Impact of photoexcitation on secondary electron emission a Monte Carlo study Wenkai Ouyang1Xiangying Zuo1and Bolin Liao1

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Impact of photoexcitation on secondary electron emission: a

Monte Carlo study

Wenkai Ouyang,1Xiangying Zuo,1and Bolin Liao1, ∗

1Department of Mechanical Engineering,

University of California, Santa Barbara, CA 93106, USA

(Dated: October 27, 2022)

Abstract

Understanding the transport of photogenerated charge carriers in semiconductors is crucial for

applications in photovoltaics, optoelectronics and photo-detectors. While recent experimental stud-

ies 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 re-

mains 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.

∗bliao@ucsb.edu

arXiv:2210.14470v1 [cond-mat.mtrl-sci] 26 Oct 2022

I. INTRODUCTION

Understanding photocarrier dynamics accurately is critical for improving the performance

and ensuring the stability of high-eﬃciency photovoltaic (PV) cells, light-emitting diodes,

and other optoelectronic devices. For example, the performance of PV cells critically de-

pends on the minority carrier diﬀusion process [1–3], and eﬃcient hot photocarrier col-

lection provides a possible means to boost the PV eﬃciency above the Shockley-Queiser

limit[4]. These processes can only be probed and better understood by experimental tools

that can detect light-induced carrier behaviors with combined high spatial and temporal res-

olutions. One candidate is the high-energy pulsed electron beam generated by illuminating

a photocathode with a pulsed laser source[5]. These short electron pulses, with durations

down to sub-picoseconds, can be accelerated to high energy, and thus, ﬁnely focused to

sub-nanometer spatial resolution. Techniques based on the pulsed electron beam, such as

ultrafast electron diﬀraction (UED) and ultrafast transmission electron microscopy (UTEM)

have been used to visualize photocarrier dynamics with great success [6–9]. In 2010, Zewail

and coworkers invented scanning ultrafast electron microscopy (SUEM) by combining the

temporal resolution of short electron pulses with the spatial resolution of scanning electron

microscopy (SEM)[10–12]. Compared to UED and UTEM, SUEM excels in surface sen-

sitivity and is particularly suitable to study surface photocarrier transport[13–16], surface

acoustic waves[17,18] and surface defect-carrier interactions[19].

In SUEM, a femtosecond infrared laser source is split and converted into a pump beam

(typically 515 nm) and an ultraviolet probe beam (typically 343 nm or 257 nm). Figure 1(a)

depicts the working principle of SUEM. The pump pulse directly hits the sample surface

and causes structural, electronic, or thermal changes, such as photocarrier excitation, surface

photovoltages[16,20], topographical distortions and temperature rise[21]. In the meantime,

the ultraviolet probe beam is focused onto the electron gun inside an SEM column to generate

short electron pulses, which are accelerated and focused onto the sample surface. The

time delay between the optical pump pulses and electronic probe pulses is controlled by a

mechanical delay stage. The probe electron pulses, or the “primary electrons” (PEs), excite

secondary electrons (SEs) from the sample surface, which are collected by an Everhart-

Thornley detector (ETD), and the number of SEs emitted from each location on the sample

surface is used to form an image. SUEM contrast images are then generated by comparing

the change in the SE images as a result of photoexcitation. By controlling the time delay

between the optical pump pulses and the electronic probe pulses, SUEM contrast images

depicting the change of the sample at a given time after the photoexcitation can be recorded.

For example, Najaﬁ et al. observed the interfacial carrier dynamics at a silicon p-n junction

with SUEM and reported ballistic carrier transport that cannot be explained by conventional

drift-diﬀusion models[22]. Liao et al. measured the hot carrier transport in amorphous

silicon using SUEM and observed the spatial separation of electrons and holes[14]. In these

studies, the high spatial-temporal resolution of SUEM enables the investigation of photo-

induced local microscopic physical processes on a wide range of materials. However, in order

to quantitatively interpret these photophysical processes, it is critical to fully understand

the underlying mechanisms for the SUEM image contrasts.

Figure 1. Schematic of SUEM experiment and image formation. (a) A schematic showing

a typical setup of the SUEM instrumentation. The green line denotes the optical pump beam while

the purple line denotes the optical beam to generate the probe electron pulses. The electron pulses

travel down the SEM column and are focused onto the sample through the electron optics. (b)

Image formation mechanism in an SUEM. The primary electrons excite secondary electrons from

the sample, which are collected by an ETD. The optical pump pulse generates photocarriers in the

bulk and near the surface, which modulate the secondary electron yield. The changes in the local

secondary electron yield as a result of photoexcitation are used to form SUEM contrast images.

