Growing Anatase and Rutile Titania on C-Cut Sapphire Using Pulsed-Laser Deposition

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Growing Anatase and Rutile Titania on c-cut Sapphire using Pulsed-Laser Deposition

Alexandra V. Gordienko, inener-fizik po special~nosti fizika kondensirovannogo sosto ni vewestva i nanosistem

A Thesis

Physics

Submitted to the Graduate Faculty of Texas Tech University in Partial Fulﬁllment of the Requirements for the Degree of

MASTER OF SCIENCE

Approved

Anthony B. Kaye Chair of Committee

David Lamp

Charles Myles

Mark Sheridan Dean of the Graduate School

Acknowledgments

This research made use of the TTU College of Arts & Sciences Microscopy Center. I would like to thank Dr. D. Unruh for his assistance with a number of aspects related to XRD. I would like to thank Keller Andrews for his thoughtful discussions, and Dr. A. Kaye for his mentorship and encouragement throughout the thesis process. Finally, I would like to thank Dennis and Brooke Harris as well as Vladimir and Svetlana Gordienko for their daily support.

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Contents

Acknowledgments ...... ii

Abstract ...... v

List of Tables ...... vi

List of Figures ...... vii

1. Introduction ...... 1 1.1 Introduction ...... 1 1.2 Overview ...... 3 1.3 My Contribution to the Field ...... 4

2. Producing the Diﬀerent Phases of TiO2 ...... 6 2.1 Major Production Methodologies ...... 6 2.1.1 Techniques in Common ...... 6 2.1.2 Sol-gel techniques ...... 8 2.1.3 Sputtering ...... 10 2.1.4 Chemical Vapor Deposition ...... 11 2.1.5 Pulsed-Laser Deposition ...... 13 2.1.6 Other Techniques ...... 15 2.2 Film Growth Considerations ...... 17 2.2.1 Crystallographic Match and Cost ...... 18 2.3 A Brief Review of Titania Production Techniques ...... 22 2.4 Pulsed-Laser Deposition: Our System, Methods, and Results . . . . . 23 2.4.1 Our System ...... 23

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2.4.2 Methodology ...... 24 2.4.3 Characterization ...... 28 2.4.3.1 Atomic Force Microscopy ...... 28 2.4.3.2 Scanning Electron Microscopy ...... 31 2.4.3.3 Raman Spectroscopy ...... 31 2.4.3.4 XRD ...... 35 2.4.4 Final Production Protocols ...... 39

3. Summary and Future Work ...... 40 3.1 Summary ...... 40 3.2 Future Work ...... 40

Bibliography ...... 42

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Abstract

In this thesis, I present a review of the growth of tetragonal phases of titanium dioxide on diﬀerent substrates. I also report a pulsed-laser deposition growth protocol that facilitates the growth of both anatase and rutile phases of titania without changing the substrate or target material. Finally, I also demonstrate the development of the ﬁrst recipe for growth of anatase titania on a sapphire substrate.

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List of Tables

2.1 Pulsed-Laser Deposition Variables ...... 14

2.2 Comparison of Thin-Film Growth Techniques ...... 16

2.3 Lattice Matches to Typical Substrate Materials ...... 20

2.4 Example Thin Film Results ...... 26

2.5 Raman Shift Peak Identiﬁcation ...... 34

2.6 TiO2 ﬁlm growth conditions ...... 39

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List of Figures

2.1 The general process of ﬁlm producing via the sol-gel technique ...... 9

2.2 Schematic of sputtering system used to create thin ﬁlms...... 10

2.3 Schematic of a typical low pressure hot wall chemical vapor deposition reactor used in coating silicon substrates...... 12

2.4 Schematic representation of the fundamental transport and reaction steps underlying metalorganic chemical vapor deposition...... 12

2.5 Schematic representation of the Kaye Research Group pulsed-laser deposition system...... 14

2.6 Sketch of how an atomic force microscope works ...... 30

2.7 Raman spectra of thin-ﬁlm samples...... 33

2.8 X-ray diﬀraction patterns for the rutile (top, red) and anatase (bottom, blue) ﬁlms...... 36

2.9 X-ray diﬀraction pattern of the mixed-phase ﬁlm from Sample 5...... 38

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Chapter 1.

Introduction

1.1 Introduction

Titanium dioxide (titania; TiO2) is a material that has been studied carefully over the last 100 years (Vegard, 1916). Since its ﬁrst production, titania has become one of the most widely used white pigments. Pearlescent-eﬀect pigments are based on TiO2, and when combined with metallic pigments, it is often used in car paints, since the combination of titania-based pearlescent pigments and metallic pigments creates an illusion of optical depth. Titania pigments are also used in decorative objects that are intended to imitate natural pearls, cosmetics, and in critical areas of security printing. Titania pigments are also used in almost all white paint and most red-colored candy.

With so many applications, TiO2 has become a popular material to study.

In 1972, Fujishima and Honda discovered the possibility to split water using TiO2 electrodes. Since its initial publication (Fujishima and Honda, 1972), the Fujishima and Honda paper has been cited over 18,000 times, and it has changed the landscape of photocatalytic science and industry: a whole range of new applications has now been discovered. Among new applications are: using TiO2 as self-cleaning coatings (i.e., when the coating breaks down the organic dirt after exposure to UV light and makes the surface hydrophilic so water spreads evenly on the glass (Roméas et al., 1999); as coatings that are used in environmental applications to clean both air and water (Di Fonzo et al., 2008; Lin et al., 2008); as components of various sensor devices (Bao et al., 2008; György et al., 2005); as a gate dielectric in MOSFET technologies

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(Kim et al., 2006; Xie et al., 2010); and, as the basis for energy-eﬃcient solar cells (Mincuzzi et al., 2009; Park et al., 2000).

Titanium dioxide has three stable crystallographic forms, two of which, rutile and anatase, are tetragonal; the third phase (brookite) is orthorhombic. It is well established that properties of titania, and hence the performance of coatings and devices based on it, are different depending upon the crystallographic phase of the titania. For instance, the rutile phase is a more stable form and it scatters light more efficiently (Thiele and French, 1998). Anatase is used in production of solar cells because of its surface chemistry (Hoffmann et al., 1995; Park et al., 2000), and because this phase has roughly twice the photocatalytic power of the rutile phase (Luttrell et al., 2014).

