Young-Joon, Han
Description
Laser processing is now regarded as a promising tool to reduce
the cost and complexity of fabricating the formation of localized
contacts between heavily doped silicon and metal, features which
have become an important element in high efficiency silicon solar
cells, such as a passivated emitter and rear cell (PERC) and an
interdigitated back contact cell (IBC). However, characterization
of localized features with conventional PV characterization tools
is...[Show more] challenging, mainly due to the limitations of spatial
resolution. This thesis develops and applies novel
characterization methods to these localized features using low
temperature micro-photoluminescence spectroscopy (μ-PLS). This
technique demonstrates that localized features, even single laser
pulse processed regions typically tens of micrometres in scale,
can be investigated directly without the need for specific sample
structures and their electronic properties can be mapped
spatially in the sub-micrometre regime.
Utilizing the sub-micron precision of these measurements, the
laser-induced crystallographic damages were investigated at
various positions within the laser-processed region, particularly
at specific points such as the boundary/edge of processed and
unprocessed regions. It was found that the edge, or pulse
overlapped regions, were significantly more defective than the
centre region. The impact of laser parameters, such as laser
pulse fluence and number of repeat pulses, on laser-induced
damage was also analysed. Significantly different levels of
defect-related PL signals were observed after laser processing of
the two different substrate surface conditions. This suggests
that wafer surface preparation can be an important factor
impacting on the quality of laser-processed silicon.
The doping profiles of thermally boron-diffused silicon samples,
which have Gaussian function type doping profiles, can be
estimated from the measured PL spectra alone. The wavelength of
the doping-related PL peak (doping peak) has a reliable and
simple linear relationship with the surface dopant density on a
semi-log plot. The PL intensity of the doping peak also shows a
linear relationship with the doping depth metric (depth factor),
but only after considering the reduction of PL intensity due to
enhanced incomplete dopant ionization at low temperature. Doping
profiles can be easily reconstructed based on these two linear
relationships and their
vi
accuracy was verified by comparisons with existing doping
profiles (via ECV profiling). Mapping of the surface dopant
density and the depth factor of micron-scale locally diffused
features was undertaken using 2-D mapping with μ-PLS
measurements at 2 μm spatial resolution.
This method was also applied to 532 nm laser-doped silicon to
show its effectiveness on locally laser-doped features. The
doping profiles of laser-doped silicon were also successfully
estimated from PL spectra measurements alone, along with 2-D maps
of the surface dopant density and the depth factor of the
laser-doped silicon. In addition, the impact of temporal pulse
parameters, such as pulse duration and temporal pulse shapes, on
the doping profiles and recombination properties of laser-doped
silicon were investigated. By correlating defect-related PL band
counts with the quantified recombination parameters determined by
the luminescence-coupled numerical device simulations, it was
shown that μ-PLS measurements are able to perform quantitative
measurements of recombination properties.
The last chapter of this thesis demonstrates an application of an
advanced laser doping process using a stack of intrinsic
amorphous silicon (Si:H(i)) and boron-doped amorphous silicon
(a-Si:B). The results showed that this stack is able to provide
excellent surface passivation as well as a sufficient amount of
dopant source for laser doping.
The method presented in this thesis is a very effective, simple
and rapid characterization for analysing localized features, in
particular spatially inhomogeneous laser-processed features on
the micron-scale. This method enables the observation of the
variation in properties within localized features which is not
possible using conventional methods. It allows for a more
in-depth study of laser processing and promotes further
development of laser technologies for high efficiency cell
fabrication.
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