Pyrolytic carbon coated black silicon

Carbon is the most well-known black material in the history of man. Throughout the centuries, carbon has been used as a black material for paintings, camouflage, and optics. Although, the techniques to make other black surfaces have evolved and become more sophisticated with time, carbon still remains one of the best black materials. Another well-known black surface is black silicon, reflecting less than 0.5% of incident light in visible spectral range but becomes a highly reflecting surface in wavelengths above 1000 nm. On the other hand, carbon absorbs at those and longer wavelengths. Thus, it is possible to combine black silicon with carbon to create an artificial material with very low reflectivity over a wide spectral range. Here we report our results on coating conformally black silicon substrate with amorphous pyrolytic carbon. We present a superior black surface with reflectance of light less than 0.5% in the spectral range of 350 nm to 2000 nm.

It has been recently demonstrated that CVD with light hydrocarbons (e.g. acetylene 16 and methane 17 ) can be employed not only for fabrication of carbon fiber-reinforced carbon but also for deposition of the nanometer thin PyC films on dielectric substrates. This experimental finding has opened a wide range of new applications of PyC including electrochemical measurements 16 , conductive vias through dielectric 19 , carbon-silicon Schottky-barrier diodes 20 and shielding of the electromagnetic radiation in a wide spectral range 13 .
Despite the fact that amorphous carbon inclusions make electrical conductivity of PyC films somewhat lower than that of graphene 16,17,21 , the transmittance of a PyC film resembles that of graphene. In particular, the transmittance of PyC is near constant in the near-IR spectral range and a wide absorption peak is located at 270 nm, at the same wavelength as for graphene 12,22 . Moreover, catalyst free synthesis of PyC permits coating of metal and dielectric substrates of any shape.
In this paper, we demonstrate a novel superior black surface for the spectral range of UV to mid-IR that employs PyC-enhanced bSi structure. By developing an inexpensive and scalable technique we achieved conformally coated bSi surface with the PyC film thickness of 25 nm and demonstrated that such coating enormously suppresses reflectance in the IR. The obtained results allow us also to revisit the long-standing discussion on the mechanism of the CVD of PyC from light hydrocarbons. In particular, the demonstrated conformal growth on micro-and nanostructured substrates indicates that the surface activated growth process is the dominant mechanism of the PyC film synthesis.
For the demonstration of ultra-thin PyC formation on Si, we deposit 25 nm thick layer of PyC on a bare Si substrate. Figure 1a,b show the SEM images of the PyC layer grown on a bare Si substrate. The film grows uniformly all around the substrate covering the sample surface with a thin layer of PyC. Although, the PyC layer is only 25 nm thick, it has crucial impact on the reflectance of the Si. Specifically, one can observe from Fig. 1c that the deposition of ultra-thin PyC layer makes the reflectance nearly wavelength independent in the range from 400 nm to 2000 nm. In UV, however, the reflectance decreases rapidly due to the strong absorption in the PyC film. This absorption resembles the M-saddle point absorption of graphene and has its maximum around the same wavelength of 270 nm (see Supplementary information).
In the characterization of PyC with Raman measurements, the disorder induced D peak (1350 cm −1 ) and graphitic G mode (1600 cm −1 ) are conventionally used to characterize the crystallinity of the fabricated film 23,24 . The measured Raman spectrum shown in Fig. 1d (and Fig. S1, Supplementary information) has a weak D' mode at around 1500-1550 cm −1 , which is a signature of the amorphous carbon 25 .
Moreover, in contrast to Raman spectrum of highly crystalline graphite and graphene, the 2D' peak in PyC film, shown in Fig. 1d is greatly widened and significantly suppressed. Analysis of the Raman spectra The reflectance of bSi after PyC coating, shown in Fig. 3a, changes drastically in the IR (λ > 1000 nm). Specifically, while bare bSi reflects more than 80% of the incident light from 1200 nm to 2000 nm (Fig. 3a), the reflection of bSi coated with PyC film is as low as 0.5% (Fig. 3c). With the wavelength higher than 2000 nm, the reflectance of thin PyC film coated bSi increases as the bSi microstructures are not deep enough for the corresponding wavelengths (see Supplementary information). It is worth noting that, although bare Si has lower reflectance compared to PyC coated Si in the VIS spectral range (Fig. 1), the anti-reflecting properties of PyC coated bSi in VIS-IR spectral range is superior to that of bare bSi (Fig. 3b).
