Mapping Carrier Dynamics on Material Surfaces in Space and Time using Scanning Ultrafast Electron Microscopy

Abstract

Selectively capturing the ultrafast dynamics of charge carriers on materials surfaces and at interfaces is crucial to the design of solar cells and optoelectronic devices. Despite extensive research efforts over the past few decades, information and understanding about surface-dynamical processes, including carrier trapping and recombination remains extremely limited. A key challenge is to selectively map such dynamic processes, a capability that is hitherto impractical by time-resolved laser techniques, which are limited by the laser's relatively large penetration depth and consequently these techniques record mainly bulk information. Such surface dynamics can only be mapped in real space and time by applying four-dimensional (4D) scanning ultrafast electron microscopy (S-UEM), which records snapshots of materials surfaces with nanometer spatial and subpicosecond temporal resolutions. In this method, the secondary electron (SE) signal emitted from the sample's surface is extremely sensitive to the surface dynamics and is detected in real time. In several unique applications, we spatially and temporally visualize the SE energy gain and loss, the charge carrier dynamics on the surface of InGaN nanowires and CdSe single crystal and its powder film. We also discuss the mechanisms for the observed dynamics, which will be the foundation for future potential applications of S-UEM to a wide range of studies on material surfaces and device interfaces.

Scanning ultrafast electron microscopy: A novel technique to probe photocarrier dynamics with high spatial and temporal resolutions

“…The surface sensitivity can be further enhanced by reducing the accelerating voltage of the electron beam [47]. This high surface sensitivity enabled, for example, the study of surface states and surface morphology and their effects in photocarrier recombination in indium gallium nitride (InGaN) nanowires [31,49], multinary copper indium gallium selenide (CIGSe) nanocrystals [32] and CdSe [50]. When used in the environmental SEM mode, SUEM can also study photocarrier dynamics on sample surfaces in the presence of water vapor and other gases [29].…”

Charge dynamics in aluminum oxide thin film studied by ultrafast scanning electron microscopy

“…The nanometer escape depth of the SE probe [19,20] gives the potential to address dynamics at surfaces and interfaces of today's nano-scale devices, where many applications rely on the interplay between semiconductors and insulators. Measurements performed with USEM have shown that SE are sensitive to the excitation of electrons in semiconductors triggered by optical pulses [21][22][23], but the technique has never been applied to insulators. In this paper we report the first USEM measurement on a thin film of alumina on silicon.…”

Super-diffusion of excited carriers in semiconductors

“…For example, pump-probe microscopy with visible light is inherently limited by diffraction, and it remains a non-routine approach in spite of recent progress3. Studies of the spatiotemporal evolution of excited carriers have recently become possible using scanning ultrafast electron microscopy (SUEM)4567, a technique that combines the spatial resolution of electron microscopy and the time resolution of ultrafast lasers.…”