Scanning Ultrafast Electron Microscopy Research Articles

Imaging hot photocarrier transfer across a semiconductor heterojunction with ultrafast electron microscopy

Semiconductor heterojunctions have gained significant attention for efficient optoelectronic devices owing to their unique interfaces and synergistic effects. Interaction between charge carriers with the heterojunction plays a crucial role in determining device performance, while its spatial-temporal mapping remains lacking. In this study, we employ scanning ultrafast electron microscopy (SUEM), an emerging technique that combines high spatial-temporal resolution and surface sensitivity, to investigate photocarrier dynamics across a Si/Ge heterojunction. Charge dynamics are selectively examined across the junction and compared to far bulk areas, through which the impact of the built-in potential, band offsets, and surface effects is directly visualized. In particular, we find that the heterojunction drastically modifies the hot photocarrier diffusivities in both Si and Ge regions due to charge trapping. These findings are further elucidated with insights from the band structure and surface potential measured by complementary techniques. This work demonstrates the tremendous effect of heterointerfaces on hot photocarrier dynamics and showcases the potential of SUEM in characterizing realistic optoelectronic devices.

Probing Photocarrier Dynamics in Wide Bandgap Semiconductors with UV-Pumped Scanning Ultrafast Electron Microscopy

Probing Photocarrier Dynamics in Wide Bandgap Semiconductors with UV-Pumped Scanning Ultrafast Electron Microscopy

A Femtosecond Electron-Based Versatile Microscopy for Visualizing Carrier Dynamics in Semiconductors Across Spatiotemporal and Energetic Domains.

Carrier dynamics detection in different dimensions (space, time, and energy) with high resolutions plays a pivotal role in the development of modern semiconductor devices, especially in low-dimensional, high-speed, and ultrasensitive devices. Here, a femtosecond electron-based versatile microscopy is reported that combines scanning ultrafast electron microscopy (SUEM) imaging and time-resolved cathodoluminescence (TRCL) detection, which allows for visualizing and decoupling different dynamic processes of carriers involved in surface and bulk in semiconductors with unprecedented spatiotemporal and energetic resolutions. The achieved spatial resolution is better than 10nm, and the temporal resolutions for SUEM imaging and TRCL detection are ≈500fs and ≈4.5ps, respectively, representing state-of-the-art performance. To demonstrate its unique capability, the surface and bulk carrier dynamics involved in n-type gallium arsenide (GaAs) are directly tracked and distinguished. It is revealed, in real time and space, that hot carrier cooling, defect trapping, and interband-/defect-assisted radiative recombination in the energy domain result in ordinal super-diffusion, localization, and sub-diffusion of carriers at the surface, elucidating the crucial role of surface states on carrier dynamics. The study not only gives a comprehensive physical picture of carrier dynamics in GaAs, but also provides a powerful platform for exploring complex carrier dynamics in semiconductors for promoting their device performance.

Picosecond carrier dynamics in InAs and GaAs revealed by ultrafast electron microscopy.

Understanding the limits of spatiotemporal carrier dynamics, especially in III-V semiconductors, is key to designing ultrafast and ultrasmall optoelectronic components. However, identifying such limits and the properties controlling them has been elusive. Here, using scanning ultrafast electron microscopy, in bulk n-GaAs and p-InAs, we simultaneously measure picosecond carrier dynamics along with three related quantities: subsurface band bending, above-surface vacuum potentials, and surface trap densities. We make two unexpected observations. First, we uncover a negative-time contrast in secondary electrons resulting from an interplay among these quantities. Second, despite dopant concentrations and surface state densities differing by many orders of magnitude between the two materials, their carrier dynamics, measured by photoexcited band bending and filling of surface states, occur at a seemingly common timescale of about 100 ps. This observation may indicate fundamental kinetic limits tied to a multitude of material and surface properties of optoelectronic III-V semiconductors and highlights the need for techniques that simultaneously measure electro-optical kinetic properties.

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

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. Particularly, we examine two photoinduced effects: the generation of photocarriers 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-illumination and paves the way toward a quantitative interpretation of the SUEM contrasts.

Persistent hot carrier diffusion in boron arsenide single crystals imaged by ultrafast electron microscopy

<h2>Summary</h2> Cubic boron arsenide (BAs) is promising for microelectronics thermal management because of its high thermal conductivity. Recently, its potential as an optoelectronic material is also being explored. However, it remains challenging to measure its photocarrier transport properties because of small sizes of available high-quality crystals. Here, we use scanning ultrafast electron microscopy (SUEM) to directly visualize the diffusion of photocarriers in BAs single crystals. Surprisingly, we observed ambipolar diffusion at low optical fluence with persistent hot carrier dynamics for above 200 ps, which can likely be attributed to the large frequency gap between acoustic and optical phonons, the same feature that is responsible for the high thermal conductivity. At higher optical fluence, we observed spontaneous electron-hole separation. Our results show BAs is an attractive optoelectronic material combining high thermal conductivity and excellent photocarrier transport properties. Our study also demonstrates the capability of SUEM to probe photocarrier transport in emerging materials.

