Passivating solar cells with
SiNx:H has been so far a scarcely understood effect that
can only be optimized for cell production in an empirical way. In this
study we determine the structural properties of SiNx:H layers
with Fourier Transform Infrared (FTIR) measurements and relate these
to both the deposition parameters and its passivating qualities for
solar cells. Furthermore we determined the relations between the hydrogen
diffusion in the SiNx:H and the structural properties of
these layers.
The Si-N, Si-H and N-H bond
densities for layers deposited with either nitrogen (N2),
ammonia (NH3) or deuterated ammonia (ND3|) and
silane (SiH4) are affected by the N/Si flow ratio and the
pressure p in a similar way although the differences in dissociation
energy and rate cause different deposition mechanisms. Comparing the
Si-N and Si-H bond densities found for NH3 and ND3
grown layers, we find that roughly 25% of the hydrogen in the SiNx:H
layers stems from the ammonia precursor gas, while 75% stems from the
silane.
We show that Si-N bond density
is an important parameter governing both the bulk and surface passivation
of the SiNx:H layers. In spite of the different deposition
mechanisms, the same relations hold between the H-diffusion coefficient,
Si-N bond density and passivating qualities of SiNx:H layers
deposited with either N2 or NH3. The best bulk
and surface passivating layers have a relatively low hydrogen diffusion
coefficient due to a high Si-N bond density. We find optimum values
for bulk and surface passivation for Si-N bond densities of 1.3*1023
cm-3, regardless of the type of SiNx:H used and
regardless of the starting wafer quality. Lower Si-N bond densities
result in layers with a more open structure and this will probably lead
to H2 formation during annealing. These H2 molecules
will effuse into the ambient during firing, and do not contribute to
the passivation of solar cells. Higher Si-N bond densities result in
a too dense structure, prohibiting an effective diffusion of H-atoms
into the bulk of the solar cells.
This study further indicates that FTIR analysis gives us a
quick and reliable tool to check the quality and properties of SiNx:H
layers. This will allow optimization of SiNx:H deposition
systems without having to make complete solar cells.