For that same reason, anatase is the preferred crystallographic phase for gas- sensing applications since photocatalytic activity is shown to enhance the sensitivity of gas sensors (Yang et al., 2003). Rutile titania could potentially become a better choice for use in solar cells because it is a less expensive material to produce [in fact, most production techniques of synthetic titania yield rutile as a result (Mo and Ching, 1995)], and it scatters light more effectively than anatase. However, the annealing procedure(s) can significantly affect the photocatalytic properties of rutile films (Luttrell et al., 2014), so the growth protocols and the substrate materials selected for the production of titania are critically important. Additionally, the dielectric properties of titania are directly related to the ratio between anatase and rutile phases present in the film. Specifically, dielectric constants increase with the increase of rutile to anatase ratio (Kim et al., 2006), which is significant when titania is used in MOSFET technologies.

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1.2 Overview

Over the last 100 years, a number of methods have been developed to produce titania, each one optimized for the ﬁnal form required. Each of these methods will be summarized and compared to each other in Chapter 2., below.

We used pulsed laser deposition as our growth method, and the principal goal of this study was to grow pure samples of both tetragonal phases of titania – anatase and rutile – using the same substrate and the same laser target. Because c-cut sapphire is one of the most commonly used in optical applications, and because sapphire is an excellent crystallographic match with rutile titania, we elected to use sapphire as our substrate. This obviously means that because the anatase phase has significantly different crystallographic parameters, it requires significantly different growth conditions compared to those used to grow the rutile phase.

The impetus of this study was two-fold: First, a colleague has suggested growing layered films of TiO2, VO2, and TiO2 to studying how the optical and electrical properties of stack varied from simple VO2 films. From prior work, we know that c-cut sapphire is a substrate that shows both excellent switching amplitude and a very sharp (almost square) transition, so it was natural to choose this substrate for the production of the layered composite material. Since we do not know how each crystallographic phase of titania would affect the properties of the resulting system, we were interested in growing both phases of titania on this substrate. A literature review suggested that even though there are certain parameters that are known to be ideal for anatase and rutile growth on different substrates (e.g., glass, Si, and

SiO2), growth of titania ﬁlms on sapphire always lead the growth of the rutile phase. The second reason for this study was to test our understanding of crystallographic

3 Texas Tech University, Alexandra V. Gordienko, December 2016 growth using the pulsed-laser deposition technique, and speciﬁcally, if we were able to selectively grow each phase without changing the substrate or the laser target. Once proven, these techniques and procedures could be applied to other, more complex materials in which the crystallographic phase plays an even more critical role (e.g.,

La2CuO4 and similar materials).

After studying the literature for similar studies that used the same substrate for different phases of the material, we concluded that the main parameters that could affect the phase of titania are growth temperature, ambient gas pressure and laser pulses repetition rate. Some studies used a mix of different gases (Ar, N2 and O2) in their annealing processes, but most of them showed almost negligible results, so all of the annealing was done with the conditions as close to the growth conditions as it was possible. Because anatase transforms to rutile transformation at annealing temperatures between 600◦C and 700◦C, our search for rutile recipe was in the temperature region above this temperature; similarly, our anatase recipe was focused below this temperature.

1.3 My Contribution to the Field

In this study I showed that the growth of anatase titania on c-cut sapphire substrates, thought to be impossible, is, in fact, possible. I believe that this result is interesting for applications in which titania is used for its optical properties, since this method dramatically decreases the cost of substrates for anatase by a factor of ∼ 20. My growth method does not require a change of substrate or laser target, so it provides an opportunity for me to study diﬀerences in properties of two tetragonal phases of titania, as the eﬀects caused by the substrate and the target

4 Texas Tech University, Alexandra V. Gordienko, December 2016 are all eliminated. My results also conﬁrmed the possibility of selectively growing speciﬁc crystallographic phases of materials by changing only growth conditions of the pulsed-laser deposition technique.

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Chapter 2.

Producing the Diﬀerent Phases of TiO2

2.1 Major Production Methodologies

Over its 100-year history, two main growth methodologies have been employed and perfected to produce bulk titanium dioxide (titania; TiO2); these methodologies are solid state reactions (Mincuzzi et al. (2009), Di Fonzo et al. (2008), Kubo et al. (2001)) and sol-gel methods (Anderson et al., 1988; Du et al., 2002). Over the same time, a wider variety of thin-ﬁlm production techniques have been similarly developed and perfected; these include sol-gel techniques (Djaoued et al., 2002; Kim et al., 2006), sputtering (Wicaksana et al., 1992), chemical vapor deposition (Lee et al., 1995), and pulsed-laser deposition [PLD; Choi et al. (2004); Lin et al. (2008); Luca et al. (2006); Luttrell et al. (2014)]. While each of these growth techniques has their advantages and disadvantages (discussed in the subsections below), PLD has become one of the most common, resulting in more than 100 publications over the last ten years from the date of this thesis.

2.1.1 Techniques in Common

While the wide array of thin-ﬁlm production protocols span a wide range of chemistry and physics, there are a number of techniques that are common to all. These include:

Substrate cleaning Substrates must be clean, smooth, and dry prior to depositing

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ﬁlms on them. The most common technique for cleaning the substrates is a general chemical method that involves removing whatever contaminants are on the substrate with a solvent and then eliminating the solvent. This typically involves cleaning the surface of the substrate with an alcohol or with acetone to remove organic contaminants, rinsing the substrates with deionized water, and

ﬁnally drying them with dry nitrogen gas (N2). For most of our substrates, we employ an ultrasonic cleaner to achieve the highest cleanliness levels, so individual substrates are placed in a beaker ﬁlled with acetone, and the beaker is placed in an ultrasonic cleaner, heated to 30◦C, degassed for 5 minutes, and then ultrasonically cleaned for 25 minutes. After the acetone cleaning,

the substrates are rinsed with deionized water, and dried with N2 just prior to deposition. Clean substrates that are not for immediate use are typically stored under methyl alcohol in a screw-top container.

In some relatively rare cases, we use an acid “piranha-solution” based cleaning procedure. For this procedure, sulfuric acid is mixed with hydrogen peroxide and deionized water in a volume ratio of 3:1:10. This method is only applied when the substrates are signiﬁcantly contaminated with organic materials, like oil. A detailed discussion of the protocols can be found in the Kaye Group Standard Operating Procedure Manual (Kaye, 2016).

Cleaning can also be accomplished using plasma-activated gases using our group’s homemade plasma etcher, but this process alters the fundamental nature of the substrate, and were therefore not used in the experiments presented in this thesis.

Sintering Sintering, a process that involves heating a deposited ﬁlm with the goal of reducing porosity, is a step common to sol-gel and chemical vapor deposition-

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based techniques. It may be considered a “pre-annealing” step (see below), or it may be used to drive certain chemical reactions that do not occur at the higher temperatures of annealing. Sintering may be conducted in a laboratory-air environment, or can be accomplished under inert gases, or even under vacuum. The speciﬁcs of the sintering process are highly dependent upon the protocol being used and the desired outcome.

