Advanced Ceramic Materials · 2016. 8. 13. · Scrivener Publishing 100 Cummings Center, Suite 541J...

Advanced Ceramic Materials

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Advanced Ceramic Materials

Edited by

Ashutosh Tiwari, Rosario A. Gerhardt and Magdalena Szutkowska

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v

Contents

Preface xiii

Part 1 Design, Processing, and Properties

1 Development of Epitaxial Oxide Ceramics Nanomaterials Based on Chemical Strategies on Semiconductor Platforms 3A. Carretero-Genevrier, R. Bachelet, G. Saint-Girons, R. Moalla, J. M. Vila-Fungueiriño, B. Rivas-Murias, F. Rivadulla, J. Rodriguez-Carvajal, A. Gomez, J. Gazquez, M. Gich and N. Mestres1.1 Introduction 41.2 Integration of Epitaxial Functional Oxides

Nanomaterials on Silicon Entirely Performed by Chemical Solution Strategies 81.2.1 Integration of Piezoelectric Quartz Thin

Films on Silicon by Soft Chemistry 101.2.2 Controllable Textures of Epitaxial

Quartz Thin Films 131.2.3 Integration of Functional Oxides by

Quartz Templating 171.2.4 Highly Textured ZnO Thin Films 21

1.3 Integration of Functional Oxides by Combining Soft Chemistry and Physical Techniques 22

1.4 Conclusions 23Acknowledgments 26References 26

vi Contents

2 Biphasic, Triphasic, and Multiphasic Calcium Orthophosphates 33Sergey V. Dorozhkin2.1 Introduction 342.2 General Definitions and Knowledge 382.3 Various Types of Biphasic, Triphasic,

and Multiphasic CaPO4 402.4 Stability 422.5 Preparation 442.6 Properties 512.7 Biomedical Applications 532.8 Conclusions 59References 60

3 An Energy Efficient Processing Route for Advance Ceramic Composites Using Microwaves 97Satnam Singh, Dheeraj Gupta and Vivek Jain3.1 Introduction 983.2 Historical Developments in Materials Processing by

Microwaves 993.3 Introduction to Microwave Heating Process 101

3.3.1 Microwave–materials Interaction Theory 1023.3.2 Microwave Heating Mechanisms 104

3.4 Heating Methods by Microwaves 1073.4.1 Direct Microwave Heating 1073.4.2 Microwave Hybrid Heating 1083.4.3 Selective Heating 1093.4.4 Microwave-assisted Processing of Materials 109

3.5 Advantages/Limitations of Microwave Material Processing 1103.5.1 Highly Energy Efficient Processing Method 1103.5.2 Better Quality of Processed Materials 1133.5.3 Cleaner Energy Processing 1143.5.4 Compact Processing Unit 1143.5.5 Restriction in Processing of All Varieties

of Materials 1153.5.6 Restrictions in Processing of Complex Shapes 1153.5.7 Non-uniformity in Heating 1153.5.8 Human Safety Issues 115

Contents vii

3.6 Application of Microwave Heating in Composite Processing 1163.6.1 Recent Review of Work Carried Out in

MMC/CMC/Alloys/Ceramic Processing by Microwaves 119

3.6.2 Microwave Melting/Casting of Metals/ Metal Matrix Composites 127

3.7 Future Prospectives 1303.8 Conclusion 133References 133

Part 2 Ceramic Composites: Fundamental and Frontiers

4 Continuous Fiber-reinforced Ceramic Matrix Composites 147Rebecca Gottlieb, Shannon Poges, Chris Monteleone and Steven L. Suib4.1 Introduction 1484.2 Parts of a CMC 149

4.2.1 Fibers 1504.2.2 Interphase 1514.2.3 Matrix 152

4.3 Modern Uses of CMCs 1544.4 History 1554.5 Ceramic Fibers 158

4.5.1 Oxide Fibers 1584.5.1.1 Alumina Fibers 1594.5.1.2 Stabilized Alumina Fibers 1604.5.1.3 Alumina Silicate Fibers 1604.5.1.4 Other Oxide Fibers 164

4.5.2 Non-oxide Fibers (SiC) 1644.5.2.1 Oxidation 1644.5.2.2 Irradiation 1654.5.2.3 Sintering 165

4.5.3 Carbon Fibers 1664.5.3.1 Polyacrylonitrile 1674.5.3.2 Pitch 167

4.6 Interface/Interphase 1684.6.1 Requirements 1694.6.2 Non-oxide 1704.6.3 Oxide 171

viii Contents

4.7 Matrix Materials 172 4.7.1 Carbon 172 4.7.2 Silicon Carbide 175 4.7.3 Oxides 178

4.8 Matrix Fabrication Techniques 179 4.8.1 Polymer Impregnation and Pyrolysis 180 4.8.2 Chemical Vapor Infiltration 181 4.8.3 Melt Infiltration 183 4.8.4 Slurry Infiltration 184 4.8.5 Metal Oxidation 185

