We demonstrated low‐resistive GaInN‐based tunnel junctions with high In mole fractions (over 0.35) grown by metalorganic vapor phase epitaxy (MOVPE). A 2 nm heavily Mg‐doped Ga0.6In0.4N/15 nm heavily Si‐doped GaN tunnel junction was grown on a standard‐sized (310 × 310 µm2) LED. The tunnel junction showed 0.06 V drop at 20 mA (26 A/cm2), corresponding to 4 × 10−3 Ωcm2 as the resistivity. This result indicates that MOVPE‐grown tunnel junctions showed an almost comparable resistivity as conventional p‐contact. We also found that the tunnel junction resistivity is decreased to 4 × 10−4 Ω cm2 with an increase of current density up to 5 kA/cm2. A tunneling through midgap states was suggested from forward‐biased I–V characteristics of transmission line model test structures with the tunnel junctions.

We fabricated nitride-based micro-LED arrays with a small number of fabrication process steps by using a combination of tunnel junctions and patterned n-GaN cathode lines. A use of the combination enables us to skip a couple of process steps required in standard LED array fabrication. A 10 × 10 channel matrix-addressable LED array with a 10 × 16 µm2 emission regions and a 25 µm pitch lengths showed uniform operating voltages and light output intensities, indicating good yield due to the small number of process steps used. In addition, microdisplay of over 1000 (14 × 72) channels was successfully demonstrated. The new array structure with the tunnel junction and n-GaN cathode line provides a high density and a high yield simultaneously.

We have investigated two approaches for an alternative hole injection with a tunnel junction targeting deep UV-LEDs. One was an AlGaN-based tunnel junction. We fabricated the AlGaN-based tunnel junctions with various AlN mole fractions (0～0.2) grown on conventional blue-LEDs by MOVPE. A 7.5 nm heavily Mg-doped GaN/15 nm heavily Si-doped Al0.2Ga0.8N tunnel junction showed a large voltage drop, 5.31 V at 20 mA, under reverse bias. The other was a GaInN-based tunnel junction. We prepared Ga0.6In0.4N tunnel junctions with various thicknesses and Si doping levels grown on the blue LEDs by MOVPE. A 2 nm heavily Mg-doped Ga0.6In0.4N/3 nm heavily Si-doped GaN tunnel junction showed only 0.12 V drop at 20mA under reverse bias. Since an absorption of the thin GaInN tunnel junction was estimated to be less than 10 %, such a tunnel junction with small bandgap and thin layer thickness is a practical approach to obtain a low resistive and low absorptive hole injection in the deep UV-LEDs.