Epitaxial growth of excitonic single crystals and heterostructures: Oxides and nitrides

Excitons in a semiconductor are Coulomb interaction-bound pairs of excited electrons in the conduction band and holes in the valence band, which can either be free bosonic particles with well-defined integer spins, called the free excitons or bound at defect/impurity sites, called bound excitons. Theory predicts several fascinating collective phenomena emanating from excitons, such as Bose–Einstein condensation, high-temperature superconductivity, and strongly correlated excitonic insulator states. There are also proposals to utilize excitons for transferring and processing information. This new paradigm of electronics is expected to be more energy efficient and compatible with optical communication. However, exciton binding energy is an important factor to be considered in realizing the excitons at room temperature (RT). In this respect, certain nitride and oxide semiconductors, such as GaN, InN, and AlN and ZnO, TiO₂, and Cu₂O, are especially interesting as the excitonic binding energy in these materials is sufficiently high, which facilitates their survival above RT. By harnessing and controlling the excitonic behavior, researchers can engineer materials with specific functionalities, leading to innovations in materials science and device fabrication. Here, we review recent developments toward the understanding of excitons in certain nitride and oxide semiconductors as well as their heterostructures and nanostructures.

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