Current Opinion in Solid State and Materials Science
Crystallization of amorphous complex oxides: New geometries and new compositions via solid phase epitaxy
Introduction
Crystallization from amorphous precursors presents an approach to synthesizing crystals with previously unattained geometries, microstructures, and compositions. Crystallization from a seed is termed solid phase epitaxy (SPE), an epitaxial process that has been extensively explored in the context of elemental and alloy semiconductors [1]. Much of the progress to date in the field has employed SPE in planar geometries and has yielded epitaxial crystalline thin film materials. There are, however, many recent indications that a similar approach can be extended to complex geometries by employing new strategies for depositing amorphous materials, deploying crystal nucleation sites, and directing the crystallization front in three dimensions. In part, these efforts draw inspiration from the formation of crystalline complex oxides such as CaCO3 and Ca5(PO4)3(OH) in biological systems, either from or on organic templates, a key biological phenomenon [2], [3], [4]. A series of recent studies have begun to address fundamental questions concerning SPE of an expanded range materials with nanometer-scale dimensions in one or more directions. Additional fundamental insight is particularly important in oxide materials for which the composition includes multiple cations in the chemical formula, termed complex oxides. Control of SPE of complex oxides offers the potential to form new materials with compositions and three-dimensional (3D) structures that are tailored to challenging applications. For example, SPE can lead to oxides exhibiting exceptional ionic transport and quantum electronics in novel geometries. Realizing this potential, however, requires a detailed and quantitative understanding of the kinetic processes involved in the transformation at a fundamental level.
Complex oxides are particularly amenable to fundamental studies of the interplay between the unit-cell-scale crystal structure and crystallization transformation kinetics because insights can be systematically developed by understanding trends across systems of increasing compositional complexity. Initial efforts to explore the crystallization of complex oxides from amorphous precursors have already shown that SPE can be applied to model oxides drawn from several important classes, including the binary oxides, perovskites, spinels, and pyrochlores reviewed here. Complex oxides incorporate multiple metal cations, exhibit complex crystal structures, and have profoundly useful properties and functionalities. These structures and properties include various forms of ferroic order, superconductivity, oxygen conductivity, catalytic function, and quantum transport. They also provide potential for fabrication of crystalline substrates with controlled lattice parameter for growth of epitaxial semiconductors [5], [6], [7], [8], [9]. A wide palette of complex oxide compositions have similar crystal structures, which allows their electronic, optical, magnetic, and ionic properties to be selected and varied within a set of similar processing conditions. Additional control over the functionality of oxides arises from microstructural features such as interfaces, defects, domain boundaries, and field-induced variation of magnetic or electrical dipole moments. Interfaces in complex oxides exhibit emergent properties due to discontinuity in the polarization or in subtler features such as the symmetry of octahedral rotations [10], [11], [12]. Complex oxides can have extremely large responses to applied stresses, including changes in crystal structure, or large changes in the concentration and mobility of point defects [13], [14]. The further exploitation of SPE in complex oxides requires the interplay of expertise in materials synthesis, atomic level characterization, and the design and measurement of structures exhibiting new or unusual transport phenomena.
At present, the synthesis of crystalline oxide materials in the form of nanostructures and thin films largely employs epitaxial growth techniques that are highly developed and widely used, but which are limited in the range of compositions that can be accessed because of kinetic phenomena such as surface diffusion and phase separation [15], [16], [17]. The surface temperatures that result in rapid surface diffusion in conventional epitaxial growth using pulsed laser deposition (PLD), sputtering, or chemical vapor deposition (CVD) can lead to phase separation of targeted materials [15]. Competing phases, including polymorphs and structurally simpler oxides involving the constituent atoms, limit the scope of compositions and slow the pace of realization of new materials. In compounds such as the pyrochlore iridates, for example, chemical decomposition impedes in situ epitaxial growth of desired phases [18]. Other systems exhibit the nucleation or formation of competing phases in epitaxial growth. These limitations can be alleviated to yield kinetically metastable compositions via SPE, in which diffusive processes during crystallization are largely suppressed.
