Milisavljevic and Wu BMC Mat (2020) 2:2 https://doi.org/10.1186/s42833-020-0008-0 BMC Materials REVIEW Open Access Current status of solid-state single crystal growth Iva Milisavljevic and Yiquan Wu* Abstract Fabrication of single crystals has long been limited to melt- and solution-growth techniques. However, in recent years solid-state single crystal growth (SSCG) has appeared as a promising alternative to the conventional techniques due to its cost-efectiveness and simplicity in terms of processing. Moreover, the SSCG technique has enabled the fab- rication of single crystals with complex chemical compositions and even incongruent melting behavior. A recently proposed mechanism of grain boundary migration known as the “mixed control mechanism” and the associated prin- ciples of microstructural evolution represent the basis of the SSCG technique. The mixed control mechanism has been successfully used to control the key aspects of the SSCG technique, which are the grain growth and the development of the microstructure during the conversion process of the single crystal from the polycrystalline matrix. This paper explains in brief basis of the mixed control mechanism and the underlying principles of microstructural evolution in polycrystalline materials and provides a comprehensive overview of the most recent research on single crystal materi- als fabricated via the solid-state single crystal growth technique and their properties. Keywords: Single crystal, Solid-state single crystal growth, SSCG, Mixed control mechanism, Microstructural evolution Introduction Single crystals have found extensive use in optical, elec- A need for single crystal fabrication tronic, optoelectronic, and other applications. Specif- Single crystals are one of the most important groups of cally, single crystal semiconductors are one of the most materials due to their continuous, uniform, and highly- widely researched and used materials. Tese materials ordered structure which enables them to possess unique have been applied for various electronic and optoelec- properties. In many aspects, single crystal materials can tronic devices and components, such as light-emitting be found to be advantageous over polycrystalline materi- diodes (LEDs), photodetectors, wide-bandgap devices, als, and many properties which are found in single crys- high-power lasers, consumer electronics, and more tals cannot be replicated in polycrystals [1]. Currently, [2, 3]. For example, current computer chip production even with the technological developments of advanced is not possible without high-quality single crystal sili- polycrystalline materials which are designed for specifc con (Si) wafers [4]. Due to their outstanding optical and applications, the electrical, optical, thermal, mechanical, electronic properties, single crystals of III–V semicon- and other properties of single crystals still remain supe- ductors, such as GaAs, GaN, InP, InAs, and others, are rior. For these reasons, it is not surprising that single an integral part of devices for application in fber-optic crystals, and the methods for their fabrication, are a topic communication, wireless and satellite communication, of interest among many researchers. solid-state lighting, and more [2]. Te importance of sin- gle crystal alumina, also known as sapphire, as well as *Correspondence: [email protected] yttrium aluminum garnet (YAG), for laser materials has Kazuo Inamori School of Engineering, New York State College also been demonstrated through numerous applications. of Ceramics, Alfred University, 2 Pine Street, Alfred, NY 14802, USA Sapphire has been used in the electronics industry both © The Author(s) 2020. This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creat iveco mmons .org/licen ses/by/4.0/. The Creative Commons Public Domain Dedication waiver (http://creat iveco mmons .org/publi cdoma in/ zero/1.0/) applies to the data made available in this article, unless otherwise stated in a credit line to the data. Milisavljevic and Wu BMC Mat (2020) 2:2 Page 2 of 26 as a passive substrate material and active device (e.g. sil- Growth from melt is the most commonly used method icon-on-sapphire); likewise, it is used for rocket domes, and is based upon the solidifcation and crystallization optical data storage, radiation detection, LED devices, of a melted material. Te Czochralski and Bridgman optical windows, and other applications [5]. On the other methods are the two most utilized melt-growth tech- hand, YAG single crystals, and especially Nd3+-doped niques. Te Czochralski method (Cz) is, for example, YAG, are known for their important application in solid- very important for the production of single crystals for state laser devices, such as waveguide lasers [6] and single electronic and optical applications, such as silicon and crystal fbers for high-power lasers [7], as well as scintil- germanium single crystals, as well as some fuoride and lation crystals, and others. Piezoelectric single crystal oxide single crystals [13]. Single crystal growth from melt materials, which were initially developed and utilized allows for the fabrication of large single crystals of excel- as transducers for sonar devices and medical ultrasonic lent quality in a relatively short time when compared to diagnostic devices, have also been applied in sensors, other growth techniques [14]. However, the melt-growth actuators, medical transducers, energy harvesters, and technique shows some disadvantages as well, such as dif- more [8, 9]. As it can be seen, single crystal materials are fculties in maintaining a stable temperature during the capable of covering a wide variety of applications, which crystal growth and in achieving very high melting points range from scientifc- and research-related to daily life. for some materials, achieving chemical homogeneity, Another important use of single crystal materials is as especially in the case when multiple elements are present substrates for flms of diferent materials; this enables a in the system, reactivity of the melted material with the whole new collection of applications. Single crystals can crucible, and high costs of production and equipment. be used not only as a mechanical support or a surface Unlike the melt-growth technique, in which the mate- at which layer or layers of materials are being deposited rial is melted frst, the solution-growth technique but can also act as a single crystal seed during epitaxial involves the dissolution of the material to be crystallized growth [10], when the deposited flm takes on orientation within a suitable solvent or fux (e.g. PbO, PbF 2, Bi2O3, of the substrate, and sometimes even a lattice structure. Li2O, Na2O, K2O, KF, P2O5, etc.) [13, 15]. Out of all the Likewise, the fabrication of single crystal epitaxial flms solution-growth techniques, high-temperature solution- on various substrates, which are a vital part of a wide growth, also known as fux-growth, has been the most range of devices for electronic, optoelectronic, magneto- utilized technique for the fabrication of single crystals optic, and many other applications, although very chal- thus far. Tis technique is especially convenient for mate- lenging, is an important goal in the thin flm industry due rials that incongruently melt or when melt-growth tech- to the numerous advantages of single crystal flms [11]. niques cannot be applied. Te main advantage of this As technological development increases, the need for technique is that the crystals are grown below their melt- high-quality single crystal materials, both in bulk and in ing temperatures and the growth of the crystal occurs thin flms, grows simultaneously. Te availability of vari- spontaneously through nucleation or crystallization on a ous single crystal materials has enabled the development seed. On the other hand, the crystal growth rates for the of a new generation of electronic, optical, optoelectronic, solution-growth method are much slower than that of and other devices. However, growth of high-quality sin- the melt-growth method and the presence of fux ions is gle crystals, with stable and reproducible quality, low unavoidable in the crystal. Growth of single crystals via defect density, with various chemical compositions and the fux method has found many important applications sometimes even extreme thermodynamic properties is in the production of single crystal materials such as gar- still one of the greatest challenges today [12]. Further- nets, various laser crystals, including borates, LiNbO3, more, techniques which are currently used for growing BaTiO3, BaB2O4, and more complex systems such as single crystals experience many processing-related dif- Sr Ba Nb O , Pb Ba Nb O , and others [13]. 1−x x 2 6 1−x x 2 6 fculties despite the technological advancements made Vapor-phase