the 2theta scan gives you information on the strain and size of a crystallite while a rocking curves shows you the mosaicity and diffuse scattering due to defects.

Good question Fahad!

Typically "FWHM of the rocking curve" is from Omega only type decoupled scan (DETECTOR AT A FIXED 2THETA) while "FWHM of conventional 2theta scan" is usually from Omega-2Theta type coupled scan.

Experimental observations with various mono crystals indicate that the Bragg profiles from both methods are nearly identical using a 2D real time XRD imaging system. They are just different methods of recording the reciprocal space data.

I've seen many explanation for the difference but fail to observe the differences experimentally between the two observation methods and the resulting Bragg XRD profiles.

I look forward to other points of view. I'll post more relevant data once this discussion heats up :-)

In the example below of GaSb (004) symmetric reflection for sample W20110221 (3rd from top), the Bragg profile for Omega only decoupled scan (green) is superimposed on the Omega-2Theta coupled scan (blue) after correcting the ordinate axis (Omega=Theta vs. 2Theta).  The  "rocking curve topographs" (Bragg Peak Position Map and FWHM Maps) appear to be "identical" as well. Do review and comment!

One other difference is the fact that the reflection is "optimized" in the case of "the FWHM of the rocking curve analysis". I'm still trying to figure out the real meaning (crystallographic) of the term "optimizing" with the conventional equatorial scan diffractogram using a 0D scintillation/proportional counter/detector. Any input clarifying this issue would be appreciated.

https://www.flickr.com/photos/85210325@N04/14550209752/

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

Mr. Ananth,

When you run a coupled 2theta-theta (or 2theta-omega) scan you scan over a line in the (001) direction in reciprocal space.

When you run a decoupled omega scan at a constant 2theta, you scan over an arc of a constant radius in reciprocal space.

For a constant thickness of an epitaxial single crystal film it is my understanding (and I'd be happy to hear other thoughts) that 2theta FWHM is a measure of the distribution of d-spacing values whereas an omega FWHM is a measure of the distribution of orientations in the film ("mosaicity" / preferred orientation of the thin film). Both may indicate the quality of thin films in the sense of uniformity and disorder, but I believe that they represent slightly different physics (although closely related). Particularly when strain is involved.

Lior! "preferred orientation of the thin film" - I imagine you mean spatially for a "flat" sample surface. If so, then the relative (spatial) Bragg Peak Position would be the measure of relative preferred orientation, wouldn't it? The FWHM is always related to "mosaicity" or "excess dislocation density" conventionally. However, we do notice that in mono crystals "sub-grain" structures and "twinning" may also contribute to the FWHM of each individual coherently diffracting domain from the sample surface. Using a 2D topographic technique as we have this would be the individual Pixel location on the sample surface. 2D XRD methods provide information about both real space and reciprocal space simultaneously. The XRD Rocking Curve Techniques allow us to deconvolute these two information streams (real & reciprocal) effectively.

Note that using the 2D real time XRD imaging system we are able to resolve down to a 27um Pixel size spatially on the sample surface. This depends on the XRD optics as well as geometry used and may be enhanced further using asymmetric reflections &/or by placing the 2D detector at an acute angle to the diffracted beam rather than perpendicular (conventional). 

The example below uses both Om-2Th coupled scan (NIST 2000 SRM Si/Si-Ge/Si) as well as Om-only decoupled scan (for Pt. thin film on Si) in the case of XRR. What would be the expected difference in this case?

I'm yet to repeat the 2 methods with the same sample to really compare in the specific case of XRR :-)

Repeated inspections of various (hkl)'s for a multitude of samples show no distinguishable difference between the two profiles. It would be a stretch to think no difference existed based on the diverse Nano structural causes for changes in FWHM as being discussed here. I'm intrigued!

The Bragg profile is always "the distribution of d-spacing values" in a specific direction. The real question is, how do we infer various Nano structural causes from the profile shape and its other characteristics such as FWHM, Peak Position, Shape, etc.? One way is to use a standard Bragg profile to compare with and then correlate the deviation from the IDEAL BRAGG PROFILE to various Nanostructural morphologies, features or attributes. We attempted just that using a LEPTOS simulated "IDEAL" profile for GaAs (004) reflection. I'll post some data soon for you erudite folks to scrutinize and critique. BOLO!

I continue to learn :-)

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_Yoneda_Spots

 Ravi, let me explain what I meant using two examples (setting aside the 2D detectors sales pitch)

Assuming a fully relaxed single crystal thin film with a relatively smooth surface:

Case A - the film contains randomly-distributed impurities/off-stoichiometries/vacancies resulting in a slightly broader distribution of the lattice constant. I expect this to have a larger effect on the coupled (2θ) scan than on the decoupled (ω) scan.

Case B - the film isn't as defected as Case A, but the 00L orientation isn't exactly constant across its surface (for example, if the substrate isn't a very high quality single crystal), or it features some buckled/domain structure (I agree that "mosaicity"/"preferred orientation" is a silly term to use in epitaxial films, I should've used quotation marks...). I expect to see a larger broadening in ω here versus Case A (and some broadening in 2θ)

Just my two cents, I'm no expert.

