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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 19, Iss. 27 — Dec. 19, 2011
  • pp: 26308–26317
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Broadband antireflective germanium surfaces based on subwavelength structures for photovoltaic cell applications

Jung Woo Leem, Young Min Song, and Jae Su Yu  »View Author Affiliations


Optics Express, Vol. 19, Issue 27, pp. 26308-26317 (2011)
http://dx.doi.org/10.1364/OE.19.026308


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Abstract

We fabricated the germanium (Ge) subwavelength structures (SWSs) using gold (Au) metallic nanopatterns dewetted by rapid thermal annealing and inductively coupled plasma etching in SiCl4 plasma for Ge-based photovoltaic cells. Using the optimized Au nanopatterns as an etch mask, the Ge SWSs were formed by varying the etching parameters to achieve the better antireflection properties. The reflectance of Ge SWSs depended strongly on their period, height, and shape which are closely related to the refractive index profile between air and the Ge substrate. The tapered cone Ge SWSs reduced considerably the reflectance compared to the samples with a truncated cone shape as well as the Ge substrate due to the linearly graded refractive index distribution from air to the Ge substrate. The Ge SWS with the tapered cone shape and high height exhibited a dramatic decrease in the reflectance (i.e., <10%) over a wide wavelength region of 350-1800 nm, thus leading to a low solar weighted reflectance of ~3.6%. The reflectance was also lower than ~8.8% at a wavelength of 633 nm in the incident angle range of 15-85°. The measured reflectance data of Ge SWSs showed similar trends to the calculated results in a rigorous coupled wave analysis simulation.

© 2011 OSA

1. Introduction

Germanium (Ge), which has a relatively high charge-carrier mobility and high absorption coefficient, is one of the most useful semiconductor materials in optical and optoelectronic device applications such as image sensors, photodetectors, and solar cells [1

1. R. Kaufmann, G. Isella, A. Sanchez-Amores, S. Neukom, A. Neels, L. Neumann, A. Brenzikofer, A. Dommann, C. Urban, and H. von Känel, “Near infrared image sensor with integrated germanium photodiodes,” J. Appl. Phys. 110(2), 023107 (2011). [CrossRef]

3

3. W. Guter, J. Schöne, S. P. Philipps, M. Steiner, G. Siefer, A. Wekkeli, E. Welser, E. Oliva, A. W. Bett, and F. Dimroth, “Current-matched triple-junction solar cell reaching 41.1% conversion efficiency under concentrated sunlight,” Appl. Phys. Lett. 94(22), 223504 (2009). [CrossRef]

]. Furthermore, the low energy bandgap (0.66 eV) of Ge allows for absorbing the photons in a wide wavelength range up to ~1800 nm. In solar cells, especially, the Ge is usually used as the substrate and bottom cell of monolithic tandem multi-junction solar cells as well as stand-alone cells in hybrid lighting systems [4

4. M. Yamaguchi, T. Takamoto, and K. Araki, “Super high-efficiency multi-junction and concentrator solar cells,” Sol. Energy Mater. Sol. Cells 90(18-19), 3068–3077 (2006). [CrossRef]

7

7. N. E. Posthuma, J. van der Heide, G. Flamand, and J. Poortmans, “Development of low cost germanium photovoltaic cells for application in TPV using spin on diffusants,” AIP Conf. Proc. 738, 337–344 (2004). [CrossRef]

]. Also, Ge single-junction solar cells can be employed as a receiver in thermophotovoltaic systems, where a narrow-band radiation spectrum (i.e., a wavelength range of 1500-1600 nm) originated from a heat source is used [8

8. T. Nagashima, K. Okumura, and M. Yamaguchi, “A germanium back contact type thermophotovoltaic cell,” AIP Conf. Proc. 890, 174–181 (2007). [CrossRef]

,9

9. J. van der Heide, N. E. Posthuma, G. Flamand, W. Geens, and J. Poortmans, “Cost-efficient thermophotovoltaic cells based on germanium substrates,” Sol. Energy Mater. Sol. Cells 93(10), 1810–1816 (2009). [CrossRef]

]. However, the surface reflectivity of Ge is very high (i.e., >35%) due to the Fresnel reflection caused by its high refractive index. Thus, for practical device applications, efficient antireflection coatings (ARCs) are required to enhance the light absorption by suppressing Fresnel reflection losses. Various kinds of semiconductor nanostructures such as nanowires and nanorods were demonstrated for ARCs [10

10. P. Yu, C. H. Chang, C. H. Chiu, C. S. Yang, J. C. Yu, H. C. Kuo, S. H. Hsu, and Y. C. Chang, “Efficiency enhancement of GaAs photovoltaics employing antireflective indium tin oxide nanocolumns,” Adv. Mater. 21(16), 1618–1621 (2009). [CrossRef]

,11

11. Y. J. Lee, D. S. Ruby, D. W. Peters, B. B. McKenzie, and J. W. P. Hsu, “ZnO nanostructures as efficient antireflection layers in solar cells,” Nano Lett. 8(5), 1501–1505 (2008). [CrossRef] [PubMed]

]. Unfortunately, these nanostructures lead to the larger surface recombination losses caused by the increased surface area, which may degrade the performance of devices. However, these losses can be somewhat reduced by a proper surface passivation of nanostructures [12

12. T. Hanrath and B. A. Korgel, “Chemical surface passivation of Ge nanowires,” J. Am. Chem. Soc. 126(47), 15466–15472 (2004). [CrossRef] [PubMed]