4.9 Toughness of CMCs 185 4.9.1 Fiber/Matrix Interface 186 4.9.2 Modes of Failure 186 4.9.3 Energy-Absorbing Mechanisms 187 4.9.4 Stress Testing of Composites 188

4.10 Applications 188 4.10.1 Brakes and Friction 190 4.10.2 Biomedical Applications 191

Acknowledgments 193References 193

5 Yytria- and Magnesia-doped Alumina Ceramic Reinforced with Multi-walled Carbon Nanotubes 201Iftikhar Ahmad and Yanqiu Zhu5.1 Introduction 2025.2 Dispersions and Stability of MWCNTs 2025.3 Influence of Yytria (Y2O3) Doping on MWCNT/Al2O3

Nanocomposites 2055.3.1 Densification and Microstructure Development 2055.3.2 Mechanical Performance and

Toughening Mechanism 2105.4 Magnesia (MgO)-Tuned MWCNT/Al2O3

Nanocomposites 2155.4.1 Role of MgO on the Densification and

Microstructural Features 2155.4.2 Effect of MgO on the Grain Size and

Fracture Behavior 2175.4.3 Mechanical Response of MgO-Doped

MWCNT/Al2O3 Nanocomposite 221

Contents ix


6 Oxidation-induced Crack Healing in MAX Phase Containing Ceramic Composites 231Guoping Bei and Peter Greil6.1 History of Crack Healing in Ceramics 2326.2 High-temperature Crack Healing in MAX Phases 233

6.2.1 MAX Phases 2336.2.2 Crack Healing in Al-contained MAX Phases 234

6.2.2.1 Ti3AlC2 2346.2.2.2 Ti2AlC 2356.2.2.3 Cr2AlC 238

6.3 Lower-temperature Crack Healing in MAX Phase-based Ceramics 2416.3.1 Oxidation Behavior of Ti2Al(1–x)SnxC MAX

Phase Solid-solution Powders 2416.3.2 Oxidation-induced Crack Healing in

Thermal-shocked Ti2SnC MAX Phase 2446.3.3 Crack Healing in Ti2Al0.5Sn0.5C–Al2O3

Composites 2496.4 Conclusions 255Acknowledgments 256References 256

7 SWCNTs versus MWCNTs as Reinforcement Agents in Zirconia- and Alumina-based Nanocomposites: Which One to Use 261M.H. Bocanegra-Bernal, C. Dominguez-Rios, A. Garcia-Reyes, A. Aguilar-Elguezabal and J. Echeberria7.1 Introduction 2627.2 Single-walled Carbon Nanotubes 2667.3 Multi-walled Carbon Nanotubes 2697.4 The Effects of CNTs Types on the Mechanical

Properties of Al2O3- and ZrO2-based Ceramics 2747.5 Why SWCNTs? or Why MWCNTs? 2857.6 Conclusions 287Acknowledgments 289References 289

x Contents

Part 3 Functional and Applied Ceramics

8 Application of Organic and Inorganic Wastes in Clay Brick Production: A Chemometric Approach 301Milica V. Vasić, Zagorka Radojević, and Lato Pezo8.1 Introduction 3028.2 Materials and Methods 305

8.2.1 Raw Materials and Laboratory Brick Samples 3058.2.2 Macro Oxides Content of the Used Raw Materials 3068.2.3 Response Surface Method 3078.2.4 Fuzzy Synthetic Evaluation Algorithm 3088.2.5 Artificial Neural Network modeling 309

8.3 Results and Discussion 3128.3.1 Characteristics of Raw Materials 3128.3.2 Changes Observed in Shaping and

Drying in the Air 3148.3.3 Characteristics of Fired Products 3188.3.4 RSM and ANOVA Analysis 3218.3.5 Neurons in the ANN Hidden Layer 3238.3.6 Simulation of the ANNs 3258.3.7 Principal Component Analysis 3288.3.8 Optimization 330


9 Functional Tantalum-based Oxides: From the Structure to the Applications 337Sebastian Zlotnik, Alexander Tkach and Paula M. Vilarinho9.1 Functional Materials: Current Needs 3389.2 Importance of Tantalum and Tantalum-based Oxides 3429.3 Properties of Alkali Tantalates 343

9.3.1 Crystal and Electronic Structures 3439.3.2 Thermochemistry 347

9.4 Processing of Alkali Tantalate Ceramics for Electronic Applications 351

9.5 Potential Applications of Alkali Tantalates 3589.5.1 Sodium Tantalate as a Photocatalyst 3589.5.2 Lithium Tantalate as a Piezoelectric Biomaterial 366

Contents xi

9.6 Conclusions 370Acknowledgement 371References 371

10 Application of Silver Tin Research on Hydroxyapatite 385Ewa Skwarek10.1 Introduction 386

10.1.1 Properties of Silver 386 10.1.2 Application of Silver 387 10.1.3 Hydroxyapatite (HAP)–Silver 391

10.2 Materials and Methods 399 10.2.1 Synthesis of Hydroxyapatite Using the

Co-precipitation Method 399 10.2.2 Synthesis of Silver-doped Hydroxyapatite 400 10.2.3 Characteristics of Surfaces of Obtained Materials 400