Lior! I note your apathy for 2D detectors in XRD. Have you used one yet? What type? Most folks in present day XRD have nearly identical or sympathetic point of view as yours and aren't shy about wearing their "emotion" openly:-)

Glad you only noted "setting aside the 2D detectors sales pitch" in general and not specifically for the AXIS-TAS25 (Advanced XRD Imaging System) made by Onsight Technology USA. Cool product! Leading edge of technology! World Leader! Check it out with an open mind :-)

"I should've used quotation marks", it's ok. I don't mind :-)

So, I understand your feelings quite clearly. If I were you, I'd feel the same way. But by being open minded 3 decades ago at RU and JHU, I discovered a lot. My objective is to correlate spatial Nano structure with the 3D XRD reciprocal space data and not to reduce 2D data into 0D as in conventional diffractograms and be all confused and ambivalent with interpretation. 

"Assuming a fully relaxed single crystal thin film with a relatively smooth surface". This is symptomatic of the challenge. I'd rather not assume, I'd prefer to quantify and compare with a "standard".

Only 2 decades ago did I discover the following from one of my favorite books: 

X-ray Diffraction In Crystals, Imperfect Crystals and Amorphous Bodies by Andre Guinier. 1962 - Worth acquiring for every XRD collection!

(pp. 162-163) 6.3.5. "Comparison between the Photographic and Counter Methods

It is obvious that the Geiger Counter and the ionization chamber have the advantage of precision and sensitivity. They alone permit a quantitative study of scattering. However, point by point measurements in reciprocal space are lengthy, while the photographic method gives directly a picture of the scattering in a single experiment over a large surface of reciprocal space. The resolving power of the film is excellent. Thus, sharply defined scattering regions, for example on rows or on planes of reciprocal lattice, show up very clearly on photographic film but are difficult to detect with a counter.

Let us note also that the counter is "blind". In many cases inaccuracies in the setup produce stray scattering which is easy to detect on the photographic film but which can spoil a series of experiments made with a counter.

The two methods are therefore complementary and it would not be advisable to neglect one in favor of the other. The photographic method is admirably suited to the qualitative exploration of an unknown pattern, since one can then find totally unexpected phenomena. The counter is necessary for quantitative measurements on a pattern which is already known qualitatively."

This was printed in 1962. This has to be considered in light of the present day 2D, (any make and model you prefer :-), detectors that combine the benefits of the counter & the photographic film in one. These writings indicate some causes for the present day inertia of moving from the 0D to the modern 2D detectors. The spatial "blindness" of the 0D scintillation counter/detector is the principal reason even experienced experts using XRD are ambivalent about the 0D XRD data/results

Let us set emotions aside and focus on the challenge: Correlating material Nano structure with reciprocal and real space data using NDE XRD. "difference between the FWHM of conventional 2theta scan and the FWHM of the rocking curve analysis"

BTW I have a huge emotional attachment to this topic, I confess :-) Its a love affair of my lifetime!

We look forward to a continued robust discussion. I'd expect a bit of both Case A & Case B for the two thin film examples I've used. I prefer making no assumptions. Besides, noting my "self promotion", have you actually reviewed the rich data sets and analyses I've publicly displayed. I do encourage all with knowledge & experience in XRD to review, critique and help us all learn!

""mosaicity"/"preferred orientation" is a silly term to use in epitaxial films". No, not at all! Quite the contrary! Let us discuss with specific examples.

"I expect to see a larger broadening in ω here versus Case A (and some broadening in 2θ" Got experimental evidence? Shouldn't be all that hard to reproduce, show us :-)

https://www.researchgate.net/post/Is_it_possible_to_determine_the_type_morphology_of_defects_in_single_crystals_by_the_shape_of_the_Bragg_XRD_Rocking_Curve_Profile

https://www.flickr.com/photos/85210325@N04/19855148646/

Lior! Nice work! I'm very interested!

"Combining functional oxides with conventional semiconductors provides the potential for transformational advancement of microelectronic devices. Harnessing the full spectrum of oxide functionalities requires current transport between the oxide and the semiconductor. This aspect is addressed by controlling the electronic barrier at an interface between a ferroelectric oxide, BaTiO3, and germanium. For the aligned conduction bands of germanium and BaTiO3, as measured by spectroscopy, current transport is controlled by the barrier between the top metal electrode and the bands of the BaTiO3. Capacitance–voltage analysis of metal–oxide–semiconductor devices further shows that the semiconductor's Fermi level can be moved by a field effect. These results demonstrate a viable approach for electronically bridging the functionalities of oxides directly with a common semiconductor"

Article Transport at the Epitaxial Interface between Germanium and F...

Lior! "XRD was done using a monochromated Rigaku SmartLab diffractometer with a Cu kα rotating anode source and fit using GLOBALFIT 2.0 software."

Is "GLOBALFIT 2.0 software" a FREEWARE? Please post a link if available.

Just trying to understand the surface sampling size used in your case for signal convolution utilizing the conventional "equatorial scan" diffractogram. I must assume that your XRD rocking curve data required "optimization" of the reflection. 

• Did you use the substrate reflection or the film reflection to "optimize"?
• Did you use "maximizing" the integrated intensity as criterion for "optimizing" the reflection?

Do note that mostly all of our XRD data was acquired with a conventional X-ray sealed tube generator

"ω here versus Case A (and some broadening in 2θ)"

Let's remember ω=θ for symmetric reflections :-)