Silicon Nanoholes for High

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Light Trapping in Conformal Graphene/Silicon Nanoholes for High Performance Photodetectors Jun Yang, Linlong Tang, Wei Luo, Jun Shen, Dahua Zhou, Shuanglong Feng, Xingzhan Wei, and Haofei Shi ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b08268 • Publication Date (Web): 25 Jul 2019 Downloaded from pubs.acs.org on July 26, 2019

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ACS Applied Materials & Interfaces

Light Trapping in Conformal Graphene/Silicon Nanoholes for High Performance Photodetectors Jun Yang1,2, Linlong Tang1,*, Wei Luo1, Jun Shen1, Dahua Zhou1, Shuanglong Feng1, Xingzhan Wei1,* and Haofei Shi1 1 Chongqing

Key Laboratory of Multi-scale Manufacturing Technology, Chongqing Institute

of Green and Intelligent Technology, Chinese Academy of Sciences, Chongqing 400714, P.R. China. 2

University of Chinese Academy of Sciences, Beijing, 100049, P.R. China

Corresponding Author * Prof. *

Xingzhan Wei, E-mail address: [email protected]

Dr. Linlong Tang, E-mail address: tll@ cigit.ac.cn

ABSTRACT: Hybrid graphene/silicon heterojunctions have been widely utilized in photodetectors, due to their unique characteristics of high sensitivity, fast response, and CMOS-compatibility. However, the photoresponse is restricted by the high reflectance of planar silicon (up to 50%). Herein, an improved graphene/Si detector with excellent light absorption performance is proposed and demonstrated by directly growing graphene on the surface of silicon nanoholes (SiNHs). It is shown that the combination of SiNHs with conformal graphene provides superior interfaces for efficient light trapping and transport of the photoexcited carriers. A high absorption of up to 90% was achieved, and the conformal graphene/SiNHs based photodetectors exhibited a higher photoresponsivity (2720 A/W) and faster response (~6.2 μs), comparing with the counterpart of the planar graphene/Si, for which the corresponding values are 850 A/W and 51.3 μs. These results showcase the vital role of 1 ACS Paragon Plus Environment

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the material morphology in optoelectronic conversion and pave the way to explore novel high-performance heterojunction photodetectors.

KEYWORDS: Graphene, Photodetector, Light trapping, Silicon nanostructures, Conformal

INTRODUCTION Graphene has attracted intensive attention in the past decades due to the unique optoelectronic properties, such as broadband absorption, Dirac cone band dispersion, high carrier mobility, and electrochemically tunable Fermi level1-2. Recently, tremendous efforts have been made to fabricate high performance graphene photodetectors for wide spectral response, ranging from the visible to the infrared3-5. However, the intrinsic low optical absorption of graphene (~2.3% per layer) has limited its photoresponsivity to a very low level of ~10 mA/W, which is much smaller than those of traditional semiconductor photodetectors69.

Combining graphene with conventional semiconductors to form van der Waals

heterojunctions offers an alternative pathway to develop high performance photodetectors. Until now, various semiconductors, including Si, Ge, GaAs, CdSe, PbS, and GaN, have been successfully applied to construct hybrid graphene/semiconductor junction photodetectors, in which the photoresponsivity is mainly determined by light absorptions and carrier transport behaviors of the semiconductors (instead of the graphene)10-18. Among these detectors, the hybrid graphene/silicon photodetectors are quite promising because of their high sensitivity, fast response, and CMOS-compatibility19-22. However, it is well known that a planar silicon surface has a high reflectance up to 50%, which severely limits the absorption of light in a graphene/silicon junction and further restricts the efficiency of the detector23-26. Therefore, the enhancement of light absorptions in graphene/silicon detectors by light trapping engineering is of great importance for the improvement of the detector performances.

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Over the past few years, various silicon nanostructures have been proposed for light trappings, such as nanowire, nanohole, and nanopyramid arrays, porous silicon, and silicon quantum dots, which exhibit excellent light-trapping performances with low reflectance and high absorptance26-34. And it is demonstrated that such light trapping nanostructures could significantly boost the efficiencies of silicon photoconductors and photovoltaic cells35-38. Inspired by this idea, researchers have combined silicon nanostructures with graphene to improve the performances of graphene/silicon heterojunction photovoltaic cells and detectors, and considerable improvements of the efficiencies are observed experimentally39-42. For instance, Fan et al. reported the use of the graphene/Si nanowire Schottky junction to enhance light harvesting39, Kim et al. showed that the graphene/porous Si photodiode had an enhanced sensitivity at the ultraviolet region40, and Yu et al. demonstrated that the graphene/Si quantum dots photodiode exhibited a superior performance due to the enhanced optical absorptions and the increased built-in potential33-34. However, the graphene growth processes typically require high temperature and catalytic metal substrates (e.g., copper, nickel). Moreover, the graphene transfer process usually needs polymers (e.g., PMMA), which are totally incompatible with nanostructured silicon, making the graphene/nanostructured silicon junction detectors face the following problems: (1) The graphene transfer process would inevitably induce both residue resins and damages, especially on silicon nanowire or nanohole arrays, which leads to a bad surface coverage and a relatively high electron-hole recombination rate; (2) Graphene films are horizontally stacked on substrates and large portions of them are suspended on the silicon nanowires or nanoholes, which reduce the effective contact areas of graphene/silicon heterojunctions and further cause the inefficient injections of carriers from the silicon to the graphene. Therefore, given the growing importance of the light-trapping graphene/silicon heterojunction detectors, it is imperative to develop new methods to solve these problems. Herein, a method of direct growth of graphene films on the surface of silicon nanoholes (SiNHs) through plasma enhanced chemical vapor deposition (PECVD), was proposed to 3 ACS Paragon Plus Environment


