Dual-colour device expands possibilities for LEDs

A semiconductor device capable of emitting two distinct colours has been created by a group of researchers in the US, potentially opening up the possibility of using LEDs universally for cheap and efficient lighting.

The proof-of-concept device, which was presented on 3 May in IOP Publishing’s journal Semiconductor Science and Technology, takes advantage of the latest nano-scale materials and processes to emit green and red light separated by a wavelength of 97nm – a significantly larger bandwidth than a traditional semiconductor.

Furthermore, the device is much more energy efficient than traditional LEDs as the colours are emitted as lasers, meaning they emit a very sharp and specific spectral line - narrower than a fraction of a nanometre - compared to LEDs which emit colours in a broad bandwidth.   

One of the main properties of semiconductors is that they emit light in a certain wavelength range, which has resulted in their widespread use in LEDs. The wavelength range in which a given semiconductor can emit light - also known as its bandwidth - is typically limited in the range of just tens of nanometres. For many applications such as lighting and illumination, the wavelength range needs to be over the entire visible spectrum and thus have a bandwidth of 300nm.   

Single semiconductor devices cannot emit across the entire visible spectrum and therefore need to be ‘put’ together to form a collection that can cover the entire range. This is very expensive and is, to a large extent, the reason why semiconductor LEDs are not yet used universally for lighting.

In the study the researchers, from Arizona State University, used a process known as chemical vapour deposition to create a 41µm nanosheet made from cadmium sulphide and cadmium selenide powders, using silicon as a substrate.

Lead author of the study, Cun-Zheng Ning, said: 'Semiconductors are traditionally "grown" together layer-by-layer, on an atom-scale, using the so-called epitaxial growth of crystals. Since different semiconductor crystals typically have different lattice constants, layer-by-layer growth of different semiconductors will cause defects, stress, and ultimately bad crystals, killing light emission properties.'

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