Subsurface Nanoimaging by Broadband Terahertz Pulse Near-Field

Dec 1, 2014 - A review. Time-resolved, pulsed terahertz spectroscopy has developed into a powerful tool to study charge carrier dynamics in semiconduc...
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Subsurface Nanoimaging by Broadband Terahertz Pulse Near-Field Microscopy Kiwon Moon, Hongkyu Park, Jeonghoi Kim, Youngwoong Do, Soonsung Lee, Gyuseok Lee, Hyeona Kang, and Haewook Han* Department of Electrical and Computer Engineering, POSTECH, San 31, Hyoja-dong, Nam-gu, Pohang, Kyungbuk 790-784, Korea ABSTRACT: Combined with terahertz (THz) time-domain spectroscopy, THz near-field microscopy based on an atomic force microscope is a technique that, while challenging to implement, is invaluable for probing low-energy light-matter interactions of solid-state and biomolecular nanostructures, which are usually embedded in background media. Here, we experimentally demonstrate a broadband THz pulse near-field microscope that provides subsurface nanoimaging of a metallic grating embedded in a dielectric film. The THz near-field microscope can obtain broadband nanoimaging of the subsurface grating with a nearly frequency-independent lateral resolution of 90 nm, corresponding to ∼λ/ 3300, at 1 THz, while the AFM only provides a flat surface topography. KEYWORDS: Terahertz, near-field microscopy, subsurface nanoimaging, nanospectroscopy

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nanospectroscopy.20−23 Such a TDS-based s-SNOM is important because high-power continuous-wave sources are usually neither continuously tunable17 nor available to many researchers.18 Moreover, a submicrometer resolution has been demonstrated only by s-SNOM based on THz-TDS20,21 and a semianalytic model has also been developed for the quantitative analysis of image contrast.28−31 These TDS-based s-SNOMs can easily be used for the pump−probe spectroscopy of lowenergy dynamics with nanoscale resolution. Recently, infrared subsurface near-field imaging has been demonstrated using sSNOM techniques.32−36 Extending such a near-field imaging technique to the THz region would result in a breakthrough in studying the low energy dynamics of solid-state and biomolecular nanostructures because these nanostructures are usually embedded in barrier layers. In this work, we experimentally demonstrate broadband THz near-field nanoimaging of a subsurface metallic grating using the s-SNOM based on THz-TDS. Our THz s-SNOM system was constructed by combining a homemade tapping-mode AFM and a conventional THz-TDS system. A schematic depicting the principle of operation is shown in Figure 1. Using off-axis parabolic mirrors, the incident THz pulse was focused on an AFM probe. The pulse scattered from the probe tip was focused on a THz photoconductive antenna to measure the far-field. To attain nanoscale resolution we fabricated a tungsten probe with an apex diameter of less than 100 nm by using an electrochemical etching method.37 The probe was attached to one prong of a quartz tuning fork, which was mechanically dithered using a phase locked loop (PLL). The time constant for the lock-in detection was ∼300

erahertz time-domain spectroscopy (THz-TDS) has recently been a powerful tool for probing fundamental low-energy dynamic processes in solid-state materials and devices. Understanding such low-energy THz dynamics has become crucial for developing next-generation electronic and optical devices.1−6 To realize THz-TDS with subwavelength resolution, various types of near-field imaging techniques have been combined with THz-TDS.7−24 By using a hollow metallic aperture a resolution of 3 μm was achieved.7 A dynamic aperture, generated by a focused laser beam on a semiconductor substrate, has also been used to achieve microscale resolution.8 Electro-optic crystals have been widely used as near-field probes, enabling vectorial near-field mapping for studying nanogaps and metamaterials.9−11 However, the electro-optic approach has a fundamental limitation because it requires the sample to be located directly on the surface of the electro-optic crystal, and the lateral resolution is limited to the diameter of the focused laser beam. At present, scattering-type scanning near-field optical microscopy (s-SNOM) seems to be the most viable technique that can offer nanoscale resolution and broadband THz spectroscopy simultaneously.17−24 In the s-SNOM technique, the metal tip of an atomic force microscope (AFM) is used as a near-field probe, which is illuminated by the incident THz field. Because of the lightningrod effect, the incident field is localized approximately to the size of the tip apex.24−27 However, the SNOM signal becomes very weak as the probe size is reduced to a submicrometer scale for higher resolution. Hence, intense continuous-wave THz lasers have been used to study the local carrier density of semiconductor devices. 17 Otherwise, THz intersubband dynamics of InAs quantum dots have been investigated using the absorption spectra from infrared s-SNOM.18 On the other hand, there have also been many efforts to adapt a conventional THz-TDS system so that it can be used for broadband coherent © 2014 American Chemical Society

