Near-field ultrafast nanoscopy of carrier dynamics in silicon nanowires

doi:10.21203/rs.3.rs-1996133/v1

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Near-field ultrafast nanoscopy of carrier dynamics in silicon nanowires

https://doi.org/10.21203/rs.3.rs-1996133/v1

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Carrier distribution and dynamics in semiconductor materials often govern their physics properties that are critical to functionalities and performance in industrial applications. The continued miniaturization of electronic and photonic devices calls for new tools to probe carrier behavior in semiconductors simultaneously at the picosecond time and nanometer length scales. Here, we develop pump-probe scattering-type scanning near-field optical microscopy (s-SNOM) to characterize the carrier dynamics in semiconductor nanowires. By coupling experiments with the point-dipole model, we resolve the size-dependent photoexcited carrier lifetime in individual silicon nanowires. We further demonstrate local carrier decay time mapping in silicon nanostructures with a sub-50 nm spatial resolution. Our pump-probe s-SNOM enables the nanoimaging of ultrafast carrier kinetics, which is an important step in advancing the future design of a broad range of electronic, photonic, and optoelectronic devices.

Semiconductor technology has stimulated the exponential progress of microelectronics in the last half-century. Silicon-based complementary metal-oxide-semiconductor (CMOS) integrated circuits form the backbone of modern technology¹. In addition to the prevalence in electronic devices, silicon nanostructures are widely exploited in photonics for versatile control of light propagation, detection, and modulation^2–5. Recently, emerging low-dimensional materials, particularly silicon nanowires (SiNWs), have also opened new possibilities for broad applications in electronics, photonics, photovoltaics, and photoelectrochemistry^6–9.

For most semiconductor materials, free-carrier concentration determines their electrical and optical properties, such as conductivity, refractive index, and absorption coefficients^10–12. In addition, the performance of transistors and semiconductor devices primarily depends on the spatial and temporal carrier distribution^13–15. Therefore, resolving the charge transport and carrier dynamics in semiconductor nanostructures is essential for designing future electronic and photonic devices.

Time-resolved terahertz spectroscopy has been widely applied to reveal carrier dynamics in bulk and nanostructured semiconductors^16,17. In addition, pump-probe microscopy was developed to probe the site-specific carrier transport and recombination^18–20. However, the spatial resolution of this microscopic method is limited by optical diffraction. Alternatively, tip-based scattering-type scanning near-field optical microscopy (s-SNOM) is capable of mapping free-carrier distribution in semiconductor nanostructures with a nanoscale resolution^21,22.

Here, we report the spatiotemporal nanoimaging of the carrier dynamics in semiconductor nanowires using pump-probe s-SNOM. We unravel the photoexcited carrier recombination dynamics by combining ultrafast near-field measurements and theoretical modeling. Moreover, this method enables the spatial mapping of carrier lifetime with a sub-50 nm resolution. Our results provide a new route to probe carrier behaviors in nanoscale materials and devices, which is of great significance to understanding the optoelectronic properties and practical functionality of semiconductor nanostructures.

Figure 1a shows the schematic of our pump-probe s-SNOM setup. A 400 nm pump beam and an 800 nm probe beam with a controlled delay time Dt are directed to an atomic force microscope (AFM) tip with an oscillating frequency Ω (see Methods and Supplementary Fig. 1 for more details on the setup). The backscattered probe beam is detected and demodulated at higher harmonics of the tapping frequency (nΩ) to suppress background noise and amplify the near-field signal²³. The samples in this study are SiNWs synthesized via an electroless etching method (Inset in Fig. 1i)²⁴. Figure 1b and Figs. 1c-g show the topography of a SiNW and the time-resolved s-SNOM imaging at different time delays, respectively. The high spatial overlap of the height and s-SNOM profiles indicates the successful detection of near-field signals (Fig. 1h). The s-SNOM time snapshots distinctly exhibit a dynamic change in the intensity of scattered light (Fig. 1j). Specifically, the s-SNOM signal amplitude undergoes an intense increase (Fig. 1d), followed by a decay process (Figs. 1e,f) and a slow rise at longer delay times (Fig. 1g). This trend can also be quantitatively visualized in the transient s-SNOM signal presented in Fig. 1i.

