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Abstract
Sub-bandgap near-infrared silicon (Si) photodetectors are key elements in integrated Si photonics. We demonstrate such a Si photodetector based on a black Si (b-Si)/Ag nanoparticles (Ag-NPs) Schottky junction. This photodetector synergistically employs the mechanisms of inner photoemission, light-trapping, and surface-plasmon-enhanced absorption to efficiently absorb the sub-bandgap light and generate a photocurrent. The b-Si/Ag-NPs sample was prepared by means of wet chemical etching. Compared to those of a planar-Si/Ag thin-film Schottky photodetector, the responsivities of the b-Si/Ag-NPs photodetector were greatly enhanced, being 0.277 and 0.226 mA/W at a reversely biased voltage of 3 V for 1319- and 1550-nm light, respectively.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
The sub-bandgap near-infrared (NIR) silicon (Si) photodetector (PD) is a key element in integrated Si photonics for telecommunication and data communication due to its good compatibility with current complementary metal-oxide-semiconductor processes [1–3]. Three main mechanisms, i.e., inner photoemission [2,4–8], two-photon absorption [9–11], and defect-mediated absorption [12–14], have been employed to enable Si to absorb sub-bandgap NIR light. Unfortunately, the inherent nonlinear feature of photo-absorption makes the sub-bandgap photo-electric conversion very inefficient compared to a visible Si PD [3,15]. How to improve the photo-absorption and photo-charge generation and transport becomes a main challenge for practical use of sub-bandgap NIR Si PDs. A number of approaches have been proposed and demonstrated for this purpose, such as Fabry-Pérot cavity design [16], waveguide configuration [17], and plasmonic enhancement [18]. Nonetheless, novel and facile approaches are still in demand for higher-efficiency sub-bandgap NIR Si PDs. Black Si (b-Si), i.e., Si that has been highly textured at surface, possesses the features of strong anti-reflection and light-trapping [19–21]. Steglich et al. [22] enhanced NIR absorptivity, and hence the responsivity, of a PtSi PD with b-Si. Juntunen et al. [23] used b-Si to achieve a visible Si PD with an extraordinarily high external quantum efficiency.
In this work, we propose another type of b-Si PD based on a b-Si/Ag nanoparticles (Ag-NPs) Schottky junction, which synergistically employs the inner photoemission mechanism to absorb NIR light, and the light-trapping structure of b-Si as well as the surface plasmons (SPs) from Ag-NPs to enhance the absorption and photo-charge generation. To reduce the surface recombination of b-Si, which arose from its large specific area [24,25], the surface of b-Si was passivated by SiO2 at the front and by Al2O3 at the rear [26–28]. The purpose of this work is to demonstrate a facile method to fabricate a sub-bandgap Si PD with high photo-electric conversion efficiency that works in the optical communication wavelength range and at room temperature. This method utilizes a relatively simple fabrication process and is suitable for mass production with low cost.
2. Experiments
High-resistivity crystalline Si(100) wafers (double-sided polished, 10 × 10 × 0.5 mm3 in size, 5000 Ω⋅cm) were used as the PD substrate. The Si wafer was degreased in acetone solution and then ultrasonically cleaned in ethanol and deionized water successively. Next, 3-nm-thick Ag layers were evaporated onto both the front and back sides of the Si wafer by resistance heating in a vacuum chamber with a base pressure lower than 8 × 10−4 Pa. After immersing the Si wafer in a solution of HF:H2O2:H2O = 1:5:10 in volume ratio for 300 s at room temperature, b-Si layers were formed on both sides of the Si wafer via a chemical etching process catalyzed by Ag [29]. The back side of the wafer was then dipped in a solution of AgNO3:HF:H2O2:H2O = 10:18:7:65 in volume ratio for 300 s and blown dry by nitrogen. Upon this process, Ag-NPs were made on the back side of the wafer. For comparison, 5-nm-thick Ag thin films were evaporated onto the back sides of some b-Si samples by resistance heating. Before depositing the electrodes, the front surface of the sample was passivated with 5-nm-thick SiO2 by means of e-beam evaporation, while the rear surface was passivated with 10-nm-thick Al2O3 layers by means of atomic-layer-deposition (ALD) (Beneq TFS 200). An 80-nm-thick indium-tin-oxide (ITO) layer was then deposited by e-beam evaporation onto the front side as the front electrode. A 2-μm-thick Al layer was evaporated by resistance heating as the rear electrode. Thermal annealing in nitrogen at 480°C for 5 min was conducted to finalize the preparation of the b-Si PD. The absorption spectra were measured using UV-vis-NIR spectrometers (Ideaoptics, PG2000-Pro-EX and NIR2500) with integrating spheres. The surface morphology was measured with scanning electron microscopy (SEM) (Philips, XL 30). The photoelectric responsivity of the PDs was measured under NIR light illumination using a source meter unit (Keithley, SMU2400). The sub-bandgap NIR light sources were 1319- and 1550-nm laser diodes (CNI Laser, MIL-H-1319 and 1550) and a supercontinuum source (YSC Photonics, SC-Pro).
