Advanced Texturing of Si Nanostructures on Low Lifetime Si Wafer

SUHAILA SEPEAI, A.W.AZHARI, SALEEM H.ZAIDI, K.SOPIAN Solar Energy Research Institute (SERI),

Universiti Kebangsaan Malaysia (UKM), 43600 Bangi, Selangor, MALAYSIA

suhaila_sepeai@yahoo.com

Abstract - Currently, close to 90% of the global photovoltaic (PV) production is based on crystalline silicon (Si). Incomplete light absorption due to the high surface reflection is the key factor that fundamentally limit the Si solar cell performance. Texturing of crystalline silicon (c-Si) has been instrumental in the development of high efficiency c-Si solar cells. Surface texturing in Si wafer is used to enhance the amount of light absorbed into devices by reducing reflection losses. Surface texturing scatters light inside the semiconductor in order to trap it inside the wafer, and therefore increases the short circuit current as well as the efficiency in the solar cell. In this paper, four types of texturing processes were used to obtain nanostructures in Si wafer. The acid –based chemical, acid-based vapor, alkaline-based chemical and metal assisted chemical etching (MACE) technique have been performed to obtain the Si nanostructures. The morphological and optical characterization of the Si nanostructures has been carried out by means of field emission scanning microscopy (FE-SEM) and UV-VIS-NIR spectrometer. The results show that the pyramid size is in a range of 550-730 nm was obtained from alkaline based chemical method, while the nanopillars was produced by MACE technique.

Key-Words : - Lifetime, Si wafer, Nanostructures, Texturing, Photon Absorption 1 Introduction Crystalline silicon, in its single crystalline or multi-crystalline (mc) format, dominates the photovoltaic (PV) industry. PV energy generation cost is still higher than energy-conversion costs of carbon-based fossil fuels. Since the price of silicon wafer accounts for almost 50 % of the energy conversion cost, historically, reducing Si wafer thickness has been successful approach. However, incomplete optical absorption in thinner Si wafers results in lower efficiencies. Therefore, there is an urgent need for the complete light absorption in thinner Si wafer, and Si nanostructures promise an elegant solution. Si nanostructure are able to enhance potential of light absorption and higher efficiency. Si nanostructures have the capability to have extremely high absorption. In addition, the Si nanostructures are able to achieve high efficiency without anti-reflection coatings (ARC) layer by making nano-sized texture. Since the nanostructure are actually smaller than the light wavelengths striking the Si, there is no sudden change in the light density and the light doesn’t reflect back off the surface of the Si wafer [Nature]. Si nanostructures have many applications including broadband reflection

reduction in solar cells [1-5], and templates for heteroepitaxial growth [6-11]. Due to extremely low broadband reflection, Si nanostructures act the same function as ARC layer in Si solar cell. A number of low-cost, large area Si nanostructure synthesis methods have been developed. These include anisotropic, deep reactive ion etching processes to form high aspect ratio, nanoscale columnar structures, metal-assisted, anisotropic etching, and controlled anisotropic acidic etching of silicon [12]. In this paper, four different methods, namely acid –based chemical, acid-based vapor, alkaline-based chemical and metal assisted chemical etching (MACE) are demonstrated to obtain nanostructures on low-grade Si wafer. The use of low-grade Si wafer is due to an attempt on producing high efficiency solar cell at low fabrication cost. The Si nanostructures formation also leads to the cheaper manufacturing process by the elimination of ARC deposition Plasma Enhanced Chemical Vapour Deposition (PECVD). 2 Methodology A p-type <100> Si wafer with a sheet resistivity ranging between 1 ohm/cm and 10 ohm/cm was