,13

13. Y. Dan, K. Seo, K. Takei, J. H. Meza, A. Javey, and K. B. Crozier, “Dramatic reduction of surface recombination by in situ surface passivation of silicon nanowires,” Nano Lett. 11(6), 2527–2532 (2011). [CrossRef] [PubMed]

]. Recently, subwavelength structures (SWSs) inspired by the moth-eye effect [14

14. P. B. Clapham and M. C. Hutley, “Reduction of lens reflexion by the “Moth Eye” principle,” Nature 244(5414), 281–282 (1973). [CrossRef]

] have been widely studied as an alternative of the conventional thin film ARCs because they can efficiently reduce the surface reflection in the wide ranges of wavelength and incident angle [15

15. Y. M. Song, S. J. Jang, J. S. Yu, and Y. T. Lee, “Bioinspired parabola subwavelength structures for improved broadband antireflection,” Small 6(9), 984–987 (2010). [CrossRef] [PubMed]

21

21. J. W. Leem, D. H. Joo, and J. S. Yu, “Biomimetic parabola-shaped AZO subwavelength grating structures for efficient antireflection of Si-based solar cells,” Sol. Energy Mater. Sol. Cells 95(8), 2221–2227 (2011). [CrossRef]

].

Meanwhile, the thermal dewetting of thin metal films is one of useful and simple processes for high-density self-assembled metal nanoparticles over a large area, which can form the etch mask for dry etching in the nanostructure fabrication [22

22. J. M. Lee and B. I. Kim, “Thermal dewetting of Pt thin film: Etch-masks for the fabrication of semiconductor nanostructures,” Mater. Sci. Eng. A 449–451, 769–773 (2007). [CrossRef]

25

25. J. W. Leem, J. S. Yu, Y. M. Song, and Y. T. Lee, “Antireflection characteristics of disordered GaAs subwavelength structures by thermally dewetted Au nanoparticles,” Sol. Energy Mater. Sol. Cells 95(2), 669–676 (2011). [CrossRef]

]. To achieve a good light absorption for solar cell applications, the geometric shape, size, and height of surface structures should be optimized [26

26. Y. Li, J. Zhang, and B. Yang, “Antireflection surfaces based on biomimetic nanopillared arrays,” Nano Today 5(2), 117–127 (2010). [CrossRef]

,27

27. J. Zhu, C. M. Hsu, Z. Yu, S. Fan, and Y. Cui, “Nanodome solar cells with efficient light management and self-cleaning,” Nano Lett. 10(6), 1979–1984 (2010). [CrossRef] [PubMed]

]. Although studies on antireflective nanostructures of various materials such as silicon, indium tin oxide, silicon nitride, gallium nitride, and gallium arsenide were reported [17

17. E. S. Choi, Y. M. Song, G. C. Park, and Y. T. Lee, “Disordered antireflective subwavelength structures using Ag nanoparticles for GaN-based optical device applications,” J. Nanosci. Nanotechnol. 11(2), 1342–1345 (2011). [CrossRef] [PubMed]

,18

18. K. C. Sahoo, Y. Li, and E. Y. Chang, “Shape effect of silicon nitride subwavelength structure on reflectance for silicon solar cells,” IEEE Trans. Electron. Dev. 57(10), 2427–2433 (2010). [CrossRef]

,23

23. S. Wang, X. Z. Yu, and H. T. Fan, “Simple lithographic approach for subwavelength structure antireflection,” Appl. Phys. Lett. 91(6), 061105 (2007). [CrossRef]

25

25. J. W. Leem, J. S. Yu, Y. M. Song, and Y. T. Lee, “Antireflection characteristics of disordered GaAs subwavelength structures by thermally dewetted Au nanoparticles,” Sol. Energy Mater. Sol. Cells 95(2), 669–676 (2011). [CrossRef]

], there has been very little work on the Ge. In this work, we studied systematically the fabrication and characteristics of the Ge SWSs by inductively coupled plasma (ICP) etching using gold (Au) nanoparticles as an etch mask to form various surface profiles, with theoretical investigations using the rigorous coupled wave analysis (RCWA) simulation. The influence of the incident angle of light on the reflectance and solar weighted reflectance was also explored for solar cell applications.

2. Experimental details

Figure 1
Fig. 1 Schematic diagram of the process steps for the fabrication of SWSs on Ge substrates using Au nanomask patterns. The tow-view SEM images of thermally dewetted Au nanoparticles and the refractive index profile of truncated and cone-shaped Ge SWSs are also shown.
shows the schematic diagram of the process steps for the fabrication of SWSs on Ge substrates using Au nanomask patterns. The top-view SEM images of thermally dewetted Au nanoparticles by rapid thermal annealing (RTA) process for Au film thicknesses of 5, 10, and 15 nm, respectively, and the refractive index profile of truncated and tapered cone Ge SWSs are also shown. Before cutting into substrate pieces with a size of ~1.7 × 1.7 cm2, 2 inch p-type (100) Ge wafers were cleaned by acetone and methanol, rinsed in de-ionized water, and subsequently dried in a flowing nitrogen gas. To fabricate the etch nanomask patterns, first, the Au films of different thicknesses were directly deposited on Ge substrates by using a thermal evaporator (KVE-T2000, Korea Vac. Tech. Ltd.) under a background pressure of 2 × 10−6 Torr at room temperature. The thicknesses of Au films were 5, 10, and 15 nm. The deposition rate was set to ~0.5 Å/s as monitored by a quartz crystal oscillator. To achieve the desirable Au etch nanomask patterns, the samples were thermally dewetted by using a RTA system (KVR-2000, Korea Vac. Tech. Ltd.) at a temperature of 450 °C for 2 min in nitrogen environment. From the SEM images in Fig. 1, the Au films turned into rounded lens-like nanoparticles. This phenomenon is because the surface energy of metal films exceeds the sum of the surface energy of the semiconductor substrate and the interfacial energy between two layers [22

