Terahertz Emission from Vertically-aligned Silicon Nanowires

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Terahertz Emission from Vertically-aligned Silicon Nanowires Yong Jae Cho, Gyeong Bok Jung, Yoon Myung, Han Sung Kim, Young Suk Seo, and Jeunghee Park * Department of Materials Chemistry, Korea University, Jochiwon, 339-700, Korea, Republic of ABSTRACT Large-area vertically aligned silicon nanowire (Si NW) arrays were synthesized with a c ontrolled length (0.3 ~ 9 µm) by the chemical etching of n-type silicon substrates. Upon their excitation using a fs Ti-sapphire laser pulse (800 nm), their THz emission intensity exhibits strong dependence on their length; the intensity increases sharply up to a length of 3 µm and then decreases slightly, due to the complete absorption of the optical pum p power. The Raman scattering spectrum exhibits the same behavior as that of the THz emission. We suggest that the field enhancement by localized surface plasmons induces more efficient THz emission or Raman scattering for the longer Si NWs. The photocurre nt measured in a photoelectrochemical cell showed consistently the length dependence wit h a maximum value at the length of 5 µm. INTRODUCTION One-dimensional (1D) nanostructures such as nanowires (NWs) and nanobelts have emerged as one of the most important building blocks for nanotechnology.1-3 Success of various applications as high-performance optoelectronic devices, field-effect transistors, logic circuits, nonvolatile memories, and biosensors, was reported in enormous pioneered works. Furthermore, they also enabled important basic studies on the size and shape-dependent nanosize effect, which determine their unique electrical or optical properties. Lately, the semiconducting nanostructures were suggested to become an efficient terahertz (THz) radiation emitter, due to the collective oscillations of conductive electrons, confined to the surface of nanostructure, so called localized surface plasmons (LSPs).4-7 Seletskiy et al. reported the enhanced emission of THz radiation from free-standing InAs nanowires (NWs), compared to that of the bulk.5 They suggested the high-efficiency dipole radiation perpendicular to the NW length, by having its surface parallel to the direction of charge transport. The enhanced THz emission of nanosize semiconductors was also investigated by a few number of research groups. He et al. reported the enhancement of the THz emission from the surface of nanosize ZnSe grains and attributed it to the local field enhancement effect.6 There was also a report on the enhanced THz emission from porous InP membranes.7

Recently, Si nanowire (Si NW) array was successfully synthesized by a metal-nanoparticleassisted catalytic etching technique, and their promising applications such as the photovoltaic cells or lithium ion battery, biosensors were also investigated by a number of research groups.8-19 In fact, flat Si surface have a high natural reflectivity with a strong spectral dependence, so is not suitable for optoelectronic devices. In order to minimize the reflection losses, subwavelength (including nanoscale) surface texture, so called “blac

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