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used. The Si wafer was initially cleaned by dipping into solution of hydrofluoric acid (HF) and nitric acid (HNO3) in a ratio of 1:100 for 10 minutes. After rinsing with deionized water, it was then dipped into HF and water (H2O) in a ratio of 1:50 for 1 minute. The wafers were then subjected to the texturing process. In this research, we used four different methods, namely acid –based chemical, acid-based vapor, alkaline-based chemical and metal assisted chemical etching (MACE). acid –based chemical, acid-based vapor used the same solution, that is HF:HNO3. For alkaline-based chemical, the Si wafer was immersed in that solution for 3 hours, while for alkaline-based vapour, the Si wafer was exposed to the vapor of the solution for 24 hours. For a alkaline-based chemical, the solution of KOH:IPA:H2O, where IPA is iso-propil alcohol (IPA) in the ratio of 1:5:125 has been used. The texturing temperature was set at 70˚C for 30 minutes. For MACE method, Ag assisted chemical etching technique has been employed to grow the nm-scale structures on p-type <100> Si wafers. Two etch process variations have been investigated. In the first approach, a two-step process is used in which Ag is first deposited on the wafers in a mixture of AgNO3/HF solution at various solutions concentration and temperature. This followed by etching with Ag as the mask in a solution of HF/H2O2 for various time intervals. In the second approach, deposition and etching are continued in a single step. For the characterization of the Si nanostructured, cross section and top view images by Scanning Electron Microscope (SEM) and reflectance measurement analyzed the outcome from that experiment. 3 Results and Discussion Table 1 summarizes electrical properties of measured Si wafer using Hall Effect measurement. This measurement is critical since it determines the bulk concentration of the Si wafer, as well as clarifies the grade of Si wafer used either low or high grade Si wafer. From the measurement, it was found that the bulk concentration measured was 1.21E16 cm-3 for 200 µm thickness wafer. According to Goetzberger et al. 1998, for doping less than 1017 cm-3, typical for most Si devices, radiative recombination plays virtually no role, and carrier lifetime is determined by the impurity level. While for a doping level greater than 1018 cm-3, the Auger recombination become dominant. Since

Auger process is assigned as lifetime, the relationship obtained empirically by Kendall (quoted in Rohatgi et al. 1984), is frequently used at the moment, accordingly, to which the lifetime in this range is calculated as;

τ =τo

1+ND

7E15

(Eq.1)

In this equation the carrier lifetime τo in pure, undoped silicon was assumed to be 400 µs. Therefore, from the Hall Effect Measurement of bulk concentration and resistivity value, it can be concluded that the Si wafer used in this experiment was low lifetime Si wafer.

Table 1. Hall effect measurement of Si wafer. Parameter Unit 200 µm

Bulk concentration cm-3 1.21E+16 Mobility cm2/Vs 5.35E+02

Resistivity ohm-cm 9.67E-01 Conductivity 1/ohm-cm 1.03E+00

Fig.1. Chemically-etched Si wafer from (a) top and (b) cross sectional view.

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Fig.2.Vapor-textured Si wafer from (a) top and (b)

cross sectional view. Alkaline-based wet-chemical texturing was investigated in detail. A solution KOH:IPA:H2O solution, where IPA is iso-propyl alcohol and H2O is water with a ratio of 1:5:125 was found to be highly effective. The texturing time was varied in a range of 10-90 minutes while the temperature was varied between 40-80 °C. Fig. 3 shows the SEM images of wet texturing process variation with time. From these images, it is clearly shown that alkaline texturing produces pyramid-like features. Following optimization of process parameters, it was determined that pyramid texture with a 30-minute texturing process leads to uniformly-etched pyramidal surfaces. Longer etching such as at 90 minutes shows that most of the pyramids are etched off. Fig.4 shows that texturing at 70°C exhibited the best pyramid pattern, size and uniformity. Fig.5 shows the SEM image of pyramid textures at a magnification of 1000. The pyramid size is in a range of 550-730 nm.

Fig.3. Top surface image of texturing pattern with a

variation on time at 80°C.

Fig.4. Top surface image of texturing pattern with

a variation on texturing temperature.

Fig.5. SEM image of pyramid texture at a

magnification of 1000x. Fig. 6 shows the AFM images of (a) acid –based chemical, (b) acid-based vapor and (c) alkaline-based chemical. It can be seen that roughness value for the chemically-etched surface is 310.72 nm, 206.82 nm and 773.97 nm. This is show that the alkaline based chemical texturing method are the most suitable method for photon absorption enhancement in Si solar cell.

Fig.6.AFM images of (a) acid –based chemical, (b) acid-based vapor and (c) alkaline-based chemical.

