Hot-Carrier Cooling in High-Quality Graphene Is Intrinsically Limited by Optical Phonons
Various emerging technologies for optoelectronic applications, in particular in the field of data communication, take advantage of the ultrafast dynamics of photo-excited charge carriers in graphene. After reaching an excited state by photon absorption, these carriers undergo a multi-step relaxation phase, in which they release the extra energy they acquired and return to their ground state. First, scattering between carriers occurs, which leads to a state with an elevated electronic temperature. Then, these hot charge carriers cool down to room temperature via various processes. Large research efforts have been devoted to the identification of the physical mechanisms that drive this cooling phase, which depends on how graphene is prepared and by the materials that surround it. The cooling processes include interactions with graphene optical and acoustic phonons, and with substrate phonons in nearby materials. A deep understanding of the phenomena governing this phase is key to graphene’s use in optoelectronic devices.
In a paper recently published in ACS Nano, the cooling dynamics of hot carriers are studied in two technologically relevant material systems based on high-quality graphene. This work, carried out by an international team of researchers from various institutes in Spain, Italy, Germany, UK, Belgium and China, where Molecular Spectroscopy Department at Max Planck Institute for Polymer Research participated, revealed the crucial role played by a cooling process in particular, which is always present in graphene, thus providing the intrinsic limit of the lifetime of its electronic excitations.
Using three different time-resolved optical measurement techniques, the researchers analysed the behaviour of photo-induced hot carriers in WSe2-encapsulated graphene, where graphene is sandwiched between two layers of tungsten diselenide, and in suspended graphene, grown by chemical vapour deposition. Both systems are meant for applications requiring very high quality of graphene, which implies large charge mobility. In these materials, a cooling timescale of tens of picoseconds was expected, because the so-called “disorder-assisted supercollision cooling” is relatively inefficient in high-mobility graphene, and because, in the two cases under study, graphene interacts weakly with phonons in its environment. The experimental results, though, showed that in both samples the relaxation of hot carriers occurs in a few (2-4) picoseconds, a much shorter time than expected.
To explain these outcomes, the authors of the study suggest that another cooling mechanism intrinsic to graphene, involving optical phonon emission, is at work. Specifically, hot carriers initially decay by emitting optical phonons, and, in turn, these optical phonons couple to acoustic phonons, producing a slower decay. In the meanwhile, the electronic system undergoes a continuous rethermalization (a process that leads to thermal equilibrium). This enable constant emission of optical phonons, which therefore provides a continuous heat sink. The researchers also developed an analytical model of this cooling pathway, which fits well with the experimental results and indicates what are the key parameters to play with for slowing down or accelerating the charge carriers relaxation.
This study provides fundamental insights into the hot-carrier cooling dynamics in high-quality graphene, which will be extremely useful for the future development of graphene-based optoelectronic devices.