||Scott Newman, Jonathan Wright, Chad Gallinat, Ryan Enck, Anand V Sampath, Hongen Shen, Meredith L Reed, Michael L Wraback
Currently, there is a great interest in developing wurtzite, InGaN-based green LEDs and LDs for display and general lighting applications. However, these devices suffer from lower external quantum efficiencies (EQE) and efficiency droop at lower current densities than blue InGaN-based LEDs , possibly due to the polarization fields related to the larger positive polarization charge at the p-(Al)GaN/n-InGaN interface in higher Indium composition Ga-polar InGaN/GaN heterostructures, which leads to larger electron leakage and poorer hole injection, as well as degradation of higher indium composition InGaN layers and InGaN/GaN quantum well intermixing associated with the higher temperatures required for the growth of the wider bandgap AlGaN and GaN capping layers in conventional p-up Ga polar devices. These problems can be alleviated by reversing the polarization fields and creating a negative polarization charge at the p-(Al)GaN/n-InGaN interface by growing a p-side-down LED on the Ga-polar plane. Furthermore, our simulations show that one can design a Ga-polar, p-side-down GaN:Mg/InGaN:Si single heterostructure (SH) LED that will have a two-dimensional hole gas (2DHG) at the heterojunction. Together with a large conduction band offset, this structure is expected to effectively confine electrons and holes near the heterojunction under operation up to large current densities.
We previously used Hydride Vapor Phase Epitaxy (HVPE) to produce p-side-down SH LEDs emitting at 480 nm with only 10% EQE droop at 500 A/cm2.  In this work, we used Molecular Beam Epitaxy to produce SH LEDs on sapphire and HVPE GaN templates from TDI (Oxford Instruments) with longer peak emission wavelengths ranging from 530 – 580 nm. At 100 mA, 0.25 mm2 LEDs with peak emission wavelengths of 530, 558, and 579 nm had voltages of 13.3, 8.3, and 7.2 V, respectively. These high voltages result primarily from a fabrication process that included a standard RIE dry etch to make contacts to the bottom GaN:Mg layer, for which the p-contacts were always Schottky. This data is consistent with reports in the literature that dry etching p-GaN produces n-type surface defects [3-5]. To avoid making contacts to etched p-GaN, a SiO2-masked regrowth process is under development. Pulsed testing with a 1% duty cycle of a 0.10 mm2 LED emitting at 550 nm showed minimal efficiency droop up to 90 mA (nominally 90 A/cm2), the maximum current tested. Most of the emission from this device came from the perimeter of the n-InGaN mesa due to current crowding in the p-GaN layer, leading to a much larger current density. These results suggest that development of n-InGaN/p-GaN p-side down Ga-polar LEDs may be a promising approach for extending the emission to longer wavelengths with reduced droop and greater integrity of the InGaN active region without the necessity of a high termperature-grown (Al)GaN capping layer.