Passivation of Dislocations

Dislocations are localized interruptions in a crystal s periodic network. These interruptions result in dangling bonds. Dislocations can be localized at a point, along a line or over an area. In the latter case, with the Fermi level pinned near midgap, an areal dislocation forms two Schottky barriers 

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First, I shall describe the hydrogenation method I used and then consider the passivation of surface states and that of bulk dangling bonds, including grain boundaries, dislocations and point defects. 

Breakdown of passivation and pitting. The local breakdown of passivity of metals, such as stainless steels, nickel, or aluminum, occurs preferentially at sites of local heterogeneities, such as inclusions, second-phase precipitates, or even dislocations. The size, shape, distribution, as well as the chemical or electrochemical dissolution behavior (active or inactive) of these heterogeneities in a given environment, determine to a large extent whether pit initiation is followed either by repassivation (metastable pitting) or stable pit growth.27... 

A number of the well-known radiation-induced centers in silicon, which are known to involve broken bonds, are also neutralized by atomic hydrogen. For exanple, the A-center (oxygen+vacancy coitplex, -0.18 eV), and divacancy level (E -0.23 eV) may be passivated [65]. Point defects produced by the Q-switched ruby-laser-annealing of both n-type [66] and p-type [67] Si surfaces are neutralized to the melt depth of - 1 /mi by plasma exposures of just 10 minutes at 100°C. And there has been a lot of work on the passivation of the electrical activity associated with dislocations (and their attendant point defects) and grain boundaries [66-76], which is so helpful in making photovoltaic solar cells from polycrystalline materials. While these defects may involve broken bonds, it should be remembered that these... 

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

In this paper, we have given a summary of some of the aspects of the work done in our laboratory on hydrogen passivation in polycrystalline silicon ribbon. We have shown that the dominant defects being passivated are dislocations, that many of the dislocation-related defects can be passivated quite readily using a Kaufman ion source, and that passivation can proceed to depths > 200 pm in some cases with diffusivities down dislocation arrays > 10- cm /sec. Several examples have been chosen to demonstrate the enormous utility of the EBIC technique in studying passivation. 

The activation of dislocation sources at the crack tip breaks the passive film and a localized dissolution takes place on the activated slip plane. 

The step geometry plays an important role on the nucleation mechanism. That the passivation of silicon surface with hydrogen prevents dislocation formation. 

Finally, some macroscopic properties should be revisited. As an example, the brittle to ductile transition could be investigated in the light of the existence of perfect dislocations. HREM and TEM experiments should be devoted to examine the very first steps of dislocation nucleation at crack tips. Passivation treatments... 

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