Annealing Annealing is the process of final crystallization of a deposited film, and is used in virtually all of the film growth methods discussed below. Annealing can relieve the strain caused by a poor lattice mismatch, and can decrease the number of defects in the film as the surface morphology changes (nucleation increases the grain size).

Although several of our systems can anneal films in situ, we typically anneal films in our homemade vacuum furnace. Like sintering (above), annealing may be conducted in a laboratory-air environment, or can be accomplished under inert gases, or even under vacuum. The specifics of the annealing process are highly dependent upon the protocol being used and the desired outcome.

As these are all part of the “best practices” of thin-ﬁlm growth, they will not be discussed in any of the individual growth protocol subsections (below).

2.1.2 Sol-gel techniques

Unique among the methods described in this thesis, sol-gel techniques are, at their heart, wet chemistry techniques that rely upon both organic and inorganic chemistry to produce a “ﬁnal” sol – a colloidal suspension of solid particles in a liquid

8 Texas Tech University, Alexandra V. Gordienko, December 2016 in which the number of dispersed particles is small enough that the interactions are dominated by short-range forces – and then relies upon sintering and annealing to produce the final crystalline film (Brinker and Scherer, 1990). In between the wet chemistry techniques and the various heat treating steps, the liquid precursor needs to be spread onto the substrate; in our case, this is done by spin-coating (judged to be the best method of those available to produce the kinds of films in which our group is typically interested).

The various steps involved in sol-gel-produced materials are shown in Fig- ure 2.1, below.

Figure 2.1. The general process of ﬁlm producing via the sol-gel technique (Brinker and Scherer, 1990).

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2.1.3 Sputtering

Sputtering is a physical vapor deposition (PVD) process used for depositing materials under low-vacuum conditions (i.e., pressures between 1 and 10 mTorr). In this technique, a target is bombarded with energetic ions, typically those from an inert gas (e.g., argon). The collisions of the ions and the the target causes some of the atoms on the target surface to be freed from the surface and ejected into the experimental space (i.e., “sputtered” off of the surface). These ejected atoms then travel some distance until they reach the substrate and start to condense into a film. In basic sputtering, a DC voltage is placed between the target (cathode) and the substrate (anode). This voltage is what creates the argon ions and directs the target atoms to the substrate (Ohring, 2002). Adding a magnetic field to this process near the target area confines the plasma near the target, limiting potential damage to the substrate in a process called “magnetron sputtering.” A schematic representation of the described system is shown in Figure 2.2, below.

Figure 2.2. Schematic of sputtering system used to create thin ﬁlms (graphic credit: http://lnf-wiki.eecs.umich.edu/wiki/Sputter_deposition).

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2.1.4 Chemical Vapor Deposition

Chemical vapor deposition (CVD) is the process of chemically reacting a compound of a material to be deposited with other gases to produce nonvolatile solid that deposits on a substrate. In such processes, reactants are transported through gas inlets to the reaction zone where they produce reactive species that are transported to the substrate surface. The reactants and their by-products are absorbed by the surface and film formation process starts. The volatile by-products of surface reactions are then desorbed and transported away from the reaction zone. CVD processes rely on gas-phase and gas-solid chemical reactions to produce thin films (Ohring, 2002). One of the largest advantages of CVD-based techniques is that they do not have to be done under vacuum. There are, however, a number of limitations for this method, such as thermodynamic and kinetic limitations (e.g., supersaturation of the reactant gas, high substrate temperatures required for creating crystalline films, etc.), problems with the flow of the gaseous reactants, and the production of potentially dangerous by-products. Schematic representations of a typical CVD system and the steps of deposition process are shown in Figures 2.3 and Figure 2.4, below.

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Figure 2.3. Schematic of a typical low pressure hot wall CVD reactor used in coating silicon substrates. Adapted from (Pierson, 1999)

Figure 2.4. Schematic representation of the fundamental transport and reaction steps underlying MOCVD. Adapted from (Jensen and Kern, 2012)

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2.1.5 Pulsed-Laser Deposition

Pulsed-laser deposition (PLD) is a physical vapor deposition process carried out in an ultra-high vacuum system. In this process, a high-powered laser is focused on the target material inside the growth chamber and as each laser pulse strikes the target, it creates a plasma plume – small amount of ablated material – which then traverses the growth chamber and deposits onto a substrate (typically located directly above the target) (Eason, 2007). If the target and the substrate were stationary during this process, ﬁlm thickness would be largely non-uniform because the ablation plume is directed normal to the target. However, the combination of raster scanning of the laser beam across the target and rotating both the substrate and the target ensures that uniform ﬁlms can be produced.

In many PLD growth protocols, one or more background gases are required. The background gas serves two purposes: first, it provides reactive species for the flux (e.g., oxygen for formation of oxide films), without which the desired films could not form. Secondly, the gas reduces the kinetic energy of the plume species and thus both slows down and physically shrinks the plasma plume. It is important to note that the pressure of ambient gas can affect the phase of the ablated material, and the amount of the ambient gas required for the formation of a given crystallographic phase depends on the thermodynamic stability of that phase. Controlling the gas flow while maintaining the required growth pressures can be challenging.

The type of ﬁlm grown depends on a wide variety of variables, all of which can be controlled independently; these variables are listed in Table 2.1, below.

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Table 2.1. Pulsed-Laser Deposition Variables

Laser Variables Growth Chamber Conditions Wavelength System base pressure Laser pulse energy Typical growth pressure Laser pulse width Process gas(es) available Laser energy density on the target System temperature control Number of laser pulses Laser aim point mobility Target Conditions Substrate Conditions Target material composition Substrate material composition Target mechanics/mobility Substrate temperature Number of targets available Substrate mechanics/mobility Target-to-substrate distance Target-substrate geometry

Figure 2.5. Schematic representation of the Kaye Research Group pulsed-laser deposition system.

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2.1.6 Other Techniques

There are a number of other methods that can be used, but they are less common, some of these are: spray pyrolysis, chemical anodization, and others. Almost all of these methods have a common disadvantage: the thickness of the resulting film is limited by some factors, thickness is not always uniform, can not be controlled and is hardly predicted. Most of the described techniques are not performed in a vacuum chamber, so contaminants are highly possible. Most of the described techniques do not provide any tools for controlling the phase and orientation of the resulting films. A summary of the various pros and cons for the main thin-film growth techniques described above are summarized in Table 2.2, below.