achieve transfer-free conformal graphene/SiNHs photodetectors for the first time. It is demonstrated that the conformal graphene/SiNHs structures have an excellent light absorption performance with absorptions up to 90%, and also possess high power absorption densities, resulting in more photocarrier generations per unit volume in the SiNHs than planar silicon. Based on such artificial nanostructures, conformal graphene/SiNHs photodetectors are then fabricated, and it is observed that the responsivity and the response speed of the photodetector are improved considerably compared to conventional graphene/Si detectors, on account of the higher absorptions, higher carrier densities, and shorter transport paths for photoexcited carriers. The proposed conformal graphene/SiNHs structures provide excellent interfaces for both light trapping and carrier transport, representing a new route to improve the performance of graphene/semiconductor hybrid photodetectors.

RESULTS AND DISCUSSION Preparation and morphological properties. Conformal graphene/SiNHs were prepared following the process illustrated in Figure 1. Firstly, silicon nanoholes were prepared by a silver-assisted etching method with discretely distributed silver nanoparticles (AgNPs), as shown in Figure 1a-c. Subsequently, the conformal graphene was directly grown on SiNHs by low pressure radio frequency (RF) plasma enhanced chemical vapor deposition (PECVD) technique reported in our previous works43-45, as illustrated in Figure 1d,e. Finally, the conformal graphene/SiNHs based photodetectors were fabricated via photolithography, vacuum evaporation and dry etching technologies (Figure 1f,g). The preparation details are presented in the experimental section. Figure 2 shows the SEM and AFM images of the conformal graphene/SiNHs. To guarantee the fabrication of SiNHs with requirable feature sizes, a dilute AgNO3 solution and a reasonable deposition time are essential, thus the deposited nanoscale AgNPs can be sparsely and discretely distributed on the silicon surface. The corresponding SEM images are 4 ACS Paragon Plus Environment

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presented in Figure S1, which shows an AgNPs layer with particle size ranging from 50 nm to 300 nm is formed on the planar silicon. In the AgNPs-assisted etching process, the silicon covered by AgNPs was etched along the vertical direction, while other parts without the coverage were nearly without erosion, as depicted in Fig. 2a. Interestingly, due to the random distribution of AgNPs, both SiNHs and silicon nanocones were simultaneously obtained (see Figure S2). Subsequently, multilayer graphene was directly grown on the structured silicon surface by the above-mentioned PECVD method. The results are shown in Figure 2b, and we can see that the SiNHs are conformally covered with a thin layer of directly-grown graphene. The cross-sectional image of the graphene/SiNH structures is presented in Figure 2c, showing that the SiNHs are vertically aligned and the depths of them are about 1~2 μm. Besides, such silicon nanoholes exhibit an inverted taper shape, which enables an excellent light harvesting performance24, 29, 46. The surface roughness of the graphene/SiNHs was characterized by AFM (Fig. 2d). It is clear that the conformal graphene/SiNHs is extremely rough, and the rootmean-square roughness (Rrms) reaches 127 nm, which is also beneficial for reducing reflection and trapping more light. TEM measurement was carried out aiming at the morphology of graphene in the conformal graphene/SiNHs structures, and the corresponding images are shown in Figure 3a and S3. It is clear that graphene has been conformally grown on both the tops and sidewalls of the silicon nanocones, and the graphene at the apexes is thicker than the sidewalls, indicating the carbon atoms have more probability to adhere to the upper parts during the PECVD process. Then, we adopted a higher magnification TEM to clearly image the graphene details, and the corresponding images are shown in Figure 3b,c. The graphene appears as clear multilayer nanosheets with an interlayer spacing of 0.34 nm, and the apexes and sidewalls of the nanocones have more than 10 and around 5~7 layers, respectively. The multilayer nanosheets herein are justifiably classified as graphene as reported in other literatures47-48, because they have an analogous Raman and optoelectric characteristic. It should be also noted 5 ACS Paragon Plus Environment


that some defects inevitably exist in these nanosheets, which can be inferred from the Raman spectra with strong D and D' band peaks, as illustrated in Figure 3d. Otherwise, there is nearly no distinguishable difference between the spectra of graphene/Si and conformal graphene/SiNHs, which further verifies that graphene can be conformally grown on the silicon surfaces with different morphologies. Light trapping in the conformal graphene/SiNHs structures. Figure 4a depicts the reflection spectra of SiNHs samples before and after graphene growth, and the spectra of planar silicon and graphene/Si were also measured as a control experiment. The reflections of SiNHs are only 10%~15% in the wavelength range of 400~800 nm, which are relatively lower than the case of planar silicon (35%~55%), signifying the SiNHs have an excellent antireflection capability. After the growth of graphene, the reflections of the conformal graphene/SiNHs are further reduced to less than 10%, due to the generation of extra absorption through graphene. Meanwhile, the spectra in wider range were measured in the wavelength range of 380-2400 nm, as depicted in Figure S4. It shows that the absorptance is high in the range of 380-1100 nm, whereas declines rapidly during 1100-1200 nm which corresponds to the bandgap of Si (1.1 eV), and finally reduces to about 5% in the range of 1200-2400 nm. Such trend shows a good agreement with the absorptance of Si, indicating the absorptions of our samples are mainly contributed by Si rather than graphene. Obviously, the light trapping behaviors of both SiNHs and conformal graphene/SiNHs can be attributed to their nanostructures, as shown in Figure 5a,c. These artificial nanostructures can provide more surface areas for light absorption, and simultaneously suppress light reflection by the tapered shapes of SiNHs, which have an inherently graded refractive index profile. To be specific, the volume density gradually changes from air to silicon according to the effective medium theory26, resulting in a gradient change of the equivalent refractive index from 1 (air) to 3.5 (bulk silicon). We derived the absorption spectra from the reflectance and transmittance data, as depicted in Figure 4b. It is evident the 6 ACS Paragon Plus Environment

