Scanning Photocurrent Imaging and Electronic Band Studies in Silicon Nanowire Field Effect Transistors

Ahn, Y. H.; Dunning, James E.; Park, Jiwoong

doi:10.1021/nl050631x

Cited by 262 publications

(292 citation statements)

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“…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.…”

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confidence: 99%

“…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.…”

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confidence: 99%

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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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Imaging Ultrafast Carrier Transport in Nanoscale Field-Effect Transistors

et al. 2014

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“…In the presence of potential modulations along the CNT, the photovoltage is a measure of the local potential gradient in the excitation region, which separates the photogenerated electrons and holes. 15 By scanning the laser spot across the sample and assembling images of the photocurrent in a computer, we obtain a sensitive microscopic probe for local electronic transport parameters of CNTs. A photocurrent image of a semiconducting CNTFET under external bias has been reported before.…”

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Imaging of the Schottky Barriers and Charge Depletion in Carbon Nanotube Transistors

et al. 2007

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The photovoltage produced by local illumination at the Schottky contacts of carbon nanotube field-effect transistors varies substantially with gate voltage. This is particularly pronounced in ambipolar nanotube transistors where the photovoltage switches sign as the device changes from p-type to n-type. The detailed transition through the insulating state can be recorded by mapping the open-circuit photovoltage as a function of excitation position. These photovoltage images show that the band-bending length can grow to many microns when the device is depleted. In our palladium-contacted devices, the Schottky barrier for electrons is much higher than that for holes, explaining the higher p-type current in the transistor. The depletion width is 1.5 μm near the n-type threshold and smaller than our resolution of 400 nm near the p-type threshold. Internal photoemission from the metal contact to the carbon nanotube and thermally assisted tunneling through the Schottky barrier are observed in addition to the photocurrent that is generated inside the carbon nanotube.

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“…12 We also monitor the reflected light intensity simultaneously to determine the absolute To improve the performance we used a lock-in technique with an intensity-modulated laser (λ = 781 nm with 20 kHz modulation, unless otherwise noted). The signal-to-noise ratio is dramatically improved this way and we can reliably detect a lowlevel current signal (< 10 pA) measured with a low light intensity.…”

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Photocurrent Imaging of p−n Junctions in Ambipolar Carbon Nanotube Transistors

et al. 2007

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We use scanning photocurrent microscopy (SPCM) to investigate the properties of internal p-n junctions as well as local defects in ambipolar carbon nanotube (CNT) transistors. Our SPCM images show strong signals near metal contacts whose polarity and positions change depending on the gate bias. SPCM images analyzed in conjunction with the overall conductance also indicate the existence and gate-dependent evolution of internal p-n junctions near contacts in the n-type operation regime. To determine the p-n junction position and the depletion width with a nanometer scale resolution, a Gaussian fit was used. We also measure the electric potential profile of CNT devices at different gate biases, which shows that both local defects and induced electric fields can be imaged using the SPCM technique. Our experiment clearly demonstrates that SPCM is a valuable tool for imaging and optimizing electrical and optoelectronic properties of CNT based devices.In semiconducting nanostructures, including carbon nanotubes (CNTs) 1, 2 and nanowires (NWs), 3 the electronic properties at various interfaces, especially metal contacts, play a crucial role, often dominating the overall performance of nanostructure-based devices. [4][5][6][7] In addition, the presence and properties of defects can cause various abnormalities in the conductance properties. To fully understand the overall conductance behavior of a nanostructure with high spatial resolution, various scanning probe microscopy (SPM) techniques have been utilized to locally perturb or electrically contact the target structures. 7,8 More recently, scanning photocurrent microscopy (SPCM) has been successfully applied to study electrical and photoelectric properties of a number of linear nanostructures, including carbon nanotubes 9, 10 and semiconducting nanowires. 11,12 The information generated by SPCM includes the band structure near nanostructure/metal interfaces, 12 carrier relaxation dynamics, 13 and detection of local defects. 10 In this paper, we report SPCM measurements of CNT transistors at different gate biases (V G 's) to investigate the electronic band structure. Our measurements reveal that the peak photocurrent (PC) spots near the metal contacts move with gate bias in the n-type regime. We measure with nanoscale resolution the intensity and polarity of these PC spots in conjunction with DC conductance to investigate the dynamics of the internal p-n junctions. We also study the effect of various local potential variations such as naturally existing defects and induced electrical field due to a partial suspension of the CNTs.Our CNT transistors were fabricated using standard photo-and electron beam lithography techniques with CVD-grown semiconducting CNTs. CNTs were first synthesized directly on a 220 nm thick thermal oxide layer on a conducting Si substrate that is also used as a back gate. Metal evaporation and liftoff were used to define drain and source electrodes. We used either Cr/Au (5/45 nm) or Ti (50 nm) for the electrodes.Our SPCM setup uses a diffraction-...

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Scanning Photocurrent Imaging and Electronic Band Studies in Silicon Nanowire Field Effect Transistors

Cited by 262 publications

References 19 publications

Imaging Ultrafast Carrier Transport in Nanoscale Field-Effect Transistors

Imaging Ultrafast Carrier Transport in Nanoscale Field-Effect Transistors

Imaging of the Schottky Barriers and Charge Depletion in Carbon Nanotube Transistors

Photocurrent Imaging of p−n Junctions in Ambipolar Carbon Nanotube Transistors

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