1. Introduction

Over the past few decades, the semiconductor silicon (Si) has been universally recognized as one of the basic materials for optoelectronic devices in the modern electronics industry due to its abundant reserves and exceptional optoelectronic performance. Exhibiting remarkable optoelectronic properties and physical and chemical stability, Si plays crucial roles in various applications, including optoelectronic detectors [1–3], image sensors [4,5], solar cells [6–10], and more. However, commercial Si still suffers from limitations, such as its narrow band gap of 1.12 eV, rendering it transparent to incident light with wavelengths exceeding 1100 nm, which severely restricts its application in the infrared range. Additionally, its intrinsic high reflectivity, even within its photosensitive wavelength of 300∼1000 nm, reaching up to 40%; when coming to the infrared range beyond its band gap absorption edge, it can surpass 60%, which results in significant energy loss, ultimately compromising its photosensitivity and quantum efficiency. Although the narrow bandgap materials, such as InGaAs, HgCdTe, and graphite, have been extensively studied and applied in the field of optoelectronic detectors, their high cost may potentially limit practical applications in the future [11–15]. Therefore, exploring methods to narrow the band gap of Si and enhance its infrared absorption may be the key to broadening its application.

Black Si, a derivative material of Si, exhibiting unique optical and electrical properties due to the light-trapping effect, is potential to be an ideal candidate material for photodetector devices. Since the processed surface turns into deep black due to the extremely low reflection results from the produced periodic microstructures, it is thus called black Si. As the processed layer is covered with micro/nano-structures, the black surface effectively extends the travel path of the incident light, and thus provides the photons with more opportunities to penetrate Si and then be absorbed, which is supposed to exhibit excellent photoresponse in the sensing detection. Since it was discovered by Mazur’s group in 1998 [16], black Si has garnered significant attention and undergone extensive research due to its remarkable potential in the fields of photoelectric conversion and communication [17,18]. The preparation methods of black Si can mainly be divided into wet and dry processes. Specifically, wet processes include chemical etching [19–21], while dry processes encompass laser etching [18,22] and reactive ion etching [23,24]. Among these methods, the technique for black Si processing by femtosecond (fs) laser ablation stands out because of the high processing accuracy. Given that the pulse width of the fs laser is only about hundreds of femtoseconds, the thermal effect during the laser ablation process can be efficiently avoided. Hyperdoping black Si processed in a specific atmosphere such as SF₆ has been widely investigated for these years, and numerous outstanding research achievements have emerged [25–29]. However, apart from the high cost and environmental unfriendliness resulting from the toxic nature of SF₆, the industrial application of the hyperdoped black Si in infrared detection is constrained by several issues. Firstly, the material’s background of extremely high free carrier concentration and complex structural defects, although it is beneficial to narrow the band gap, it may directly enhance noise in the real infrared detection for the Si-based photodetectors [30,31]. Secondly, chalcogenide doped black Si fabricated in the atmosphere tends to exhibit poor thermal stability. Following annealing, the enhanced absorption observed in black Si may be diminished again as the structural defects repaired [32,33]. Therefore, in order to circumvent the challenges associated with chalcogenide hyperdoping, we propose a simple processing method for the fabrication of black Si, employing fs laser processing in water. It was reported [34] that the thermal conductivity of water exceeds the conductivity of a standard gas by more than one order of magnitude, and the water vaporizes and dissociates upon contact with the substrate surface. Due to the higher thermal conductivity of water, coupled with its vaporization and dissociation processes, a significant amount of heat is transferred away from the molten layer, thereby reducing its lifespan in water. The presence of water and the subsequent acceleration in the cooling rate of the molten layer facilitate the freezing of nanometer-scale patterns onto the surface before dissipation, thereby enabling the formation of low spatial frequency ripples that are unattainable in gaseous conditions.

