Silicon Nanocanyon: One-Step Bottom-Up Fabrication of Black Silicon

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Surfaces, Interfaces, and Applications

Silicon Nanocanyon: One-step bottom-up fabrication of black silicon via in-lasing hydrophobic self-clustering of silicon nanocrystals for sustainable optoelectronics Seunghyun Back, Seongbeom Kim, Seung Gap Kweon, Jong Eun Park, Song Yi Park, Jin Young Kim, and Bongchul Kang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b11483 • Publication Date (Web): 27 Sep 2018 Downloaded from http://pubs.acs.org on October 9, 2018

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ACS Applied Materials & Interfaces

Silicon Nanocanyon: One-step bottom-up fabrication of black silicon via in-lasing hydrophobic self-clustering of silicon nanocrystals for sustainable optoelectronics Seunghyun Back1†, Seongbeom Kim2†, Seung Gap Kweon1, Jong Eun Park3, Song Yi Park4, Jin Young Kim4, Bongchul Kang1* 1

Department of Mechanical System Engineering, Kumoh National Institute of Technology, Gumi, 39177, Korea

2

Department of Mechanical Design Engineering, Kangwon National University, Samcheok, 25913, Korea 3

Department of Mechanical Engineering, Korea Advanced Institute of Science and Technology, Daejeon, 34141, Korea

4

Department of Energy Engineering, School of Energy and Chemical Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan 44919, Korea

† Equally contributed in this work

* [email protected] (Tel. 82-54-478-7400, Fax. 82-54-478-7319)

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Abstract We report a novel one-step bottom-up fabrication method for multiscale-structured black Si, which is characterized by randomly distributed microscale Si layers covered with sub-100 nm protrusions with submicron boundary grooves. The unique multiscale structure, suggested as a “nanocanyon,” effectively minimizes light reflection over a broad spectrum by diversifying the scattering routes from the nano-textured surface to the wide distributed boundary micronanoscale grooves. This structure was achieved by hydrophobic clustering and local aggregation of instantaneously melted Si nanocrystals on a glass substrate under laser irradiation. This method can replace the complicated conventional silicon processes, such as patterning for selective Si formation, texturing for improved absorption, and doping for modifying the electrical properties, because the proposed method obviates the need for photolithography, chemical etching, vacuum processes, and expensive wafers. Finally, black Si photo-sensor arrays were successfully demonstrated by a low-cost solution process and a laser growth sintering technique for micro-channel fabrication. The results show the great potential of the proposed fabrication method for low-cost and sustainable production of highly sensitive optoelectronics and as an alternative to conventional wafer-based photosensor manufacturing techniques.

KEYWORDS: Silicon nanocrystals, Black silicon, Hydrophobic clustering, Optoelectronics, Sustainable manufacturing.


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Manuscript 1. Introduction Silicon (Si) is the most important semiconducting material and is widely used in photosensors,1,2 photovoltaics,3,4 electronics,5,6 optical components,7,8 and bio products.9,10 Si is mainly utilized in two forms, a sliced wafer or thin film deposited on substrates. The form is chosen depending on its intended applications (i.e., a wafer transistor for memory devices and a thin film transistor for flat panel displays).4-6 Recently, the demand for Si has dramatically increased for photo-sensitive devices, including photo-sensors and photovoltaics, as highperformance imaging devices with high contrast are required for the rapid development of display devices and the need for eco-friendly energy sources is increasing.11,12 However, complicated and expensive processes, such as photolithography, vacuum deposition, and toxic chemical etching, are required to produce these Si-based devices in large quantities. In addition, a texturing process is inevitably needed to improve the light absorption efficiency of Si layers and the cost of raw materials is higher than that of other semiconductors owing to the limited shapes of materials that are typically supplied.13,14 Furthermore, safety issues related to HF leakage and the casualties that occur in the manufacturing fields imply that that current Si fabrication methods cause significant social issues. Black Si is considered a breakthrough in maximizing the absorption efficiency of Si wafers over a wide spectrum from ultraviolet (UV) to infrared (IR) by introducing nano- and microripples onto the wafer surface. Because the fine ripples enhance the scattering of incident light and elongate the optical path length infinitely, the resulting reflectance is suppressed compared to that of a bare silicon wafer.15-18 Various fabrication methods for black Si have been studied to develop high-contrast imaging sensors and high-efficiency photovoltaics over recent decades. The major techniques, which are well established in practical applications, are