10.3 Results and Discussion 402 10.3.1 The Results of XRD and Surface 402 10.3.2 Zeta Potential at the Hydroxyapatite/NaNO3

Electrolyte Solution Interface 404 10.3.3 Surface Charge Density 408 10.3.4 Adsorption of Silver Ions on Hydroxyapatite 410 10.3.5 Kinetics of Ag+ Ions Adsorption on the

Hydroxyapatite Surface 41310.4 Conclusion 414References 415

Index 419

Preface

Ceramic materials are inorganic and nonmetallic porcelains, tiles, enam-els, cements, glasses and refractory bricks. Today, the term “ceramics” has gained a wider meaning as a new generation of materials which influence our lives; electronics, computers, communications, aerospace and other industries rely on them for a number of uses. In general, advanced ceramic materials include electroceramics, optoelectronic ceramics, superconduc-tive ceramics and the more recently developed piezoelectric and dielectric ceramics. Due to their features, including their mechanical properties and decorative textures, they can be considered for environmental uses and energy applications, as well as for use in bioceramics, composites, function-ally graded materials, intelligent ceramics and so on. This book has a trans-disciplinary readership that spans students, engineers, scholars, scientists, physicists, chemists, life scientists and beyond. The volume brings together innovative methodologies and strategies adopted in the research and devel-opment of advanced ceramic materials and offers a comprehensive view of cutting-edge research on ceramic materials and technologies.

A wide range of processing methods used to generate ceramic mate-rials for a variety of functional, structural and biomedical applications are described in this book. The book starts with an excellent review of solution-based methods that can be used to deposit epitaxial films of oxide nanomaterials for microelectronics applications and is followed by a detailed description of tantalum oxides and related phases and their poten-tial use in solar cells and other applications. In the next chapter there is a discussion of the basics of microwave processing which contains a brief summary of its history in various materials and a description of the recent work on hybrid microwave sintering of metal matrix composites contain-ing absorbing ceramic materials.

The next chapters focus on structural applications, starting with a description of continuous fiber ceramic matrix composites, where typical matrix and filler materials are discussed along with the inter-facial layers needed to induce crack deflection and improved fracture toughness. The following two chapters deal with the addition of carbon

xiii

xiv Preface

nanotubes (single-wall and multiwall) into bulk alumina and zirconia and how the characteristics of the nanotubes as well as the processing methods used can affect the resultant properties. Next the detection of crack healing in MAX phase ceramics and their enhanced properties as a result of the incorporation of these unique materials are discussed.

Additional chapters investigate the effect of the additives on manufac-turability and biocompatibility. In the first chapter of this section, waste materials from a variety of industries are incorporated into ceramic brick for sustainable manufacturing. The authors propose the use of an artificial neural network optimization program for identifying the conditions that work best for each additive. The next chapter focuses on the importance of different additives to improve the bioactivity of calcium orthophosphates used in medical implants, followed by a chapter investigating the effect of silver additions to hydroxyapatite for improved antifungal and antibacte-rial responses using a variety of surface controlled schemes.

This book is written for readers from diverse backgrounds across the fields of chemistry, physics, materials science and engineering, medical science, pharmacy, environmental technology, biotechnology, and bio-medical engineering.

EditorsAshutosh Tiwari, PhD, DSc

Rosario Gerhardt, PhDMagdalena Szutkowska, PhD

July, 2016

Part 1DESIGN, PROCESSING,

AND PROPERTIES

3

Ashutosh Tiwari, Rosario A. Gerhardt and Magdalena Szutkowska (eds.) Advanced Ceramic Materials, (3–32) © 2016 Scrivener Publishing LLC

1Development of Epitaxial Oxide Ceramics

Nanomaterials Based on Chemical Strategies on Semiconductor Platforms

A. Carretero-Genevrier1*, R. Bachelet1, G. Saint-Girons1, R. Moalla1, J. M. Vila-Fungueiriño2, B. Rivas-Murias2, F. Rivadulla2,

J. Rodriguez-Carvajal3, A. Gomez4, J. Gazquez4, M. Gich4 and N. Mestres4

1Institut des Nanotechnologies de Lyon (INL) CNRS— Ecole Centrale de Lyon, Ecully, France

2Centro de Investigación en Química Biológica y Materiales Moleculares (CIQUS), Universidad de Santiago de Compostela, Santiago de Compostela, Spain

3Institut Laue-Langevin, Grenoble Cedex 9, France 4Institut de Ciència de Materials de Barcelona ICMAB, Consejo Superior de