Scanning ultrafast electron microscopy reveals photovoltage dynamics at a deeply buried p−Si/SiO2 interface

The understanding and control of charge carrier interactions with defects at buried insulator/semiconductor interfaces is essential for achieving optimum performance in modern electronics. Here, we report on the use of scanning ultrafast electron microscopy (SUEM) to remotely probe the dynamics of excited carriers at a Si surface buried below a thick thermal oxide. Our measurements illustrate a previously unidentified SUEM contrast mechanism, whereby optical modulation of the space-charge field in the semiconductor modulates the electric field in the thick oxide, thus affecting its secondary electron yield. By analyzing the SUEM contrast as a function of time and laser fluence we demonstrate the diffusion mediated capture of excited carriers by interfacial traps.

Transient Strain-Induced Electronic Structure Modulation in a Semiconducting Polymer Imaged by Scanning Ultrafast Electron Microscopy.

Understanding the optoelectronic properties of semiconducting polymers under external strain is essential for their applications in flexible devices. Although prior studies have highlighted the impact of static and macroscopic strains, assessing the effect of a local transient deformation before structural relaxation occurs remains challenging. Here, we employ scanning ultrafast electron microscopy (SUEM) to image the dynamics of a photoinduced transient strain in the semiconducting polymer poly(3-hexylthiophene) (P3HT). We observe that the photoinduced SUEM contrast, corresponding to the local change of secondary electron emission, exhibits an unusual ring-shaped profile. We attribute the observation to the electronic structure modulation of P3HT caused by a photoinduced strain field owing to its low modulus and strong electron-lattice coupling, supported by a finite-element analysis. Our work provides insights into tailoring optoelectronic properties using transient mechanical deformation in semiconducting polymers and demonstrates the versatility of SUEM to study photophysical processes in diverse materials.

Spatiotemporal Imaging of Thickness-Induced Band-Bending Junctions.

van der Waals materials exhibit naturally passivated surfaces and an ability to form versatile heterostructures to enable an examination of carrier transport mechanisms not seen in traditional materials. Here, we report a new type of homojunction termed a "band-bending junction" whose potential landscape depends solely on the difference in thickness between the two sides of the junction. Using MoS2 on Au as a prototypical example, we find that surface potential differences can arise from the degree of vertical band bending in thin and thick regions. Furthermore, by using scanning ultrafast electron microscopy, we examine the spatiotemporal dynamics of charge carriers generated at this junction and find that lateral carrier separation is enabled by differences in the band bending in the vertical direction, which we verify with simulations. Band-bending junctions may therefore enable new optoelectronic devices that rely solely on band bending arising from thickness variations to separate charge carriers.

Carrier density oscillation in the photoexcited semiconductor

The perturbation of a semiconductor from the thermodynamic equilibrium often leads to the display of non-linear dynamics and formation of spatiotemporal patterns due to the spontaneous generation of competing processes. Here, we use scanning ultrafast electron microscopy to show that the transport of hot carriers in the strongly excited semiconductor slows down by turning into an oscillatory process; this is evidenced by the expansions and contractions in the second moment of the distribution. We attribute such a response to the electric field generated by the spatial separation of photo-excited electrons and holes under intrinsic and photo-induced fields; we then introduce a transport model that mimics the experimental observation. Our finding provides a direct imaging evidence for the electrostatic oscillation of hot carriers in the highly excited semiconductor and offers new insights into their dynamics in space and time.

Probing Surface Photovoltage Effect Using Photoassisted Secondary Electron Emission.

While the properties of surfaces and interfaces are crucial to modern devices, they are commonly difficult to explore since the signal from the bulk often masks the surface contribution. Here we introduce a methodology based on scanning electron microscopy (SEM) coupled with a pulsed laser source, which offers the capability to sense the topmost layer of materials, to study the surface photovoltage (SPV) related effects. This method relies on a pulsed optical laser to transiently induce an SPV and a continuous primary electron beam to produce secondary electron (SE) emission and monitor the change of the SE yield under laser illumination. We observe contrasting behaviors of the SPV-induced SE yield change on n-type and p-type semiconductors. We further study the dependence of the SPV-induced SE yield on the primary electron beam energy, the optical fluence, and the modulation frequency of the optical excitation, which reveal the details of the dynamics of the photocarriers in the presence of the surface built-in potential. This fast, contactless, and bias-free technique offers a convenient and robust platform to probe surface electronic phenomena, with great promise to probe nanoscale effects with a high spatial resolution. Our result further provides a basis to understand the contrast mechanisms of emerging time-resolved electron microscopic techniques, such as the scanning ultrafast electron microscopy.

