Direct Imaging of Free Carrier and Trap Carrier Motion in Silicon Nanowires by Spatially-Separated Femtosecond Pump–Probe Microscopy

Gabriel, Michelle M.; Kirschbrown, J.; Christesen, Joseph D.; Pinion, Christopher W.; Zigler, David F.; Grumstrup, Erik M.; Mehl, Brian P.; Cating, Emma E. M.; Cahoon, James F.; Papanikolas, John M.

doi:10.1021/nl400265b

Cited by 128 publications

(148 citation statements)

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“…Alternatively, optical ultrafast measurement techniques have been widely used to investigate charge carrier dynamics with a time resolution determined by the optical pulse width (down to a few femtoseconds) 9 . Recently, researchers have reported visualizing charge carrier movements in free-standing Si NWs using an ultrafast pump-probe imaging technique 10,11 . The carrier diffusion motions induced by a pump pulse located in the middle of the NWs were visualized; however, these optical measurements are limited for interrogating the carrier dynamics in operating devices because they strongly depend on the non-linear properties of materials and they are frequently obscured by the substrate signals.…”

Section: *Electronic Mail: Ahny@ajouackrmentioning

confidence: 99%

Imaging Ultrafast Carrier Transport in Nanoscale Field-Effect Transistors

et al. 2014

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One-dimensional nanoscale devices, such as semiconductor nanowires (NWs) and singlewalled carbon nanotubes (SWNTs), have been intensively investigated because of their potential application of future high-speed electronic, optoelectronic, and sensing devices 1-3 .To overcome current limitations on the speed of contemporary devices, investigation of charge carrier dynamics with an ultrashort time scale is one of the primary steps necessary for developing high-speed devices. In the present study, we visualize ultrafast carrier dynamics in nanoscale devices using a combination of scanning photocurrent microscopy and timeresolved pump-probe techniques. We investigate transit times of carriers that are generated near one metallic electrode and subsequently transported toward the opposite electrode based on drift and diffusion motions. Carrier dynamics have been measured for various working conditions. In particular, the carrier velocities extracted from transit times increase for a larger negative gate bias, because of the increased field strength at the Schottky barrier. *Electronic mail: ahny@ajou.ac.kr 2The transit time of the charge carriers is a crucial factor limiting the high frequency response of nanoscale devices; however, traditional radio-frequency measurements are often limited by the high impedance or the RC constants of the devices [4][5][6][7][8] . Alternatively, optical ultrafast measurement techniques have been widely used to investigate charge carrier dynamics with a time resolution determined by the optical pulse width (down to a few femtoseconds) 9 . Recently, researchers have reported visualizing charge carrier movements in free-standing Si NWs using an ultrafast pump-probe imaging technique 10,11 . The carrier diffusion motions induced by a pump pulse located in the middle of the NWs were visualized; however, these optical measurements are limited for interrogating the carrier dynamics in operating devices because they strongly depend on the non-linear properties of materials and they are frequently obscured by the substrate signals. Consequently, these techniques are not ideal for low-dimensional systems with NWs thinner than the optical spot size (<100 nm) or with SWNTs.Scanning photocurrent microscopy (SPCM) techniques have been introduced as powerful tools for investigating local optoelectronic characteristics, such as metallic contacts, defects, interfaces, and junctions [12][13][14][15][16][17][18][19] . We were able to collect localized electronic band information that is not disturbed by signals originating from the substrate, and hence, compared with conventional optical pump-probe techniques, SPCM can provide a higher signal-to-noise ratio. Only recently, ultrafast pump-probe photocurrent techniques have been demonstrated for studying carrier dynamics in carbon nanotube devices by using a collinear pump and probe beams, focused at the same position 20,21 . In addition, ultrafast phenomena in graphene and GaAs NWs have been investigated by measuring terahertz radiation that results from...

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Section: *Electronic Mail: Ahny@ajouackrmentioning

confidence: 99%

Imaging Ultrafast Carrier Transport in Nanoscale Field-Effect Transistors

et al. 2014

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“…A range of ultrafast laser-based techniques is nowadays available for probing the evolution of electronic, optical, structural and magnetic properties of solids after a sudden perturbation such as optical excitation, providing invaluable information on the mutual coupling of electronic, nuclear and spin degrees of freedom, as well as of transport properties. Despite femtosecond temporal resolution, the investigation of ultrafast processes in nanoscaled, low-dimensional systems additionally requires high spatial resolution [4][5][6] as well as high sensitivity sufficient for investigating small sample volumes, that is, femtosecond probe pulses strongly interacting with the sample. Electrons with sub-keV kinetic energies, here referred to as low-energy electrons, exhibit exceptionally high scattering cross-section and a de Broglie wavelength on the order of 1 Å, which, in principle, allows for achieving atomic resolution both in imaging as well as diffraction approaches.…”

mentioning

confidence: 99%

Femtosecond electrons probing currents and atomic structure in nanomaterials

Müller¹,

Paarmann²,

Ernstorfer³

2014

Nat Commun

123

158

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The investigation of ultrafast electronic and structural dynamics in low-dimensional systems such as nanowires and two-dimensional materials requires femtosecond probes providing high spatial resolution and strong interaction with small volume samples. Low-energy electrons exhibit large scattering cross-sections and high sensitivity to electric fields, but their pronounced dispersion during propagation in vacuum so far prevented their use as femtosecond probe pulses in time-resolved experiments. Here, employing a laser-triggered point-like source of either divergent or collimated electron wave packets, we developed a hybrid approach for femtosecond point projection microscopy and femtosecond low-energy electron diffraction. We investigate ultrafast electric currents in nanowires with sub-100 femtosecond temporal and few 10 nm spatial resolutions, and demonstrate the potential of our approach for studying structural dynamics in crystalline single-layer materials.

