THz emission was observed from the vertically aligned silicon nanowire (Si NW) arrays, upon the excitation using a fs Ti-sapphire laser pulse (800 nm). The Si NWs (length = 0.3 ~9 μm) were synthesized by the chemical etching of n-type silicon substrates. The THz emission exhibits significant length dependence; the intensity increases sharply up to a length of 3 μm and then almost saturates. Their efficient THz emission is attributed to strong local field enhancement by coherent surface plasmons, with distinctive geometry dependence.
©2010 Optical Society of America
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 . 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 . There was also a report on the enhanced THz emission from porous InP membranes .
Recently, Si nanowire (Si NW) array was successfully synthesized by a metal-nanoparticle-assisted 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 [8–14]. In fact, flat Si surface has 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 “black Si”, was investigated for higher efficiency solar cells and other optoelectronic devices [15–17]. It was reported that these black Si can be an efficient emitter of THz radiation, since the surface of Si grains has the advantage of an extremely high absorption over a wide wavelength range . Therefore, it is expected that the uniformly-grown vertically-aligned Si NW arrays can be practically developed as more efficient THz emitters than the black Si, and provide a fundamental insight for the LSPs of 1D semiconductor nanostructures. Herein, we measured the strong THz emission from the vertically-aligned n-type Si NW array (synthesized by a metal-nanoparticle-assisted catalytic etching technique), showing a remarkable length-dependence behaviors in the range of 0.3 - 9 μm.
2. Sample preparation & experimental setup
The (100) Si wafer (n-type 1-20 Ωcm) pieces (size = 2 × 2 cm2) were etched with 5% HF aqueous solution, and immediately placed into a 0.005 M AgNO3/4.8 M HF solution for 1 min to form a uniform Ag nanoparticle coating . They were washed with water to remove the extra Ag ions and then immersed in a 4.8 M HF/0.4 M H2O2 etchant. After 5-30 min etching in the dark, the wafers were washed repeatedly with water and immersed in dilute HNO3 aqueous solution to dissolve the Ag catalysts. The wafer surfaces were in deep black, and their back sides in gray. The wafers were washed with 5% HF again to remove the oxide layer and then cleaned with water and dried under N2 flow. The NW products were analyzed by scanning electron microscopy (SEM, Hitachi S-4700), field-emission transmission electron microscopy (TEM, Jeol JEM 2100F and FEI TECNAI G2 200 kV), and high-voltage TEM (HVEM, Jeol JEM ARM 1300S, 1.25 MV). The length was controlled in the range from 0.3 to 9 μm, using an etching time.
Figure 1a shows the SEM micrograph of high-density 3 μm-long Si NW array. The TEM images reveal that the surface of the Si NWs is smooth and their average diameter is 150 ± 30 nm (Fig. 1b and inset). They consist of highly crystalline Si nanocrystals grown along the  direction. The distance between the (200) planes is about 2.7 Å, which is consistent with that of cubic Si (JCPDS Card No. 80-0018; a = 5.392 Å). The bottom inset shows the corresponding Fast Fourier-transform (FFT) electron diffraction (ED) pattern generated from the inversion of the TEM image using Digital Micrograph GMS1.4 software (Gatan Inc.). Figure 1c shows the side viewed SEM micrograph of the Si NW array samples having the uniform length of 0.3 ± 0.1, 1 ± 0.2, 2 ± 0.2, 3 ± 0.3, 5 ± 0.5, and 9 ± 1 μm. The TEM images show that the average diameter is nearly the same, irrespective to the length.
Figure 2 shows a schematic diagram for the experimental setup used to detect the THz emission from the samples using reflection geometry. A Ti:sapphire laser is used as a light source, operating at a center wavelength of 800 nm with a repetition rate of 76 MHz and a pulse width of 190 fs. The polarization was parallel to the plane of incidence for best pump absorption, using a half-wave plate. The laser beam passes through a half-wave plate, after being reflected by mirrors, and is split into a pump beam and a probe beam by a beam splitter. The pump beam goes through a chopper, while the probe beam is delayed with respect to the pump beam by means of a scanning optical delay line. A variable attenuator is used for the pump beam to vary the excitation power. The laser spot size is approximately 3 mm at the surface of the sample. The THz radiation from the surface of the sample oriented at an angle of incidence of 45° is collected and focused onto the detector using a pair of parabolic mirrors. The detection antenna was a Hertzian dipole antenna, which had a gap of 5 μm on low temperature-grown GaAs (LT-GaAs). The optical power (probe beam) incident on the LT-GaAs receiver is 5 mW.
