ICFO solves photoexcited graphene ‘puzzle’

15 May 2018

Barcelona-based group says understanding mechanism will boost graphene-based light detectors.

Using graphene as a light-sensitive material in light detectors can offer significant improvements with respect to existing materials commonly used nowadays, say a team of researchers at ICFO, Barcelona, who have made a breakthrough in understanding graphene’s light-converting mechanisms.

The work has been published in Science Advances

For example, graphene can detect light of almost any wavelength, and typically it produces an extremely fast electronic response within 1 x 10^-12 seconds. Thus, comments ICFO in its latest announcement, “in order to properly design graphene-based light detectors it is crucial to understand the processes that take place inside the graphene after it absorbs light.”

ICFO researchers ICREA Prof. at ICFO Frank Koppens and Klaas-Jan Tielrooij, in collaboration with scientists from other European research centers, say they have now succeeded in understanding these processes.

Conductivity rises and falls

The Science Advances paper gives a full explanation of why, in some cases, graphene conductivity increases after light absorption and in other cases, it decreases. The researchers state: “For many of the envisioned optoelectronic applications of graphene, it is crucial to understand the sub-picosecond carrier dynamics immediately following photoexcitation and the effect of photoexcitation on the electrical conductivity—the photoconductivity.

The researchers show that this behaviour correlates with the way in which energy from absorbed light flows to the graphene electrons: After light is absorbed by the graphene, the processes through which graphene electrons heat up happen extremely fast and with a very high efficiency. For highly-doped graphene (where many free electrons are present), ultrafast electron heating leads to carriers with elevated energy – hot carriers – which, in turn, leads to a decrease in conductivity.

Interestingly enough, for weakly-doped graphene (where not so many free electrons are present), electron heating leads to the creation of additional free electrons, and therefore an increase in conductivity. These additional carriers are the direct result of the gapless nature of graphene – in gapped materials, electron heating does not lead to additional free carriers.

This simple scenario of light-induced electron heating in graphene can explain many observed effects. Aside from describing the conductive properties of the material after light absorption, it can explain carrier multiplication, where – under specific conditions – one absorbed light particle (photon) can indirectly generate more than one additional free electron, and thus create an efficient photoresponse within a device.

The results of the paper, in particular, understanding electron heating processes accurately, will definitely mean a great boost in the design and development of graphene-based light detection technology. The research work was funded by the E.C. under Graphene Flagship, as well as by a Mineco Young Investigator grant.

Technical observations

The Science Advances paper abstract details the researchers’ approach and gives a summary of their findings: “We have distinguished two types of ultrafast photo-induced carrier heating processes: At low (equilibrium) Fermi energy (E_F ≲ 0.1 eV for these experiments), broadening of the carrier distribution involves interband transitions. At higher Fermi energy (E_F ≳ 0.15 eV), broadening of the carrier distribution involves intraband transitions. Under certain conditions, additional electron-hole pairs can be created for low E_F, and hot carriers for higher E_F.

So how could this highly academic research be transferred into product and market developments? The Scientific Advances article concludes: “The insights from our work lead to direct input for optimizing graphene-based photodetector devices.”