22. J. M. Lee and B. I. Kim, “Thermal dewetting of Pt thin film: Etch-masks for the fabrication of semiconductor nanostructures,” Mater. Sci. Eng. A 449–451, 769–773 (2007). [CrossRef]

]. Clearly, the average size and period of Au nanoparticles can be roughly controlled by the RTA condition as well as the Au film thickness. The average diameter and correlation distance of the disorderly patterned Au nanoparticles were estimated using a commercial image processor (ImageJ 1.42q, NIH), thus exhibiting approximately 57.1 ± 29, 132.5 ± 70, and 237.8 ± 100 nm and 110, 245, and 430 nm for Au film thicknesses of 5, 10, and 15 nm, respectively. Using the Au nanopatterns as the etch mask, the SWSs were fabricated on Ge substrates by an ICP (Plasmalab System 100, Oxford) etching process. The etching conditions (i.e., RF power, additional ICP power, process pressure, and etching time) were changed to obtain a desirable etched profile of antireflective Ge surface structures. The thickness of Au films was very thin and the Au nanomask patterns were considerably removed during the ICP etching process. Particularly, for cone-shaped Ge SWSs, the Au nanomask patterns were almost removed. Nevertheless, the remaining Au nanopatterns after the etching were eliminated by Au etchant solution. As shown in the refractive index profile of Fig. 1, the effective refractive index is gradually varied from air (nair = 1) to the Ge substrate (average nGe~4.8 at wavelengths of 350-1800 nm) via Ge SWSs, which is correlated with their effective volume fraction [23

23. S. Wang, X. Z. Yu, and H. T. Fan, “Simple lithographic approach for subwavelength structure antireflection,” Appl. Phys. Lett. 91(6), 061105 (2007). [CrossRef]

,28

28. A. J. Jääskeläinen, K. E. Peiponen, J. Räty, U. Tapper, O. Richard, E. I. Kauppinen, and K. Lumme, “Estimation of the refractive index of plastic pigments by Wiener bounds,” Opt. Eng. 39(11), 2959–2963 (2000). [CrossRef]

]. The refractive index of truncated cone Ge SWSs is abruptly changed at the interface of air/Ge SWSs, whereas the tapered cone Ge SWSs have a linear refractive index change from air to the Ge substrate. These different index profiles would modify the surface reflection properties [21

21. J. W. Leem, D. H. Joo, and J. S. Yu, “Biomimetic parabola-shaped AZO subwavelength grating structures for efficient antireflection of Si-based solar cells,” Sol. Energy Mater. Sol. Cells 95(8), 2221–2227 (2011). [CrossRef]

,29

29. D. G. Stavenga, S. Foletti, G. Palasantzas, and K. Arikawa, “Light on the moth-eye corneal nipple array of butterflies,” Proc. Biol. Sci. 273(1587), 661–667 (2006). [CrossRef] [PubMed]

]. The structural shape and morphology of the fabricated Ge SWSs were observed by using a scanning electron microscope (SEM, LEO SUPRA 55, Carl Zeiss) with an accelerating voltage of 10 kV. The total reflectance was measured by using UV-vis-NIR spectrophotometer (Cary 5000, Varian) with an integrating sphere at near-normal incidence angle of 8°. For angle-dependent reflectance measurements, the spectroscopic ellipsometry (V-VASE, J. A. Woollam Co. Inc.) was used at angles of 15-85° for linearly polarized light incidence in specular mode.

3. Results and discussion

Figure 2(a)
Fig. 2 (a) SEM images of etched Ge SWSs using Au nanomask patterns for Au film thicknesses of (i) 5 nm, (ii) 10 nm, and (iii) 15 nm, (b) contour plot of the variation of calculated reflectance spectra as a function of the period of Ge SWSs, (c) electric field intensity distribution of the Ge SWS at a wavelength of 1000 nm at normal incidence, and (d) measured reflectance spectra of the corresponding Ge SWSs.
shows the 30°-tilted oblique-view SEM images of the etched Ge SWSs using Au nanomask patterns for Au film thicknesses of (i) 5 nm, (ii) 10 nm, and (iii) 15 nm. The corresponding cross-sectional SEM images are shown in the insets. The samples were etched with 50 RF power at 5 mTorr for 10 min in 5 sccm SiCl4 plasma. It can be evidently observed that the Au nanopatterns were transferred directly onto the Ge substrates by the ICP etching, which results in Ge SWSs, as can be seen in Figs. 1 and 2. As the Au film thickness was increased from 5 to 15 nm, the Ge SWSs were changed from the truncated cone nanopillars to the cylindrical ones while their average height had similar values of about 107 ± 8 nm. This can be explained by the fact that the thinner Au nanoparticles with a small size (i.e., for 5 nm Au films) are more quickly removed at the edge of nanoparticles compared to the thicker and larger ones. In the etched Ge SWGs, the volume fraction ratios (i.e., the ratio of the volume occupied by the constituent semiconductor material to the total volume), which were roughly estimated from the SEM images with the help of the image processor, were approximately 0.7, 0.55, and 0.49 for 5, 10, and 15 nm of Au films, respectively. The theoretical analysis on the antireflective characteristics of the fabricated Ge SWSs was carried out using the RCWA simulation [30

