[Ifeffit] Give me some advices

[Leandro Araujo]

Hi Kefan,

you mentioned a .bin file? I couldn't find any file attached to your
last email...

Before getting down to business, let me tell you that Ge/Si QDs grown
by MBE were "EXAFSed" (if such a term exists) in detail by Kolobov &
co-authors and it would be interesting to check your data against that
published in [Z. H. Sun, S. Q. Wei, A. V. Kolobov, H. Oyanagi, and K.
Brunner, Phys. Rev. B 71, 245334 (2005)].

Getting down to the topics:

1- Important point: I assumed your EXAFS experiments were performed at
low temperature (~ 10K) to minimize thermal disorder. Is this
assumption correct? If you measured at room temperature, then it would
explain why you do not see much action above k ~ 10 inverse angstrom,
since thermal disorder could be damping your not-so-strong EXAFS
signal. I'd suggest you to always measure the QDs at the lowest
possible temperature to make the spectra clearer and the
post-processing (including fitting) easier.

2- Your concern about GeOx in your samples is well justified. That
"deep" in the XANES region, just after the white line, is
characteristic of Ge-O bonds and is not present in the bulk forms of
Ge (crystalline and amorphous); I'm not sure about bulk SiGe, though.
It would not be surprising to have Ge-O in your samples since O is a
fast diffuser and your capping layer is just 15 nm. At 500oC or more O
atoms can surely get into your QDs. Although O binds preferentially to
Si rather than Ge, you may not have enough Si to "grab" all the O. My
suggestion is that you should try some XANES linear combination fits
using your QDs as unknowns and bulk crystalline Ge, bulk crystalline
SixGe1-x and bulk crystalline GeO2 as standards. It is easy to perform
such fits using Athena, check the manual again. You can also check
Bruce's excellent example available online at:
http://cars9.uchicago.edu/~ravel/software/doc/Athena/html/examples/aucl.html
My guess is that your samples contain mixed environments similar to
the mentioned standards. You can also try using amorphous standards
instead of the crystalline ones.

3- To verify the presence of crystalline Ge and/or Ge-O bonds in your
samples, performing XPS or Raman measurements (if available) would be
of invaluable help.

4- I think there is no need to truncate the data after 10 inverse
angstrom. Just close the k-space window right after the last
oscillations (before the "flat" region) and you should be fine. Keep
in mind that the number of independent parameters will be small in
this case since you will get a k region of ~ 6 inverse angstroms (from
4 to 10 more or less).

Good luck,
Leandro



On Dec 18, 2007 1:35 PM,  <wang.kefan at gmail.com> wrote:
> Hi  Leandro
> Thank you for your helpful advices. The title should be change to "Please
> give me some advices" , and I am sorry for this error.
>
> The attached .prj file can be opened by Athena rightly by changing the
> download file's extend name .bin  to .prj, if someone else want to join our
> discuss.
>
> Then go to our topics:
>
> The QDs samples were prepared by MBE and XAFS measurements, as you guessed,
> were performed at a synchrotron facility in fluorescence mode. Since the
> Si/Ge intermixing happened frequently during growth, there is certainly Si
> in so-called Ge quantum dots(QDs, or nanocrystal). However, now I am very
> caring for if the GeOx in the QDs. The XAFS spectra show that there may be
> GeOx in the Ge QDs, but since the QDs were covered in situ by a 15nm Si cap
> layer the oxidizing process of QDs by air should not happen. This
> inconsistency between XAFS and experiments is the real reason for that I
> want to extract the truth from strict softwae analysis.
>
> 1, I have the bulk Ge and diluted SiGe standard spectra, but they do not
> resemble with these spectra as you mentioned. I am sure that the QDs is a
> combination of Ge and Si elements, and am not clear if the O exists. The
> growth temperature is beyond 500oC, and annealing was performed from
> 550-800oC(all temperature uncalibrated with +/- 50oC ). The amorphous Ge
> seems to be impossible, but can not exclude it. In situ RHEED also can not
> tell me if the deposited Ge is crystallizing.
>
> 2, If I truncate the k to 10 A-1, the fit result is accepted or not?
>
>
> Kefan
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