to synthesize Sr

RuO

(SRO). Therefore, first the principles of PLD will be discussed together with the most important parameters for this research and their general influence on the PLD process. These parameters are the substrate temperature, the background pressure and the target material. Then the choice for the substrate material will be described.

In section 3.2 a description of the phase diagram will be given, which comments on the chemical aspects required for the synthesis of the correct phase of Sr

Ru

O

. To analyse whether this synthesis has been done correctly, multiple analysis techniques will be utilised. Each of these techniques can provide information about the different properties of the thin film. And each of these properties shows some information about the quality of the film. These properties are: the crystallinity, the growth kinetics and the transport properties. The different techniques used to analyse these properties are described in section 3.3.

3.1 Pulsed Laser Deposition

PLD is a deposition technique for producing thin films on a substrate which uses a high energetic, pulsed laser. This laser is focused through an optical setup on a target, which is located together with the substrate in an ultra-high vacuum (UHV) chamber. A mask is used to select the homo- geneous part of the laser. When the laser beam hits the target, the energy is absorbed by the top layers of the material and converted into thermal, chemical and mechanical energy[8][12]. This results in an electronic excitation, ablation and exfoliation of the surface. A plasma is then formed which will deposit particles on the substrate. These particles have a certain diffusion length, de- pending on the substrate temperature and kinetic energy of the particles, within which they find a binding location. Preferred is a location where the energy is minimized.

It is possible to add a background gas to influence the kinetic energy of the particles and to influence the dynamics of the plasma. This background gas also promotes reactions at the films surface to suppress the amount of vacancies. For instance oxygen and nitrogen might be required to create oxides and nitrides without vacancies, since a vacancy of one of these anions mostly goes paired with a cation vacancy. Argon can be used as an inert gas to influence the plasma dynamics while leaving the surface of the thin film untouched.

A systematic setup is shown in figure 3.

Figure 3: A systematic PLD setup[13].

3.1.1 Influence of the substrate temperature

The temperature of the substrate has a significant influence on the crystallinity of the grown

film. For instance, films grown at room temperature are usually amorphous[14]. To increase the

incoming particle mobility, and thus the diffusion length, a higher substrate temperature can be

3.1 Pulsed Laser Deposition 3 THEORETICAL BACKGROUND

chosen. Other functions of the temperature are the desorption time of particles on the surface and the nucleation rate[15]. With a higher temperature there is an increased possibility that volatile materials will evaporate from the substrate surface.

3.1.2 Influence of background pressure

The pressure of the background gas determines three factors:

1. The scattering of the plasma plume;

2. The kinetic energy of the incoming particles;

3. The range of the plasma before thermalisation.

At lower pressures the plasma plume is very wide and undefined, because no gas is present which can confine the plume. The kinetic energy of the incoming particles on the substrate will be very high. Higher pressures result in a narrower, more defined plasma plume, because of the confinement from the background pressure. Also, higher pressures will decrease the kinetic energy of the incoming particles and decrease the thermalisation range.

The kinetic energy has a big influence on the films grown. Depending on the incident particles energy, atoms may ‘absorb’ on the surface more gently, at lower energies, or could be embed- ded more violently and even damaging of the pre-existing film by sputtering atoms at higher energies[14]. The resulting kinetic energy will also have an influence on the diffusion length of the incoming atoms, which will influence what the possible binding locations for the particle are.

As stated before, to grow oxide rich films, which have no oxygen vacancies, a background pressure of oxygen is required[16].

Besides the total pressure, multiple gasses can be mixed to influence film growth, hence the importance of partial pressures. What can for instance be achieved with partial pressures, are studies of how a partial pressure of a certain gas influences the resulting film, while keeping total pressure conditions, and thus the plasma dynamics and range, the same.

3.1.3 Target

The composition of the target material determines the composition of the thin film which will be grown, since this is the material which will be deposited on the substrate. The thin films grown for this report were grown using a single sintered Sr

RuO

target. An X-Ray Diffraction (XRD) scan of this target is shown in figure 4. This scan shows us that the target consists of Sr, Ru and O in the expected proportions, 2:1:4 with a maximum error of 5%.

Figure 4: An XRD scan of the target material. The red line represents the target, while the blue line

represents the expected graph for Sr

RuO

powder. The target was rotated during the measurement to

average out the different unit cell directions which results from the technique used to produce this target,

3.2 Phase diagram 3 THEORETICAL BACKGROUND

3.1.4 Substrate

A few substrate parameters are of importance when selecting a suitable substrate material, namely the lattice constants and the termination. A brief description of the influences of these parameters is given below followed by a discussion of the substrate material.

Lattice constants

The most important properties when choosing a substrate for hetero-epitaxial growth are the lat- tice constants. The lattice constants will mainly determine the direction in which the thin film will grow. Choosing a substrate with the right lattice parameters will minimize the amount of defects created by strain on the film which is important for maintaining the quality of the resulting film[12][17].

Termination

Termination is a factor which determines the initial stages of hetero-epitaxial growth. Some com- ponents of materials favour being grown on a certain termination over another component in the incident material[18]. This may result in certain growth characteristics. For instance, Choi et al.[19] showed that when growing SrRuO

on a TiO

terminated SrTiO

substrate, a transition of termination takes place during the deposition of the first unit-cell layers.