To understand the evolution of the s-SNOM signal over the increasing delay time, we adopt a point-dipole model to analyze the scattered light (see Supplementary Note 1)^22,25,26. Briefly, the scattered field E_sc by the tip is derived from the dipolar near-field interaction between the AFM tip and the sample with the dielectric function e, yielding^25,26

${\mathbf{E}}_{\text{s}\text{c}}\propto A{e}^{i\phi }{\mathbf{E}}_{\text{i}\text{n}}=$$\frac{\alpha \left(1+\beta \right)}{1-\frac{\alpha \beta }{16\pi {\left(a+z\right)}^{3}}}{\mathbf{E}}_{\text{i}\text{n}}$ (1)

Here the measured scattering intensity $S\propto {A}^{2}$, $\phi$ is the phase of scattered light, a is the AFM tip radius, z is the tip-sample distance, $\alpha =4\pi {a}^{3}$ is the polarizability of the tip, and $\beta =(\epsilon -1)/(\epsilon +1)$, respectively (Fig. 2a). The dielectric function e is further calculated as a function of free carrier density N based on the Drude model (Supplementary Fig. 2)²⁷

$$\epsilon (\omega ,N)={\epsilon }_{\infty }-\frac{{{\omega }_{p}\left(N\right)}^{2}}{\omega (\omega +i{\Gamma })}$$

where ${\epsilon }_{\infty }=11.7$ for silicon, ${\omega }_{p}$ is the plasma frequency and a function of the carrier concentration N (Supplementary Note 2), and ${\Gamma }$ is the damping rate. In addition, the effect of temperature is considered by the Jellison-Modine model²⁸ (Supplementary Fig. 3, also see Supplementary Note 2 for more details).

Figure 2b shows the calculated s-SNOM intensity with an increasing free-carrier concentration N from 10¹⁸ to 10²² cm^− 3 at the wavelength of 800 nm. As N increases, the calculated s-SNOM signal exhibits a minimum at N ~ $6\times {10}^{19}$ cm⁻³ and then reaches its maximum at N ~ $1.2\times {10}^{20}$ cm^− 3. This behavior arises from the resonant near-field interaction between the AFM probing tip and the free carriers in SiNWs²⁵. The experimental transient s-SNOM signal can be well fitted by the point-dipole model assuming a biexponential decay of photoexcited carriers, i.e., $N\left(\varDelta t\right)=N\left(0\right)\times ({A}_{1}{e}^{-\frac{\varDelta t}{{t}_{1}}}+{A}_{2}{e}^{-\frac{\varDelta t}{{t}_{2}}})$ (Fig. 2c). The biexponential kinetics (t₁, t₂) are attributed to the carrier recombination and diffusion²⁹, and the average carrier lifetime t_avg can be calculated by ${t}_{\text{a}\text{v}\text{g}}=({A}_{1}+{A}_{2})/({A}_{1}/{t}_{1}+{A}_{2}/{t}_{2})$. The corresponding phase change is also consistent with the experimental results (Supplementary Fig. 4). We exclude the effect of laser heating as the temperature increase is negligible with respect to the s-SNOM amplitude (Supplementary Note 3 and Supplementary Fig. 5). In addition, the variation of tip-sample distance due to thermal expansion of SiNWs only leads to slight changes in the s-SNOM intensity (Supplementary Note 4 and Supplementary Fig. 6). These results verify the dominant role of carrier density in the transient s-SNOM measurements. We further validate the point-dipole model by measuring the same SiNW with different pump power (Fig. 2d), where two curves can be well fitted with different initial carrier densities and the same decay kinetics (Supplementary Table 1).

We then apply pump-probe s-SNOM to investigate the carrier dynamics in individual SiNWs with varying geometries. Figure 3a shows the transient s-SNOM signals measured in different nanowires with various widths. The corresponding scanning electron microscope (SEM) images are shown in Fig. 3b. All experimental curves are well fitted with the point-dipole model, and the decay times are summarized in Fig. 3c. The carrier lifetime shows a linear increase with the increasing size of the SiNWs, indicating that surface recombination dominates in semiconductor nanowires³⁰. The surface recombination velocity (SRV) can be calculated from carrier lifetime t_avg as SRV = d/4t_avg, where d is the SiNW width³¹. The linear fitting gives a surface recombination velocity of 2.2 $\times$ 10⁴ cm/s, which is consistent with previous reports^29,32.