based on chemical etching for selective corrosion.15,19 More advanced techniques to improve the process and absorption efficiency, such as metal-assisted chemical etching,16,20,21 reactive ion etching,22-24 laser irradiation,17,25,26 and the Fray-Farthing-Chen-Cambridge process,18,27,28 are also based on the chemical corrosion of a Si or SiOx wafer.29 These methods have disadvantages in terms of eco-friendly and low-cost fabrication of photo-detecting Si layers because of poor sustainability due to the large wastage of toxic chemicals, high energyconsumptive and complicated processes for multiscale structure fabrication, and the need for a mask template for selective black Si formation. Femtosecond-laser (fs-laser) treatment is a versatile technique that can be used to improve the light absorption of Si wafers. Spike-like Si microstructures, which are covered with Si nanoparticles of diameter 10–50 nm, were fabricated by irradiating a Si wafer with fs-laser pulses in a SF6 environment. This unique structure was shown to suppress surface reflection, and both sulfur atoms and structural defects contributed to enhancing the absorption in the sub-band gap region of Si.17 However, the fs-laser source is highly expensive and unstable. Furthermore, the processing rate is quite slow due to an infinitesimal material removal rate per pulse, and additional etching processes are required to remove the internal and surface damages that result from the intensive laser pulses. Therefore, a novel alternative method to fabricate black Si that obviates the need for vacuum depositions, chemical etchings, photolithography, and wafers is required to improve the sustainability and safety of the process and to enhance its cost-effectiveness and performance. In this paper, we report a one-step fabrication method for multiscale-structured black Si via the in situ laser-induced hydrophobic clustering of Si nanocrystals (Si NCs) on a glass substrate. Microscale Si layers, which are covered with sub-100 nm protrusions, are randomly distributed, separated by micro/nanoscale grooves. This unique multiscale structure is referred to as a “nanocanyon” in this study. The structure appear black in contrast to the ACS Paragon Plus Environment

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typical silver-gray Si wafer and effectively minimize light reflection over a broad spectrum. Additional processes to modify their electronic mobility were not required because the impurities contained within the Si NC solution were also thermally incorporated in the black Si structure during the hydrophobic clustering process. As a result, the fabricated black Si has a great potential for application in cost-effective and sustainable production of large-area and thin photosensitive devices as an alternative to conventional wafer-based devices, without the use of toxic chemicals, vacuum depositions, and complicated process steps.

2. Results and discussion A few production methods of Si NCs has been reported, such as ball milling or chemical etching of bulk silicon,30,31 plasma dissociation of from gaseous precursor,32 and solutionphase synthesis based on the high temperature decomposition of liquid precursor.33 To prevent the impurity involvement and improve the productivity of NCs, Si NCs were synthesized by laser pyrolysis of silicon precursor gases because the Si NCs can be produced without being exposed to contaminants in vacuum chamber in a continuous manner. A continuous wave (CW) CO2 laser of a wavelength of 10.6 µm and SiH4 gas were used as a laser source and precursor, respectively. As shown in Fig. 1-(a), the Si NCs were created by laser-induced thermal dissociation of SiH4 molecules and subsequent nucleation and growth of the Si NC in vacuum. The reactor was designed with three orthogonal arms for the laser beam path, a precursor gas inlet, and an observation port. Si NCs were captured in a collector that was detachable from the line. The size of the NCs was controlled by adjusting the laser power and gas flow rate, and determining the rate of thermal growth of the Si nuclei.34 As shown in Fig. S1, the Si NCs had a well-ordered crystalline structure. Si NCs of approximately 45 nm dimeter used in this study (see Fig. S2) were dispersed in ethanol in a



