08 May 2013
Such technology could be used for solid state lighting, full color displays, and sources for fluorescence bio and chemical detection.
The proof-of-concept device, which was first published 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.
The device is more energy efficient than traditional LEDs because the two wavelengths are emitted as lasers, with sharp and specific spectral lines that are narrower than a fraction of a nanometer. This may be compared to LEDs that emit colours across a broader 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 bandwidth of a given semiconductor is typically limited to a few tens of nanometers. But 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 (from blue to red).
Single semiconductor devices cannot emit across the entire visible spectrum so they need to be combined to cover the entire range. This is expensive and is, to a large extent, the reason why semiconductor LEDs are not yet used universally for lighting. In this study, the researchers, from Arizona State University, used a process known as chemical vapour deposition to create a 41µm-long nanosheet made from CdS and CdSe powders on silicon substrates.
Lead author of the study, Professor Cun-Zheng Ning, commented, “Semiconductors are traditionally grown together layer by layer, on an atomic scale, by 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 poor crystals, killing light emission properties.” This is why current LEDs cannot have different semiconductors within them to generate red, green and blue colours for lighting.
However, recent developments in nanotechnology mean that structures such as nanowires, nanobelts and nanosheets can be grown to tolerate much larger mismatches of lattice structures, and thus allow different semiconductors to be grown together without too many defects.
Ning added, “Multicolor light emission from a single nanowire or nanobelt has been achieved before but what is important in our work is that we have developed lasers at two distinct wavelengths. To physically put together several separate lasers of different colors is too costly to be useful and thus our proof-of concept experiment becomes interesting and potentially important technologically. In addition to being used for solid state lighting and full color displays, such technology can also be used as light sources for fluorescence bio and chemical detection.”
About the Author
Matthew Peach is a contributing editor to optics.org.