Investigaciones Científicas CSIC, Campus UAB Catalonia, Spain

AbstractThe technological impact of combining substrate technologies with the properties of functional advanced oxide ceramics is colossal given its relevant role in the development of novel and more efficient devices. However, the precise control of interfaces and crystallization mechanisms of dissimilar materials at the nanoscale needs to be further developed. As an example, the integration of hybrid struc-tures of high-quality epitaxial oxide films and nanostructures on silicon remains extremely challenging because these materials present major chemical, structural and thermal differences. This book chapter describes the main promising strate-gies that are being used to accommodate advanced oxide nanostructured ceram-ics on different technological substrates via chemical solution deposition (CSD) approaches. We will focus on novel examples separated into two main sections: (i) epitaxial ceramic nanomaterials entirely performed by soft chemistry, such as nanostructured piezoelectric quartz thin films on silicon or 1D complex oxide nanostructures epitaxially grown on silicon, and (ii) ceramic materials prepared by combining soft chemistry and physical techniques, such as epitaxial perovskite

*Corresponding author: [email protected]

4 Advanced Ceramic Materials

oxide thin films on silicon using the combination of soft chemistry and molecular beam epitaxy. Consequently, this chapter will cover cutting-edge strategies based on the potential of combining epitaxial growth and CSD to develop oxide ceramics nanomaterials with novel structures and improved physical properties.

Keywords: Epitaxial growth, thin-film growth, silicon, perovskites, solution chemistry, molecular beam epitaxy, oxide nanostructures, magnetic oxide nanowires, quartz thin films, octahedral molecular sieves

1.1 Introduction

Single-crystalline thin films of functional oxides exhibit a rich variety of properties such as ferroelectricity, piezoelectricity, superconductiv-ity, ferro- and antiferro-magnetism, and nonlinear optics that are highly appealing for new electronic, opto-electronic and energy applications [1, 2]. Over the past few years, tremendous progress has been achieved in the growth of functional oxides on oxide substrates (such as LaAlO3, SrTiO3, Al2O3, MgO, and scandates) [3, 4]. As a result, to date, it is pos-sible to control the epitaxial growth at the unit cell level, which has led to new phenomena arising from the engineering of novel interfaces [5–8]. However, to fully exploit their properties, functional oxides should be effectively integrated on a semiconductor platform like silicon, germa-nium or III/V substrates, which are compatible with the electronics industry. The controlled epitaxial growth of functional oxide layers on semiconductor substrates is a challenging task as a result of the strong structural, chemical, and thermal dissimilarities existing between these materials. In spite of the difference in lattice parameters and thermal expansion coefficients, the major difficulty to engineer epitaxy is linked to the necessity of preventing the formation of an amorphous interfa-cial layer during the first stages of the growth (e.g. SiO2 or silicates on Si, depending of the atmosphere), which hinders any further epitaxy. Additionally, the cations of most oxide compounds can easily inter-diffuse into the silicon substrate giving rise to the formation of spuri-ous phases at the interface [9]. To overcome these major challenges, it is required to use a stable buffer layer, which can act simultaneously as a chemical barrier preventing ionic inter-diffusion and as a structural template favoring epitaxy.

In this context, McKee et al. [10] demonstrated the possibility to grow epitaxial SrTiO3 (STO) films on Si(001) by molecular beam epitaxy (MBE) with Sr passivation strategy. This work sets the basis to integrate STO and related perovskites on silicon for monolithic devices. Consequently, most

Development of Epitaxial Oxide Ceramics Nanomaterials 5

of the research on crystalline functional oxides such as STO [11], lead zir-conate titanate PbZr0.52Ti0.48O3(PZT) [12], BaTiO3 (BTO) [13–17], LaCoO3 (LCO) [18], and La0.7Sr0.3MnO3 (LSMO) [19] integrated with Si has been based on an STO buffer layer epitaxially grown on Si(001) by MBE.

For decades, the integration of functional oxides onto a silicon platform has been identified as an important route to improve and widen the perfor-mances of microelectronics and nanoelectronics devices. A clear example is the successful preparation of two-dimensional electron gas at interfaces between LaAlO3 and SrTiO3 (STO) on Si(001). In this case, the STO film acts simultaneously as a buffer layer and as an active part of the functional heterostrucuture [20]. Moreover, 2D electron gases at the interface have also been demonstrated using LaTiO3 [21] and GdTiO3 [22, 23] grown on STO-buffered Si. Functional non-volatile BTO-based ferroelectric tunnel junctions (FTJ) on Si(001) substrates with a tunneling electroresistance (TER) ratio over 10,000% have been recently demonstrated by pulsed laser deposition (PLD) [24] and MBE [25] growth methods. In both cases, this was accomplished by including a thin layer of STO as an epitaxial template on silicon. In addition, concomitant ferroelectric and antiferromagnetic behaviors were demonstrated on single-crystal BiFeO3 (BFO) films grown on STO on Si(100) using PLD [26] and MBE [27].

Integration of self-assembled vertical epitaxial nanocomposites thin films on Si substrates has been reported for multiferroic or magnetic memory and logic devices. The growth of La0.7Sr0.3MnO3–ZnO perovskite–wurtzite and CeO2–BTO fluorite–perovskite vertical nanocomposites on a Si substrate by PLD was described using a TiN/SrTiO3 bilayer buffer layer [28, 29]. The respective magnetoresistance and ferroelectric properties matched those of similar films grown on single-crystal STO. In addition, perovskite–spinel magneto-electric BFO–CFO vertical nanocomposites were successfully integrated on Si using two different buffered substrates: Sr(Ti0.65Fe0.35)O3/CeO2/YSZ/Si and 8 nm STO/Si [30].

