Photovoltage dynamics of the hydroxylated Si(111) surface investigated by ultrafast electron diffraction

We present a method to measure transient photovoltage at nanointerfaces using ultrafast electron diffraction. In particular, we report our results on the photoexcitation of carriers and their ensuing relaxations in a hydroxyl-terminated silicon surface, which is a standard substrate for fabricating molecular electronics interfaces. The transient surface voltage is determined by observing Coulomb refraction changes induced by the modified space-charge barrier within a selectively probed volume by femtosecond electron pulses. The results are in agreement with ultrafast photoemission studies of surface-state charging, suggesting a surface charge relaxation mechanism closely coupled to the bulk carrier dynamics near the surface that can be described by a drift-diffusion model. This study demonstrates an ultrafast diffraction method for investigating interfacial processes with both charge and structural resolutions.