Realizing a SnO₂-based ultraviolet light-emitting diode via breaking the dipole-forbidden rule

Although many oxide semiconductors possess wide bandgaps in the ultraviolet (UV) regime, currently the majority of them cannot efficiently emit UV light because the band-edge optical transition is forbidden in a perfect lattice as a result of the symmetry of the band-edge states. This quantum mechanical rule severely constrains the optical applications of wide-bandgap oxides, which is also the reason why so few oxides enjoy the success of ZnO. Here, using SnO ₂ as an example, we demonstrate both theoretically and experimentally that UV photoluminescence and electroluminescence can be recovered and enhanced in wide-bandgap oxide thin films with 'forbidden' energy gaps by engineering their nanocrystalline structures. In our experiments, the tailored low-temperature annealing process results in a hybrid structure containing SnO₂ nanocrystals in an amorphous matrix, and UV emission is observed in such hybrid SnO₂ thin films, indicating that the quantum mechanical dipole-forbidden rule has been effectively overcome. Using this approach, we demonstrate the first prototypical electrically pumped UV-lightemitting diode based on nanostructured SnO₂ thin films.