SPE relies on the suppression of higher-energy-barrier homogeneous nucleation within the amorphous state and favors the lower-barrier process of crystallization from a crystalline seed, as shown schematically in Fig. 1(a) and illustrated for the perovskite complex oxide BiFeO3 in Fig. 1(b) [19]. SPE can produce crystals with smooth surfaces and has kinetic processes amenable to fundamental study, as illustrated for the perovskite oxides EuTiO3 and SrTiO3 in Fig. 1(c) and (d), respectively [20], [21]. SPE is carried out under entirely different processing conditions than epitaxial growth from the vapor, and thus opens new avenues for materials synthesis. The longer-range surface mass transport that leads to phase separation in conventional epitaxy, for example, can be inhibited during SPE because atomic surface diffusion is reduced at the low deposition temperatures used to form and process the amorphous layers. There are complex challenges associated with understanding the kinetics of SPE, particularly in probing and ultimately understanding structural and chemical phenomena occurring during the growth process. Mastering these phenomena will enable realization of the full potential of SPE and the unique materials it can produce. The complexity of this challenge requires advances in oxide materials synthesis, characterization, theory, nanomaterials manipulation, and in the utilization of novel oxides in key applications.
The fundamental phenomena of SPE have been most widely investigated in group IV semiconductors consisting of Si or Si/SiGe alloys, where SPE has a key role in silicon processing technologies [1]. The fundamental thermodynamic phenomenon underpinning SPE in Si is the high free energy of the amorphous form in comparison with the crystal, as illustrated in Fig. 2 [22]. The difference in free energy between amorphous and crystalline forms renders the amorphous material metastable and favors crystallization. A fundamentally similar free energy difference also applies to the crystallization of amorphous oxides. Extensive experimental studies of SPE in group IV semiconductors have revealed the dependence of the rates of crystallization on the crystallographic orientation of the amorphous/crystalline interface [23], on the concentrations of Ge in SiGe alloys and of dopants [24], [25], and the local stress state [26]. Molecular dynamics simulations have probed the roles of point defects such as interstitials [27], the nucleation of crystalline islands on the (0 0 1) amorphous/crystalline interface [28], and the dependence of the velocity of the crystallization front on pressure and mechanical stress [29].
Other observations, however, point to the complexity of the SPE of some compounds, such as GaAs, for which SPE does not lead to single crystal films [30], [31], [32]. Only a thin layer of the order of tens of nanometers of GaAs recrystallizes epitaxially before the growth of GaAs crystals nucleated elsewhere in random orientations consume the amorphous layer and form a polycrystalline thin film [30], [31], [32]. The polycrystallinity of GaAs layers crystallized by SPE can be attributed to the covalent and directional bonding of compound semiconductors, and extremely low deviations from stoichiometry required to form these materials [31]. In contrast, many oxides crystallize over a broad range of stoichiometry with small changes in structure and can be oxidized under ambient conditions, making them a particularly favorable class of materials in which to explore and to exploit SPE methods. Furthermore, the differences in chemical bonding within oxides in comparison with semiconductors leads to fundamental differences in the kinetics and diffusion and mechanisms of ordering [33].
In contrast with single-component systems, in which systematic investigations have elucidated many of the atomic-scale processes of SPE, much less extensive mechanistic insight is available for compounds, particularly complex oxides. There is a strong existing base of knowledge associated with the devitrification of bulk oxides [34], [35], [36], [37], [38], [39], [40], [41], [42], [43], [44], [45], [46], [47] and a much smaller body of literature on crystallization of amorphous oxide thin films, including in Al2O3 and other model systems [48], [49], [50], [51]. As a result, the present state of understanding of structural reordering during the transition of complex oxides from disorder to order often lacks detailed mechanistic insight. SPE has already been demonstrated in a few complex oxide systems, for example as in Fig. 1(b)–(d) in thin-film layers of the perovskites BiFeO3 [19], [52], EuTiO3 [20], and SrTiO3 [21], [53], [54], on SrTiO3 single crystal substrates. Compounds with greater structural complexity synthesized by SPE include pyrochlore iridates [18], Y3Fe5O12 (YIG), and doped YIG for optical applications [55]. Despite these demonstrations of SPE, the underlying kinetic and atomic-level processes enabling SPE of these materials and the extension to other systems are not yet completely understood. The fundamental issues that are beginning to be addressed thus include the velocities of amorphous/crystalline interfaces, elemental redistribution during crystal growth, and factors that promote or suppress nucleation.
Complex oxide SPE has the potential to lead to the synthesis of complex oxides in intricate 3D shapes such as membranes, pillars, foams, and one-dimensional interfaces, useful in present and future applications. There are initial indications from work in simple oxides that the realization of 3D geometries will be possible. The crystallization of amorphous Al2O3 in the form of hollow crystalline domes with nanoscale wall thicknesses [56] and from isolated seeds has been recently demonstrated [51]. Crystallization in nanoscale geometries introduces additional kinetic issues such as the dependence of the growth velocity on the local composition, the influence of the specific geometry and crystal-to-wall interfacial energies, and the stress arising from volume changes during the amorphous-to-crystalline transformation [14], [19], [50].