30. M. G. Moharam and T. K. Gaylord, “Rigorous coupled-wave analysis of planar-grating diffraction,” J. Opt. Soc. Am. 71(7), 811–818 (1981). [CrossRef]

]. In calculations, the 5th order diffraction was used to obtain the diffraction efficiency of Ge SWSs, which is a sufficient number to stabilize the results numerically, and the incident light entered from air into the Ge SWS at normal incidence. Figure 2(b) shows the contour plot of the variation of calculated reflectance spectra as a function of the period of Ge SWSs. The three-dimensional simulation model used in this calculation, which is shown in the inset of Fig. 2(b), was constructed by the mean shape of the SEM image, assuming the truncated cones with a height of 110 nm and the six-fold hexagonal symmetry structure for simplicity. The apex diameter was set to 0.5 of the bottom diameter in the truncated cone. In calculations, the diameter of nanopillars was increased to the ratio value of 0.55 to the period of Ge SWSs in consideration of average diameter and correlation distance of the disorderly patterned Au nanoparticles. As the period of Ge SWSs is increased, the low reflectance band is shifted to the longer wavelength region [31

31. S. A. Boden and D. M. Bagnall, “Tunable reflection minima of nanostructured antireflective surfaces,” Appl. Phys. Lett. 93(13), 133108 (2008). [CrossRef]

]. However, the reflectance becomes higher at periods above 400 nm in the wavelength region of >600 nm. At periods around 250 nm, the reflectance of <10% is maintained in the wavelength range of 350-700 nm, as can be seen in Fig. 2(b). The electric field intensity distribution of the Ge SWS with a period of 250 nm at a wavelength of 1000 nm at normal incidence used in the calculation of Fig. 2(b) is shown in Fig. 2(c). The electric field is absorbed at the relatively short optical distance in the Ge due to its absorption.

Figure 2(d) shows the measured reflectance spectra of the corresponding Ge SWSs. The reflectance of Ge substrate is also shown for comparison. Due to the large difference in refractive index between air and the Ge, the surface reflectivity of Ge substrate was higher than 35% at wavelengths of 350-1800 nm. For the etched Ge SWSs, the reflectance is lower than that of the Ge substrate due to the gradual index change from air to the Ge substrate via Ge SWSs as displayed in Fig. 1. Above λ~1800 nm, the abrupt increase in reflectance spectra is attributed to the backscattered light from the back surface of the Ge substrate since the Ge is transparent below its energy bandgap [32

32. M. L. Kuo, D. J. Poxson, Y. S. Kim, F. W. Mont, J. K. Kim, E. F. Schubert, and S. Y. Lin, “Realization of a near-perfect antireflection coating for silicon solar energy utilization,” Opt. Lett. 33(21), 2527–2529 (2008). [CrossRef] [PubMed]

]. For Au nanopatterns of the 5 nm film, the reflectance of the Ge SWS is lower than that of other samples in the short wavelength range of 500-650 nm. In contrast, the low reflectance wavelength region was approximately 650-1000 nm for Au nanopatterns of the 10 nm film. This means that the low reflectance band is shifted towards the longer wavelength region as the average period of SWSs is increased. For the Ge SWS etched using Au nanopatterns of the 15 nm film, the reflectance spectrum had a similar shape over a wide wavelength region of 350-1800 nm, keeping the lower reflectance values. This results from the lower average height and longer average period of SWSs [31

31. S. A. Boden and D. M. Bagnall, “Tunable reflection minima of nanostructured antireflective surfaces,” Appl. Phys. Lett. 93(13), 133108 (2008). [CrossRef]

], closer to the flat substrate. Although there are some discrepancies between the measured and calculated results due to the simplicity of the geometric simulation model for the actually fabricated structures, the overall trend appears to be similar. Therefore, for Au nanomask patterns, the film thickness of 10 nm was chosen in this experiment because it provides a relatively low reflectance over a broad wavelength range of 650-1800 nm.

Figure 3
Fig. 3 (a) 10°-tilted side-view SEM images and (b) measured reflectance spectra of the etched Ge SWSs using Au nanomask patterns for RF powers of (i) 25 W, (ii) 50 W, (iii) 75 W, and (iv) 100 W.
shows (a) 10°-tilted side-view SEM images and (b) measured reflectance spectra of the etched Ge SWSs using Au nanomask patterns for RF powers of (i) 25 W, (ii) 50 W, (iii) 75 W, and (iv) 100 W. The process pressure and etching time were 5 mTorr and 10 min, respectively. As the RF power was increased from 25 to 100 W, the etching rate was increased from 8.3 to 20 nm/min owing to the increased ion energy flux, and the height of Ge SWSs became higher from 83±6 to 200±14 nm while their volume fraction ratio was decreased from ~0.58 to ~0.5. The shape of etched structures was also dependent on the RF power. The nanopillars in Ge SWSs were more tapered with increasing the RF power because the Au nanomask patterns were more quickly eroded from the edges due to the high RF power during the etching process. These tapered nanopillars with a cone shape in SWSs have a linearly graded refractive index profile, which results in the lower reflectance over a wide wavelength region than that of the cylindrical shaped SWSs [18

18. K. C. Sahoo, Y. Li, and E. Y. Chang, “Shape effect of silicon nitride subwavelength structure on reflectance for silicon solar cells,” IEEE Trans. Electron. Dev. 57(10), 2427–2433 (2010). [CrossRef]

]. As expected, at an RF power of 100 W, the more tapered Ge SWS with taller nanopillars exhibited a relatively low reflectance as shown in Fig. 3(b), leading to an average reflectance of ~16.7% at wavelengths of 350-1800 nm.