If the substrate has no single-termination, multiple initial growth modes could take place at the same time, which is not beneficial for the quality of the resulting film.

Substrate Material

A number of substrates have been used to grow Sr

RuO

on. (La

Sr

)(Al

Ta

)O

(LSAT) has been used by Krockenberger et al.[3], while SrTiO

(STO) has been used by Ohinishi et al.[10]

and LaAlO

(LAO) by Zurbuchen et al.[5].

When comparing the lattice parameters from LSAT (3.869 ˚ A,) STO (3.905 ˚ A) and LAO (3.79

˚ A), with the lattice parameter of our thin film, Sr

RuO

(3.87 ˚ A), it can easily be concluded that LSAT has the smallest lattice mismatch (0.2%). But on the other hand, no single termination can be achieved when using LSAT as a substrate, while this is possible when using STO or LAO.

Another problem that can be encountered, when using aluminium rich substrates, is the suppression of superconductivity. This all, and the fact that the STO substrates were abundantly available to us, led to the choice of using STO as a substrate.

The STO substrates were treated with acetone for 10 minutes, ethanol for 10 minutes, hydrolysis for 30 minutes and BHF for 30 seconds, followed by annealing at 950

C for 1.5 hours. This resulted in TiO

terminated surfaces with unit step edges.

3.2 Phase diagram

Jacob et al.[20] documented a 3D oxygen potential diagram, which can be found in figure 5. With this figure it is possible to determine which pressure and temperature is required for the synthesis of Sr

RuO

, when the ratio of strontium oxide (SrO) and ruthenium oxide (RuO

) is known. For example, at η

= 0.1, (T/K)

∗ 10

= 10 and log(P

/P

) = –6, the resulting equilibrium is a mixture of SrO and Sr

RuO

.

The composition of our target results in an η

/(η

+η

) value of

. Due to the stoichiometric ablation of PLD the material that will be deposited on the target will be about the same ratio.

Around this η

/(η

+ η

) value it can be seen that every pressure above 10

is associated with

the correct Sr

RuO

phase for the temperature range that is depicted (925K to 1350K). More on

the chosen temperatures and pressures will be described in section 5.1.

3.3 Characterization 3 THEORETICAL BACKGROUND

Figure 5: Three-dimensional oxygen potential diagram for the system Sr-Ru-O as a function of composition and temperature. P

is the standard pressure (1 bar)[20].

3.3 Characterization

A number of methods have been used to characterise the thin films. A brief description of the utilised methods is given below.

3.3.1 Atomic Force Microscopy (AFM)

To be able to analyse the surface morphology of the grown film, AFM will be utilised. AFM is an analytical method which can determine the surface structure of thin films with atomic resolution.

To achieve this the film is moved under a tip, which is 1 atom thick at the end. This tip is attached to a cantilever which vibrates typically at 100 to 400 kHz, this is known as tapping mode. The tip is repulsed by atomic forces generated by the film as soon as it comes near. By aiming a laser at the cantilever and measuring the amplitude of the vibrations of the reflection from this laser through an optical sensor, one can measure differences in height. This can be translated into a height map which represents the surface of the film[12]. In a different mode of the AFM, the contact mode, the cantilever does not vibrate. Now only the atomic forces influence the tip and thus cause changes in reflection of the laser from the cantilever. These now directly translate into height differences.

A computer is then used to translate the height information into an image.

The AFM scans have supplied information about the morphology of the films and substrates.

3.3.2 Reflection High-Energy Electron Diffraction (RHEED)

RHEED is a technique used during deposition to be able to see layer-by-layer growth. An electron gun source aimed at the sample under a low angle almost parallel to the substrate is used, together with a photoluminescent detector at the other side of the sample also under a low angle. The electrons from the electron gun strike the sample and diffract from atoms at the surface of the sample. A small fraction of these electrons interfere constructively and form patterns on the detector. Diffraction patterns created this way are the image of the position of atoms on the sample surface[21]. There are three growth modes that are visible in these patterns, island growth, layer- by-layer growth and steady-state growth, all indicating whether or not growth is stoichiometric.

By analysing the intensity curves, RHEED provided information about the growth kinetics, like

3.3 Characterization 3 THEORETICAL BACKGROUND

by an increase in intensity where smoothing takes place. Steady-state growth is visible as a more constant RHEED intensity, where the incoming particles diffuse constantly.

3.3.3 X-Ray Diffraction (XRD)

XRD is a non-destructive technique that can provide information about all issues related to the crystal structure of bulk solids including lattice constants and crystallography, identification of unknown materials, orientation of single crystals and preferred orientation of polycrystals, defects, stresses, etc[12].

When X-ray beams with a certain wavelength are emitted onto a material, with a spacing between layers (d), diffraction, or constructive interference, occurs when the distance travelled by the reflected beams differ by a complete number times the wavelength (Bragg’s Law). Since every crystal has its own lattice parameters, it is possible to distinguish them through intensity peaks[22].

The angle between the interface and the detector (theta) can be varied in order for the d- spacings between polycrystalline materials to satisfy Bragg’s Law conditions. By plotting the angular position of the detector and the resulting intensities of the diffracted peaks a pattern is produced, which is unique for every sample[12].