3. Results and discussion
Figure 3a displays the time-domain waveform of THz emission from the Si wafer and 0.3~9 μm-long Si NW array, upon an irradiation of maximum-power (450 mW) pumping fs Ti:sapphire laser pulses. The THz emission from the Si wafer is much weaker than that from the NW samples. Figure 3b shows their corresponding Fourier-transform frequency spectrum of THz radiation (in logarithm scale). Figure 3c represents the peak intensity of THz radiation as function of NW length, indicating a sharp increase up to a length of 3 μm and almost saturation afterwards. Figure 3d displays the dependence of the THz emission intensity on theincident pump beam power (100 - 450 mW), measured from the 1, 3 and 7 μm-long Si NW array. The THz radiation intensity increases almost linearly with the incident pump-beam power.
When photocarriers are excited by ultrashort laser pulses, the ultrafast charge transport is driven by the intrinsic surface depletion field of the semiconductor and/or by the photo- Dember field originated from the difference of the diffusion velocities between the electrons and holes [18–20]. This impulsive field change can produce coherent plasma oscillations, generating the THz radiation. Since the Si NW array have the surface parallel to the direction of charge transport, the dipole radiation will emit perpendicular to the NW length, not suffer total internal reflection . Therefore, the Si NWs can act as efficient antennas due to a strong field enhancement by the coherent LSPs, as the incident electric field of the laser is parallel to the NW axis . Additionally, in the vertically-aligned NW array structure, the multiple reflections along the surface can take place and lead to a strong pump beam absorption where the confinement of the electron-hole pair should result in large changes of the local potential. As a result, the stronger optical absorption of the longer Si NW array produces the higher-intensity THz emission.
The damping mechanism is inherent importance when plasma oscillations are used to generate THz pulses. Since the plasma oscillations are collective excitations, the nonradiative lifetime of the plasmons is the damping time of the collective carrier motion. Hasselbeck et al. reported the observation of Landau damping in the InSb top layers separated from the (100) InSb substrate with a 40 nm barrier of Al0.1In0.9Sb . As the thickness of the InSb top layer decreased from 1500 to 200 nm, collective plasma behavior is lost - the plasma becomes Landau damped. They also reported the loss of THz emission signal in the short InAs NWs and explained it by a strong Landau damping - spatial confinement that forces the electrons to interact primarily as individual nanosize wires . Therefore, we suggest that the reduced intensity of the shorter Si NWs would be dominated by such Landau-damped plasma. The maximum THz radiation intensity is observed for the 3 μm Si NW arrays. In order to get evidence for the photon absorption, we measured the UV-visible absorption spectra of the Si NW array having a different length, as shown in Fig. 4a . A UV-visible-NIR absorption spectrometer (Varian, Cary 1000) was used to measure the absorption % of the Si NW array in a reflectance mode. The absorption (%) at a wavelength of 800 nm (excitation wavelength) as a function of NW length shows a continuous increase up to the length of ~3 μm, and almost saturation (~100%) afterwards (Fig. 4b). It indicates that the Si NWs longer than ~3 μm are not able to absorb more photons and thus cannot generate more carriers. As a result, the 3 μm appears as an optimum length for maximum THz emission, due to the complete absorption of the pumping laser power.
The THz radiation due to Dember photocurrent is proportional to not only the difference in the mobility of the electrons and holes and the gradient of the carrier density, but also the pump intensity . The THz radiation intensity from black Si increases almost linearly with the incident pump-beam power (0 - 2 W), due to the photo-Dember effect . Therefore the linear power dependence of the present Si NW array is compatible with such photo-Dember effect. This linear power dependence of the THz emission intensity implies that the maximum pump-beam power of 450 mW is below the saturation level for the 3 μm-long Si NW array. Thus the shorter Si NW sample has potential to produce the stronger THz radiation, when more intense excitation source used.
In summary, we measured the THz emission of the vertically-aligned n-type Si NW arrays. The Si NWs were synthesized with a controlled length (0.3 ~9 μm), by the Ag-nanoparticle-assisted catalytic etching technique. The intensity of the THz emission from the Si NW arrays increased sharply for a length up to 3 μm and then almost saturated. The maximum intensity of the THz emission at the length of 3 μm can be ascribed to the complete absorption of pumping power. A strong field enhancement by coherent LSPs induces the efficient THz emission perpendicular to the NW surface. The THz spectrum measurement can provide valuable information for the geometry dependence of the LSPs when building most efficient NW-based optoelectronic devices.
This study was supported by the WCU (World Class University) program through the NRF funded by the Ministry of Education, Science, and Technology (R31-10035), and MKE under the auspices of the ITRC support program supervised by the IITA (2008-C1090-0804-0013). This work was supported by the Photonics 2020 research project through a grant provided by GIST (Gwangju Institute of Science Technology). The HVEM (Daejeon), XRD (Taegu), and XPS (Pusan) measurements were performed at the KBSI. The experiments at the PLS were partially supported by MOST and POSTECH.
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