1. Introduction

Reflection is a hindrance for the performance of devices, such as photovoltaic devices, flat panel displays and optical-electrical sensors, which causes a decrease in the photon energy conversion efficiency for these devices [1–3]. The study of a broadband antireflection surface (BARS) has been widely carried out to overcome this problem. Many surface texturing techniques have been used to reduce optical reflection at silicon-air interface and enhance optical absorption, such as laser texturing, optical interference lithography, and reactive ion etching [4–6]. Among them, in the presence of elemental sulfur gas, femto-second pulsed laser has been used to form a black silicon surface, which exhibits near-unity absorption over a broad wavelength range [7]. However, this technique has a high production cost and potential environmental pollution because of using the expensive femto-second laser and poisonous gas. In order to satisfy the requirements of industrial applications of BARS, its fabrication has to be low cost and effective. Recently metallic NPs which support surface plasmons have been used in thin-film solar cells to increase light trapping [8–11]. Sevenfold enhancement of the absorption at λ = 1200 nm was reported after covering a silicon wafer with silver nanostructures [12]. Short-circuit current enhancement of 29% was demonstrated by directly forming silver NPs on the rear surface of a silicon thin-film solar cell [13]. On the other hand, Ag-Au composite NPs have been widely studied in localized surface plasmon resonance (LSPR) sensors because of their distinctive optical characteristic from monometallic Ag or Au NPs, such as stronger near-field and larger extinction cross section [14–16]. But no research has still been performed to investigate how the broadband antireflection performance can be affected by Ag-Au alloy NPs, which support surface plasmon resonances different from those of monometallic NPs (here we name it as alloy nanoparticle plasmon resonance), and neither has their combination with laser surface texturing enhanced light trapping been investigated. In this paper, the Ag-Au alloy NPs are fabricated on the laser textured monocrystalline silicon surface by dewetting process. The textured surface is formed by nanosecond pulsed laser in the ambient environment. By combining the enhanced light trapping effect provided by laser textured surface and the alloy nanoparticle plasmon resonance, an average reflectance (AR) down to 3.9% over the main solar spectrum (300~1200 nm) is achieved, which demonstrates a decrease of ~50% and ~90% as compared to the corresponding monometallic NPs and the original flat Si surfaces, respectively. It is a low cost and effective technique with extensive potential applications in security, photovoltaic devices, flat-panel displays and optical-electric sensors.

2. Experiments

The metallic NPs were fabricated on (i) flat and (ii) textured Si surfaces (both are shown in Fig. 1 ). To study the morphological evolution of the NPs from mono-metal to alloy and the influence of only NPs (excluding the surface laser-textured patterns) on the antireflection performance, monometallic and alloy NPs were primarily fabricated on flat Si surfaces. Single-side polished mono-crystalline silicon wafers were placed in the chamber of an e-beam evaporator for depositing the metallic thin films. Two samples were coated with a single layer of 30 nm Ag and 15 nm Au thin films, respectively. There was another one sample coated with two layers: 15 nm Au film on top of 30 nm Ag film. The NPs were formed by a thermal annealing (dewetting) process in a furnace with the presence of N₂ atmosphere. The sample with the single layer of Ag thin film was annealed at 200 °C for 3 hours, while the samples with Au film and Ag-Au films were annealed at 800 °C for 1 hour. The samples were characterized by a UV-VIS-NIR spectrophotometer (Shimadzu, UV-3600) for reflection spectra measurement, and the spectra data were measured at an incident angle of 8° with an integration sphere configuration.

 

Fig. 1 Cross-section schematics of NPs on (a) flat and (b) textured Si substrates.

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3. Results and Discussion

As can be seen in Fig. 2 and Table 1 , on the flat Si substrates, the monometallic Ag NPs present a large size with a broad size distribution and low density, which has a mean diameter (MD) of 154 nm, a standard deviation (SD) of 54 nm and a surface coverage (SC) of 24%, while the Au NPs show a smaller size with a narrower size distribution and higher density (MD ~21 nm, SD ~7 nm, and SC ~44%). As compared to the monometallic NPs, the Ag-Au alloy NPs have a broader size distribution, which shows a mean diameter (MD) of 86 nm with a standard deviation (SD) of 82 nm, and a surface coverage (SC) of 42%. As for the shape anisotropy, which was defined as the average ratio of the major to minor axes of the NP cross section in the plane of the substrate, it is 1.66 for Ag-Au, 1.33 for Au and 1.22 for Ag NPs. It is obvious that the Ag-Au NPs have larger sizes and shape distributions than corresponding monometallic NPs.