Scan Types

Four different scans were made using XRD for each of the thin films. The first was a full angle scan of the grown film. When analysing these scans it was possible to determine what phases of SRO were present in the films, and whether crystals of other materials were present. From these scans it was also possible to determine whether the peaks moved slightly from their theoretical position, which provided information about the crystallinity of the film.

To analyse the peak shifts, the uncorrected unit cell height, calculated from each of the different peaks of one sample, was plotted against the Nelson-Riley number[23]. From this plot it was possible to determine the correct unit cell height by fitting a linear correlation of the form A · x + B, where B equals this unit cell height. The uncorrected unit cell heights were calculated using Bragg’s law:

d = 1 2 · n · λ

sin θ (1)

Where n is an integer determined by the order given, λ is the wavelength of the utilised radiation, in this case Copper K Alpha (0.15418nm), and θ is half the angle at which a peak was determined by the XRD scan. The corresponding Nelson-Riley numbers were calculated as follows:

n

= 1

2 ·  cos

θ

θ + cos

θ sin θ



(2) Where again θ is half the angle at which a peak was determined by the XRD scan.

The second scans were more detailed scans of the SRO006 and STO002 peaks, where Laue oscillations were visible. These oscillations showed whether the individual layers in the film have the same thickness. When they differed strongly in height, fewer, or even no oscillations were visible. This information was used to compare the quality of the films.

The third scans were reflectivity scans where, because of the difference in refractive indices between the film and the substrate, it was possible to determine the thickness of the films.

The last scan was a so called phi-scan. For this scan an XRD peak direction which is not in the SRO00X direction has to be selected, in our case the SRO204 direction. Since this direction has only four directions of symmetry when the material has been grown epitaxially, it is possible to determine whether epitaxial growth has taken place. Epitaxial growth will be visible as four peaks which have the same intensity and are located 90

apart. Unfortunately for only one sample the phi-scan was performed, due to a lack of time.

3.3.4 Resistivity Measurement (PPMS)

By adding contacts onto the thin film and applying a voltage over these contacts, it is possible

to measure resistance. With a helium cooling system (PPMS) it is possible to do temperature

dependent four-point resistivity measurement.

3.3 Characterization 3 THEORETICAL BACKGROUND

A good measurement standard is called residual-resistivity ratio (RRR). This is the ratio of the resistance at 300 K and 0 K. However, since 0 K cannot be obtained, a higher temperature will be used.

The RRR and temperature dependent resistance of all substrates, provided information about the defects and conductance in the crystals. At higher temperatures the phonon interaction dom- inates the conducting behaviour, while at lower temperatures the impurities and dislocations will dominate this. Two RRR values were obtained per substrate, one in each direction along substrate edges. The two obtained values are classified as the horizontal direction and the vertical direction.

These directions however, are arbitrary, since no physical reference was given to the substrates to determine their direction. A systematic figure of the locations of the bonded electrodes and measured units is given in figure 6.

Figure 6: Systematic figure of the RRR measurement electrode locations and the respective connected

electrodes

4 RESEARCH QUESTIONS

4 Research Questions

Theoretically, it would be possible to synthesize Sr

RuO

with any of the parameters specified in section 3.2. But since these parameters also have an influence on the kinetics of growth of the PLD process, it is more complicated.

To be able to grow this material in its superconducting state using pulsed laser deposition, the optimal deposition parameters need to be determined. This study focused on the two parame- ters which have most influence on the resulting films, namely the substrate temperature and the background pressure.

This resulted in the following two research questions:

• What is the substrate temperature dependence of the crystallinity, the growth kinetics and the transport properties of Sr

RuO

?

• What is the background pressure dependence of the crystallinity, the growth kinetics and the

transport properties of Sr

RuO

?

5 EXPERIMENTS AND RESULTS

5 Experiments and Results

The experiments conducted and their results are described in the following section. First is com- mented on the different deposition parameters. Following, the results are split into three groups of properties: crystallinity, growth kinetics and transport properties.

5.1 Experiments

All thin films were grown at 2Hz with a pulse duration of 25ns at a fluency of around 2.2 J/cm

. The laser which was used was a KrF (248nm) laser. The target-to-substrate distance was 5cm.

The substrates used had low miscut angles ranging from 0.05

to 0.25

, no pits and the phase of the AFM scan indicates a single terminated substrate, meaning the BHF and annealing treatment was performed successfully. Only the substrate of sample 5 shows a few etch pits, for which the assumption was made that this small amount would not have a significant influence on the results.

The AFM figures of these substrates pre-deposition can be found in appendix A.1. After growth, samples were cooled down at 50

C/min at the same pressure they were grown. The variable deposition parameters which were used are depicted in table 2. Sample 3 was used for both the temperature dependence analysis and the background pressure dependence analysis. With sample 6 and 7 partial oxygen pressures, combined with argon, were used, to determine the influence of partial pressures.

Sample T

(

C) P

(mbar) t

1 900 10

O

15

2 950 10

O

15

3 1000 10

O

15

4 1050 10

O

15

5 1080 10

O

15

6 1000 10

Ar & 10

O

20 7 1000 10

Ar & 10

O

20

8 1000 10

O

20

9 1000 10

O

28

Table 2: The variable deposition parameters.

A side note must be made for sample 4, where it is possible that the data might not be representable. Most characterisation methods show an anomaly for sample 4, which is bad sign.