The strong IR absorption of PyC coated bSi is very similar to the results obtained by using strongly doped silicon 9 , but the absorption mechanism is very different. The strong doping increases the number of free electrons in silicon and thus increases light absorption inside the silicon substrate. Since the bSi surface is acting as an anti-reflecting surface, the light is trapped and absorbed by the strongly doped silicon substrate. At light doping, the silicon itself does not absorb as seen from the reflectance spectrum in Fig. 3a. Instead, the absorbing carbon layer, which is only 25 nm thick, interacts with the incident light and absorbs it at the surface of the bSi substrate. Thus, the performance of the PyC-coated structure is actually not dependent on the absorption of the substrate.
Since the IR absorption in lightly doped silicon is relatively weak, the strong reflection at near IR originates from the backside reflection of the bare and black silicon substrate. In order to demonstrate this, we measured reflectance of black and bare Si with a spectralon disk as a backside reflector. This configuration highlights the influence of the 25 nm thick PyC film on the reflectivity. Specifically, we showed that both bare and black Si reflect about 80% and 50% of the incident IR radiation with and without backside reflector, respectively. However, the reflectance of the PyC coated samples measured with or without the spectralon disk was the same, which is an evidence of very strong IR radiation absorption in the ultra-thin PyC film. Although, in our experiment we did not measure the dependence of the reflectivity on the angle of incidence, we believe that the PyC coating should suppress reflection also at oblique incidence. This is because 25 nm thick PyC layer is thin enough not to affect to a few micron deep features on the bSi surface.
Since the in-plane size of intertwined graphene nanoribbons in PyC is about a few nanometers, strong electron scattering makes the electrical conductivity of PyC lower than that of graphene 16,17,21 and also suppresses electroand nonlinear-optical effects 12 . Thus one may expect that absorbance of graphene coated bSi will outperform PyC coated bSi. However, coating of the pyramidal bSi surface conformally by graphene is hardly achievable. Furthermore, monolayer thick graphene is not enough for sufficient absorbance. Meanwhile, PyC offers a reliable conformal coating method as 25 nm thick PyC layer doesn't make changes to the structural appearance of bSi.
The conformality of PyC is attributed to the nature of the CVD process, where the active sites are randomly distributed over the surface. Hydrocarbon molecules on these active sites are absorbed followed by hydrogen desorption 26 . Thus, one may expect that this mechanism can result in synthesis of the solid PyC layer that will precisely follow the shape of a substrate. In the initial stage of the CVD process, the methane decomposes forming carbon dimers (C2) in gaseous atmosphere 14,26,27 . Interaction of dimers with other gaseous species in the CVD chamber leads to the formation of aromatic C6-based compounds by C2→ C6 route (other routes are also possible 27 ) that attach on the substrate forming new active sites for substrate-gas interaction. Specifically, when a massive aromatic species land on the substrate, they may continue evolving thus forming active surface sites that initiate growth of the carbon rings both at the edges and on the top of basal planes 26 . Such surface activated synthesis explains the conformal growth of PyC over the arbitrary shaped substrate surface.
In summary, we have demonstrated a wide band anti-reflecting surface by combining bSi with PyC thin film. The structural integrity of the substrate was not compromised and enhanced light absorption was demonstrated over a wide band. Absorption and Raman spectroscopic measurements confirm that the PyC film is uniform over the substrate and the structure is amorphous. One may expect a lower DC conductivity and optical nonlinearity compared with highly oriented graphite films and graphene 12 . However, the excellent uniformity and extreme conformality of PyC films over large areas indicates dominant heterogeneous growth mechanism. The advantages of PyC coating make it preferable and practical for many potential applications. In particular, biocompatibility and chemical inertness of PyC coated bSi suggest use in biomedical sensors and increase the sensitivity in e.g. electrochemical experiments, where the molecule adhesion plays a crucial role. Moreover, since an ultra-thin PyC film is robust and easily holds its weight 28 , it could be used as a membrane material for a free-standing gratings in e.g. micro-and nanoelectromechanical systems.

