Enhanced injection efficiency and light output in bottom tunnel-junction light-emitting diodes using UID GaN spacers

TL;DR

This study demonstrates that bottom tunnel-junction (TJ) GaN-based LEDs with UID GaN spacers exhibit significantly improved injection efficiency and light output, attributed to optimized carrier transport and polarization effects, promising high-performance laser diodes.

Contribution

The paper reveals the mechanisms behind enhanced injection efficiency in bottom-TJ GaN LEDs using optimized heterojunctions and UID GaN spacers, supported by detailed energy band analysis.

Findings

Higher injection efficiency and output power in bottom-TJ devices across 5 orders of current density.

State-of-the-art differential resistance below 7x10^-4 Ω·cm^2 with UID GaN spacers.

Improved hole injection and electron blocking due to polarization effects in bottom-TJ structures.

Abstract

Recently, the use of bottom-TJ geometry in LEDs, which achieves N-polar-like alignment of polarization fields in conventional metal-polar orientations, has enabled enhancements in LED performance due to improved injection efficiency. Here, we elucidate the root causes behind the enhanced injection efficiency by employing mature laser diode structures with optimized heterojunction GaN/In$_{0.17}$Ga$_{0.83}$N/GaN TJs and UID GaN spacers to separate the optical mode from the heavily doped absorbing p-cladding regions. In such laser structures, polarization offsets at the electron blocking layer, spacer, and quantum barrier interfaces play discernable roles in carrier transport. By comparing a top-TJ structure to a bottom-TJ structure, and correlating features in the electroluminescence, capacitance-voltage, and current-voltage characteristics to unique signatures of the N- and Ga-polar polarization heterointerfaces in energy band diagram simulations, we identify that improved hole injection at low currents, and improved electron blocking at high currents, leads to higher injection efficiency and higher output power for the bottom-TJ device throughout 5 orders of current density (0.015 - 1000 A/cm$^2$). Moreover, even with the addition of a UID GaN spacer, differential resistances are state-of-the-art, below 7x10-4 $\Omega$cm$^2$. These results highlight the virtues of the bottom-TJ geometry for use in high-efficiency laser diodes.