Also the sample was dropped and heated twice from a low temperature to 1050

during growth.

It is included in the discussions for the completeness of the report.

5.2 Crystallinity

The overall XRD information is shown in figures 7 (temperature dependence) and 8 (pressure dependence), displaying the following results. First of all, all substrate material is clearly STO, judging from the peak heights. Secondly, all samples show atleast some Sr

RuO

peaks meaning the right Ruddlesden-Popper phase of SRO was grown, where, in this case, the 006 peak is the strongest.

Sample 4 is missing peaks of SRO002 and SRO004, while sample 2, 5 and 8 have an unwanted peak between SRO006 and STO002. Sample 5 is the sole sample that has two unwanted peaks at 35

and 73

. These peaks are most probably SrO peaks, originating from the fact that at high temperatures, in oxygenated environments, RuO

and RuO

become more volatile. Removing the Ru from Sr

RuO

leaves our perpetrator, SrO.

The temperature seems to have more influence on the overall XRD scan than the background

pressure in the ranges researched.

5.2 Crystallinity 5 EXPERIMENTS AND RESULTS

Figure 7: XRD θ − 2θ scan from 10

to 110

. Samples 1 through 5 are placed above each other

Figure 8: XRD θ − 2θ scan from 10

to 110

. Samples 3 and 6 through 9 are placed above each other

5.2.1 XRD Zoom Analysis

Figures 9 and 10 give a more detailed view of SRO006 and STO002. This allows us to closely examine the Laue oscillations around SRO006. The oscillations are for sample 1, 3 and 5 well- defined and symmetrical, for sample 4 and 8 asymmetrical but well-defined and sample 2, 6, 7 and 9 are not very well-defined. In the case of sample 2, 5 and 8 it also allows us to take a close look at the 44.6

peak, which is probably a silver (Ag002 44.09–44.40

) peak, originating from the silver paint used to thermal contact the substrates. Moreover, noticeable are minor peak shifts in the 2θ regime, in the order of tenths of degrees.

Sample 3 is the least asymmetrical where on both sides of the peak the oscillations are best

defined, this indicates a relative small difference in thickness of layers. This concurs with the

relative intensity of its SRO006 peak compared to the STO002 peak intensity, found in table 3.

5.2 Crystallinity 5 EXPERIMENTS AND RESULTS

Figure 9: XRD θ − 2θ zoom from 41

to 48

. Samples 1 through 5 are placed above each other

Figure 10: XRD θ − 2θ zoom from 41

to 48

. Samples 3 and 6 through 9 are placed above each other

A high percentage indicates the thin film has high intensity diffraction and since a well-defined thickness in layers has less scattering these two properties are intertwined.

For all other samples the same trend is visible, high intensity Laue oscillations compare with a

high peak intensity percentage, sample 6 being the only exception, and low intensity oscillations

compare to a low peak percentage intensity. There appears to be no linear temperature dependence

visible. We do see that samples grown on either 10

or 10

mbar at 1000

C show the most

promising results. However, to be able to draw conclusions, more information is needed from

samples grown at pressures between 10

or 10

mbar.

5.2 Crystallinity 5 EXPERIMENTS AND RESULTS

Sample T

(

C) I

(counts/s) I

(counts/s) I

(%)

1 900 61355 1.05E+06 5.84

2 950 43265 1.09E+06 3.96

3 1000 55610 1.08E+06 5.14

4 1050 33095 1.01E+06 3.27

5 1080 26420 9.08E+05 2.90

Sample P

(mbar) I

(counts/s) I

(counts/s) I

(%)

6 10

Ar & 10

O

55075 1.08E+06 5.07

7 10

Ar & 10

O

24475 1.05E+06 2.33

8 10

O

60050 1.13E+06 5.32

9 10

O

23760 3.20E+06 0.74

Table 3: Values of the intensity of SRO006 and STO002 peaks and relative percentage (I

= I

/I

)

5.2.2 XRD Peak Shifts

An interesting piece of information that can be extracted from the XRD scan is how well the lattice parameter of the grown thin film matches the theoretical value, as stated in section 3.3.3. The shifts found for all samples are displayed in table 4. These values are corrected for the shift in the STO002 peak, which can result from small misalignments of the XRD device. This means that all peaks are shifted with respect to the STO002 peak’s theoretical value, as for example has been done with the SRO006 peaks for all samples in figure 9.

A negative angle shift corresponds to a shift to the left in the figure, while a positive shift corresponds to a shift to the right. When taking a closer look at table 4 the data seems to lack any pattern. Only the SRO002 and SRO008, for the temperature dependence samples, and SRO002 and SRO0010, for the pressure dependence samples, peaks seem to shift to the left in most cases, while all other peaks shift to the right. When comparing the amount all peaks shift in one sample, no pattern is observed either.