Methods
Sample preparation. In order to demonstrate the conformal PyC deposition, we coated a bare silicon substrate and bSi with PyC films. The bSi was formed in a cryogenic, inductively coupled plasma reactive ion etcher (ICP-RIE; Plasmalab System 100, Oxford Instruments, UK) using the following optimized parameters: 40 sccm SF 6 , 18 sccm O 2 , 6 W forward power, 1000 W ICP power, − 110 °C, 10 mTorr pressure and helium backside Similar to the bare Si, the bSi reflectance is about 80% at wavelengths above 1000 nm. However, coating of bSi with PyC reduces the reflectance down to less than 0.5%. (b) Although, bSi reflects less than 0.5% in the visible spectral range, there is a reflection peak around 375 nm where the reflectance becomes more than 1%. In PyC coated bSi this peak is suppressed while the absorption stays higher than 99.5%. (c) bSi coated with PyC has reflectance less than 0.5% in the 1200 nm to 2000 nm spectral range while the reflectance of uncoated bSi is more than 80%.
Scientific RepoRts | 6:25922 | DOI: 10.1038/srep25922 cooling. Details of bSi fabrication are reported in previous studies 29,30 . The silicon used in the experiment was lightly p-doped, 0.5 mm thick silicon wafers.
The PyC coatings of the bare Si and bSi were performed by using a hot wall CVD system described in ref. 10. The system employs methane/hydrogen mixture as a carbon source and is equipped with a cylindrical quartz chamber, a tubular furnace (Carbolite CTF 12/75/700) and a computer controlled vacuum pump and gas flow controllers. The thickness of the fabricated PyC films is determined by the methane concentration in the hydrogen-methane gas mixture and pressure in the CVD chamber.
Before heating the oven, the CVD chamber was purged twice with nitrogen and once with hydrogen. In the first stage of the process, the chamber was heated up to 700 °C during one hour in hydrogen atmosphere (7 mbar). At 700 °C, it was pumped down to vacuum and the hydrogen was replaced with hydrogen-methane gas mixture. After the gas exchange the chamber was heated from 700 °C to 1100 °C in 40 min. At 1100 °C, the catalyst-free spontaneous methane decomposition starts. This process is initiated by methane radicals that form new hydrocarbon molecules, which are combined into massive polycyclic aromatic structures and deposited onto substrate 2,3 . After the process both sides of the substrate were coated with the PyC film. In our experiments we did not observe any dependence on the sample position inside the chamber, i.e. PyC films of the same thickness were deposited on the both sides of the horizontally and vertically oriented substrates. The gravity is not involved in the deposition process, i.e. it is unlikely dominated by landing of massive hydrocarbon particles on the substrates. The dependence of the deposited PyC film thickness on methane concentration can be found in ref. 17.
In order to demonstrate that developed technique can be employed for PyC deposition on arbitrary substrates, we also coated one-dimensional gratings fabricated from silica, silicon and TiO 2 substrates (see Supplementary information).
Characterization. The optical reflectance characterization for PyC was done by using spectrophotometer (Perkin Elmer lambda 1050 with 8/d geometry using spectral range from 350 nm to 2000 nm. Measurement was done using a spectralon disk as a backside mirror and about 1 mm × 10 mm beam spot size. Raman spectroscopy was done by Renishaw inVia Raman microscope with excitation wavelength of 514 nm. The excitation beam intensity was kept low in order to avoid heat induced phenomena in the carbon film. Scanning electron microscopy was done by SEM Zeiss Supra 40 and Leo 1550 Gemini.