Photoexcitation in scanning ultrafast electron microscopy is a well-known technique to image charge-carrier transfer across numerous semiconductor surfaces. The contrast mechanism generally relies upon the higher-probability emission of secondary electrons from the excited electrons or, in the presence of oxide thin films, band-relaxation which introduces a field that affects secondary electron emission. Here, we present the case where the laser beam induces heterogeneous ultrafast contrast features despite a lack of heterostructures such as junctions. We attribute the heterogeneity of these responses to variations in oxide-bulk interlayer defects that influence the magnitude and direction of band-bending. Furthermore, we hypothesize the high repetition rate used for investigating these phenomena results in the introduction of nonrest equilibrium states at high enough fluences. Rigorous controls and randomized time point scans have demonstrated the ultrafast nature of these contrast features is real and the intensity variation can be directly correlated to either charge carrier concentration (bare semiconductor) or interlayer trap state concentration (oxide thin film).
Du et al. (Tue,) studied this question.