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“…Ref. 38 indicates that the defect density in CVD-grown MoS 2 monolayers is ~10 13 cm −2 , very close to n sat . This supports the idea that defect and trap states dominate carrier relaxation in our MoS 2 monolayers, although we cannot rule out contributions from the hot phonon effect.…”

Section: Resultsmentioning

confidence: 81%

“…Here, we use ultrafast optical microscopy (UOM) 12,13 to measure ultrafast carrier dynamics in atomically thin single molybdenum disulfide flakes grown by chemical vapor deposition (CVD). By tuning the probe photon energy through the MoS 2 band gap (both indirect and direct), our UOM measurements show that conduction and valence band states are rapidly populated on a sub-picosecond (ps) time scale in a MoS 2 monolayer after photoexcitation at 3.1 eV, consistent with previous work [14][15][16][17][18] .…”

mentioning

confidence: 99%

“…The normalized photoinduced change in transmission (Δ T/T), which is directly proportional to the photoexcited carrier population at a given point in time and space 15,21,27 , is measured at the detector. Real time monitoring of both focused spots and the sample position using a CCD camera with a 50X objective and accompanying beam expander allows us to selectively measure ultrafast carrier dynamics in individual nanomaterials, removing the detrimental influence of inhomogeneous broadening 12,13 . Images of the atomically thin monolayer (0.6 nm in thickness) MoS 2 flakes taken with our UOM system are shown in Fig.…”

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Ultrafast optical microscopy of single monolayer molybdenum disulfide flakes

et al. 2014

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We have performed ultrafast optical microscopy on single flakes of atomically thin CVD-grown molybdenum disulfide, using non-degenerate femtosecond pump-probe spectroscopy to excite and probe carriers above and below the indirect and direct band gaps. These measurements reveal the influence of layer thickness on carrier dynamics when probing near the band gap. Furthermore, fluencedependent measurements indicate that carrier relaxation is primarily influenced by surface-related defect and trap states after above-bandgap photoexcitation. The ability to probe femtosecond carrier dynamics in individual flakes can thus give much insight into light-matter interactions in these twodimensional nanosystems.There has been an explosion of recent interest in the quasi-two-dimensional (2D) transition metal dichalcogenides (TMDCs), which are layered materials with strong in-plane covalent bonding, leading to atomically thin layers within their structure 1 . Their unique 2D geometry allows for the design and fabrication of complicated structures at desirable positions within optoelectronic devices, much more efficiently than other low-dimensional nanomaterials 1,2 . In particular, molybdenum disulfide (MoS 2 ) has received much recent attention due to its excellent electronic 3 and optical properties 4,5 . Most notably, the indirect bandgap in the bulk (~1.2 eV) changes to a direct bandgap (~1.9 eV) as the number of atomic layers is reduced to one, causing MoS 2 monolayers to generate strong photoluminescence [4][5][6][7] . Therefore, unlike graphene, which does not have an intrinsic band gap 8 , TMDCs are especially promising for potential optoelectronic applications, such as photo-transistors 9 , photodetectors 10 , and electroluminescent devices 11 . Many of these applications will depend on a detailed knowledge of the optical properties and carrier relaxation dynamics, which can be elucidated by photoexciting carriers and probing their relaxation as a function of layer thickness with femtosecond time resolution.Here, we use ultrafast optical microscopy (UOM) 12,13 to measure ultrafast carrier dynamics in atomically thin single molybdenum disulfide flakes grown by chemical vapor deposition (CVD). By tuning the probe photon energy through the MoS 2 band gap (both indirect and direct), our UOM measurements show that conduction and valence band states are rapidly populated on a sub-picosecond (ps) time scale in a MoS 2 monolayer after photoexcitation at 3.1 eV, consistent with previous work [14][15][16][17][18] . Pump fluence-dependent measurements reveal that subsequent carrier relaxation in our samples is primarily due to surface-related defects and trap states, not the Auger processes observed in previous measurements on MoS 2 and other semiconductor nanosystems 12,[18][19][20][21][22][23] . We also observed an increase in the carrier relaxation time with an increase in the number of MoS 2 layers, likely due to the well known layer-dependent changes in the electronic structure 16,24 . Our UOM measurements of photon energy-a...

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Direct Imaging of Free Carrier and Trap Carrier Motion in Silicon Nanowires by Spatially-Separated Femtosecond Pump–Probe Microscopy

Cited by 128 publications

References 26 publications

Imaging Ultrafast Carrier Transport in Nanoscale Field-Effect Transistors

Imaging Ultrafast Carrier Transport in Nanoscale Field-Effect Transistors

Femtosecond electrons probing currents and atomic structure in nanomaterials

Ultrafast optical microscopy of single monolayer molybdenum disulfide flakes

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