We present a simple, cost-effective, large scale fabrication technique for antireflective disordered subwavelength structures (d-SWSs) on GaAs substrate by Ag etch masks formed using spin-coated Ag ink and subsequent inductively coupled plasma (ICP) etching process. The antireflection characteristics of GaAs d-SWSs rely on their geometric profiles, which were controlled by adjusting the distribution of Ag etch masks via changing the concentration of Ag atoms and the sintering temperature of Ag ink as well as the ICP etching conditions. The fabricated GaAs d-SWSs drastically reduced the reflection loss compared to that of bulk GaAs (>30%) in the wavelength range of 300-870 nm. The most desirable GaAs d-SWSs for practical solar cell applications exhibited a solar-weighted reflectance (SWR) of 2.12%, which is much lower than that of bulk GaAs (38.6%), and its incident angle-dependent SWR was also investigated.
©2012 Optical Society of America
Gallium arsenide (GaAs), which has high absorption coefficient and electron mobility, is an important semiconductor material used for optoelectronic device applications including vertical-emitting lasers, image sensors, photodetectors, and solar cells [1–4]. In particular, GaAs based solar cells have attracted considerable attention due to their superior efficiency and high radiation hardness (i.e., ideal for space applications) [4,5]. However, their high Fresnel reflection (>30%), which originates from the large refractive index mismatch between air (nair = 1) and GaAs (nGaAs~3.6), limits the efficiency of GaAs solar cells. An antireflective structure is inevitably necessary to enhance the efficiency of GaAs solar cells by suppressing unwanted surface reflection losses and thereby increasing the transmission of sunlight into the solar cells. Recently, biomimetic subwavelength structures (SWSs), which can minimize reflection losses, have attracted great interest as a promising candidate for high efficiency optoelectronic devices due to their long-term stability, broadband wavelength and omnidirectional antireflection properties [5–14]. Etch masks with nano-scale dimensions are crucial to fabricate SWSs, and several lithography techniques such as e-beam, interference, and nanoimprint lithography have been usually employed to form nano-scale etch masks [6,7,9,10]. However, these techniques are complex, expensive, and can only pattern over a small area, making them inadequate for practical solar cell applications. To avoid these drawbacks of lithography based techniques, metal nanoparticles formed by a thermal dewetting process of metal films were explored as an etch mask to fabricate SWSs [11–13]. Unfortunately, this method still has critical drawbacks such as long processing time and the requirement of sophisticated equipment for metal evaporation and thermal dewetting. In addition, the temperatures for the thermal dewetting process of metal films are excessively high (i.e., at least 500 °C); such high temperatures can deteriorate the performance of devices. To address the aforementioned drawbacks, a simple, fast, and economical nanopatterning technique without lithography process is required for realizing SWSs.
In this work, we focused on simplified fabrication of SWS and its effects on optical properties. To achieve this aim, we employed spin-coated Ag ink, which can form nano-scale Ag masks via a sintering process using a hotplate , to fabricate GaAs disordered SWSs (d-SWSs). This lithography-free approach for creating SWSs is a simple, cost-effective, and high throughput method. In order to effectively improve the light harvesting in GaAs solar cells by incorporating SWSs, the distribution, shape, and height of SWSs should be optimized because the reflection properties are strongly correlated with the geometric parameters of SWSs. Therefore, the geometric profiles and antireflection characteristics of GaAs d-SWSs, which were made under different sintering conditions of spin-coated Ag ink with different Ag concentration and subsequent inductively coupled plasma (ICP) etching using various etching conditions, were systematically investigated. The incident angle-dependent reflectance and solar-weighted reflectance (SWR) were also investigated for solar cell applications. All the analyses were done by taking into account of SWSs effects on optical properties. For this reason, the passivation layer was not used, however, it is necessary to minimize surface recombination losses caused by introducing surface nanostructures .