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light trapping effect is produced in both SiNHs and conformal graphene/SiNHs, leading to greatly improved absorptions (>90%) in comparison with the cases of planar Si and planar graphene/Si (~50%). In order to further clarify the mechanism of high absorptions, we numerically modeled the light-trapping behaviors of SiNHs with and without graphene. The obtained reflection and absorption spectra are shown in Figure 5e,f. The modeling spectra exhibit a similar trend with the experimental results (Figure 4a,b) for both SiNHs and conformal graphene/SiNHs, further confirming the excellent light trapping roles of these nanostructures. Note that the simulated reflectances of SiNHs are slightly higher than the experimental results, which could be attributed to the different surface roughness between simulation and experiment. More detailedly, the sidewalls and apexes of SiNHs are assumed to be perfectly smooth in the simulation, while the metal-assisted etching process in the experiment would inevitably bring in tiny burrs and concave pits on the surfaces which are beneficial for reducing reflection. The simulated electric field and power absorption density distribution of the planar Si and SiNHs are illustrated in Figure 5b,d. For the case of planar Si, the highest magnitude of the electric field reaches 1.65 V/m at the Air/Si interface (the incident electric field is set to be 1 V/m), due to the high reflection of Si. The power absorption density exponentially decreases in the direction perpendicular to the silicon surface, and a maximum value of 595 W/m3 is produced in the silicon layer near the interface, as shown in Figure 5b. In comparison, the highest electric field is only 1.15 V/m for the SiNHs structures (Figure 5d), which is smaller than that of the planar Si case, indicating the reflection is reduced. However, the electric field distribution of the entire SiNHs is higher than that of planar Si, resulting in higher power absorption densities with a maximum value of 1679 W/m3. Since the absorbed photons would excite electron-hole pairs in silicon, therefore, more photoexcited carriers per unit volume can be generated due to the higher power absorption density, which tends to improve the photodetection efficiencies. 7 ACS Paragon Plus Environment


Photoelectric properties. On the basis of controllable preparation of conformal graphene/SiNHs, the photodetectors were fabricated through a photolithography and lift-off process, and the relevant schematic view and SEM image of the typical conformal graphene/SiNHs photodetector is shown in Figure 6a,b, respectively. Moreover, a high magnification SEM was employed to image the detector, which shows that the Au electrodes, SiNHs substrate, and conformal graphene/SiNHs channel are clean and can be easily identified (see Figure S5). An energy dispersive spectrometer (EDS) measurement was also conducted to map element distributions (Figure S6), which are in good accordance with the high-magnification SEM images. As a comparison, two other types of photodetectors based on planar graphene/Si and SiNHs arrays were fabricated, and the I-V electrical characteristics were measured, as shown in Figure S7. Obviously, both the planar graphene/silicon and conformal graphene/SiNHs photodetectors exhibit ohmic contact behaviors with linear I-V relations, while the SiNHs detector shows a curved I-V relation, suggesting Schottky contacts are formed between the Au electrodes and SiNHs. Figure 6c shows the photoresponse of three types of photodetectors based on the planar graphene/Si, SiNHs, and conformal graphene/SiNHs at 635 nm. Clearly, all devices can be readily switched on or off with excellent reproducibility. The photocurrent of conformal graphene/SiNHs photodetector is up to 34 μA, which is much higher than counterparts of planar graphene/Si (17 μA) and SiNHs (10 μA). The relevant photoresponse mechanism for graphene/SiNHs detector is provided in Figure 6d to explain the photocurrent enhancement. Due to the difference of Fermi level, a portion of electrons in SiNHs would move into the graphene and cause an upward bending of energy levels in silicon near the interface, thus, a related built-in electric field is formed which is directing from the silicon to the graphene. Upon the illumination of light, the photocarriers generated in SiNHs will be separated by the built-in field, leading to the injection of holes into the graphene channel. These additional 8 ACS Paragon Plus Environment

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injected holes from SiNHs will change the conductivity of graphene, further inducing the variation of the current in graphene and effectively leading to a quantum gain. The fundamental role of the conformal graphene layer herein is to serve as a photogating channel for the hybrid photodetectors. And the photoresponsivity (R), one of the crucial parameters of a photodetector, can be extracted from R = IP / P, where IP is the photocurrent and P is the incident light power. Notably, a photoresponsivity up to 2720 A/W is obtained at Vds = 1 V bias, which is obviously higher than planar graphene/Si (850 A/W) and SiNHs (1360 A/W) based photodetectors. And the detectivity (D*) can be calculated to be 1.25 × 1011 cm·Hz1/2·W-1 from the formula

