Macropores in p-Type Silicon

Macropore formation on p-type silicon electrodes was first observed for anodization in water-free mixtures of anhydrous HF and an organic solvent [Pr7, Ril]. Later it was observed that organic HF electrolytes with a certain fraction of water [Pol, We5], or even non-organic, aqueous HF electrolytes [We2, Le21], are also sufficient for the formation of macropores on p-type Si electrodes. This indicates that macropore formation on such electrodes cannot be ascribed to the chemical iden- 

For macropore formation on low doped p-type substrates, the applied bias and current density are coupled a change in the applied bias produces a corresponding change in anodization current density. For macropore growth on p-type Si 

3 mA cm-2, 90 min, RT). (e) An n-type substrate anodized under similar conditions is shown for comparison (10% HF, 5 12 cm, 

The growth of a macropore on a p-type substrate can be initiated by artificial etch pits. The growth of predefined pore arrays is observed to be more stable than the growth of random pores on flat electrodes [Chl6, Le21]. If a slit is used for pore initiation the formation of trenches separated by thin walls has been observed on (100) p-type substrates [Oh5]. Note that for slits along the (110) direction the walls become (110) planes, in contrast to trenches produced by alkaline etchants, for which only (111) oriented walls can be formed on (110) oriented silicon substrates. 

In conclusion it can be said that the flexibility of pore array design on low doped p-type Si is less than that for macropore formation on n-type substrates, because of the limitations in array porosity and substrate doping range. 

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Fukami K, Kobayashi K, Matsumoto T, Kawamura YL, Sakka T, Ogata YH (2008) Electrodeposition of noble metals into ordered macropores in p-type silicon. J Electrochem Soc 155 D443-D448... 

Note that during macropore formation in p-type silicon electrodes the pore tip current density is usually well below JPS, and so Eqs. (9.1) to (9.5) are not applicable to p-type macropore formation [Le21]. 

An interesting question is whether such well-ordered pore arrays can also be produced in other semiconductors than Si by the same electrochemical etching process. Conversion of the macropore formation process active for n-type silicon electrodes on other semiconductors is unlikely, because their minority carrier diffusion length is usually not large enough to enable holes to diffuse from the illuminated backside to the front. The macropore formation process active in p-type silicon or the mesopore formation mechanisms, however, involve no minority carrier diffusion and it therefore seems likely that these mechanisms also apply to other semiconductor electrodes. 

Vyatkin A., Starkov V., Tzeitlin V., Presting H., Konnle J., Konig U. Random and ordered macropore formation in p-type silicon. J. Electrochem. Soc. 2002.149(1), G70-G 76. 

R. B. Wehrspohn, F. Ozanam, and J.-N. Chazalviel, Nano- and macropore formation in p-type silicon,... 

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)...

Christophersen M, Carstensen J, Foil H (2000a) Crystal orientation dependence of macropore formation in p-type silicon using organic electrolytes. Phys Status Solidi A 182 103-107 Christophersen M, Carstensen J, Feuerhake A, Foil H (2000b) Crystal orientation and electrolyte dependence for macropore nucleation and stable growth on p-type Si. Mat Sci Eng B 69-70 194-198... 

Slimani A, Iratni A, Chazalviel J-N, Gabouze N, Ozanam F (2009) Experimental study of macropore formation in p-type silicon in a fluoride solution and the transition between macropore formation and electropolishing. Electrochim Acta 54 3139-3144 Smith RL, Collins SD (1992) Porous silicon formation mechanisms. J Appl Phys 7LR1-R22 Starkov W (2003) Ordered macropore formation in silieon. Phys Status Solidi (a) 197 22-26 Steiner P, Lang W (1995) Micromachining applications of porous silicon. Thin Solid Films 255 52-58... 

Kobayashi K, Harraz FA, Izuo S et al (2006) Microrod and microtube formation by electrodeposition of metal into ordered macropores prepared in p-type silicon. J Electrochem Soc 153 C218-C222... 

An electric field in the semiconductor may also produce passivation, as depicted in Fig. 6.1c. In semiconductors the concentration of free charge carriers is smaller by orders of magnitude than in metals. This permits the existence of extended space charges. The concept of pore formation due to an SCR as a passivating layer is supported by the fact that n-type, as well as p-type, silicon electrodes are under depletion in the pore formation regime [Ro3]. In addition a correlation between SCR width and pore density in the macroporous and the mesoporous regime is observed, as shown in Fig. 6.10 [Thl, Th2, Zh3, Le8]. 

