Enhancement of antireflection property of silicon using nanostructured surface combined with a polymer deposition
Silicon (Si) nanostructures that exhibit a significantly low reflectance in ultraviolet (UV) and visible light wavelength regions are fabricated using a hydrogen etching process. The fabricated Si nanostructures have aperiodic subwavelength structures with pyramid-like morphologies. The detailed morphologies of the nanostructures can be controlled by changing the etching condition. The nanostructured Si exhibited much more reduced reflectance than a flat Si surface: an average reflectance of the nanostructured Si was approximately 6.8% in visible light region and a slight high reflectance of approximately 17% in UV region. The reflectance was further reduced in both UV and visible light region through the deposition of a poly(dimethylsiloxane) layer with a rough surface on the Si nanostructure: the reflectance can be decreased down to 2.5%. The enhancement of the antireflection properties was analyzed with a finite difference time domain simulation method.
KeywordsSilicon nanostructure Antireflection Hydrogen etching Silicon-based polymer
Recently, antireflection (AR) techniques have been widely used in various applications such as solar cells [1, 2, 3], electro-optical devices , sensors , and lenses  to significantly suppress the reflective loss at the interface of two media. In particular, in solar cells using crystalline silicon (Si) modules, AR has been a significant research focus due to the enhancement of photo-conversion efficiency [1, 2]. Despite excellent conversion efficiency in crystalline Si solar cells, the high refractive index (n = 3.4) of Si has limited the efficient utilization of sunlight [7, 8]. This is because more than 30% of incident sunlight is scattered or reflected from the Si surface due to a large discontinuity of n between the air and Si interface.
In order to reduce the reflection from the air-material interface, the n of the two media should be similar or changed smoothly at the interface. Nature has its own strategy to effectively reduce reflection: for example, nanostructured surface on a moth eye [6, 9]. Such biological nanostructured surfaces can create a composite comprising air and a material, where n gradually changes from the air to the material because effective n depends on the volume fraction of the two media. Furthermore, it is important to note that moth eyes are satisfied that they have the optimal AR conditions using two-dimensional subwavelength structures [4, 10] and tapered morphologies [4, 11].
So far, several types of biomimetic nanostructured surfaces with excellent AR properties have been developed using electron-beam lithography, laser interference lithography, and nanoimprint lithography [12, 13, 14]. However, these techniques require expensive devices and complicated procedures. Moreover, there have been few papers that describe simple post-treatments to further reduce the reflection from the material surface, although some post-treatment methods have been reported including oxygen treatments for improving the abrasion resistance of the coating , NH3-heat processes followed by a trimethylchlorosilane modification to enhance the scratch resistance and moisture resistance , and the effects of heat, laser, and ion post-treatments on HfO2 single layers . Here, we present a hydrogen etching approach to fabricate pyramid-shaped Si nanostructures that exhibits a comparatively low reflectance at the wavelength regions of ultraviolet (UV) and visible (Vis). The aspect ratio and two-dimensional spacing of Si nanostructures can be controlled by changing the etching condition. In addition, the reflectance was further reduced by depositing a Si-based polymer on the fabricated Si nanostructures, which also induce more uniform reflectance behavior over UV and Vis regions.
Results and discussion
Formation mechanism of the pyramid-shaped Si nanostructures can be explained as follows. An annealing of a Si plate under hydrogen environment weakens the bonds between Si atoms. As a result, SiH x gases and radicals are formed easily through the reaction with hydrogen gas, leading to the etching of the Si plate . During the hydrogen etching process, both etching and redeposition of the Si atoms/radicals occur and the Si surface was reproduced to have the most energetically stable shapes [18, 21]. The (100) surface of Si is more rapidly etched than (110) and (111) surfaces . As a result, pyramid-shaped Si nanostructures of which side faces comprise energetically stable (111) crystalline surfaces are formed . However, non-perfect etching occurred at a relatively low annealing temperature of 1,100°C. Furthermore, SiH x gases and radicals formed at such a low temperature can be redeposited on the Si nanostructure [18, 24], leading to the formation of the bump-like structures on the apexes of the pyramid-like nanostructures as shown in Figure 3c.
The AR properties of the non-compact nanopyramid structure and the effect of the buffer layer on the AR properties were analyzed with FDTD simulation. As shown in the simulation results (Figure 6c), the reflectance from the air-Si interface increases as the spacing (d in Figure 6a) between the neighboring nanopyramids increases. In addition, the FDTD simulation result also shows that a PDMS buffer layer further reduces the reflectance: the reflectance was reduced by approximately 5% over all the wavelength regions. These simulation results correspond well with the experimental results shown in Figure 7. In addition, although a buffer layer is deposited on the Si nanostructure, a reflection occurs at the surface of the buffer layer because of the difference in n between air and the PDMS buffer layer (see the small step in Figure 5c). However, we observed that surface of a PDMS layer was not perfectly flat. As shown in the AFM image (Figure 6b), the PDMS layer has a rough surface with the roughness of approximately 20 nm. This rough surface was naturally formed when the PDMS layer was coated on the Si nanostructures through the doctor blade technique. This rough surface of the PDMS layer induces a diffused reflection like the Si nanostructures on a Si plate and thus, the reflectance at the interface between air and PDMS layer is decreased . The FDTD simulation result clearly demonstrates this fact (Figure 6d): relatively uniform low reflectance was obtained by the rough surface of the PDMS layer on the fabricated Si nanostructures (black line in Figure 6d). However, a flat surface of the PDMS layer with the thickness of 1 μm could induce the fluctuated and slightly high reflectance (blue line in Figure 6d) compared to that of the rough PDMS surface. These are because constructive and destructive interferences between reflections from the flat PDMS surface and the Si nanostructures are alternately occurred due to the flat surface of the PDMS layer (inset of Figure 6d). On the other hand, the rough surface of the PDMS layer could randomly scatter the reflections from the PDMS surface and the Si nanostructures, and thus, these arbitrarily scattered reflections by the rough PDMS surface could be dissipated through the destructive inference of themselves. Therefore, Si nanostructures and a PDMS buffer layer with a rough surface can dramatically improve the AR properties of a Si surface (Figure 7).
Pyramid-shaped Si nanostructures were fabricated on a Si plate using a hydrogen etching process. Due to the nanopyramid structure, the Si surface exhibited a significantly low reflectance at UV and visible light regions. Furthermore, the discontinuity of neff at the air-Si interface could be reduced through the deposition of a Si-based polymer with a rough surface. Consequently, the AR properties of the Si nanostructures were further enhanced. The hydrogen etching method combined with a polymer coating can be easily scalable to a large surface and is a cheap process. Therefore, we believe that this method is useful for the practical applications to electro-optical devices that require low AR surfaces.
This work was supported by a National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (no. 2012–0009523).
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