For omnidirectional antireflection properties of the Ge SWSs, the incident angle (θi) of the linearly polarized light was varied from 15 to 85°. Figure 6(a)
Fig. 6 (a) Measured reflectance of the optimized Ge SWS at 15 min of etching time under different angles of the incident light for a wavelength of 633 nm, (b) contour plot of calculated angle dependent reflectance spectra of the optimized Ge SWS, and (c) photograph images of (i) Ge substrate and (ii) optimized Ge SWS for the samples (left) and the lower-magnified 30°-tilted side view SEM image of (ii) (right).
shows the measured reflectance of the optimized Ge SWS at 15 min of etching time under different angles of the incident light for a wavelength of 633 nm. For the incident angles of θi = 15-70°, the reflectance remained below 2.5% and then it significantly increased up to ~8.8% at θi = 85°. However, these values were much lower than that of the Ge substrate (i.e., ~47.7% at λ~633 nm) at near-normal incidence. The contour plot of calculated angle dependent reflectance spectra of the corresponding structure in Fig. 5(b) is shown in Fig. 6(b). The period, height, and bottom diameter of Ge SWSs were assumed to be 250, 700, and 200 nm, respectively. The calculated reflectance depends strongly on the incident angle over a wide wavelength region of 350-1800 nm, especially at incident angles above θi = 70°. However, the reflectance remains roughly below 20% at wavelengths of 350-1400 nm at incident angles less than θi = 70°. At wavelengths around 633 nm, the reflectance decreases up to θi = 50° and it abruptly increases up to θi = 85°. Although there are some differences between the experimentally measured and theoretically calculated results, a similar overall trend is observed. Figure 6(c) shows the photograph images of (i) Ge substrate and (ii) optimized Ge SWS for the samples (left) and the lower-magnified 30°-tilted side-view SEM image of (ii) (right). The surface of the Ge SWS clearly appeared to be dark black due to its low surface reflectivity in comparison with the Ge substrate. As shown in the SEM image of Fig. 6(c), it can be observed that the taller and tapered cone SWS is uniformly formed on the Ge substrate over a large area.

4. Conclusion

Acknowledgments

This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (No. 2011-0003857 and No. 2010-0025071).

References and links

1.

R. Kaufmann, G. Isella, A. Sanchez-Amores, S. Neukom, A. Neels, L. Neumann, A. Brenzikofer, A. Dommann, C. Urban, and H. von Känel, “Near infrared image sensor with integrated germanium photodiodes,” J. Appl. Phys. 110(2), 023107 (2011). [CrossRef]

2.

L. Tang, S. E. Kocabas, S. Latif, A. K. Okyay, D. S. Ly-Gagnon, K. C. Saraswat, and D. A. B. Miller, “Nanometre-scale germanium photodetector enhanced by a near-infrared dipole antenna,” Nat. Photonics 2(4), 226–229 (2008). [CrossRef]

3.

W. Guter, J. Schöne, S. P. Philipps, M. Steiner, G. Siefer, A. Wekkeli, E. Welser, E. Oliva, A. W. Bett, and F. Dimroth, “Current-matched triple-junction solar cell reaching 41.1% conversion efficiency under concentrated sunlight,” Appl. Phys. Lett. 94(22), 223504 (2009). [CrossRef]

4.

M. Yamaguchi, T. Takamoto, and K. Araki, “Super high-efficiency multi-junction and concentrator solar cells,” Sol. Energy Mater. Sol. Cells 90(18-19), 3068–3077 (2006). [CrossRef]

5.

I. Prieto, B. Galiana, P. A. Postigo, C. Algora, L. J. Martínez, and I. Rey-Stolle, “Enhanced quantum efficiency of Ge solar cells by a two-dimensional photonic crystal nanostructured surface,” Appl. Phys. Lett. 94(19), 191102 (2009). [CrossRef]

6.

N. E. Posthuma, J. van der Heide, G. Flamand, and J. Poortmans, “Emitter formation and contact realization by diffusion for germanium photovoltaic devices,” IEEE Trans. Electron. Dev. 54(5), 1210–1215 (2007). [CrossRef]

7.

N. E. Posthuma, J. van der Heide, G. Flamand, and J. Poortmans, “Development of low cost germanium photovoltaic cells for application in TPV using spin on diffusants,” AIP Conf. Proc. 738, 337–344 (2004). [CrossRef]

8.

T. Nagashima, K. Okumura, and M. Yamaguchi, “A germanium back contact type thermophotovoltaic cell,” AIP Conf. Proc. 890, 174–181 (2007). [CrossRef]

9.