In this study, black Si with multi-scale surface structures is directly fabricated using fs pulses under water. Experimental results reveal that the surface morphology of black Si undergoes a unique evolution as the ablation laser fluence increases. Notably, higher pulse energy yields the gradual emergence of multi-scale laser-induced periodical surface structures (LIPSSs), characterized by the presence of low spatial frequency ripples, in general termed as ‘ripples’ and micron sized grooves. The optical properties of generated black Si have been thoroughly examined, demonstrating a significant reduction in reflectance and a remarkable increase in absorptance as compared to unprocessed Si. It is worthy to note that the black Si exhibits a responsivity of approximately 50.2 mA/W under a bias of 10 V when exposed to incident light with a wavelength of 1550 nm. Furthermore, the external quantum efficiency (EQE) of this device attains an impressive value of 4.02%, which is another crucial factor in optoelectronic devices.

2. Materials and methodology

Figure 1 reports the schematic diagram of the experimental setup. The fs laser pulses are generated by an amplified Yb:KGW laser system (PHAROS-2mJ, Light Conversion, Lithuania), producing linearly polarized pulses with a wavelength of 1030 nm and a duration of 210 fs. The laser system has a maximum repetition rate of 200 kHz. A frequency converter (HIRO PH1F3) is employed to generate laser wavelength at 515 nm. The desired number of laser pulses is selected using an electromechanical shutter (Vincent Associates, LS6). A beam expander consists of a concave and a convex lens is used is used to increase the diameter of the laser beam. The beam is then filtered by a 4f system, where it is focused on the center of an iris using a convex lens and subsequently made parallel by another convex lens.. Furthermore, the pulse energy is regulated using a system comprising a half-wave plate (HWP) and a Glan-Taylor polarizer (GTP). A beam splitter is placed in the beam path, redirecting a part of the beam towards a beam analyzer (DataRay, WinCamD-LCM CMOS beam profiler) to evaluate the quality of the beam before irradiation.

 

Fig. 1. Schematic diagram of the fs laser ablation experimental setup for the black Si in water.

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The black Si is processed under water environment by the femtosecond laser, using a custom setup, and the corresponding schematic is shown in Supplement 1, Fig. S1. The underwater irradiation setup consists of a water source from which the water is transported to the processing container via a conduit, facilitated by the force of gravity. To regulate the water flow rate, a throttling valve is employed, ensuring a controlled flow speed of approximately 100 mm³/s. The container where the sample is processed, is made from Polyvinyl chloride. This cube shaped container of size 20 × 20 × 15 mm³is designed to securely hold the sample target with the assistance of two diagonally placed clamps. The water inlet and outlet are located on both sides of the container, both elevated 5 mm above the sample surface. Consequently, the fixed water level within the container remains at 5 mm, while the volume of water consistently measures 2 mL. Lastly, the waste-water containing the processing debris is collected in a designated waste water collection unit after the processing container.

To fabricated black Si, a commercially available N-type (111) Si target (resistivity of 20-35 Ω/cm), measuring 10 × 10 mm and a thickness of 500 µm is used. The target is placed in a container mounted on a three-dimensional stage (MMC-110, Micronix, USA), which offers a minimum precision of 100 nm and a maximum translation speed of 2.5 mm·s⁻¹. During laser processing, the sample is irradiated with laser pulses at a frequency of 1kHz, a fluence of 0.03 J·cm⁻², 0.09 J·cm⁻², and 0.18 J·cm⁻² respectively for three different samples, and a scanning speed of 2.5 mm/s with a step separation of 10 µm between two adjacent lines. The final focused laser beam size on the target is around 40 µm in diameter, and every 16 pulses are shot at the same position. After laser treatment, the sample is thermally annealed for 30 minutes in N₂ gas within a muffle furnace at a temperature of 873 K.

