Phosphor-free polychromatic emission InGaN light-emitting diode

Abstract

Broadband white light is indispensable for applications involving general illumination and displaying, a task conventionally fulfilled by fluorescent light sources. Light-emitting diodes (LEDs) based on the Group-III nitrides have been taking over that role in recent years, despite the fact that LEDs are inherently monochromatic sources with spectral line-widths in the range of 20 to 50nm. The most adopted industrial solution is to shift part of the light emitted by a blue InGaN chip into longer wavelength, using color converters such as phosphors. To achieve a white light, the Stokes shift is usually above 100nm. The tradeoff is a dramatic energy loss during the color-conversion process. Also, the relatively shorter lifetimes of phosphors compared to that of the LED chips reduces chromatic reliability of such devices. Researches have long been working for a phosphor-free white LED.

It is known that c-plane grown InGaN multi-quantum wells (MQWs) experience high strain due to the lattice mismatch between the wells and the barriers. The strain-induced piezoelectric field narrows the effective energy band gap and lead to a red shift of the emission, known as the quantum-confined Stark effect (QCSE). It has been shown that the QCSE can be partially eliminated by releasing the strain via nanoscale structures such as nanopillars. Studies on the strain-relax effect of nanopillars have shown a blue shift of 70-200 meV, depending on the size of the pillars. 200 meV corresponds to about 50nm in wavelength in the visible band, which may provide another color-converting solution for white lighting if further enhanced.

Nanosphere lithography (NSL) uses nanospheres as the lithography mask. It has been widely studied and applied in recent years since it offers ordered nanopatterns in a simple and low-cost way. The key process of NSL is the formation of the mask from spheres that are initially in colloidal form. This mainly takes use of the self-assembly phenomenon during the evaporation of the dispersant solvent, attributed to the attractive capillary forces. Till now, based on the self-assembly process, nanopatterns with sub-100nm feature size have been achieved.

By taking advantage of NSL, a phosphor-free polychromatic LED structure comprising a combination of high-density microstructured and nanostructured InGaN MQWs is proposed and demonstrated. Polychromatic emission is realized by taking advantage of the low-dimensional-induced strain-relax effect, combining light emitted from strain-relaxed nanotips at wavelengths shorter than the as-grown by as much as 80 nm with longer-wavelength light emitted from the larger unscathed microdisks. The localized emission characteristics have been characterized by spatially-resolved near-field photoluminescence (nf-PL) spectroscopy which allows both the photoluminescence (PL) signal and spectrum from individual nanotips to be optically-resolved and distinguished from emission at the larger-dimensioned regions. The far-field optical properties are evaluated through electroluminescent (EL) spectroscopy, from which an EL spectra containing two peaks of 80nm difference in wavelength can be achieved by fabricating the micro/nano structures on high In content InGaN MQWs that emit at center wavelengths of ~575 nm. The continuous broadband spectrum is characterized by International Commission on Illumination (Commission internationale de l'éclairage, CIE) coordinates of (0.39, 0.47) and a color rendering index (CRI) of 41.