Mechanisms of dislocation generation and methods of crystal growth are two historically rich areas of scientific study. These two fields converge in the area of metamorphic epitaxial materials, where the goal is to produce high-performance devices that contain high densities of crystal defects in regions of the engineered material away from the active areas. Metamorphic epitaxy is a form of thin-film growth, where the lattice structure of the layer and substrate are mismatched, and its defining characteristic is that any elastic strain in the overlayer has been relaxed by the deliberate introduction of dislocations at the film–substrate interface. Metamorphic growth enables novel combinations of relaxed single-crystal materials to realize novel functionality and performance in numerous technological areas, including lasers, photovoltaics, transistors, and quantum computing. Many of the devices described in this issue are impossible to realize using the traditional approach of avoiding dislocation generation; instead, they rely on metamorphic epitaxy to attain high performance.

Multijunction solar cells have proven to be capable of extremely high efficiencies by combining multiple semiconductor materials with bandgaps tuned to the solar spectrum. Reaching the optimum set of semiconductors often requires combining high-quality materials with different lattice constants into a single device, a challenge particularly suited for metamorphic epitaxy. In this article, we describe different approaches to metamorphic multijunction solar cells, including traditional upright metamorphic, state-of-the-art inverted metamorphic, and forward-looking multijunction designs on silicon. We also describe the underlying materials science of graded buffers that enables metamorphic subcells with low dislocation densities. Following nearly two decades of research, recent efforts have demonstrated high-quality lattice-mismatched multijunction solar cells with very little performance loss related to the mismatch, enabling solar-to-electric conversion efficiencies over 45%.

We analyze the optical properties of composite materials that combine nanowire and nanolayer platforms. We revisit effective-medium theory (EMT) description of wire materials with high filling fraction positioned in anisotropic unit cells and present a simple numerical technique to extend Maxwell–Garnett formalism in this limit. We also demonstrate that the resulting EMT can be combined with transfer-matrix technique to adequately describe photonic band gap behavior, previously observed in epitaxially grown semiconductor multilayer nanowires.

Quantum information and computing are at the forefront of computer science, but their implementation relies on significant developments in materials science. In particular, suitable, lattice-matched substrates for two promising approaches—electrostatically defined quantum dots in Si/SiGe heterostructures, and superconducting circuits containing Josephson junctions—do not exist. Instead, these approaches rely on metamorphic substrates. In this article, we focus on the general structure and requirements of SiGe quantum dot heterostructures, the demands they impose on the underlying substrate, and the impact that properties of the metamorphic substrate have on device performance. Superconductor Josephson junction materials are briefly discussed in a similar fashion, and opportunities for future developments in both systems are described.

The epitaxial integration of III–V optoelectronic devices on silicon will be the enabling technology for full-scale deployment of silicon photonics and the key to improving communication systems. Silicon photonics also offer new opportunities for the realization of ultracompact and fully integrated sensing systems operating in the mid-infrared (MIR) regime of the spectrum. In this article, we review recent developments, through several approaches, in the direct metamorphic epitaxial growth of various III–V materials-based lasers on silicon substrates. We show that GaAs-based 1.3-μm III–V quantum dot lasers and GaSb-based MIR quantum-well lasers grown on silicon substrates can operate with low threshold current density and high operating temperature, which hold promise for the future.