We demonstrate the
first electrically injected AlGaN-based ultraviolet-B
resonant-cavity light-emitting diode (RCLED). The devices feature
dielectric SiO2/HfO2 distributed Bragg reflectors
enabled by tunnel junctions (TJs) for lateral current spreading. A
highly doped n++-AlGaN/n++-GaN/p++-AlGaN TJ and a top n-AlGaN current spreading layer are used as transparent
contacts, resulting in a good current spreading up to an active region
mesa diameter of 120 μm. To access the N-face side of the device,
the substrate is removed by electrochemically etching a sacrificial
n-AlGaN layer, leading to a smooth underetched surface without evident
parasitic etching in the n- and n++-doped layers of the
device. The RCLEDs show a narrow emission spectrum with a full width
at half-maximum (FWHM) of 4.3 nm compared to 9.4 nm for an ordinary
LED and a more directional emission pattern with an angular FWHM of
52° for the resonance at 310 nm in comparison to ∼126°
for an LED. Additionally, the RCLEDs show a much more stable emission
spectrum with temperature with a red-shift of the electroluminescence
peak of about ∼18 pm/K and a negligible change of the FWHM
compared to LEDs, which shift ∼30 pm/K and show spectrum broadening
with temperature. The demonstration of those devices, where a highly
reflective mirror is spatially separated from an ohmic metal contact,
opens up a new design space to potentially increase the poor light
extraction efficiency in UV LEDs and is an important step toward electrically
injected UV vertical-cavity surface-emitting lasers.