Tunnel junction (TJ) LEDs are limited by a high voltage penalty introduced by the TJ, caused by wide depletion widths, high bandgaps, and high effective hole mass. InGaN-based interlayers (ILs) in Ga-polar c-plane devices are used to improve tunneling efficiency by reducing the tunneling bandgap, but they experience a tradeoff: better electrical performance at the expense of greatly increased optical absorption, especially detrimental for VCSEL and EELD cavities. Avoiding these losses requires extraordinarily precise cavity positioning, which significantly increases processing complexity. Thus, it is an important endeavor to reduce the voltage penalty introduced by TJs without inviting greater optical losses or more stringent processing requirements that hinder large-scale production.
Researchers at the University of California, Santa Barbara have improved on the conventional TJ structure and interlayer design to produce III-N devices that reach new heights in electrical efficiency and optical transparency. The novel TJ structure employs 1D narrow bandgap materials formed on or above the p-type layer to lower the tunneling barrier. An Mg-doped AlGaN interlayer (IL) with the key advantage of tensile strain greatly reduces the effective hole mass on the p-side of the tunneling region, which improves the tunneling probability without introducing excess loss that is typically observed with interlayers. These new design features are suitable for TJ devices of any device architecture, including emitters in the ultraviolet (UV) range and laser diodes such as VCSELs and EELDs. This technology can become the seminal invention that ushers in new generations of low-loss TJ LEDs, VCSELs, and EELDs.