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Light, as one of the most common media of information transfer, is closely related to our daily life. From incandescent to solid-state lighting based on light-emitting diodes (LEDs), artificial light sources keep on improving in terms of energy efficiency. Faced with increasing energy shortages nowadays, LED technology has attracted much interest due to its significantly higher energy transfer efficiency compared with traditional light sources. LEDs, being semiconductor devices based on the fundamental p-n junction, produce light with the energy released during the radiative recombination processes of its carriers. By adjusting the band gap of the semiconductor through a variety of means, different colours of light can be emitted. Though LEDs have higher energy utilization efficiency compared with traditional light sources, a single LED chip emits only monochromatic (single-colour) light due to the fixed band gap value of the respective semiconductor. This is far from meeting the requirements of the two major artificial lighting applications: lighting and displaying, which need polychromatic light. The most commonly adopted solution today is the use of phosphor materials to convert part of the emitted light into light of other colours at longer wavelengths. The trade-off of this approach is unavoidable substantial energy losses during the colour conversion process, which in turn result in heat generation that further reduces the internal quantum efficiencies of the LED. Also, the phosphor has a shorter lifetime than the LED chip itself, and degradation of the phosphor causes the colour of the emitted light to gradually change throughout its lifespan.

Schematic diagrams showing the essential steps for the fabrication of polychromatic LED structures

It has been found that changes to the strain of a semiconductor material will modify the energy band structure of the semiconductor, which in turn will affect the colour of the emitted light. The strain can be adjusted by non-uniform nano-structuring of the quantum wells embedded in a LED chip, making polychromatic emission from a single chip possible.

In our study, by using the nanosphere lithography (NSL) technique, nano-rods of different diameters were fabricated on the surface of an InGaN LED chip, which forms the heart of a typical LED light bulb. The nano-rods penetrated into the quantum wells and resulted in non-uniformly distributed strain. This gave rise to localized emission properties across the emission area which were studied at a nanoscale level using scanning near-field optical microscopy (SNOM). Photoluminescence was used to study the far-field emission of the chip. The microphotographs taken show the dual-colour emitting nature of the nano-structured chips, including green and blue, the combination that constitutes polychromatic light. Further exploration of this technique to develop whitelight-emitting chips is being investigated. This technology may prove to be an important milestone towards achieving single-chip non-phosphor polychromatic LED lighting with high efficiency and reliability.

The research team comprises Mr Cong FENG, Dr Jian-an HUANG and Dr Hoi Wai CHOI (from left to right)

Dr Hoi Wai CHOI
Department of Electrical and
Electronic Engineering

The University of Hong Kong