The morphology of the sample is analyzed using 3D confocal microscopy (Smartproof 5, Carl Zeiss, Germany) and SEM (JSM-7500F, JEOL). The reflection and absorption of the sample in the spectral range of 200-2500 nm is characterized using a spectrophotometer (Lambda 950, Perkin Elmer, USA) equipped with an integrating sphere (LISR-UV). The sample undergoes Raman spectroscopy, and Photoluminescence (PL) spectroscopy (FTPL10) using a Raman spectrometer (XploRA Plus, HORIBA Jobin Yvon, France) with a 5 mW excitation source at a wavelength of 532 nm. An X-ray photoelectron spectrometer (XPS, ESCALAB250Xi, ThermoFisher Scientific, USA) by Mg Kα irradiation and an energy-dispersive spectrometer (EDS model EDAX by AMETEK) are also used to analyze the elemental composition. In addition, 300 nm thick Au electrodes are evaporated onto the sample surface to facilitate photodetection. To enhance the contact between the Au electrode and the black Si, a thermal annealing process is conducted at 773 K in an N₂ gas environment for a duration of 1 min. Subsequently, the photoelectric characteristics of the metal-semiconductor-metal (MSM) infrared detector device are assessed using a Source Meter Unit Instrument (Keithley 2635B, a Tektronix company, USA).

3. Results and discussion

This study investigates the photoelectric characteristics of black Si fabricated by fs laser in water. Initially, the structure and morphology of the obtained black Si are illustrated using SEM and 3D confocal microscopy. Additionally, Raman and PL spectroscopy as well as the XPS and EDS are employed to analysis the phase transition and elements composition. Subsequently, the absorptance and reflectance of black Si are measured and compared with the unprocessed Si. Finally, a MSM infrared detector is successfully fabricated utilizing the black Si, followed by a comprehensive examination of its optoelectronic properties.

3.1 Topographical properties and structural characteristics

Figure 2(a) ∼ (f) showcase SEM images and 3D confocal microscopy images illustrating the surface morphology of black Si, clearly depicting the structural evolution of the LIPSSs as the fs laser fluence increases. Considering the black Si is processed in a liquid water environment, the external temperature drops faster than the internal part of the ablated Si, resulting in the formation of a surface tension gradient. The heat flow induced by the gradient may promote the formation of elongated-shaped LIPSSs on the surface [35]. When Si is irradiated by a fs laser, the energy is mainly conducted to the electrons rather than the lattices, which leads to an increase in the concentration of excited electrons. Once the concentration of excited electrons attains a critical threshold, electromagnetic waves emerge. This phenomenon triggers a transformation on the surface of Si, converting it into a metallic state and giving rise to surface plasmon polaritons (SPPs) [36]. The interaction between SPPs and subsequent laser pulses contribute to the formation of the LIPSSs. As displayed in Fig. 2(a), the black Si exhibits quasi-regular, sub-wavelength ripples with an arrangement direction perpendicular to the polarization of the fs laser at a fluence of 0.03 J·cm⁻². The period of the ripples, $\mathrm{\Lambda }$, is approximately 240 ± 30 nm, significantly shorter than the wavelength of the incident laser light, λ = 515 nm. When the incident laser increases to a fluence of 0.09 J·cm⁻², as reported in Fig. 2(b), wide stripes begin to emerge underneath the surface, accompanied by the separation of ripples along the direction perpendicular to the ripples. These stripes, called grooves in the latter content, are therefore oriented approximately parallel to the laser polarization direction with longer period of approximately Λ=2 µm, evidencing a multi-scale structures formation of the black Si. This multi-scale surface structures may potentially generate a synergistic effect on the anti-reflection property, thus enhancing absorption. Figure 2(d) presents a zoomed-in view, revealing a high-resolution image of the perpendicular ripples formed atop the wide grooves, which offers a clear demonstration of the bi-scale structures exhibited by the black Si. In compared to subwavelength ripples, the formation mechanism of multi-scale surface structures also consisting of grooves follows a distinct process. The previously formed ripples can serve as a layer with a progressively changing refractive index which may facilitate the immersion of incident light into the deeper part of the black Si layer [37], leading to an uneven distribution of the harvested energy between surface ripples and the bulk. By accurately controlling the incident laser energy, the interaction between the fs laser and SPPs can be reinstated on the ripple-structured surface, thereby inducing the formation of micron sized grooves along the direction perpendicular to ripples [36]. Up on increasing the laser fluence to 0.18 J·cm⁻², as shown in Fig. 2(c), the orientation of the long-range orderly grooves becomes distorted and is no longer consistently parallel to the polarization of laser. In this case, the black Si exhibits a completely irregular structured surface, which is attributed to the high incident energy and the anti-reflection effect of the grooves. These factors amplify the influence of nonlinear effects and other possible contributing factors [38,39]. Energy accumulates, and ablation occurs within the deeper regions of the surface structures, resulting in irregular morphology and a rougher surface [30].