Here we review recent progress in the development of fundamental understanding of the fundamental phenomena underpinning SPE and in the exploration of SPE in complex oxides of different compositions. A summary of the amorphous oxide compositions, crystallized morphologies and orientations, crystalline nucleation sites, and crystallization temperatures appears in Table 1. We further review the development of methods for characterizing the structure and composition on the appropriate length scale and for controlling complex oxide crystal nucleation at the nanoscale. Finally, we describe challenges in extending oxide SPE to new compositions and applications of SPE-derived materials in thermal transport engineering, quantum electronics, and as substrates for heteroepitaxy.
Section snippets
Amorphous structure and nucleation from amorphous phases
Crystallization fundamentally involves the reconfiguration of atomic bonding at the atomic scale. In SPE, the initial state for this reconfiguration is the amorphous oxide structure. A further understanding of this amorphous structure and the interface between amorphous and crystalline complex oxides has the potential to lead to an improved understanding of the crystallization process. Studies of amorphous Al2O3, amorphous SrTiO3, and amorphous oxide semiconductors illustrate key features of
Binary oxides: Al2O3, TiO2, and ZrO2
Studies of compositionally simple binary oxides provide useful insight into the range of phenomena that can be expected to occur in complex oxide SPE. Planar thin films of amorphous Al2O3 crystallize into the γ phase of Al2O3 upon heating and subsequently convert from the γ phase to the α phase at high temperature [96]. The γ-to-α transformation occurs via the motion of a planar growth front [97]. In materials in which Al is alloyed with Fe or Cr, the velocity of the γ-to-α transformation
New compositions
The expansion of the basic concepts of SPE to new materials has the potential to enable the synthesis of a range of oxide compounds for which effective synthesis routes are not yet available. A few particularly exciting examples are (i) the formation of rare-earth iridate magnetic materials, for which SPE has the potential to resolve challenges in obtaining the correct anion composition during epitaxial growth and (ii) the formation of complex multi-metal-ion layers providing a lattice match
Conclusion
Oxide crystallization via SPE changes the kinetics of crystal growth and allows new compositions and geometries to be realized. The rapid development of deposition, characterization, theoretical, and computational methods has already enabled exciting advances towards the nanometer scale control of crystallization. These methods have provided new fundamental insight into the relevant phenomena of crystal growth, such as nucleation rates and interface velocities, and now promise to enable the
Acknowledgement
This research was supported by US National Science Foundation through the University of Wisconsin Materials Research Science and Engineering Center (DMR-1720415).
References (143)
- et al.
Solid-phase epitaxy
- et al.
Solid phase epitaxy of EuTiO3 thin films on SrTiO3 (100) substrates with different oxygen contents
J. Cryst. Growth
(2013) - et al.
Atomistic examinations of the solid-phase epitaxial growth of silicon
J. Cryst. Growth
(2009) - et al.
Thermal and devitrification behavior of CaO-Ga2O3-SiO2 glasses
J. Eur. Ceram. Soc.
(2000) - et al.
Effect of some rare-earth oxides on structure, devitrification and properties of diopside based glasses
Ceram. Int.
(2009) - et al.
Nanocrystal formation and photoluminescence in the Yb3+/Er3+ codoped phosphosilicate glasses
J. Non-Cryst. Solids
(2014) - et al.
Applicability of the adiabatic nucleation theory to glasses
J. Non-Cryst. Solids
(2010) - et al.
Glass transition temperature and devitrification study of barium germanate glasses
J. Non-Cryst. Solids
(1997) - et al.
Effects of annealing on the structure and composition of electron-beam-evaporated tin oxide films
Thin Solid Films
(1984) - et al.
Ion-implantation and annealing of crystalline oxides and ceramic materials
Nucl. Instrum. Meth. B
(1988)
Incorporation of air-cavity into sapphire substrate and its effect on GaN growth and optical properties
J. Cryst. Growth
Structural ordering of ultra-thin, amorphous aluminium-oxide films
Surf. Sci.
Epitaxial growth of barium titanate thin films on germanium via atomic layer deposition
J. Cryst. Growth
Precursors and chemistry for the atomic layer deposition of metallic first row transition metal films
Coord. Chem. Rev.
