[Ifeffit] Question on GaN Artemis fit (esp. So2 factor)

[Alejandro X Levander]

Dear ifeffit community;
I am working on modeling the local structure of amorphous GaN(1-x)As(x)
alloys. After helpful learning experiences at the SSRL XAS Summer School and
reading Scott Calvin's doped material help page, I was able to model all of
the data for the As K edge I had collected. However, I am now having issues
modeling the Ga K edge spectra despite using similar methods. I expect the
Ga to be tetrahedrally coordinated and bonded to a mixture of As and N based
on the previously determined overall composition.

To determine the SO2 parameters for the Ga K edge I am using data collected
in fluorescence mode of a thin film (690 nm) of GaN on a sapphire (Al2O3 -
500um) substrate as a reference sample. I have fixed the first and second
shell amplitudes in order to determine SO2. The SO2 parameter for the
nitrogen shell appears too low to be physically correct (~0.5). When I use
these values for SO2 to try and model my amorphous films, I get highly
unexpected coordination values much larger than four when summed for As and
N bonding since the model is compensating for the low SO2. I am concerned
there may be an issue with the data processing in Athena (e.g.
self-absorption or deglitching) or multiple-scattering, but cannot pin down
the problem.

I have attached the GaN (sample code sn281) Athena and Artemis files and an
example of trying to fit the Ga K edge of amorphous GaN(0.55)As(0.45)
material in Artemis (sample code sn404). The amorphous thin film was 220 nm
thick on a 500um pyrex glass substrate measured in fluorescence mode.

As a side note, I have also been having trouble with using ATOMS for the
wurtzite crystal structure. Despite trying multiple appropriate .cif files
from ICSD and making sure to use 1/3 instead of 0.33334 when I run ATOMS,
there appear to be rounding errors that turn one coordination into three.
Thus I have been manually changing the feff input.

Any help is greatly appreciated,
Alejandro
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