Zhong et al. Nanoscale Research Letters (2018) 13:316 https://doi.org/10.1186/s11671-018-2741-9 NANOEXPRESS Open Access Enhanced near-infrared absorber: two-step fabricated structured black silicon and its device application Hao Zhong1, Nasir Ilyas2, Yuhao Song2, Wei Li1* and Yadong Jiang1 Abstract Silicon is widely used in semiconductor industry but has poor performance in near-infrared photoelectronic devices because of its high reflectance and band gap limit. In this study, two-step process, deep reactive ion etching (DRIE) method combined with plasma immersion ion implantation (PIII), are used to fabricate microstructured black silicon on the surface of C-Si. These improved surfaces doped with sulfur elements realize a narrower band gap and an enhancement of light absorptance, especially in the near-infrared range (800 to 2000 nm). Meanwhile, the maximum light absorptance increases significantly up to 83%. A Si-PIN photoelectronic detector with microstructured black silicon at the back surface exhibits remarkable device performance, leading to a responsivity of 0.53 A/W at 1060 nm. This novel microstructured black silicon, combining narrow band gap characteristic, could have a potential application in near-infrared photoelectronic detection. Keywords: Black silicon, Light absorptance, Band gap, Device responsivity Background silicon in the near-infrared wavelength, the etched silicon Until now, many micro- and nanostructured black silicon structures will be doped by ion implantation after DRIE materials can also be manufactured by using DRIE treat- process. Under certain conditions, the black silicon arrays ment and ion implantation, aiming to reduce light reflect- can be obtained, and the resulted sulfur dopants contained ance and enhance the near-infrared absorptance [1–5]. within the silicon lattice will eventually cause significant DRIE process is usually carried out in a mode of cyclic absorptance of below band gap radiation [10, 11]. etching-passivation steps with a photoresist mask which As one of the most important material in semicon- can enable the silicon microfabrication of high-aspect ra- ductor industry, black silicon has been widely used in tio structures. Generally, this approach utilizes F-based sensitive photoelectronic detectors, solar cells, biochem- plasmas such as SF6 for fast isotropic etching and then ical sensors, display devices, and optical communication switches to a sidewall passivating cycle using C4F8 [6–8]. objects [12–20]. Micro- and nanostructures of black sili- During the subsequent etching cycle, the passivating film con have been the focus of intense researches in recent is preferentially removed from the bottom of the groove years due to their extensive device application. A Si-PIN due to ion bombardment, while preventing the etching of photoelectronic detector with black silicon at the front the sidewalls [9]. Finally, the alternating of etching and surface has been investigated in our early study [21]. passivating cycles could form the specific geometries of This device structure has a wide depletion layer so that the etched silicon structures, which depend mainly on it can reduce the influence of carrier diffusion move- mask size, gas flow, electrode power, process time, cycle ment and achieve the purpose of improving device sensi- times, and so on. In order to enhance the absorption of tivity and response speed. It is also noticed that using black silicon as a photosensitive surface is very difficult for the generated carriers to be collected by P layer to * Correspondence: [email protected] 1State Key Laboratory of Electronic Thin Films and Integrated Devices, output photocurrent through electrode, resulting in a University of Electronic Science and Technology of China, Chengdu 610054, relatively low visible light response compared with a China traditional Si-PIN detector. So, it appears a query that if Full list of author information is available at the end of the article © The Author(s). 2018 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Zhong et al. Nanoscale Research Letters (2018) 13:316 Page 2 of 8 a Si-PIN photoelectronic detector with black silicon at (NIR2500) equipped with an integrating sphere (Idea the back surface could complete two tasks at one time, Optics, IS-20-5). The detector responsivity was mea- i.e., to increase device sensitivity not only in the sured by using an optical power meter (OPHIR, Vega), near-infrared but also in the visible wavelength? an optical chopper (Scitec Instruments, Model-300CD), In this article, we report the light absorptance en- and a Keithley2636B apparatus under the dark room en- hancement and narrower band gap characteristic of vironment. In order to ensure the accuracy of the meas- microstructured black