The integration of functional oxides on germanium has recently received a great attention for high-speed and low-power device applications [31], as a result of the higher electron and hole mobility of germanium over silicon [32]. Indeed, a germanium-based ferroelectric field effect transistor was produced recently [33]. In this case, an ultrathin (20 Å) STO layer was first deposited on the Ge substrate. This layer imposes an in-plane compressive strain on BTO to overcome the tensile strain caused by the thermal expan-sion mismatch between both materials, therefore providing BTO films on Ge with out-of-plane polarization.

The development of freestanding oxide devices based on microelec-tromechanical systems (MEMS) technologies using standard silicon


micromachining techniques was possible from SrTiO3/Si structures. Thus, the fabrication of integrated free-standing LSMO microbridges for low-power consumption pressure sensors [34] and uncooled bolometers [35] was recently demonstrated.

The direct growth of functional oxide film on silicon has proved to be also an effective way of integration without epitaxy. In this context, a field effect transistor preserving magnetoelectric functionality on a silicon-integrated device based on a La0.825Sr0.175MnO3/Pb0.2Zr0.8TiO3 (LSMO/PZT) bilayer directly grown by PLD on non-processed Si substrate has been demon-strated by Fina et al. [36]. The measured modulation of the magnetic and transport properties of LSMO upon PZT ferroelectric switching is large, despite the polycrystalline nature of the structure.

Yttrium-stabilized zirconia (YSZ) has also shown to be a very effective buffer layer to integrate functional oxide layers on Si(001) despite a lattice mismatch of about 5% and because it scavenges the native oxide on the sub-strate surface and reduces the native SiO2 oxide layer, with controlled oxygen partial pressure. These characteristics favor the formation of an epitaxial rela-tion with the silicon substrate [37–39], thus making possible the integration of functional ferromagnetic spinel oxides [40–42] and ferroelectric perovskite oxides [43]. The use of an YSZ template substrate has also permitted the fab-rication of all-oxide, free-standing, heteroepitaxial, and piezoelectric MEMS on silicon by using PbZr0.52Ti0.48O3 as the active functional material [44]. Recently, optimized growth conditions and subsequent functional oxides deposition have been shown on a silicon wafer scale (>4") using PLD [45].

The opportunities of combining functional oxides with integrated photo-nic devices and circuits are equally enormous. In spite of the recent advances made on silicon photonics, many limitations still need to be solved [46]. The integration of electro-optical active oxides will allow extending the silicon photonics platform to engineer nonlinear materials, which can be effectively used for tuning, switching, and modulating light in extremely dense pho-tonic circuits. Examples of that are: the fabrication of electro-optical switches based on oxides with metal-to-insulator transitions (e.g. VO2) [47], optical insulators based on magnetic oxides (e.g. Co-substituted CeO2−δ and Co/Fe-substituted SrTiO3−δ) [48], and high-speed modulators based on oxides with the strong Pockels coefficients (e.g. BaTiO3) [49, 50]. Moreover, the inte-gration of PZT layers on GaAs substrates is highly interesting for optoelec-tronic applications considering, for instance the modulation of the optical properties of GaAs-based heterostructures through the strain induced by a piezoelectric layer [51]. Analogously, an epitaxial buffer layer of STO initially grown by MBE is needed for the successful epitaxial integration of the ferro-electric PZT on GaAs [52, 53]. BTO has also been successfully integrated on


GaAs using MBE and showed good ferroelectric characteristics when mea-sured by piezoresponse force microscopy (PFM) [54].

In the past decades, most of the works on crystalline oxides thin films growth on semiconductors have been based on a layer-by-layer approach to heteroepitaxy. The main techniques used to this purpose have been MBE or PLD after adjusting the growth conditions during the deposition to avoid semiconductor surface oxidation or cationic interdiffusion at the interface. However, for future applications in industry, chemical deposition methods such as metal–organic chemical vapor deposition (MOCVD), chemical solutions and sol–gel-based processes, and atomic layer deposition (ALD) show clear advantages over MBE or PLD. These advantages are mainly due to the scalability and low cost of chemical deposition-based methods. ALD entails the sequential delivery of precursors or reagents that either adsorb to saturation coverage or undergo selective ligand reactions, which are self-limiting for the film growth [55, 56]. This growth technique can provide atomic layer control and allows the deposition of ultrathin conformal films onto very high-aspect-ratio structures.

