  Understanding the transport of photogenerated charge carriers in
semiconductors is crucial for applications in photovoltaics, optoelectronics
and photo-detectors. While recent experimental studies using scanning ultrafast
electron microscopy (SUEM) have demonstrated that the local change in the
secondary electron emission induced by photoexcitation enables direct
visualization of the photocarrier dynamics in space and time, the origin of the
corresponding image contrast still remains unclear. Here, we investigate the
impact of photoexcitation on secondary electron emissions from semiconductors
using a Monte Carlo simulation aided by time-dependent density functional
theory (TDDFT). Particularly, we examine two photo-induced effects: the
generation of photo-carriers in the sample bulk, and the surface photovoltage
(SPV) effect. Using doped silicon as a model system and focusing on primary
electron energies below 1 keV, we found that both the hot photocarrier effect
immediately after photoexcitation and the SPV effect play dominant roles in
changing the secondary electron yield (SEY), while the distribution of
photocarriers in the bulk leads to a negligible change in SEY. Our work
provides insights into electron-matter interaction under photo-illumiation and
paves the way towards a quantitative interpretation of the SUEM contrasts.