The initial stages of template-controlled CaCO3 formation revealed by cryo-TEM
Science
Biomimetic CaCO3 mineralization using designer molecules and interfaces
Chem. Rev.
Polymer-controlled growth rate of an amorphous mineral film nucleated at a fatty acid monolayer
Langmuir
Multiferroics: progress and prospects in thin films
Nature Mater.
Ferroelectric thin films: review of materials, properties, and applications
J. Appl. Phys.
A high-mobility electron gas at the LaAlO3/SrTiO3 heterointerface
Nature
ScAlMgO4: an oxide substrate for GaN epitaxy
Mat. Res. Soc. Symp. Proc.
Band gap engineering based on MgxZn1-xO and CdyZn1-yO ternary alloy films
Appl. Phys. Lett.
Emergent phenomena at oxide interfaces
Nat. Mater.
Octahedral rotation-induced ferroelectricity in cation ordered perovskites
Adv. Mater.
Nanosecond phase transition dynamics in compressively strained epitaxial BiFeO3
Adv. Electron. Mater.
“Stretching” the energy landscape of oxides: effects on electrocatalysis and diffusion
MRS Bull.
Strain effects on oxygen migration in perovskites
Phys. Chem. Chem. Phys.
Self-assembled heteroepitaxial oxide nanocomposite thin film structures: designing interface-induced functionality in electronic materials
Adv. Funct. Mater.
Magnetic phase formation in self-assembled epitaxial BiFeO3-MgO and BiFeO3-MgAl2O4 nanocomposite films grown by combinatorial pulsed laser deposition
ACS Appl. Mater. Interfaces
Review of magnetoelectric perovskite-spinel self-assembled nano-composite thin films
J. Mater. Sci.
Odd-parity magnetoresistance in pyrochlore iridate thin films with broken time-reversal symmetry
Sci. Rep.
Crystallization engineering as a route to epitaxial strain control
APL Mater.
Distinct Nucleation and Growth Kinetics of Amorphous SrTiO3 on (001) SrTiO3 and SiO2/Si: a Step toward New Architectures
ACS Appl. Mater. Interfaces
Materials Fundamentals of Molecular Beam Epitaxy
Modeling two-dimensional solid-phase epitaxial regrowth using level set methods
J. Appl. Phys.
Stressed multidirectional solid-phase epitaxial growth of Si
J. Appl. Phys.
Composition dependence of solid-phase epitaxy in silicon-germanium alloys – experiment and theory
Phys. Rev. B
Pressure-enhanced crystallization kinetics of amorphous Si and Ge – implications for point-defect mechanisms
J. Appl. Phys.
Molecular-dynamics simulations of solid-phase epitaxy of Si: growth mechanisms
Phys. Rev. B
Atomistic simulations of solid-phase epitaxial growth in silicon
Phys. Rev. B
Ion-implantation and low-temperature epitaxial regrowth of GaAs
J. Appl. Phys.
Epitaxial regrowth of thin amorphous GaAs layers
Appl. Phys. Lett.
Solid-phase regrowth of amorphous GaAs grown by low-temperature molecular-beam epitaxy
Appl. Phys. Lett.
Physical Ceramics: Principles for Ceramic Science and Engineering
Devitrification properties of lead borate glasses
Phase Trans.
Thermal, structural and crystallization study of niobium potassium phosphate glasses
Mater. Res.
Crystallizing phases and kinetics of crystal growth in potassium tetragermanate glass
J. Mater. Sci. Lett.
Devitrification theory and glass-forming phase diagrams of fluoride compositions
Adiabatic nucleation in devitrifying “fragile” oxide glasses
Phys. Stat. Solidi C
Toward modeling phosphate tellurate glasses: the devitrification and addition of gadolinium ions behavior
J. Phys. Chem. A
Cited by (23)
Microstructure and porosity evolution of alkali activated slag at various heating temperatures
2020, Journal of Materials Research and TechnologyCitation Excerpt :Electron back scatter diffraction (EBSD) is a technique which has provided a detailed understanding of the crystallography microstructural behavior of polycrystalline material [20]. This advanced technique performed with scanning electron (SEM) was utilized to measure crystal orientation in ceramic fabricated samples [21,22] among other areas of steel [23,24] and electronic materials [25,26], but which has not previously analysed for the alkali activated materials. Furthermore, the details of phase mapping, grain boundary characteristic, grains size, crystallographic texture and strain analysing within the materials are easily obtained using EBSD.
Lateral solid phase epitaxy of yttrium iron garnet
2024, Physical Review Materials