Dr. Ebrahim Najafi

Chemistry and Chemical Engineering Division

California Institute of Technology

Pasadena, California

Ultrafast Imaging of Charge Carrier Dynamics at the Semiconductor Surfaces and Interfaces

Charge carrier dynamics at the semiconductor surfaces and interfaces often determine the overall performance of electronic, photovoltaic and thermoelectric devices. However, these interfacial dynamics are not fully understood primarily because traditional optical spectroscopies lack the sufficient spatiotemporal resolution and hence measure only the average behavior at near equilibrium. Scanning ultrafast electron microscopy (SUEM) overcomes such limitations by allowing the 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 electrons and holes at the silicon surface by SUEM; we explored the super-diffusion of these carriers that lasted tens of picoseconds and ultimately transformed into the steady-state diffusion. Furthermore, 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 and we observed 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.

Biography:

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.