As explained above, SUEM contrast images reﬂect the change in the number of emit-

ted SEs (SE yield, or SEY) from each location on the sample surface as a result of

photoexcitation[12], as illustrated in Fig. 1(b). Thus, the key to understanding SUEM

contrast mechanisms is to examine how the SE generation, transport, and emission pro-

cesses are aﬀected by photoexcitation. Currently, several SUEM contrast mechanisms have

been proposed, which can be categorized into bulk eﬀects and surface eﬀects[12], as sum-

marized in Fig. 2. The bulk eﬀects are illustrated in Fig. 2(a). When electrons and holes

are generated by photoexcitation in the bulk, the photo-excited electrons possess a higher

energy and have a higher probability of escaping as SEs after interacting with the PEs. This

mechanism indicates that the photo-illuminated area should show a higher SEY and, thus,

a “bright” contrast in the SUEM images. This mechanism, although not quantitatively

examined so far, has been used to interpret the early SUEM results[13,14]. In addition,

the photogenerated bulk carriers can also collide with SEs during their transport to the

surface and prevent them from escaping the material surface, leading to a reduction of the

SEY and a “dark” SUEM contrast. This mechanism was used to explain the dark SUEM

contrasts observed on GaAs surfaces[23]. Furthermore, the photoexcitation can also change

the sample surface voltage and cause the surface electronic energy band to bend, known as

the surface photovoltage eﬀect (SPV) [24]. This eﬀect is graphically illustrated in Fig. 2(b).

On semiconductor surfaces, defects and dangling bonds can pin the surface Fermi level and

lead to the bending of electronic bands near the surface that is associated with a surface

electronic ﬁeld. This surface ﬁeld can facilitate or hinder the transmission of SEs across

the sample surface, depending on the band bending direction. This eﬀect is well known to

cause diﬀerent SEM image intensities from n-type and p-type surfaces[24]. Photoexcited

photocarriers can compensate for the surface bending (the SPV eﬀect), and thus modify the

SE escape probability and the SEY. Fig. 2(b) shows the scenario in n-type silicon, where

the surface bands bend upward near the surface under the dark condition, hindering the

SE escape. In this case, SPV compensates for the surface band bending and lowers the

surface barrier for SEs to escape, leading to a higher SEY and a bright SUEM contrast. Li

et al. experimentally veriﬁed the impact of the SPV eﬀect by observing a brighter (darker)

SEM contrast in n-type (p-type) silicon under illumination compared to the dark state [20].

The SPV eﬀect at an internal interface was also recently studied using SUEM[16]. Despite

these previous eﬀorts, there is no quantitative understanding of the relative contribution of

these mechanisms to the SUEM contrast. This lack of understanding poses an obstacle to

obtaining quantitative information from SUEM images. Therefore, it is necessary to develop

a computational approach to clarify the contributions from various contrast mechanisms.

In this work, we implemented a Monte Carlo simulation assisted with time-dependent

density functional theory (TDDFT) to quantitatively study the SUEM image contrast under

Figure 2. Proposed SUEM Contrast Mechanisms. (a) Schematic illustrating the SUEM

contrast mechanism due to the generation of photocarriers in the sample bulk. The presence

of photocarriers with higher energy can both increase the generate rate of secondary electrons

and scatter the secondary electrons during their transport. (b) Schematic illustrating the surface

photovoltage eﬀect in n-type silicon. The black lines depict the energy band diagram in the dark

condition, where χ0is the electron aﬃnity and Vsis the surface band bending due to Fermi level

pinning. The red dashed lines depict the energy band diagram under photo-illumination. The

photocarriers compensate for the surface band bending and reduces the eﬀective potential barrier

for SEs to escape. SPV: surface photovoltage. Ec: conduction band bottom. Ev: valence band

top. EF: Fermi level.

photoexcitation with both bulk photocarrier eﬀects and the surface SPV eﬀect in the model

system silicon and with PE energy from 50 eV to 1 keV. In particular, we used TDDFT

to compute the eﬀect of photoexcitation on the electron energy loss function (ELF) in

silicon, and then used the ELF as input to a Monte Carlo simulation to evaluate the impact

of photoexcitation on SEY. Our study laid the foundation for the future development of

SUEM as a quantitative imaging tool.

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Impactofphotoexcitationonsecondaryelectronemission:aMonteCarlostudyWenkaiOuyang,1XiangyingZuo,1andBolinLiao1,1DepartmentofMechanicalEngineering,UniversityofCalifornia,SantaBarbara,CA93106,USA(Dated:October27,2022)AbstractUnderstandingthetransportofphotogeneratedchargecarriersinsemiconductorsiscruci...

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Impact of photoexcitation on secondary electron emission a Monte Carlo study Wenkai Ouyang1Xiangying Zuo1and Bolin Liao1

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