|About this Abstract
||2010 Electronic Materials Conference
||TMS 2010 Electronic Materials Conference
||M7, Ultrafast Transient Absorption Microscopy Studies of Carrier Dynamics in Epitaxial Graphene
||Libai Huang, Gregory Hartland, Li-Qiang Chu, L Luxmi, Randall M Feenstra, Chuanxin Lian, Kristof Tahy, Huili Grace Xing
|On-Site Speaker (Planned)
Energy exchange between the electrons and phonons is particularly important to electron transport, and understanding this process will be vital for the realization of future graphene-based electronics. Epitaxial growth is a very promising approach for practical applications, as it has the ability to prepare graphene on a large scale and supported on a substrate. However, epitaxially grown graphene is highly inhomogeneous, with variations in the sample thickness occurring over length scale of a few micrometers. It is also not clear how substrate interactions affect the carrier dynamics. To pave the road for electronic devices based on epitaxial graphene, characterization methods with high spatial resolution are needed to understand these effects.
Here we present transient absorption microscopy as a novel tool to characterize graphene, and to interrogate the charge carrier dynamics. This technique has the ability to directly image carrier dynamics with a diffraction-limited spatial resolution and a time resolution of ~ 200 fs. The intensity of the transient absorption signal is shown to correlate with the number of graphene layers. The carrier cooling exhibits a bi-exponential decay, consisting of an instrument-response limited fast decay time τ1 (< 0.2 ps) and a slower decay time τ2. The value of τ2 was found to increase with increasing pump fluence. The fast decay is assigned to coupling between the electrons and optical phonons in graphene, and the slower decay is attributed to the hot phonon effect. At high pump intensities the slow decay reaches a limiting value, which is assigned to the relaxation time of the optical phonons. The contribution of the slow component to the overall decay was found to vary with spatial position in the sample. This is attributed to differences in coupling between the graphene and the substrate. Transient absorption images at different delay times also reveal variation in optical phonon lifetime that was not related to graphene thickness. These results point to transient absorption microscopy as a potentially important tool for characterizing the electrical conductivity of graphene through measurement of the lifetime of optical phonon modes excited by charge carrier relaxation.