Ultra-wide-bandgap AlGaN homojunction tunnel diodes with negative differential resistance

Abstract

The power efficiencies of state-of-the-art AlxGa1-xN deep-ultraviolet (UV) emitters operating in the <300 nm wavelength region are currently limited to a few percent in part due to limitations in the series and contact resistance which result in excessive drive voltages. AlxGa1-xN tunnel contacts and tunnel junctions in deep-UV devices are a promising route toward increasing these efficiencies by improving the contact resistances, hole injection, and reducing optical absorption by removing undesirable p-GaN contact layers. However, due to doping inefficiencies, standalone tunnel diodes have not been realized in the form of homojunction AlxGa1-xN. In this work, AlxGa1-xN (0.19 ≤ x ≤ 0.58) homojunction tunnel diodes are fabricated with high reverse bias current densities, and one device with x = 0.19 demonstrates a negative differential resistance at ∼2.4 V. AlxGa1-xN p++/n++/n tunnel diodes are compared to reference p++/i/n diodes to provide clarity about the role of tunneling conduction vs leakage conduction. Transmission electron microscopy verifies that heavy doping does not result in visible defects such as Mg precipitates and allows for subsequent epitaxy, critical for buried tunnel junction structures. Increasing the bandgap energy of AlxGa1-xN for higher Al content tunnel junctions decreases the tunnel current, but still allows sufficient conduction necessary for future improvements in deep UV emitter efficiencies.

Pulse and Compliance Current-Dependent Negative Differential Resistance in BaTiO3/Nb:SrTiO3 Heterojunctions

“…For example, the intensity and location of NDR may be different for different samples, and the NDR may shift or even disappear during the cyclic voltage scanning. Various mechanisms, such as charge trapping and detrapping, ,, filament formation and rupture, , redox reaction, , ferroelectric polarization switching, , charge carrier, and tunneling, have been proposed to account for the NDR effect. Ferroelectric polarization switching-induced NDR, as an innovative and alternative mechanism, has a great advantage for highly reliable NDR devices.…”

Temperature-Dependent Carrier Recombination and Efficiency Droop of AlGaN Deep Ultraviolet Light-Emitting Diodes

“…The mechanisms involved in efficiency droop in AlGaN-based DUV LEDs at high currents and/or high temperatures are still not clear. Experimental results have revealed that multiple sources, e.g., extend dislocations, quantum confined Stark effect (QCSE), carrier delocalization, Auger recombination, and carrier leakage, lead to the lower EQE and severer efficiency droop in AlGaN-based DUV LEDs than those in InGaN-based visible LEDs [10], [12]- [14]. In AlGaN-based DUV LEDs, the aluminum-rich EBL tends to prevent holes from entering MQWs, leading to a poor hole-injection efficiency [12], whereas the high-energy carriers are inclined to drift out of high-defect-density MQWs due to the insufficient restriction [15].…”

A 310 nm Optically Pumped AlGaN Vertical-Cavity Surface-Emitting Laser

“…To be suitable for VCSELs, a tunnel junction needs to have low resistivity, be stable at high current densities, and have very low optical absorption, all of which is especially challenging to achieve for UV devices. Nevertheless, recent advances in AlGaN-based tunnel junctions − demonstrate their potential as a viable solution for electrical injection of UV VCSELs.…”