Hi Alejandro,
On Fri, Sep 2, 2011 at 6:08 PM, 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
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