Sample SRO002 (

) SRO004 (

) SRO006 (

) SRO008 (

) SRO0010 (

) SRO0012 (

)

1 -0.17 0.22 0.06 -0.14 0.31 0.23

2 -0.19 0.32 0.08 -0.14 0.21 0.17

3 -0.01 0.18 0.14 0.06 0.03 0.33

4 N/A N/A 0.00 -0.30 N/A 0.03

5 0.07 0.10 0.20 0.22 0.32 0.45

3 -0.01 0.18 0.14 0.06 -0.15 0.33

6 -0.03 0.24 0.16 0.12 -0.01 0.43

7 -0.03 0.14 0.14 0.17 0.2 0.48

8 -0.17 0.26 0.16 0.04 -0.43 0.41

9 0.11 -0.02 0.07 0.09 0.05 0.18

Table 4: Shifts in SRO peaks from their theoretical value, corrected for the shift in the STO002 peak Since a shift to the right means a smaller lattice parameter than the theoretical value, for which we do not have an explanation, our expectation is that the actual theoretical thin film lattice parameter value is smaller than the bulk value, c = 12.7323 ˚ A, used in our XRD scan.

When comparing our results to a smaller lattice parameter, all peak shifts will become negative, meaning the measured film lattice parameter is larger than a thin film without vacancies. Since oxygen vacancies often go paired with cation vacancies and all temperature dependence samples were grown at low oxygen pressure, it is possible for vacancies to be present and thus to influence the lattice parameter.

The sample with the smallest deviation in angles seems to be the sample grown at 10

mbar O

, which is in line with the oxygen and cation vacancies conclusion.

An analysis to used to obtain more data on peak shifts is called Nelson Riley, as presented

5.2 Crystallinity 5 EXPERIMENTS AND RESULTS

in section 3.3.3. The table with c-axis values, table 5, is obtained from the graphs in appendix A.2. Sample 4 displayed an absurdly high value for the c-axis when Nelson Riley is applied, it is therefore not included in the results.

When the c

values are compared, it can be observed that high oxygen pressures have the smallest deviation. This comes, once more, from the fact that oxygen and cation vacancies are most probably the smallest at higher oxygen pressures. The other error values do not display an obvious trend: where the sample grown at 10

O

pressure is expected to have a smaller error, it does not. The rest of the samples are quite constant in their error, except for sample 3 and 5.

Their low error value most probably has more to do with statistics than a general trend.

Sample T

(

C) c-axis (nm) c

(%)

1 900 1.2678 0.4265

2 950 1.2688 0.3479

3 1000 1.2708 0.1908

4 1050 N/A N/A

5 1080 1.2750 -0.1390

Sample P

(mbar) c-axis (nm) c

(%) 6 10

Ar & 10

O

1.2683 0.3872 7 10

Ar & 10

O

1.2678 0.4275

8 10

O

1.2676 0.4422

9 10

O

1.2748 -0.1233

Table 5: C-axis values with their error in percentages compared to the bulk SRO value (1.27323 nm), obtained with Nelson Riley analysis

5.2.3 Epitaxial Phi-Scan

The phi-scan done by XRD of sample 6 indicates that growth was epitaxial since a periodicity of

90

was obtained when aligned with the SRO(204) direction.

5.3 Growth Kinetics 5 EXPERIMENTS AND RESULTS

5.3 Growth Kinetics

The RHEED patterns obtained during growth, as found in appendix A.3, start out directly with layer-by-layer growth (except for sample 5) where the oscillation time is obtained from and the pulses can be counted in the first peak. The oscillation time was about 9-11 seconds for samples grown at 10

mbar, 15-16 seconds for samples grown at 10

mbar and 20-21 seconds for samples grown at 10

mbar. The layer-by-layer growth slowly transitions into steady-state growth. This has its origin in the high temperature parameter, which provides an environment where a high diffusion of atoms is present.

The point at which the layer-by-layer growth switches to steady-state growth is different for all samples. These points can be found in table 6 (unfortunately no RHEED pattern was obtained for sample 6). Krockenberger et al.[3] argue it is beneficial for the film quality to maintain layer- by-layer growth. Furthermore the longer layer-by-layer growth is present, the more information can be distilled from the measurement. From the pressure dependent samples, it can be concluded that when grown at 1000

C the percentage of the length of layer-by-layer growth oscillations is constant. From the temperature set, the sample grown at 1000

C seems to be the sample with the longest oscillations.

Sample T

(

C) Time Before Transition (s)

1 900 220

2 950 205

3 1000 380

4 1050 180

5 1080 300

Sample P

(mbar) Time Before Transition (s)

6 10

Ar & 10

O

N/A

7 10

Ar & 10

O

200

8 10

O

510

9 10

O

780

Table 6: Time before layer-by-layer growth transitions into steady-state growth

From the oscillation time and time grown an estimate can be made on how thick the thin film will be. In fifteen minutes about 90 layers of half a unit cell (6.3 ˚ A) are grown, totalling to 63 nanometer and 52 nanometer of thin film grown at respectively 9 seconds per layer and 11 seconds per layer. Where 15–16 seconds per layer was grown for 20 minutes the calculated thickness value was found totalling 47–50 nm and for growing at a speed of 20–21 seconds per layer a thickness of 50–53 nm is expected after 28 minutes. The growth time is longer at higher pressures, where the kinetic energy and the thermalisation range is decreased, as mentioned in section 3.1.2, since less particles reach the substrate.

Comparing the calculated temperature dependent thin film thickness values (52–63 nm) to the values found through the XRD reflectivity scan (45–53 nm), a large deviation can be found. Once the layer-by-layer growth transitions to steady-state growth, it is no longer possible to monitor the growth speed as easily. We expect high temperature volatility and lacking the ability to stick to play a major role during steady-state growth, resulting in thinner films than expected. Since the steady-state growth does not start at the same time for all samples, thickness varies throughout the samples. The deviation found while analysing the pressure dependent samples, 47–50 nm at 10

mbar and 50–53 nm at 10

mbar compared to respectively 49–53 nm and 53 nm, is not as significant. We think that the higher the temperature becomes, the more significant it is to the thickness of the film.

