Interfacial Carrier Dynamics of Graphene on SiC, Traced by the Full-Range Time-Resolved Core-Level Photoemission Spectroscopy at SPring-8 BL07LSU
I. Matsuda Group
Time evolutions of the Dirac Fermions in graphene layers have attracted both academic and technological interests due to observations of the various intriguing opto-electric phenomena, such as carrier multiplications and generation of a terahertz laser. Nowadays, graphene layers can be epitaxially grown on a SiC substrate in wide area and such non-equilibrium carriers in the Dirac bands have been directly probed by time-resolved photoemission spectroscopy. On the other hand, the previous studies focused only on electronic evolutions within a graphene layer and little examination was made on carrier dynamics through the graphene/substrate interfaces. Unveiling dynamic roles of the graphene/SiC interface is necessary to understanding the opto-electric properties in SiC based detectors, for example.
In the present research, we conducted measurements of time-resolved core-level photoemission spectroscopy by a combination of laser and synchrotron radiation [1]. The experiment was performed at SPring-8 soft X-ray beamline BL07LSU with the beamline laser station, BL07LASER. Figure 1 shows temporal variations of peak positions of C 1s and Si 2p during relaxation of the surface photovoltage effect, induced by the optical pumping with a laser pulse. Two C 1s components are assigned to a graphene layer (G) and a SiC substrate (SiC), while one Si 2p component originates from a SiC substrate. Thus, the high-resolution core-level spectra selectively distinguish dynamical information between overlayer and substrate. While the whole relaxation of the electron-hole recombination process takes 100 nanoseconds, one can also find difference in temporal variation between C and SiC in the subnanosecond region. This is because carrier lifetime in the graphene layers take longer than that of the typical interface-state of SiC by the bottleneck effect of Dirac cones. When there is a buffer layer between graphene and SiC, electron-hole recombination is dominantly held at the interface and the lifetime shortens one-order of the magnitude [1]. The selective evaluations of carrier dynamics in non-uniform samples, such as heterojunctions, allow one to design and develop novel optoelectric devices.
References
- [1] T. Someya, H. Fukidome, N. Endo, K. Takahashi, S. Yamamoto, and I. Matsuda, Appl. Phys. Lett. 113, 051601 (2018).