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Table 2.2. Comparison of Thin-Film Growth Techniques

Process Capabilities Limitations (pros) (cons) Sol-gel Very little equipment required; pro- Stoichiometry difficult to control; duction time is very short; large ar- inclusion of impurities can be prob- eas can be covered quickly and eas- lematic; minimal control over film ily; wide range of materials can be thickness; very thin films (close to produced; cost is considered mini- critical thickness) almost impossi- mal compared to other methods ble to produce Sputtering Wide range of materials can be pro- Cost is considered high compared duced with few impurities; deposi- to other techniques; thin films are tion rates can reach ∼ 100 Å/s, so harder to produce that thick films thicker films can be created due to the high growth rates Chemical Capable of producing large-area, Cost is considered very high com- Vapor relatively uniform films with very pared to other techniques; use Deposition low levels of impurity; film den- of and production of potentially sity can be quite good; no vacuum dangerous materials is a concern; equipment is required mostly used to deposit dielectrics (metals are not deposited with this technique); control of film thickness can be a challenge Pulsed- Capable of scaling to very large area Cost is considered very high com- Laser production of very uniform film; pared to other techniques; film Deposition film stoichiometry can be controlled growth rates can be limited (i.e., it “easily” and the films have virtually takes a long time to make a film); no impurities; typical grain sizes are operation of complicated, expensive very small, and films are very dense vacuum equipment can be challeng- and uniform, even down to the crit- ing ical thickness; a wide range of materials can be produced; film thickness is trivially controlled, so a very wide range of film thicknesses can be produced

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2.2 Film Growth Considerations

There are at least ﬁve major things to consider once you have elected your speciﬁc growth method:

1. Lattice mismatch is one of the top two concerns. Lattice mismatch occurs when a crystal material is epitaxially grown on a substrate that has different lattice parameters and is especially important for the growth of thin films since it creates defects in crystal structure. These defects, can, however, be partially compensated by strain when thicker films are produced (although the production of thicker films can result in other issues), and this strain may be at least partially released during the annealing process.

2. The coefficient of thermal expansion is also an important factor in the production of defect-free films since it affects the lattice parameters of both the film and the substrate, creating expansion mismatch. Since temperatures during the growth process can span ∼ 1, 000◦C, matching coefficients of thermal expansion is important for growth considerations as well as considering how the film will be used in its final form.

3. In some applications it is important to match the optical properties of the film with the optical parameters of the substrate; e.g., substrates that have low coefficients of transmission in the visible portion of the electromagnetic spectrum would not be applicable for films used in optical spectrum, despite the fact that they may be less expensive to use or have more attractive physical or chemical features. As an example, which silicon may have a number of properties that make it useful for a wide variety of electronic applications, it is opaque in the

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visible part of the spectrum, so it is not useful for those applications which

would require a transparent substrate for wavelengths 250 nm . λ . 1 µm.

4. The same principle applies to the electrical conductivity of the substrate. Specif- ically, since one of the major applications of titania is in MOSFET technologies

in which TiO2 is used as a gate dielectric, it is important that the ﬁlm be deposited on a semiconducting substrate (Campbell et al., 1997; Kim et al., 2006; Xie et al., 2010).

5. Another important parameter to consider is the cost of the substrate. In this age of highly-restricted research funds and hyper-engineered production processes that eliminate virtually all waste, cost is a factor across the board. While some potential substrate materials may have a superior lattice match with the selected titania ﬁlm, the cost of those substrates is so high that it makes it impractical to use even in the research lab, and therefore impossible for use in commercial production settings.

There are other parameters that may need to be considered as well (e.g., ﬁlm adhe- sion to the selected substrate, required surface roughness, and surface morphology demands), but these are typically less important than those listed above.

2.2.1 Crystallographic Match and Cost

Based on a detailed literature survey, the most common PLD substrates used for growing titania are glass, SiO2 (quartz), LaAlO3 (LAO), SrTiO3 (STO), Ti, Si,

GaAs and c-cut α-Al2O3 (i.e., c-cut sapphire). Anatase TiO2 is typically grown on a wide variety of substrates (including glass, quartz, STO, LAO and Si); similarly,

18 Texas Tech University, Alexandra V. Gordienko, December 2016 rutile can be grown on most of the substrates listed above, but Janisch et al. (2005) claim that the use of two of these substrates (GaAs and c-cut sapphire) leads to growth of rutile only. The same group claimed that the production of anatase TiO2 on c-cut sapphire was “impossible.” To elucidate the reasons behind these trends, we calculated the lattice mismatch between the mentioned substrates and anatase and rutile phases of titania, using the relation

a − a = f s , (2.1) as

in which is the lattice mismatch, af is the lattice constant of the film material, and as is the lattice constant of the substrate material. In this context, lattice constants refer to one of the physical dimensions of the unit cells in the lattice of the materials that are parallel to the growth surface. Thus, the lattice mismatch here represents a basic “structural compatibility” between the film and the substrate. Ideally, the substrate should be chosen so that the lattice mismatch is minimal; otherwise, strains developed in the film during the growth process will be larger. These strains can also cause undesirable defects in the subsequent epitaxial layers.

The results are shown in Table 2.3, along with the approximate cost of 1 cm2 of each of the substrates (as estimated from the MTI Corp. online catalog retrieved on 27 June 2016).

It is apparent from Table 2.3 why many researchers producing anatase TiO2 would select LaSrAlO4 or SrTiO3 as their substrate. However, these represent two of the three most expensive substrates on our list; the third best lattice match is

Al2O3, which is available at a signiﬁcantly lower cost. We note for completeness that because SrTiO3 is a perovskite-type structure with alternating layers of SrO and

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Table 2.3. Lattice Matches to Typical Substrate Materials

Substrate a ε (Anatase) ε (Rutile) Approx. Costa Material (Å) (%) (%) (USD/cm2) b c Al2O3 4.759213 −20.54 −3.51 2.31 GaAs 5.65325d −33.09d −18.69 3.08 e e LaAlO3 5.3646 −29.51 21.01 9.10 f f LaSrAlO4 3.75664 0.61 22.19 21.03 Si 5.4307g −30.31h −15.19 1.67 i i SiO2 4.912 −23.20 −7.56 2.00 j k SrTiO3 3.905268 −3.72 15.74 55.00 Ti 2.95111l 28.08m 55.51 23.51 a Substrate costs were estimated from the MTI Corp. online catalog on 20 June 2016 b Dobrovinskaya et al. (2009) c Reeber and Wang (2000) d Blakemore (1982) e Howard et al. (2000) f Kawamura et al. (2015) g Hössinger (2000) h Watanabe et al. (2004) i Ackermann and Sorrell (1974) j Schmidbauer et al. (2012) k de Ligny and Richet (1996) l Wood (1962) m Spreadborough and Christian (1959)

TiO2, one of the layers can be terminated using a wet solution, leaving an atomically

flat substrate. Such termination is possible because SrO is a basic oxide and TiO2 is an acidic oxide, so by controlling the pH level of the terminating solution, it is possible to remove one of the materials and leave the other one untouched (Kawasaki et al., 1994; Kennedy and Stampe, 2003). Thus an atomically flat substrate with orientation in (100) plane is created by terminating the substrate at TiO2 layer, leading to a virtually perfect match between the substrate and the film.

