Measurement of Ultrafast Carrier Dynamics in Epitaxial Graphene
- PDF / 272,973 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 80 Downloads / 220 Views
1081-P06-04
Measurement of Ultrafast Carrier Dynamics in Epitaxial Graphene Jahan M. Dawlaty, Shriram Shivaraman, Mvs Chandrashekhar, Michael G. Spencer, and Farhan Rana Electrical and Computer Engineering, Cornell University, Philips Hall, Cornell University, Ithaca, NY, 14853 ABSTRACT Using ultrafast optical pump-probe spectroscopy, we have measured carrier relaxation times in epitaxial graphene layers grown on SiC wafers. We find two distinct time scales associated with the relaxation of nonequilibrium photogenerated carriers. An initial fast relaxation transient in the 70-120 fs range is followed by a slower relaxation process in the 0.41.7 ps range. The slower relaxation time is found to be inversely proportional to the degree of crystalline disorder in the graphene layers as measured by Raman spectroscopy. We relate the measured fast and slow time constants to carrier-carrier and carrier-phonon intraband and interband scattering processes in graphene. INTRODUCTION Graphene is a two dimensional (2D) atomic layer of carbon atoms forming a honeycomb crystal lattice [1,2]. It is a zero-bandgap semiconductor with a linear energy dispersion relation for both electrons and holes [2]. The unusual electronic and optical properties of graphene have generated interest in both experimental and theoretical arenas [2-4]. The high mobility of electrons in graphene has prompted a large number of investigations into graphene based high speed electronic devices, such as field-effect transistors, pn-junction diodes and transistors, and terahertz oscillators, and also into low noise electronic sensors [5-8]. Recently, epitaxial growth of graphene by thermal decomposition of SiC surface at high temperatures has been investigated [4]. This technique can provide anywhere from a few monolayers of graphene to several (>50) layers on the surface of a SiC wafer. Graphene layers grown by this technique have demonstrated structural and electronic properties similar to those of graphene layers obtained by micromechanical cleaving techniques, including the massless Dirac-like energy dispersion relation for electrons and holes and carrier mobilities in the few tens of thousand cm2 / Vs range [4,9]. Epitaxial growth of graphene on SiC provides a technique to obtain large area multilayers that can be used for studies, such as ultrafast optical spectroscopy, that are difficult to conduct on monolayers. In this paper, we present results from measurements of the ultrafast dynamics of photoexcited carriers in graphene for the first time. Ultrafast studies of carrier dynamics in other forms of carbon, such as carbon nanotubes and bulk graphite, have been reported in the past [10,11]. The results presented in this paper are relevant for understanding carrier intraband and interband scattering mechanisms in graphene and their impact on proposed and demonstrated graphene based electronic and optical devices.
EXPERIMENT The graphene samples were epitaxially grown on the carbon face of semi-insulating 6HSiC wafers using techniques similar to those th
Data Loading...
