Fig. 1. (a) Schematic of a UVC LED with a tunnel heterojunction in reverse bias configuration for hole injection. (b) Equilibrium band structure diagram (0 V bias voltage) of a TJ-LED with 8 nm GaN:Si interlayer.

Fig. 2. (a) Normalized emission spectra of fully transparent MOVPE-grown TJ-LEDs with GaN interlayer (heterojunction) and without interlayer (homojunction) emitting at 268 nm. The inset shows microscope images of the heterojunction TJ-LED in (b) the “off” state and (c) the “on” state of the device.

Fig. 3. Microscope images taken by a UV-sensitive camera for TJ-LEDs at a DC current of 5 mA. (a) A square mesa geometry with a top n-contact at the perimeter and (b) a finger-shaped mesa geometry with central stripe top n-contact are measured from the top. The electric contact is provided by the small metal electrodes only. The entire mesa area of the top AlGaN:Si semiconductor is homogeneously emitting with a slight enhancement under the stripe contact.

Fig. 4. Experimental LIV curves for tunnel heterojunction LEDs with emitting area of 0.15  mm2 featuring 1, 4, and 8 nm thick interlayers as a function of the injected current under cw operation. The black curves represent the tunnel homojunction LED.

Fig. 5. Operation voltage and external quantum efficiency in bottom emission configuration of UVC LEDs with tunnel junction measured at a cw current of 5 mA as a function of the interlayer thickness.