As previously mentioned, an STO buffer layer grown by MBE is a required step for the epitaxial integration of many oxide materials. In this context, the growth of crystalline oxides on semiconductors by combining physical and chemical methods is also a matter of current research [55]. As an example, a combined MBE (to grow first a four-unit cell thick STO buffer layer) and ALD growth method to deposit crystalline oxide thins films on Si(001) including TiO2, BaTiO3, SrTiO3, and LaAlO3 was developed [57–60]. In addition, the deposition of ferroelectric Pb(Zr)TiO3 using chemical solution spin coating on STO-buffered Si and GaAs grown by MBE was also demonstrated [61, 62].

The use of Ge or GaAs substrates makes possible to grow epitaxial perovskite oxides directly via ALD [63], compared to silicon substrates. In this case, a post-deposition annealing at high temperatures is required for crystallization. Recent improvements in the crystalline quality of oxides grown on Ge using ALD highlight the potentiality of this growth method as a scalable integration route of functional oxides for microelectronics technology. Indeed, epitaxial STO and Al-doped STO films up to 15 nm thick with a high degree of crystallinity were grown on the Ge(001) sub-strates via ALD for high-mobility Ge-based transistors [64]. ALD growth of epitaxial SrHfO3 on Ge as a high-k dielectric material has also been demonstrated [65]. Likewise, high-quality epitaxial LaLuO3 and La2−xYxO3 thin films were achieved on GaAs (111) by ALD, and GaAs MOS capaci-tors made from this epitaxial structures showed very good interface quality with small frequency dispersion and low interface trap densities [66, 67].


Nevertheless, the integration of functional oxides on semiconductors entirely performed by chemical methods is still in its early stages.

In this chapter, we present recent promising strategies used to accom-modate advanced oxide nanostructures on silicon substrates via chemical solution deposition (CSD) routes. Two different approaches are proposed, namely the growth of nanostructured oxides entirely by chemical solutions and the combination of soft chemistry and MBE. These two approaches along with relevant examples that will be further discussed in this chapter are displayed in Figure 1.1.

1.2 Integration of Epitaxial Functional Oxides Nanomaterials on Silicon Entirely Performed by Chemical Solution Strategies

Integrating functional oxides nanomaterials as active materials in devices importantly depends on the capability to incorporate crystalline metal

Figure 1.1 General schematic diagram representing all the processes, oxide nanomaterials integrated on silicon, and applications discussed in this book chapter.

Dip-coating

Spin-coating

Template filling

Epitaxial porous films

Films heterostructures

Nanowires/nanorods

Piezoelectrics

Thermoelectrics

Resistive switchersHigh T ferromagnetism

Mechanical switchers

Ferroelectrics

Chemicalprocess

Nanostructures& thin films

Magneticnano-oxide

MBE

Chemical solution grow

th

Silicon

Nano-oxideelectronics


oxides into silicon structures. This feature represents a hard challenge issue because the matching between dissimilar (structurally, thermally, and in general chemically reactive) oxides with silicon in hybrid struc-tures is difficult. As mentioned in the precedent section, one of the most important difficulties stems from the fact that the oxygen partial pressure and silicon temperature must be controlled to avoid the formation of an amorphous SiO2 or silicates crystalline oxide layers at the first stage of growth, which might inhibit epitaxy [68]. In this direction, most of the precedent works on the integration of oxide materials on silicon follow the conventional MBE or PLD techniques that provide advanced control of the interfaces and growth processes [68]. These physical methods are able to develop interface engineering strategies to grow functional oxides thin films on Si and other semiconductor platforms. However, MBE and PLD methodologies are limited to the synthesis of complex oxide materi-als under the form of thin films. As a consequence, top-down approaches consisting on expensive lithography and more recently new, sophisticated and tedious electron and ion beam lithographies are needed to develop epitaxial oxide nanostructures with controllable shapes and morpholo-gies. Additionally, controlled synthesis of epitaxial ternary and quaternary metal oxide nanostructures on silicon is challenging due to the difficulty on controlling the precursor reactions and achieving a homogeneous final stoichiometry.

As an alternative, CSD methods are very convenient since they offer a bottom-up strategy to produce nanostructures with large material diversity, easy setup, and good control over stoichiometry. In addition, it makes possible the use of dopants and the possibility of coating large and uniform areas, which have proved to be a highly-flexible procedure for the fabrication of electronic oxide films and nanostructures [69–75]. However, few efforts have been devoted to integrate functional oxides on semiconductors by using this technique. In a classical CSD method, the synthesis process and growth mechanism that allows to prepare epitaxial oxide nanostructures and thin films on different technological substrates is based on three different stages: (i) The synthesis of a stable and stoichio-metric chemical precursor solution; (ii) the deposition of the precursor solution on a substrate either through spin coating, dip coating, or spray coating; and (iii) a thermal treatment to remove the solvent, allowing the densification and final epitaxial crystallization of oxide nanostruc-tures and thin films. Chemical solution methods include a large vari-ety of techniques such as sol–gel techniques, chelation, metal–organic decomposition, polymer-assisted deposition (PAD), and hydrothermal methods [68].