The RHEED snapshots of the temperature dependent samples, post-deposition and after cool- ing down, all display the typical bright 3 spots in the middle of the diffraction pattern, except for sample 4.

The roughness in the AFM figures match the RHEED snapshots, which can both be found in

appendix A.4. In sample 1 second order diffractions were seen, indicating a slight bit of 3D growth.

5.4 Transport Properties 5 EXPERIMENTS AND RESULTS

Sample 4, showed a 3D surface RHEED pattern and sample 1, 2, 3 and 5 showed a 2D RHEED pattern. This was confirmed by the AFM scans. When taking a look at the RMS of the roughness determined from the AFM data, a general trend can be seen. The RMS goes from 0.40 up to 0.83 nm while the temperature is increased from 900 to 1080

C. From figure 12a one can conclude that roughness increases when temperature increases.

(a) Temperature dependency of the RMS (b) Pressure dependency of the RMS

Figure 12: Roughness obtained from AFM pictures

When analysing the RHEED snapshots of the pressure dependent set of samples, sample 3, 6 and 8 are quite similar. They display a type of roughness between 2D and 3D, where sample 8 is the most 2D and sample 3 the most 3D of the three. Sample 7 has a 3D surface, while sample 9 has a pattern with split spots nudging towards a 3D surface. Comparing these snapshots to the roughness values (the Root Mean Square (RMS) values, as seen in figure 12b) one observes a clear pattern: growing at higher pressures leaves a rougher surface than at lower pressures.

5.4 Transport Properties

From the PPMS results the RRR can be attained. The PPMS curves from which the values were obtained can be found in appendix A.5. The RRR for all samples, except for sample 4 and 9, can be found in table 7. The anisotropy in RRR is most probably related to the substrate orientation, which will be elaborated on in section 5.4.2. However, first an analysis will be given of the RRR results depending on the varied parameters.

5.4.1 Residual Resistance Ratio Analysis

When looking closer at the different results, a trend can be seen. Samples 1–3 and 5–8 show an almost linear drop to just below 50K and then show an upsweep, some bigger than others. Sample 4 has a huge increase in resistivity at low temperature up to a resistivity of 50kΩ in the horizontal direction and 200kΩ in the vertical direction. This essentially means the layer is insulating. Sample 9 showed odd behaviour right from the start, which is why the measurement was done twice with new bonds. Twice it showed insulating behaviour at all temperatures.

The upsweep as observed in almost all PPMS curves at lower temperatures can be compared to the behaviour of semiconductors, where electrons at a low temperature are no longer excited into the conductance band. This results in less charge carriers being available, resulting in a higher resistance at these temperatures. The samples in which this upsweeping effect was least visible were samples 3, 5 and 6, in one of the two directions. The RRR of these three samples are 4.47, 6.26 and 7.50.

It appears as if samples grown at low temperatures have an overall low RRR and high oxygen

5.4 Transport Properties 5 EXPERIMENTS AND RESULTS

Sample T

(

C) T

(

C) RRR

RRR

1 900 2.91 1.6111 1.3609

2 950 2.91 0.9634 0.8857

3 1000 2.70 2.0014 4.4767

4 1050 N/A N/A N/A

5 1080 2.51 6.2572 1.3182

Sample P

(mbar) T

(

C) RRR

RRR

6 10

Ar & 10

O

7.72 1.6903 7.5004 7 10

Ar & 10

O

7.59 1.4394 2.0708

8 10

O

8.30 1.8773 1.3117

9 10

O

N/A N/A N/A

Table 7: Numerical results obtained from the PPMS characterisations for the temperature analysis (top 5) and for the pressure analysis (bottom 4)

Because at lower temperatures the transport behaviour is dominated by the defects in the thin film, the thin films which show larger upsweeps when cooled down, will most likely contain more defects or contaminations in their atomic structure. All samples, except for sample 6 in one direction, show upsweeps, which means all grown films have structural defects, except for sample 6 in one direction.

5.4.2 Residual Resistance Ratio Anisotropy

It is evident that some samples show a big difference between the horizontal RRR and the vertical RRR, and the differences in initial and final resistances between the two different measurement directions in the results of the resistivity measurements. In table 8 an overview of the substrate step- edge angles, the difference in resistance at room temperature (∆R

), the difference in resistance at around 8K (∆R

) and the relative thin film RRR difference (RRR

) is given for the relevant samples. The values to calculate ∆R

and ∆R

can be found in table 10 in appendix A.6.

Since for this comparison it is convenient to take the RRR values all at the same temperature, namely about 8K, different RRR values for samples 1 through 5 were taken than shown in table 7. These RRR values, with the respective temperatures, are shown in appendix A.6, table 9. The step-edge angle is defined as the smallest angle to a vertical or horizontal direction. The angle- corrected AFM images from which these angles were determined are displayed in appendix A.6, figure 51. The relative difference between the two RRR values is defined as:

RRR

=  RRR

RRR

− 1



· 100% (3)

Where RRR

is the maximum value of RRR

and RRR

, and RRR

is the minimal value.