It is also worth noting that according to the data in Table 2.3, sapphire is the

20 Texas Tech University, Alexandra V. Gordienko, December 2016 best match for the rutile phase of titania, which explains why most research groups assume in their studies that regardless of growth method and growth conditions only rutile phase can be obtained when using c-cut sapphire as a substrate for titania growth.

The main conclusion here is that the ideal substrates for rutile and anatase growth are c-cut sapphire and LSAO (or STO, which is the next closest one in lattice mismatch for anatase), respectfully. This observation is confirmed by the statistics of publications that used PLD as their growth methods: most papers describe the growth of rutile on c-cut sapphire and anatase on LSAO and STO (Dabney et al., 2008; Hsieh et al., 2002; Le Boulbar et al., 2014; Luttrell et al., 2014). However, these spectacular crystallographic matches both come at considerable cost and require a relatively complex growth protocols to obtain both phases of titania on the same substrate. For example, in 2002, Hsieh et al. obtained both anatase and rutile TiO2 on SrTiO3 substrates, but while the anatase phase was deposited directly on the substrate, the rutile phase was grown by oxidizing titanium nitride films (Hsieh et al., 2002). However, we believe that this mixed-target and/or mixed-substrate approach is not be applicable for many studies, since the optical and electrical properties of the film can be affected by the substrate material and it becomes difficult (if not impossible) to compare the properties of phases when substrates and/or targets are changed, since there is no way to tell if the differences in the overall device performance are caused by the film itself, by the substrate material, or by some film-substrate interaction.

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2.3 A Brief Review of Titania Production Techniques

Because titania has so many applications and because the phase of titania significantly affects the performance of the resulting product (see §1.1, above), there have been numerous studies on the growth of titania using various methods (as described in §2.1). A detailed review of the literature reveals that researchers tend to favor using a pure titanium target with a silicon substrate to grow anatase titania (see, e.g., Di Fonzo et al., 2008, Luca et al., 2006, and György et al., 2007), whereas typical growth protocols for rutile thin films require rutile titania targets and either glass or silicon substrates (see, e.g., György et al., 2007, Dzibrou et al., 2008, and Long et al., 2008). Kitazawa et al. (2006), Luttrell et al. (2014), and Le Boulbar et al. (2014) used c-cut Al2O3 as their substrate for growing rutile TiO2, but in every instance in which researchers used Al2O3 as a substrate, it was exchanged for

LaAlO3 to produce anatase TiO2. According to Luca et al. (2006), Janisch et al.

(2005), and references therein, growing TiO2 on Al2O3 leads to either anatase TiO2, mixed-phase films, or brookite films. For studies that considered multiple distinct crystallographic forms, researchers changed either the PLD target (see, e.g., Hsieh et al. (2002) who used rutile TiO2 and TiN as their targets and Ohshima et al. (2006) who used Ti, TiO2 and TiN as their targets) or the substrate (see, e.g., Luttrell et al., 2014, Kitazawa et al., 2006, and Le Boulbar et al., 2014) to achieve their goal. Such ambiguity may be why different groups found vastly different growth parameters to be ideal for the same TiO2 crystallographic phase (cf. Hsieh et al., 2002, Dzibrou et al., 2008, Long et al., 2008, and Choi et al., 2004). Specifically, Hsieh et al. found the best growth pressure and temperature combination to be 10 mTorr and

◦ 800 C (at 5 Hz laser pulse rate) for their anatase ﬁlms that were grown on SrTiO3

22 Texas Tech University, Alexandra V. Gordienko, December 2016 substrates; Dzibrou et al. and Choi et al. grew their titania films on glass and the first group found the optimal temperature and pressure for anatase titania to be 30 mTorr and 500◦C, while the same combination of parameters led to rutile and anatase mix for the second group. These discrepancies in the “best” protocols for a given crystallographic phase are, as stated above, most likely due to changing substrates and/or targets; these changes make it impossible to adapt the described recipes to titania growth on different substrates, or to judge wether the change in optical and electrical properties of the material is caused by a different phase or the effects are caused by the substrate or even a substrate-film interaction.

This led us to the heart of this thesis: specifically, if you can grow stoichiometrically- correct, specific crystallographic states of TiO2 using the identical target and the identical substrate, the effects of the target and substrate can be eliminated, and this can lead us to a much better understanding of the physics behind the growth of titania.

2.4 Pulsed-Laser Deposition: Our System, Methods, and

Results

2.4.1 Our System

We grew thin ﬁlms using custom-built pulsed-laser deposition system with a Coherent COMPex Pro KrF excimer laser (λ = 256 nm, pulse width = 25 ns) held

◦ at a 45 angle to a rotating TiO2 mixed-phase target inside a custom-built growth chamber (total volume∼ 36 L) with a base pressure of 6.5 × 10−11 Torr. The thin

23 Texas Tech University, Alexandra V. Gordienko, December 2016

ﬁlm growth process was typically started near a pressure of ∼2 × 10−9 Torr.

The target was 1-in. in diameter, 0.25-in. thick, and 99.99% pure (SuperCon- ductor Materials) TiO2 ceramic disk containing a 50%-50% mix of anatase and rutile phases. The target was rotated at ∼3 rpm during the entire growth process.

In every case, we grew the ﬁlm on a 10 × 10 mm c-cut (0001) Al2O3 substrate that was heated from back side with a platinum wire heater; temperature was measured with a thermocouple placed in a representative position, and the substrate was rotated at ∼3 rpm (in a direction counter to the target) during the entire deposition process.

2.4.2 Methodology

Over the course of this study, the parameters that were varied were the:

• laser pulse energy [220 mJ to 300 mJ];

• number of laser pulses to obtain the desired ﬁlm thickness;

• laser energy density, controlled both by changing the pulse energy and by re- focusing the laser spot on the target (from 1.5 to 2.0 J · cm−2);

• target to substrate distance [from 5 cm to 8 cm];

• substrate temperature [from 250◦C to 825◦C];

• growth pressure [from ∼1 × 10−4 Torr to 100 mTorr];

• annealing temperature [from 250◦C to 900◦C];

• annealing pressure [from 5 mTorr to 100 mTorr]; and

• annealing time [from 1 hr to 2 hr].