3. Results and discussion
Figure 1(a) is a bird’s-eye view SEM image of the back side of the b-Si after Ag-NPs deposition. Numerous nano-pillars are seen at the surface and the formation of the b-Si layer is confirmed. Figure 1(b) shows a cross-sectional SEM image of this sample, which enables statistical measuring of the aspect ratio of the nano-pillars. The average aspect ratio obtained is 8.3 ± 0.6. From both Figs. 1(a) and 1(b), Ag-NPs are found to adhere to the Si nano-pillars, forming Si/Ag-NPs Schottky junctions [2,4–8]. The average diameter of the Ag-NPs is 161.0 ± 4.0 nm.
Fig. 1 (a) Bird’s-eye view and (b) cross-sectional SEM image of back side of b-Si sample after Ag-NPs deposition.
Figures 2(a) and 2(b) depict the absorption spectra for planar Si, planar Si loaded with Ag-NPs, b-Si with Ag thin film, and b-Si with Ag-NPs. The Ag thin film and Ag-NPs were located at the back side of the sample, not facing directly toward the impinging light. In the 400–1100-nm regime as shown in Fig. 2(a), the absorptivity is considerably enhanced for the b-Si/Ag-NPs sample due to its light-trapping structure [30]. For the sub-bandgap regime in which the wavelength is larger than 1100 nm as shown in Fig. 2(b), almost no absorption is found for planar Si, as expected. The absorption of the planar Si at approximately 1200 nm is due to the involvement of phonons in the light absorption by indirect-bandgap semiconductors. However, for planar Si with Ag-NPs, sub-bandgap absorption can be seen due to an inner photoemission process, in which hot electrons induced by the impinging sub-bandgap light at the Ag-NPs can overcome the Schottky barrier and be emitted onto the conduction band of Si, completing the course of sub-bandgap NIR absorption [2,4–8]. For b-Si with Ag-NPs, the sub-bandgap absorption is significantly enhanced, as both the inner photoemission and light-trapping mechanisms work synergistically. It is noticed that the average sub-bandgap absorptivity of the b-Si with Ag-NPs here is ~80%. This is evidently higher than that in [22], where silicide thin films rather than nanoparticles were used for Schottky photo-detection [22]. For comparison, we made one more sample, i.e., b-Si loaded with a 5-nm-thick Ag thin film, and measured its absorption spectrum as plotted in Figs. 2(a) and 2(b) as well. In addition, the Ag thin film was at the back side of the b-Si sample. Clearly, the sub-bandgap absorption for b-Si with Ag thin film is considerably weaker than that of b-Si with Ag-NPs. It is known that, in contrast to the case of b-Si with Ag thin film, localized SPs could be induced at the interface of b-Si and Ag-NPs under light illumination as the wavelength matching can be more readily satisfied [31]. The formed SP band could promote the energy of hot electrons on Ag-NPs, and help more electrons be emitted to b-Si [32]. This could explain that more sub-bandgap NIR light was absorbed by b-Si with Ag-NPs than that by b-Si with Ag thin film. Therefore, for the sample of b-Si loaded with Ag-NPs, there are actually three mechanisms, i.e., inner photoemission via Schottky junctions, light trapping, and SP-enhanced absorption, working synergistically to absorb light in the sub-bandgap NIR regime.
Fig. 2 Absorption spectra for planar Si, planar Si loaded with Ag-NPs, and b-Si loaded with Ag film and Ag-NPs, in wavelength regimes of (a) 400–1100 nm and (b) 1200–2200 nm.