 

Fig. 2 SEM images (a-c) and size distribution (d-f) of NPs on flat Si surfaces, NPs size formed by thermal annealing of (a, d) 30 nm Ag, (b, e) 15 nm Au, and (c, f) 15nm-Au/30nm-Ag thin films.

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Table 1. Morphological Parameters and Optical Performance of NPs on Flat Si Surfaces^a

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Figure 3 shows the measured reflection spectra of the samples. It shows that when compared to the original flat Si surface without coated NPs, both monometallic and alloy NPs can improve the performance of broadband antireflection of the flat Si surfaces, but the alloy NPs can enhance the antireflection performance even more. The Au and Ag monometallic NPs mainly enhance the antireflection performance in the visible range and at the wavelengths below the band gap of silicon (wavelengths below 800nm for Ag NPs, and 1100nm for Au NPs), while the Ag-Au alloy particles enhance the antireflection performance in the spectral range of 300 ~1200 nm. The alloy NPs improve the antireflection property with a reduction of 55 and 45% in average reflectance over the wavelength range of 300 ~1200 nm as compared to the original flat Si surfaces and monometallic NPs, respectively.

 

Fig. 3 Measured reflection spectra of flat Si surfaces coated with metallic NPs, NPs are formed by thermal dewetting of 30 nm Ag, 15 nm Au, and 15nm-Au/30nm-Ag thin films.

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The other group of samples was then prepared to investigate the influence of the combination of the NPs and laser-textured patterns, where NPs were fabricated on laser-textured Si surfaces. Silicon wafers were textured by a pulsed 1064 nm fiber laser, which was configured at a pulse width of 1.5 ns, a repetition rate of 100 kHz and a power of 4.2 W. The surfaces of samples were scanned in horizontal, vertical and two diagonal directions. A reference sample was also processed by the same laser texturing. The samples were then put inside the chamber of the e-beam evaporator for the thin-film deposition and the furnace for thermal dewetting as the previous group of samples, i.e., Ag monometallic NPs were formed by annealing the 30 nm Ag thin film at 200 °C for 3 hours, Au monometallic and the Ag-Au alloy NPs formed by annealing 15 nm Au and 30nm-Ag/15nm-Au bi-layers films at 800 °C for 1 hour.

As can be seen in Fig. 4 , when compared to the NPs formed on flat Si surfaces shown in Fig. 2, the NPs on textured surfaces are much smaller and their shapes are more anisotropic. For laser-textured samples, the Ag monometallic NPs presents a mean diameter (MD) of 39 nm and a surface coverage (SC) of ~30%, the Au NPs with a MD of 16 nm and a SC of ~27%, but the Ag-Au alloy particles tend to be lower density (SC ~10%) with a size between the monometallic Ag and Au NPs (MD ~30 nm).

 

Fig. 4 SEM images of NPs formed on laser-textured Si surfaces, NPs formed by thermal annealing of (a) 30 nm Ag, (b) 15 nm Au, and (d) 15nm-Au/30nm-Ag thin films.