24 Texas Tech University, Alexandra V. Gordienko, December 2016

The laser wavelength constant at 256 nm; we could change the wavelength by changing the excimer gas, but this is both (a) beyond the scope of this thesis, and (b) because of the current worldwide shortage of neon (one of the buffer gases), outside the budget of most programs. The laser pulse width remained constant at 25 ns, because our system is not equipped with appropriate stretchers or compressors. This pulse width is typical for systems similar to ours, and this constraint is not judged to be significant. The ambient gas we used in every growth protocol and during the annealing processes was molecular oxygen (O2). If our system was upgraded with an atomic oxygen source (see §3.), this would have been used instead (in order to produce higher-quality films at lower pressures). However, the presence of oxygen is required in both the growth and annealing processes because ambient gas slows down the the ablation plume expansion, so a larger part of the plume is deposited on the substrate. Finally, both the target and substrate materials were kept constant, because this formed the crux of the study, i.e., to develop growth protocols for both rutile and anatase titania, and to able to ascertain which growth parameters determine the phase of titania when grown using this method. Table 2.4 summarizes the growth parameters of five of our most interesting samples.

When we considered the various published protocols used in other PLD studies, we noticed that the three main parameters that researchers manipulate were temperature, pressure and repetition rate, although the latter is less commonly viewed as an important growth parameter (Dabney et al., 2008; György et al., 2007; Kitazawa et al., 2006). By using the general trends observed by others, we were able to (relatively) quickly arrive at our initial’ growth protocols for both anatase and rutile. In any case, it must be noted that even if we were to copy another researcher’s growth protocol exactly, we would not reproduce their result – such is the black magic that

25 Texas Tech University, Alexandra V. Gordienko, December 2016

Table 2.4. Example Thin Film Results

Sample Number Condition 1 2 3 4 5 Laser Variablesa Pulse energy (mJ) 296 222 222 222 222 Repetition Rate (Hz) 50 5 5 20 20 Fluence J · cm−2 2.0 1.5 1.5 1.5 1.5 Number of shots (thousands) 100 40 40 70 40 Growth Chamber, Target, and Substrate Conditionsb Chamber pressure (mTorr) 35 5 5 50 10 Substrate temperature (◦C) 250 700 825 300 700 Target-to-substrate distance (mm) 70 60 60 50 60 Annealing Conditionsc Pressure (mTorr) 35 5 5 50 10 Temperature (◦C) 250 900 900 300 900 Time (hr) 1 2 2 1 2 Resulting phase Anatase Rutile Rutile Anatase Mixed Phases a The laser beam struck the rotating target at angle of ∼ 45◦ and was rastered across the target with raster period of 4.2 s. b See §2.4.1 for additional details on the target, the substrate, and the growth conditions. c All high-temperature annealed ﬁlms were annealed in the Kaye Research Group homemade vacuum tube furnace under a partial atmosphere of research-grade oxygen; ﬁlms annealed at low temperatures were annealed in situ. is PLD.

One of the biggest issues with PLD is the potential ablation of micron-size particles, that sometimes occurs when the penetration depth of the laser beam into the target material is large. The problem is usually resolved by selecting an appropriate target material (that is dense and strongly absorbs the laser wavelength) for the chosen laser wavelength, or sometimes can be partially resolved by reducing the energy density of the laser and increasing the target to substrate separation distance. The first few attempts at generating stoichiometrically-correct, complete thin films were unsuccessful, served as a means to determine the correct combination of laser energy, fluence, and target-to-substrate distance for both phases. This step is essen- tial because some of the first films, when retrieved from the PLD system, appeared “sputtered” – with large “globs” of material on the surface of an otherwise apparently

26 Texas Tech University, Alexandra V. Gordienko, December 2016 smooth film. Multiple factors can contribute to such an appearance, but mainly, it is the fact that different targets absorb fixed wavelength differently, and if the laser wavelength of our system is fixed, the only way to change the amount of material ablated with each pulse was by changing the pulse energy and the fluence on the target. These parameters were necessarily restricted within a certain range; lower energy density on the target results in a smaller plasma plume, which in turn requires a slower deposition rate. In some cases, films with over 400,000 shots were not thick enough for them to be characterized by powder XRD. The size of the ablation plume also depends on the pressure of the ambient gas, but pressure is also one of the key parameters that determines the crystallographic phase of the resulting material, so it could only be adjusted slightly. The final energy and distance we chose for both of the ideal recipes were: 252 mJ and 60 mm for anatase and 222 mJ and 65 mm for rutile. The other successful protocols had slightly different temperatures and pressures, but all of the films presented in Table 2.4 appeared reasonably smooth and sputter-free when examined by eye.

Due to the limitations in readily available characterization methods (namely, the film XRD at the Texas Tech Nano Tech Center, which could not give us spectra that we could confidently call a pure phase), some of the first films we successfully characterized were ∼ 1 µm thick so that they could be characterized using powder XRD instrument in the Texas Tech University Department of Chemistry and Biochemistry.

As it was mentioned above, this study did not cover different annealing procedures and all of the anatase films were annealed at the growth temperature and pressure, while rutile films were annealed at the growth pressure and a slightly higher

27 Texas Tech University, Alexandra V. Gordienko, December 2016 temperature. All of the rutile films were annealed in the Kaye Research Group vacuum furnace (base pressure of ∼ 1 × 10−4 Torr), and all of the anatase films were annealed in the growth chamber. We had to transfer the rutile film to the furnace, because keeping the heater at temperatures required for annealing, even at low oxygen pressures, could potentially damage the system, while our furnace was designed for us to reach annealing temperatures in excess of 1, 200◦C.

2.4.3 Characterization

All of the films were initially examined visually; the films that appeared to clean (vice those that appeared “sputtered”), were then examined using the atomic force microscope (AFM) to ensure the films were continuous and to determine the roughness of each film. Continuous films were then examined with using powder x-ray diffraction (XRD) to determine both their crystallinity and the specific crystallographic phases present in the sample. Film thickness was later measured using a profilometer and confirmed with via scanning electron microscopy (SEM). Since the XRD peaks of anatase and brookite are similar, we confirmed our final results using Raman spectroscopy. Each of these characterization instruments and techniques are discussed in the sections below.

2.4.3.1 Atomic Force Microscopy

Atomic force microscopy works by tapping the very sharp tip of a cantilever along the sample and measuring its response. In tapping mode a stiﬀ cantilever is used, and the tip is a few nanometers away from the sample. In this mode,

28 Texas Tech University, Alexandra V. Gordienko, December 2016 the cantilever is “tapped” vertically while it scans the sample, and the changes in vibration amplitude are detected (see Figure 2.6). The displacement of the cantilever causes the displacement of the laser beam on the photodiode. The photodiode usually has four sections and the combinations of sums on the four sections determine the height of the cantilever at each particular point. For more information, see (Ohring, 2002).