In order to determine the best texturing method, the spectral reflectance measurement of chemically-

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etched, vapor-etched and wet texturing was also investigated. The textured Si wafer reflectance was compared to polished Si wafer. The spectral reflectance measurements are plotted in Fig. 7. From the data, it is observed that textured wafers reflect substantially less light than the polished surface in the visible light (range from 400-700 nm). The polished wafer shows the highest reflectance with an averaged value of 0.772. This is followed by vapor-etched, chemically-etched and alkaline-textured with averaged value of 0.591, 0.167 and 0.104 respectively. Therefore, the alkaline-texturing was determined to be the best technique to absorb more photons in Si wafer through reduced reflection.

Fig.7. Optical reflectance measurements on low

lifetime Si wafer.

Fig. 8 shows the Si nanopillars that produced by MACE technique. The Si nanopillars that subjected to two different concentrations of AgNO3 shows that structural uniformity deteriorates as the concentration of AgNO3 is reduced as shown in Fig. 8 (a), whereas higher concentration of AgNO3 induces the formation of uniform nm-scale pillars as shown in Fig. 8 (b) [Ayu]. This method was not optimized yet and will be further investigated in details due to the excellent results on Si nanostructures formation by this technique.

Fig. 8. SEM image of cross-section of Si nanopillars

for different etchant composition of HF/AgNO3 4 Conclusion Four different methods of acid –based chemical, acid-based vapor, alkaline-based chemical and metal assisted chemical etching (MACE) are successfully demonstrated to obtain nanostructures on low-lifetime Si wafer. The best methods for Si nanostructures formation are alkaline-based chemical and MACE. The choice of this method will be used for future advanced Si solar cell fabrication. It is hoped that it may lead to an increase in the photon absorption. Further research on optimization of MACE technique will be reported out in the future. Acknowledgement This work has been carried out with the support of the Malaysia Ministry of Science, Technology and Innovation (MOSTI) under the FRGS grant. References: [1] D. S. Ruby and Saleem H. Zaidi, “Metal catalyst techniques for texturing silicon solar cells”, 6, 329, 296 B1, issued December, 2001. [2] Saleem H. Zaidi and J. M Gee, "Enhanced light absorption of solar cells and photodetectors by diffraction,” 6, 858, 462 B2(2005). [3] Saleem H. Zaidi, “Enhanced optical absorption and radiation tolerance in thin-film solar cells and photodetectors”, 10/298,694 (2005). [4] Douglas S. Ruby, William K. Schubert, J. M. Gee, and Saleem H. Zaidi, “Silicon cells made by self-aligned, selective emitter, plasma- etchback process”. [5] Saleem H. Zaidi, D. S. Ruby, and J. M. Gee, IEEE Trans. Elect. Dev. 48, 1200 (2001).

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[6] Saleem Zaidi and S. R. J. Brueck, Nanoscale Fabrication by Interferometric Lithography Proc. SPIE 3740 Optical Engineering for Sensing and Nanotechnology 340-343 (1999). [7] Saleem H. Zaidi, James M. Gee, Douglas S. Ruby, and S. R. J. Brueck, Characterization of Si Nanostructured Surfaces, Proc. SPIE 3790 Engineered Nanostructural Films and Materials, 151- 159 (1999). [8] Ganesh Vanamu, A. K. Datye and Saleem. H. Zaidi, Epitaxial Growth of High- Quality Ge films on Nanostructured Silicon Substrates, Applied Physics Letters, 88, (2006). [9] Ganesh Vanamu, A. K. Datye, R. L. Dawson and Saleem. H. Zaidi, High quality GaAs films on Ge/SiGe on nanopatterned Si using interferometric lithography, Applied Physics Letters, 88, 1, (2006). [10] Ganesh Vanamu, A. K. Datye and Saleem. H. Zaidi, High quality Ge/SixGe1- x on nano-scale patterned Si structures, Journal of Vacuum Science & Technology B 23, 1622-1629 (2005). [11] Saleem H. Zaidi, “Nanostructures for Hetero-epitaxial Growth on Silicon Substrates”, 6, 835, 246 B2, issued on Dec. 28, 2004. [12] A. W. Azhari, B. T. Goh, Suhaila Sepeai, M. Khairunaz, K. Sopian and Saleem H. Zaidi. Synthesis and Characterization of Self-Assembled, High Aspect Ratio nm- Scale Columnar Silicon Structures, 9th IEEE Photovoltaic Specialists Conference, Tampa, Florida.

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