In this section, we present important and recent advances concerning the epitaxial growth of functional oxides nanostructures and nanostruc-tured thin films on silicon entirely performed by chemical solution strate-gies. We will show that CSD methodology can be used as a new chemical strategy in which the devitrification of amorphous SiO2 native layer on silicon permits the integration of different functional oxide nanostruc-tures in air atmosphere. Further epitaxial stabilization of new oxide nano-structures on silicon with enhanced ferromagnetic and electric properties can then be achieved by using this novel chemical approach, supporting the validity and generality of this methodology for the fabrication of func-tional oxide films and nanostructures on silicon. We present the studies conducted on epitaxial growth of piezoelectric quartz nanostructures on silicon [76], as a model system, even though this growth mechanism can further be applied to the integration of other different oxide nanomateri-als on silicon. Indeed, the possibility to generate epitaxial quartz films on silicon by taking advantage of the good epitaxial relation of these crystallo-graphic structures during a catalytic devitrification process of SiO2 native layer makes possible to extend this procedure to other functional oxide nanostructures. More specifically, using alkaline earth cations in the pre-cursor solution is the key to promote the catalytic devitrification of amor-phous SiO2 native layer and consequently the crystallization into α-quartz during the thermal treatment. The α-quartz layer acts as template for the epitaxial growth of single-crystalline oxide nanowires of different compo-sitions, oxide thin films and more importantly allows the direct integra-tion on silicon substrates [77, 78]. Thus, this methodology exhibits a great potential and offers new strategies to integrate novel oxide compounds totally performed by chemical routes with unique, electric, magnetic, or optical properties.

1.2.1 Integration of Piezoelectric Quartz Thin Films on Silicon by Soft Chemistry

Quartz is one of the few materials that have an outstanding combination of properties, i.e. (i) abundant in nature, (ii) environmentally friendly, (iii) piezoelectric with high-quality factor, (iv) low solubility, (v) high hardness, and (vi) stress compensated. As a result, α-quartz is extremely used for applications in industry such as glassmaking, foundry, or hydraulic fracturing and also in a wide range of fields including micro-electronics or telecommunications. Therefore, α-quartz is an important material for microelectronics industry since it is selected to fabricate oscil-lators and transducers that constitute any electronic device. However, to


date, α-quartz is exclusively synthesized by hydrothermal methods, which produce big crystals making impossible to decrease their size below a thickness of 10 μm [79], and for most applications, these crystals need to be bonded on Si substrates. This feature represents an important barrier for the microelectronic industry since thinner monocrystalline quartz plates are currently highly demanded to produce faster device operation, higher-frequency filtering, or transducers with lower detection levels and improved sensitivity.

In spite of the technological needs, epitaxial quartz nanostructures on silicon are not yet developed. As an alternative, chemical CSD method-ology appears as a bottom-up approach capable to prepare quartz nano-structures by taking advantage of all the benefits of soft chemistry [80]. Nevertheless, silica has more than 11 polymorphs that make extremely dif-ficult the crystallization of pure α-quartz phase from an amorphous SiO2 gel. As a consequence, the synthesis of quartz using CSD requires a critical control over different parameters, such as the choice of precursors, catalyz-ers, thermal treatment, and humidity [81–84].

Recently, we have deciphered the mechanism behind the devitrification–crystallization process of α-quartz by studying 3D amor-phous silica monoliths as a model system containing different doping levels of Sr2+ catalyst by in situ neutron thermodiffractometry [85]. Silica monoliths of specific catalyst composition were prepared by sol–gel pro-cess from alcohol/water solutions of soluble silicic acid precursors together with surfactant structure-directing-agents films. These studies showed, for the first time to our knowledge, the dynamic interaction between silica glass and Sr2+ catalysts and even crystalline phase changes that take place during quartz growth in real time. Particularly, these studies provided evidences that quartz formation is not driven by the presence of inter-mediate silicate phases and that a precise doping level of Sr2+ cations is needed to assist the quartz crystallization during the thermal treatment. Figure 1.2 shows the thermally activated devitrification–crystallization of amorphous silica monoliths assisted by different doping levels of Sr2+ cata-lyst with respect to Si (1%, 2%, 6%, and 12%) and monitored by neutron thermodiffractometry. Importantly, only silica monoliths containing a 6 atomic percent of Sr2+ catalyst produced the direct observation and synthe-sis of pure quartz polymorph crystallization at relative low temperature. At this Sr2+ concentration within the silica monoliths, pure quartz crys-tallization is ensured in a wide range of temperatures. Below this critical concentration (


1 and 2 atomic percent of Sr2+ are used, amorphous patterns are observed during the neutron thermo diffractometry (Figure 1.2a and b), indicating the non- crystallization of the silica monolith. Conversely, above 6 atomic percent of Sr2+, i.e. 12% Sr2+, excess of the catalyst produced the crystalliza-tion of cristobalite polymorph at lower temperature, in competion with the crystallization of quartz (see Figure 1.2d).

