† Corresponding author. E-mail:
Project supported by the National Natural Science Foundation of China (Grant Nos. 11574408, 11504439, 61627814, and 61675238), the National Key Research and Development Program of China (Grant No. 2017YFB0405402), the National Instrumentation Program of China (Grant No. 2012YQ14000508), and the Young-talent Plan of State Affairs Commission, China (Grant No. 2016-3-02).
The absorption responses of blank silicon and black silicon (silicon with micro/nano-conical surface structures) wafers to an 808-nm continuous-wave (CW) laser are investigated at room temperature by terahertz time-domain spectroscopy. The transmission of the blank silicon shows an appreciable change, from ground state to the pump state, with amplitude varying up to 50%, while that of the black silicon (BS) with different cone sizes is observed to be more stable. Furthermore, the terahertz transmission through BS is observed to be strongly dependent on the size of the conical structure geometry. The conductivities of blank silicon and BS are extracted from the experimental data with and without pumping. The non-photo-excited conductivities increase with increasing frequency and agree well with the Lorentz model, whereas the photo-excited conductivities decrease with increasing frequency and fit well with the Drude–Smith model. Indeed, for BS, the conductivity, electron density and mobility are found to correlate closely with the size of the conical structure. This is attributed to the influence of space confinement on the carrier excitation, that is, the carriers excited at the BS conical structure surface have a stronger localization effect with a backscattering behavior in small-sized microstructures and a higher recombination rate due to increased electron interaction and collision with electrons, interfaces and grain boundaries.
Black silicon (BS) is a term used to describe silicon with micro (and/or nano)-scale conical surface structures that are induced by short and ultrafast laser pulse irradiation or plasma etching.[1–3] Because of its strong absorption in the visible light and infrared regime, BS has drawn a great deal of attention for its optoelectronic applications like light harvesting in solar cells and light-emitting devices.[4,5] The BS formed in SF6, featuring a narrowed band gap due to the sulfur doping, has a broad spectral band of absorption, extending to the THz.[6] For BS structured in air, without band gap modification the strong spectral absorption band ends at 1200 nm, making it potentially useful for enhancing the emission of THz radiation from natural surface emitter.[7] However, the absorption of the enhanced THz emission, changed by BS micro (nano)-structured surface, to the best of our knowledge, has not been comprehensively studied, which is probably because the identifying of localization effects and ultrafast carrier dynamics of micro- (or nano-) structured surface is not trivial, and they exhibit process-dependent and surface-sensitive properties.
Terahertz time-domain spectroscopy (THz-TDS) is an excellent technique for studying optical properties of various materials including semi-conductors and conducting polymers in forms of nanoparticles, nanowires, nanodisks, hollow spheres, etc.[8–13] It probes the far-infrared (30 μm–3 mm, 0.1 THz–10 THz) region of the spectrum, which closely matches typical carrier scattering rates of 1012 s−1 to 1014 s−1.[14–16] Furthermore, THz transmission or absorption is sensitive to the density and transportation of carriers. Thus, THz-TDS can directly infer the influence of micro (nano)-scale disorder and size effect on carrier motion from the response of surface conical structure to electromagnetic field in the terahertz frequency range. In this study, we report the absorption responses of blank silicon and BS wafers to an 808-nm continuous-wave (CW) laser at room temperature by terahertz time-domain spectroscopy system (THz-TDS).
The micro (or nano)-structured surface geometry used in the experiment was fabricated by direct-laser-writing (DLW) technique on a double-side polished 〈100〉 Si wafer with a resistivity of 1000 Ω⋅cm. Compared with the conventional fabrication methods such as plasma or chemical etching, DLW is simple and cost effective.[17–19] In addition, the DLW technique allows for precise control over the dimensions of the surface micro-structures and enables systematic and highly reproducible studies of its geometry-dependent optical properties. In air, the BS was micro-structured by a solid-state nano-second laser (Enpon-Nano-H532) with a 355-nm wavelength and 6-ns pulse duration. The wafers were ultrasonically cleaned in an acetone bath, and then dried in a nitrogen gas flow. The laser fluence was fixed at 11.8 J/cm2. By controlling the laser scanning speed, overlap ratio and intervals between successive laser scan tracks, we obtained four micro-structured BS surfaces (numbered 01, 02, 03 and 04, accordingly), with increasing unity size as shown in Fig.
We measured the total (specular and diffuse) reflectance (R) and transmittance (T) to determine the absorptance (A = 1 − R − T) of the structured surfaces in spectrum range from 620 nm to 1050 nm. The measurements were performed by using a spectrophotometer equipped with an integrating sphere detector. Figure
We characterized absorption tunabilities of blank silicon and BS by using a terahertz time-domain spectroscopy system with a couple of photoconductive antennas for both terahertz generation and detection as shown in Fig.
The transmitted THz spectra of two different structure types, i.e., blank silicon and BS, without and with optical pumping, are shown in Figs.
The BS absorption at 808-nm pump beam is twice more than the blank silicon absorption. Furthermore, the THz-TDS experimental results show that, without pump, the THz transmission is independent of surface micro-structure. Thus, we attribute the abnormal THz absorption by BS under optical pumping to special carrier dynamics in surface micro-structure.
In order to explain the proposed mechanism of photo-generated carrier dynamics and validate our hypothesis, the real part of conductivity of the refractive index n of the sample through the relationship:
What is more, we have calculated the transient frequency-dependent complex photoconductivity parameters
The Drude model is ordinarily suitable for two-dimensional free electron gas with complete momentum randomization following elastic scattering events. However, in the 10-μm photo-excited layer and micro/nano-conical surface structures in BS samples, more photo-excited electrons reflect from interfaces and surfaces. Such a backward scattering event can be modeled by modifying the Drude model according to Smith by including the persistence of velocity parameter to describe the scattering event.[32,33] The Drude Smith model is given by
Based on the curve fitting of photoconductivity σr(ω) to the Drude–Smith model, fitting parameters of electron density N, characteristic scattering time τ, and back-scattering rate c1 are derived and summarized in Table
According to Fig.
The optically induced changes in THz transmission and absorption of silicon wafers with two types of surfaces, i.e., unstructured blank silicon and micro-structured BS, are investigated at room temperature by using an NIR pump-terahertz probe system with an 808-nm CW laser pump. It is shown that BS structure causes not only a strong anti-reflection effect but also significant absorption enhancement at 808-nm wavelength due to localization effect and light trapping. The increases of the surface area and the photo-generated carrier concentration contribute to the absorption enhancement. Remarkably, with the optical pumping, the THz transmission spectra of blank silicon and BS are very different in the whole range of 0.5 THz–2.5 THz. It is proposed based on our measurements along with the model fitting that the carriers excited in BS surface with conical structure should have a stronger localization effect according to the backscattering behaviors in small-size micro-structures and the higher recombination rate due to increased electron interactions with other electrons, the interfaces and grain boundaries. Therefore, in the bigger conical geometry structure the excited carriers may move around more easily and as a consequence less collision would occur, which is related to higher THz absorption. Our work paves the way for a better understanding of the influence of the different geometrical- and optical-induced absorption of THz wave and the future applications of the black silicon.
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