D

*

R A

2q I dark . The photoresponse enhancement of the

conformal graphene/SiNHs detector can be attributed to the light trapping effect and the conformal junction. As described above in Figures 4 and 5, the light trapping effect is beneficial for producing high absorption (above 90%) and power absorption density, therefore resulting in more photocarrier generations per unit volume. What’s more, the efficient separation and collection of photocarriers are also responsible for the high performance. Compared with the planar graphene/Si and SiNHs detectors, the conformal graphene/SiNHs detectors can provide a shorter transport path for the photoexcited carriers in SiNHs to inject into the conformal graphene channel, resulting in less charge recombination. More importantly, the conformal graphene/SiNHs detectors possess excellent stability. After exposed in air for 6 months, nearly no major degradation occurred (Figure S8), indicating the detector is stable even without sealing. The broadband photoresponse of the conformal graphene/SiNHs photodetector was investigated. As shown in Figure S9, the detector has an obvious photoresponse in all the measured wavelengths, and the normalized responsivity versus wavelength is plotted, which shows a maximum responsivity at around 900 nm. In addition, the photoresponse at 1550 nm was measured, as presented in Figure S10. The photocurrent (~0.2 A) is two orders smaller 9 ACS Paragon Plus Environment


than that at 635 nm (34 A), and the rise and recovery time is much longer, which signifies that the intrinsic response of graphene is not good and the SiNHs are critical for the optoelectric response. At last, the photoresponse speeds of three kinds of photodetectors were measured with respect to the light modulation frequencies at 1 Hz, 100 Hz, and 2500 Hz, and the corresponding results for the conformal graphene/SiNHs photodetector are shown in Figure 7. For all switching frequencies, the photodetectors can switch fastly between “On” and “Off” states (Figure 7a), and the relative balance (Imax-Imin)/Imin shows a small attenuation (about 10%) even at 2500 Hz (Figure 7b), suggesting the detectors could work in high frequencies. Then, the response time is extracted from the response curve, where the rise and recovery times are determined from the time interval for the response to rise (decline) from 10% (90%) to 90% (10%) of the peak value49, respectively. As shown in Figure 7c,d, the rise time τr and the recovery time τf are measured to be 6.2 μs and 8.6 μs, respectively. In contrast, τr and τf for planar graphene/Si detectors are 51.3 μs and 120 μs, respectively, and for SiNHs detectors are 17.6 μs and 64.2 μs (See Figure S11). Therefore, the conformal graphene/SiNHs photodetectors are much faster than the other two detectors, because the carriers generated in SiNHs can inject into the nearby conformal graphene more rapidly. This phenomenon is analogous to the reported silicon nanowires/PEDOT:PSS heterojunction structures49-51. Specifically, the conformal graphene/SiNHs heterojunction herein constructs a shorter transport path, which can efficiently collect holes generated from SiNHs and inject them into the conformal graphene channel. Compared with the previously reported graphene/Si photodetectors, the conformal graphene/SiNHs detector in this paper shows a comprehensive performance with high responsivity and low response time, as presented in the Table S1. Thus, a photodetector with both high responsivity and fast response speed in the visible region has been demonstrated through the direct growth of graphene on the most versatile semiconductor 10 ACS Paragon Plus Environment

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(silicon). It should be noted that such mechanism is with universal compatibility with other kinds of semiconductors, such as germanium, gallium arsenide, et. al., which paves the way to explore high-performance hybrid heterojunction photodetectors in other interested wavelength regions.

CONCLUSION To summarize, a novel type of conformal graphene/SiNHs heterojunction photodetectors was fabricated by the directly conformal growth of graphene on silicon nanoholes via PECVD. Significant enhancement of light absorption up to 90% was achieved due to the efficient light trapping in the conformal graphene/SiNHs structures. Meanwhile, the responsivity of the conformal graphene/SiNHs detector is enhanced by 2 times compared to the SiNHs detector. Also, the response speed of the conformal graphene/SiNHs detector becomes even faster. These results originate from the lower reflections, higher absorption densities, and shorter carrier

transport

paths

in

conformal

graphene/SiNHs

detectors.

The

conformal

graphene/SiNHs photodetectors provide new ideas for the development of high performance hybrid photodetectors.

METHODS Fabrication of silicon nanoholes. SiNHs were prepared by a traditional two-step silver assisted etching method. Firstly, n-type (100) single-side polished silicon wafers (thickness 525±10 μm, resistivity 1-10 Ω▪cm) were successively cleaned by ultrasonication in deionized (DI) water, acetone, ethanol, and DI water. The native oxide layer was removed by immersing the wafer in dilute aqueous hydrofluoric acid (HF, 5%) for 3 min. Next, the naked silicon wafers were dipped in a mixed aqueous solution of HF (4.6 mol/L) and AgNO3 (0.005 mol/L) for 60 seconds, reducing Ag+ to independently distributed silver nanoparticles (AgNPs), which assists the following wet selective etching by HF. Subsequently, the AgNPs-coated silicon wafer was immersed into an aqueous etchant containing HF (4.6 mol/L) and hydrogen 11 ACS Paragon Plus Environment


