The growth of high-quality single crystals remains a challenging endeavor of materials science. Crystals of suitable size (from fiber crystals with diameters of tens of micrometers up to crystalline ingots or blocks with volumes up to 1 m3) and perfection (free from precipitates, inclusions, and twins with good uniformity and low concentration of dislocations) are required for fundamental research and practical implementation in microelectronic circuits, electro-optic switches and modulators, solid-state lasers, light-emitting diodes, sensors, and many other devices. In this introductory article of this issue of MRS Bulletin, we describe the two main challenges of today's crystal growth, namely (1) the production of well-established crystalline materials with improved structural perfection and larger size at a lower cost and (2) the bulk growth of new categories of materials with extreme thermodynamic characteristics, such as a very high melting point, high melting dissociation pressure, incongruent phase diagram, and anisotropic segregation. The subsequent six articles provide examples of how the crystal growers took up these challenges, which led to new experimental approaches and technological advances.

Bridgman or vertical gradient freeze (VGF) crystal growth processes have several advantages compared to other melt growth methods, especially the possibility to achieve a low level of thermal stress and low dislocation densities in the grown crystals. However, crystals grown in contact with a crucible usually suffer from mechanical stress during cooling, reducing the structural quality. The “detached” or “dewetted” Bridgman growth avoids this problem and has recently been investigated in more detail as a promising tool to improve crystal quality. Detached growth, where the crystal is separated from the crucible wall by a gap of 10–100 μm, was found originally in some microgravity experiments going back to 1975. Considerable improvements of crystal quality were reported for those cases; however, the reasons for the detachment were not fully understood. In the last 10–15 years, theoretical investigations as well as new experiments have shown beyond a doubt that detached growth can, in principle, be achieved in Earth's gravity with the same advantages that were demonstrated in the crystals grown under microgravity. It could be shown that the ability to achieve detachment depends on a complex interplay of the wetting of the melt with the crucible and the crystal as well as the pressure balance in the system, including the hydrostatic pressure, the gas pressure above the melt, and the pressure below the melt. It turns out that for stable detachment, only, specific combinations of meniscus shape, gap size, wetting angle, growth angle, and pressures work. The conditions that lead to detachment are thus highly specific for a given system.

The present and future demands of industrial bulk crystal growth from the melt are concentrated on improved crystal quality, increased yield, and reduced costs. To meet these challenges, the size of the melt volume must be markedly increased. As a result, violent convective perturbations appear within the melts due to turbulent heat and mass flows. They disturb the single crystal growth and give rise to compositional inhomogeneities. The application of external force fields is an effective method to dampen and control these flows. After introducing different stabilizing variants, such as constant and accelerated melt rotation, mechanical vibrations, and electric current, this article focuses on the use of magnetic fields. Nonsteady fields became very popular because, in this case, the needed strength of the magnetic induction is much lower than for steady fields. A new low-energy low-cost technology that combines heat and magnetic field generation in one module placed close to the melt crucible is introduced.

Aluminum nitride (AlN) and gallium nitride (GaN) play an essential role in modern electronics, particularly in optoelectronics. Highly efficient light-emitting devices covering the ultraviolet to green spectral region are fabricated from these materials. Despite all efforts, the growth of large-size and high-quality AlN and GaN crystals for substrates, which are thermally and lattice-matched to the AlGaN-based device structures, is still in its infancy. This is due to the high equilibrium vapor pressure of nitrogen above these compounds, which requires growth techniques employing either the vapor phase or liquid solutions. The best commercially available GaN substrates show a high dislocation density of >105 per cm2 and strong bowing with a radius of curvature smaller than 10 m. This article reviews current growth techniques that look promising and may become commercially viable.

The micro-pulling-down technique is a crystal growth method that has been mostly developed since 1992. The general scheme of the growth system is relatively simple: the melt (oxide, fluoride, metal) residing in a crucible is transported in downward through microcapillary channel(s) made in the bottom of the crucible. Two driving forces (capillary action and gravity) support the delivery of the melt to the liquid/solid growth interface formed under the crucible due to a properly established temperature gradient. Appropriate configuration of the crucible bottom allows for controlling of the crystal shape (fibers, rods, tubes, plates) and the dimensions of the crystals' cross sections that range approximately from 0.1 to 10 mm. A great number of scientifically and industrially important optical crystal fibers have been successfully produced using this method.

The renaissance of double tungstate (DW) and double molybdate (DMo) crystals is due to the vigorous recent progress in growth and characterization of these materials in the form of both bulk crystals with centimeter dimensions and epitaxial layers. “Double” refers to compounds having two ions with the generic composition MT(XO4)2 (where M = Li+, Na+, or K+; T = Ln3+, Y3+, or Bi3+; and X = W6+ or Mo6+). These compounds crystallize primarily in two phases belonging to the monoclinic (C2/c) and tetragonal (l4) structures. The specific properties of both types of crystals and the major technological challenges associated with the Czochralski and top-seeded solution growth of rare-earth doped crystals are presented. The performance of specific crystals is discussed in terms of their applications as tunable, thin-disk, and ultrashort (<100 fs) pulsed lasers or as nonlinear elements for laser Raman shifting.

Relaxor-based single crystals of complex perovskite solid solutions, Pb(Mg1/3Nb2/3)O3–PbTiO3 [PMN–PT] and Pb(Zn1/3Nb2/3)O3–PbTiO3 [PZN–PT], exhibit extraordinary piezoelectric performance, with extremely high piezoelectric coefficients, very large electromechanical coupling factors, and exceptionally high strain levels. These materials outperform the currently used Pb(Zr1–xTix)O3 [PZT] ceramics, making them the materials of choice for the next generation of electromechanical transducers for a broad range of advanced applications. In this article, recent major advances in the development of piezocrystals are reviewed in terms of crystal growth, piezoelectric properties, crystal chemistry, domain structure, and device applications.