weight ratio of 10%. The Si NC solution was uniformly coated on a glass substrate and irradiated with a CW laser of 1070 nm wavelength in an inert environment, as shown in Fig. 1(b). As a result, black Si patterns were selectively fabricated in the laser-irradiated regions on the glass substrate. Fig. 1(c) depicts the sequential thermal transitions of Si NCs under laser irradiation and shows the corresponding scanning electron microcopy (SEM) images. As shown in Fig. S3, the Si NCs deposited on the glass were melted around a melting temperature of bulk phase (approximately 1689 K) by absorbing the intensive incident laser light of Si NCs, which led to their instantaneous aggregation with each other.35 Here, only Si NCs more strongly absorbed the laser light than glass substrate because of a high transmittance of glass for NIR wavelength. The clustering of melted Si NCs led to the formation of larger individual Si micro and nano-droplets, resembling Si islands (Fig. 1(c)(ii)), on the glass substrate because the Si droplet characterizes the hydrophobicity on a glass.36,37 As shown in Fig. S4, the micro- and nano-droplets, which were formed by melting, migration, and aggregation of Si NCs, exhibited a contact angle of over 80 ° on the glass substrates. Consequently, with continued laser-induced hydrophobic clustering and recrystallization of Si NCs, microscale Si structures, which were randomly distributed on the substrate with separation by micron/submicron grooves, were formed, as shown in Fig. 1(c)(iii). This structure is distinguished from the continuous Si NC layers, which are created on hydrophilic substrates such as Si wafer and carbonic surface via uniform agglomeration of NCs without directional clustering due to the hydrophilicity by chemical affinity and the low contact angle, respectively (Fig. S5).38,39 The microscale Si structures were covered with surface protrusions with a size below 100 nm, which originated from the surface residuals of insufficiently sintered Si NCs. Here, the substrate did not suffer thermal damage because the bulk glass substrate, which has a much larger thermal capacity and higher transparency than that of the Si NCs, does not directly interact with both the melted Si NCs and laser light.


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Overall, the Si structure was composed of three different scale regime; flat micro-structures, submicron grooves, and surface protrusions of sub-100 nm, which were expressed as microrocks, submicron-valleys, and nano-textured surfaces, respectively (Fig. 1(c)-(iii)). The unique multiscale structure led to a significant change in the optical properties of the Si layer for broad spectrum light. The micro-rocks with nano-protrusions suppressed reflection by enhancing the scattering of light, which increased the resultant absorption efficiency. In addition, the irregularity and aperiodicity of the micro-rocks, which also contributed to light scattering within and between the submicron-valleys, led to a further reduction in the reflectance throughout the wide spectrum range. This multiple scattering routes facilitate to efficiently improve the light absorption on Si layer, compared to single route scattering system of conventional black Si requiring repetitive and complicated etching processes, as shown in Fig. S6.40 As detailed in the experimental procedures shown in Fig. 2(a), the Si NC solution was uniformly coated onto a glass substrate by precisely translating a blade across the surface. This method enables the fabrication of large-area film. By irradiating the coated, yellowishbrown NCs with a focused laser spot, the Si NCs were locally changed into a matte black color. We found that Ar gas blown onto the laser-irradiated zone prevents and retards the thermal oxidation of Si NCs during the laser sintering process. This drastic change in the Si NCs to an achromatic material, which implies the improvement of light absorption, is caused by a photo-thermally induced structural transformation of the Si NCs. Subsequently, distilled water was sprayed onto the sample to clearly remove residual Si NCs from the substrate by bombarding it with water droplets (Fig. S7). Fig. 2(b) shows the absorbance of Si NCs coated onto a glass substrate with respect to wavelength change from UV to near-infrared (NIR). Since Si NCs exhibit a strong absorption



