Grown Strained Ca2fe2o5 Thin Films

Synthesis and Cation Stoichiometry Effects of MBE- Grown Strained Ca2Fe2O5 Thin Films

A Thesis

Submitted to the Faculty

Drexel University

Yizhou Yang

In partial fulfillment of the

Requirements for the degree

Master of Science in Material Science and Engineering

May 2018

Dedication

This work is dedicated to the memories of my grandfather Huici Yang (1933-2018)

Acknowledgement

Work on this thesis was supported by the National Science Foundation

(grant number CMMI-1562223). Film synthesis utilized the RHEED instrument, which was acquired through an Army Research Office DURIP grant (W911NF-14-

1-0493).

Table of Contents

2.1 Perovskite oxides...... 5 2.2 Oxygen deficient perovskites ...... 6

2.2.1 Effect of Epitaxial Strain on CaFeO2.5 ...... 11

2.2.2 Magnetic and Electronic Properties of Ca2Fe2O5 ...... 15

2.2.3 Off-Stoichiometry of CaFexO2.5 ...... 16 2.3 Oxyfluorides ...... 18 2.3.1 Cation substitution ...... 19 2.3.2 Anion substitution ...... 20

3.1 Molecular Beam Epitaxy ...... 23 3.1.1 Substrate preparation and loading ...... 24 3.1.2 Effusion cells and QCM rate measurement ...... 26 3.1.3 Reflection High-Energy Electron Diffraction (RHEED) ...... 28 3.1.4 Oxygen Mass Flow Controller ...... 29 3.2 X-ray Diffraction and Reflectivity ...... 30 3.2.1 X-ray Diffraction ...... 30 3.2.2 X-ray Reflectivity ...... 32 3.2.3 X-ray Photoelectron Spectroscopy (XPS) ...... 32 3.2.4 Rutherford Backscattering Spectrometry (RBS) ...... 34

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3.3 Spectroscopic Ellipsometry ...... 35 3.4 Fluorination ...... 37

4.1 Synthesis of Ca2Fe2O5 films ...... 40 4.2 The effect of cation off-stoichiometry ...... 47

4.3 Strain effects on Ca2Fe2O5 ...... 50 4.4 Fluorination of CFO films ...... 53

List of Tables

Table 2.1 Strain induced by lattice mismatch between substrates and Ca2Fe2O5 films...... 14 Table 4.1 List of samples ...... 41 Table 4.2 The binding energies of Fe 2p3/2 of each bond involved in the fluorination process...... 58

List of Figures

Figure 1.1 Fuel cells and other electric generators. Reproduced from Ref. [7] ...... 2 Figure 1.2 Elements that can be accommodated within the perovskite structure. Reproduced from Ref. [8] ...... 3 Figure 2.1 Schematic structure of a perovskite oxide. Reproduced from Ref. [14] 5 Figure 2.2 Schematic of the expansion of the SrTiO3- lattice upon incorporation of an oxygen vacancy (red circles: O atoms; blue diamonds: Ti atoms). [15] [16] Figure reproduced from Ref. [15] ...... 7 Figure 2.3 ADF STEM image of [110]c-oriented LSCO thin film grown on LSAT substrate. Figure reproduced from Ref. [18]...... 8 Figure 2.4. In the brownmillerite unit cell, the left graph shows the b-axis direction is repeated every 4 layers. In the right graph, black solid lines mark the unit cell of the brownmillerite structure while white dashed lines mark the unit cell of the related cubic perovskite...... 9 Figure 2.5 (a) Schematic of left and right tetrahedral chain. (b) Pnma symmetry that has L and R chain alternating between layers. (c) Pbcm symmetry that has L and R chain alternating within layers. Reproduced from Ref. [20]...... 10 Figure 2.6 (a) Temperature dependent magnetization of SCO thin films. (b) magnetization hysteresis loops of SCO films at 10K. (c) Temperature dependent resistivity. Reproduced from Ref. [21] ...... 11 Figure 2.7 Blue dots represent the substrate, and red dots represent the film. (a) Film and substrates having matched in-plane lattice. (b) Film is compressed to align with the substrate. Reproduced from Ref. [16] The film is not uniform thus the out- of-plane lattice of the film changes...... 12 Figure 2.8 (a) X-ray diffraction pattern of Ca2Fe2O5 films on SrTiO3 (STO), (La0.3Sr0.7)(Al0.65Ta0.35)O3 (LSAT), LaAlO3 (LAO), and LaSrAlO4 (LSAO); the film peaks are shown in blue. (b) Schematic of film orientation on STO and LSAT. c) Schematic of film orientation on LAO and SLAO. Reproduced from Ref. [22]...... 13 Figure 2.9. Schematics showing (a) oxygen vacancies parallel to the substrate and (b) oxygen vacancies perpendicular to the substrate...... 14 Figure 2.10 (a) Temperature dependence of resistivity measurement of Ca2Fe2O5 under applied magnetic fields. [27] (b) M(H) loops of La-doped Ca2-xFe2O5. Reproduced from Ref. [28] ...... 16 Figure 2.11 Off-stoichiometry impacts on the lattice parameter of LaFeO3 thin films. Reproduced from Ref. [30] ...... 17

Figure 2.12 X-ray diffraction patters of Ca2-xLaxFe2O5. [28] ...... 18 Figure 2.13 (a) Lattice parameters for La1-xSrxMnO3. (b) Temperature dependence of resistivity for La1-xSrxMnO3; Tc represents the critical temperature for the ferromagnetic phase transition. Reproduced from Ref. [31] ...... 20 Figure 2.14 (a) XRD plot for SrFeO3-δ (002) as-grown film, SrFeO3-δFδ (002) fluorinated film, and SrFeO3 (002) ozone annealed film. Reproduced from Ref. [33] 2 (b) Optical absorption spectra of SrFeO3-xFx, the inset shows (αhv) vs hv curves for SrFeO2F and LaFeO3 film. (c) Schematic showing density of states for SrFeO3- xFx. Reproduced from Ref. [34] ...... 21 Figure 2.15 (a) Spin-coat method used for fluorination of thin films: A polymer solution is used for spin coating, then the sample is annealed, then the polymer layer is removed. Fluorine atoms diffuse into the sample in step 3. (b) Vapor transport method: in a quartz tube, a fluoropolymer is placed at the upstream position relative to the sample. A carrier gas (Ar) carries the fluorine content through the quartz tube; fluorine reaches the sample and diffuses through it. Reproduced from Ref. [35] 22 Figure 3.1 (a) photo and (b) schematic example of the molecular beam epitaxy system used in this thesis...... 23 Figure 3.2 Photo of sample holder from the load-lock. [36] ...... 25 Figure 3.3 Metal sources heating up process for (a) calcium and (b) iron...... 26 Figure 3.4 Geometry of Staib Instraments RHEED. [37] ...... 29 Figure 3.5 The schematic diagram of diffraction of X-rays by a crystal (Bragg condition) [38]...... 30 Figure 3.6 Sample graph for X-ray reflectivity [40] ...... 32 Figure 3.7 Iron(III) 1s and 2p spectra. Due to the high-spin of Fe3+, there is a multiplet-split for Fe2p spectra...... 33 Figure 3.8 Example plot of RBS measured from a CaFeO2.5 film on MgO...... 35 Figure 3.9 (a) Photo of fluorination setup. (b) Schematic of fluorination process. Argon gas flows into the quartz tube, while inside the tube, there is a boat like container which containing fluoropolymer and film...... 37 Figure 4.1 (a) The substrate temperature and MBE main chamber pressure throughout the growth. (b) Shuttering sequence for both calcium and iron for the first two unit cells...... 43 Figure 4.2 RHEED images and their intensity scan plot of a (a) plain LSAT substrate, (b) stoichiometric Ca2Fe2O5, (c) Ca0.9FeO2.5 on LSAT, (d) Ca0.79FeO2.5 on STO, (e) CaFe0.88O2.5 on STO, and (f) CaFe0.76O2.5 on STO. The lower panels are the intensity scans corresponding to the red dashed line...... 44