2. Experimental details
A diluted solvent-based Ag ink, which consists of soluble Ag clusters containing Ag atoms of 10% wt., was spin-coated on a single-side polished (100) GaAs substrate of size 25 × 25 mm2. Since the antireflection characteristics of SWSs strongly rely on the distribution of the SWSs [5–14], differently distributed Ag etch masks were formed to obtain desirable GaAs d-SWSs by adjusting the Ag ink ratio in a mixed solution of isopropanol and Ag ink (i.e., Ag ink ratios of 50%, 35%, and 25%) as well as by adjusting the sintering temperature of the as-coated Ag ink. The sintering process was carried out on a hotplate (HHP-411, Iuchi Seiei Dou Co., Japan) at various temperatures of 150 °C, 200 °C, 250 °C, and 300 °C for 5 min. Figure 1(a) shows the top-view field-emission scanning electron microscope (FE-SEM, S-4700, Hitachi, Japan) images of the formed Ag masks corresponding to the Ag ink ratio and the sintering temperature. Corresponding 45° tilted view SEM images are also shown in the insets. As can be seen in Fig. 1(a), the Ag masks were randomly agglomerated on the GaAs substrate to achieve an energetically stable state when the surface energy of Ag is sufficient for dewetting [14,16,17]. It can be observed that the Ag masks change into rounded shapes with the increase of the sintering temperature. The temperature at which the as-coated Ag ink starts to separate to form round shaped isolated Ag masks increased with increasing Ag ink ratio due to the relatively high concentration of Ag atoms (i.e., with larger thickness of as-coated Ag ink layer) . Thus, the Ag masks were not fully turned into rounded shapes (partially linked) at a 50% Ag ink ratio due to the increased Ag ink ratio and the insufficient surface energy of Ag. It can also be clearly seen that the distribution, size, and interspacing of the Ag masks were changed with the adjustment of Ag ink ratio and the sintering temperature. This means that the distribution and interspacing of the individual d-SWSs can be coarsely controlled by adjusting the Ag ink ratio and the sintering temperature without any additional process steps. It is noteworthy that compared to previously reported techniques [11–13], these metal masks can be prepared at much lower temperature. Thus, degradation problems in device performance owing to high thermal treatment may be minimized as well.
Since the antireflection characteristics of SWS depend on the density of the SWSs, which in turn is closely related to the density of the etch mask, the fill factor (i.e., packing fraction) of the GaAs surface with the Ag masks for various Ag ink ratios and sintering temperatures were estimated using a commercial image processing program (ImageJ 1.42q, NIH). As can be seen Fig. 1(b), the fill factor decreased with decreasing Ag ink ratio and increasing sintering temperature. This means that the packing fraction of the GaAs surface (i.e., the density of GaAs d-SWSs) by d-SWSs can be adjusted by varying the Ag ink ratio and the sintering temperature.
The GaAs d-SWSs were fabricated by an ICP (Plasmalab System 100, Oxford Instrument Co., UK) etcher using the Ag etch masks in an SiCl4 plasma ambient. The ICP etching conditions such as radio-frequency (RF) power, flow rate of additional Ar, and etching time were varied to obtain excellent antireflective GaAs surface structures, while the ICP power, process pressure, and flow rate of SiCl4 were fixed at 0 W, 2 mTorr, and 7.5 sccm, respectively. The Ag etch masks were partially removed during the ICP etching process, especially when the thickness of the Ag masks was small (i.e., low Ag ink ratio and sintering temperature). After the ICP etching process, the samples were immersed in a diluted mixture of KI and I2 chemical etchant at room temperature to completely remove the residual Ag masks; this was followed by rinsing with deionized water and drying under a flow of N2 gas. A schematic diagram of the process steps for fabricating the GaAs d-SWSs using spin-coated Ag ink and ICP etching is presented in Fig. 1(c).
3. Results and discussion
Figure 2(a) shows 45° tilted view SEM images of the GaAs d-SWSs fabricated by using spin-coated Ag inks; samples were sintered at a temperature of 250 °C for 5 min, with different Ag ink ratios of 50%, 35%, and 25%. Corresponding cross-sectional SEM images are also shown in the insets. The ICP etching was carried out at 75 W RF power for 210 s. It can be seen in Fig. 1(a) and 2(a) that the Ag etch masks were transferred onto the GaAs substrate, resulting GaAs d-SWSs with different distributions, sizes, and interspacing. It was also found that the GaAs d-SWSs using a 25% Ag ink ratio have partially collapsed SWSs. This can be explained by the fact that thinner Ag etch masks having small size eroded more rapidly by dry etching from the edge of masks compared to thicker and larger masks [12, 16]. The estimated average height of the fabricated GaAs d-SWSs was 292 ± 15 nm, 271 ± 32 nm, and 229 ± 68 nm for50%, 35%, and 25% Ag ink ratios, respectively. The interspacing of the fabricated GaAs d-SWSs was increased by increasing the Ag ink ratio, as can be seen in the insets of Fig. 2(a). It is known that the antireflection characteristics of SWSs are related to the period (i.e., interspacing) of SWSs. A theoretical investigation of period dependent antireflection characteristics of GaAs SWSs was performed using rigorous coupled-wave analysis . The simulation model was constructed by the mean shape of the SEM image, and this shape is assumed for simplicity to be a truncated cone with a height of 300 nm, a ratio of upper diameter to bottom diameter of 0.3, and a six-fold hexagonally symmetric structure. Figure 2(b) shows the contour plot of the calculated reflectance spectra as a function of the period of the GaAs SWSs. As the period of the GaAs SWSs increases, the low reflectance region shifts to the longer wavelength region . For a period under 350 nm, the reflectance remains less than ~2% over the entire wavelength range.
