Quantum dots: Clever alloys for wider light emission

A  team of KACST researchers has developed a promising new method of preparing quantum dot (QD) crystals that  improves the consistency of production and provides precise control over the optical properties of the QDs for use in photonic applications.

QDs are synthetic crystals measuring just a few nanometres in diameter. They have attracted great interest in recent years for use in devices that range from photovoltaic cells to high-definition TV screens and lasers, due to the possibility of tuning their properties as required. For example, scientists can decide precisely which wavelengths of light are absorbed and emitted by QDs simply by changing their size and molecular structure. 

The tuneability of QDs comes from the way electrons are excited into conduction energy bands when light is shone on them. Then, because the excited electrons are confined within the small space of the QDs, the electrons drop back down to the lower valence band, emitting light of specific wavelengths. 

QDs are frequently grown on gallium-arsenide substrates, but the emission wavelengths of these QDs are limited because the gallium and indium atoms mix during the fabrication process, reducing the overall size of the QDs. Various alloy “strain reducing layers,” including those made from gallium arsenide antimonide (GaAsSb), have been trialled to prevent this mixing. However, the growth of these layers on QDs has been difficult to replicate, because the arsenide and antimonide atoms compete with one another, resulting in a subtly different layer every time. 

Now, Abdelmajid Salhi at the University of Manchester, UK,  with colleagues at the KACST National Nanotechology Research Centre in Riyadh, have demonstrated a method of fine-tuning the optical properties of indium arsenide (InAs) QDs, by adding an alloy layer that can shift or extend the range of emission wavelengths. 

Salhi and co-workers realised that they could create more consistent GaAsSb layers by disposing of the normal random alloy growth methods. Instead, they embraced the new technology of so- digital alloying, which works in a manner similar to a 3D printer, building up alternating layers of gallium arsenide (GaAs) and gallium antimonide (GaSb) one at a time. This enabled the reproduction of near-identical layers on different QDs.

What’s more, by varying the speed of deposition, or the growth order of the layers, the researchers were able to produce QDs with different concentrations of antimony and different structures. These various types of QDs showed different ranges of emission, with some reaching to infrared wavelengths of around 1,500 nanometres.

1. Salhi, A., Alshaibani, S., Alaskar, Y., Albadri, A., Alyamani, A., & Missous, M.  Tuning the optical properties of InAs QDs by means of digitally-alloyed GaAsSb strain reducing layers. (2019)