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Figure 4.3. Fitted and experimental RBS data from (a) CaFe0.88O2.5 and (b) CaFe0.76O2.5. (c) Comparison between the CaFe0.76O2.5 and CaFe0.88O2.5 normalized experimental data...... 45 Figure 4.4 X-ray reflectivity plots for (a) Ca2Fe2O5 on the LSAT substrate, (b) Ca2Fe2O5 on the STO substrate...... 46 Figure 4.5 (a) X-ray diffraction for cation deficient Ca2Fe2O5 films on STO substrates. The peaks around 46.5o are the substrate 002, while the peaks between 49o and 50o are from the films. (b) Rocking curve measurements obtained from the 002 reflection for calcium deficient films...... 47 Figure 4.6. Relationship between the out-of-plane lattice constant and iron concentration in the films...... 49 Figure 4.7 (a) X-ray diffraction pattern of CFO thin films on SLAO, LAO, LSAT, and STO substrates. The film peaks are indicated by arrows. (b) CFO film out-of- plane lattice parameters as a function of the substrate lattice parameter; inset shows schematics of the vacancy ordering directions. (c) Omega scans and resultant FWHM value for films on each substrate...... 51 Figure 4.8 (a) Schematic of the hypothesized evolution of grain boundaries before and after air annealing. (b) X-ray diffraction of a Ca2Fe2O5 -SLAO film before and after ~300oC hotplate air annealing for 12 hours...... 53 Figure 4.9. Post fluorination samples (a) on a STO substrate, fluorine is measured only in the surface layers (~top 22nm) of the sample, the overall average change in out-of-plane lattice parameter is 0.37%. (b) On a LSAT substrate, 15% F is uniformly distributed through the film (CaFeO1.825F0.675). The change of out-of- plane lattice parameter is 0.16%. (c) On a SLAO substrate, 15%F is present near the surface, then decreases to ~8% at the center of the filmCaFeO2.14F0.36. The change of the average out-of-plane lattice is 0.08%. Peaks at 44° are the sample holder for XRD measurement...... 56 Figure 4.10 Optical absorption measured before and after fluorination for films on (a) LSAT, (b) SLAO, and (c) STO. The upper panel of each figure is the optical absorption and the lower panel is the difference spectra...... 57 Figure 4.11 (a) Fe 2p XPS spectra before and after fluorination for CFOF-LSAT; (b) the Fe 2p3/2 peak position comparison. (c) Fe 2p spectra before and after fluorination for CFOF-SLAO; (d) the Fe 2p3/2 peak position comparison. Black lines are the experimental data and red dash lines are the fitted data in (c) and (d)...... 59

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Abstract

The physical properties of perovskite oxide (ABO3) thin films can be tailored through cation deficiency, epitaxial strain, and anion substitution. Similar approaches should be applicable to A2B2O5 brownmillerite materials that consist of alternating layers of BO6 octahedra and BO4 tetrahedra, however, such strategies have yet to be explored in detail in brownmillerite films. In this thesis, Ca2Fe2O5 thin films are synthesized using molecular beam epitaxy, and anion substitution is explored through vapor transport fluorination. Cation off-stoichiometric effects, oxygen vacancy channel orientation, and fluorine substitution are studied through characterization methods such as X-ray diffraction, X-ray reflectivity, X-ray photoelectron spectroscopy, ellipsometry, and Rutherford backscattering spectrometry. The results suggest that Ca deficiency leads to minimal changes to the lattice parameter, while Fe deficient films exhibit c-axis expansion. Epitaxial strain is shown to control the orientation of the vacancies, which is consistent with previous reports. For fluorination, the mechanism is dependent on the orientation of vacancies. When the oxygen vacancies are oriented parallel to the film/substrate interface, fluorination is dominated by fluorine substitution for oxygen, while the mechanism for perpendicularly oriented oxygen vacancies is a combination of fluorine substitution and insertion into anion vacancy sites.

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Introduction

The development of human society highly depends on the energy that can be utilized on earth. In 2015, among the total 97.7 quadrillion Btu of energy that was consumed within the U.S., 36% was based on petroleum, 29% on natural gas,

16% on coal, 9% on nuclear electric power, and only 10% was renewable energy.

[1] Fuel cells, a form of renewable energy that have advantages such as low pollution emission, high efficiency, and low cost, have drawn a lot of attention in the past decades. Compared with other fuel cells such as polymer electrolyte fuel cell and phosphoric acid fuel cell, solid oxide fuel cells have the best power generation efficiency and the supply power as shown in Figure 1.1. To further improve the efficiency of solid oxide fuel cells, considerable research is currently directed toward finding new materials to serve as the cathode, anode, and electrolyte. [2] [3] [4] [5] [6]

Figure 1.1 Fuel cells and other electric generators. Reproduced from Ref. [7].

Metal oxides with the formula unit A2B2O5 derive from the perovskite structure, ABO3, which consist of A-site, B-site cations, and anions as shown in

Figure 1.2. The properties of perovskites can easily be tailored as required for different applications, making this a very versatile material family. The A2B2O5 brownmillerite family is an anion deficient perovskite with ordered oxygen vacancies which gives brownmillerite materials desirable properties such as high oxygen ion conductivity, low electronic conductivity, and stability at high temperatures. These properties make brownmillerites promising candidates for electrolytes in solid oxide fuel cells.

Figure 1.2 Elements that can be accommodated within the perovskite structure. Reproduced from Ref. [8].

This thesis will first present the details on synthesizing Ca2Fe2O5 brownmillerite thin films through the molecular beam epitaxy method.

Characterization of the samples is performed by X-ray diffraction, X-ray reflectivity, Rutherford backscattering, X-ray photoelectron spectroscopy, and ellipsometry. The structural effects of cation off-stoichiometry, a common problem in oxide film deposition, will be presented by comparing the characterization of stochiometric and off-stochiometric samples. Finally, by applying fluorination methods to epitaxially strained Ca2Fe2O5 films, the role of ordered oxygen vacancy orientation on topochemical fluorination will be presented. These results provide

3 fundamental insights into brownmillerite film synthesis and characterization needed to apply such materials to thin film solid oxide fuel cells. [9] [10] [11] [12]

Background

2.1 Perovskite oxides

Perovskites are a crystal class with the general chemical formula of ABX3, where the A-site usually contains a large alkali or rare earth metal cation, the B-site usually contains a smaller transition metal cation, and X is an anion such as nitrogen, oxygen, or a halogen. [13] As shown in Figure 2.1, at every corner of a perovskite unit cell, there is a BX6 octahedra, and the A-site cation is located at the center of the three-dimensional corner-connected octahedral network.

Figure 2.1 Schematic structure of a perovskite oxide. Reproduced from Ref. [14].

The Goldschmidt tolerance factor (t) indicates the stability of the perovskite structure:

푟 +푟 푡 = 퐴 0 Equation 2.1 √2(푟퐵+푟0)

5 where rA, rB, and r0 each represent the ionic radii of the A, B cations and X anions.

For the perovskite structure to be stable, materials must have a tolerance factor between 0.7 to 1. For example, perovskites that have tolerance factor greater than

1, such as BaNiO3, crystallize in a hexagonal structure. Cubic perovskites such as

SrTiO3, usually have tolerance factor between 0.95-1. Other perovskites that have factor less than 0.95, such as GdFeO3, usually have an orthorhombic or rhombohedral structure.

A central motivation for studying perovskite oxides is the customizable physical properties of these materials. Especially in transition metal perovskites, by interchanging the atoms in the A-site or the anion site, the B-site valence state can be modified, resulting in changes of the physical behavior such as electrical conductivity, magnetic ordering, and optical absorption.

2.2 Oxygen deficient perovskites

Perovskites that are oxygen deficient have the chemical formula of ABO3-δ, where δ is the amount of oxygen missing per formula unit. Oxygen stoichiometry has a significant impact on determining the properties of perovskite materials. In a stoichiometry SrMnO3 perovskite, oxygen has valence state of -2, and strontium has valence state of +2, which gives a nominal +4 state on manganese. However, if there are oxygen vacancies present (eg. δ = 0.5), the valence state of manganese is reduced from +4 to +3. Figure 2.2 shows when a local oxygen ion is removed, the

6 nominal charge of -2 leaves a positively charged environment at the vacancy. Due to the impact of the Coulombic repulsion force between atoms with the same charge, the nearby titanium B-site cations move away from the vacancy, whereas four nearby oxygen ions are attracted towards the vacancy. [15]

Figure 2.2 Schematic of the expansion of the SrTiO3- lattice upon incorporation of an oxygen vacancy (red circles: O atoms; blue diamonds: Ti atoms). [15] [16] Figure reproduced from Ref. [15].