peroxide (0.2 mol/L) at room temperature (Figure 1b). In this step, vertically-aligned silicon nanoholes with different depths can be fabricated by controlling the etching time. The asfabricated SiNHs samples were immersed into the diluted HNO3 and HF solution respectively, to remove the AgNPs and SiO2 on the surfaces of SiNHs, and the schematic representation of the corresponding SiNHs is shown in Figure 1c. Preparation of Conformal Graphene/SiNHs. The low pressure RF-PECVD technique was adopted to fabricated conformal graphene on SiNHs. Briefly, the as-obtained SiNHs samples were placed in the quartz tube of PECVD, and the furnace was heated to 750 °C at the working pressure of 42 Pa. The SiNHs samples were annealed at 700 ℃ in H2 for 30 min to clean the surfaces. Next, the graphene was conformally grown on the surface of SiNHs (Figure 1e) when the gas flow of CH4 and H2 was chosen to be 6 and 4 sccm, respectively, and the RF power and growth time were 250 w and 10 min. Photodetector fabrication. Photolithography, vacuum evaporation, and dry etching technologies were separately adopted to fabricate the conformal graphene/SiNHs based photodetectors. 3 nm Cr and 50 nm Au films were successively deposited onto the conformal graphene/SiNHs to form ohmic contact by using e-beam evaporation. Both the source and drain electrodes were patterned via a lift-off process. Subsequently, the conformal graphene film was patterned by photolithography and etched by O2 plasma with 100 W RF power and 20 sccm gas flow in an RIE system for 3 min. Characterization. The surface morphologies of the samples were characterized by JEOL JSM-7800F scanning electron microscopy (SEM) operating at 5~10 kV. Furthermore, the TEI Talos F200S G2 Field emission transmission electron microscopy (TEM) with a 200 kV working voltage was adopted to observe the subtle details. The TEM samples were prepared by mechanically scratching the surface of graphene/SiNHs samples and transferring to TEM grids. Atomic force microscopy (AFM, Bruker Dimension Edge) measurement was 12 ACS Paragon Plus Environment

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performed to characterize the roughness of the graphene/Si, SiNHs and conformal graphene/SiNHs, and a Raman spectroscopy (Renishaw inVia Reflex) with a laser excitation wavelength of 532 nm was employed to measure the quality of graphene. The reflectance and absorbance spectra of planar silicon, graphene/Si, SiNHs and conformal graphene/SiNHs were measured by a UV-Vis-NIR spectrophotometer (PerkinElmer, Lambda 950) with an integrating sphere. Lastly, the photoresponse characteristics of the fabricated photodetectors were evaluated using a semiconductor characterization system (Keithley 4200) under dark and light illumination conditions, in which a laser of 635 nm wavelength was used as the light source. Modeling. The modeling of the optical spectra and electric field distribution for the conformal graphene/SiNHs was conducted by finite element method via COMSOL Multiphysics software. In the modeling, periodic boundary conditions were imposed in the xand y-direction, while two perfectly matched layers were applied in the z-direction. The geometrical parameters in the modelings were set in accordance with the experiment. For the silicon holes, the radius of the top end face, the radius of the bottom end surface, and the height were randomly distributed in the range of 100~300 nm, 10~100 nm, and 1~2 m, respectively, and the period is 50~350 nm larger than the diameter of the top end surface. Besides, the geometrical parameters for the silicon nanocones were set as follows: the radius of the end surface and the height were distributed in the range of 50~200 nm and 350~650 nm, respectively, and the period was 50~300 nm larger than the diameter of the nanocone end surface. The numbers were generated by the random function generator in COMSOL. 100 groups of silicon nanoholes and nanocones were simulated, and the relevant absorption and reflection spectra were averaged to fit the experiment. ASSOCIATED CONTENT Supporting Information 13 ACS Paragon Plus Environment


Supplementary material including SEM images of silver nanoparticles, silicon nanoholes and silicon nanocones, cross-sectional SEM image of SiNHs, TEM images of graphene on silicon nanostructures, SEM images and EDS mapping images of the photodetector based on conformal graphene/SiNHs, electrical characteristics of photodetectors in photoconductor mode, and response speed comparison of different photodetectors. AUTHOR INFORMATION Corresponding Author *

Prof. Xingzhan Wei, E-mail address: [email protected]

*

Dr. Linlong Tang, E-mail address: tll@ cigit.ac.cn

Author Contributions The manuscript was written through the contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interests.

ACKNOWLEDGEMENTS This work was supported by the National Natural Science Foundation of China (NSFC 11574308), the Basic Science and Frontier Technology Research Program of Chongqing (cstc2016jcyjA0315, cstc2017shmsA1471) and Hundred-Talent Program of Chinese Academy of Sciences.


