Laser Texturing
Laser texturing is a two-step process that facilitates isotropic texturing of a silicon surface regardless of crystallographic orientation [1]. Firstly, a pulsed laser is used to ablate silicon from the bulk regions. Laser pulses can be modulated with the scan speed or stage motion to allow direct control of the space between pits. Next, the silicon is chemically etched using sodium hydroxide (NaOH) solution to smooth out the ablated pits and remove any slag that may have been deposited during laser processing.
Figure 1: Scanning electron microscopy image of a laser textured silicon wafer (image courtesy of Dr Malcolm Abbott)
Although laser textured silicon can achieve lower reflectance than alkaline textured (random pyramid) silicon wafers, the adverse impact of lifetime defects formed from the laserprocess are a disadvantage of this process.
Plasma Texturing (Reactive Ion Etching)
Plasma texturing, sometimes called reactive ion etching is a technique which uses ion bombardment from a plasma to etch the silicon surface. Commonly used plasma precursors include sulfur hexafluoride (SF6/O2), nitrous oxide (N2O) and chlorine (Cl2) gas mixtures. Other mixtures such as nitrous trifluoride (NF3/N2) and oxygen (O2/N2) gas mixtures have also been reported [2,3]. The plasma texturing process self-masking as residual slag from the etching process can be redeposited; lowering the etch rate in localised areas. This facilitates the formation of a texture.
Although the resulting reflectance can be significantly lower on mc-Si (15-22% before ARC deposition) when compared to acidic texturing, the process is costly and lower in throughput. Furthermore, reactive ion bombardment of the surface can lead to lattice damage of the underlying bulk silicon
Metal-Assisted Texturing
Metal-assisted etching has been used to etch silicon surfaces to form a series of small and deep pores. The resulting low reflectance of raw silicon (~2%) typically makes the surface look black and consequently silicon textured via this process is sometimes referred to as ‘black silicon’. The low reflectance also means that an antireflection coating is not necessary for optical purposes.
In order to fabricate black silicon via metal-assisted texturing, noble metal nanoparticles such as silver (Ag) or gold (Au) are deposited onto the surface of the silicon; either through means of electroplating or under vacuum via thermal evaporation, sputtering or e-beam evaporation [4]. Electroless methods involving nanoparticles suspended in solution or through silver ion-containing compounds (AgNO3) can also be used. For instance, Liu et al. reported a method for metal-assisted texturing using silver nitrate, hydrofluoric acid and hydrogen peroxide solution [4]. In this process, silver ions on the surface of the silicon inject holes into the silicon underneath it, forming silicon oxide or silicon dioxide. The hydrofluoric acid solution can then etch the oxide and remove it from the silicon. As the process continues over time, the metal ion sinks deeper into the silicon creating a porous structure. Nitric acid can then be used to remove the metal nanoparticles from the surface once texturing is complete.
Figure 2: An illustration of the metal-assisted chemical etch process: (1) the reduction of an oxidative agent (such as H2O2) catalyzed by a noble metal particle; (2) the injection of the holes generated during the reduction reaction, into the silicon substrate, with the highest hole concentration underneath the metal particle; (3) the migration of holes to silicon sidewalls and surfaces; and (4) the removal of oxidized silicon via HF [4].
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[2] – J.B. Park, J.S. Oh, E. Gil, S.-J. Kyoung, J.-S. Kim, G.Y. Yeom, Plasma texturing of multicrystalline silicon for solar cell using remote-type pin-to-plate dielectric barrier discharge, J. Phys. D. Appl. Phys. 42 (2009) 215201. doi:10.1088/0022-3727/42/21/215201. Available: https://doi.org/10.1088/0022-3727/42/21/215201
[3] – J. Yoo, J.-S. Cho, S. Ahn, J. Gwak, A. Cho, Y.-J. Eo, J.-H. Yun, K. Yoon, J. Yi, Random reactive ion etching texturing techniques for application of multicrystalline silicon solar cells, Thin Solid Films. 546 (2013) 275–278. Available: https://doi.org/10.1016/j.tsf.2013.02.045
[4] – X. Liu, P.R. Coxon, M. Peters, B. Hoex, J.M. Cole, D.J. Fray, Black silicon: fabrication methods, properties and solar energy applications, Energy Environ. Sci. 7 (2014) 3223–3263. Available: https://doi.org/10.1039/C4EE01152J
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[5] – L. Yalçin and R. Öztürk, “Performance comparison of c-Si, mc-Si and a-Si thin film PV by PVsyst simulation,” J. Optoelectron. Adv. Mater., vol. 15, no. 3–4, pp. 326–334, 2013. Available: researchgate.net
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