Sample Step-edge ∆R

(Ω) ∆R

(Ω) RRR

(%) angle (

)

1 0 3.34 5.38 17.22

2 40 2.85 6.27 8.45

3 20 2.88 -6.01 122.08

5 10 158.32 182.00 339.47

6 0 16.56 19.22 343.73

7 0 12.44 19.75 43.87

8 10 2.19 9.94 43.12

Table 8: Substrate step-edge angles, resistance differences and RRR values

5.4 Transport Properties 5 EXPERIMENTS AND RESULTS

Even though no main pattern is visible and thus no definite conclusions can be drawn, it seems as if there is an anisotropy of the RRR

and of the absolute resistance differences with respect to the step-edge angle. A low angle results in no step edges having to be crossed by the measurement current in one direction, and all step edges having to be crossed in the other direction (figure 13a).

Whilst a step edge angle which is close to 45

, results in the same amount of step edges have to be crossed by each of the measurement currents (figure 13b).

Especially sample 2, with an angle of 40

distinguishes itself with a very low difference in RRR values. This whilst especially samples 5 and 6 show a very large difference. The low value of sample 1 and the high value of sample 3 cannot be explained through this theory, but there are most likely more factors involved in the resulting RRR

.

An overall observation from the ∆R

and ∆R

is that the latter all have a higher value than the former, except with sample 3. This is due to one of the two directions always increasing its resistance more in the lower temperature area, whilst the other increases less and even in some cases continues to decrease. Sample 2 also again shows a consequent low difference in ∆R values compared to the others. Again there are some values which seem to contradict this theory, namely the ∆R

of sample 9, and the ∆R

values of sample 1 and 3, but again, most likely there are more factors involved which determine the behaviour of the ∆R values.

The expectation is that these effects are the result of phase boundaries in the thin film which manifest directly above the step-edges of the substrate. These will affect the density of charge carriers which can pass through them at lower temperatures.

(a) Unit step angle of 0

(b) Unit step angle of 45

Figure 13: Systematic view of different unit-step directions (red) with the electrode locations and

current measurement directions (grey)

6 CONCLUSIONS

6 Conclusions

The goal of this research was to synthesize Sr

RuO

thin films using pulsed laser deposition.

Looking at the results, this has proven to be successful. However, since the PLD parameters have an influence on the growth kinetics and the resulting films, the following research questions were drawn up.

• What is the substrate temperature dependence of the crystallinity, the growth kinetics and the transport properties of Sr

RuO

?

• What is the background pressure dependence of the crystallinity, the growth kinetics and the transport properties of Sr

RuO

?

The trends found concerning the different properties, dependent on either the substrate tem- perature or the background pressure, are depicted below.

Substrate temperature dependence of the crystallinity

Growth temperatures above 1050

C seem to provide an environment where RuO

and RuO

are volatile, which can lead to unwanted SrO formation and less stoichiometric growth of SRO.

Also, the phi-scan done by XRD of sample 6 indicates that growth was epitaxial. No conclusion can be bound to this fact, it can only be hoped that all samples grown were epitaxial. Since sample 6 was grown at 1000

C it can only be said that growth at 1000

C is epitaxial under certain circumstances.

Futhermore, the temperature appears to have more influence on the overall XRD scan than the background pressure in the ranges researched.

Substrate temperature dependence of the growth kinetics

The time before the transition from layer-by-layer growth to steady-state growth takes place can be maximized at a certain temperature, this is expected to be near 1000

C. Raising or lowering the temperature will decrease the time layer-by-layer growth takes place.

Also it can be concluded that roughness increases when temperature increases.

Substrate temperature dependence of the transport properties

It appears samples grown at low temperatures have an overall low RRR. The best results are obtained at higher temperatures, ≥1000

C, where the sample grown at 1050

C is an exception.

Background pressure dependence of crystallinity

To grow thin film SRO with the smallest difference in lattice parameter from bulk SRO, growing at a high oxygen pressure seems to offer the best results. In our case the best results were obtained at 10

mbar O

, which is in line with the theory on oxygen and cation vacancies.

The samples with the most constant unit cell heights were grown at 10

or 10

mbar O

. However, to be able to draw conclusions, more information is needed from samples grown at pres- sures between 10

or 10

mbar.

Background pressure dependence of the growth kinetics

The background pressure does not play a major role in the time before layer-by-layer growth transitions to steady-state growth.

Also, comparing the roughness values, as seen in figure 12b, a clear pattern can be observed:

growing at higher pressures leaves a rougher surface than at lower pressures.

Background pressure dependence of the transport properties

It appears samples grown at high oxygen pressures do not stimulate high RRR. The best results

are obtained at pressures around 10

mbar argon combined with 10

oxygen.

6 CONCLUSIONS

Residual Resistivity Ratio Anisotropy

Although no exact results where obtained, an anisotropy in the resistance values and the RRR

measurements of the thin films seems to be present. This is expected to be a result of phase

boundaries in the thin films due to the step-edges of the substrate.

7 RECOMMENDATIONS

7 Recommendations

7.1 Growth Optimalisation

The temperatures around 1000

C, according to the measurements, have the most potential for yielding superconducting Sr

RuO

thin films. It may be useful to determine the effects of the temperature around this value with lower increment steps, since Krockenberger et al. suggested that only a very narrow temperature range yields a good quality film[3] and since the thin film grown at 1050

C did not seem very reliable.

