Quantitative Analysis for Efficiency Studies in III-Nitride Light Emitting Diodes (LEDs) using Electron Emission Spectroscopy (EES)
As the average luminous efficacy of light emitting diodes (LEDs) has increased over the years, the energy performance of LEDs has surpassed preceding lighting technologies such as incandescent and fluorescent lighting. One way to reduce the cost per lumen.hour would be to maximize the wall-plug efficiency (WPE), where the internal quantum efficiency (IQE), η_rad, plays a major role. η_rad represents the number of photons created per injected electron-hole (e-h) pair and is known to peak at a low current density ~1-10 A cm-2. This reduction in efficiency at higher current densities has been referred to as efficiency droop, or simply droop, posing a roadblock to full penetration of the lighting market and for applications in display technologies.An effective technique to study efficiency droop is electron emission spectroscopy (EES). EES is capable of measuring and detecting hot electrons generated in the active region of an LED, thus allowing direct investigation of the recombination, and scattering processes in the device. Electrons were detected to be emitted from a side valley at ~0.9 eV above the Γ-valley of GaN, which can only be generated by Auger recombination. The intensity of these hot electrons was showed to increase with increasing droop, thereby providing direct evidence of Auger recombination as the cause of droop. The work presented here concerns taking the experimental technique towards a full quantitative approach. Using the light output power of an LED as a proxy for active region carrier density n, we were able to directly obtain the power law dependence of the various valley peaks on n, distinguishing between 2-body trap-assisted Auger recombination (TAAR) and 3-body band-to-band Auger combination. Efficiency and thermal droop studies in c-plane blue and green III-nitride LEDs were made to quantify the prevalence of TAAR and investigate the sources of the defects. We report on detection of a new high energy upper valley at ~1.7 eV above Γ-valley, which was predominantly generated by TAAR. Its detection was contingent on a low number of pre-well InGaN/GaN superlattice (SL) period, higher [In] quantum wells, and presence of AlGaN in the active region – indicating the defect reduction capabilities of SLs and presence of a deep trap at the AlGaN/(In)GaN interfaces. Through systematic thinning of the p-region by growth or by ex-situ etching, we present work leading towards obtaining the absolute TAAR and 3-body Auger electron currents for full LED recombination physics quantification.