Epitaxial Growth of Complex Metal Oxides

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Ideal for those interested in the improvements in epitaxial growth processes that have resulted in higher quality films of complex metal oxides and the further advances and applications they have created for electronic and optical purposes Fakta. Ta kontakt med Kundesenteret. Avbryt Send e-post. Les mer. Om boka Epitaxial Growth of Complex Metal Oxides The atomic arrangement and subsequent properties of a material are determined by the type and conditions of growth leading to epitaxy, making control of these conditions key to the fabrication of higher quality materials.

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In reality, both types of systems show complex physics and chemistry with an immense number of functionalities [ 1 ]. Irrespective of their structural symmetry, three types of crystalline solids can be found either naturally or artificially, i. These include polycrystals, single crystals, and thin films. As a result, polycrystals are occasionally considered dirty materials, and they show unusual behaviors at low temperature due to disorder.

Polycrystals can be nearly several centimeters in size. Schematics of a a polycrystal, b single crystal, and c thin film. Polycrystals have many grains, whereas the crystal orientation in single crystals is uniform. Moreover, thin films are grown on structurally compatible metal oxide substrates. Polycrystalline figure was taken from Ref. Therefore, it becomes easy to determine the various directions of a crystal and measure its properties along a particular direction. As a result, single crystals are regarded to be the cleanest and are very popular among the material science community as they reflect the exact properties of a material.

Single crystals are nearly a few mm in size. For the most part, the floating-zone method, Czochralski method and Bridgman-Stockbarger method are used to grow high-quality single crystals [ 3 ]. A detailed analysis of these methods is beyond the scope of this chapter. Thin films consist of very few layers of a solid material. Details about various thin-film deposition techniques are discussed later in this chapter. Most thin films are made of oxides, particularly transition metal oxides TMOs. TMO thin films are one of the most investigated research topics in condensed matter physics as they show a variety of phenomena, e.

Among the various types of TMOs, perovskites are a class of materials that shows almost all the properties mentioned above. Sr-cation is at the center; Ti-cation is surrounded by six O-anion forming TiO6 octahedra cage. Reprinted with permission from [11, 20, 26]. Perovskites, named after the Russian Mineralogist Count Lev Aleksevich von Perovski, have a general unit cell crystal structure of the ABO 3 type, where the A-site is an alkaline earth or rare earth metal and the B-site is a transition metal element e.


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The structure can easily accommodate a wide range of valence states in both A- and B-sites e. This octahedral cage is the most important part of the crystal structure of these materials because the hopping of electrons from one d -orbital of the transition metal element to another d -orbital depends on the shape, size and position of this octahedron; thus, it affects the physics and chemistry of the material and the appearance of variety of phenomena [ 21 , 22 , 23 , 24 ].

In bulk polycrystals and single crystals, the shape, size and position of the BO 6 octahedra can be manipulated externally by inducing chemical pressure replacing A-site or B-site cations with other transition metal elements , or by partial oxygen pressure changing the pressure from the atmospheric one [ 6 ].

Cations with different sizes lead to the distortion of the crystal lattice, which is usually quantified as the Goldsmith tolerance factor [ 27 ] t f , given by. The stability and distortion of a crystal structure is indicated by the value of t f. For a perfect cubic structure, t f is 1. Structure still remains cubic for 0. For more lower value of t f it forms other types of crystal structures, resulting in structural transitions to orthorhombic, or rhombohedral states that have lower symmetry than the cubic state.

However, as a result of chemical substitutions, disorder is introduced into the materials, which in most cases suppresses and even destroys the properties of a material. These difficulties can be overcome in a unique way in thin films by a disorder-free clean route approach. This can be achieved by growing thin films on substrates that are structurally compatible but have different cubic pseudo-cubic lattice constants. Once a strain effect is induced in a film, due to the change of energy scales of various degrees of freedoms lattice, charge, spin, and orbital , it shows novel properties that cannot be found in parent bulk compound.

This means that novel quantum-correlated phenomena can be obtained by the strain engineering of oxide heterostructures, which broadens the field and our understanding of condensed matter physics. In the next section, we will discuss how to grow such atomically controlled high-quality thin films and induce the strain effect in TMO heterostructure.

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In recent decades, significant advances have been made in synthesizing epitaxial thin films in the laboratory using various deposition techniques [ 5 ]. The PLD technique is probably the most commonly used method for growing oxide thin films [ 5 , 30 , 31 , 32 ]. Films are grown inside a high-vacuum chamber. A homemade or commercially available polycrystalline target is ablated by an energy source typically a KrF laser with a wavelength of nm or a frequency-doubled Nd:YAG laser with a wavelength of nm.

