Visualization of carrier dynamics in p(n)-type GaAs by scanning ultrafast electron microscopy

Abstract

Four-dimensional scanning ultrafast electron microscopy is used to investigate doping-and carrier-concentration-dependent ultrafast carrier dynamics of the in situ cleaved single-crystalline GaAs(110) substrates. We observed marked changes in the measured timeresolved secondary electrons depending on the induced alterations in the electronic structure. The enhancement of secondary electrons at positive times, when the electron pulse follows the optical pulse, is primarily due to an energy gain involving the photoexcited charge carriers that are transiently populated in the conduction band and further promoted by the electron pulse, consistent with a band structure that is dependent on chemical doping and carrier concentration. When electrons undergo sufficient energy loss on their journey to the surface, dark contrast becomes dominant in the image. At negative times, however, when the electron pulse precedes the optical pulse (electron impact), the dynamical behavior of carriers manifests itself in a dark contrast which indicates the suppression of secondary electrons upon the arrival of the optical pulse. In this case, the loss of energy of material's electrons is by collisions with the excited carriers. These results for carrier dynamics in GaAs(110) suggest strong carrier-carrier scatterings which are mirrored in the energy of material's secondary electrons during their migration to the surface. The approach presented here provides a fundamental understanding of materials probed by four-dimensional scanning ultrafast electron microscopy, and offers possibilities for use of this imaging technique in the study of ultrafast charge carrier dynamics in heterogeneously patterned micro-and nanostructured material surfaces and interfaces.scanning electron microscopy | ultrafast phenomena | charge dynamics imaging R ecent advances in four-dimensional (4D) ultrafast electron microscopy (UEM) have made it possible to investigate nonequilibrium electronic and structural dynamics with atomicscale spatial resolution and femtosecond temporal resolution (1). Unlike UEM, which operates in the transmission mode, scanning UEM techniques exploit the time evolution of secondary electrons (SEs) produced in the specimen, and provide additional marked advantages over the transmission mode. These include a relatively facile sample preparation requirement, an efficient heat dissipation, a lower radiation damage, and an accessibility to low-voltage environmental study (2, 3). Since its development this technique has been used to study carrier excitation dynamics in several prototypical semiconducting materials surfaces. In these studies, image contrast was monitored as a function of time, and it was found that Si exhibits a bright contrast in the image at positive times without appreciable dynamics at negative times, whereas CdSe displays bright contrast at positive times and dark contrast at negative times (2). However, the correlation between the measured time-dependent SE intensity and electronic structure of the material of interes...

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

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

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Real-Space Mapping of Surface Trap States in CIGSe Nanocrystals Using 4D Electron Microscopy

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“…The dark contrast at negative time can be attributed to the diffusion of plasmon-excited carriers that are generated from deeper areas of the sample by electron impact, which can be perturbed by electron-hole pairs generated by the clocking photon pulse, or to different scattering processes. 58,62 At positive time delays, the 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 9 optical pulse promotes a fraction of the valence-band electrons to the conduction band.…”