Optical polarization of UV-A and UV-B (In)(Al)GaN multiple quantum well light-emitting diodes

The optical polarization of the in-plane emission of c-plane oriented (In)(Al)GaN multiple quantum well light emitting diodes in the spectral range from 288 nm to 386 nm has been investigated by electroluminescence measurements. The intensity of transverse-electric polarized light relative to the transverse-magnetic polarized light decreases with decreasing emission wavelength. This effect is attributed to the different electronic band structures in the active region of the light emitting diodes. A changing aluminum and indium mole fraction in the (In)(Al)GaN quantum wells results in a rearrangement of the valence bands at the Γ-point of the Brillouin zone. For shorter wavelengths the crystal-field splitoff hole band moves closer to the conduction band relative to the heavy and light hole bands and as a consequence the transverse-magnetic polarized emission increases. Moreover, the in-plane polarization is shown to depend on the injection current. The correlation between the in-plane polarization and the injection current has been found to be different for light emitting diodes with InGaN and (In)AlGaN multiple quantum wells. The results highlight that polarization effects need to be considered when optimizing the light extraction from ultraviolet light emitting diodes in the (In)AlGaN materials system.