When the target is ablated, it produces a highly energetic plasma plume from the target. This highly energetic plume contains ions and molecules that are then deposited onto the substrate surface, which is attached on a substrate holder and placed opposite the target along the same out-of-plane axis. The substrate temperature, which is controlled by a heater, is determined from outside the chamber using a pyrometer. Gaseous atoms condense on a template created by the substrate to form a single crystal.

During this process, one needs to fulfill the growing conditions, e. The in-situ growth process can be monitored in real time by using the reflection high-energy electron diffraction RHEED method.

MBE CLUSTER GROWTH

Reprinted with permission from Refs. The advantages of the PLD technique are: 1 in-situ stoichiometric transfer of composition from target to substrate; 2 compatible materials can be grown under oxygen pressures ranging from ultra-high vacuum UHV to atmospheric pressure; 3 materials ranging from ultra-thin homoepitaxial thin films to artificial superlattices can be grown with nanometer precision; 4 depending on the availability of the target material, a wide variety of films can be grown; and 5 materials are grown in a compact and inexpensive chamber.


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  7. Therefore, to grow the highest quality epitaxial thin films, one should be aware of these facts. There are many groups around the world who have been pioneers in growing artificial epitaxial single-crystalline thin films of the highest quality. Molecular beam epitaxy MBE is also a method used to grow high-quality epitaxial thin films [ 5 , 34 ]. Cho [ 35 ].

    The only difference is the target material. The spray duration is individually controlled for each beam by shutters. Once all the deposition conditions are satisfied, the ejected molecules travel to the substrate surface, condenses and form a single-crystalline thin film compatible with the substrate crystal structure.

    One of the main advantages of MBE thin film growth is its extreme cleanliness; i. Substrates seem to be the basis of all thin film growth. Choosing a suitable metal oxide substrate is an important factor for growing high-quality epitaxial thin films as the structure and properties of a thin film depends on the underlying substrate and the interfacial interaction between the substrate and film [ 38 ].

    Growth and interfacial properties of epitaxial oxides on semiconductors: ab initio insights

    When choosing a metal oxide substrate for growing epitaxial films, one should consider the following factors: Lattice matching between the substrate and film, which is important for the growth of most natural state films structural compatibility. No chemical reaction between the elements of the substrate and film chemically compatibility. Thermal-expansion matching between the substrate and film, as films are generally grown at high temperatures good thermal-expansion match.

    Surface quality of the substrate e. For most ABO 3 perovskites, their lattice constants range from 3.

    Fortunately, there are many perovskite single-crystal metal oxide substrates available commercially with lattice constants ranging from 3. Among various available perovskite substrates, insulating SrTiO 3 is the most popular one. There are also a broad range of substrates available with similar structures to that of SrTiO 3 while possessing different lattice constants and crystal orientations. With a judicial choice of substrate, various atomically controlled high-quality thin films can be grown.

    Reprinted and adapted with permission from Ref. Copyright Materials Research Society. Strain engineering is a unique way to create the novel functionalities in epitaxial oxide thin films [ 40 , 41 , 42 , 43 ]. This is defined as the typical strain effect in thin films.

    Under these epitaxial strain scenarios, the properties of functional oxide thin films can be drastically altered. Thus, elastic strain is a viable route to observe materials with exceptional properties that cannot be observed in their bulk form by any other means [ 44 , 45 ]. Although it looks simple, the intrinsic mechanism of the appearance of novel functionalities induced by the strain effect is quite complex to understand. In the next section, we briefly discuss about the intrinsic mechanism of the strain effect in perovskite thin films.

    On the other hand, for tensile strained films, lattices expand along the in-plane direction and shrink along the out-of-pane direction. Reprinted with permission from Ref. In post-Moore era, electronic devices with multifunctionality may offer a new alternative to replace the current silicon-based technology because the additional value the devices would generate from multifunctionality may create an economically viable path superseding the miniaturization limit of silicon electronic devices.

    In this perspective, oxide electronics based on multifunctional properties of transition metal oxides looks promising [ 46 , 47 ]. Even more exciting is the fact that advanced thin film growth techniques with atomic controllability provide further opportunities to design and synthesize artificial complex transition metal oxide heterostructures and superlattices to bring forth emergent physical properties, normally not seen in bulk states. This is perhaps due to a lack of knowledge about fully resolved atomic structures, especially the position of non-trivial oxygen atoms, as no experimental tool has yet been developed for the direct observation of oxygen atoms.

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