“Silicon is the workhorse of consumer electronics,” explains team leader Edward Sargent. “It is ubiquitous in our everyday life and is an excellent material for electronics. However, since it does not absorb light efficiency and is blind to IR radiation beyond wavelengths of 1100 nm, it cannot be used for important optoelectronics applications such as optical communications and night vision.

“In our new work, we have made a phototransistor based on room-temperature processed, low cost, semiconducting colloidal quantum dots (CQDs). These structures add to silicon new functionalities – such as for example the possibility of tuning the wavelengths at which the device absorbs light.”

Sargent and colleague Valerio Adinolfi began by fabricating silicon-junction FETs. To do this, the researchers grew a thin epitaxial p-type silicon layer on top of an n-type type silicon substrate. They then contacted the epitaxial layer using source and drain electrodes made from aluminium. Finally, they deposited a layer of colloidal quantum on top of the p-type silicon channel by a technique called spin coating.

QDs act as an IR photosensitive gate

“The quantum dots act as an IR photosensitive gate that modulates the p-type silicon channel based on the amount of infrared light absorbed, Sargent tells nanotechweb.org. In fact, the FET exploits a photovoltage that arises at the silicon CQD interface to control junction electronics. And it is this that allows it to modulate the conductivity of the silicon channel in proportion to incoming light at wavelengths below that of silicon’s bandgap.

“The device allows for detection of IR light directly on silicon,” explains Sargent. “It is also particularly sensitive with a gain of more than 104 electrons per photon at IR wavelengths and nearly 106 in the visible range of frequencies. Its response time is around 10 microseconds, which is five orders of magnitude better than previous silicon-colloidal quantum dot-based phototransistors.”

The Toronto team, reporting its work in Nature doi:10.1038/nature21050, is now busy trying to improve the photovoltage FET by making the colloidal CQD layer sensitive to longer wavelengths.