Ultrafast electron imaging of surface charge carrier dynamics at low voltage.

The performance of optoelectronic devices strongly depends on charge carrier dynamics on top of surfaces of the absorber layers. Unfortunately, this information cannot be selectively probed using conventional ultrafast laser spectroscopic methods, due to the large penetration depth (tens of nm to μm) of the photon pulses in the pump-probe configurations. Therefore, ultrafast time-resolved approaches that can directly and selectively visualize the behavior of the surface carrier dynamics are urgently needed. Here, we introduce a novel methodology of low-voltage scanning ultrafast electron microscopy that can take ultrafast time-resolved images (snapshots) of the surface of materials at the sub-nanometer level. By this approach, the surface of the photoactive materials is optically excited and imaged, using a pulsed low-voltage electron beam (1 keV) that interacts with the surface to generate secondary electrons with an energy of a few eV, and that are emitted only from the top surface of materials, providing direct information about the carrier dynamics and the localization of electron/holes in real space and time. An outlook on the potential applications of this low voltage approach in different disciplines will also be discussed.

Real-Space Mapping of Surface-Oxygen Defect States in Photovoltaic Materials Using Low-Voltage Scanning Ultrafast Electron Microscopy.

Ultrathin layers of native oxides on the surface of photovoltaic materials may act as very efficient carrier trapping/recombination centers, thus significantly affecting their interfacial photophysical properties. How ultrathin oxide layers affect the surface and interface carrier dynamics cannot be selectively accessed by conventional ultrafast transient spectroscopic methods due to the deep penetration depth (tens to thousands of nanometers) of the pump/probe laser pulses. Herein, scanning ultrafast electron microscopy (S-UEM) at a low voltage of 1 keV electrons was recently developed at KAUST to selectively map the ultrafast charge carrier dynamics of a few layers on the top surfaces of photovoltaic materials. Unlike high-voltage 30 keV experiments, at 1 keV, the depth of detected secondary electrons (SEs) underneath the surface is significantly reduced 5 times, thus making it possible to visualize the dynamics of charge carrier from the uppermost surface of photoactive layers. More specifically, this new approach has been employed to explore and decipher the tremendous impact of native oxide layers and oxygen defect states on charge carrier dynamics in space and time simultaneously at sub-nanometer levels on several photovoltaic material surfaces, including Si, GaAs, CdTe, and CdZnTe single crystals. Interestingly, the contrast in the SEs time-resolved difference images switched from a dark "energy-loss mechanism" to a bright "energy-gain mechanism" only by removing the layers of surface oxides. Moreover, the charge carrier recombination was estimated and found to be dramatically affected by the native oxide layers. The density functional theory (DFT) calculations demonstrate that the work function of oxygen-terminated Si surface also increases slightly upon optical excitation and makes for less SE intensity, providing another piece of evidence that the origin of the dark contrast observed on these material surfaces should be assigned to the surface oxide formation, mainly oxygen defect states in the band gap and/or work function increment. Our findings provide a new method and pave the way for evaluating the effect of surface morphology and defects, including but not limited to native oxide, on charge carrier dynamics, and interfacial properties of photovoltaic absorber layers.

Ultrafast imaging of surface-exclusive carrier dynamics in silicon

Understanding the dynamics of charge carriers at the semiconductor surfaces and interfaces is fundamental to the further development of photocatalytic, photovoltaic, and optoelectronic devices. Here, we study the surface photovoltage (SPV) dynamics in intrinsic and doped silicon using scanning ultrafast electron microscopy (SUEM). SUEM is a surface sensitive technique that allows the direct imaging of carriers at ultrafast time scales, thereby elucidating their spatiotemporal response to optical excitation. We first discuss the mechanism of image formation in SUEM. We then use these images to show that carrier dynamics on the silicon surface depends strongly on the doping type and concentration, though not always dictated by SPV. The numerical simulation of the drift-diffusion model suggests that this is due to the formation of complex transport processes, driven by intrinsic and photoinduced fields in the excited volume. This work refines our current understanding of the surface-exclusive dynamics in semiconductors by introducing a means to study their evolution in space and time and providing a model to explain the underlying mechanism.

Visualization of Charge Carrier Trapping in Silicon at the Atomic Surface Level Using Four-Dimensional Electron Imaging.

