Question

(b) P-type doping in deep ultraviolet AlGaN based LEDs suffer from low hole mobility.

(i) Sketch a design of a tunnel junction for a DUV LED operating at 280 nm.

Explain how the tunnel junction operates in a DUV LED.

Answer 1

The photoelectric properties and physical mechanism of AlGaN-based deep ultraviolet light emitting diodes (DUV-LEDs) with the superlattice p-type doping layer (PSL) are studied numerically and compared with the Al-composition (50%) conventional p-type layer AlGaN-based DUV-LEDs. The extraordinary design of DUV-LEDs with varied barrier PSL have been investigated by Advance Physical Model of Semiconductor Devices (APSYS) software. Through comparing the internal quantum efficiency (IQE), light output power, electroluminescence intensity (EL), distributions of carrier concentration and the energy band diagrams. As a result of hole injection augment and electronic leakage reduction, the property of AlGaN-based DUV-LED with the PSL has enhanced significantly. Moreover, the 55%- Al-composition of superlattice barrier p-type doping layer greatly reduces the effective potential height for holes in valence band, which is beneficial for hole injection from PSL. The new structure improves the properties of DUV-LED and shows remarkable output performance.

Tunnel Junction Operation in a DUV LED:

TUV LEDs are fabricated using plasma a assisted molecular beam epitaxy (MBE). Non-equilibrium hole injection into the active region of the UV LED structures are demonstrated through light emissions ranging from UV-A to UV-C LEDs. Low tunnel junction resistances below 2 mΩ have been achieved for AlGaN tunnel junctions with Al composition as high as 75%. The optimization of the p-type layers led to the demonstration of high on-wafer efficiencies, which are 3.37% (external quantum efficiency) and 1.62% (wall-plug efficiency) for UV-A LEDs, and 2.8% (EQE) and 1.1% (WPE) for UV-B LEDs. The device performance is comparable to state-of-the-art UV LEDs, demonstrating the great potential in the applications for high power and high efficiency UV emitters.

A novel metal/semiconductor tunnel junction structure employing reflective Al-based metal stack is further demonstrated, as an approach that can be better adopted in other material production technologies, especially metal-organic chemical vapour
deposition (MOCVD). The effects of Ni and Al contacts on both the electrical and optical performance of the tunnel-injected UV LEDs are compared. High external quantum efficiency and wall-plug efficiency of 2.65% and 1.55% are obtained at the emission wavelength of 325 nm. While further studies in validating their applications toward shorter emission wavelength are necessary, the metal/semiconductor tunnel junction structure provides a novel solution to the p-type contact challenges faced by conventional UV LEDs.