In the A2B2O5 brownmillerite structure, for every six oxygen atoms, there is one missing. Also, compared to disordered oxygen-deficient perovskites, the oxygen vacancies in brownmillerites are ordered. Instead of each layer containing

BO6 octahedra, in the brownmillerite structure, there are alternating layers of BO4 tetrahedra and BO6 octahedra. The typical tolerance factors for a brownmillerite composition are between 0.94 and 1.01. [17] Figure 2.3 shows the STEM image of a brownmillerite (La0.5Sr0.5)2Co2O5 thin film grown on a LSAT substrate. Compare

7 to the perovskite LSAT, the LSCO film exhibits significant ordered oxygen vacancies. [18]

Figure 2.3 ADF STEM image of [110]c-oriented LSCO thin film grown on LSAT substrate. Figure reproduced from Ref. [18].

Due to the structural distortion and octahedral rotation that is caused by oxygen deficient, the size of brownmillerite unit cell is different from its related perovskite structure. In each brownmillerite unit cell, the b-axis direction contains four layers of B-site atoms, the first and the third layers consist of rotated octahedra, and the second and fourth layers consist of tetrahedra that are pointing in opposite directions. Therefore, the b-axis lattice parameter (b) in brownmillerite unit cell is four times then the related pseudocubic perovskite structure (bp). As shown in

Figure 2.4, the brownmillerite in-plane lattice (a) is √2 times larger than the related cubic perovskite in-plane lattice parameter (ap) due to the distortions.

Figure 2.4. In the brownmillerite unit cell, the left graph shows the b-axis direction is repeated every 4 layers. In the right graph, black solid lines mark the unit cell of the brownmillerite structure while white dashed lines mark the unit cell of the related cubic perovskite.

There are many previous studies of the structure and material properties of brownmillerites. D’Hondt et al. reported the preferred tetrahedral chain order in the

Sr2Fe2O5 brownmillerite [19]. Using transmission electron microscopy, the study discovered two different orders, left (L) and right (R) chains, within the tetrahedral layers as shown in Figure 2.5a. The tetrahedral chain ordering influences the oxygen vacancy channel between the tetrahedral chains. [19] Young and

Rondinelli’s study showed that the chain order would lead to different space groups.

If all chains are in the same direction, the structure is I2bm, if there is an alternation within each layer, then the structure is Pbcm as shown in Figure 2.5b. If the alternation happens between each layer, the result is a Pnma structure as shown in

Figure 2.5c. Young’s research also indicates that Ca2Fe2O5 prefers Pnma symmetry,

9 while Sr2Fe2O5 prefers Pbcm symmetry. These preferences are based on the rotation of tetrahedral chains.

Figure 2.5 (a) Schematic of left and right tetrahedral chain. (b) Pnma symmetry that has L and R chain alternating between layers. (c) Pbcm symmetry that has L and R chain alternating within layers. Reproduced from Ref. [20].

From the perspective of physical properties, Jeen et al., reported epitaxial growth of brownmillerite Sr2Co2O5 (SCO) single crystal films on SrTiO3 substrates by pulsed laser epitaxy [21]. X-ray absorption spectroscopy (XAS) measurements show that the Co L-edge shifts to a higher energy state when the SCO film is oxidized from brownmillerite (BM) structure (Co3+) to a perovskite (P) structure

(Co4+). As shown in Figure 2.6a and b, the magnetization of BM-SCO has a constant value of zero. However, when the material is transformed to the oxidized structure, it shows ferromagnetism which is brought about by Co4+. Figure 2.6c presents the temperature dependent electronic transport properties. BM-SCO

10 showed a highly insulating behavior while in P-SCO, the resistivity is significantly reduced. [21]

Figure 2.6 (a) Temperature dependent magnetization of SCO thin films. (b) magnetization hysteresis loops of SCO films at 10 K. (c) Temperature dependent resistivity. Reproduced from Ref. [21] .

2.2.1 Effect of Epitaxial Strain on CaFeO2.5

In epitaxial deposition, a mismatch in the in-plane lattice parameters between the film and the substrate can result in a biaxially strained film, as shown in Figure 2.7. When the film’s bulk in-plane lattice parameter is larger than that of the substrate, the film will be compressed which will result in an increased out-of- plane lattice parameter. Similarly, if the film is under tensile strain, then the out-of- plane lattice parameter will decrease.

Figure 2.7 Blue dots represent the substrate, and red dots represent the film. (a) Film and substrates having matched in-plane lattice. (b) Film is compressed to align with the substrate. Reproduced from Ref. [16]. In this example, the film is not uniform thus the out-of-plane lattice of the film changes as a function of thickness.

Bulk Ca2Fe2O5 has a distorted orthorhombic structure with lattice parameters a = 5.427 Å, b = 14.763 Å, and c =5.597 Å. As discussed in Figure 2.4, when this film is growing along the b direction, the related perovskite would have

1 in-plane lattice parameter ap = cp = 푎, while the out-of-plane lattice parameter is √2

1 bp = b. 4

To investigate strain-related property changes, Inoue and coauthors grew

Ca2Fe2O5 films on different substrates to apply varying amounts of strain to the films [22]. The X-ray diffraction measurement shown in Figure 2.8 was used to obtain the out-of-plane lattice parameters of the strained films. The results indicate there are two different orientations for the as-grown films. For the STO and LSAT

1 substrates, the films grow vertically with the in-plane lattice parameter equal to 푎 √2

1 = 3.837 Å, and the out-of-plane lattice equal to b =3.691 Å. For the LAO and 4

SLAO substrates, the films grow horizontally which has relaxed in-plane lattice

1 1 equal to b =3.691 Å, while the out-of-plane lattice parameter equals to 푎 = 4 √2

3.837 Å as shown in Figure 2.8b and c. [22]

Figure 2.8 (a) X-ray diffraction pattern of Ca2Fe2O5 films on SrTiO3 (STO), (La0.3Sr0.7)(Al0.65Ta0.35)O3 (LSAT), LaAlO3 (LAO), and LaSrAlO4 (LSAO); the film peaks are shown in blue. (b) Schematic of film orientation on STO and LSAT. c) Schematic of film orientation on LAO and SLAO. Reproduced from Ref. [22].

The lattice strain which is induced by the lattice mismatch can be calculated as:

푎 −푎 휀 = s f × 100% Equation 2.2 푎f

where 푎s is the substrate in-plane lattice, and 푎f is the bulk CFO lattice parameter.

Table 2.1 gives a summary of the values of the substrates and CFO in-plane lattice

13 parameters. Comparing the values of 3.836 Å and 3.691 Å to the substrate in-plane lattice parameters, and also the growth orientation, one finds that the CFO films tend to grow under tensile strain, and orient to minimize the strain value.

Table 2.1 Strain induced by lattice mismatch between substrates and Ca2Fe2O5 films.

Substrate in- Bulk CFO Lattice Lattice Mismatch plane Lattice Parameter (Å) induced lattice strain Parameter (Å) LSAT 3.868 3.837 0.81% STO 3.905 3.837 1.77% LAO 3.793 3.691 2.76% SLAO 3.756 3.691 1.76%

Figure 2.9. Schematics showing (a) oxygen vacancies parallel to the substrate and (b) oxygen vacancies perpendicular to the substrate.

As discussed earlier, the oxygen vacancies in the brownmillerite are ordered, therefore, a change in the growth orientation will result in the change of oxygen vacancy orientation as shown in Figure 2.9. Young’s research investigated

14 the impact of the vacancy orientation on the band gap. [20] For the parallel orientations (SLAO, LAO), an increase of tensile strain would result in an increase of the band gap, while for the perpendicular orientation (LSAO, STO), the band gap would decrease with the increasing of tensile strain. This results from the connectivity of the Fe-O-Fe bond angle between tetrahedral iron and octahedral iron (ΘOT). For instance, in the parallel vacancy orientation, ΘOT is reduced from

180o with increasing tensile strain, and this results in decreasing overlap of Fe d orbitals and O p orbitals, which gives a larger band gap. [20]

2.2.2 Magnetic and Electronic Properties of Ca2Fe2O5

Detailed magnetic and electronic properties of bulk Ca2Fe2O5 (BM-CFO) has been studied thoroughly in the past. At room temperature, BM-CFO shows G- type antiferromagnetism, with a weak ferromagnetic magnetization. [23] When the temperature reaches the Neel temperature, 725 K, BM-CFO transitions to a paramagnetic state. [24] Moreover, at T = 1180 K a significant in-plane lattice expansion is found due to a structural phase transition from Pnma to an incommensurate Imma structure, and att T = 1308 K, observation of a dilatometric anomaly has been made. [25] BM-CFO is an electronic insulator, and has oxygen- ion-related conductivity. However, if BM-CFO is oxidized to CaFeO3 (P-CFO), there is a metal-insulator transition at 290 K. [26] The red curve and the lower inset

15 in Figure 2.10a shows the insulating behavior of BM-CFO, while the black curve in Figure 2.10b shows the antiferromagnetic behavior.