When observing the influences from the different pressures, the PPMS measurements indicate that slightly increased argon pressures of 10

mbar result in a film with a high RRR. The XRD measurements show that increased oxygen pressures of 10

mbar give consistent unit-cell volumes throughout the film, however the resistivity measurements of this sample show a larger upsweep at lower temperatures. The expectation that can be drawn from this is that somewhere around partial oxygen pressures of 10

and partial argon pressures between the range of 10

to 10

an optimal pressure values can be found. So it will be useful to determine the effects of these ranges with a higher resolution.

7.2 Other Recommendations

The resistance of sample 6 had not yet reached a lowest value at 2K. Therefore, because no lower temperatures could be reached with the system used for this paper, the suggestion is to cool it down to even lower temperatures and to determine what effects this has on the resistance and thus the RRR.

Since oxygen vacancies might be present in the samples which were grown at lower background pressures, we recommend post-annealing the current samples, which showed a high RRR value already, in oxygen, especially sample 3 and 6 might have potential to reach higher RRR values and to even show signs of superconductivity.

Possible patterns in the anisotropy of the RRR

and absolute resistance differences with respect to the substrate step-edge direction have been found. However no definite conclusions can currently be drawn since no note was made on the direction of the substrates during the resistivity measurement, and also since the other influences on the directional dependence of the RRR and absolute resistance values are not known. To be able to draw conclusions about these patterns it is recommended to perform four point measurements of the resistance in multiple directions on one sample, as shown systematically in figure 14, over a temperature range from 293 to 2 Kelvin. As the connected electrodes, the remaining electrode locations in figure 14 should be connected in a similar fashion. It is important that the direction of the substrate step-edges is known. Something that can also be investigated is the influence of film thickness on this possible phenomena.

Lastly, another analysis technique which was not utilised in this report can be used to determine

the quality of the grown films and find dislocations and phase boundaries. This technique is the

Transition Electron Microscope (TEM). It can show the locations of atoms and their structure

with very high detail and determine whether the distance between dislocations is more than the

coherence length of 66nm.

7.2 Other Recommendations 7 RECOMMENDATIONS

Figure 14: Systematic figure of the RRR measurement electrode locations for measuring the anisotropy

in the RRR and the overall resistance

References

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A Appendix

A.1 AFM Pre-deposition

(a) Sample 1 (b) Sample 2

(c) Sample 3 (d) Sample 4

(e) Sample 5 (f) Sample 6

(g) Sample 7 (h) Sample 8

(i) Sample 9

Figure 15: AFM Images of the substrate pre-deposition

A.2 Nelson Riley Analysis

Figure 16: Nelson Riley analysis done on the XRD results of sample 1

Figure 17: Nelson Riley analysis done on the XRD results of sample 2

Figure 18: Nelson Riley analysis done on the XRD results of sample 3

Figure 19: Nelson Riley analysis done on the XRD results of sample 4, it is missing peak shifts and can

Figure 20: Nelson Riley analysis done on the XRD results of sample 5

Figure 21: Nelson Riley analysis done on the XRD results of sample 6

Figure 22: Nelson Riley analysis done on the XRD results of sample 7

Figure 24: Nelson Riley analysis done on the XRD results of sample 9

A.3 RHEED Data

Figure 25: Complete RHEED data measured during growth of sample 1

Figure 26: Complete RHEED data measured during growth of sample 2

Figure 27: Complete RHEED data measured during growth of sample 3

Figure 28: Complete RHEED data measured during growth of sample 4

Figure 29: Complete RHEED data measured during growth of sample 5

Figure 31: Complete RHEED data measured during growth of sample 8

Figure 32: Complete RHEED data measured during growth of sample 9

A.4 RHEED Snapshots and AFM Post-deposition

(a) RHEED Snapshot post-deposition at low tem- perature

(b) AFM post-deposition

Figure 33: Roughness analysis of sample 1

(a) RHEED Snapshot post-deposition at low tem- perature

(b) AFM post-deposition

Figure 34: Roughness analysis of sample 2

(a) RHEED Snapshot post-deposition at low tem- perature

(b) AFM post-deposition

Figure 35: Roughness analysis of sample 3

(a) RHEED Snapshot post-deposition at low tem- perature

(b) AFM post-deposition

Figure 36: Roughness analysis of sample 4

(a) RHEED Snapshot post-deposition at low tem- perature

(b) AFM post-deposition

Figure 37: Roughness analysis of sample 5

(a) RHEED Snapshot post-deposition at low tem- perature

(b) AFM post-deposition

Figure 38: Roughness analysis of sample 6

(a) RHEED Snapshot post-deposition at low tem- perature

(b) AFM post-deposition

Figure 39: Roughness analysis of sample 7

(a) RHEED Snapshot post-deposition at low tem- perature

(b) AFM post-deposition

Figure 40: Roughness analysis of sample 8

(a) RHEED Snapshot post-deposition at low tem- perature

(b) AFM post-deposition

Figure 41: Roughness analysis of sample 9

A.5 PPMS Curves

Figure 42: PPMS curve of sample 1

Figure 43: PPMS curve of sample 2