Received: October 17, 2014 Revised: November 19, 2014 Published: December 1, 2014 549

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Letter

Figure 1. THz s-SNOM system. The tip−sample distance was dithered at the oscillation frequency of a quartz tuning fork and precisely controlled by a tapping-mode AFM system.

Figure 2. Peak intensity images from approach curves above the sample surface. (a) E1, (b) E2, and (c) E3 (d,e) approach curves on the Au and Si sections. The size of the images is 800 nm by 200 nm. The interaction distances were 38 nm (88 nm), 15 nm (26 nm), and 13 nm (18 nm) for E1, E2, and E3, respectively, where the dotted lines shows the e−1 levels relative to the maximum signals.

ms. As a result, the probe-sample distance was precisely modulated by h(t) = d0 + a(1 − cos Ωt)/2, where a, d0, and Ω are the modulation depth (∼100 nm), the minimum probesample distance, and the modulation frequency (27 kHz), respectively. The minimum probe-sample distance d0 was precisely controlled by the tapping-mode AFM system. The entire experimental setup was enclosed in a dry-air purged chamber. The total electric field at the probe tip is the sum of the incident field (Ei), the specularly reflected field (Er) at the sample surface, and the scattered field (Es) that includes all the information about near-field interaction.20−22 The scattered field is a nonlinear function of the probe-sample distance, and thus Es is the sum of the harmonics of Ω. To extract Es from Etot, the nth-harmonic component (En) of the photocurrent in the THz antenna was measured by demodulation at nΩ.17−22 We used a metallic grating embedded in a dielectric layer. A 30 nm thick gold grating with a period of 800 nm was fabricated on an insulating Si substrate using holographic lithography38 followed by e-beam evaporation. After depositing a Si3N4 layer, the sample surface was flattened using a chemical-mechanical polishing process. Figure 2 shows the line-scan images for the peak intensities of scattered fields using the approach curves of E1, E2, and E3 where the subsurface grating depth was ∼30 nm. The depth of the embedded gold grating varied over the sample surface, and moving to a position with a thicker Si3N4 layer resulted in a slightly reduced image contrast. Because the depth of the grating was much smaller than the THz wavelength, the multiple reflections due to the multilayer sample structures were not observed. The scattered signal only became significant when the probe was near the surface, which is clear evidence of near-field interaction. We also measured the probe-sample interaction distance at which the signal reduced to 1/e with respect to the maximum signal. This means that clear near-field imaging is possible only when the probe-to-sample distance is shorter than the interaction distance. Above the Au (Si) section, the interaction lengths were 38 nm (88 nm), 15 nm (26 nm), and 13 nm (18 nm) for E1, E2, and E3, respectively. The near-field imaging contrast decreased with increasing demodulation order. If the resolution is important, E2 and E3 may be preferred to E1. However, for subsurface imaging with a thick coating layer, E1 seems to be the reasonable choice

because the interaction distance is much longer than the highorder demodulations. The near-field peak intensity images of E1, E2, and E3, along with the AFM topographic image, are shown in Figure 3. Because of the polishing process, no sign of the subsurface grating structure was observed in the AFM image. However, in the near-field images the metallic grating under the flat surface was clearly revealed. The subsurface lateral resolutions were estimated to be 90, 90, and 80 nm for E1, E2, and E3, respectively. The small spatial offsets in the E1, E2, and E3 images are due to the long-term drift of the nanostage during the measurements. In the AFM image, we observed small protrusions, which were clearly imaged in the near-field images, as well. The protrusions are due to the silica residuals of the polishing process. With these protrusions, the resolution is better than 40 nm, but this is a result of the surface topography rather than an effect of constituent material. Therefore, we ignore the protrusions and conclude that the resolution of our THz s-SNOM is ∼90 nm for the near-field imaging. In general, the scattered signal intensity strongly depends not only on d0 but also on the local surface profile and the material distribution. This means that even tiny nanometer-sized surface protrusions can cause large changes in the scattering signal, leading to topographical artifacts.39,40 Therefore, for near-field imaging of nanostructures we need to precisely control the probe-sample distance using an AFM system. We found that the lateral resolution was not enhanced by the higher-order demodulation. Compared with our previous study,20 this is an unexpected result, but it can be explained as follows. In a tapping-mode AFM, the dithering direction is orthogonal to the surface. Therefore, by the nonlinear nature of the probe-sample interaction, the interaction distance decreases with increasing demodulation order. This effectively enhances the near-field confinement in the normal direction to the sample surface. However, the lateral near-field confinement is mostly determined by the probe size rather than the demodulation order, particularly for subsurface imaging, because higher-order demodulation has a shorter interaction length. For flat but compositionally inhomogeneous samples, the high-order demodulation does not improve the lateral 550

