We developed a novel GaAs subwavelength structure (SWS) as an antireflective layer for solar cell applications. The GaAs SWS patterns were fabricated by a combination of nanosphere lithography (NSL) and reactive ion etching (RIE). The shape and height of the GaAs SWS were controlled by the diameter of the SiO2 nanospheres and the etching time. Various GaAs SWS were characterized by the reflectance spectra. The average reflectance of the polished GaAs substrate from 200nm to 800nm was 35.1%. However, the average reflectance of the tapered GaAs SWS was reduced to 0.6% due to scattering and moth-eye effects.
©2011 Optical Society of America
The development of novel technologies for solar cell applications has been on the rise due to high oil prices and environmental issues. Especially, GaAs solar cells have been highlighted due to their high efficiency, compared with conventional Si-based solar cells [1,2]. In addition, the high radiation hardness of GaAs makes it an ideal for space applications. However, polished GaAs surfaces have a very large reflectivity, which makes surface antireflection very important to increase the efficiency of GaAs solar cells [2,3]. Antireflection coatings (ARCs) such as SiNx, SiOx and TiOx have been typically used to reduce the reflectance of solar cells [4,5]. ARC is usually deposited by plasma-enhanced chemical vapor deposition (PECVD), sol-gel method or sputtering . However, these processes require expensive vacuum or plasma equipments. In addition, these coatings are only effective over a limited spectral range, and have several other problems such as mechanical and thermal stability, adhesion and thermal mismatch.
Recently, subwavelength structure (SWS), which refers to surface relief gratings with a size smaller than the wavelength of the incident light, has been widely investigated to reduce reflectance and increase transmission [6–10]. Deep and tapered SWS has been shown to be greatly effective in reducing the reflectance over a wide spectral range due to a reduction in Fresnel reflection by the gradual and continuous change of the refractive index from air to substrate. The pyramid structures also increase the probability of trapping the incident light. In addition, the absence of foreign materials in the SWS substrate enhances the long-term stability of solar cells, especially in harsh environments.
GaAs structures have been usually fabricated using a dry-etching process such as reactive ion etching (RIE) and inductively coupled plasma (ICP) etching [11–13], where the sub-wavelength patterns have been defined by various methods such as e-beam lithography, nanoimprint lithography and laser interference lithography [3,9]. E-beam lithography is useful for the fabrication of very small and accurate patterns. However, e-beam lithography is expensive and shows low throughput. Nanoimprint lithography and laser interference lithography can easily be applied to large areas, but the processes are very complex and expensive. In this paper, we employed SiO2 nanosphere lithography to fabricate nano-sized patterns on GaAs. Nanosphere lithography has many advantages such as high throughput and low cost and is a relatively simple process compatible with conventional microelectronics fabrication facility [14,15].
2. Experimental details
SiO2 nanospheres were synthesized by the hydrolysis of tetraethylorthosilicate (TEOS) in ethanol and deionized water using an ammonia catalyst . The diameter of the SiO2 nanospheres was adjusted by controlling the mole fraction of TEOS, DI water and ammonia. SiO2 nanospheres with a diameter of 350nm, which were confirmed by scanning electron microscopy (SEM), were used in these experiments. Figure 1 (a) shows a schematic diagram of the process used to fabricate the GaAs SWS. SiO2 nanospheres were spin-coated on the GaAs substrate [N-type, (100), SCSA] as shown Fig. 1 (b). Then, the SiO2 nanospheres were thinned by RIE to control the distance between the patterns. The etching gas consisted of a mixture of SF6 (10sccm) and O2 (10sccm). The pressure and etching time was 200mTorr and 10 seconds, respectively. After RIE etching at an etching power of 50W and 75W, the diameter of the SiO2 nanospheres was reduced to 310nm and 260nm, respectively. As shown in Fig. 1 (c), SiO2 nanospheres were successfully thinned by RIE etching at an etching power of 75W. Finally, GaAs SWS was fabricated by Cl-based dry-etching using the RIE technique (Fig. 1 (d)). The etching gas consisted of a mixture of BCl3 (5sccm) and Cl2 (30sccm). The etching power was 100W, and the pressure was 5mTorr. Etching times of 30 seconds, 60 seconds and 90 seconds were compared. After the dry-etching process, the residue of SiO2 nanospheres was removed by wet etching using a HF-based solution. The effects of GaAs SWS were assessed by the reflectance measurement. The reflectance spectra from 200nm to 800nm were obtained using a spectrophotometer (Cary 5000, Varian). The resolution was about 0.03nm (at UV/VIS) and 0.02nm (at NIR), respectively.