To manifest the existence of localized SPs for b-Si with Ag-NPs, the electromagnetic field distribution and the absorption spectrum of the plasmonic system were simulated using the finite-element method in COMSOL MultiPhysics [33]. Figures 3(a)–3(d) exhibit the simulated distributions of the SP electromagnetic field around a half-sphere Ag-NP surrounded by Si and Al2O3 environments under the illumination of 1319- and 1550-nm light. In our calculations, the periodic boundary conditions were adopted to simulate the nanoparticle array. For simplicity, the diameter of the Ag-NPs was chosen as 150 nm. The Ag-NP was located on the surface of a flat Si wafer. The Ag-NP and the flat Si surface were covered by Al2O3, as schematically shown in Figs. 3(a) and 3(c). We can see that the electric field is highly enhanced around the Ag-NP. This enhancement can promote the absorption of the incident light as well as the efficiency of the inner photoemission, thus increasing the responsivity of the PD [32].
Fig. 3 Electric field intensity distribution in Al2O3/Ag-NP/Si at (a) x-z and (b) x-y planes under illumination of 1319-nm light and at (c) x-z and (d) x-y planes under illumination of 1550-nm light. (e) Simulated absorption spectrum of Al2O3/Ag-NPs/Si, together with measured absorption spectra of Si and Si with Ag-NPs in Al2O3 and their difference spectrum.
The SP electromagnetic field results from the localized SPs that are induced by the impinging light, and its wavelength should be close to that of the SP resonance [31]. In the simulated absorption spectrum shown in Fig. 3(e), an absorption peak appears at approximately 1180 nm, which is a clear signature of the SP effect. For experimental identification, the SP resonance was tested for the similarly sized Ag-NPs located between Al2O3 and Si by measuring the absorption spectra of the planar Si with and without Ag-NPs that were covered by Al2O3, and then taking their difference as shown in Fig. 3(e). The difference spectrum reveals a SP resonance absorption peak at approximately 1260 nm, indicating the formation of localized SPs at the interface between Si and Ag-NPs. This experimental result roughly agrees with the simulated one. The difference between the absorption peaks in the simulated and measured spectra could be due to the diverse size and shape distributions of the experimentally deposited Ag-NPs [31].
Schematic of the b-Si/Ag-NPs PD is presented in Fig. 4. The b-Si layer at the front surface was solely for anti-reflection and covered by the SiO2 passivation layer and ITO front electrode. The b-Si/Ag-NPs was formed at the rear surface and covered by the Al2O3 passivation layer and Al back-electrode. The visible light was mainly absorbed near the front surface, through which most of the sub-bandgap light traveled straightforwardly through the Si substrate and was illuminated on the b-Si/Ag-NPs at the rear surface. Schottky junctions between b-Si and Ag-NPs were formed at the rear to absorb the sub-bandgap NIR light. The electrons in Ag-NPs could be excited by the NIR light and then cross the Schottky junction, producing a photon current.
Figures 5(a) and 5(b) give the photocurrent-reversely biased voltage, or I-V, curves of the b-Si/Ag-NPs PD under the illumination of 1319- and 1550-nm light, respectively. Here, the photocurrent indicates the current induced by light illumination, i.e., the dark current has been subtracted from the total current. For comparison, the I-V curves of a planar Si with 5-nm-thick Ag thin-film PD with a similar geometric design as Fig. 4 are presented. Although Schottky junctions of Si and Ag thin film exist in the planar Si PD, because of the lack of light-trapping structure its sub-bandgap photocurrent is found to be quite lower than that of the b-Si one for all of the illumination light and the entire biased voltage range here. Comparing the photocurrent at the two wavelengths, we can see that the photocurrent of the b-Si/Ag-NPs PD at 1319 and 1550 nm are close, while the photocurrent of the Si/Ag-film PD at 1319 nm is much higher than that at 1550 nm. This result of larger photocurrent improvement at 1550 nm than that at 1319 nm confirms that the b-Si/Ag-NPs structure can effectively trap the incident light and promote the detector efficiency.
Fig. 5 I-V curves for b-Si/Ag-NPs PD, b-Si/Ag thin film PD, and planar Si/Ag-NPs PD under illumination of (a) 1319- and (b) 1550-nm light.