 

Fig. 2. SEM images of the black Si ablated by the fs laser at different zoomed views (a) ∼ (d); 3D confocal microscopy reconstructions and the profile line based on a displacement map technique of the black Si (e); height distribution histogram of the laser produced microstructures with surface roughness analysis in the inset (f); Raman spectra of the black Si both with and without annealing compared to the unprocessed Si at the range of 400-600 cm⁻¹ (g) and 100-500 cm⁻¹ (h); PL spectra of the black Si compared to unprocessed Si (i).

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The morphology of the processed sample exhibits periodic structures distributed over a wide range of scale, ranging from the grooves with features in the micrometer range to ripples with typical sub-wavelength size. This multi-scale morphology can generate a synergistic effect on the anti-reflection property of the black Si surface. On one hand, the large-scale, micron-sized grooves can effectively capture incident light, enhancing the efficiency of photons penetration into the Si. When the typical spatial period of the surface features $\mathrm{\Lambda }$ is close to the incident light wavelength $\mathrm{\lambda }$, absorption is enhanced due to strong scattering effects. Whereas, for $\mathrm{\Lambda }$ > $\mathrm{\lambda }$, anti-reflection performance is improved by light trapping in the surface grooves. On the other hand, the ripples decorating on the micron-sized grooves may serve as a uniform layer of subwavelength structures, leading to a smoother transition of the refractive index and consequently leading to a significant reduction in reflection.

The multi-scale morphology and surface roughness of black Si are examined in detail using 3D confocal microscopy, offering a comprehensive view of its structural characteristics. The average height of the grooves can be easily measured along the section profile, as reported in Fig. 2(e), along with the surface profile in the inset. It is noteworthy that the black Si showcase parallel and regular ridge structures arranged in an array, the width and height are approximately 2 µm and 1.35 µm, respectively, with smaller ripples adorning the surface. Although the grooves are of comparable size, they exhibit a more intricate texture rather than a smooth surface. Further details on the height distribution and surface roughness are reported in Fig. 2(f). As depicted in the height distribution histogram, the grooves exhibit the height value at approximately 1.35 µm, displaying a normal distribution pattern on both sides of this value. The root-mean-square (RMS) roughness factor Sq, representing the average roughness, is determined to be 0.17 µm. The skewness factor Ssk indicates the bumpy trend of the black Si, whose value (-0.03) suggests that the height of peaks is generally higher than that of grooves. The kurtosis factor Sku represents the sharpness of the LIPSSs, indicating that the peaks are quite sharp. The maximum peak height Sp and trough depth Sv are approximately 1.41 µm and 1.23 µm, respectively, while the peak-to-valley height disparity Sz is the sum of Sp and Sv. The arithmetical mean height Sa represents the height differences between the LIPSSs and the average plane.

