UC Santa Barbara

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 states, such as point defects and dangling bonds, which are largely introduced during plasma-based device patterning. Due to high surface-area-to-volume ratios, these effects become ever more important for microLEDs. In addition to the surface recombination loss problem, there are challenges related to growing high quality, high In-content InGaN for full color RGB displays, most notably for red emitters. The 11% lattice mismatch between InN and GaN leads to reduced crystal quality. Moreover, large piezoelectric fields in the III-nitrides reduce the radiative efficiency and lead to poor emission characteristics. The goals of this Ph.D. project were to (i) eliminate surface-related nonradiative recombination losses and (ii) develop strategies for mitigating strain in InGaN/GaN LEDs for improved efficiency and emission characteristics.

Dielectric passivation and chemical treatments have been used to alleviate surface recombination losses; these approaches, however, have mostly focused on devices with lateral dimensions of 20 µm and larger due to fabrication and optical characterization challenges. A new cleanroom processing scheme and high sensitivity optoelectronic testing system were developed in order to fabricate and test ultrasmall microLEDs in the relevant size ranges. Chemical etching and Al2O3 dielectric passivation were used to minimize nonradiative sidewall defects in InGaN/GaN microLEDs (mesa diameter 2-100 µm). Analysis of the position and shape of EQE curves for all devices suggested size-independent carrier recombination rate coe_cients, suggesting an enhancement in the light extraction e_ciency (LEE). These extraction bene_ts had not been experimentally observed before due to overwhelming surface recombination losses and/or large device dimensions. Ray tracing simulations determined that reducing the mesa diameter increased internal light propagation directionality, leading to fewer total internal reection (TIR) losses, thus confirming the experimental trend of higher EQE for small devices—a trend that is atypical. Given that device performance may actually increase with decreasing mesa diameters when surface losses are minimized, new device applications become viable. One application is sub-micron LEDs as a post-growth, top-down method of strain mitigation, which has been theorized to have many benefits, but in practice has always been limited by surface losses.

When mesa dimensions are reduced to the sub-micron scale, the average strain state in InGaN/GaN structures may be relieved. However, the mesa diameter-strain relationship is not well known. To investigate this relationship, strain in InGaN/GaN multiple-quantum well (MQW) light emitters was relaxed through a colloidal lithography and top-down plasma etching platform. The colloidal lithography was performed using Langmuir-Blodgett dip-coating with silica masks (d = 170, 310, 690, 960 nm) and a Cl2/N2 inductively coupled plasma etch to produce nanorod structures. The InGaN/GaN MQW nanorods were characterized using x-ray diffraction (XRD) reciprocal space mapping (RSM) to quantify the degree of relaxation. A peak relaxation of 32% was achieved in the smallest diameter feature (120 nm after etching). Power dependent photoluminescence (PDPL) showed blue-shifted quantum well emission, which suggests via reduced piezoelectric field.

The cleanroom process previously discussed for microLEDs (mesa diameter = 2-100 µm) was scaled down to smaller dimensions. A combined electron beam lithography and photolithography approach will be employed to fabricate these devices, which is currently under development. In addition to device processing, optical characterization techniques are being conducted to better understand key properties of sub-micron LEDs. For instance, a reduced piezoelectric field due to strain relaxation is expected to alter the carrier dynamics and ultimately the overall device behavior. In order to isolate the carrier dynamics from overall device EQE characteristics, time-resolved and time-integrated photoluminescence (PL) spectroscopy experiments will be conducted. Specifically, the PL experiments will be used to decouple changes to the radiative and nonradiative recombination rates as the mesa dimensions are reduced. The dielectric passivation and chemical treatments previously developed will continue to be used to minimize surface recombination losses, which not only lower the radiative efficiency but also have the potential to dominate the carrier dynamics. By altering both the temperature and laser pump power (i.e. carrier density) in passivated mesa structures, the various recombination pathway rates may be decoupled from one another.