Title: Mitigating surface losses and strain in InGaN microLEDS
Advisor: Mike Gordon
III-nitride microLEDs have gained much attention as a replacement for organic-LED (OLED) and liquid crystal (LCD)-based displays due to the former’s tunable bandgap, high defect tolerance, long lifetimes and superior efficiency. High pixel density, next-generation displays also demand efficient and low cost red-green-blue (RGB) pixels with lateral dimensions below 5 μm, for which OLED and LCD platforms become intractable. Unfortunately, nitride-based LEDs become inefficient as device dimensions shrink due to nonradiative surface recombination losses, which are largely introduced during plasma-based device patterning. Moreover, producing efficient red nitride-based LEDs with suitable emission characteristics for displays is notoriously difficult owing to the 11% lattice mismatch between InN and GaN. The goals of this thesis are to (i) eliminate nonradiative surface recombination losses and (ii) develop strategies for mitigating strain-related issues in InGaN/GaN LEDs.
This talk will first highlight our recent work on fabricating microLEDs down to 2 μm diameters. Chemical etching and dielectric passivation were used to minimize nonradiative sidewall defects in InGaN/GaN microLEDs. The measured external quantum efficiency (EQE) increased as the diameter was decreased—behavior that is atypical of microLED EQEs. Analysis of the EQE versus current density behavior determined enhanced backside light extraction efficiency (LEE), which was confirmed by ray tracing analysis.
Next, the microLED cleanroom process is extended to the nanoscale regime to assess the limitations of the observed improved EQE behavior. Literature reports on nanoscale LEDs have also shown other potentially useful and interesting properties, but there lacks a systematic understanding of the diameter-related dependence and quantification of overall device performance. The practicality of such devices has been limited by surface recombination losses as well as processing and characterization challenges—all of which have been greatly advanced over the course of my Ph.D. X-ray diffraction (XRD) determined the presence of strain relaxation in sub-micron mesa structures and, furthermore, that relaxation increased as the mesa diameter was decreased. Photoluminescence (PL) techniques are also being developed to better understand how strain relaxation affects the carrier dynamics and emission characteristics and how these factors correlate to the overall device properties.