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Imaging carrier transport in nanoscale devices using femtosecond photocurrent microscopy

Imaging carrier transport in nanoscale devices using femtosecond photocurrent microscopy

초록/요약

Investigation of charge carrier dynamics with an ultrashort time scale is one of the primary steps necessary for developing high-speed electronic devices, in particular, with future nanomaterials such as semiconducting nanowires (NWs) and single-walled carbon nanotubes (SWNTs) and graphenes. The transit time of the charge carriers is the ultimate factor limiting the high frequency response of nanoscale devices; however, traditional radio-frequency measurements are often restricted by the high impedance or the RC constants of the devices. Alternatively, optical ultrafast measurement techniques can be used to investigate charge carrier dynamics with a time resolution determined by the optical pulse width. In nanoscale devices, localized effects such as metallic contacts (ohmic, Schottky), junctions (heterojunctions, P-N junctions), defects, and chemical interactions, strongly influence the overall device performance due to their nanoscale size. Therefore, it is also important to understand the carrier dynamics in conjunction with the local characteristics in various operating conditions. In order to understand these phenomena, we present a novel technique of femtosecond scanning photocurrent microscopy (fs-SPCM) in which we combine scanning photocurrent microscopy and pump-probe measurements. Using this technique, we can visualize ultrafast carrier movements in one-dimensional nanoscale devices, such as etched Si NWs from silicon-on-insulator wafers, Si NW grown using chemical vapor deposition (CVD), carbon nanotubes, and graphene transistors. We measured the transit times of ultrafast carriers generated near one metallic electrode and immediately thereafter transported toward the other side electrode through drift and diffusion processes. First, we introduce a variety of measurement techniques, such as two-dimensional (2D) spatio-temporal imaging and pump-probe photocurrent correlation imaging methods. Because our fs-SPCM technique can freely control the x-y axis locations of the pump and probe beam independently by using two pairs of galvanometers, these provide information about the movements of the charge carriers as a function of time, as well as localized energy band information in nanoscale devices. In particular, a 2D spatio-temporal imaging is very useful tool to directly observe the rate of change of the carriers according to the probe position for the fixed pump position, which provides information of the escape time and the carrier transit time very effectively. We were able to measure the carrier transit times (1–100 ps) for Si NW, SWNT, and graphene devices with different channel lengths ranging from 1 to 8 μm. We found that the carrier velocity for SWNT and graphene devices is an order of magnitude larger than that of Si NWs, which implies that the intrinsic cut-off frequency of individual SWNT and graphene devices could reach 1 THz for 1-μm channel length. Besides, we believe that our technique can be applied to a variety of semiconducting devices including the commercialized systems. Most importantly, we investigated carrier dynamics in various working conditions, such as source-drain and gate biases in nanoscale field-effect transistors. Gate-dependent measurements reveal that the carrier velocity changes linearly with the applied gate bias in accordance with increase in the electric field strength in the Schottky barrier. We also observed drift-like motion, in which the average velocity did not change noticeably with changes in the channel length. This finding is a significant deviation from the ordinary transient drift-diffusion model and has been atttributed to both surface recombination effects and the unique transport properties at high carrier kinetic energies. Conversely, the source-drain bias control results in the tailoring of the ultrashort electrical pulses, potentially useful for generating short electrical pulses. Based on these results, it will be possible to design novel devices in which we create ultrafast carrier pulses and simultaneously control their movements. Our work represents an important step toward understanding ultrafast dynamics in various nanoscale devices and toward developing future high-speed electronic devices. The results are presented, along with future goals to reduce the pulse-width in order to fabricate faster devices.

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목차

Chapter 1 1Introduction 1
Chapter 2 Ultrafast dynamics in nanoscale devices 4
2.1 Carbon nanotube and semiconductor nanowire field effect transistors 4
2.1.1 Carbon nanotubes and silicon nanowires 4
2.1.2 Field effect transistors based on low-dimensional nanomaterials 8
2.2 Carrier dynamics: diffusion vs. drift 11
2.3 Characterization studies of the nanoscale devices using scanning photocurrent microscopy 14
2.4 The basic principles of pump-probe technique 18
2.5 Femtosecond dynamics on 1D NWs 20
2.5.1 Haynes-shockley experiment 20
2.5.2 Carrier dynamics studies using ultrafast pump-probe microscopy 23
2.5.3 Carrier dynamics studies using femtosecond photocurrent microscopy 28
Chapter 3 Device fabrications and imaging methods of carrier dynamics 29
3.1 Device fabrication 29
3.2 Fabrication of SWNT and Si NW FETs 33
3.3 Experiment setup of high-speed scanning photocurrent microscopy 36
3.4 High-speed SPCM images of Si NW FET for various gate voltages 39
3.5 Experiment setup of femtosecond scanning photocurrent microscopy 43
3.6 Conclusion 47
Chapter 4 Imaging ultrafast carrier transport in Si NWand SWNT field-effect transistors 48
4.1 Introduction 48
4.2 2D probe-photocurrent (Ipr) image as a function of time delay 50
4.3 Study of carrier dynamics on Si NW FETs using 2D spatio-temporal imaging 51
4.4 Pump-induced photocurrent changes in the middle of NWs 55
4.5 Transit time measurements on SWNT and grapehene devices 59
4.6 Pump-probe correlation SPCM image 63
4.7 Conclusion 66
Chapter 5 Imaging ultrafast carrier transport at various operating conditions in Si NW and SWNT field-effect transistors 68
5.1 Introduction 68
5.2 Pump-power dependence 69
5.3 Gate-dependent carrier transport phenomena 72
5.4 Source- drain bias voltage dependent carrier transport phenomena 79
5.5 Analysis based on transient drift-diffusion model 83
5.6 Conclusion 87
Conclusion 89
Appendix Suspended single-walled carbon nanotube fluidic sensors 91
A.1 Introduction 91
A.1 Fabrication of SWNT fluid sensors 93
A.2 flow sensing results using a supported SWNT device 95
A.3 flow sensing results for the partially suspended SWNT device 98
A.4 Conclusion 107
References 108
Publications list 121

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