To further study the structural defects of black Si ablated by fs laser irradiation, Raman and PL spectral analysis are carried out and the results are depicted in Fig. 2(g) ∼ (i). The Raman spectra, providing valuable insights into the vibrational modes and molecular structure of materials, are reported in Fig. 2(g) and (h). Given that annealing is an essential step in the preparation of a photodetector as it may help to form a unique Schottky barrier contact between the metal electrode and the processed layer [40], Raman spectra is acquired for the black Si both with and without annealing are carried out. Notably, all the samples, including unprocessed Si, exhibit distinct peaks at 520 cm⁻¹, corresponding to the vibrational mode of monocrystalline Si (Si-I) [41,42]. However, a significant reduction in peak intensity is observed in black Si compared to the unprocessed Si. This can be attributed to the defects induced by fs laser irradiation, which alter the surface morphology and cause a phase transformation from monocrystalline Si to polycrystalline or amorphous Si [43]. Moreover, as illustrated in the enlarged section of the 100 cm⁻¹ to 500 cm⁻¹ range, shown in panel (h) of Fig. 2, the black Si exhibits three distinct Raman modes, attributable to Si-I, the pressure-induced polycrystalline phases (Si-III and Si-XII), and the amorphous phase (a-Si). Apart from the original monocrystalline Si-I, the emergence of Raman signatures associated with a-Si, Si-III, and Si-XII is indicative of an extensive presence of structural imperfections caused by the fs laser processing. It has been documented that during the processing of black Si, the pressure wave generated by fs laser irradiation triggers a phase transformation on the Si surface, resulting in the conversion of a portion of crystalline Si (Si-I) into a-Si and polymorphic crystal structures such as Si-III and Si-XII [40]. Nonetheless, upon annealing, the a-Si, Si-XII, and a fraction of Si-III revert back to Si-I, as demonstrated by significant increases in the Raman mode intensity at 303 cm⁻¹ and 520 cm⁻¹. As was reported that the annealing process served to mitigate and restore structural defects, ultimately enhancing carrier mobility without producing a notable alteration to the Si surface [40].

Figure 2(i) shows the PL spectra, which represent the emission of light after absorbing photons [29]. The PL spectrum of black Si exhibits a distinct difference from unprocessed Si, as there are three prominent peaks at wavelength of 558 nm (P558), 544 nm (P544) and 538 nm (P538), respectively [44]. After the ablation of Si by fs laser, the peak intensity of P558 and P544 decreases, while P538 increases. Notably, a new peak at 533 nm (P533) appears nearby, exhibiting even higher intensity than P538. This may be due to the formation of new chemical bonds, dangling bonds and trap states within the amorphous or polycrystalline Si, which alter the vibrational modes of black Si and lead to distinct changes in the Raman spectra. However, fs laser processing may introduce various defects, such as di-vacancy V-V, multi-vacancy centers, vacancy-oxygen complexes, clusters of vacancies, and vacancy-impurity associations [30]. These defects may induce new energy levels, thereby enhancing the photon trapping capability of black Si and promoting absorption in both the visible and infrared spectral bands.

The EDS spectra and the XPS analyses are conducted on the underwater processed black Si to characterize the elemental composition and chemical states. For comparative purpose, the similar analyses of the unprocessed Si as well as the black Si processed at same laser fluence in air are also performed. As reported in Fig. 3(a) ∼ (c), although the O content in the black Si processed in water condition is significantly lower than that processed in air. The presence of 4.1% oxygen content indicates its incorporation within the Si substrate, which is considerably higher than the 1.2% observed in unprocessed Si. Moreover, the XPS spectra presented in panels (d) and (e) of Fig. 3 confirm the alteration in the chemical state of Si in the processed Si compared to the unprocessed Si shown in panel (f). The Si 2p core-level profile exhibits two distinct peaks, corresponding to Si° and Si⁴⁺, respectively. Notably, the peaks located approximately at 103.4 eV, being attributed to SiO₂, occupy a significant portion of the black Si spectrum. Although its proportion in the water-processed black Si is slightly lower than that processed air, there is unequivocal evidence that oxygen has been introduced into the black Si, as indicated by the significant change in shape of the peaks compared to the unprocessed Si. Therefore, it can be inferred that plenty of Si-Si bonds in the interface layer have been replaced by Si-O bonds. This result is consistent with the report by Jun Ren [45], where the XPS spectrum analysis showed the appearance of silicon oxides under water, indicating rapid quenching from a high-temperature state, most likely induced by the collapse of the bubble, which generated a high-speed liquid jet and additional shock pressure that could further carry away the ablation debris.

 

Fig. 3. EDS analyses comparing the black Si processed in air (a) and in water (b) using the same fs laser fluence, alongside the unprocessed Si as a reference (c). XPS spectra are also presented, showing the Si 2p spectrum of these three samples: black Si processed in air (d), in water (e), and the unprocessed Si (f).

