MSE Seminar Series: Ebrahim Najafi, CalTech
Friday, January 27, 2017
1:00 p.m.-2:00 p.m.
Room 2108, Chemical and Nuclear Engineering Building
Imaging of Charge Carrier Dynamics at Semiconductor Surfaces and Interfaces by
Ultrafast Electron Microscopy
A thorough understanding of the interfacial carrier dynamics in semiconductors is essential to the further development of nanoelectronic, optoelectronic and photovoltaic technologies. Since there is a constant supply of energy during the operation of a typical device, carriers are often driven away from equilibrium, where their spatiotemporal behavior becomes nonlinear and chaotic. In addition, the relaxation pathway of these excited carriers involves the exchange of energy, transport in the space, and complete disappearance due to recombination. The control and manipulation of these events at the nanoscale, whose timescale spans from a few femtoseconds to hundreds of microseconds, will enable the design and fabrication of complex devices with supreme performance and efficiency. Conventional spectroscopy techniques are unable to study such processes at the nanoscale due to their insufficient spatiotemporal resolution. Scanning ultrafast electron microscopy (SUEM) overcomes such limitation by allowing high resolution imaging of the electronic and structural dynamics at the femtosecond, picosecond and nanosecond timescales. In this work, we investigated the spatial dynamics of charge carriers in silicon by SUEM; specifically, we explored the initial super-diffusion of carriers following optical excitation which lasted tens of picoseconds and ultimately transitioned into the steady-state surface diffusion. Interestingly, at large excitation densities, electrons and holes showed non-linear transport behavior, resulting in the emission of plasma waves. Finally, we studied charge carrier dynamics at the silicon p-n junction which showed that charge separation was not limited to the junction and began tens of micrometers away; in addition, the separated charges remained localized in both space and time across the junction for tens of nanoseconds.
Ebrahim Najafi is a postdoctoral scholar in Professor Ahmed Zewail’s group at the California Institute of Technology in Pasadena, California. His work focuses on the development of ultrafast electron microscopy techniques and their applications in studying the ultrafast phenomena in semiconductor thin films and nanomaterials.