[Ifeffit] DWF's for amorphous InP
Claudia Schnohr
css109 at rsphy1.anu.edu.au
Thu Apr 5 00:12:46 CDT 2007
Hey Matt,
thank you very much for your comments.
Indeed, what we hope to determine from our measurements are relative
differences between a number of samples measured in the same way (same
beamline, same run etc) just as you have suggested.
The problem with the amorphous samples is that due to the correlation
between DWF and coordiantion number and due to the overlap of the peaks
(In-In and In-P) different ratios of the two DWFs might still give the
same (or at least a very similar) fit. Therefore, the fitting procedure
might choose one ratio for sample A and another ratio for sample B even
though their spectra look basically the same. That is why we thought to
have some relation between the two DWFs would be handy and it is always
worth a try to ask other people about their experiences :)
At the moment, I try using different (multiple) k-weights and various
k-ranges to see how the system behaves and hopefully I am able to pick a
setting that allows for a good (reliable) fitting of the data even without
assuming a relation between the two DWFs.
Thanks again for your help,
Claudia
> Dear Claudia,
>
>> We think that restraining the DWF's might be a good approach.
>> Unfortunately, the crystalline and amorphous systems do not have similar
>> DWF's due to the large amount of disorder in the amorphous phase. From
>> previous studies it is known, that the DWF of an amorphous semiconductor
>> is roughly (!) twice the DWF of the crystalline phase. Hence, fixing the
>> In-P DWF to twice the crystalline value (from a standard we also
>> measured)
>> would be a a first approach. My question is whether there maybe is a
>> better way of relating/restraining the DWFs, as for example suggested by
>> Crozier, Rehr and Ingalls (X-ray Absorption, Koningsberger and Prins,
>> Wiley & Sons, 1988). They derive a formula for the DWF that contains the
>> reduced mass and an integral over the projected density of states. If
>> one
>> could make a reasonable assumption about the integral (which is the
>> problem) it would be possible to correlate the two DWF's. Or maybe there
>> is another way to make an educated guess (by computing with FEFF or so).
>
> If I might jump here, I think this question is closely related to the
> one from Ricardo Faccio earlier today, and other questions about
> standards and reference compounds we've seen recently.
>
> It is generally difficult to get accurate and meaningful amplitude
> (values for N and sigma2) from an a priori modeling of an EXAFS
> spectrum. There are many reasons for this, including experimental
> problems (that is, measuring the amplitude of chi(k) with high
> accuracy) and theoretical problems (that is, understanding with high
> accuracy all the physical terms other than N and sigma2 that influence
> the amplitude of chi(k)).
>
> It seems natural to want to improve the accuracy of N and sigma2
> through "better modeling" including trying different k-weights for
> the fit. This can even work to a small extent, and it is impressive
> that it does work and that Artemis helps you do this easily. But it
> won't improve the accuracy of N and sigma2 by a lot.
>
> But since you bring up some older work on Debye-Waller Factors and are
> working on a classic EXAFS problem like site disorder in an amorphous
> semiconductor, I'd like to bring up a point that at one time was
> taught earlier on in EXAFS analysis:
> Absolute measures of N and sigma2 have much larger uncertainties
> than
> relative measures of N and sigma2.
>
> In the pre-FEFF days, very few people would measure a single spectra
> and expect (dare) to publish N and sigma2 for it. Instead, two
> spectra would be measured with similar instrumentation, analyzed
> together, and N and sigma2 would be compared. One common approach
> was to vary the temperature, and if there was no phase change, N could
> be assumed to be invariant, and you would extract the temperature
> dependence of sigma2 (within a constant offset). Another was to
> measure a well-characterized crystal (say, crystalline InP) and
> compare N and sigma2 between the two phases. Even with FEFF, these
> are good approaches.
>
> The drawback to these approach are that they're limited to single
> scattering and need a good reference sample and spectra with which to
> compare your unknown. But it would certainly be possible to analyze
> spectra of amorphous semiconductor in this way.
>
> The key is to look for relative differences. For sure, start by
> modeling data for crystalline InP to get some idea of the thermal
> component of sigma2 for the In-P bond. Then, if at all possible,
> measure InP at a few different low temperatures: All the variation
> between those spectra will be due to the thermal part of sigma2, not
> the static disorder, and not N. Once you get that far, you'll
> probably want to do a multi-data-set fit constraining N for In-In and
> In-P to be the same for all temperatures.
>
> Hope that helps,
>
> --Matt
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--
Claudia S. Schnohr
Department of Electronic Materials Engineering
Research School of Physical Sciences and Engineering
The Australian National University
Canberra, ACT 0200
AUSTRALIA
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