Broadband emission from an ensemble of nano-pillars with multiple diameters

Abstract

Generating white light from monochromatic light sources is commonly achieved via one of two common methods: exciting fluorescence phosphors from a shorter wavelength LED, or mixing light from three or more LED chips, commonly known as RGB LEDs. Phosphor efficiency degrade over time, and have lifetimes shorter than the chip itself. RGB LEDs require turning on three or more p-n junctions and suffer from color mixing issues. We introduce a promising approach towards achieving phosphor-free white light emission, tapping on strain engineering and nanoscale processing. The proposed approach makes use of a long wavelength chip, which is invariably strained due to the high Indium content. By relaxing the built-in strain in a controllable manner, through the formation of dimension and site-controlled nano-pillars, the emission wavelength of individual pillars of varying sizes will blue-shift towards short wavelengths. The extent of blue-shift (strain relaxation) depends on a number of factors including dimension and lateral ion penetration. Nano-pillars of a continuum of dimensions are patterned by nanosphere lithography, making use of a nanosphere colloid containing spheres with a wide range of diameters. The resultant structure contains an ensemble of nano-pillars each emitting a slighting different wavelength according to its dimension, producing a continuous broadband spectrum with FWHM of 72.23nm. With the right mix of nano-pillar dimensions, different shades of white light can be generated from a single array, representing a viable single-chip phosphor-free white light generating solution. Most importantly, the strain-relaxed nanostructures offer both high internal quantum efficiencies and light extraction efficiencies.