silicon fabricated by two-step urement, we carried out calibration before test and each process: DRIE combined with PIII. The effect of differ- of these measurements was performed on a few samples ent etching process on the light absorptance in the (usually 4 to 6). wavelength range from 400 to 2000 nm have been stud- ied, and the detector based on this microstructured Results and Discussion black silicon at the back surface has also been investi- Figure 3 gives the typical SEM images of aligned micro- gated with an emphasis on device responsivity in the structured silicon arrays which are perpendicular to the wavelength of 400~1100 nm. surface of substrate for three different mask sizes. It is clearly shown that the top view of the microstructured Methods silicon is not actually standard circles due to the fact As shown in Fig. 1a, uniform and periodic distributing that DRIE process mainly depends on the mask size and cylindrical arrays were chosen to research the optical the quality of photolithography technique. Then, in properties of microstructured silicon by FDTD Simula- order to investigate the effect of etching process on light tion software. Figure 1b represents the relationship be- absorptance of microstructured silicon, we change the tween the absorbance and four different model sizes of cycle times as 30, 70, and 100 times on mask III testing the microstructured silicon after optimized simulation, samples under the conditions of etching with SF6 for 3 s, in which four models have the same cylinder diameter and passivating with C4F8 for 2 s, every cycle time. (D =4 μm) but different center distance (T1 =12 μm, T2 According to the different etching rate ratio between =10μm, T3 =8μm, T4 =6μm). photoresist and silicon, the etching depth can be con- As shown in Fig. 2, based on the above optimized trolled by process parameters as long as the photoresist simulation results, three different photolithography mask is thick enough as a mask. It can be seen from Fig. 4 that were designed by varying the mask size named as mask I the height of cylinders increases with the number of (D =4 μm, T =6μm), mask II (D =4μm, T =8 μm), and cycle times, in which the height from the top to the sub- mask III (D =4 μm, T =10 μm), respectively. Then strate are about 1.87 μm, 2.35 μm, and 3.15 μm, respect- photoresist NR9-1500PY was applied to deposit circular ively. It is well known that in DRIE process, although arrays mask on the polishing surface of silicon pieces there are passivating gases to protect the side wall of the (15 × 15 cm2), which were cut from n-type silicon wafers etching target, it is still more or less accompanied by lat- with a thickness of 500 μm and a resistivity of 2500 to eral etching. This is the reason why the resulting morph- 3000 Ω · cm. In order to investigate the effect of etching ologies are not ideal cylindrical arrays. Obviously, the process on light absorptance of microstructured silicon, morphologies of these microstructured silicon arrays can we moved the testing samples resulted from mask III be well controlled by varying lithography process and into process cavity (DRIE, ICP-100D) and changed the etching cycle times. cycle times, in which SF6 was used as etching gas and Figure 5a represents the light absorptance of micro- C4F8 as passivating gas, for 30 times, 70 times, and 100 structured cylindrical arrays at different cycle times times, differently. After etching processes, the silicon without PIII process. It is shown that C-Si with etched pieces were removed of any residual photoresist under silicon arrays, compared to ordinary C-Si, can enhance the atmosphere of oxygen to ensure that the silicon sur- light absorptance throughout the wavelength range from face was clean and smooth. In order to enhance the ab- 400 to 2000 nm to a certain extent. The sample etched sorptance of microstructured silicon, especially in for 100 cycle times presents the highest absorptance, up near-infrared band, the testing samples resulted from to 70% in the NIR range (800 to 2000 nm), and the aver- mask III hereafter were doped with sulfur elements age absorptance of this sample reaches 55% in the wave- through PIII process under the condition of 1.0E + length range from 400 to 2000 nm. This is due to the − 15 cm 2 implantation dose and 800 eV implantation en- multiple reflection and absorption of microstructured ergy, respectively. silicon (as in Fig. 6). In the course of incident light being The morphologies of black silicon were characterized reflected continuously on the side surface of the cylinder, by a field emission scanning electron microscope (SEM, the absorption path length of incident light increases, JSM-7500F). The light absorptance was obtained at resulting in the absorptance enhancement.