Elastic and inelastic scattering in graphene studied by microwave transport and noise
Andreas Betz (LPA)

Infos Complémentaires

en salle de conférence E244 / Conf IV

Vendredi 28 août à 14h

Résumé :

This thesis discusses the elastic and inelastic scattering in monolayer graphene, investigated by means of microwave carrier dynamics and noise.

We study in a first part the high frequency properties of graphene field-effect transistors on different substrates. Particular interest lies in the figures of merit like e.g. the transit frequency f_T, defining the transistor’s current amplification capabilities, and the transconductance g_m representing its gate sensitivity. High values are obtained for both parameters in GHz measurements. We find in particular that these figures remain substantial even in miniaturised devices.

We introduce top-gated graphene field-effect capacitors as a probe of the elastic scattering mechanisms in graphene. Employing similar techniques as in the transistor experiments, we are able to directly access the diffusion constant D and its dependence on carrier density. The latter is the signature of the scattering mechanism present in the graphene sheet.
Our novel GHz experiments reveal a constant transport scattering time as a function of energy which is in disagreement with conventional theoretical predictions, but supports the random Dirac mass disorder mechanism.

Furthermore, we study inelastic scattering of charge carriers by acoustic phonons in graphene which is among the first realisations of such an experiment in a genuine two-dimensional geometry. A broadband cryogenic noise thermometry setup is used to detect the electronic fluctuations, the current noise, from which we extract the average electron temperature T_e as a function of Joule power P. At high bias we find P\propto \Sigma T_e^4 as predicted by theory and which is the tell-tale sign of a 2D phonon cooling mechanism. From a heat equation analysis of data in a broad bias range, we extract accurate values of the electron-acoustic phonon coupling constant \Sigma. Our measurements point to an important effect of lattice disorder in the electron-phonon energy relaxation.

en salle de conférence E244 / Conf IV