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Theoretical Analysis of Electron Relaxation of Sub-THz Optically Generated Carriers in Monolayer Graphene Mediated by Longitudinal Acoustic Phonons
KTH, School of Electrical Engineering and Computer Science (EECS).
2018 (English)Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
Abstract [en]

Electron relaxation is the decay process that electrons experience when falling back to a state of lower energy. By comparing the scattering events that would deplete an energy level of carriers with the generation phenomena associated with an incident radiation, it is possible to predict whether the material fulfills saturable absorption conditions. With this goal in mind, a twodimensional layer of carbon atoms arranged in a honeycomb lattice structure, called graphene, has revealed exceptional opto-electronic properties thanks to its unique semimetal behavior. The thesis investigates carrier relaxation dynamics mediated by longitudinal acoustic phonons of an electron population pumped at sub-meV energies in an ideal graphene monolayer. The theoretical results derived by Fermi’s Golden Rule reveal the exceptional symmetry of the phonon-absorption and emission transitions taking place between two states, k and k’. In absence of relaxation mechanisms other than electron-acoustic phonon interactions, the simulations highlight the strong impact of the operational temperature on the final outcomes, predicting decay times of µs and even ms time scale, and variation rates of the electron distribution in the order of kHz/cm2 and tens of Hz/cm2.

Abstract [sv]

Elektronrelaxering är processen som en elektron genomgår när den återgår till ett lägre energitillstånd. Genom att jämföra spridningshändelser med energi motsvarande en energinivå för laddningsbärare med genereringsfenomenet förknippat med inkommande strålning är det möjligt att förutsäga om ett material uppfyller absorbtionskraven för att vara mättnadsbart. Med detta mål i åtanke, har ett tvådimensionellt lager av kolatomer i en hexagonal gitterstruktur, kallat för grafen, avslöjats ha exceptionella opto-elektroniska egenskaper tack vare dess unika beteende som semimetall. Detta masterexamensarbete undersöker laddningsbärares relaxeringsdynamik, i detta fall styrd av longitudinella, akustiska fononer för en elektronpopulation som befinner sig på sub-meV-energinivåer i ett idealt grafenlager. De teoretiska resultat som tagits fram från Fermis Gyllene regel avslöjar exceptionell symmetri för fonon-absorption och energiemissionsövergångar mellan två tillstånd k och k’. När andra relaxeringsmekanismer än den mellan elektroner och akustiska fononer bortses ifrån, framhäver simuleringarna den starka påverkan drifttemperatur har på slutresultatet. Simuleringarna förutsäger sönderfallstider i µs och även i ms-skala, samtvariationsgrader i elektrondistributionen i storleksordningen kHz/cmoch tiotals Hz/cm2.

Place, publisher, year, edition, pages
2018. , p. 96
Series
TRITA-EECS-EX ; 2018:19
Keywords [en]
Electron relaxation, graphene monolayer, Fermi’s Golden Rule, electron distribution, longitudinal acoustic phonons.
Keywords [sv]
Elektronrelaxering, grafenlager, Fermis Gyllene regel, elektrondistribution, longitudinella, akustiska fononer.
National Category
Electrical Engineering, Electronic Engineering, Information Engineering Computer and Information Sciences
Identifiers
URN: urn:nbn:se:kth:diva-239449OAI: oai:DiVA.org:kth-239449DiVA, id: diva2:1265419
Subject / course
Information and Communication Technology
Educational program
Master of Science - School of Electrical Engineering (EES) - Master of Science - Research on Information and Communication Technologies
Examiners
Available from: 2018-11-23 Created: 2018-11-23 Last updated: 2018-11-23Bibliographically approved

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CiteExportLink to record
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