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REFERENCES (1) Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Katsnelson, M. I.; Grigorieva, I. V.; Dubonos, S. V.; Firsov, A. A. Two-Dimensional Gas of Massless Dirac Fermions in Graphene. Nature 2005, 438 (7065), 197-200. (2) Geim, A. K.; Novoselov, K. S. The Rise of Graphene. Nat. Mater 2007, 6 (3), 183-191. (3) Koppens, F. H.; Mueller, T.; Avouris, P.; Ferrari, A. C.; Vitiello, M. S.; Polini, M. Photodetectors Based on Graphene, Other Two-Dimensional Materials and Hybrid Systems. Nat. Nanotech 2014, 9 (10), 780-793. (4) Liu, C. H.; Chang, Y. C.; Norris, T. B.; Zhong, Z. Graphene Photodetectors with UltraBroadband and High Responsivity at Room Temperature. Nat. Nanotech 2014, 9 (4), 273-278. (5) Mueller, T.; Xia, F.; Avouris, P. Graphene Photodetectors for High-Speed Optical Communications. Nat. Photonics 2010, 4, 297-301. (6) Yao, J. D.; Zheng, Z. Q.; Shao, J. M.; Yang, G. W. Stable, Highly-Responsive and Broadband Photodetection Based on Large-Area Multilayered WS2 Films Grown by PulsedLaser Deposition. Nanoscale 2015, 7 (36), 14974-14981. (7) Yao, J.; Zheng, Z.; Yang, G. All-Layered 2D Optoelectronics: A High-Performance UV– vis–NIR Broadband SnSe Photodetector with Bi2Te3 Topological Insulator Electrodes. Adv. Funct. Mater. 2017, 27 (33), 1701823. (8) Xie, C.; Mak, C.; Tao, X.; Yan, F. Photodetectors Based on Two-Dimensional Layered Materials Beyond Graphene. Adv. Funct. Mater. 2017, 27 (19), 1603886. (9) Long, M.; Wang, P.; Fang, H.; Hu, W. Progress, Challenges, and Opportunities for 2D Material Based Photodetectors. Adv. Funct. Mater. 2018, 1803807. (10) Liu, F.; Kar, S. Quantum Carrier Reinvestment-Induced Ultrahigh and Broadband Photocurrent Responses in Graphene-Silicon Junctions. ACS Nano 2014, 8 (10), 10270-10279. (11) Konstantatos, G.; Badioli, M.; Gaudreau, L.; Osmond, J.; Bernechea, M.; Garcia de Arquer, F. P.; Gatti, F.; Koppens, F. H. Hybrid Graphene-Quantum Dot Phototransistors with Ultrahigh Gain. Nat. Nanotech. 2012, 7 (6), 363-368. (12) Goossens, S.; Navickaite, G.; Monasterio, C.; Gupta, S.; Piqueras, J. J.; Pérez, R.; Burwell, G.; Nikitskiy, I.; Lasanta, T.; Galán, T.; Puma, E.; Centeno, A.; Pesquera, A.; Zurutuza, A.; Konstantatos, G.; Koppens, F. Broadband Image Sensor Array Based on Graphene–CMOS Integration. Nat. Photonics 2017, 11 (6), 366-371. (13) Zeng, L. H.; Wang, M. Z.; Hu, H.; Nie, B.; Yu, Y. Q.; Wu, C. Y.; Wang, L.; Hu, J. G.; Xie, C.; Liang, F. X.; Luo, L. B. Monolayer Graphene/Germanium Schottky Junction as 15 ACS Paragon Plus Environment


High-Performance Self-Driven Infrared Light Photodetector. ACS Appl. Mater. Inter. 2013, 5 (19), 9362-9366. (14) Yang, F.; Cong, H.; Yu, K.; Zhou, L.; Wang, N.; Liu, Z.; Li, C.; Wang, Q.; Cheng, B. Ultrathin Broadband Germanium–Graphene Hybrid Photodetector with High Performance. ACS Appl. Mater. Inter. 2017, 9 (15), 13422-13429. (15) Luo, L.-B.; Hu, H.; Wang, X.-H.; Lu, R.; Zou, Y.-F.; Yu, Y.-Q.; Liang, F.-X. A Graphene/Gaas Near-Infrared Photodetector Enabled By Interfacial Passivation With Fast Response And High Sensitivity. J. Mater. Chem. C 2015, 3 (18), 4723-4728. (16) Lu, Y.; Feng, S.; Wu, Z.; Gao, Y.; Yang, J.; Zhang, Y.; Hao, Z.; Li, J.; Li, E.; Chen, H.; Lin, S. Broadband Surface Plasmon Resonance Enhanced Self-Powered Graphene/Gaas Photodetector with Ultrahigh Detectivity. Nano Energy 2018, 47, 140-149. (17) Gao, Z.; Jin, W.; Zhou, Y.; Dai, Y.; Yu, B.; Liu, C.; Xu, W.; Li, Y.; Peng, H.; Liu, Z.; Dai, L. Self-Powered Flexible and Transparent Photovoltaic Detectors Based on Cdse Nanobelt/Graphene Schottky Junctions. Nanoscale 2013, 5 (12), 5576-5581. (18) Jin, W.; Ye, Y.; Gan, L.; Yu, B.; Wu, P.; Dai, Y.; Meng, H.; Guo, X.; Dai, L. SelfPowered High Performance Photodetectors Based on Cdse Nanobelt/Graphene Schottky Junctions. J. Mater. Chem. 2012, 22 (7), 2863-2867. (19) Chen, Z.; Cheng, Z.; Wang, J.; Wan, X.; Shu, C.; Tsang, H. K.; Ho, H. P.; Xu, J.-B. High Responsivity, Broadband, and Fast Graphene/Silicon Photodetector in Photoconductor Mode. Adv. Opt. Mater. 2015, 3 (9), 1207-1214. (20) An, X.; Liu, F.; Jung, Y. J.; Kar, S. Tunable Graphene-Silicon Heterojunctions for Ultrasensitive Photodetection. Nano Lett. 2013, 13 (3), 909-916. (21) ShoichiroFukushima; MasaakiShimatani; SatoshiOkuda; ShinpeiOgawa; YasushiKanai; TakaoOno; KazuhikoMatsumoto. High Responsivity Middle-Wavelength Infrared Graphene Photodetectors Using Photo-Gating. Appl. Phys. Lett. 2018, 113 (6), 061102. (22) Lin, F.; Chen, S.-W.; Meng, J.; Tse, G.; Fu, X.-W.; Xu, F.-J.; Shen, B.; Liao, Z.-M.; Yu, D.-P. Graphene/GaN Diodes for Ultraviolet and Visible Photodetectors. Appl. Phys. Lett. 2014, 105 (7), 073103. (23) Gao, Y.; Cansizoglu, H.; Polat, K. G.; Ghandiparsi, S.; Kaya, A.; Mamtaz, H. H.; Mayet, A. S.; Wang, Y.; Zhang, X.; Yamada, T.; Devine, E. P.; Elrefaie, A. F.; Wang, S.-Y.; Islam, M. S. Photon-Trapping Microstructures Enable High-Speed High-Efficiency Silicon Photodiodes. Nat. Photonics 2017, 11 (5), 301-308. (24) Han, S. E.; Chen, G. Optical Absorption Enhancement in Silicon Nanohole Arrays for Solar Photovoltaics. Nano Lett. 2010, 10 (3), 1012-1015. 16 ACS Paragon Plus Environment