Figure 2.10 (a) Temperature dependence of resistivity measurement of Ca2Fe2O5 under applied magnetic fields. [27] (b) M(H) loops of La-doped Ca2-xFe2O5. Reproduced from Ref. [28].

2.2.3 Off-Stoichiometry of CaFexO2.5

Although molecular beam epitaxy is a precise way of controlling thin film growth, variables such as the chamber pressure, source temperature, and the sensitivity of the quartz crystal microbalance (QCM) crystal affect the cation stoichiometry of perovskite and brownmillerite films. Previous work on cation deficient perovskites suggests off-stoichiometry can alter interfacial conductivity, film lattice expansion [29], and optical properties [30]. In the study of cation off- stochiometric LaFeO3 thin films, the most significant change that results from the off-stoichiometry is the out-of-plane lattice parameter. Figure 2.11a shows the c-

16 axis parameter changes in LaFeO3 with either iron or lanthanum deficiency. The results suggest that compared with A-site deficiency, B-site deficiency has much less impact on the out-of-plane lattice parameter. When there is excessive A-site cation concentration or B-site vacancies, the charge balance will be reached through formation of oxygen vacancy or reduction of B-site valence state. [30]

Figure 2.11 The impact of cation off-stoichiometry on the c-axis lattice parameter of LaFeO3 thin films. Reproduced from Ref. [30].

For brownmillerite materials, though there is close to no research that has been conducted on the effect of cation off-stoichiometry. Phan’s research on La- doped Ca2Fe2O5 showed a lattice parameter expansion resulting from the reduction of B-site iron. Figure 2.12 shows the XRD pattern of Ca2-xLaxFe2O5; the film peak at about 33.5o shifts towards the lower 2θ position which indicates the out-of-plane

17 lattice expansion. The additional peaks in the XRD data suggest possible phase separation. [28] However, in this study, the A:B ratio is 1:1 and thus the influence of cation off-stoichiometry was not reported.

Figure 2.12 X-ray diffraction patterns of Ca2-xLaxFe2O5. Reproduced from Ref. [28].

2.3 Oxyfluorides

The electronic properties of perovskites mainly depend on the electron configuration of the B-site transition metal. To tailor the valence state of the B-site metal, past researchers have mainly focused on two approaches, cation substitution and anion substitution.

2.3.1 Cation substitution

As an example of cation substitution, we first consider manganites. Pure

2+ LaMnO3 is an insulator. However, if Sr is substituted onto the A-site, the hole concentration increases, leading to a transition from an insulator to a metal. Figure

2.13a shows that the in-plane lattice parameter (a) contracts with increasing amount of Sr substitution, while the out-of-plane (c) lattice behaves oppositely as the in- plane parameter. The structure of La1-xSrxMnO3 changes from orthorhombic (with

Pbnm symmetry) to rhombohedral (with 푅3̅푐 symmetry) as a function of x. Figure

2.13b shows the insulator to metal transition behavior due to the increasing amount of Sr substitution. In LaMnO3 (x=0), the material shows a typical insulator resistivity curve, while at x=0.175, the material starts showing metallic behavior.

The critical temperature indicates a ferromagnetic phase transition that increases with increasing amounts of Sr. In LaMnO3, Mn has a +3 state; after the Sr doping, the Mn valence state increases towards +4.

Figure 2.13 (a) Lattice parameters for La1-xSrxMnO3. (b) Temperature dependence of resistivity for La1-xSrxMnO3; Tc represents the critical temperature for the ferromagnetic phase transition. Reproduced from Ref. [31].

2.3.2 Anion substitution

During anion substitution oxygen is subsstituted by other anions that have a different valence state, such as fluorine. In the example of fluorinated SrFeO3-δFδ, the valence state of Sr is +2, and O is -2. In the SrFeO3 structure, Fe has nominal valence state of +4, however, if oxygen is substituted with fluorine which has valence state of -1, Fe moves to a lower oxidation state. This is supported by Sr and

Fe K-edge X-ray absorption which conducted by Berry et al. [32] The bottom two plots in Figure 2.14a show that the lattice expands when iron’s oxidation state lowers from +4 to +3. [33] Figure 2.14b shows when the fluorine concentration is

~ 0.6, there is a new absorption peak at 1 eV. As shown in the inset graph, iron in

LaFeO3 and SrFeO2F has the same valence state, and both materials have direct band gaps. Figure 2.14c shows that the energy of transition A, assigned to the

20 valence to conduction band transition, increases with increasing fluorine content.

The increased band gap suggests that with increasing fluorine substitution in the perovskite, the material becomes more insulated.

Figure 2.14 (a) XRD plot for SrFeO3-δ (002) as-grown film, SrFeO3-δFδ (002) fluorinated film, and SrFeO3 (002) ozone annealed film. Reproduced from Ref. 2 [33]. (b) Optical absorption spectra of SrFeO3-xFx, the inset shows (αhv) vs hv curves for SrFeO2F and LaFeO3 film. (c) Schematic showing density of states for SrFeO3-xFx. Reproduced from Ref. [34].

In order to introduce fluorine content into the perovskite structure, two methods are used as shown in Figure 2.15. The methods are a spin coating and a vapor transport approach. X-ray diffraction measurements show that the film fluorinated with spin coating methods has a more dramatic peak shift than film

21 fluorinated with vapor transport method. However, the vapor transport method gives better crystallinity of the film, and the fluorine concentration in the film is more uniform. [35] Based on these results, the vapor transport method was used in the course of this thesis.

Figure 2.15 (a) Spin-coating method used for fluorination of thin films. A polymer solution is used for spin coating, then the sample is annealed, and then the polymer layer is removed. Fluorine atoms diffuse into the sample in step 3. (b) Vapor transport method: in a quartz tube, a fluoropolymer is placed at the upstream position relative to the sample. A carrier gas (Ar) carries the fluorine vapor through the quartz tube; fluorine reaches the sample and diffuses into it. Reproduced from Ref. [35].

Experimental techniques

3.1 Molecular Beam Epitaxy

Thin film deposition is commonly achieved by the following methods: molecular beam epitaxy (MBE), chemical vapor deposition (CVD), pulsed laser deposition (PLD), and sputtering. In contrast to CVD and PLD, MBE is conducted in an ultra-high vacuum environment (<1×10-8 Torr). Though the deposition speed of MBE is relatively low (~60 sec/monolayer), MBE can control the film composition very precisely. In this thesis, CaFeO2.5 thin films are deposited with

MBE.

a) b)

Figure 3.1 (a) Photo and (b) schematic of the molecular beam epitaxy system used in this thesis.

Figure 3.1 shows a photo and schematic illustration of the MBE. Calcium and iron sources are sublimed using effusion cells, and the oxygen source is

23 introduced and controlled through the mass flow control system. For every layer of

CaFeO2.5, the fluxes of calcium and iron must be dosed to the substrate in equal part to avoid cation off-stoichiometry. The deposited atoms arrange into the perovskite-like structure due to the epitaxial constraint of the substrate.

3.1.1 Substrate preparation and loading

On the day before growth, substrates are prepared. Substrates with various in-plane lattice parameters including (001) (LaAlO3)0.3(Sr2AlTaO6)0.7 (aLAST =

3.868Å), (001) SrTiO3 (aSTO = 3.905Å)), (001) LaAlO3 (aLAO = 3.793Å), and (001)

SrLaAlO4 (aSLAO=3.756Å) are cleaned by rinsing in acetone for 15 mins, following by rinsing in isopropyl alcohol for 15 mins. Leitsilber 200 Silver paint is used as the adhesive between the substrates and the tungsten stub which is stable under high growth temperature condition (tungsten melting temperature=3422oC). Then the assembled stub and sample is placed on a hot-plate for 30 minutes at 155oC to cure the silver paint.

Samples are transferred into the main chamber through the load-lock (LL) chamber. When the turbopump of the LL is turned off, the LL goes to atmosphere pressure, and substrates can be loaded through the LL gate. Then the turbopump is turned back on again to reach the ultra-high vacuum state which is a similar pressure as the main chamber. During the loading process, nitrogen gas is introduced into the load-lock to purge any contamination that might go into the LL

24 during sample loading. By using LL as an intermediate transfer chamber, the pressure in the main chamber and at the metal sources can stay low to avoid any contamination or perturbation.

