Reports: G10
45982-G10 Ultrafast Probing of Structures, Functions, and Pathways for Light-Excited Amorphous Silicon in Potential Energy Landscape
We investigate the charge transfer mechanism at the silicon/silicon-oxide interface using an ultrafast electron diffractive voltammetry approach. The increase of photoinduced interface potential is determined based on the refraction effect of the submerged electron beam through monitoring the shift of diffraction peaks on the ultrafast timescale. We attribute the rise of interface potential to a photo-induced hot electron transfer from the subsurface silicon to the oxide surface.
In a previous study, we have shown that the surface voltage can relax on 100 ps timescale based on the return of the refracted diffraction peaks with the oxide thickness of ~ 2nm, chemically grown on etched, stepped Si/SiO2/OH surface. Here, we investigate a flat interface, and primarily focus on resolving the charge injection processes through examining the power dependence of the injection time. Since we use 800 nm and 400 nm as excitation sources, which has a very deep penetration depth (> micron) in contrast to previous UV studies, so that the surface lattice heating effect is significantly reduced (estimated to be less than 400 K). In contrast, the transient electronic temperature is much greater, thus it is expected that the charge transfer can be promoted by the hot electron channels. Because of this, we can concentrate on the field-induced Coulomb refraction effect on the diffraction peak shift to evaluate the charge dynamics. We also evaluate the extent of the surface charge accumulation that extend into the vacuum region above the surface using an electron point projection imaging approach, which allows us to map out the electron density distribution and its evolvement as a function of time.
We conclude, based on the power-dependence study of the refraction as well as a dynamical imaging of the photoelectron density distribution, that multiple channels jointly lead to a transient charge redistribution that can be characterized by a surface dipolar field across the SiO2 layer and a near surface field attributed by photoemitted electrons. We determine that the dipolar field strength to be on the level of 0.1V/nm, corresponding to a charge density of 1013 e/cm2. The mechanism establishing this surface charge is a coherent 2+1 multiphoton absorption that leads to a direct injection of electrons into the conduction band of SiO2, causing surface charge accumulation. Below a threshold fluence, the mutiphoton process is ineffective, and the surface charging has a linear power dependence, which can be explained by a hot electron tunneling process. In contrast, the surface emission is nearly linear, showing no threshold behavior. The photoemitted electrons have a Maxwellian-like thermal velocity profile, that causes a finite extension into the vacuum above the surface, but the electrons eventually return to the surface on a few hundred ps timescale.