For our measurements, we used an Asylum Research MFP-3D-BIO atomic force microscope (AFM) and Bruker MSNL probes with nominal tip radii of 2 nm to characterize the surface morphology of each sample. Individual locations on each sample for a 5 µm × 5 µm scan were selected from within a randomly-located 20 × 20 µm scan taken near the center of each sample to minimize edge effects. For each scan, the rate was set to 0.5 Hz, the scan angle was fixed at 90◦, the set point was held to 1 V, and the integral gain was set to 10. The resolution of the final 25 µm2 region was set to 512×512, meaning that lateral features as small as ∼10 nm could be resolved. For reference, the typical roughness of the c-cut Al2O3 substrates was measured to be 0.098 nm. All measurements were obtained in tapping mode.

Once the raw images were obtained, all of the large features (& 4 nm) were masked off, and the image was flattened using a third-order fit to remove image shadowing effects.

The typical root mean square(rms) roughness of our anatase samples was

< 1 nm (for reference, the typical roughness of the c-cut Al2O3 substrates was measured to be 0.098 nm), and the typical roughness of our rutile samples was measured to be < 1 nm as well.

29 Texas Tech University, Alexandra V. Gordienko, December 2016

Figure 2.6. Sketch of how an atomic force microscope works (ﬁgure courtesy of Henderson and Oberleithner, 2000).

30 Texas Tech University, Alexandra V. Gordienko, December 2016

2.4.3.2 Scanning Electron Microscopy

The principle of scanning electron microscopy is based on the fact that the electrons interacting with atoms of the sample emit secondary electrons, which can be detected. Electrons emitted from a filament are drawn to the anode and focused by condenser lenses into a small spot size beam. The beam is directed using deflection coils. As the electrons hit the sample the decelerate and transfer their energy to atomic electrons and the lattice. The most common method of data collection on SEM is detecting secondary electrons emitted by atoms of the sample. The detector consists of a scintillator that detects charged particles and a photomultiplier that amplifies the signal and produces an electrical output signal (Ohring, 2002).

SEM normally makes an excellent tool for determining the film roughness, but in our case, the films were so smooth that the nominal process of coating them with gold (to make them conducting) produced a rougher surface, and imaging without a conducting coating layer did not produce useful results (the electrons only damaged the surface). However, we were able to use SEM to measure the thickness of each film by breaking the sample in half and imaging the cross section. The nominal thickness of our final anatase sample was measured to be 100 nm and the thickness of the final rutile sample was 90 nm.

2.4.3.3 Raman Spectroscopy

Raman spectroscopy technique is based on inelastic scattering of photons emitted by a laser on the atoms and molecules, which causes change in wavelength of scattered light. A “clean” laser beam (usually generated with a monochromator or

31 Texas Tech University, Alexandra V. Gordienko, December 2016 a laser line filter) is normally focused on the sample and the light scattered from the sample is collected, while the light at the wavelength of the laser is filtered out by notch- and/or longwave-pass filters. The signal beam is therefore a change from the original laser wavelength is detected, and this change is normally represented as the “Raman shift” (the difference between inverse of scattered and incident wavelengths) with the most disgusting units of wavenumbers (Smith and Dent, 2005).

In this work, we used an α-WITec system with excitation from a 532-nm Nd:YAG laser and a 100X objective lens with an NA = 0.90. The signal is picked up with a 1024 × 127 pixel peltier-cooled CCD camera with a resolution of four wavenumbers (the device and methodology are described in (Bennet et al., 2013). The results are shown in Figure 2.7, in which the rutile, anatase, and sapphire substrate signals are shown in red, blue, and green, respectively. The peaks in both the rutile and anatase signals are described in full in Table 2.5, below, in which each peak, identiﬁed by the wavenumber of the Raman shift center is given along with the corresponding photon mode that caused the peak.

32 Texas Tech University, Alexandra V. Gordienko, December 2016

1500

1000 Raman Intensity

500

0 200 400 600 800 Raman Shift

Figure 2.7. Raman spectra of thin ﬁlms samples of rutile (red), anatase (blue), and Al2O3 (sapphire) substrate (green). See the text for the discussion of the peaks.

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Table 2.5. Raman Shift Peak Identiﬁcation

TiO2 Raman Shift Mode Crystallographic Center Identiﬁcation Phase (cm−1) * Anatase (blue) 155 Eg(1) * 198 Eg(2) † 383 Al2O3 substrate Eg * 401 B1g(1) † 421 Al2O3 substrate A1g † 453 Al2O3 substrate Eg * 523 B1g(2) + A1g † 581 Al2O3 substrate Eg * 641 Eg(3) † 644 Al2O3 substrate A1g † 755 Al2O3 substrate Eg Rutile (red) 238 Several multi-photon modes‡ † 383 Al2O3 substrate Eg † 421 Al2O3 substrate A1g * 442 Eg † 453 Al2O3 substrate Eg † 581 Al2O3 substrate Eg * 610 A1g † 755 Al2O3 substrate Eg * Frank et al. (2012) † Misra et al. (2001) ‡ Strong second-order Raman scattering is observed in this region, so the broad peak near 238 cm−1 is the result of the contribution of a number of multiphonon modes. This fea- ture is characteristic of rutile phase titania; for additional details, see (Frank et al., 2012).

34 Texas Tech University, Alexandra V. Gordienko, December 2016

2.4.3.4 XRD

X-ray diﬀraction (XRD) is a technique commonly used for determining the phase of material. In XRD, samples are irradiated by X-rays, which are then diﬀracted into characteristic directions by the atoms of the sample; the resulting data is given in terms of x-ray intensities as a function of scattering angle. This al- lows comparing the resulting peaks with known materials or in some cases, building a 3-D map of electron density in the crystal.

All of the films were analyzed using a Rigaku Ultima 3 powder XRD system to both inspect quality of the films in terms of both crystallinity and crystallographic phase. The samples were all measured in the range 20◦ ≤ 2θ ≤ 60◦ with a step size of 0.02◦ and an integration time of 0.6 s per step. The x-ray diffraction patterns are plotted in Figure 2.8, in which our best rutile and anatase patterns are shown on the top and bottom, respectively. The rutile pattern has been shifted vertically for visual clarity. Patterns were initially identified with the JadeTM software package1; the anatase phase matched with PDF#97-015-4604 Djerdj and Tonejc (2006) and the rutile phase matched with PDF#03-065-1119 . Each peak in the XRD patterns is identified with the Miller index identified by Jade. Note that both patterns are clean; there are no anatase peaks in the rutile pattern and vice versa. The peak labeled with a dagger (†) in the rutile pattern (top panel) is from the aluminum sample holder and is not part of the film. From this work, we see that the growth protocols in Table 2.6 produce a clean rutile film with a preferred (110) orientation and an anatase film with a preferred (004) orientation.