The ultrathin thickness (∼1-2 nm) of the native oxide layer on silicon surfaces, which acts as efficient trapping centers, precludes the possibility of studying its impact on the surface-charge carrier dynamics by conventional time-resolved laser spectroscopic techniques because of the large penetration depth of the pump and probe pulses. Here, we use four-dimensional scanning ultrafast electron microscopy (4D S-UEM) with unique surface sensitivity to directly visualize the charge carrier dynamics on Si(100) crystals before and after surface treatment (which removes the native oxide layer) in real space and time simultaneously. Our time-resolved snapshots of the top surface and Kelvin probe-force microscopy results demonstrate that the oxide layer can be formed within minutes after surface treatment, creating undesirable surface-trap states that destroy the population of photogenerated charge carriers on the surface and possibly at the device interface. This new surface observation provides critical photophysical insights into how a few atomic layers of oxide can dramatically influence charge carrier recombination dynamics in silicon solar cells.

Extraordinary Carrier Diffusion on CdTe Surfaces Uncovered by 4D Electron Microscopy

Summary Herein, we describe the selective mapping of surface charge carriers, achieved using scanning ultrafast electron microscopy, for visualization of light-triggered carrier dynamics on the nanometer scale, diffusing on the surface of CdTe, a leading semiconductor in commercial photovoltaics. We reveal a novel diffusion model of charge carriers on surfaces. The nature of charge diffusion varies significantly, from superior diffusion to virtually trapped, depending on the crystal orientation. For instance, carriers on the (110) crystal orientation displayed a diffusion coefficient of ∼40,000 cm2·s−1 (i.e., ∼104 times larger than measured in the bulk). In contrast, the (211) polar facets formed oxidative layers that, in turn, formed mid-surface-trap states that nearly arrested the charge diffusion completely. Complementary techniques and theoretical modeling explain the acute sensitivity of charge carriers to surface orientation and termination. Our findings reveal the enormous untapped potential for improved CdTe-based devices and optoelectronic materials in general through facet management.

Scanning ultrafast electron microscopy: Four-dimensional imaging of materials dynamics in space and time

Abstract

Imaging the Reduction of Electron Trap States in Shelled Copper Indium Gallium Selenide Nanocrystals Using Ultrafast Electron Microscopy

Insertion of alkali metal ions, especially Na, is a well-established method to significantly increase the power conversion efficiency of copper indium gallium selenide (CIGSe)-based photovoltaic devices. However, although it is known that Na ions mostly reside on the surface of CIGSe layer following diffusion, the exact mechanism of how Na affects the carrier dynamics of CIGSe still remains ambiguous. This is mainly due to the unavailability of suitable surface-sensitive techniques. Herein, we employ four-dimensional scanning ultrafast electron microscopy (4D S-UEM), which has the unique capability of mapping the charge carrier dynamics in real time and space selectively on the materials surfaces, to directly observe the effect of Na insertion on the carrier dynamics of shelled CIGSe film. It is found that an additional layer of NaF to the thin film of ZnS-shelled CIGSe nanocrystals not only increases the grain size and improves the texture of the film but, more importantly, reduces fast electron trap cha...

(Invited) Emerging Two-Dimensional Materials for Electronic and Photonic Device Applications

In this talk, I will discuss our recent work in developing novel electronic and photonic devices based on the emerging two-dimensional (2D) materials. I will first present our work in understanding the fundamental electronic and optical properties of low-symmetry 2D materials such as black phosphorus (BP) using a newly developed scanning ultrafast electron microscopy (SUEM) technique. Our recent study of bandgap tuning in BP and the demonstration of BP based optical detectors will also be presented. In the second half of my talk, I will discuss our work on developing an atomically-thin resistive memory device based on two-dimensional materials that demonstrates record low current/power operation with promising potential applications in ultra low power computation and memory.

Imaging Localized Energy States in Silicon-Doped InGaN Nanowires Using 4D Electron Microscopy

Introducing dopants into InGaN NWs is known to significantly improve their device performances through a variety of mechanisms. However, to further optimize device operation under the influence of large specific surfaces, thorough knowledge of ultrafast dynamical processes at the surface and interface of these NWs is imperative. Here, we describe the development of four-dimensional scanning ultrafast electron microscopy (4D S-UEM) as an extremely surface-sensitive method to directly visualize in space and time the enormous impact of silicon doping on the surface-carrier dynamics of InGaN NWs. Two time regimes of surface dynamics are identified for the first time in a 4D S-UEM experiment: an early time behavior (within 200 ps) associated with the deferred evolution of secondary electrons due to the presence of localized trap states that decrease the electron escape rate and a longer time scale behavior (several ns) marked by accelerated charge carrier recombination. The results are further corroborated by ...