To test the role of SPs, the I-V curves of another b-Si PD with 5-nm-thick Ag thin film on the surface of b-Si at the rear are given. For this PD, localized SPs are difficult to form due to the failure of wavelength matching [31]. Indeed, the photocurrent of this PD is lower than that of b-Si PD, which can generate SPs for all of the illumination light and the entire biased voltages here. It is also noticed that for the b-Si PD with Ag-NPs the general photocurrent decreases when the illuminating wavelength increases from 1319 to 1550 nm at all the biased voltages. This is because, with increasing wavelength, the energy gained by the hot electron at the Fermi level of the Ag-NP from the impinging light decreases progressively, so the probability of the electron to overcome the Schottky barrier decreases. Furthermore, with increasing wavelength, the separation between the wavelength of the SP resonance and that of the illuminating light becomes larger, making the SP-induced electromagnetic field less intense, as already indicated in Figs. 3(a)–3(d). As a result, the energy gained by the hot electron at the Fermi level of Ag-NP from the SP band decreases, which further reduces the absorptivity, and hence, the photocurrent.
The responsivity R of the PD is defined as
where Ip is the photon-induced current and Po the power of the illuminating light. As compared to those of a planar Si PD with Ag thin film of similar geometric design, the responsivities of the b-Si/Ag-NPs PD were greatly enhanced, being 0.277 and 0.226 mA/W at the reversely biased voltage of 3 V for impinging light with wavelengths of 1319 and 1550 nm, respectively. The responsivities of the b-Si/Ag-NPs PD for illuminating wavelengths of 1319 and 1550 nm at the reversely biased voltages of 3, 5, and 10 V are tabulated in Table 1. With the reversely biased voltage increased up to 10 V, the responsivity of the b-Si/Ag-NPs PD increases.
Table 1. Responsivities of b-Si/Ag-NPs PD for illuminating wavelengths of 1319 and 1550 nm at reversely biased voltages of 3, 5, and 10 V.
The dark current density of the b-Si/Ag-NPs PD is 0.84 mA/cm2 at the reversely biased voltage of 10 V. This value is relatively low among inner photoemission PDs [3]. The noise equivalent power (NEP) limited by the shot noise from the dark current is calculated as
where Id is the dark current, e the charge of an electron, and Δf the bandwidth. Assuming a bandwidth of 1 kHz and a detection area of 1 cm2, the NEP of the b-Si/Ag-NPs PD at the reversely biased voltage of 10 V is 9.4 × 10−7 and 1.1 × 10−6 W for wavelengths of 1319 and 1550 nm, respectively. The corresponding specific detectivity of the b-Si/Ag-NPs PD is 3.4 × 107 and 2.8 × 107 for wavelengths of 1319 and 1550 nm, respectively. The responsivity will be enhanced by further optimizing the PD thus the specific detectivity will also be increased.The responsivity of b-Si/Ag-NPs PD at the reversely biased voltages of 3, 5, and 10 V in wavelength range of 1250−1650 nm is shown in Fig. 6. With increasing wavelength, the responsivity decreases, which is due to decreasing inner photoemission efficiency with increasing wavelength.
Fig. 6 Responsivity of b-Si/Ag-NPs PD at reversely biased voltages of 3, 5, and 10 V in wavelength range of 1250−1650 nm.
4. Conclusions
In this paper, we demonstrated a Si photodetector based on a black Si/Ag nanoparticles Schottky junction. This photodetector synergistically employed the mechanisms of inner photoemission, light trapping, and surface-plasmon-enhanced absorption to efficiently absorb sub-bandgap light and generate a photocurrent. The average absorption of the near-infrared sub-bandgap light reached 80% for the b-Si/Ag NPs samples. Compared with those of a planar Si/Ag-NPs Schottky PD, the responsivities of the b-Si/Ag-NPs PD were greatly enhanced.
Funding
CIOMP-Fudan University joint fund (FC2017-001); the National Natural Science Foundation of China (51472051); the Shanghai Science and Technology Committee (18JC14111500).
Acknowledgments
We thank Mr. Yi-Zhen Chen, Dr. Lei Ma and Mr. Wen-Jie Zhou for their help with the simulation. We also thank Dr. Jin Zhou and Mr. Dong-Hai Zhang for their help with the measurement of responsivity.
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