Macropores arrays

Fig. 9.9 SEM micrograph of an n-type silicon electrode with an etched macropore array (5 2 cm, (100), 3 V, 350 min, 2.5% HF). Pore growth was induced by a square pattern of pits produced by standard lithography and subsequent alkaline etching (inset upper right). In order to measure the depth dependence of the growth rate, the current density was periodically kept at 5 mA cm 2 for 45 min and then reduced to 3.3 mA crrf2 for 5 min. This results in a periodic decrease in the pore diameter, as indicated by the white labels on the left-hand side. After [Le9].

Fig. 9.11 Optical micrograph of macropore arrays with (a) densely and (b) widely spaced pores (n-type, 40 Q, cm, 4% HF, 143 min, 2 V).

In contrast to the micro- and mesoporous regimes, for which only a few empirical laws for the growth rate and porosity are available, the detailed pore geometry for macropore arrays in n-type silicon can be pre-calculated by a set of equations. This is possible because every pore tip is in a steady-state condition characterized by = JPS [Le9]. This condition enables us to draw conclusions about the porosi-... 

Fig. 9.15 Cross-sections of macropore arrays parallel to the electrode surface (a) for a square, (b) for an ordered and (c) for a random pattern. The pores (black squares)...

The experimentally observed parabolic increase in pore depth and linear decrease in concentration shown in Fig. 9.18 c indicate that Eq. (9.6) is valid [Le9]. The macropore growth rate decreases linearly with l according to Eq. (9.6). If, therefore, a constant pore diameter is desired for a macropore array, a decrease in etching current or illumination intensity, respectively, with time is required. 

Design Rules and Limits of Macropore Array Fabrication... 

Not all desirable macropore array geometries can be achieved by the electrochemical etching process. This section gives the upper and lower limits for pore dimensions and a few design rules. Macropores in n-type substrates will be discussed first [Lel7]. 

In conclusion it can be said that the limits of macropore array formation are in some way complementary to the limitations of plasma etching [Ja3]. The latter technique gives a higher degree of freedom in lateral design, while the freedom in vertical design and the feasible pore aspect ratios are limited. 

The short-pass characteristic of Eq. (10.11) is evident. Measurements of the spectral transmittance of macropore arrays with the pore axis parallel to the inci-... 

Fig. 10.13 The measured transmittance of macropore arrays of pore diameter d and pore length I, with the pore axis parallel to the light beam. The data are corrected for porosity (p = l). The transmittance of a single...

FIGURE 8.25. The relationship between pore diameter and n-type doping density of the silicon substrate for stable formation of macropore arrays. The upper and lower limits of stable pore formation are shown as a function of substrate resistivity (dashed lines). (Reprinted from Lehmann and Griining. 1997, with permission from Elsevier Science.)... 

V. Lehmann and U. Griining, The limits of macropore array fabrication. Thin Solid Films 297, 13,... 

Fig. 3 LHS Euclidean macropore array in p-type silicon (Kim et al. 2009) RHS Fractal-like oxide replica of tire pore volume in n-type silicon (Tondare et al. 2008)...

Kim JH, Kim KP, Lyu HK, Woo SH, Seo HS, Lee JH (2009) Three dimensional macropore arrays in p-type silieon fabrieated by electrochemical etching. J Korean Phys Soc 55(1) 5-9 Loni A, Canham LT (2013) Exothermic phenomena and hazardous gas release during thermal oxidation of mesoporous silicon powders. J Appl Phys 113 173505 Lysenko V, Vitiello J, Remaki B, Barbier D (2004) Gas permeability of porous silicon nanostructures. Phys Rev E 70 017301... 

Ao X, Tong X, Kim DS, Zhang L, Knez M, Muller F, He S, Schmidt V (2012) Black silicon with controllable macropore array for enhanced photoelectrochemical performance. Appl Phys Lett 101 111-901... 

Lehmann V, Griining U (1997) The limits of macropore array fabrication. Thin Solid Films 297 13 Lehmann V, Stengl R, Luigart A (2000) On the morphology and the electrochemical formation mechanism of mesoporous silicon. Mater Sci Eng B 69-70 11 Ogata YH, Kobayashi K, Motoyama M (2006) Electrochemical metal deposition on silicon. Curr Opin Solid State Mater Sci 10 163... 

Macropore arrays found applications in various fields, some of which are listed in Table 4. 

Table 4 Main application domains of macropore arrays ...

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... 

Fig. 4 Pore density versus silicon electrode doping density for porous silicon layers of different geometries. Notice that macropores are essentially obtained on low to moderately doped substrates. The dashed line shows the pore density of a triangular pore pattern with a pore pitch equal to two times the SCR width for a 3 V applied bias. Note that only macropores on n-type substrates may show a pore spacing significantly exceeding this limit. The regime of stable macropore array formation on n-Si is indicated by a dot pattern. Doping type and etching current density (in mA/cm ) are indicated in the legend (After Lehmann 1993)...

Fig. 5 Examples of regular and ordered macropore arrays, (a) Two-dimensional macropore array wifli an intentional line defect (From Griining et al. 1996) (b) array of pores grown on n-Si (10 cm ) die pore initiation pattern shown in die inset has been produced by photoUthography and alkaline etching (From Lehmann et al. 2000)...

Kordas K, Remes J, Leppavuori S et al (2001) Laser-assisted selective deposition of nickel patterns on porous silicon substrates. Appl Surf Sci 178 93-97 Koynov S, Brandt MS, Stutzmann M (2006) Metal-induced seeding of macropore arrays in silicon. Adv Mater 18 633-636... 

Bobo P et al (2012) Novel method of separating macroporous arrays from p-type silicon substrate. J Semi Cond 33(4) 0430004-l... 

Chao KJ et al (2000) Formation of high aspect ratio macropore array on p-type silicon. Electrochem Solid State Lett 3(10) 489-492... 

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