Page 16 of 27

Page 17 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(25) Wang, K. X.; Yu, Z.; Liu, V.; Cui, Y.; Fan, S. Absorption Enhancement in Ultrathin Crystalline Silicon Solar Cells with Antireflection and Light-Trapping Nanocone Gratings. Nano Lett. 2012, 12 (3), 1616-1619. (26) Zhu, J.; Yu, Z.; Burkhard, G. F.; Hsu, C.-M.; Connor, S. T.; Xu, Y.; Wang, Q.; McGehee, M.; Fan, S.; Cui, Y. Optical Absorption Enhancement in Amorphous Silicon Nanowire and Nanocone Arrays. Nano Lett. 2009, 9 (1), 279-282. (27) Hong, L.; Wang, X.; Zheng, H.; He, L.; Wang, H.; Yu, H.; Rusli. High Efficiency Silicon Nanohole/Organic Heterojunction Hybrid Solar Cell. Appl. Phys. Lett. 2014, 104 (5), 053104 (28) Lin, C.; Martinez, L. J.; Povinelli, M. L. Experimental Broadband Absorption Enhancement In Silicon Nanohole Structures with Optimized Complex Unit Cells. Opt. Express, 2013, 21(105): A872-A882 (29) Chen, T.-G.; Yu, P.; Chen, S.-W.; Chang, F.-Y.; Huang, B.-Y.; Cheng, Y.-C.; Hsiao, J.C.; Li, C.-K.; Wu, Y.-R. Characteristics of Large-Scale Nanohole Arrays for Thin-Silicon Photovoltaics. Prog. Photovoltaics 2014, 22 (4), 452-461. (30) Tsakalakos, L.; Balch, J.; Fronheiser, J.; Korevaar, B. A.; Sulima, O.; Rand, J. Silicon Nanowire Solar Cells. Appl. Phys. Lett. 2007, 91 (23), 233117. (31) Cao, L.; Fan, P.; Vasudev, A. P.; White, J. S.; Yu, Z.; Cai, W.; Schuller, J. A.; Fan, S.; Brongersma, M. L. Semiconductor Nanowire Optical Antenna Solar Absorbers. Nano Lett. 2010, 10 (2), 439-445. (32) Peng, K.-Q.; Wang, X.; Li, L.; Hu, Y.; Lee, S.-T. Silicon Nanowires for Advanced Energy Conversion And Storage. Nano Today 2013, 8 (1), 75-97. (33) Yu, T.; Wang, F.; Xu, Y.; Ma, L.; Pi, X.; Yang, D. Graphene Coupled with Silicon Quantum Dots for High-Performance Bulk-Silicon-Based Schottky-Junction Photodetectors. Adv. Mater. 2016, 28 (24), 4912-4919. (34) Ni, Z.; Ma, L.; Du, S.; Xu, Y.; Yuan, M.; Fang, H.; Wang, Z.; Xu, M.; Li, D.; Yang, J.; Hu, W.; Pi, X.; Yang, D. Plasmonic Silicon Quantum Dots Enabled High-Sensitivity Ultrabroadband Photodetection of Graphene-Based Hybrid Phototransistors. ACS Nano 2017, 11 (10), 9854-9862. (35) Peng, K.-Q.; Wang, X.; Li, L.; Wu, X.-L.; Lee, S.-T. High-Performance Silicon Nanohole Solar Cells. J. Am. Chem. Soc. 2010, 132 (20), 6872-6873. (36) Garnett, E.; Yang, P. Light Trapping in Silicon Nanowire Solar Cells. Nano Lett. 2010, 10 (3), 1082-1087. (37) Lu, W.; Wang, C.; Yue, W.; Chen, L. Si/PEDOT:PSS Core/Shell Nanowire Arrays for Efficient Hybrid Solar Cells. Nanoscale 2011, 3 (9), 3631-3634. 17 ACS Paragon Plus Environment


