Passivation of Grain Boundaries

C. H. Seager and D. S. Ginley, Passivation of grain boundaries in polycrystalline silicon, Appl. Phys. Lett. 34(5), 337, 1979. 

Fig. 5. EBIC micrograph of recombination in an EFG ribbon sample showing passivation of grain boundaries and intragranular dislocation arrays.

From polarization curves the protectiveness of a passive film in a certain environment can be estimated from the passive current density in figure C2.8.4 which reflects the layer s resistance to ion transport tlirough the film, and chemical dissolution of the film. It is clear that a variety of factors can influence ion transport tlirough the film, such as the film s chemical composition, stmcture, number of grain boundaries and the extent of flaws and pores. The protectiveness and stability of passive films has, for instance, been based on percolation arguments [67, 681, stmctural arguments [69], ion/defect mobility [56, 57] and charge distribution [70, 71]. 

Potentiostatic methods, being capable of detecting differences in corrosion and passivation behaviour of various parts of a heterogeneous surface, have been applied to the electrochemical determination of grain boundary corrosion... 

Seager, C.H., and Ginley, D.S., (1982). Fundamental Studies of Grain Boundary Passivation in Polycrystalline Silicon with Application to Improved Photovoltaic Devices, Sandia Report, SAND82-1701, p. 19-21. 

Pits are found to develop particularly in the region of grain boundaries which themselves are readily open to attack ( intergranular corrosion ). Thus, it is the Cu-depleted regions that are the major cause of the intergranular damage of Al-Cu alloys. The A1 itself remains passive for a certain time and potential, but even when... 

The reasons for these features are manifold and again related to the presence of grain-boundary phases. We often have a very complex passive oxidation behavior with leaching of grain-boundary constituents to the scale, crystallization, internal oxidation, bubble formation, scale cracking, and other features. All these phenomena are different for different systems and vary with the amount and exact composition of the additives. For detailed information the reader is referred to the textbooks and proceedings already mentioned and some reviews, which cite papers from a vast amount of literature [140,152,153]. 

Fig. 4. EBIC micrographs illustrating depth of grain boundary passivation. Hydrogen ions were incident from the top. (a) Shows a typical boundary before any passivation (b) and c) show other boundaries after hydrogen passivation. 

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