Figure 3.2 Photo of sample holder from the load-lock. [36]

Samples are transferred to the sample holder (Figure 3.2) temporarily located at the top sector of the main chamber from the interlock, and it is wheeled down to the bottom sector of the main chamber, where all five effusion cells are aiming to the center of the sample. The substrate is heated using a TDK-Lambda

GEN60-12.5 power supply. A multimeter is connected to the substrate thermal couple to read the in-situ substrate temperature. While ramping up the temperature of the substrate, the silver paint on the stub is likely to outgas which leads to a spike of the chamber pressure. Thus, after the power supply reaches 3 A, the temperature is raised with a 0.25 A increment until the desired temperature to avoid any pressure spikes in the chamber.

3.1.2 Effusion cells and QCM rate measurement

The MBE has 5 separate effusion cells which point directly to the center of substrate holder. Each cell contains one metal source: calcium, iron, strontium, manganese, and lanthanum. For the CaFeO2.5 growth specifically, only calcium

(99.5% Calcium shot, redistilled, Alfa Aesar, CAS#7440-70-2) and iron (99.95%

Iron slug, Alfa Aesar, CAS#7439-89-6) sources are used. While calcium and iron

o o are heated to the desired temperature, (TCa=500 C, TFe=1225 C) the other sources stay at room temperature with their cell shutters closed.

Figure 3.3 Metal sources heating up process for (a) calcium and (b) iron.

A recirculating water chiller is used to provide cooling water to the shell outside the k-cell. At the beginning of the process, the surface of the calcium might be slightly oxidized or contaminated from the previous growth. Thus, by increasing the source temperature, the surface layers sublime, leading to spikes of the main chamber pressure. The solution to this issue is use a 3-stage heating process as

26 shown in Figure 3.3a. The heating up process is continued when a stable pressure is observed at 40oC and 200oC. Eventually the Ca source can be heated up to desired temperature (around 500oC). For heating up the Fe source, the 3-stage process used for Ca is not needed due to less pressure fluctuations during heating up. Fe is heated with a rate of 0.5 degree per second until the desired temperature (around 1250oC), as shown in Figure 3.3b. When both sources are heated, the main chamber pressure are stabilized at approximately 5×10-8 Torr.

During growth, the parameters that control the amount of deposited metal are the effusion cell temperature and the shutter time which is remotely controlled by an Arduino program. The shutter time is calculated based on its proportionality to the rate that is measured by a quartz crystal microbalance. The crystal in the

QCM detects small changes of mass. During the measurements, one source is sublimed to the surface of the crystal, and the film thickness along with time is recorded. The last eight minutes of the measurement is used in calculating the atomic flux, using

Thickness −Thickness rate = t t−480 Equation 3.1 QCM 480s where t represents the duration in seconds. From the rate measurement, the shutter time can be calculated as:

substrate surface density timeshutter = . Equation 3.2 K−value∗ rateQCM

The substrate surface density is the number of cations on each layer of the substrate where a and b are the in-plane lattice parameter:

1 substrate surface density = . Equation 3.3 푎∗푏

The K-value is the correction factor for composition stoichiometry, and it is measured by X-ray photoelectron spectroscopy and Rutherford backscattering (see section 3.3 for details), and X-ray reflectivity. The correct K-value is critical to obtain the right stoichiometry of calcium and iron.

3.1.3 Reflection High-Energy Electron Diffraction (RHEED)

Since MBE gives precise control on the layer deposition at the atomic scale, the in-situ surface monitoring technique for crystal growth becomes very important.

In RHEED, an electron beam generated from an electron gun is directed toward the substrate surface as shown in Figure 3.4. Based on the surface structure and the atomic spacing, the diffracted intensity of the reflected beam would be either constructive or destructive. The dot-like or line-like diffraction pattern depends on the geometry of the surface. In this thesis, a RHEED system consisting of an electron gun (STAIB instruments Mod B11000037P05), and a camera (kSA 400) that records images of the pattern from the RHEED screen.

Figure 3.4 Geometry of Staib Instruments RHEED gun. [37]

3.1.4 Oxygen Mass Flow Controller

To oxidize the film during growth, oxygen is introduced into the chamber though a mass flow controller (MFC). Before each growth, the oxygen lines are purged and filled three times. Oxygen enters the chamber through the needle valve, at which time the pressure in the chamber increases accordingly. With the MFC, the amount of oxygen introduced into the chamber is controlled. For Ca2Fe2O5 brownmillerite films, the growth pressure is usually 2.5×10-6 Torr, while the rate of oxygen input is 0.075 sccm.

3.2 X-ray Diffraction and Reflectivity

3.2.1 X-ray Diffraction

X-ray diffraction is a common non-destructive method for measuring the crystal structure. To probe the atomic spacing of a crystal, X-rays, a form of electromagnetic radiation which has shorter wavelength (λ) than the spacing between atoms is used.

Figure 3.5 A schematic diagram of diffraction of X-rays by a crystal (Bragg condition) [38].

As shown in Figure 3.5, the path difference of two incident waves is Δ. When Δ =

1 nλ, the two outgoing waves are in-phase, and when Δ = nλ, outgoing waves cancel 2 out, which is out-of-phase, where n is an integer. When the reflected beams are all in-phase, there will be a detectable diffraction peak. Bragg’s law provides the relationship between the lattice parameter (d) and the incident angle (θ):

2dsinθ = nλ Equation 3.4

For a crystal with known structure, the position of the diffraction peak can be calculated. Samples in this study are measured with monochromatic X-rays, and from the 0 0 2 diffraction peak for the thin film, the c-lattice parameter which indicates the out-of-plane atomic spacing is be calculated. [38]

For this study, a Rigaku SmartLab X-ray diffraction system is used. The wavelength of X-ray is 1.54 Å. The setup includes a parallel beam mode with a two-bounce Ge(220) monochromator module. The program GenX is used to fit

XRD data [39]; the program simulates diffraction from a model of thin film samples grow over substrates.

3.2.2 X-ray Reflectivity

Figure 3.6 Sample graph for X-ray reflectivity, with the physical implications for data features given in colored text. Reproduced from Ref. [40].

With the same diffractometer as X-ray diffraction, X-ray reflectivity (XRR) can also be measured. XRR measurements are conducted in the low angle range

(2θ = 0.5-4o). The purpose is to measure the thickness and the surface roughness of the thin film. Figure 3.6 gives an example of XRR data set: the critical angle depends on the electron density of a surface; the width of each fringe gives the thickness of the film; the intensity decay with increasing angle indicates the surface roughness of a sample.

3.2.3 X-ray Photoelectron Spectroscopy (XPS)

In the photoelectric effect, when a photon which contains high enough energy hits an atom, an electron will be ejected. In XPS experiments, the kinetic

32 energy (KE) of the emitted electron is measured. With known electron kinetic energy, the binding energy (BE) can be derived as [41]:

K. E.XPS = Eph − ϕXPS − B. E.XPS Equation 3.5 where Eph is the initial photo energy, and ϕXPS is the work function of the material.

For each element, its stationary state has a specific number of electrons that can be emitted. [42] To obtain depth-dependent information, an ion beam is used to etch away layers of the surface, while spectra are collected before and after the etching process.

Fe 2p3/2

Fe 2p1/2

Fe 1s

670 680 690 700 710 720 730 740 750 Binding Energy (eV)

Figure 3.7 Iron(III) 1s and 2p spectra. Due to the high-spin of Fe3+, there is a multiplet-split for Fe 2p spectra.

Figure 3.7 shows a sample iron spectrum containing the photoelectron intensity as a function of the binding energy. Due to the electrostatic interaction between the valence electrons and the nucleus, removal or addition of electronic charge can result in a decrease or increase of the binding energy. [43] Therefore, by measuring the binding energy of Fe 2s peak, the chemical state of iron can be determined. The XPS measurements are conducted on Physical Electronics

VersaProbe 5000 XPS and AES spectrometer by Jiayi Wang at Drexel University

Central Research Facility.

3.2.4 Rutherford Backscattering Spectrometry (RBS)

Compared to XPS, RBS is a non-destructive ion beam analysis method for measuring the composition of a sample. The beam used in RBS consists of alpha particles (4He+) with 2 MeV energies. During the material analysis, the energy and flux of the backscattered ions are measured. [42]

8000

7000

6000

5000

4000

Counts 3000 Fe 2000 Ca

1000

-1000 0 500 1000 1500 Channel

Figure 3.8 Example plot of RBS measured from a CaFeO2.5 film on MgO.

