When do we use the small angle x-ray (two-theta less than 5 degrees) for thin film structural study,Is it possible observe x-ray peaks at higher angles? why ?
Dear Robert ,
Thanks. but how it is possible observe Bragg peaks at higher angle (2 theta larger than 5 degrees) for thin film has locally a periodic structure of several ordered layers while we use from low -angle?
please explain more.
Regards,
Abbas
Hi Abbas,
Its possible if your films are under stresses.
Just read any basic book like C. Suryanarayan, M.G. Norton, "X-ray Diffraction: A Practical Approach", Plenum Press New York (1998)
Dear all, it is extremely important to consider the experimental geometry carefully - as I understood the original question, a symmetric setup is considered, i.e. incidence and diffracted angles are identical. In that case, a thin film may have "Bragg-like" peaks as Robert Metzger has motivated above. If the angle is increased in such a geometry (i.e. larger incidence and exit angles are considered) then usually the peak intensities of "real" Bragg peaks are small, due to the small number of interfering net-planes for a thin film sample. If the films are disordered in addition, then the peak intensitites may even vanish, especially in the case of light lements, where the X-ray scattering cross section is small.
In those situations, the use of an asymmetric geometry makes more sense: The incidence angle is fixed to a small value (typically well below 5° down to only 0.1°), and only the exit angle is scanned. This is the so-called Grazing incidence (GI) XRD geometry. Here the interaction path with the atoms of the thin film sample is much larger, and thus the intensity of the Bragg peaks as well. So such an asymmetric geometry is favourable for thin film analysis in many cases!
Hope this helps to clarify the situation, Dirk
Here are examples of XRD observations in the "Small Angle" and "Wide Angle" range. Both the NIST 2000 SRM (standard reference material) and a sputtered Pt thin film on Si (001) sample were examined. Both symmetric and asymmetric geometry were explored. The examination depth may be changed by altering the incidence angle to preferentially examine varying depths in the sample surface as Dirk has pointed out.
Abbas! Post your data, if available, for review.
It is possible to get Bragg peaks at low angle only if the periodicity is very large and the incident beam wavelength is very small. Generally, this is not the case. I'd think, if the d-spacing is very large then the use of a large wavelength would allow for easier measurement at higher angles away from the interference of the "Main Beam".
https://www.researchgate.net/post/Would_anyone_have_a_clue_as_to_the_origin_of_unexpected_spots_in_the_real_time_2D_XRR_signal_from_thin_film_samples_on_substrates_Yuneda_Spots
https://www.researchgate.net/post/What_is_the_best_way_to_interpret_REAL_TIME_3D_X_Y_o-2TH_o_ph_ch_reciprocal_space_relative_intensity_data_from_HRXRD_rocking_curve_analyses
Dear, yes it is possible to observe 2 theta higher than 5 degrees. For example, YIG ; 2 theta angle starts at 20 deg.
If you refer to small angle X-ray scattering (SAX), which I guess is used for what you mean by thin film structural study, than you do not observe any peaks at higher angles, since that works in the total reflexion °region, < 0.3°Th (experts correct me...) and you do not make it with a powder diffractometer. If you are using a powder diffractometer and make a 2Th vs. I(cps) scan, than 5°(2Th) is NOT low angle, is a lot higher than 1°(2Th) and there you may have peaks belonging to real crystal structures... or you just see artifacts of a misaligned goniometer and slit system.
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