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The measured reflection spectra of this group of samples are shown in Fig. 5 . When compared to the reference sample which was laser-textured without coated metal NPs, the samples coated with monometallic NPs show a higher average reflectance and demonstrate deterioration in the antireflection performance. This is a big different from the monometallic NPs coated on the flat Si surfaces where the NPs cause a decrease in average reflectance. The difference in the antireflection performance of the monometallic NPs on different substrates (i.e. on flat and textured surfaces, here we call it “substrate effect”) is an interesting and complicate phenomenon, and the detail mechanism will be investigated further in the future research. One probable reason is supposed as following: the laser-textured pattern causes not only changes in particles’ shape, size and surface coverage, but also a change in refractive index of effective surrounding medium of the NPs. Each of these changes plays some role to influence the SPR property of the NPs, such as tuning SPR wavelength and modifying the SPR bandwidth. In this experiment, it can be observed from the reflection spectra that the SPR bandwidth of the monometallic NPs on the flat surface is broader in our interested wavelength range (i.e. λ< 800 nm for Ag NPs, and λ< 1100 nm for Au NPs), while the SPR bandwidth of the monometallic NPs on the textured surface is narrower in the interested wavelength range (i.e. 750 nm <λ< 1100 nm for Ag NPs, and λ< 900 nm for Au NPs). As a result, in our interested wavelength range (300~1200 nm), for the flat surface, the effect of plasmon resonance dominated, which causes a broadband antireflection, and thus a reduction in the average reflectance; while for the textured surface, most wavelengths are out of LSPR band, so the plasmon effect does not play an important role any more, but the light trapping effect attributed to the surface texturing is a main factor [12]. Because of their small sizes, the monometallic NPs may not possess enough light trapping capability to compensate for the loss of the shaded surface textures. As a result, the average reflectance increases, especially at the long wavelength region, the gold particles cause sharp increase in the reflection. However, similarly to the flat surfaces with alloy NPs, the alloy-NPs-covered laser-textured sample presents low and steady reflection in the entire investigated wavelength range. The alloy laser-textured sample holds an average reflectance of 3.9%, which shows a decrease of 47, 52 and 55% in average reflectance when compared to the laser-textured reference sample, Ag-NPs-coated sample, and Au-NPs-coated sample, respectively. It is obvious that the Ag-Au alloy particles show higher improvement in the performance of broadband antireflection than monometallic NPs, which is in accordance with the NPs on a flat Si wafer. Therefore, it can be concluded that the Ag-Au alloy NPs can make a more obvious improvement (~50%) in the broadband antireflection performance than the corresponding monometallic NPs, no matter whether the samples have undergone the laser texturing. However, if the samples were textured by laser, the performance of broadband antireflection can be further improved (45% for flat Si surface versus 55% for textured Si surface). The opinion that Ag-Au alloy NPs improve the antireflection performance was also verified by the Ag50-Au10 alloy NPs and their corresponding monometallic NPs, as shown in Fig. 5.

 

Fig. 5 Measured reflection spectra of laser-textured Si surfaces coated without and with NPs (Ag, Au, and Ag-Au). NPs were formed by the annealing of 30 nm Ag, 15 nm Au, and 15nm-Au/30nm-Ag thin films, (Inset) NPs were formed by the annealing of 50 nm Ag, 10 nm Au, and 10nm-Au/50nm-Ag thin films.

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The broadband ultralow reflectance demonstrated by the alloy-particles covered textured samples can be mainly attributed to the enhanced light trapping provided by the surface textures and the particle plasmon resonance. It is well known that for NPs with a size much smaller than the wavelength of light, the extinction is dominated by the absorption, so at the LSPR wavelength (${\lambda }_{LSPR}$), the light absorption in alloy NPs is enhanced and causes a decrease in reflection. For those NPs with a larger size, the extinction is dominated by the scattering, and the absorption in high-index substrate is enhanced at the wavelength of LSPR of metallic particles due to the enhanced scattering cross section and near-field coupling [10–12]. It is also well known that the surface plasmon resonance wavelength of metal NPs depends on the size, shape, and particle material [17]. Compared to the monometallic particles, the alloy NPs have a broader size and shape distribution, and thus have a broader surface plasmon resonance band. Therefore, the alloy NPs shows much better broadband antireflection property due to the enhanced absorption either in metal NPs or in Si surfaces at the broadband LSPR wavelengths. Meanwhile, the alloy particles tend to be in flatter shape, at a smaller distance from the substrate, and thus increase its near-field coupling.

3. Conclusions

In summary, the Ag-Au alloy NPs were fabricated on flat and laser-textured Si surfaces by thermal dewetting process, which demonstrates an enhanced broadband antireflection property of ~50% as compared to the monometallic NPs. By combining two light-trapping-enhanced approaches, textured surface and alloy nanoparticle plasmon resonance, average reflectance down to 3.9% over the spectral range of 300 ~1200 nm was obtained, which shows a reduction of ~50% and ~90% as compared to the corresponding monometallic NPs and the original flat Si surfaces, respectively. This fabrication method is low cost and effective, which provides a feasible and economy solution for BARS applications in security, photovoltaic devices, flat-panel displays and optical-electric sensors.