for short wavelengths below 300 nm (magenta region in Fig. 2(a)), conventional laserinduced annealing processes, such as low temperature poly-crystallization (LTPS) and excimer laser annealing (ELA), were used to improve the electron mobility in the fabrication of thin film transistors. This restructuring was performed through reconstruction of the microstructure from amorphous to polycrystalline and is based on lasers with a UV wavelength shorter than the Si bandgap.41,42 Visible (VIS) wavelength lasers also show a similar effect as that demonstrated by UV lasers because of a small absorption peak near 400 nm, which corresponds to the surface plamson resonance mode of the Si NCs.43 However, in this method, such laser sources are inappropriate for generating the hydrophobic clustering effect of Si NCs because the NCs are instantaneously transformed into a continuous Si layer without individual clustering. This is due to intensive photo-thermal melting and the sudden solidification of Si NCs. A delay time is required to transport the melted Si NCs and maintain the momentary liquid phase to allow the Si NCs to form hydrophobic melted droplets on the glass surface (Fig. S4). Therefore, the NIR laser, which is less absorbed by the Si NCs than UV and VIS wavelengths are, is more suitable for hydrophobic clustering and multi-scale structuring, as shown in Fig. 2(b). The X-ray diffraction (XRD) and energy-dispersive X-ray spectroscopy (EDS) measurements in Fig. 2(c) show the chemical and crystallographic characteristics of the laser-processed black Si compared with the glass substrate, Si NC, and the specimen fabricated without an inert gas assistance. The XRD peaks of black Si agree well with those of crystalline Si. The width (FWHM) of XRD peak corresponding to the (111) crystal plane of black Si was 0.1507°, which is smaller than that of Si NC (0.2295°); this implies that the Si grains grew to approximately 95 nm in the laser sintering process according to Debye-Scherrer equation. The broad and slight increase in the XRD intensity around 22° is caused by a background effect of the glass substrate through the submicronvalleys.41,44 The EDS analysis shown in the inset of Fig. 2(c) provides the spatial distribution


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of Si in the laser-irradiated region compared to the surrounding Si NCs. It was found that the oxidation of black Si was limited during its formation, which followed the thermal melting and clustering, since there was no change in the concentration and distribution of Si in the black Si region when compared to the surrounding area. This chemical constancy is due to the presence of Ar gas, which prevents from thermal oxidation of Si NCs in the sintering process. In the absence of Ar gas, the major peak decreased as compared with the Si NCs’ peak because a large portion of the Si NCs was thermally oxidized and converted into SiO2 by interactions with ambient oxygen in the air during the melting and restructuring processes. Overall, it is clear that this method creates a black Si structure with a multiscale polycrystalline phase. The Ar gas plays another critical role in the formation of the multiscale structure. Fig. 3(a) shows the variation in the microstructure of Si NCs depending on the Ar gas assistance. The molten Si drops that were oxidized without the assistance of an inert gas, contacted with the glass substrate with a high hydrophilicity owing to a similarity in their chemical compositions; thus, they were well wetted on the glass with a low contact angle and spread to the surrounding area, forming a liquid-film.36,37 As a result, a continuous layer with a smooth surface, which showed the distinct structural properties of black Si characterized by a nanocanyon structure, was formed when the sample was irradiated with a laser in ambient air. Regarding their structural differences, the visual appearance and color of the laser-processed regions were considerably distinct, as revealed by microscopy observations (Fig. 3(a)). The morphology of the nano-canyon structure was characterized by cross-sectional SEM and surface profiling by atomic force microscopy (AFM). The thickness of the nano-canyons fluctuated between 1.4 and 1.8 µm, which was approximately one-fourth of that of the coated Si NCs (5.5 µm), as shown in Fig. S8. The thickness shrinkage corresponds to the volume reduction of internal nano-voids of the original Si NC coating layer, which were closed by the ACS Paragon Plus Environment