3. Results and discussion
Figure 2 (a) ~(c) show SEM images of GaAs SWS after RIE etching for 30 seconds (SWS1), 60 seconds (SWS2) and 90 seconds (SWS3), respectively, using SiO2 nanospheres with a diameter of 310nm. Figure 2 (d) ~(f) show SEM images of GaAs SWS after RIE etching for 30 seconds (SWS4), 60 seconds (SWS5) and 90 seconds (SWS6), respectively, using SiO2 nanospheres with a diameter of 260nm. The height of GaAs SWS after RIE etching for the same time was almost equal. In our etching condition, the etching rate of the GaAs and SiO2 nanospheres was 9nm/s and 4.5nm/s, respectively. In Fig. 2, the shape of the GaAs SWS after RIE etching for the same time varied depending on the diameter of the SiO2 nanospheres.
Figure 3(a) ~(c) show cross-sectional SEM images of GaAs SWS1, SWS4 and SWS5, respectively, which were obtained using the focused ion beam (FIB) technique. In the case of RIE etching for 30 seconds, cylindrical GaAs SWS was fabricated as shown in Fig. 2(a) and (d). The exact cross-sectional structures of the cylindrical GaAs SWS as SWS1 and SWS4 are shown in Fig. 3 (a) and (b). The SWS2 and SWS3 seem a cylindrical shape with a tapered top surface (Fig. 2(b) and (c)). In contrast, GaAs SWS5 and SWS6 had a tapered cone shape (Fig. 2(e) and (f)). The cross-sectional SEM image of the tapered SWS5 is shown in Fig. 3(c). As shown in Fig. 3(c), the etching depth of GaAs SWS was about 500nm. A cone shape is known to be very effective in reducing the reflectance of GaAs substrates due to a gradual change of index and multi-scattering events. In the case of cylindrical GaAs SWS, incident light has a high probability of reflection from the top surface in Fig. 3(a). However, incident light onto the cone shaped GaAs SWS showed less reflectivity in Fig. 3(c) .
The effects of these specific structures were compared through reflectance measurements (Fig. 4 ). The average reflectance of polished GaAs substrates from 200nm to 800nm was about 35.1%. The average reflectance of GaAs SWS after RIE etching for 30 seconds, 60 seconds and 90 seconds by SiO2 nanospheres with a diameter of 310nm were 12.8% (SWS1), 1.9% (SWS2) and 0.6% (SWS3), respectively. The average reflectance of GaAs SWS after RIE etching for 30 seconds, 60 seconds and 90 seconds by SiO2 nanospheres with a diameter of 260nm were 6.1% (SWS4), 1.1% (SWS5) and 0.8% (SWS6), respectively. As shown in Fig. 4, the GaAs SWS with a cone shape were more effective in reducing the reflectance than the GaAs SWS with cylinder shape.
The reduction in reflectance by SWS was largely caused by two effects [6,10]. Firstly, at shorter wavelengths less than the spacing of the SWS, the redistribution of the incident light will lower the reflectance since the spacing of the protuberances is not sufficiently small. Secondly, at longer wavelengths, the reflectance was reduced by the gradual change in the refractive index by suppressing the Fresnel reflection, [(n1-n2)/(n1 + n2)]2, where n1 and n2 are the refractive indices of the each material. These combined effects were capable of reducing the reflectance of GaAs substrates through over a wide range of wavelengths. These are in good agreement with SWS1~SWS6 in Fig. 2, 3 and 4 since the change in refractive index from air to GaAs is more abrupt in SWS1 and SWS4, compared with other structures in Fig. 2 and 3. Another important component to reduce the reflectance is the ratio between the height and wavelength . Therefore, the higher structures can lower the reflectance further. The height in SWS3 and SWS6 is higher than that in SWS2 and SWS5, which is in good agreement with reflectance results in Fig. 4. Wilson and Hutley reported that the reflectance of SWSs can be very low for the wavelengths higher than spacing and less than 2.5 × (height) at normal incidence , which is consistent with our experimental results (Fig. 4). Figure 5 shows the images from (a) SWS1, (b) SWS2 and (c) SWS3. The reflected characters of these SWSs were imaged by the reflection of a LCD monitor. The brightness of the reflected character at the center was gradually degraded by an increasing in the etching time, which is consistent with the results in Fig. 4. The reflected characters in the edge of Fig. 5 (b) and (c) were slightly bright because SWS were not uniform at the edges. with a diameter of 310nm and RIE etching for (a) 30 seconds, (b) 60 seconds and (c) 90 seconds.
Tapered GaAs SWS was successfully fabricated by combining nanosphere lithography and RIE. Six types of GaAs SWS were demonstrated by varying the diameter of SiO2 nanospheres and the etching conditions. The reflectance of GaAs substrates was reduced from 35.1% to 0.6% by tapered GaAs SWS. We report that the tapered GaAs SWS was very effective in reducing the reflectance and increasing the absorption due to graded-refractive index and multi- scattering effects. This simple antireflective technique shows a great potential for GaAs-based solar cells.
This research was supported by Future-based Technology Development Program (Nano Fields) through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2011-0029328).
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