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

Matt Newville newville at cars.uchicago.edu
Tue Sep 6 11:17:43 CDT 2011


Hi Alejandro,


On Fri, Sep 2, 2011 at 6:08 PM, Alejandro X Levander
<alevander at berkeley.edu> wrote:
> 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

I didn't quite understand everything you were doing, but:

  - It does look like the amplitude of k^2 chi(k) is much larger (4x)
for the SN281 sample than the SN404 sample.

  - I don't quite understand why you are applying different S02
parameters *per path*  / bond.  That is, even
for the SN281 sample, the artemis project has different S02 for Ga-N
and Ga-Ga.

The usual approach would be to apply a single S02 for all paths, and
even for all Ga spectra measured under the same conditions.

Hope that helps,

--Matt



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