III-Nitride Micro and Nano Structures for Solid State Lightning

Abstract

Visible light emitting diodes (LEDs) are widely used in daily consumer electronics systems, such as general lighting, displays, communication, sensing, and also biomedical applications. To mitigate the ever increasing technology demand, there are tremendous on-going efforts in improving material properties and micro-fabrication techniques. In general, visible LEDs are environmentally friendly, robust and reliable light emitters with small device footprint, and are capable of delivering high luminous efficacy.

Typically, LEDs rely on group-III-nitride materials to generate visible light. One of the techniques to generate white light is to coat blue LEDs with yellow phosphor, or ultraviolet (UV) LEDs with red-green-blue (RGB) phosphor. Other scheme relies on combination of RGB LEDs, where high brightness green and blue LEDs are generally grown on robust sapphire substrate. But the current challenges in high threading dislocation density of III-Nitride materials on sapphire or hetero-substrate, phosphor degradation, and bulk-LED mechanical design constraints imposed by the supporting substrate wafer motivate further scientific investigations into strain-engineering, novel reliable phosphor-semiconductor, color-tuning techniques, and transferrable III-nitride vertical LEDs.

The current research presents a significant step towards the utilization of annealed porous GaN as a template for subsequent growth of fully relaxed GaN-based epitaxy materials. In our study, we observed significant compressive strain relaxation of 0.41 ± 0.04 GPa in annealed porous GaN fabricated using UV-assisted electroless etching. Moreover the use of GaN nanoparticles with large wavelength tunability and 10 µm InGaN microstructures with different indium composition ushers a new way of making reliable phosphor for white light generation. We also investigate the epitaxial lift-off of InGaN LED structures by selectively etching unintentionally doped GaN sacrificial buffer layer. High GaN/InGaN etching selectivity of 100/1 and with GaN lateral etch-rate of 5 µm/min was achieved using the photo assisted electroless etching process. The kinetics of electron hole transfer in the diffusion limited etching reaction is discussed. Transferred LEDs onto flexible and glass substrates showed ~10 times higher optical power output, 2 times lower series resistance and a lower turn-on voltage than bulk LEDs fabricated from the same wafer. This innovative technique offers a low cost optoelectronic platform for the formation of pixelated red, green and blue (RGB) display on any flexible, transparent or rigid substrates. The technique will also enable new platform for sensing, wearable electronics/optoelectronics and biomedical applications.