Broadband Nanorod+Microdisk Light-emitting Diodes

Abstract

The spectral line-width of an InGaN LED chip is typically in the range of 20 to 50 nm, which is far from sufficient for general illumination applications. The most-commonly adopted solution for spectral broadening is the introduction of color-conversion materials such as phosphors. However, these materials generally have shorter lifetimes than the LED chips themselves. Also, stokes shift energy losses during the color conversion process reduces the efficiencies of such devices. Researchers have long been striving for a non-phosphor approach towards polychromatic emission. Exemplary works include LEDs containing sets of quantum wells emitting at different wavelengths, quantum dots of varied dimensions as well as nano-pyramids emitting different wavelengths from different facets. It is known that the Quantum-confined Stark effect (QSCE) exists in the InGaN/GaN multiple quantum wells (MQW) due to the piezoelectric field induced by strain. It has been shown that the strain can be partially relaxed by nano-structuring, resulting in a blue-shift of the spectrum. Our proposed LED consists of a densely-packed array of microdisks and nanorods with a range of dimensions, fabricated on an InGaN/GaN LED structure of high emission wavelength of ~570 nm (high Indium content in the wells). The larger non-strain-relaxed microdisks will emit at longer wavelengths, while the smaller strain-relaxed nanorods emit at shorter wavelengths. A mixture of such microdisks and nanorods on the same wafer will therefore produce polychromatic emission. Based on this approach, a single-chip non-phosphor polychromatic LED structure with spectral FWHM of ~94nm is demonstrated.