Pore Arrays

FIGURE 8.21. Cross sectional morphology of PS formed on p-Si (a) 25flcm, 40% HP, 30mA/cm (b) 25Gem, 30% HP, lOOm/Ucm (c) 0.01 Gem, 20% HP, 30mA/cm (d) 0.01 Gem, 20% HP, lOOmA/cml (Reprinted from Beale et 1985, with permission from Elsevier Science.) 

FIGURE 8.22. Cross-section morphology of PS layers formed in 1% HP solution at different potentials. After Zhang. (Reproduced by permission of The Electrochemical Society, Inc.) 

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Fig. 6.7 Calculated values of porosity as a function of the pore diameter to pore pitch ratio for pore arrays of different pore shapes and pore patterns.

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. 

Note that Eq. (9.1) applies to pore arrays as well as to randomly distributed pores. For simple orthogonal or hexagonal arrays of macropores with one pore per unit cell of the pattern, the porosity can be defined locally as the ratio between the cross-sectional area of the pore AP and the area of the unit cell AU as shown in Fig. 9.15 a ... 

The pore growth direction is along the (100) direction and toward the source of holes. For the growth of perfect macropores perpendicular to the electrode surface (100), oriented Si substrates are required. Tilted pore arrays can be etched on substrates with a certain misorientation to the (100) plane. Misorientation, however, enhances the tendency to branching and angles of about 20° appear to be an upper limit for unbranched pores. For more details see Section 9.3. 

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. 

It is clear from the results showed in table 2 that the Ti content is nearly constant upon silylation, indicating that Ti does not leach out from the mesoporous catalyst during silylation procedure. Also, the structural integrity of the pore array is preserved as it is deduced from the XRD patterns (figure 1) which remains unchanged after silylation. 

Jessensky, O., Muller, F., and Gosele, U., Self-organized formation of hexagonal pore arrays in anodic alumina. Appl. Phys. Lett. 72,1173 (1998). 

The XRD patterns of the calcined samples are shown in Figure 1. In the small angle range (SAXRD) it can be observed that (i) sample A shows three broad dilfraction peaks, indicating the formation of a mesophase with a hexagonal pore arrangement (ii) sample D shows diffraction peaks, which can be deconvoluted into three peaks also attributable to the hexagonal pore array ... 

Mesoporous carbon was obtained by sucrose carbonization in the pores of MCM-4 silica spheres with subsequently dissolution of the silica. The carbon was impregnated with the ZSM-5 synthesis gel and the crystallization was carried out under hydrothermal conditions. After burning off the carbon, ZSM-5 with a bimodal mesopore system showing mean diameters around 2 and 30 nm was obtained. Nevertheless, the hexagonal pore array of the MCM-41 was not reproduced in the ZSM-5. 

The structure of the PAA layer before removing from the aluminum substrate (Fig. 2). The structure represents almost parallel pores array with average pore diameter and distance between cells 100 and 300 nm, correspondingly. 

The novel synthesis method of AI-MCM-41, using tetraethoxysilane (TEOS) and sodium aluminate (NaAlOa) as Si and AI sources, was studied. Dodecyltrimethyl ammonium bromide was employed as template and the initial gel was hydrothermally treated at 100 C. The influence of reaction conditions such as surfactant/Si molar ratio and synthesis time have been evaluated. The samples showed well-defined X-ray diffraction patterns indicating a highly ordered structure of pore arrays. MCM framework was characterized by infrared analysis and a tentative assignment of bands was performed. Both XRD and FT-IR studies could be correlated and an optimal characterization of material could be reached. 

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