Enhanced near-infrared absorber: two-step fabricated structured black silicon and its device application
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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.
KeywordsBlack silicon Light absorptance Band gap Device responsivity
Until now, many micro- and nanostructured black silicon materials can also be manufactured by using DRIE treatment and ion implantation, aiming to reduce light reflectance and enhance the near-infrared absorptance [1, 2, 3, 4, 5]. DRIE process is usually carried out in a mode of cyclic etching-passivation steps with a photoresist mask which can enable the silicon microfabrication of high-aspect ratio structures. Generally, this approach utilizes F-based plasmas such as SF6 for fast isotropic etching and then switches to a sidewall passivating cycle using C4F8 [6, 7, 8]. During the subsequent etching cycle, the passivating film is preferentially removed from the bottom of the groove due to ion bombardment, while preventing the etching of the sidewalls . Finally, the alternating of etching and passivating cycles could form the specific geometries of the etched silicon structures, which depend mainly on mask size, gas flow, electrode power, process time, cycle times, and so on. In order to enhance the absorption of silicon in the near-infrared wavelength, the etched silicon structures will be doped by ion implantation after DRIE process. Under certain conditions, the black silicon arrays can be obtained, and the resulted sulfur dopants contained within the silicon lattice will eventually cause significant absorptance of below band gap radiation [10, 11].
As one of the most important material in semiconductor industry, black silicon has been widely used in sensitive photoelectronic detectors, solar cells, biochemical sensors, display devices, and optical communication objects [12, 13, 14, 15, 16, 17, 18, 19, 20]. Micro- and nanostructures of black silicon have been the focus of intense researches in recent years due to their extensive device application. A Si-PIN photoelectronic detector with black silicon at the front surface has been investigated in our early study . This device structure has a wide depletion layer so that it can reduce the influence of carrier diffusion movement and achieve the purpose of improving device sensitivity 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 output photocurrent through electrode, resulting in a relatively low visible light response compared with a traditional Si-PIN detector. So, it appears a query that if a Si-PIN photoelectronic detector with black silicon at the back surface could complete two tasks at one time, i.e., to increase device sensitivity not only in the near-infrared but also in the visible wavelength?
In this article, we report the light absorptance enhancement and narrower band gap characteristic of microstructured black silicon fabricated by two-step process: DRIE combined with PIII. The effect of different etching process on the light absorptance in the wavelength range from 400 to 2000 nm have been studied, and the detector based on this microstructured black silicon at the back surface has also been investigated with an emphasis on device responsivity in the wavelength of 400~1100 nm.
The morphologies of black silicon were characterized by a field emission scanning electron microscope (SEM, JSM-7500F). The light absorptance was obtained at room temperature using a fiber optic spectrometer (NIR2500) equipped with an integrating sphere (Idea Optics, IS-20-5). The detector responsivity was measured by using an optical power meter (OPHIR, Vega), an optical chopper (Scitec Instruments, Model-300CD), and a Keithley2636B apparatus under the dark room environment. In order to ensure the accuracy of the measurement, we carried out calibration before test and each of these measurements was performed on a few samples (usually 4 to 6).
Results and Discussion
This high absorptance mainly comes from the sulfur doping among microstructured cylindrical arrays, forming multiple impurity levels in the energy band structure of C-Si. As a result, when these induced multiple impurity levels overlap, a new impurity band is formed after broadening, which finally reduces the band gap of C-Si. The band gap can be obtained from the absorptance spectrum of the sample by Tauc mapping. The specific steps adopted are as follows:
(iv) The inflection point (the maximum point of the first derivative) is obtained by calculating the first derivative of the hv-(hvF(R∞))1/2 curve, and the tangent of the curve is made at this point. The abscissa value of the intersection of the tangent and the X axis are the band gap of the sample.
It can be seen from Fig. 8b that although the Si-PIN detector with black silicon formed on the back surface (device 2) shows a relatively little improvement responsivity in visible spectrum, the response spectrum of it gives an even higher responsivity in the wavelength range from 680 to 1100 nm with about 60 nm red shift of peak responsivity, compared with the commercial Si-PIN detector (device 1). The main reason for such a distinction is that the device structure of these two detectors (devices 1 and 2) is different. When the photon energy is greater than the band gap of C-Si, the incident light is mainly absorbed by P layer and so the generated carriers have enough energy to transit N layer. Most of the generated carriers can be collected by N+ layer to output photocurrent through electrode. In this condition, whether the back surface of the detector is introduced with or without black silicon, there will be a limited influence on the device response in the visible wavelength. Different from the detector with black silicon at the front surface , device 2 demonstrates a better response in the visible wavelength. That is why there is a relatively little improvement in visible light response according to the measured responsivity curve. Again in device 2, because the black silicon layer is set on the back surface, even if the photon energy is less than the band gap of C-Si, near-infrared light is able of penetrating P layer and absorbed by N layer, and then a large number of generated carriers are able to be collected by the N+ layer under the action of reverse bias. As a result, there will be a countable photocurrent output and the device represents a substantial responsivity increase in the near-infrared wavelength.
According to above results, our present study could provide a feasible strategy for near-infrared photoelectronic detection field, but there are still a lot of aspects that should be considered. For example, better fabrication processes and ion implantation technologies of microstructured black silicon, which could precisely control the morphologies and band gaps of the structured silicon should be explored. Furthermore, some other novel device structures of photoelectronic detector based on black silicon should be designed in order to realize a better device performance.
In summary, the microstructured black silicon materials are fabricated by two-step process: deep reactive ion etching combined with plasma immersion ion implantation. The microstructured cylindrical arrays on the surface of silicon wafers have three different sizes: mask I (D = 4 μm, T = 6 μm), mask II (D = 4 μm, T = 8 μm), and mask III (D = 4 μm, T = 10 μm), with the height of 1.87 μm, 2.35 μm, and 3.15 μm, respectively. Obviously enhanced light absorptance of black silicon has been obtained in a wide wavelength range from 400 to 2000 nm, and the maximum and average light absorptance reach 83% and 62%, respectively. These enhancements are discussed extensively based on multiple reflection, increased absorption path length, and narrow band gap. A novel Si-PIN photoelectronic detector with black silicon formed on the back surface has been fabricated, and a comparison of device responsivity has been made with one commercial device named as S1336-44BK. It is concluded that our Si-PIN photoelectronic detector with black silicon formed on the back surface has a substantial increase in responsivity, particularly in the near-infrared wavelengths, rising to 0.53 A/W at 1060 nm and 0.31 A/W at 1100 nm, respectively.
This work was supported by the National Natural Science Foundation of China (Grant No. 61421002).
HZ carried out the measurements and drafted the manuscript. NI, YS, and WL participated in the experimental design and discussed the results. WL and YJ helped to coordinate the experiments and revise the manuscript. All authors read and approved the final manuscript.
The authors declare they have no competing interests.
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