J. van der Heide, N. E. Posthuma, G. Flamand, W. Geens, and J. Poortmans, “Cost-efficient thermophotovoltaic cells based on germanium substrates,” Sol. Energy Mater. Sol. Cells 93(10), 1810–1816 (2009). [CrossRef]

10.

P. Yu, C. H. Chang, C. H. Chiu, C. S. Yang, J. C. Yu, H. C. Kuo, S. H. Hsu, and Y. C. Chang, “Efficiency enhancement of GaAs photovoltaics employing antireflective indium tin oxide nanocolumns,” Adv. Mater. 21(16), 1618–1621 (2009). [CrossRef]

11.

Y. J. Lee, D. S. Ruby, D. W. Peters, B. B. McKenzie, and J. W. P. Hsu, “ZnO nanostructures as efficient antireflection layers in solar cells,” Nano Lett. 8(5), 1501–1505 (2008). [CrossRef] [PubMed]

12.

T. Hanrath and B. A. Korgel, “Chemical surface passivation of Ge nanowires,” J. Am. Chem. Soc. 126(47), 15466–15472 (2004). [CrossRef] [PubMed]

13.

Y. Dan, K. Seo, K. Takei, J. H. Meza, A. Javey, and K. B. Crozier, “Dramatic reduction of surface recombination by in situ surface passivation of silicon nanowires,” Nano Lett. 11(6), 2527–2532 (2011). [CrossRef] [PubMed]

14.

P. B. Clapham and M. C. Hutley, “Reduction of lens reflexion by the “Moth Eye” principle,” Nature 244(5414), 281–282 (1973). [CrossRef]

15.

Y. M. Song, S. J. Jang, J. S. Yu, and Y. T. Lee, “Bioinspired parabola subwavelength structures for improved broadband antireflection,” Small 6(9), 984–987 (2010). [CrossRef] [PubMed]

16.

M. Y. Chiu, C. H. Chang, M. A. Tsai, F. Y. Chang, and P. Yu, “Improved optical transmission and current matching of a triple-junction solar cell utilizing sub-wavelength structures,” Opt. Express 18(S3Suppl 3), A308–A313 (2010). [CrossRef] [PubMed]

17.

E. S. Choi, Y. M. Song, G. C. Park, and Y. T. Lee, “Disordered antireflective subwavelength structures using Ag nanoparticles for GaN-based optical device applications,” J. Nanosci. Nanotechnol. 11(2), 1342–1345 (2011). [CrossRef] [PubMed]

18.

K. C. Sahoo, Y. Li, and E. Y. Chang, “Shape effect of silicon nitride subwavelength structure on reflectance for silicon solar cells,” IEEE Trans. Electron. Dev. 57(10), 2427–2433 (2010). [CrossRef]

19.

J. W. Leem, Y. M. Song, Y. T. Lee, and J. S. Yu, “Antireflective properties of AZO subwavelength gratings patterned by holographic lithography,” Appl. Phys. B 99(4), 695–700 (2010). [CrossRef]

20.

B. J. Kim and J. Kim, “Fabrication of GaAs subwavelength structure (SWS) for solar cell applications,” Opt. Express 19(S3Suppl 3), A326–A330 (2011). [CrossRef] [PubMed]

21.

J. W. Leem, D. H. Joo, and J. S. Yu, “Biomimetic parabola-shaped AZO subwavelength grating structures for efficient antireflection of Si-based solar cells,” Sol. Energy Mater. Sol. Cells 95(8), 2221–2227 (2011). [CrossRef]

22.

J. M. Lee and B. I. Kim, “Thermal dewetting of Pt thin film: Etch-masks for the fabrication of semiconductor nanostructures,” Mater. Sci. Eng. A 449–451, 769–773 (2007). [CrossRef]

23.

S. Wang, X. Z. Yu, and H. T. Fan, “Simple lithographic approach for subwavelength structure antireflection,” Appl. Phys. Lett. 91(6), 061105 (2007). [CrossRef]

24.

C. H. Chiu, P. Yu, H. C. Kuo, C. C. Chen, T. C. Lu, S. C. Wang, S. H. Hsu, Y. J. Cheng, and Y. C. Chang, “Broadband and omnidirectional antireflection employing disordered GaN nanopillars,” Opt. Express 16(12), 8748–8754 (2008). [CrossRef] [PubMed]

25.

J. W. Leem, J. S. Yu, Y. M. Song, and Y. T. Lee, “Antireflection characteristics of disordered GaAs subwavelength structures by thermally dewetted Au nanoparticles,” Sol. Energy Mater. Sol. Cells 95(2), 669–676 (2011). [CrossRef]

26.

Y. Li, J. Zhang, and B. Yang, “Antireflection surfaces based on biomimetic nanopillared arrays,” Nano Today 5(2), 117–127 (2010). [CrossRef]

27.

J. Zhu, C. M. Hsu, Z. Yu, S. Fan, and Y. Cui, “Nanodome solar cells with efficient light management and self-cleaning,” Nano Lett. 10(6), 1979–1984 (2010). [CrossRef] [PubMed]

28.

A. J. Jääskeläinen, K. E. Peiponen, J. Räty, U. Tapper, O. Richard, E. I. Kauppinen, and K. Lumme, “Estimation of the refractive index of plastic pigments by Wiener bounds,” Opt. Eng. 39(11), 2959–2963 (2000). [CrossRef]

29.

D. G. Stavenga, S. Foletti, G. Palasantzas, and K. Arikawa, “Light on the moth-eye corneal nipple array of butterflies,” Proc. Biol. Sci. 273(1587), 661–667 (2006). [CrossRef] [PubMed]

30.