aggregation of NCs during the laser sintering procedure. As shown in Fig. 3(b), the randomly distributed peaks and valleys are distinguishable in the AFM profile of the black Si. The valleys were characterized by a thickness of approximately 1000 nm and a broad distribution of widths from 200 to 2400 nm (see Fig. 3(c)). The diversity of the widths of the valleys contributes to the enhancement of the scattering of incident light over a wide spectrum. In addition, as the average surface roughness of the nano-canyons was approximately 138 nm, which is ten times the surface roughness obtained without the Ar gas assistance (14 nm), the rough morphological features also afforded a high absorptivity of light. Fig. 4(a) shows the spectral reflectance of the black Si structure as compared to a bare Si wafer. The overall reflectance was lower than 2% and remained almost constant for all the examined wavelengths, unlike the high reflectance and wavelength dependency observed for the Si wafer. The black Si greatly reduced light loss and improved the absorption efficiency with a change in the chromatic properties, as shown in the insets of Fig. 4(a). In particular, the low spectral sensitivity of black Si originates from the various-sized components, i.e., micro- to nano-scale components, in the black Si, which leads to light scattering over a broad spectrum. In terms of mechanical reliability, the black Si layers remained intact without peeling under a strong Scotch-tape adhesion test performed for 100 times, as shown in Fig. 4(b). We assume that this outstanding adhesion is caused by the instantaneous thermal bonding of interfacial Si atoms between the black Si and glass substrate. Furthermore, Fig. 4(c) shows letter-like black Si patterns, which indicate that not only planar patterns, but also arbitrary patterns can be fabricated by simply modifying the CAD data of a laser scanning system as a digital process. Therefore, this process enables complete pattern formation with simultaneous texturing without using additional processes and materials, which is unachievable for typical black Si fabrication techniques.


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To demonstrate the potential for sustainable and low-cost fabrication of photo-detecting devices as an alternative to conventional wafer-based opto-electonics including image sensor, illumination sensor, and photodiodes, a black Si-based photo-detecting array device was fabricated using only solution processes from the black Si layer to micro-channels without the use of photolithography, chemical etching, and vacuum deposition, which should be combined to make other functional layers, such as electrode and insulator, in the device works using Si NCs. As shown in Fig. 5(a), pairs of micro-channels were fabricated on a glass substrate to transmit electronic signals from the photo-sensor to an IC circuit by using a low-cost laser-induced growth sintering process for the hybrid suspension. The hybrid suspension was reformulated in-process from a particle-free organometallic complex via the self-generation of ultrafine nanoparticles over the coated layer by insufficient thermal decomposition, as shown in the transmission electron microscopy (TEM) image in the inset of Fig. 5(a).45-48 Because the growth sintering of the hybrid suspension selectively creates the electrodes following a series of ion precipitation, clustering, growth, and agglomeration procedures, microelectrodes with a high-surface quality, which is critical for the uniform coating of Si NCs and for inducing an ohmic contact between the electrode and the black Si layer, were fabricated. Photo-sensing arrays were formed by selectively irradiating the Si NCs, which were coated on the pre-fabricated micro-channels on a glass substrate, with laser, as shown in Fig. 5(b). The specific resistance of the black Si was recorded as 30.4 Ωcm, which is at least three hundred times higher than that of a crystallized intrinsic Si wafer (see Fig. S9). The low resistivity is owing to the impurities’ doping effect during the sintering of NCs. The Si NC solution that was synthesized in this study contained very limited impurities such as Ca, Fe, Ni, and Cu, as shown in Table S1. It is assumed that the impurities were introduced into the Si NC solution during synthesis, storage in vessels, or handling in the laboratory. These metallic impurities can be removed by employing additional purification