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3.2 Optoelectronic properties of the black Si

Optical characterizations, including absorptance and reflectance, are crucial for elucidating the optoelectronic properties of materials. In this study, the absorptance, transmittance, and reflectance are accurately measured and illustrated in Fig. 4. The optical characterizations of the unprocessed Si are also conducted for comparison purposes. As shown in Fig. 4(a), the unprocessed Si exhibits a high reflectance across the entire spectrum ranging from 200-2500 nm, averaging approximately 60%. Especially within the wavelength range of 1100-2500 nm, its reflectance can reach up to 70%. Even within its photosensitive wavelength range, specifically in the visible and near-infrared spectrum ranging of 400-1100 nm, the reflectance of unprocessed Si reaches to 40%. However, once the Si is ablated by the fs pulses, the high reflectance is entirely eliminated across the spectrum. Notably, the obtained black Si exhibits much lower reflectance at wavelength beyond 1100 nm, maintaining an average reflectance of merely 27% across the 200-2500 nm wavelength range.

 

Fig. 4. Reflectance (a); transmittance (b) and absorptance (c) spectra of the black Si compared to unprocessed Si; Schematic diagram of the enhanced absorption mechanism; the unit cell and energy band structure (d).

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The measured transmittance and absorptance characteristics, presented in Fig. 4(b) and (c), respectively, are in accordance with the reflectance data above. As one can discern, the unprocessed Si exhibits a high transmittance of approximately 30%-40% for wavelength exceeding 1000 nm. Whereas the black Si becomes completely opaque to incident light in the whole spectra, indicating that the incident light is either absorbed or reflected. The absorption spectra in Fig. 4(c) demonstrate that unprocessed Si exhibits remarkable absorptance of over 60% in the range of 500-1000 nm. However, as the wavelength exceeds 1100 nm, the absorption sharply drops and nearly reaches to 0. This phenomenon is attributed to the wide bandgap of Si, where the energy of photons captured in this wavelength range is insufficient to support the electrons transition from valence to conduction band. Conversely, the absorption of unprocessed Si is also inadequate in the short wavelength range below 400 nm due to its extremely high reflectivity of 60%-85%. Notably, the black Si can exhibit remarkable enhancement in the absorptance over the entire spectral range, particularly below 1100 nm, with an average absorptance exceeding 85%. Although the absorptance decreases rapidly when the wavelength exceeds 1100 nm, it still remains above 50%. The calculated average absorptance of black Si across the entire range stands at 71%, marking a substantial enhancement compared to unprocessed Si, which only exhibits an average absorptance of 20%. Moreover, as displayed by the red dashed lines, neither the reflection nor absorption of the black Si exhibit significant change after annealing. Consequently, the obtained enhancement in absorption can be attributed to the synergistic effects of multi-scale structures on the surface and the introduced intermediate energy levels. On one hand, as illustrated in Fig. 4(d), the surface structures, comprising both ripples and grooves, extend the light transmission path within the grooves, thus strengthening the interaction between photons and Si, effectively minimizing reflection and enhancing photon capture. Additionally, the ripples on the surface may create a gradient refractive index layer due to the formation of oxidized silicon, gradually increasing from air to bulk Si, aiding in the transmission of photons into black Si and further suppressing reflection [37]. On the other hand, the introduced intermediate energy levels due to the Urbach states or deep-level structural defects in the re-solidified surface layer may narrow the band gap of the Si and reduce the excitation energy of electrons [30,46,47]. The synergistic effect of multi-scale LIPSSs and induced defects improve the sub-bandgap absorption characteristics of black Si across a wide range of wavelengths, making it a prospective material for infrared photoelectric conversion.

3.3 MSM type infrared photodetector fabricated by black Si

To characterize the optoelectronic properties of the black Si, MSM type infrared photodetectors are fabricated, as depicted in the schematic diagram shown in Fig. 5(a). The current produced in response to the bias voltage serve as a visual indicator of photodetector performance [39]. When the photosensitive surface is exposed to light, electrons in the valence band of the Si capture photons and gain energy, enabling the transition to higher energy levels if the energy is sufficient. This process generates electron-hole pairs. A sufficient number of photogenerated electron-hole pairs present in the conduction band result in the generation of an output current response, driven by either an internal or external potential difference [48].