These results demonstrated that neutron diffraction can be a useful tool for nanotechnology, although this technique needs large amounts of material to statistically prove the different processes that take place in a solid-state catalytic reaction. In this case, the authors used 3D amorphous silica monoliths as model systems to study the Sr2+-mediated devitrifica-tion mechanism of silica. By taking advantage of the results obtained from neutron diffraction, it was possible to develop a new chemical route for the growth of epitaxial quartz films and nanostructures on silicon [76].

Figure 1.2 Thermally activated devitrification–crystallization of amorphous silica monoliths assisted by different doping levels of Sr2+ catalyst and monitored by neutron thermodiffractometry. (a) 1 atomic percent (1%) of Sr2+, (b) 2 atomic percent (2%) of Sr2+, and (c) 6 atomic percent (6%) of Sr2+. Notice that under this amount of Sr2+, amorphous silica monoliths crystallize into pure quartz polymorph at relative low temperature. (d) 12 atomic percent (12%) of Sr2+. Notice that under this doping level within the silica, the crystallization of silica results in a competition between cristobalite and quartz polymorphs.

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Quartz film synthesis involved a controlled dip-coating deposition on Si(100) substrates of a sol–gel solution containing partially hydrolyzed and condensed tetraethoxysilane (TEOS) quartz precursor in presence of cet-rimonium bromide (CTAB). Analogously to neutron diffraction experi-ments, silica films were doped with 6 atomic percent of Sr2+, incorporated as chloride salt, which produced the devitrification and crystallization into quartz of silica film. Importantly, strontium was homogeneously distrib-uted along the amorphous silica film matrix and silicon interface, which is crucial to successfully crystallize silica films into quartz. Amorphous silica films were doped either by a two-step synthesis, where strontium salt is impregnated into a mesoporous silica previously prepared, or in a single-step synthesis, where this cation is directly incorporated during gelification and drying of dip-coated films through an evaporation-induced self-assembly (EISA) process [86]. Following both strategies, epitaxial quartz films were obtained after thermal treatments in air at 1000 °C during 5 h. Quartz crystallization starts at 950 °C, where the low mismatch degree between quartz and Si(100) substrates induced a preferential assembly and epitaxial growth of α-quartz crystals during heterogeneous nucleation along the silicon surface (see Figure 1.3).

X-ray diffraction (XRD) scans and transmission electron microscopy (TEM) cross-section analysis can be used to determine the good crystallin-ity and misorientation of epitaxial α-quartz films, as shown in Figure 1.4. Notice that quartz crystallization starts within the same temperature range observed from neutron diffraction experiments in 3D silica mono-liths (Figure 1.2c and 1.4a). Inset in Figure 1.4a shows rocking curves of (100) peak at different temperatures, which confirms a complete crys-tallization at 1000 °C, achieving the lower full width at half-maximum (FWHM) value (3°) after 5 h of thermal treatment. Figure 1.4b confirms that only silica films with 6% of Sr2+ give rise to epitaxial quartz films, in agreement with the catalytic behavior of the devitrification of silica already observed from neutron diffraction. Additionally, the epitaxial relation of α-quartz films on silicon can be obtained from pole figures given by the quartz (100)||Si(100), as shown in Figure 1.4c.

1.2.2 Controllable Textures of Epitaxial Quartz Thin FilmsThe high versatility of CSD methodology makes possible the fabrication of epitaxial quartz with different textures on silicon. Indeed, CSD methods are very convenient since they provide a bottom-up strategy to engineer nano-structures with good control over the shape and morphology. An example of that is the control over the texture and porosity of epitaxial quartz thin


Figure 1.3 Schematics of the growth mechanism of epitaxial quartz thin films on (100)Si substrate. (1) Cross-sectional cartoon of the initial amorphous mesoporous silica film where the 6% of Sr2+ catalyst is homogeneously distributed along the silica matrix and silicon interface. (2) Devitrification and melting of the original amorphous mesoporous film and first crystallization above 925 °C of epitaxial α-quartz film. (3) Epitaxial quartz film formation on (100) Si substrate. After crystallization process all Sr2+ sinters and forms spherical amorphous nanoparticles of SrCO3 that are finally fixed at the surface within quartz grain boundaries.

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films. These nanostructured films can be prepared either through a single-step synthesis via a novel phase separation process or through a two-step synthesis that requires the previous preparation of a mesoporous silica film (see Figure 1.5). Evidence of these two processes is displayed in Figure 1.5, where a silica film with hexagonal close-packaged pores of 700 ± 50 nm in diameter (Figure 1.5a) yielded epitaxial α-quartz thin films that kept the initial distribution and pore size diameter (Figure 1.5b). Analogously, mesoporous quartz films were synthesized by a two-step process, where the minimum pore size to accommodate quartz crystals around the initial pore morphology is 40 nm. Below this pore size, the porosity collapses yielding the formation of dense epitaxial quartz films. This growth mecha-nism was among the first examples that prove the possibility of engineering

Advanced Ceramic Materials · 2016. 8. 13. · Scrivener Publishing 100 Cummings Center, Suite 541J...

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Transcript of Advanced Ceramic Materials · 2016. 8. 13. · Scrivener Publishing 100 Cummings Center, Suite 541J...