Figure 2(c) shows the measured hemispherical reflectance spectra of the corresponding GaAs d-SWSs in the wavelength range of 300-870 nm. The reflectance of the bulk GaAs is also shown as a reference. The hemispherical reflectance spectra were measured using a UV-VIR-NIR spectrophotometer (Cary 500, Varian, USA) equipped with an integrating sphere at near-normal incident angle of 8°. As can be seen in Fig. 2(c), the GaAs d-SWSs drastically reduced the reflection loss compared to that of the bulk GaAs (>30%) over the entire wavelength range due to the graded index profile of the d-SWSs. It was also found that the reflection minima of the GaAs d-SWSs moved to the longer wavelength region as the Ag ink ratio increased from 25% to 50% due to the increased interspacing between individual SWSs, as can be observed in Fig. 2(b). Meanwhile, the GaAs d-SWSs fabricated with an Ag ink ratio of 35% exhibited the best antireflection capability over the entire wavelength range, with an average hemispherical reflectance of 2.13% due to the small period and the mixed interspacing of the adjacent d-SWSs changing the Ag ink ratio from 50% (large period) to25% (small period). Therefore, the Ag ink ratio of 35% was chosen to form Ag etch masks for the remainder of experiments.
Figure 3 shows the measured hemispherical reflectance spectra of the GaAs d-SWSs as a function of sintering temperature. Corresponding 45° tilted view SEM images are displayed in the insets. The RF power and etching time were 75 W and 210 s, respectively, for the fabrication of the d-SWSs. It is observed that the distribution of the fabricated GaAs d-SWSs is slightly changed according to the distribution of the Ag etch masks, as can be seen in Figs. 1(a) and 3. This can be attributed to the different degree of agglomeration, corresponding to the sintering temperature, of the as-coated Ag ink. Although the change in distribution of GaAs d-SWSs due to the change in sintering temperature was not significant, the average hemispherical reflectance changed from 3.80% to 2.13%. The GaAs d-SWSs formed using the sintering temperature of 250 °C exhibited the lowest average hemispherical reflection (2.13%) among the fabricated GaAs d-SWSs at different sintering temperatures. Therefore, the sintering temperature of 250 °C was chosen to form Ag etch masks for the remainder of experiments.
The effect of RF power on the reflectance spectrum of the GaAs d-SWSs is shown in Fig. 4 . The samples were etched for 210 s. Cross-sectional SEM images of the corresponding GaAs d-SWSs are also shown in the insets. As the RF power increased from 50 W to 100 W, the average height of the GaAs d-SWSs increased from 256 ± 29 nm to 363 ± 53 nm, and the etching rate of GaAs d-SWSs increased from 73.1 to 103.7 nm/min due to the increased ion energy. The RF power also has an effect on the shape of the etched GaAs d-SWSs. The etched profile of the GaAs d-SWSs was transformed from truncated cone profile to cone-like profile as the RF power increased because the Ag etch masks were quickly eroded at higher RF power. This means that the shape of the GaAs d-SWSs can be coarsely adjusted by controlling the RF power. It is observed that GaAs d-SWSs with lower height and truncated cone profile (50 W RF power) exhibited much higher reflectance in the short wavelength range compared to other samples with larger height and cone-like profile. Thus, the average hemispherical reflectance of the GaAs d-SWSs decreased from 2.74% to 1.79% with increase of the RF power from 50 W to 100 W in the wavelength range of 300-870 nm. Though the GaAs d-SWSs fabricated with RF power of 100 W showed the lowest average hemispherical reflectance, this condition is inadequate for the fabrication of GaAs SWSs due to rapid erosion of Ag etch masks because it can cause the collapse of the SWSs. Thus, the RF power of 75 W was chosen to fabricate the GaAs d-SWSs for the following experiments.