The program SIMNRA is used for RBS analysis [44]. Figure 3.8 shows a sample RBS spectrum of a CFO thin film grown on a MgO substrate. The Ca elemental signal can be found between channels 900 and 1000, and the Fe signal can be found between channel 1000 and 1100. Counts at channels between 250 and

800 come from the MgO substrate. The RBS measurements are conducted by Dr.

Ryan Thorpe at Rutgers University in the Experimental Surface Science Group.

3.3 Spectroscopic Ellipsometry

Ellipsometry is a method for measuring the optical and dielectric properties of a sample that has relatively smooth surface. When electromagnetic waves travel

35 through the thin film, the wave speed changes, and some of the incident wave is absorbed by the sample, the rest of the wave is reflected to the air. The raw data measured from the ellipsometer is Ψ and Δ, in which tan(Ψ) is the amplitude ratio between incidence linearly polarized light and reflected elliptically polarized light, and Δ is the phase difference, the relationship can be shown as:

r̃ tan(Ψ)eiΔ = P . Equation 3.6 r̅s

Here r̃P and r̃s are the Fresnel reflection coefficient for the parallel and perpendicular plane of incidence. In this thesis, the raw data is analyzed using a

Cauchy model. The Cauchy function is used to describe the relationship between the index of refraction and the wavelength [45]:

푛 푛 푛(휆) = 푛 + 1 + 2 , Equation 3.7 0 휆2 휆4 where n0, n1, n2 are the Cauchy parameters. For weakly absorbing materials, a non- zero absorption coefficient α can be calculated as [45]:

훼 훼 훼(휆) = 훼 + 1 + 2 Equation 3.8 0 휆2 휆4 where 훼0, 훼1, 훼2 are the extinction parameters. The relationship between the absorption coefficient and bandgap energy is [34]:

n (훼ℎ푣) = 퐴(ℎ푣 − Eg) Equation 3.9 where ℎ푣 is the photon energy, Eg is the bandgap energy, n is a constant. In this study, when the valence state of iron changes, it is interesting to investigate whether the resonant frequency which is related to the band gap changes. WVASW software is used to analyze the absorption data.

3.4 Fluorination

Figure 3.9 (a) Photo of fluorination setup. (b) Schematic of fluorination process. Argon gas flows into the quartz tube, while inside the tube, there is a boat which contains the fluoropolymer and film.

To introduce fluorine into the oxygen deficient perovskite structure, a chemical vapor transport method is used. Figure 3.9 shows the schematic of the setup. In the ceramic boat, fluorine containing polymers such as poly tetrafluoroethylene

(Aldrich chemistry, LOT#BCBR4757V) and polyvinylidene fluoride (Aldrich chemistry, LOT#MKBP6851V) are used. These polymers create fluorine which is carried by the argon gas and diffuse into the film. A tube finance (Thermo Scientific

Lindber Blue M, Model#TF55030A-1) is used to heat the boat up to temperatures

37 between 200oC to 300oC, yet the maximum temperature should not exceed the melting temperature of the polymers.

Result and Discussion

A2B2O5 materials with the brownmillerite structure such as Sr2Fe2O5, Sr2Co2O5, and Ca2Fe2O5 have been studied in both bulk and thin film forms. Of particular relevance for this thesis, bulk Ca2Fe2O5 was synthesized by stoichiometric reaction of CaCO3 and metal oxides (Al2O3; Ga2O3; Fe2O3); while, Ca2Fe2O5 thin films that have been studied were mostly synthesized by the pulsed laser deposition method.

[46] [47] [20] [48] [49] [50] [51] This chapter presents the synthesis of Ca2Fe2O5 thin films using molecular beam epitaxy (MBE), and for the first time shows how cation off-stoichiometry affects material properties. Moreover, strain dependent vacancy ordering and the property changes that result from vacancy ordering will also be presented.

4.1 Synthesis of Ca2Fe2O5 films

In this thesis, Ca2Fe2O5 epitaxial thin films were used to study the effect of cation off-stoichiometry and strain on fluorination. In total, 26 thin film growths were conducted using MBE. Among all the samples, 13 of them were deposited on

(LaAlO3)0.3(Sr2AlTaO6)0.7 substrates (CFO-LSAT); 14 of them were on SrTiO3

(CFO-STO); 8 of them were on LaAlO3 (CFO-LAO); 8 of them were on SrLaAlO4

(CFO-SLAO); and 7 of them were on MgO (CFO-MgO). As shown in Table 4.1, 5 films are 80 unit cells (u.c.), 24 films are 100 u.c. and 9 films are 200 unit cells. All

40 samples were measured with in situ reflection high-energy electron diffraction

(RHEED), X-ray diffraction (XRD), X-ray reflectivity (XRR), and spectroscopic ellipsometry. Selected samples were measured by Rutherford backscattering spectroscopy (RBS) and X-ray photoelectron spectroscopy (XPS).

Table 4.1 List of samples

Sample Substrate Thickness Data presented in this Names (u.c.) chapter CFO2 LSAT 80 CFO3 LSAT 80 CFO4 STO 80 CFO6a/b LSAT/STO 80 CFO7a/b LSAT/STO 100 Figure 4.2b, 4.4ab, 4.5ab CFO8a/b LSAT/STO 100 Figure 4.2c, 4.5ab CFO10a/b LSAT/STO 100 CFO11 LAO 100 CFO12 STO 100 Figure 4.2d, 4.5ab CFO13 LSAT 100 CFO Cali1a/b STO/SLAO 100 Figure 4.2d, 4.3ac, 4.5a CFO Cali 2a/b STO/SLAO 100 Figure 4.2f, 4.3bc, 4.5a CFO14a/b LAO/SLAO 100 CFO15 LSAT 100 CFO16 LAO 100 CFO17a/b/c LSAT/STO/SLAO 100 CFO18 LAO 100 CFO19a/b STO/LAO 100 CFO20a/b LSAT/LAO 100 CFO21a/b LSAT/SLAO 100 CFO22a/b/c LSAT/STO/SLAO 200 Figure 4.7abc, Figure 4.8b CFO23a/b/c LSAT/STO/SLAO 200 Figure 4.9abc, 4.10abc, 4.11abcd CFO24a/b/c LSAT/STO/LAO 200 Figure 4.7abc

Figure 4.1 shows the substrate temperature and chamber pressure during the

MBE growth. In Figure 4.1a, the red solid line represents the substrate temperature.

o At t = 0 mins, the substrate temperature (Ts) is room temperature (~25 C), and the

-8 chamber pressure (Pc) is in an ultra-high vacuum state (~5×10 Torr). Ts is then ramped up to ~130oC (5.3 mA multimeter reading) over the course of 10 minutes.

At this point, molecular oxygen is introduced into the growth chamber before Ts

-6 goes any higher. After Pc reaches to 2.5×10 Torr, Ts is ramped up to the desired

o growth temperature (~650 C). 5 minutes after both Ts and Pc stabilize at the growth condition, the shuttering sequence for calcium and iron is started. For a 100 unit cell film, a typical growth time is 60 minutes at a growth rate of ~0.6-0.7 nm/min.

When growth is finished, both effusion cells are set back to room temperature,

o -8 while Ts cools to 130 C in 10 minutes. Then Pc is decreased to 5×10 Torr by stopping the flow of oxygen into the chamber. In the end of the process, Ts is set back to room temperature and the sample is removed from the growth chamber.

Figure 4.1b shows a sample shutter process for the first two unit cells; this sequence is then repeated for all other unit cells in the deposition. In this example, the iron shutter time is 32.6 seconds, and the calcium shutter time is 38.9 seconds. The exact shutter times are determined by pre-growth flux measurements using the quartz

42 crystal monitor. After 5 seconds delay, both iron and calcium shutters open, and the process is repeated.

Figure 4.1 (a) The substrate temperature and MBE main chamber pressure throughout the growth. (b) Shuttering sequence for both calcium and iron for the first two unit cells.

