Dissolution during PS Formation

This equation indicates that the amount of chemical dissolution relative to that of electrochemical dissolution increases with time. This is reasonable, as chemical dissolution is proportional to the surface area of PS which in turn increases with time while 

FIGURE 8.18. Relationship between amount of dissolved silicon and dipping time in HF solutions. The PS was formed in 40% HF at SOmA/cm . After Unno et al. (Reproduced by permission of The Electrochemical Society, Inc.) 

Morphology, which is determined by the distribution of materials in space, is the least quantifiable aspect of a material. It is thus very difficult to characterize morphology of PS, which has extremely rich details with respect to the range of variations in pore size, shape, orientation, branch, interconnection, and distribution. Qualitatively, the diverse morphological features of PS reported in the literature can be summarized by Fig. 8.19 with respect to six different aspects pore shape, pore orientation, shape of a pore bottom, fill of macropores, branching, and depth variation of a PS layer. 

The condition for PS formation is determined by current density and HF concentration and is essentially independent of the condition of the silicon substrate as described in Section 8.2.2. The morphology of PS, on the other hand, is determined by all factors involved in anodization, particularly the factors related to the substrate. For example, doping concentration, which does not affect the nature of electrochemical reactions, is a principal factor determining the morphology of PS. 

The morphology of the PS formed on n-Si also strongly depends on illumination conditions, that is, intensity, frequency, and direction (front or back). Very different morphologies are produced by front versus back illumination. Back illumination generally produces straight pores, whereas front illumination typically produces a two-layer PS as shown Fig. 8.19(6c). 

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The dissolution of PS during PS formation may occur in the dark or under illumination. Both are essentially corrosion processes, by which the silicon in the PS is oxidized and dissolved with simultaneous reduction of the oxidizing species in the solution. The material in the PS, which is distant from the growing front is little affected by the external bias due to the high resistivity of PS and is essentially at the open circuit potential (OCP). Such corrosion process is responsible for the formation of micro PS of certain thickness (stain film) in HF solutions containing oxidants under an unbiased condition. 

The reaction product SiF4 would be gaseous, but it reacts with two HF to Si I7 and two protons and stays in solution [Mellj. The solubility of Si 17, which is in the order of mol 1 1 is significantly reduced in the presence of alkali metal ions. Especially for Rb, K or Cs, a micrometer thick, insoluble layer of metal hexafluoro-silicate may be formed on the electrode surface [Hal2j. The divalent electrochemical dissolution reaction is dominant during PS formation. The effects of the reaction products SiFg and H2 on pore growth are discussed in Section 9.5. 

Evolution of hydrogen gas occurs during the formation of PS. It is the result of the chemical reaction responsible for the effective dissolution valence of less than 4. Figure 8.11 shows that the amount of hydrogen gas is proportional to the time of anodization. When the anodization is stopped, the hydrogen evolution still continues at a lower rate. In situ F TIR studies indicate that the H-termination of the silicon surface is preserved during PS formation on The silicon atoms on the surface are... 

During PS formation at an anodic potential, the tip of pores dissolves electrochemi-cally. The pore wall areas that are sufficiently distant from the tips where no holes are available, dissolve only chemically and at a very low rate. The chemical dissolution does not depend on potential but on the time of immersion and the total surface area of the PS. Due to the large surface area of PS, a significant amount of material may be removed by the chemical dissolution during the formation period of PS. Figure 8.18 shows that the amount of chemical dissolution increases with immersion time in the HF solution and with decreasing HF concentration. The dependence of the chemical dissolution on time has been found to follow the equation... 

Because of the different potential distributions for different sets of conditions the apparent value of Tafel slope, about 60 mV, may have contributions from the various processes. The exact value may vary due to several factors which have different effects on the current-potential relationship 1) relative potential drops in the space charge layer and the Helmholtz layer 2) increase in surface area during the course of anodization due to formation of PS 3) change of the dissolution valence with potential 4) electron injection into the conduction band and 5) potential drops in the bulk semiconductor and electrolyte. 

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