methods of Si NCs, such as acid etching, in the synthesis or post-process.49,50 Since such metallic impurities were uniformly doped into the black Si while the Si NCs were melted and aggregated with each other, the impurities contributed to a decrease in resistivity of less than 1000 Ωcm, which corresponds to that of an n-type Si wafer. It provides the potential for control of the electrical property and doping state of black Si through adjusting the kind or content of foreign additives in the Si NC solution (see Fig. S10 and S11). This result reveals another great merit, that is, the method concurrently completes the sequential processes that are normally required for the production of conventional Si wafer-based devices, such as the patterning process for selective formation of a Si sensor layer, texturing process for improving the absorption, and chemical doping for modifying the electrical properties. Fig. 5(c) shows the variation in resistance of the black Si photo-sensor array depending on the incident optical power. The dark resistance of 25.5 kΩ linearly decreased with increasing optical power; therefore, this device can detect light power without calibration. As shown in Fig. 5(d), the black Si and silver micro-channels form an ohmic contact with each other because the interfacial atoms are diffused during the laser sintering process. It was found that the linear slope of the current-voltage (I-V) sweep curves for black Si increased in specific steps with an increase in the incident optical power, unlike the Si NCs, which did not show any change. Therefore, this method, which was used to fabricate the black Si devices, facilitates the cost-effective and eco-friendly production of high-resolution optoelectronics.

3. Conclusion This study suggested a novel one-step bottom-up fabrication method of multiscalestructured black Si film for sustainable, low-cost, and scalable manufacturing of Si-based optoelectronics. In future, we believe that the innovative and efficient process can be


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extensively applied for the solution process of large-area functional Si devices, such as solar cells, transistor, LED, and battery,51-54 because the electrical properties of black Si can be modified by introducing impurities that are suitable dopants for each device applications in the Si NCs and by adjusting their composition in a controlled environment, as an alternative to conventional wafer-based devices.55-60

Experimental section Laser pyrolysis synthesis of Si NC solution. The vacuum reactor for laser pyrolysis was designed to have three orthogonal arms for a laser beam path, a precursor gas path, and viewports. A 54 W CO2 laser beam with a 10.6 µm wavelength was focused on the reaction zone which was located a few millimeters above the gas inlet nozzle. The gas inlet nozzle had two concentric tubes. The mixture of processing gases, which was composed of a SiH4, H2, and SF6 gases with flow rates of 25, 100, and 5 sccm, respectively, flowed through the inner tube, and a He carrier gas flowed through the outer tube at 2000 sccm. SF6 was used as a photosensitizer and H2 was used to suppress the detonation reaction, which could have taken place due to the dissociation of SF6. The carrier gas was used to confine the processing gas through the stream line, cool down the hot processing gas mixture, and carry the synthesized Si NCs. The gas flow rates were controlled by mass flow controllers. The as-synthesized Si NCs were captured by a filter, which was placed inside a collector that was detachable from the line. The very fine Si NCs appeared brown and were handled only inside a glove box to minimize oxidation. The as-synthesized Si NCs were dispersed in ethanol with a weight ratio of 10%. The dispersion turned into a homogeneous dark brown ink after the initiation of mild sonication, which was continued for 10 min. To break up the Si NC aggregates, strong hydraulic shear forces were applied to the dispersion using a high-speed homogenizer



(Heidolph, Silent Crusher S).

Specimen preparation and laser processing. The Si NC solution was coated to a thickness of 5.5 µm onto a 1-mm-thick glass substrate by translating the substrate, which was placed below a blade with a gap of 60 µm, with a constant velocity of 100 mm/s. The CW laser beam of 1070 nm wavelength that was emitted from a 10 W ytterbium-doped fiber oscillator was focused to a spot of 34 µm onto the specimen through 2D galvanometers and an f-theta lens with a focal length of 100 mm. Ar gas (0.05 MPa) was blown into the laser-focused region during laser irradiation. After the laser processing was completed, the specimen was cleaned by spraying distilled water for 15 s to remove the residual Si NCs on the nonirradiated region.