M. G. Moharam and T. K. Gaylord, “Rigorous coupled-wave analysis of planar-grating diffraction,” J. Opt. Soc. Am. 71(7), 811–818 (1981). [CrossRef]

31.

S. A. Boden and D. M. Bagnall, “Tunable reflection minima of nanostructured antireflective surfaces,” Appl. Phys. Lett. 93(13), 133108 (2008). [CrossRef]

32.

M. L. Kuo, D. J. Poxson, Y. S. Kim, F. W. Mont, J. K. Kim, E. F. Schubert, and S. Y. Lin, “Realization of a near-perfect antireflection coating for silicon solar energy utilization,” Opt. Lett. 33(21), 2527–2529 (2008). [CrossRef] [PubMed]

33.

D. J. Economou, “Modeling and simulation of plasma etching reactors for microelectronics,” Thin Solid Films 365(2), 348–367 (2000). [CrossRef]

34.

D. Redfield, “Method for evaluation of antireflection coatings,” Solar Cells 3(1), 27–33 (1981). [CrossRef]

OCIS Codes
(310.1210) Thin films : Antireflection coatings
(220.4241) Optical design and fabrication : Nanostructure fabrication
(050.6624) Diffraction and gratings : Subwavelength structures

ToC Category:
Solar Energy

History
Original Manuscript: September 28, 2011
Revised Manuscript: November 13, 2011
Manuscript Accepted: November 23, 2011
Published: December 9, 2011

Citation
Jung Woo Leem, Young Min Song, and Jae Su Yu, "Broadband antireflective germanium surfaces based on subwavelength structures for photovoltaic cell applications," Opt. Express 19, 26308-26317 (2011)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-19-27-26308