We further explore the capability of pump-probe s-SNOM to probe the spatially resolved carrier dynamics with a nanoscale resolution. A large, nonuniform silicon nanowire is selected as the test sample (Fig. 4a). Transient s-SNOM signals measured at different locations (P1-P4 in Fig. 4a) show distinct temporal evolution (Fig. 4b), corresponding to carrier lifetimes of 460.3, 379.9, 294.6, and 338.0 ps, respectively. The time-resolved s-SNOM images also exhibit spatially nonuniform dynamics of the nanostructure (Supplementary Fig. 7). Figure 4c presents the time-resolved s-SNOM map along the nanowire (dashed line in Fig. 4a). The carrier lifetime extracted from the spatiotemporal mapping is strongly correlated to the topography as evidenced by the qualitatively similar profiles (Fig. 4d). The probe of carrier dynamics at a spatial resolution of 35 nm is demonstrated.

We have developed a pump-probe s-SNOM as a noninvasive technique to examine the carrier dynamics in semiconductor nanostructures. The combination of ultrafast optics and near-field imaging enables the investigation of carrier kinetics with both high temporal (picosecond) and spatial (sub-50 nm) resolutions. This capability is valuable for the characterization and optimization of functional optoelectronic devices. While this work analyzed the carrier dynamics in SiNWs as a demonstration, the pump-probe s-SNOM can serve as a versatile and general optical diagnostic platform to study nanomaterials, including two-dimensional heterostructures³³ and quantum dots³⁴. It also provides a promising tool to study other non-equilibrium thermodynamic phenomena, including phase transitions^35,36, energy and charge transfer³⁷, and phonon propagation³⁸.

Experimental setup. The detailed schematic of the experimental setup is shown in Supplementary Fig. 1. The setup is developed based on a commercial near-field optical microscopy system from Molecular Vista. An 800 nm femtosecond laser beam (Spectra Physics) is split by a 9:1 beam splitter (CVI Laser Optics) into a probe beam (10%) and a pump beam (90%). The pump beam is frequency doubled by a nonlinear crystal, beta Barium borate (Eksma Optics), and then amplitude-modulated by an acoustic-optic modulator. The probe beam passes through a mechanical delay stage (Thorlabs). Both beams are merged and directed to the platinum-coated AFM tip (Molecular Vista) via a parabolic mirror. The optical intensities of pump and probe beams before entering the AFM chamber are ~ 1 mW and ~ 0.5 mW, respectively. The tip has a resonance frequency Ω at ~ 250 kHz and an apex radius of ~ 20 nm). The scattered probe light is then collected by the parabolic mirror and redirected to an avalanche photodiode detector (Thorlabs) after passing a long-pass filter (Thorlabs). The voltage signal from the avalanche photodiode detector (Thorlabs) is sent to a lock-in amplifier for signal demodulation, referenced at a frequency determined by the controller.

Characterizations. All SEM images are taken with a FEI Quanta 650 SEM.

Acknowledgments

We thank Zhengliang Su and Qiye Zheng for their assistance in the experimental setup. C.P.G. acknowledges the financial support from Laser Prismatics under the DOE SBIR Phase 2 grant DE-SC0018461. This work is partly prepared by LLNL under Contract DE-AC52-07NA27344. The nanowire preparation was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Materials Sciences and Engineering Division under Contract No. DE-AC02-05-CH11231 (EMAT program KC1201).

Author Contributions

J.L., Y.R., and C.P.G. conceived the idea and planned the experimental work. J.L., R.Y., and Y.R. worked on the experiments and analyzed the data. P.C. and J.W. synthesized the silicon nanowires. M.E. assisted in the experiments. H.K.P. contributed to the experimental design and apparatus construction. J.L. wrote the paper with inputs from all authors. C.P.G. supervised the project.

Competing interests

The authors declare no competing interests.

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Near-field ultrafast nanoscopy of carrier dynamics in silicon nanowires

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