Fabrication of photo-sensing array. An organometallic complex (CO-011, Inktec Co.) with a silver weight ratio of 10 %, which was composed of a chelate bonding structure of silver ions and carbamate amine complex, is characterized by a particle-free transparent silver ionic precursor. The solution, which was spun onto a glass substrate at 1000−3000 rpm, was baked on a hot plate at 100 °C for 30 s to reformulate it into a nanoseed/organometallic hybrid suspension through the generation of ultrafine nanoparticles over the coated layer by insufficient thermal decomposition. The hybrid suspension, which was an intermediate state that enhances the optical absorption, was irradiated with a spot laser of 34 µm diameter to selectively fabricate the micro-channels with a channel length of 5 µm. The residual suspension was removed from the substrate by immersing it into an n-hexane solution. Then, a black Si layer was formed by scanning the laser spot in plane onto the Si NC solution that was coated onto the pre-formed micro-channels.


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Characterization methods. The crystalline structure and size distribution of the Si NCs synthesized by the laser pyrolysis method were characterized by TEM and statistical analysis. The morphology and topography of the fabricated black Si was characterized by SEM and AFM. The variations in the chemical compositions of Si NCs depending on the laser processing condition were investigated by XRD. To characterize the electrical properties of the photo-sensor array, the electrical resistance flowing through micro-channels was monitored while increasing the collimated CW laser of 1070 nm wavelength.

Supporting information The Supporting Information is available free of charge on the ACS Publications website. TEM image and size distribution of Si NCs, TGA data of Si NC solution, Schematics and SEM image of multiscale microstructure formation, Microscopy of Si NC patterns fabricated on hydrophilic surfaces, Schematics of scattering effect on black Si structures, Washing effect of water spraying process, Cross-sectional SEM images of Si NCs before and after lasing, Variation of specific resistance with laser does, Chemical components in Si NC solution, EDS data for doping effect in hydrophobic clustering, Chemical characterization of Ag-doped black Si.

Acknowledgements This work was supported by National Research Foundation of Korea (NRFK) (Grant No. 2017R1D1A1B03032268).




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Figure Captions

Fig. 1. Schematics and experiment procedure of nano-canyon structure formation. (a) Schematic of a laser pyrolysis reactor and mechanism of nanocrystal synthesis inside the reaction zone. (b) Schematic of bottom-up laser fabrication of black Si patterns from Si nanocrystals on glass substrate by laser irradiation. Inset is transmission electron microscope of Si NCs. (c) Formation mechanism of nano-canyon structure via laser-induced hydrophobic clustering and sintering and corresponding scanning electron microscope images.


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Fig. 2. Experiment procedure and physical/chemical characterization of Si NCs and black Si. (a) Experiment procedure: (i) Formation of Si NC solution on glass substrate using blade coating, (ii) Irradiation of focused CW laser onto Si NC layer with assistant of Ar gas, and (iii) Removal of residual Si NCs by spraying water. (b) Spectral absorbance of Si NC. (c) Comparison of X-ray diffraction crystallography of quartz, Si NC, and laser-processed black Si with and without Ar assistance. Inset is the EDX image of laser-processed black Si (Red dot: Si).



Fig. 3. Morphological and topological characterization of nano-canyon structure. (a) Microscopies and SEM images of laser-processed pattern depending on Ar gas assistance (left: without Ar assistance, right: with Ar assistance). (b) Three- and two-dimensional AFM profile of nano-canyon black Si structure. (c) Distribution of valley width (FWHM) in black Si.


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Fig. 4. Optical and mechanical performance of black Si. (a) Spectral reflectance from UV to NIR (Inset: bare Si wafer and Si black) black Si and Si wafer. The inset shows photograph of laser-processed black Si specimen. (b) Peeling-off test using scotch tape. (c) Arbitrary black Si patterns (KIT) fabricated by controlling the path of a laser spot.



Fig. 5. Fabrication and characterization of black Si photo-sensor. (a) Schematics of fabrication of black Si photo-sensing array. (b) Demonstration of fabricated photo-sensor specimen. (c) Linear trend of resistance change of photo-sensor with respect to incident light power and the inset is the corresponding experimental set-up. (d) Sweeping curves of currentvoltage (I-V) of black Si depending on the power of incident light.


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TOC Figure


Silicon Nanocanyon: One-Step Bottom-Up Fabrication of Black Silicon

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