In situ RHEED gives real-time feedback on the surface crystallinity of the samples in the chamber during growth. Figure 4.2a shows the RHEED pattern measured from a blank substrate of LSAT (001); the RHEED pattern for blank STO

(001) is very similar to LSAT. When the surface of the deposited film is two- dimensional (2D), RHEED patterns are typically lines such as Figure 4.2b. The smooth intensity scan between the diffraction streak (there are no extra dots or lines) indicates that stoichiometric Ca2Fe2O5 has a smooth surface. Figure 4.2c,d show RHEED patterns from calcium deficient films. The RHEED pattern from the

Ca0.9FeO2.5 film (Figure 4.2c) shows half-order streaks. Since the RHEED pattern directly probes reciprocal space, the half-order lines indicate the formation of a

43 doubled unit cell at the surface in the real space. In the Ca0.79FeO2.5 film (Figure

4.2d), the RHEED pattern starts showing some transmission spots. These spots result from the electron beam incident to a rough surface with three-dimensional islands. [52] Meanwhile, for the RHEED patterns from iron deficient films (Figure

4.2e and f), the surfaces are still 2D-like. But for these films the reflection streaks are broader than the ones in Figure 4.2b, which can be explained by instead of having only single crystal on the surface, the surface consists of many small domains. [52]

Figure 4.2 RHEED images and their intensity scan plot of a (a) plain LSAT substrate, (b) stoichiometric Ca2Fe2O5, (c) Ca0.9FeO2.5 on LSAT, (d) Ca0.79FeO2.5 on STO, (e) CaFe0.88O2.5 on STO, and (f) CaFe0.76O2.5 on STO. The lower panels are the intensity scans corresponding to the red dashed line.

RBS is used to analyze the atomic concentrations (cation stoichiometry) for the as-grown samples. Considering that RBS not only measures the film, but also takes the substrate into the measurement, MgO substrates are the best choice for

RBS measurement as the elements Mg and O both have lighter atomic weights than

Ca and Fe, which allows easier analysis of the Ca and Fe peaks. Figure 4.3a and b show the RBS data and fits for the CaFe0.88O2.5 and CaFe0.76O2.5 films. Each peak from left to right is magnesium, calcium, and iron, based on the atomic weight of each elements. Figure 4.3c compares the CaFe0.88O2.5 and CaFe0.76O2.5 experimental data. The area under the calcium peak was larger for the red curve than the black curve, which indicates that the CaFe0.88O2.5 film has a greater calcium concentration than the CaFe0.76O2.5 film. Other films that were grown simultaneously to these films – those on LSAT and STO substrates – are assumed to have the same cation stoichiometry.

Figure 4.3. Fitted and experimental RBS data from (a) CaFe0.88O2.5 and (b) CaFe0.76O2.5. (c) Comparison between the CaFe0.76O2.5 and CaFe0.88O2.5 normalized experimental data.

To characterize the thickness and surface properties of the films, X-ray reflectivity was used. Samples in Figure 4.4a and b were grown at the same time, and the only differences are the substrates of the samples, CFO-LSAT and CFO-

STO. Both samples have the same critical angle, which suggests the same surface density. The thickness of CFO-LSAT is 375 Å, while the thickness of CFO-STO is

390 Å. The out-of-plane lattice parameter of CFO-LSAT is 3.723 Å, and for CFO-

STO is 3.686 Å, thus it can be calculated that the film on LSAT has 101 unit cells, and the film on STO has 105 unit cells, close to the targeted thickness of 100 unit cells. The surface roughness for CFO-LSAT is 5 Å which is 1.3% of the original thickness and for CFO-STO it is 2 Å which is 0.5% of the original thickness. The oscillation amplitude for the CFO-STO is smaller than the CFO-LSAT which suggests the density contrast of CFO-STO is less than CFO-LSAT.

Figure 4.4 X-ray reflectivity plots for (a) Ca2Fe2O5 on the LSAT substrate, (b) Ca2Fe2O5 on the STO substrate.

4.2 The effect of cation off-stoichiometry

Growth-induced cation off-stoichiometry effects have been studied on different ABO3 perovskite materials to understand how cation deficiency alters on lattice parameters, electronic behavior, ferroelectricity, interfacial conductance, and optical absorption. [30] [53] [54] [55] However, similar studies had yet to be performed on ABO2.5 films, prior to this work.

Figure 4.5 (a) X-ray diffraction for cation deficient Ca2Fe2O5 films on STO substrates. The peaks around 46.5o are the substrate 002, while the peaks between 49o and 50o are from the films. (b) Rocking curve measurements obtained from the 002 reflection for calcium deficient films.

Cation off-stoichiometric CFO films were investigated by X-ray diffraction to understand the lattice response to both Ca and Fe deficiency. Figure 4.5a compares the diffraction patterns for the as-grown 002 peaks. The iron deficient

CaFe0.76O2.5 and CaFe0.88O2.5 films have thicknesses of 25.8 nm and 26.7 nm, respectively, and the calcium deficient Ca0.9FeO2.5 and Ca0.79FeO2.5 films have

47 thicknesses of 38 nm and 33.9 nm, respectively. Figure 4.5a shows that cation deficient films have lower film intensity and broader film peaks. Also, the positions of the film peaks indicate a (~0.004 Å) contraction of the out-of-plane lattice constant for calcium deficient films compared to stoichiometric CaFeO2.5. The measured c-axis parameter values are presented in Figure 4.6. In contrast, iron deficient CaFe0.76O2.5 and CaFe0.88O2.5 have out-of-plane lattice parameters of 3.704

Å and 3.708 Å, indicating expanded c-axis parameters due to the B-site vacancies.

Increasing concentrations of iron vacancies appear to cause greater lattice expansion. The observation of c-axis lattice sensitivity of B-site deficiency agrees with previous cation off-stoichiometry studies from perovskites. In the cases of

LaFeO3 and La2-xCoTiO6-δ, A-site deficiency led to minimal changes in c-axis lattice parameter. While in the studies of LaFeO3 and (Ba0.5Sr0.5)(Co0.8Fe0.2)yO3-δ,

B-site deficiency let to expansion in the c-axis lattice parameter [30] [56] [57]

Figure 4.6. Relationship between the out-of-plane lattice constant and iron concentration in the films.

The rocking curves shown in Figure 4.5b suggest that although calcium deficient does not result in a significant out-of-plane lattice parameter change, it does lead to degraded crystallinity in the film. The omega scan detects mosaic spread along the growth direction. Higher intensity and a narrower peak indicates better crystallinity, which can be quantified by the full width half maximum

(FWHM). Using the FWHM as an indication of crystallinity is commonly applied in the study of thin film. [58] [59] For example, in the study of p-type CuO semiconductor film, electrodeposition method has a smaller FWHM value than potentiostatically deposited method, which suggested electrodeposited CuO has much better crystallinity and contained less residual stress. [60]

4.3 Strain effects on Ca2Fe2O5

When CFO films are grown on substrates with differing in-plane lattice parameters, the film’s in-plane lattice parameter will be strained due to the mismatch (assuming the film thickness is below the critical thickness for strain relaxation). Figure 4.7a shows the XRD patterns of the samples for this thesis. The out-of-plane lattice for cation stoichiometric (0 2 0) CFO-SLAO is 3.880 Å, (0 2 0)

CFO-LAO is 3.890 Å, (0 0 2) CFO-LSAT is 3.723 Å, (0 0 2) CFO-STO is 3.686

Å. These values agree with the previous work on pulsed laser deposited strained

CFO films, [20] [22] and also suggest that the oxygen vacancy ordering is along different crystallographic directions in the films on STO and LSAT compared to

SLAO and LAO as previously reported. The presumed crystallographic relationships between the film and the substrates are shown inset of Figure 4.7b.

Figure 4.7c shows the rocking curves measured from each film. Films with parallel vacancies (CFO-STO and CFO-LSAT) have better crystallinity than films with perpendicular vacancies (CFO-LAO and CFO-SLAO).

Figure 4.7 (a) X-ray diffraction pattern of CFO thin films on SLAO, LAO, LSAT, and STO substrates. The film peaks are indicated by arrows. (b) CFO film out-of- plane lattice parameters as a function of the substrate lattice parameter; inset shows schematics of the vacancy ordering directions. (c) Omega scans and resultant FWHM value for films on each substrate.

I speculate that the differences in crystallinity are related to the directions of oxygen vacancy ordering. On SLAO and LAO substrates, the oxygen vacancies lay perpendicular to the substrate. The vacancy channels have two potential directions ([100] and [010] directions). This is anticipated to lead to two distinct domain populations, as illustrated in Figure 4.8(a), which shows the top view ([001] direction) of the sample. To remove some of these grain boundaries through growth of the domains, the sample was processed by annealing on a 300oC hotplate in air for 12 hours. Figure 4.8(b) shows the measured XRD data before and after this annealing process. The post-annealed state has a higher film peak intensity which represents a better ordered crystal structure. In other words, heating the sample likely results in the migration of grain boundaries, increasing the grain size.

Figure 4.8 (a) Schematic of the hypothesized evolution of grain boundaries before and after air annealing. (b) X-ray diffraction of a Ca2Fe2O5 -SLAO film before and after ~300oC hotplate air annealing for 12 hours.

