Re: Explaining for broadening of peaks due to a shift of theta in theta-two theta scanning?

[pstephens]

Besides Joerg's suggestion of mis-cut substrates (which may be too 
expensive for use for thin film growth experiments) you could use Si 
wafers cut to (100).  The first allowed peak in that direction is (400), 
which is at about 70 degrees 2theta for Cu K-alpha.  That will still 
reflect any bremsstrahlung radiation from your x-ray tube, which can make 
a significant background.  If you have a monochromator in your optics, 
that will cut it down, but you can still get artifacts, such as lambda/2 
radiation reflecting from the (400) appearing to be a peak at the 
(forbidden) Si (200) position.

Or, maybe you can find a compromise between the Si substrate peak and 
defocusing with a smaller value of omega (theta - 2theta/2) than 3 
degrees.

Another possibility, if you are ready to rearrange your diffractometer, is 
to make the source-sample and sample-detector distances different, so that 
source, sample, and detector are on the focusing circle with theta not 
half of 2theta.  I wonder if any of the manufacturers provide this 
geometry?

^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~
Peter W. Stephens
Professor, Department of Physics and Astronomy
Stony Brook University
Stony Brook, NY 11794-3800
fax 631-632-8176

Re: Explaining for broadening of peaks due to a shift of theta in theta-two theta scanning?

[Eduard E. Levin]

Dear Huy,

as it was pointed out in previous letter by Joerg Bergmann, line broadening 
is caused by violation of theta-2theta relationship.

More info can be found in the following articles:
1. R. Berthold. Z. Angew. Phys. 7 (1955) 443.
2. H. Toraya, J. Yoshino. J. Appl. Cryst. 27 (1994) 961.

Best regards,
Eduard.

Re: Explaining for broadening of peaks due to a shift of theta in theta-two theta scanning?

[Reinhard Kleeberg]

Such an offset of the theta (omega) axis violates the para-focusing 
condition of the Bragg-Brentano geometry. You will get diffracted 
intensity from positions far below and above the gonimeter axis, i.e. 
far outside the focusing circle. The only option to reduce this effect 
is a reduction of the divergence of the primary beam. Without parallel 
beam optics, you can just reduce the opening of the equatorial slit, of 
course causing significant loss of intensity.

Reinhard Kleeberg

Huy LE-QUOC schrieb:

Dear Rietvelders,

To avoid the gigantic peaks of Si substrate (which are too dominant 
over peaks of phases in investigation on a thin film of 3 micron), we 
have tried to take a scan theta-two theta with a shift of 3 degree for 
theta (i.e. instead of theta-2theta corresponding to 5-10 degree at 
the beginning, I have set theta=8 degree and 2theta=10). In fact, we 
have been able to reduce dramatically intensity of Si peaks but the 
broadening of all peaks in the diffractogram is increased comparing to 
the normal scan without shift of theta.
We have also tried to rotate the sample around its perpendicular axis 
(angle Phi) to find the appropriate Phi where the peaks of Si are 
slightly decreased and found that the broadening of peaks is the same 
as in a normal theta-2theta scan.


Does anyone have any ideas about this increased broadening of peaks 
during a theta-2theta scan with theta shifted ?


By the way, do you know others ways, besides the razing incidence, to 
avoid the dominant peaks of substrate over the thin film  ?


Thanks a lot in advance for your any helps.

Best regards,

---
Huy LE-QUOC,
Doctorant
LPSC/UJF-Grenoble INP-CNRS, Centre de Recherche 
Plasmas-Matériaux-Nanostructures
Institut Néel/CNRS, Département Matière Condensée, Matériaux et 
Fonctions

53 rue des Martyrs, Grenoble 38026, FRANCE
Phone: +33 4 76 28 40 38
Fax:   +33 4 76 28 40 11