Metamorphic epitaxial materials
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Introduction The desire to have complete deterministic control over composition and structure is a guiding passion for many materials engineers and scientists. Those who grow epitaxial materials have mastered the technique to such a degree that by following a general set of rules, certain combinations of atoms can be combined with a high level of confidence into a predetermined lattice of a stratified material with very few dislocations. These relatively defect-free, single-crystal epitaxial heterostructures are made into a variety of devices found in cell phones, computers, and light sources, among others, that undoubtedly play a central role in modern-day life. The performance of all electronic and optoelectronic systems, circuits, and devices, whether for computation, communication, sensing, or energy conversion, are limited by the properties of the constituent materials. For some applications, bending the rules of epitaxial growth is necessary to attain the next level of performance. One example is the efficiency improvements in metamorphic multijunction solar cells, where deviating from a single lattice constant allows the device designer to choose absorber materials with a wider range of bandgap values to more optimally divide the solar spectrum. The term “metamorphic epitaxial material” describes a single-crystal thin film on a single-crystal substrate, where the film and substrate have a significant structural difference. This difference is often the relative lattice constants of the film and
substrate, but can also include film/substrate materials with different unit cells. The term metamorphic is similar to the geological and biological designation indicating a “change in form,” because there is indeed a change in the material structure due to the elastic strain relaxation and plastic deformation processes that occur during the growth process. By transcending the conventional paradigm of lattice-matched crystal growth in epitaxy, an emerging class of engineered materials is enabling devices to reach new performance levels. As illustrated schematically in Figure 1, lasers, transistors, solar cells, and even quantum computer components of the future are a few examples of these emerging metamorphic materials described in this issue of MRS Bulletin.1–4
Epitaxial growth paradigms Epitaxial growth paradigms can be generally grouped under three classes: homomorphic, pseudomorphic, and metamorphic.5 These growth paradigms are briefly reviewed, as all three are widely employed in modern heterostructures. Homomorphic growth, or homoepitaxy, is the first category, both because of its historic significance in the development of epitaxial crystal growth and surface science and because of its relative simplicity. Homomorphic epitaxy is the growth of a single-crystal thin film on a single-crystal substrate that is composed of the same material. The single-crystal substrate acts as the seed crystal that enables the growth of a coherent
Christopher J.K. Richardson, Laboratory for Physical Sciences, University of Maryland, USA; richa
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