A specific feature of macropore formation in n-type silicon is the possibility of controlling the pore tip current by illumination and not by applied bias. This adds another degree of freedom that is not available for mesopore or macropore formation on p-type substrates. The dark current density of moderately doped n-type Si electrodes anodized at low bias is negligible, as shown in Fig. 4.11, therefore all macropore structures discussed below are formed using illumination of the electrode to generate the flux of holes needed for the dissolution process. Illumination, however, is not the only possible source of holes for example, hole injection from a p-doped region is expected to produce similar results. 

Janshoff A, Dancil K-P S, Steinem C, Greiner DP, S-Y LV, Gurtner C, Motesharei K, Sailor MJ, Ghadiri MR (1998) Macroporous p-type silicon Fabry-Perot layers. Fabrication, characterization, and applications in biosensing. J Am Chem Soc 120 12108-12116... 

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... 

Lehmann V, Foil H (1990) Formation mechanism and properties of electrochemically etched trenches in n-type silicon. J Electrochem Soc 137 653-659 Lehmann V, Ronnebeck S (1999) The physics of macropore formation in Low-doped p-type silieon. J Electrochem Soc 146 2968-2975... 

Ponomarev EA, Levy-Clement C (1998) Macropore formation on p-type Si in fluoride containing organic electrolytes. Electrochem Solid St Lett 1 42-45 Ponomarev EA, Levy-Clement C (2000) Macropore formation on p-type silicon. J Porous Mat 7 51-56... 

According to the macropore formation mechanisms, as discussed in Section 9.1, the pore wall thickness of PS films formed on p-type substrates is always less than twice the SCR width. The conductivity of such a macroporous silicon film is therefore sensitive to the width of the surface depletion layer, which itself depends on the type and density of the surface charges present. For n-type substrates the pore spacing may become much more than twice the SCR width. In the latter case and for macro PS films that have been heavily doped after electrochemical formation, the effect of the surface depletion layer becomes negligible and the conductivity is determined by the geometry of the sample only. The conductivity parallel to the pores is then the bulk conductivity of the substrate times 1 -p, where p is the porosity. 

Monocrystalline, macro- and mesoporous silicon were used for the electrochemical deposition of Pt. A 10 pm thick macroporous silicon layer was formed by anodizing of p-type Si wafers of 12 Ohm-cm resistivity in an aqueous solution of HF acid and DMSO (10 46 by volume parts) at the current density of 8 mA-cm [1]. Pore channels distributed with the surface density of 6T0 cm look like long straight holes with inlet diameters of 1.5 pm. An uniform 1 pm thick mesoporous silicon layer was fabricated by anodizing of n" -type Si wafers of 0.01 Ohm-cm resistivity in a solution of HF acid, water and isopropanol (1 3 1 by volume parts) at the current density of 60 mA-cm . The mesoporous silicon sample formed looks like Si layer perpendicularly pierced through by pore channels with diameter of about 20 nm. The number of pores per square centimetre is up to 2-10 [2]. 

Thus, in general, the dissolution of silicon at an anodic current results in three regions of the silicon substrate which is exposed to the electrolyte as shown in Fig. 8.41. Such an etched layer prior to the initiation of pores is involved in all types of PS because the etching canses roughening of the surface, which is required for pore initiation. The etching phase is associated with the formation of macropores on both n and p types of silicon and with formation of micropores on For example, for n-... 

M. Christophersen, J. Carstensen, and H. Foil, Crystal orientation dependence of macropore formation p- and n-type silicon in organic electrolytes, Phys. Status Solidi, (a)l 2 1, p- 103, 2000. 

A common way to find out the nature of ohmic contact to semiconductor is to measure the value of the specific contact resistance, pc (G cm ). Zimin etal. (1995), Zimin and Komarov (1998) measured the transition resistivity of the PS surfaces prepared from both p-type and n-type sihcon wafers with Al contact. Specific contact resistance of Al and Ni to p-type PS (55 % porosity) was reported by Kanungo and co-workers (Kanungo et al. 2009a Kanungo et al. 2006). Maji et al. reported the same for Al contact to macroporous silicon (2010). P. Vinod (2005,2009,2013) studied silver ohmic contact to p-type porous silicon by quantitative measurements of the specific contact resistance. Table 2 gives the summary of the reported work on specific contact resistance/transition-specific resistivity measurements. A separate chapter in this handbook Electrical Transport in Porous Silicon reviews the various factors that influence the resistivity of the porous silicon layer itself. 

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