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

Shaheen, Basamat S.; El‐Zohry, Ahmed M.; Yin, Jun; Bastiani, Michele De; Wolf, Stefaan De; Bakr, Osman M.; Mohammed, Omar F.

doi:10.1021/acs.jpclett.9b00598

Cited by 10 publications

(16 citation statements)

References 67 publications

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“…As discussed in Figures 4 and 5, being a single crystalline or amorphous form can dramatically influence the charge carrier dynamics of Si. It is worth mentioning that our group found that 89 surface native oxides have significant impacts on the recombination of charge carriers on Si samples. As shown in Figure 6a, most of the photoexcited charge carriers recombined within 3 ns on the Si single crystal with native oxide layers (top row).…”

Section: Emerging Applicationsmentioning

confidence: 89%

“…(b) Surface potential profile of Si was measured immediately after etching (red) and exposed to air for 40 min after etching (black). KPFM images for HF-etched Si under illumination of the laser (top part) and in the dark (bottom part) were taken (c) immediately after HF etching and (d) after 40 min of air exposure . Reprinted from ref .…”

Section: Emerging Applicationsmentioning

confidence: 99%

“…KPFM images for HF-etched Si under illumination of the laser (top part) and in the dark (bottom part) were taken (c) immediately after HF etching and (d) after 40 min of air exposure . Reprinted from ref . Copyright 2019 American Chemistry Society.…”

Section: Emerging Applicationsmentioning

confidence: 99%

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Access to Ultrafast Surface and Interface Carrier Dynamics Simultaneously in Space and Time

et al. 2021

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Charge carrier dynamics at material surfaces and interfaces play a fundamental role in controlling the performance of photocatalytic reactions and photovoltaic devices; however, precise characterization of the surface dynamical properties of a material with nanometer (nm) and femtosecond (fs) spatial and temporal resolutions, respectively, is a precondition for profound understanding and is thus urgently needed. Many techniques have been developed to meet this demand, but barely any of them have simultaneous excellent surface sensitivity (depth resolution) and sufficient spatiotemporal resolutions, except for a one-of-a-kind second-generation scanning ultrafast electron microscope (S-UEM), which has been established and developed at KAUST to provide direct and controllable dynamical information about the ultrafast charge carrier dynamics and the localization of electrons and holes on the photoactive material surface and interfaces. In this feature article, the instrumentation, working principles, new capabilities, and unique applications of S-UEM in the ultrafast characterization of material surfaces and interfaces, including charge carrier injection, surface carrier diffusion, surface carrier trapping, and recombination, are systematically summarized and inspected. Future developments from both theoretical and experimental perspectives are also discussed.

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Section: Emerging Applicationsmentioning

confidence: 89%

Section: Emerging Applicationsmentioning

confidence: 99%

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Access to Ultrafast Surface and Interface Carrier Dynamics Simultaneously in Space and Time

et al. 2021

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“…Traditionally used optical techniques are incapable of meeting the nanometer-range resolution requirement to map the flow of charge carriers over interfaces, and further complications arise from bulk and surface dynamics occurring on time scales that can differ by orders of magnitude. − In ultrafast scanning electron microscopy (USEM), a focused electron beam is used to probe the dynamics of a charge carrier distribution after laser excitation, thus bringing electron beam resolution into the traditional pump–probe schemes. − With USEM, carrier dynamics has been studied in bulk materials such as Si and GaAs, in crystals including CIGSe and CdSe, and in layered materials like black phosphorus and across a silicon p–n junction. − In all schemes, low-energy, 0–10 eV, secondary electrons (SEs) are used as the probe signal. As these SEs typically have a very short, only a few nanometers, mean free path, the bulk contribution to the signal is naturally limited, leading to an exquisite sensitivity to surface-related phenomena .…”

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confidence: 99%

Lock-in Ultrafast Electron Microscopy Simultaneously Visualizes Carrier Recombination and Interface-Mediated Trapping

Garming

Bolhuis

Conesa-Boj

et al. 2020

J. Phys. Chem. Lett.

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Visualizing charge carrier flow over interfaces or near surfaces meets great challenges concerning resolution and vastly different time scales of bulk and surface dynamics. Ultrafast or four-dimensional scanning electron microscopy (USEM) using a laser pump electron probe scheme circumvents the optical diffraction limit, but disentangling surface-mediated trapping and ultrafast carrier dynamics in a single measurement scheme has not yet been demonstrated. Here, we present lock-in USEM, which simultaneously visualizes fast bulk recombination and slow trapping. As a proof of concept, we show that the surface termination on GaAs, i.e., Ga or As, profoundly influences ultrafast movies. We demonstrate the differences can be attributed to trapping-induced surface voltages of approximately 100–200 mV, which is further supported by secondary electron particle tracing calculations. The simultaneous visualization of both competing processes opens new perspectives for studying carrier transport in layered, nanostructured, and two-dimensional semiconductors, where carrier trapping constitutes a major bottleneck for device efficiency.

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“…Silicon crystal with n-type-doped silicon wafers were purchased from Norsun. The surface of crystals was treated as previously reported . Briefly, specimens with 1 cm × 1 cm size were cut by a diamond knife from phosphorus-doped n-type Si wafers (6 in., resistivity of 1–5 Ω/cm, with a doping level of ∼6.5 × 10 14 cm –3 ).…”

Section: Experimental Methodsmentioning

confidence: 99%

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

Shaheen

El‐Zohry

Zhao

et al. 2020

ACS Appl. Mater. Interfaces

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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 (10s-100s nm) 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 five 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-nm levels on several photovoltaic material surfaces, including Si, GaAs, CdTe and CdZnTe single crystals. Interestingly, the contrast in the SEs timeresolved difference images switched from a dark "energy-loss mechanism" to a bright "energygain 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 oxygenterminated Si surface also increases slightly upon optical excitation and makes for less SEs 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 bandgap, 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.

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

Cited by 10 publications

References 67 publications

Access to Ultrafast Surface and Interface Carrier Dynamics Simultaneously in Space and Time

Access to Ultrafast Surface and Interface Carrier Dynamics Simultaneously in Space and Time

Lock-in Ultrafast Electron Microscopy Simultaneously Visualizes Carrier Recombination and Interface-Mediated Trapping

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

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