4.4 Fluorination of CFO films

As presented in many previous works [33] [61] [62] [63], introducing fluorine atoms into an ferrite oxide can alter the valence state of the B-site iron. In this thesis, to study the impact of oxygen vacancy alignment, a fluorination process was performed on 200 unit cell thick films on LSAT (CFOF-LSAT), STO (CFOF-

STO), SLAO (CFOF-SLAO), and LAO (CFOF-LAO). To perform the fluorination reaction, a vapor transport method was used. [33] The fluoropolymer used to create fluorine content is PTFE which has a chemical formula of (C2F4)n. Compared to

PVF (C2H3F)n and PVDF (C2H2F2)n, PTFE does not contain hydrogen which eliminates the possibility of forming Fe-H bonds. The furnace temperature used was 230oC, which is lower than the PTFE decomposition temperature of 350oC. In this section, preliminary data including XRD measurements, fluorine concentration and iron valence state measured by XPS, and optical absorption results will be presented.

For CFOF-LAO samples, the pre-fluorinated diffraction intensity was already very weak; after the fluorination, a diffraction peak could not be measured from the film. Therefore, CFOF-LAO results will not be included in the following discussion. For CFOF-LSAT and CFOF-STO, the introduction of fluorine results in a lattice expansion as shown in the XRD scan in Figure 4.9(a) and (b). The c- axis lattice parameter of CFOF-STO film expanded from 3.687 Å to 3.700 Å while the c-axis lattice parameter of CFOF-LSAT film from 3.718 Å to 3.723 Å. However, the out-of-plane lattice parameter change in the CFOF-SLAO is very small (0.008%) which can be regarded as negligible. The differences in lattice expansion thus appears to be influenced by the alignment of the oxygen vacancies.

The XPS results from CFOF-STO indicate that F only incorporated on the surface of the film, as shown in Figure 4.9a. This is speculated that because CFO-

STO has smaller c-axis lattice parameter, it is hard for fluorine to diffuse through the film along the c-axis direction. Other possible explanation for the expansion of

54 the c-axis lattice parameter is through creation of oxygen vacancy concentration during the fluorination process via carbon-based reduction.

Figure 4.9. Post fluorination samples (a) on a STO substrate, fluorine is measured only in the surface layers (~top 22 nm) of the sample, the overall average change in out-of-plane lattice parameter is 0.37%. (b) On a LSAT substrate, 15% F is uniformly distributed through the film (CaFeO1.825F0.675). The change of out-of- plane lattice parameter is 0.16%. (c) On a SLAO substrate, 15%F is present near the surface, then decreases to ~8% at the center of the film CaFeO2.14F0.36. The

56 change of the average out-of-plane lattice is 0.08%. Peaks at 44° are the sample holder for XRD measurement.

Figure 4.10 Optical absorption measured before and after fluorination for films on (a) LSAT, (b) SLAO, and (c) STO. The upper panel of each figure is the optical absorption and the lower panel is the difference spectra.

Optical absorption was measured to determine if fluorination alters the optical properties of the films. Figure 4.10(a) and (c) show that at the first absorption edge

(~2.4 eV), the post-fluorination absorption coefficient decreases in the region of 2

– 2.5 eV which suggests the reduction of iron in the CFO. In the study of SrFeO3-

δFδ, similar absorption behavior was reported with the reduction of iron. [34]

However, this reduction of absorption is not observed in CFOF-STO, shown in

Figure 4.10c, which is consistent with the lack of fluorine for this film shown in

Figure 4.9a.

Past studies suggest that there are two possible mechanisms for the fluorination process: i) fluorine substitution for oxygen and ii) fluorine insertion into anion vacancy sites. [64] [65] In scenario i, when one fluorine atom replaces one oxygen atom, the B-site metal is reduced; while in scenario ii, the B-site metal is oxidized.

By analyzing the XPS Fe 2p spectra based on the values of various Fe-O and Fe-F binding energies in Table 4.2, the bond evolution and the mechanism during the fluorination process can be determined.

Table 4.2 The binding energies of Fe 2p3/2 of each bond involved in the fluorination process.

Bonds Binding Energy (eV) [66] Fe3+-O 711.4 Fe2+-O 709.4-710.7 Fe3+-F 713.9-714.8 Fe2+-F 711.3-711.4

Figure 4.11(a) and (c) compares the XPS Fe 2p spectra before and after fluorination on both LSAT and SLAO substrates. The satellite peaks at lower binding energy of the Fe 2p1/2 and 2p3/2 peaks suggest Fe reduction after fluorination. [67] Figure 4.11(b) and (d) are the zoomed in views of the Fe 2p3/2 peaks. CFOF-LSAT exhibits minimal peak shifts while the CFOF-SLAO peak shifts by 0.4 eV to lower energy values. The decreasing of binding energy value is an indication of Fe reduction.

Figure 4.11 (a) Fe 2p XPS spectra before and after fluorination for CFOF-LSAT; (b) the Fe 2p3/2 peak position comparison. (c) Fe 2p spectra before and after fluorination for CFOF-SLAO; (d) the Fe 2p3/2 peak position comparison. Black lines are the experimental data and red dash lines are the fitted data in (c) and (d).

The majority of Fe bonds in as-grown CFO films are the Fe3+-O bonds.

After fluorine atoms are introduced, F bonds with Fe to create the Fe2+-F bonds for scenario i, and mixed Fe3+-F and Fe2+-F bonds for a mixture of scenarios i and ii.

Additionally in scenario i, some Fe3+-O bonds become Fe2+-O due to the reduction of iron. For CFOF-SLAO film, I attribute the Fe 2p3/2 peak shifts to the formation of Fe2+-O, which suggests scenario i is dominant in the CFOF-SLAO fluorination

59 process. However, for CFOF-LSAT film, the unchanged Fe 2p3/2 peak position is attributed to the formation of Fe2+-O and Fe3+-F which cancel out any peak shift.

This suggests that a mixture of scenarios i and ii occur during the CFO-LSAT fluorination process.

Conclusion

In this thesis, I synthesized Ca2Fe2O5 using molecular beam epitaxy and studied the effects of cation off-stoichiometric effects and strain on structural properties for the first time. In films grown on SrTiO3 substrates, cation off- stoichiometry resulted in changes to the c-axis parameter and a roughening of the surface as observed by RHEED. Compared to the cation stoichiometric sample, calcium deficient films show a significant decreasing of the crystallinity, while the out-of-plane lattice parameter stays almost the same as the stoichiometric sample.

In contrast, iron deficient samples show very little sign of a change in crystallinity, but the out-of-plane lattice parameter expands with increasing amounts of iron deficiency. These results are similar to those previously reported for cation off- stoichiometric ABO3 perovskites samples. In future studies, to further determine the role that the valence state of iron plays in the changing out-of-plane lattice, measurements such as ellipsometry and XPS could be conducted. Ellipsometry would provide the bandgap change which mainly results from the B-site cation in perovskite-like structures, and from XPS, the change of the iron oxidation state could be analyzed.

The lattice mismatch between the film and the substrate imparts a biaxial strain into the Ca2Fe2O5 thin films. The oxygen vacancy ordering in the brownmillerite structure allows for two film growth directions, one with vacancies

61 horizontally aligned, and the other one with vacancies vertically aligned. The preferred orientation is decided by two conditions: first, compared to the compressive strain, tensile strain is generally preferred; second, smaller tensile strain is preferred. Following these general rules, it was found that CFO films on

SrTiO3 and LSAT grew with the vacancy planes parallel to the film/substrate interfaces, while films on SLAO and LaAlO3 grew with the vacancy planes perpendicular to the film/substrate interface.

Fluorination was used to examine the effect of vacancy orientation. While fluorination of a film on LSAT and SrLaAlO4 was successful, my preliminary data here showed for CFO-LSAT fluorination, the mechanism is a combination of fluorine substitution and fluorine insertion; while for CFO-SLAO fluorination, the mechanism is dominated by fluorine substitution. Doping fluorine directly caused the expansion of the lattice. During the fluorination process, fluorine and carbon vapor act to reduce the film, removing oxygen from CFO, while some F substitutes for oxygen. Both processes lead to the reduction of iron cations, which then expand their radii. The comparison of Fe 2p spectra before and after fluorination suggest some of the bonds are converted from Fe3+-O to Fe2+-F and Fe2+-O. For future work, detailed studies how oxygen vacancy alignment alters fluorination while utilizing more reaction variables such as temperature, duration, and different fluoropolymers would shed light into how to best fluorinate brownmillerites.

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