 

Fig. 5. Diagram of the Si-based MSM type detector and electrode size parameters on the photosensitive surface (a); I-V curves of the black Si under different laser irradiation powers (b); histogram for responsivity and EQE vary with bias and fitting curves for the relationship between photo-current and laser power for different bias values in the inset (c); I-T curve of the black Si under increased laser irradiation power from 4 mW to 28 mW (d); semilogarithmic I-V curves of dark current compared to the photo-current under 28 mW in the voltage range from -5 V to 5 V (e).

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The I-V characteristics of the black Si-based photodetector are investigated and presented in Fig. 5(b) ∼ (e). The black curve reads the dark current showing as a baseline for comparison. In this study, the photodetector is irradiated by a laser beam with wavelength of 1550 nm, subjected to varying bias voltages and power intensities. As reported in Fig. 5(b), when the bias voltage varies from -10 V to 10 V, the current initially decreases until it reaches 0 and subsequently begins to increase. The I-V curves exhibit approximately symmetrical distribution in the first and third quadrants, with almost the same current values corresponding to the same bias voltages in both positive and negative regions. Moreover, the I-V curve illustrates an increasing trend as the laser power increases from 3.9 mW to 27.7 mW. Further details are provided in the inset of Fig. 5(b), where the maximum photocurrent significantly increases to approximately 1.23 mA under 27.7 mW at 10 V. In addition, the performance of a photodetector is evaluated based on two crucial parameters: responsivity and EQE, which are accurately measured and depicted in Fig. 5(c). The inset shows the photo-response currents as a function of light power intensity under different biases. The currents are measured under different bias voltages ranging from 1 V to 10 V, while the photodetector is illuminated by a 1550 nm wavelength laser with gradually increasing power intensity. The slope of each fitted line represents the responsivity at corresponding bias voltage. The effective exposure area calculated to be 48.4 mm², resulting in a maximum responsivity and EQE of 50.2 mA/W and 4.02%, respectively, at a bias voltage of 10 V. As compared to the reports listed in Table 1, which carries out the responsivity of photodetectors based on black Si in recent years, the photodetector presented in this work shows an excellent performance, where the responsivity can reach to 50.2 mA/W@10 V, which is significantly higher than the Schottky-based photodetectors of 15 mA/W@8 V [52] and Ti-doped black Si of 3.42 mA/W@-5 V [54] under 1550 nm illumination.

Table 1. Performance comparison of photodetectors based on black Si.

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The photocurrent response under zero bias voltage serves as a pivotal metric for assessing detector performance, as it reflects the capacity of black Si to produce photo-generated carriers and demonstrates its inherent optoelectronic conversion efficiency. Figure 5(d) depicts current-time (I-T) curve of 7 cycles of on-off light switching using an external shutter, with the light power gradually increasing. It is evident that the photodetector exhibits a sensitive response to the incident light: upon turning on the light source, the response current curve rises rapidly and then stabilizes at a certain value. When the light source is removed, the I-T curve drops to 0 immediately. The tests involve gradually increasing the incident laser power to a preset value and maintaining it for 20 sec, followed by the other half circle of decreasing the laser power to 0 mW for another 20 sec. This test entails repeating the process in subsequent cycles, wherein each cycle involves a gradual increase of light power intensity. The obtained response current increases from 0.61 µA to 1.29 µA as the incident light power increases from 4 mW to 28 mW, demonstrating a clear linear correlation between the response current and the light power intensity. Undoubtedly, the photocurrent here is formed through the interaction of the built-in electric field and photons with the Si material. Due to the lack of the necessary driving force caused by the strong potential difference to effectively promote the formation of the response current, even though a large accumulation of carriers is generated through illumination, the response current under zero bias voltage is significantly lower than the current value obtained when the bias voltage is applied. Figure 5(e) compares the leakage current, revealing the I-V curves using a logarithmic scale with bias voltage ranging from -5 V to 5 V under 28 mW light irradiation. The measured leakage current is 0.017 µA, which is two orders of magnitude lower than the photocurrent of 1.68 µA, revealing a good signal-to-noise ratio of 98.8%.