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References

  1. R. Kaufmann, G. Isella, A. Sanchez-Amores, S. Neukom, A. Neels, L. Neumann, A. Brenzikofer, A. Dommann, C. Urban, and H. von Känel, “Near infrared image sensor with integrated germanium photodiodes,” J. Appl. Phys.110(2), 023107 (2011). [CrossRef]
  2. L. Tang, S. E. Kocabas, S. Latif, A. K. Okyay, D. S. Ly-Gagnon, K. C. Saraswat, and D. A. B. Miller, “Nanometre-scale germanium photodetector enhanced by a near-infrared dipole antenna,” Nat. Photonics2(4), 226–229 (2008). [CrossRef]
  3. W. Guter, J. Schöne, S. P. Philipps, M. Steiner, G. Siefer, A. Wekkeli, E. Welser, E. Oliva, A. W. Bett, and F. Dimroth, “Current-matched triple-junction solar cell reaching 41.1% conversion efficiency under concentrated sunlight,” Appl. Phys. Lett.94(22), 223504 (2009). [CrossRef]
  4. M. Yamaguchi, T. Takamoto, and K. Araki, “Super high-efficiency multi-junction and concentrator solar cells,” Sol. Energy Mater. Sol. Cells90(18-19), 3068–3077 (2006). [CrossRef]
  5. I. Prieto, B. Galiana, P. A. Postigo, C. Algora, L. J. Martínez, and I. Rey-Stolle, “Enhanced quantum efficiency of Ge solar cells by a two-dimensional photonic crystal nanostructured surface,” Appl. Phys. Lett.94(19), 191102 (2009). [CrossRef]
  6. N. E. Posthuma, J. van der Heide, G. Flamand, and J. Poortmans, “Emitter formation and contact realization by diffusion for germanium photovoltaic devices,” IEEE Trans. Electron. Dev.54(5), 1210–1215 (2007). [CrossRef]
  7. N. E. Posthuma, J. van der Heide, G. Flamand, and J. Poortmans, “Development of low cost germanium photovoltaic cells for application in TPV using spin on diffusants,” AIP Conf. Proc.738, 337–344 (2004). [CrossRef]
  8. T. Nagashima, K. Okumura, and M. Yamaguchi, “A germanium back contact type thermophotovoltaic cell,” AIP Conf. Proc.890, 174–181 (2007). [CrossRef]
  9. J. van der Heide, N. E. Posthuma, G. Flamand, W. Geens, and J. Poortmans, “Cost-efficient thermophotovoltaic cells based on germanium substrates,” Sol. Energy Mater. Sol. Cells93(10), 1810–1816 (2009). [CrossRef]
  10. P. Yu, C. H. Chang, C. H. Chiu, C. S. Yang, J. C. Yu, H. C. Kuo, S. H. Hsu, and Y. C. Chang, “Efficiency enhancement of GaAs photovoltaics employing antireflective indium tin oxide nanocolumns,” Adv. Mater. 21(16), 1618–1621 (2009). [CrossRef]
  11. Y. J. Lee, D. S. Ruby, D. W. Peters, B. B. McKenzie, and J. W. P. Hsu, “ZnO nanostructures as efficient antireflection layers in solar cells,” Nano Lett.8(5), 1501–1505 (2008). [CrossRef] [PubMed]
  12. T. Hanrath and B. A. Korgel, “Chemical surface passivation of Ge nanowires,” J. Am. Chem. Soc.126(47), 15466–15472 (2004). [CrossRef] [PubMed]
  13. Y. Dan, K. Seo, K. Takei, J. H. Meza, A. Javey, and K. B. Crozier, “Dramatic reduction of surface recombination by in situ surface passivation of silicon nanowires,” Nano Lett.11(6), 2527–2532 (2011). [CrossRef] [PubMed]
  14. P. B. Clapham and M. C. Hutley, “Reduction of lens reflexion by the “Moth Eye” principle,” Nature244(5414), 281–282 (1973). [CrossRef]
  15. Y. M. Song, S. J. Jang, J. S. Yu, and Y. T. Lee, “Bioinspired parabola subwavelength structures for improved broadband antireflection,” Small6(9), 984–987 (2010). [CrossRef] [PubMed]
  16. M. Y. Chiu, C. H. Chang, M. A. Tsai, F. Y. Chang, and P. Yu, “Improved optical transmission and current matching of a triple-junction solar cell utilizing sub-wavelength structures,” Opt. Express18(S3Suppl 3), A308–A313 (2010). [CrossRef] [PubMed]
  17. E. S. Choi, Y. M. Song, G. C. Park, and Y. T. Lee, “Disordered antireflective subwavelength structures using Ag nanoparticles for GaN-based optical device applications,” J. Nanosci. Nanotechnol.11(2), 1342–1345 (2011). [CrossRef] [PubMed]
  18. K. C. Sahoo, Y. Li, and E. Y. Chang, “Shape effect of silicon nitride subwavelength structure on reflectance for silicon solar cells,” IEEE Trans. Electron. Dev.57(10), 2427–2433 (2010). [CrossRef]
  19. J. W. Leem, Y. M. Song, Y. T. Lee, and J. S. Yu, “Antireflective properties of AZO subwavelength gratings patterned by holographic lithography,” Appl. Phys. B99(4), 695–700 (2010). [CrossRef]
  20. B. J. Kim and J. Kim, “Fabrication of GaAs subwavelength structure (SWS) for solar cell applications,” Opt. Express19(S3Suppl 3), A326–A330 (2011). [CrossRef] [PubMed]
  21. J. W. Leem, D. H. Joo, and J. S. Yu, “Biomimetic parabola-shaped AZO subwavelength grating structures for efficient antireflection of Si-based solar cells,” Sol. Energy Mater. Sol. Cells95(8), 2221–2227 (2011). [CrossRef]
  22. J. M. Lee and B. I. Kim, “Thermal dewetting of Pt thin film: Etch-masks for the fabrication of semiconductor nanostructures,” Mater. Sci. Eng. A449–451, 769–773 (2007). [CrossRef]
  23. S. Wang, X. Z. Yu, and H. T. Fan, “Simple lithographic approach for subwavelength structure antireflection,” Appl. Phys. Lett.91(6), 061105 (2007). [CrossRef]
  24. C. H. Chiu, P. Yu, H. C. Kuo, C. C. Chen, T. C. Lu, S. C. Wang, S. H. Hsu, Y. J. Cheng, and Y. C. Chang, “Broadband and omnidirectional antireflection employing disordered GaN nanopillars,” Opt. Express16(12), 8748–8754 (2008). [CrossRef] [PubMed]
  25. J. W. Leem, J. S. Yu, Y. M. Song, and Y. T. Lee, “Antireflection characteristics of disordered GaAs subwavelength structures by thermally dewetted Au nanoparticles,” Sol. Energy Mater. Sol. Cells95(2), 669–676 (2011). [CrossRef]
  26. Y. Li, J. Zhang, and B. Yang, “Antireflection surfaces based on biomimetic nanopillared arrays,” Nano Today5(2), 117–127 (2010). [CrossRef]
  27. J. Zhu, C. M. Hsu, Z. Yu, S. Fan, and Y. Cui, “Nanodome solar cells with efficient light management and self-cleaning,” Nano Lett.10(6), 1979–1984 (2010). [CrossRef] [PubMed]
  28. A. J. Jääskeläinen, K. E. Peiponen, J. Räty, U. Tapper, O. Richard, E. I. Kauppinen, and K. Lumme, “Estimation of the refractive index of plastic pigments by Wiener bounds,” Opt. Eng.39(11), 2959–2963 (2000). [CrossRef]
  29. D. G. Stavenga, S. Foletti, G. Palasantzas, and K. Arikawa, “Light on the moth-eye corneal nipple array of butterflies,” Proc. Biol. Sci.273(1587), 661–667 (2006). [CrossRef] [PubMed]
  30. M. G. Moharam and T. K. Gaylord, “Rigorous coupled-wave analysis of planar-grating diffraction,” J. Opt. Soc. Am.71(7), 811–818 (1981). [CrossRef]
  31. S. A. Boden and D. M. Bagnall, “Tunable reflection minima of nanostructured antireflective surfaces,” Appl. Phys. Lett.93(13), 133108 (2008). [CrossRef]
  32. M. L. Kuo, D. J. Poxson, Y. S. Kim, F. W. Mont, J. K. Kim, E. F. Schubert, and S. Y. Lin, “Realization of a near-perfect antireflection coating for silicon solar energy utilization,” Opt. Lett.33(21), 2527–2529 (2008). [CrossRef] [PubMed]
  33. D. J. Economou, “Modeling and simulation of plasma etching reactors for microelectronics,” Thin Solid Films365(2), 348–367 (2000). [CrossRef]
  34. D. Redfield, “Method for evaluation of antireflection coatings,” Solar Cells3(1), 27–33 (1981). [CrossRef]

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