A schematic showing various applications of excitons present in a wide band gap oxide semiconductor.
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Excitons, bound states of electrons and holes via Coulomb attraction, are usually short-lived and weakly bound in semiconductors like silicon, primarily because of the strong dielectric screening effects. On the other hand, in the case of wide band semiconductors, exciton binding energies are significantly enhanced, ranging from approximately 60 meV in ZnO to more than 100 meV in β-Ga2O3, which often exceeds the thermal energy at room temperature (kT ∼ 25 meV) (
A schematic showing various applications of excitons present in a wide band gap oxide semiconductor.
ZnO, one of the most extensively studied wide band gap oxide semiconductors, shows three clear excitonic peaks. It has perhaps served as a model system for room-temperature excitonic lasing, and the development of polariton condensates in ZnO microcavities has propelled research into solid-state Bose–Einstein condensation. In contrast, β-Ga2O3, a fourth-generation semiconductor with a relatively broad bandgap of about 4.8 eV, has much larger exciton binding energies and anisotropic excitonic behaviour, much due to its low-symmetric monoclinic crystal structure. These characteristics, along with its high breakdown field, render Ga2O3 a highly promising material for numerous possible applications, including deep-ultraviolet photonics and high-power electronics devices (
Despite their potential characteristics, retaining their excitonic properties can be challenging. This is due to the material issues, such as the presence of grain boundaries, native vacancies, and structural flaws. These material defects can act as nonradiative recombination sites that suppress excitonic emission. Excitons are typically more localised (Frenkel-type) and have a low radiative recombination efficiency in materials such as TiO2. This affects their utility in light-emitting applications but plays a crucial role in photocatalytic activity and charge separation processes (
Characterising excitonic properties in wide band gap materials has also been important for effectively understanding and designing functioning optoelectronic (excitonic) devices. Exciton lifetimes and recombination pathways can be directly measured using time-resolved photoluminescence (TRPL). High-resolution techniques such as cathodoluminescence (CL) spectroscopy and hyperspectral imaging can, on the other hand, image the exciton distribution, particularly in the vicinity of defects or interfaces.
Theoretical modelling has also been key in understanding the physics of the exciton, its origin, decay, and its engineering. State-of-the-art many-body perturbation theory, combined with the Bethe–Salpeter equation (BSE), has enabled quantitative predictions of exciton binding energies, wavefunctions, and optical spectra in excitonic wide band gap materials (
Currently, the focus is mostly on excitonic engineering, which manipulates exciton formation, transport, and recombination through nano-structuring, strain modulation, and a dielectric environment. Quantum confinement of excitons, as provided by nanowires, quantum wells, and 2D oxides, can further improve excitonic binding and charge recombination dynamics. Integrated systems of 2D materials and oxide semiconductors can offer new opportunities for ultrafast charge separation and energy transfers. These systems are key to the fields of photovoltaics and excitonic transistors.
In summary, wide-bandgap oxide semiconductors' excitonic characteristics are essential to a variety of cutting-edge technologies and are no longer only an academic curiosity. The future of light-matter interaction will be shaped by the accurate control of excitonic dynamics as growth techniques advance and theoretical-experimental integration becomes more profound.
PS: Conceptualization, Resources, Supervision, Writing – original draft, Writing – review and editing. NR: Resources, Writing – original draft, Writing – review and editing. AP: Writing – review and editing. RK: Writing – review and editing.
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