Sub-picosecond hot electron transfer in WS₂/hBN/p-Si hybrid structure revealed by energy- and time-resolved photoemission electron microscopy

Carrier transfer plays a central role in all optoelectronic applications. It can occur much faster in low dimensional materials than in conventional bulk materials, which can be exploited in developing ultrafast and high-efficient optoelectronic devices. Here, we report an ultrafast hot electron transfer study in a hybrid structure of monolayer WS₂ on p-type silicon substrate separated by thin layer of hexagonal boron nitride (hBN). We studied the ultrafast electron transfer dynamics by an energy- and time-resolved photoemission electron microscopy, and determined that photoexcited electrons transfer to p-type silicon via two paths of a direct hot-electron transfer on a sub-picosecond timescale and another one of intra-band carrier cooling and subsequent electron transfer on a timescale of a few picoseconds. The transfer rate and the relative weight of the two paths can be quantitatively determined, which depends on excitation wavelength and the thickness of the hBN layer.