Impact of photoexcitation on secondary electron emission: a Monte Carlo study

TL;DR

This study uses Monte Carlo simulations combined with TDDFT to analyze how photoexcitation influences secondary electron emission in semiconductors, clarifying the origins of contrast in ultrafast electron microscopy images.

Contribution

It introduces a detailed simulation approach to distinguish effects of photo-carriers and surface photovoltage on secondary electron yield in semiconductors.

Findings

Hot photocarriers and SPV significantly alter SEY after photoexcitation.

Bulk photocarrier distribution has negligible impact on SEY.

Insights aid in interpreting SUEM contrast mechanisms.

Abstract

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.