Abstract
The electrochemical formation of macropores in porous silicon is briefly reviewed. Various morphologies are obtained as a function of the substrate type and etching conditions. On n-Si, macropores are generally growing along preferential crystallographic directions. On p-Si, in aqueous conditions far from electropolishing, the growth direction is rather determined by the current lines in the space-charge region. A summary of macropore characteristics is given as a function of the preparation conditions. Various models have been developed in order to account for the morphologies and characteristic sizes. These joint experimental and theoretical works have provided a good understanding of macropore growth, opening the way to many applications, and the most significant ones are mentioned. An impressive level of control has eventually been achieved for the fabrication of regular macropore arrays of high aspect ratio, including the incorporation of intentional defects or pore-wall shaping.
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Introduction
According to the IUPAC standard, macropores correspond to pores exhibiting characteristic sizes (pore diameter and average distance between pores) larger than 50 nm. The term “macropore” is usually associated with smooth cylindrical pores with characteristic sizes on the order of 1 μm.
This kind of pore can be obtained under a variety of conditions and with differing morphologies (see chapter “Routes of Formation for Porous Silicon”). In this review, we focus on electrochemically etched macropores. The key parameters are the electrolyte type (aqueous (aqu), organic (org), oxidant (ox)) the HF concentration, the surfactant, the Si doping type and level (n, n+, p, p+), and in some cases the illumination (backside illumination (bsi) or frontside illumination (fsi)). Detailed reviews regarding their formation are available (Föll et al. 2002; Lehmann 2005; Chazalviel and Ozanam 2005; Lehmann 2002; and handbook chapter “Porous Silicon Formation by Anodization”).
Current-Line- and Crystallography -Driven Macropores
Two distinct classes of macropores are observed, as summarized in Table 1. Macropores obtained from n-Si always exhibit a strong growth dependence on crystallographic orientation. On p-Si, this dependency is lower (Lehmann and Rönnebeck 1999), and in aqueous conditions at low enough current density and/or high enough HF concentration, the growth turned to be determined by the direction of the current lines in the space-charge region Media et al. 2011.
Principal Application of Macropores
Macropore arrays found applications in various fields, some of which are listed in Table 4.
Design of Regular Macropore Arrays
The fabrication of regular macropore arrays requires prestructuring of the Si substrate using lithography and alkaline etching (Chao et al. 2000; van den Meerakker et al. 2000; Starkov 2003). The pitch of the prestructured hole array has to match the average spacing of random macropore arrays grown on the same substrate under similar electrochemical conditions. The width of the walls of the porous structure (which depends on the pitch structure and the pore lateral size) is mostly determined by the width of the space-charge layer (i.e., mostly dependent on substrate doping level) and the pore diameter by the etching conditions. Figures 2 and 3 give some design rules in the case of p-Si. In the case of n-Si, the pore diameter is mostly determined by the current density, i.e., the illumination level, according to Lehmann’s model (Lehmann 1993). However, diffusion effects in the liquid phase, as theoretically modeled (Barillaro and Pieri 2005), must be taken into account in order to keep the fluoride concentration stationary at the pore tips. Figure 4 gives the typical pore-density range accessible on n-Si under usual backside illumination conditions or p-Si in the dark.
Conclusions
Since the first report of Theunissen (Theunissen 1972) and the pioneering work of Lehmann in the 1990s, many efforts have been devoted to macropore fabrication by electrochemical etching. Impressive macropore arrays have been achieved, with high aspect ratios and smooth or patterned vertical walls. Examples are shown in Fig. 5. Alternative techniques have been proposed such as galvanic etching (Xia et al. 2000), stain etching (Mills et al. 2005), and metal-assisted (electro)chemical etching (Li et al. 2013). These techniques are separately reviewed in this handbook (see Chapters “Porous Silicon Formation by Galvanic Etching,” “Porous Silicon Formation by Stain Etching,” and “Porous Silicon Formation by Metal Nanoparticle-Assisted Etching”).
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Gabouze, N., Ozanam, F. (2014). Macroporous Silicon. In: Canham, L. (eds) Handbook of Porous Silicon. Springer, Cham. https://doi.org/10.1007/978-3-319-05744-6_10
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