The XRD pattern of our best anatase sample (Sample 6 in Table 2.6; shown

1MDI Jade Software. XRD pattern processing. Material Data Inc., Livermore, California

35 Texas Tech University, Alexandra V. Gordienko, December 2016

20 (110) )

15 3

(121) (011) (111)(120) (220) 10 (004)

Intensity (A.U. x10 (112)

(011) (015) (013) (020) (121) 0 20 30 40 50 60 2q (degrees)

Figure 2.8. X-ray diﬀraction patterns for the rutile (top, red) and anatase (bottom, blue) ﬁlms; see text for details. in the bottom, blue curve in Figure 2.8) is clean, showing only anatase peaks; the large peak at 38.014◦ indicates that our anatase sample has a preferred orientation in the (004) plane. This sample is similar to Sample 1 in Table 2.4, and required only minor adjustments to create our “best” anatase protocol.

One can see that the XRD pattern of rutile sample2 Sample 7 of Table 2.6; shown in the top, red curve in Figure 2.8) only contains peaks for rutile phase of titania, except for one small peak near 38.5◦. This anomalous peak matches the XRD pattern of the aluminum sample holder used in these experiments, and are not part of the ﬁlm. The sample has preferred orientation in (110) plane as evidenced by the very large peak at 27.432◦; this explains why one of the possible rutile peaks in our range [the (020) peak at 39.185◦] is not present.

2Sample 2 in Table 2.4 and Sample 7 in Table 2.6 are identical, except for the number of shots (i.e., instead of ﬁlm thickness). Sample 2 is ∼1.1 µm thick, and Sample 7 is ∼100 nm thick.

36 Texas Tech University, Alexandra V. Gordienko, December 2016

Sample 3 (Table 2.4) was grown at a higher temperature, compared to Sam- ple 7 and the XRD pattern of that sample also shows the rutile phase, the peaks were narrower, but not as tall, which suggests that the film is more crystalline (which is understandable, taken a higher growth temperature). However, the signal to noise ratio was not sufficient to determine that the film was purely rutile.

Sample 4 (Table 2.4) showed that the sample was anatase, but not all of the anatase peaks appeared in the XRD spectrum. This may be because the film had a strong preferred orientation in (004) plane, or because the signal-to-noise ratio was not sufficient to determine if the film was a pure phase or not.

Sample 5 (Table 2.4) showed both anatase and rutile peaks (see Figure 2.9), despite the high growth temperature. Even though this film was of mixed crystallographic phase, which is not the goal of this study, it was included in this thesis because it showcases the importance of growth pressure and its effect on phase of the films.

37 Texas Tech University, Alexandra V. Gordienko, December 2016

Figure 2.9. X-ray diﬀraction pattern of Sample 5 (Table 2.4) showing a pattern of mixed phases. Rutile phases are indicated by the red “R,” anatase phases are indicated by a blue “A,” and the reﬂection from the aluminum sample holder is indicated by the † symbol.

38 Texas Tech University, Alexandra V. Gordienko, December 2016

2.4.4 Final Production Protocols

As the result of this study, we determined the best protocols for production of anatase and rutile titania on c-cut sapphire using PLD. The specific protocols required to produce each film are presented in Table 2.6; the number of shots was adjusted so that the samples have the same thickness of ∼100 nm, but we were able to show that these protocols were successful in producing not only thick films, but also thin films (even reaching down to the critical thickness of titania). Generally speaking, for metals and metal oxides, the typical growth rate is ∼ 0.01 nm/pulse, and our final anatase protocol confirms this figure. The rutile film required far fewer shots to produce roughly the same thickness because it was grown at a considerably lower pressure.

Table 2.6. TiO2 film growth conditions Condition 6 7 Laser Settings Pulse energy (mJ) 296 222 Repetition rate (Hz) 50 5 Fluence (J · cm−2) 2.0 1.5 Number of shots 10,000a 3,000b Growth Conditions Chamber pressure (mTorr) 35 5 Background gas O2 O2 Substrate temperature (◦C) 250 700 Target-to-substrate distance (mm) 65 60 Annealing Conditions Pressure (mTorr) 35 5 Background gas O2 O2 Temperature (◦C) 250 900 Time (hr) 1 2 Resulting Phase Anatase Rutile a Corresponding film thickness: 100 nm b Corresponding film thickness: 90 nm

39 Texas Tech University, Alexandra V. Gordienko, December 2016

Chapter 3.

Summary and Future Work

3.1 Summary

I have shown that the growth of pure rutile and pure anatase TiO2 is possible without changing substrate materials or PLD targets, and I have demonstrated this growth on c-cut Al2O3, despite the large lattice mismatch between Al2O3 and anatase phase of TiO2. Growing anatase in this manner does, however, induce significantly more strain in the anatase film compared to the rutile film grown on the same substrate. For applications in which titania is used for its optical properties, Al2O3 substrates may be preferential as it could significantly decrease the cost of production of titania films, as well as being highly survivable.

3.2 Future Work

This work showed the possibility of growth of anatase titania on c-cut sapphire substrates, despite of a large lattice mismatch. As a follow-up of this study I am planning to investigate the degree to which the growth parameters eﬀect the phase of the resulting material and create a phase diagram for titania on c-cut sapphire substrates, with growth pressure, temperature and laser repetition rate as the parameters.

Another interesting route of investigation would be selecting our best anatase and rutile recipes and conducting a full annealing study to see how annealing would

40 Texas Tech University, Alexandra V. Gordienko, December 2016 affect the films. As a part of this study, we made an attempt to measure the lattice strain in the films, using PXRD measurements and the Scherrer equation, but because the instrument used is not intended for use on films, the results were not included in this study. Currently, I believe that choosing a certain annealing recipe could help relieve the lattice strain in both films and receive a better signal-to-noise ratio from XRD measurements.

After creating a phase map of titania and developing additional understanding regarding which parameters play a key roles in determining the ﬁnal crystallographic phase of the material during PLD growth, I am interested in making an attempt to manipulate phase of other materials. An example of one such material is La2CuO4 (LCO). LCO is a material that undergoes a reversible phase transition similar to that of VO2, which causes a change in both optical and electrical properties of that material. So far, there is only two published papers showing the experimental observation of the change in properties in LCO (Okamoto et al., 2010). These authors claim that in order to produce the correct crystallographic phase of LCO (i.e., the one that shows the phase transition) requires the use of (LaAlO3)0.3(SrAl0.5Ta0.5O3)0.7 and LaSrAlO4 as substrates because of their crystallographic match with LCO – substrates that are remarkably expensive. I believe that the growth of this material on less expensive substrates (like sapphire) could help us study the properties of this material and the perovskite-type oxides of the same type, that could potentially be interesting because of their metal-insulator transitions.

41 Texas Tech University, Alexandra V. Gordienko, December 2016

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