Complete Report - University of New South Wales

In qualitative PL imaging, only the relative variation of the PL intensity across the sample is
investigated. Such qualitative imaging was applied extensively by the buried contact group of the
Centre and within a short period PL imaging was established as a standard characterisation
technique. In fact the initial PL imaging set-up has been in such heavy demand that setting up
of a second PL imaging system is planned for the fi rst half of 2006. A further improvement
of the data acquisition speed by a factor 50 is anticipated for this new system. Within the
fi rst few months of applying PL imaging as a process monitoring tool especially the buried
contact group identifi ed various serious problems with standard processing sequences,
that had previously been used for years [4.6.10, 4.6.11]. The fact that PL imaging is fast,
simple to use and applicable to unfi nished cells turned out to be an invaluable combination
in this context. Some examples of process induced defects identifi ed with PL images are
shown in Fig.4.6.6.9. Various collaborative research projects with industry partners (see for
example [4.6.12]) aim at demonstrating the capabilities of the system and at increasing the
awareness in the PV community regarding the potential of this new technique. A mid-term
goal is the demonstration of PL imaging as an industrial in-line process monitoring tool. The
data acquisition speed in PL imaging is several orders of magnitude higher than in comparable
imaging or mapping methods, making PL Imaging the only experimental technique that can
provide high resolution images of the material quality from large area wafers in a matter of
seconds.
Figure 4.6.6.9: Examples of effective process monitoring using PL imaging. Problems with
standard processing sequences that could be identifi ed include amongst various others (from
left to right): wafer handling using tweezers, furnace contamination; cracks. Each PL image
is taken with a data acquisition time of only one second. Some more examples of process
induced defects that were identifi ed with PL imaging are shown in Section 4.3.2.
Theoretical work
One complication in the analysis of QSS-PL lifetime measurements arises from the fact
that at high carrier densities (>10 16 cm -3 ) the PL intensity no longer follows the product
of electron and hole concentrations as given by Eq.1. It is well established and theoretically
understood that the Auger coeffi cients in silicon decrease as a function of injection level due
to the reduced Coulomb attraction between electrons and holes at high carrier densities.
The reduced attraction of electrons and holes at high carrier densities also affects radiative
recombination, which leads to an injection level dependence of the radiative recombination
coeffi cient B(T,∆n). While we had earlier described the temperature dependence of B(T) at
low injection levels [4.6.13], a detailed analysis of the injection level dependence has only been
carried out very recently by P.P. Altermatt in collaboration with researchers from this Centre
[4.6.14]. The injection level dependence of B is now used in our QSS-PL lifetime system to
correctly interpret experimental photoluminescence data.
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