The effect of additional Ar flow rate on the fabricated GaAs d-SWSs is shown in Fig. 5 . The etching process was carried out for 210 s in an SiCl4 plasma without and with Ar gas (i.e., 30 and 60 sccm). Cross-sectional SEM images of the corresponding GaAs d-SWSs are also displayed in the insets. It is seen that as the Ar gas flow rate was increased from 0 to 60 sccm, the average height of the fabricated GaAs d-SWSs dropped from 271 ± 32 nm to 114 ± 7 nm. This indicates that the etch rate of GaAs decreased from 77.4 to 32.5 nm/min with the addition of Ar to SiCl4 gas because this addition prevents the action of etch reactants. With the addition of Ar gas, the average reflectance increases from 2.13% (without Ar gas) to 9.49% (60 sccm of Ar gas) due to the lower height of GaAs d-SWSs, leading to an abrupt change in the refractive index profile [5,12].
The etching time dependent hemispherical reflectance spectra of the fabricated GaAs d-SWSs are shown in Fig. 6 . The etching process was performed without Ar gas for 120, 210, and 300 s. Cross-sectional SEM images of the corresponding GaAs d-SWSs are also shown in the insets. As the etching time increased from 120 to 300 s, the average height of GaAs d-SWSs increased, from 200 ± 22 nm to 489 ± 71 nm, which decreased the average hemispherical reflectance from 3.15% to 0.71% in the wavelength range of 300-870 nm. It is well-known that SWSs with larger height exhibit better antireflection properties [6, 7, 11–14]. To investigate the effective reflection of the GaAs d-SWSs on the solar cell performance under the solar radiation spectrum, we calculated the SWR, which can be explained as the ratio of reflected photons to total incident photons, i.e., the normalization of reflectance spectra with the terrestrial air mass 1.5 global (AM 1.5G) , as given in the following equation 9]. Thus, the GaAs d-SWSs sample fabricated for 210 s is considered as a potential candidate for practical solar cells because it exhibited low and stable reflection as well as relatively low SWR of 2.12% in the entire wavelength range of 300-870 nm.
The angle-dependent antireflection property is also an important parameter to achieve reasonable light absorption during the day. To evaluate the angle-dependent SWRs of the fabricated GaAs d-SWSs with RF power of 75 W for 210 s without Ar gas, incident angle-dependent reflectance was obtained using a Cary variable angle specular reflectance accessory in specular mode over the incident angle range of 20° to 60° and a wavelength range of 300-870 nm, as shown in Fig. 7 . The contour plot of the incident angle-dependent reflectance as a function of wavelength for the corresponding structure is shown in the inset of Fig. 7. The discontinuity at 800 nm wavelength in the contour plot is due to measurement set-up.
Although the reflectance increased as the angle of incidence (AOI) increased, the reflectance remained roughly below 20%. The angle-dependent SWR remained below 5% up to an AOI of 40° and rapidly increased to ~11.8% for an AOI of 60°, as shown in Fig. 7. However, these values of AOI dependent SWRs were much lower than those of bulk GaAs substrate, showing that the fabricated d-SWSs hold great potential for use in solar cells.
We fabricated GaAs d-SWSs using spin-coated Ag ink followed by ICP etching. By adjusting the Ag ink ratio and sintering temperatures as well as the ICP etching parameters, various GaAs d-SWSs with different distributions, shapes, heights, and interspacing were fabricated in order to achieve desirable antireflective structures for practical solar cell applications. Compared to bulk GaAs, the GaAs d-SWSs drastically reduced the surface reflection losses. GaAs d-SWSs fabricated using Ag etch masks formed by an Ag ink ratio of 35% and sintering temperature of 250 °C followed by ICP etching process with RF power of 75 W for 210 s without Ar gas in an SiCl4 plasma showed the most promising results; these materials exhibited an SWR of 2.12% at near-normal AOI and remained below 5% up to an AOI of 40° in the wavelength range of 300-870 nm. This simple and low-cost fabrication technique for production of antireflective structures on a large scale shows the great potential of GaAs based solar cells.
This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (No. 2011-0017606) and by the Core Technology Development Program for Next-generation Solar Cells of Research Institute for Solar and Sustainable Energies (RISE), GIST.
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