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Figure 4. (a) Spatiotemporal image of E1(x, td) and (b) spatio-spectral image of E1(x, ω).

frequency-independent, which has not been achieved with THz s-SNOM. At THz frequencies, Au is a good conductor, and insulating Si has nearly constant refractive index with small attenuation. Moreover, the grating period is much smaller than the THz wavelength, which means that the metal grating becomes nonresonant. Therefore, the spectral images of Au and Si regions do not have any spectral features. In our measurements, the scattered THz pulses exhibit no transient oscillations within a time delay of ∼5 ps after the main peak. Therefore, the spectral images are very smooth and clean. This is very important for spectroscopic near-field material imaging and recognition because the reliable spectroscopic analysis of near-field images is difficult with transient oscillations. We have demonstrated subsurface broadband THz near-field imaging of an embedded metallic grating. The lateral resolution of the first-order demodulation was ∼90 nm, which is nearly frequency-independent, and is comparable to those of the second- and third-order demodulations. We also defined and measured the probe-sample interaction length, which is a measure of the strength of the probe-sample interaction, to determine the depth limit of subsurface near-field imaging. It seems that the first-order demodulation is preferred to higherorder demodulations for subsurface imaging, particularly for nanostructures embedded in thick films because the first-order demodulation has the longest interaction length, and a lateral resolution comparable to the higher-order demodulation. We expect that subsurface broadband THz near-field imaging will become an important tool for studying THz dynamics in nanoscale devices and nanomaterials because many of these nanostructures are embedded in host materials.

Figure 3. THz near-field peak intensity images for (a) E1, (b) E2, (c) E3, and (d) AFM topography, where the scale bar is 500 nm. (e) Nearfield profiles along the dashed lines in (a−c), where the blue, green, and red curves represent E1, E2, and E3, respectively. Topographic artifacts are observed at surface protrusions marked with dashed circles.

resolution. However, if the material change coincides with topographic step the resolution can be enhanced by the highorder demodulation because the material contrast is mixed with the topographic effect. For example, in our previous study20 a Si wafer was partially coated by a 30 nm thick gold layer. When the probe encountered the boundary from the gold side of the surface, the effective probe-sample distance immediately increased at the Au−Si boundary, and the signal intensity dropped due to the topography. This effect becomes significant as the demodulation order is increased. Therefore, we conclude that in the previous study,20 the resolution enhancement was partially caused by topographical effects. Note also that in THz s-SNOM, the high-order demodulation is not mandatory because background artifacts are negligible with the demodulation at Ω.20,28 The spatiotemporal and spatio-spectral images are shown in Figure 4, where the scanning was performed along the dashed line in Figure 3. The spatio-spectral image of E1(x, ω) was obtained from the Fourier transformation of E1(x, td). The lateral resolution of the spatio-spectral image was also nearly 551

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Present Addresses

(K.M.) Electronics and Telecommunications Research Institute, Daejeon, Korea. E-mail: [email protected]. (H.P.) Samsung Advanced Research Institute of Technology, Suwon, Korea. E-mail: [email protected]/ (J.K.) Samsung Electronics, Hwasung, Korea. E-mail: jeonghoi. [email protected]. Author Contributions

H.H. and K.M. conceived the experiments. K.M., H.P., J.K., and Y.D. performed experiments. K.M., Y.D., S.L., G.L., and H.K. analyzed the data. H.H., K.M., and Y.D. wrote the paper. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported in part by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No. 2009-0083512).

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