(38) Yan, R.; Gargas, D.; Yang, P. Nanowire Photonics. Nat. Photonics 2009, 3 (10), 569-576. (39) Fan, G.; Zhu, H.; Wang, K.; Wei, J.; Li, X.; Shu, Q.; Guo, N.; Wu, D. Graphene/Silicon Nanowire Schottky Junction for Enhanced Light Harvesting. ACS Appl. Mater. Inter. 2011, 3 (3), 721-725. (40) Kim, J.; Joo, S. S.; Lee, K. W.; Kim, J. H.; Shin, D. H.; Kim, S.; Choi, S. H. NearUltraviolet-Sensitive Graphene/Porous Silicon Photodetectors. ACS Appl. Mater. Inter. 2014, 6 (23), 20880-20886. (41) Xie, C.; Lv, P.; Nie, B.; Jie, J.; Zhang, X.; Wang, Z.; Jiang, P.; Hu, Z.; Luo, L.; Zhu, Z.; Wang, L.; Wu, C. Monolayer Graphene Film/Silicon Nanowire Array Schottky Junction Solar Cells. Appl. Phys. Lett. 2011, 99 (13): 133113. (42) Zeng, L.; Xie, C.; Tao, L.; Long, H.; Tang, C.; Tsang, Y. H.; Jie, J. Bilayer Graphene Based Surface Passivation Enhanced Nano Structured Self-Powered Near-Infrared Photodetector. Opt. Express 2015, 23 (4), 4839-4846. (43) Zhang, H.; Zhao, K.; Cui, S.; Yang, J.; Zhou, D.; Tang, L.; Shen, J.; Feng, S.; Zhang, W.; Fu, Y. Anomalous Temperature Coefficient Of Resistance In Graphene Nanowalls/Polymer Films and Applications In Infrared Photodetectors. Nanophotonics 2018, 7(5): 883-892. (44) Yang, J.; Ran, Q.; Wei, D.; Sun, T.; Yu, L.; Song, X.; Pu, L.; Shi, H.; Du, C. ThreeDimensional Conformal Graphene Microstructure for Flexible and Highly Sensitive Electronic Skin. Nanotechnology 2017, 28 (11), 115501. (45) Yang, J.; Wei, D.; Tang, L.; Song, X.; Luo, W.; Chu, J.; Gao, T.; Shi, H.; Du, C. Wearable Temperature Sensor Based on Graphene Nanowalls. RSC Adv. 2015, 5 (32), 2560925615. (46) Zhang, A.; Guo, Z. Efficient Light Trapping in Tapered Silicon Nanohole Arrays. Optik 2016, 127 (5), 2861-2865. (47) Rehman, M. A.; Akhtar, I.; Choi, W.; Akbar, K.; Farooq, A.; Hussain, S.; Shehzad, M. A.; Chun, S. H.; Jung, J.; Seo, Y. Influence of an Al2O3 Interlayer in a Directly Grown Graphene-Silicon Schottky Junction Solar Cell. Carbon 2018, 132, 157-164. (48) Jiurong Li; Qinglei Guo; Nan Zhang; Weidong Tao; Anli Xu; Siwei Yang; Zhiduo Liu; Gang Wang; Chen, D.; Ding, G. Direct Integration of Polycrystalline Graphene on Silicon as a Photodetector via PACVD. J. Mater. Chem. C 2018, 6(36): 9682-9690. (49) Liang, Z.; Zeng, P.; Liu, P.; Zhao, C.; Xie, W.; Mai, W. Interface Engineering To Boost Photoresponse Performance of Self-Powered, Broad-Bandwidth PEDOT:PSS/Si Heterojunction Photodetector. ACS Appl. Mater. Inter. 2016, 8 (29), 19158-19167.


Page 18 of 27

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(50) Liang, Z.; Su, M.; Wang, H.; Gong, Y.; Xie, F.; Gong, L.; Meng, H.; Liu, P.; Chen, H.; Xie, W.; Chen, J. Characteristics of a Silicon Nanowires/PEDOT:PSS Heterojunction and Its Effect on the Solar Cell Performance. ACS Appl. Mater. Inter.2015, 7 (10), 5830-5836. (51) Pudasaini, P. R.; Ruiz-Zepeda, F.; Sharma, M.; Elam, D.; Ponce, A.; Ayon, A. A. High Efficiency Hybrid Silicon Nanopillar–Polymer Solar Cells. ACS Appl. Mater. Inter. 2013, 5 (19), 9620-9627.



FIGURES

Figure 1. Schematic diagram of the fabrication process for the photodetector based on conformal graphene/SiNHs. (a) Deposition of AgNPs on n-Si via wet chemical method under silver nitrate solution; (b-c) Formation of SiNHs by utilizing AgNPs-assisted etching approach; (d-e) Conformal graphene on SiNHs by the method of PECVD; (f) Patterning Au source/drain electrodes via a lift-off process; (g) Graphene patterns fabricated by using lithography and oxygen-based reactive ion etching (RIE) instruments.


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Figure 2. Morphology characterization of the conformal graphene/SiNHs. (a)-(b) Low and high magnification SEM images of the conformal graphene/SiNHs. (c)-(d) Cross-sectional SEM and AFM images of the conformal graphene/SiNHs.



Figure 3. Conformal graphene on silicon nanostructures. (a)-(c) Low and high magnification TEM images of the conformal graphene/SiNHs. (d) Raman spectra of the planar graphene/Si and the conformal graphene/SiNHs.


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Figure 4. Light-trapping properties of the SiNHs and the conformal graphene/SiNHs. Reflection spectra (a) and absorption spectra (b) of the planar Si, graphene/Si, SiNHs, and conformal graphene/SiNHs.



Figure 5. The physical mechanism of the light-trapping performances and optical simulations. (a) Schematic view of the graphene/Si; (b) Simulated electric field energy distribution (left) and power absorption density (PAD, right) in 2D cross-section of the graphene/Si; (c) Schematic view of the conformal graphene/SiNHs; (d) Simulated electric field energy distribution (left) and PAD (right) of the conformal graphene/SiNHs; Simulated reflectance (e) and absorption (f) of different graphene/Si structures.


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Figure 6. (a) Schematic view of the conformal graphene/SiNHs detectors. (b) SEM image of the fabricated conformal graphene/SiNHs detector. (c) Measured photoresponse of the planar graphene/Si, SiNHs, and conformal graphene/SiNHs detectors. (d) Band-alignment of the graphene/SiNHs heterostructure.



Figure 7. (a) Photoresponse of the conformal graphene/SiNHs photodetectors at the frequencies of 1, 100, and 2500 Hz; (b) Relative balance (Imax-Imin)/Imin versus frequency of the pulsed light; (c) Rise time τr and (d) recovery time τf of the photodetector.


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