Surface and bulk passivation of multicrystalline silicon solar cells by ...

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the best characterized silicon-insulator interface. It is well-known that the interface states
of this system are mainly related to silicon dangling bond defects with a very broad
energy distribution due to the inherent variation in the bond angles and the distances of
the next neighbor atoms [63, 64].
Post-deposition anneals of PECVD layers on silicon are important in order to
obtain passivated surfaces. Hezel et at have shown that such an anneal can decrease
interface state density of SiNX Si by two orders of magnitude [65], which agrees with the
measured results of strong reduction of Seff due to post-deposition that was observed by
Leguijt et al. [66]. However, other authors have expressed different opinions. Boehme
and Lucovsky reported hydrogen loss during anneal of SiN [67].
The deposition of SiNX layers on silicon substrate leads to the formation of a
space charge region at the interface characterized by a Q of the order of 10 12 cm 2. In p-
type Si, a depletion/inversion layer is formed, while in n-type Si, the positively charged
insulator attracts majority carriers and repels minority carriers. Hence, an accumulation
layer is formed.
Low Seff of PECVD SiN-passivated Si surface is attributed to the combination of
moderately low density of interface states and a high positive charge density. Both
parameters are given in Table 2.1 for as-deposited and thermally treated silicon nitride
films [68].
Table 2.1 Positive Fixed Charge and Interface-Trap-Density of As-Deposited and
Annealed SiN-Si Interface [68]
Silicon nitride condition Q (cm) Di (cm2/eV)
As-deposited 3x10i2 2x 10i i
Thermally treated 1x1012
1x1011