Figure 6 reports the results of the photoelectric stability test for the detector, conducted by cycling the on/off I-T curves for a long period of time. The switching process comprises 50 cycles, with each cycle encompassing 10 sec of activation followed by 10 sec of deactivation, totaling 20 sec per cycle. This process is conducted under 1550 nm laser irradiation and without the application of any bias voltage. As shown in Fig. 6(a), the I-T curve of the black Si is illustrated by a green curve, while the unprocessed Si is represented by the black curve for comparison. Both exhibit regular responses to the incident light: when the light is switched on, the photocurrents increase simultaneously and maintain a constant current values; while the light source is switched off, the I-T curves drop rapidly and reach approximately 0. The black Si maintains a stable photocurrent value of 1.2 µA in each cycle, and there is no obvious decline in the response current value after 50 on/off switching cycles.

 

Fig. 6. I-T curves of the black Si under 1550 nm laser irradiation compared to the unprocessed Si with light on-off cycling, the power is 28 mW, and during the measurement no external bias voltage is applied (a); and response speed measurement: enlarged I-T curve in one cycle (b).

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The stability of the black Si-based photodetector was evaluated by analyzing its optoelectronic properties utilizing a sample that had undergone natural oxidation in ambient air for a duration exceeding three months, revealing excellent stability, as demonstrated in Fig. S2. Moreover, the photocurrent value of black Si is nearly three orders of magnitude higher than that of the unprocessed Si, as evident from the noise-signal ratio within the I-T curves. The unprocessed Si exhibits a photocurrent value of approximately 1.15 pA, overshadowed by substantial noise, making it indistinguishable from the noise signal. Conversely, black Si demonstrates a relatively smooth I-T curve and a negligible noise signal. The results clearly demonstrate that the synergistic structures decorated on the surface of black Si coupled with the introduced defect energy levels have substantial effects on the light absorption characteristics, thereby significantly enhancing the photoelectric conversion of the Si-based detector. Consequently, compared to the unprocessed Si, the black Si reveals superior performance in both optical sensitivity and stability, confirming the utilization of fs laser processing in water condition as an effective technique for enhancing the functionality of Si material.

The response speed in the time domain is comprehensively assessed based on both rising and falling time metrics. The rising time specifically refers to the period during which the photoresponse rises from 10% to 90% of its peak value, whereas the falling time characterizes the duration of the response declining from 90% to 10% [55]. In this test, the switching speed of the fabricated photodetector is thoroughly examined by alternately activating and deactivating the light source at zero bias voltage, utilizing a 1550 nm wavelength laser beam with the power of 28 mW. As depicted in Fig. 6(b), the black Si-based photodetector exhibits a rising time of τ₁= 161 ms and a falling time of τ₂= 182 ms, respectively.

4. Conclusion

In this study, we introduce a method for fabricating black Si with multi-scale periodic surface structures, containing subwavelength ripples and micron sized grooves, using fs laser ablation in water. The resulting black Si demonstrates excellent optoelectronic properties, with an average absorptance of approximately 71% across a broad wavelength range of 200-2500 nm, a significant improvement compared to the unprocessed Si with an absorptance of only 20%. Furthermore, the MSM type photodetector fabricated by black Si exhibits excellent optoelectronic properties under 1550 nm. Specifically, we achieved a responsivity of 50.2 mA/W@10 V and an EQE of 4.02%, indicating the high efficiency of the photodetector in converting incident light into electrical signals. Additionally, we evaluated the response speed of the photodetector by measuring the rising and falling times of the I-T curve, which were found to be 161 ms and 182 ms, respectively. The significant enhancement in optoelectronic performance of the black Si can be attributed to the synergistic effect of both the multi-scale surface structures and introduced defect energy levels, which contribute to an anti-reflection and light trapping properties, effectively expanding its absorption range into the infrared band due to the narrowed bandgap resulting from the introduction of intermediate energy levels. Overall, our findings highlight the potential of black Si fabricated through fs laser surface processing under water for various optoelectronic applications.