Got two questions:
1. How to choose the angle? Is that smaller the better? Since smaller angle will give less profile depth and it will thus avoid the information from the lower layer as much as possible?
2. Should I choose point focus source or line focus source for GIXRD?
The fixed angle is generally chosen to be slightly above the critical angle for total reflection of the film material.
I agree with the Mr. Ankit. To get the stronger signal from the film, the fixed value of Omega should be slightly above the Critical Angle.
Btw, I understand the point you are saying, because the information from substrate is limited when the internal total reflection happens between the film/substrate interface, right? However, I don't know if you ever run the GIXRD, because this is not easy and I'm expecting answer based on experience but not from book. Thanks anyway.
Dear Zhimin, it depends on what you want to probe. Small incidence angles imply a small penetration depth and low intensity of the Bragg peaks, not only because of the smaller penetration, but also because the illumination of the sample (intensity per area) is smaller at lower incidence angles. However, the information originates from the outer surface of your films.
In contrast, larger incidence angles will give larger penetration and larger intensity, but don't forget that all your Bragg peaks will shift due to the refraction of the X-rays within the sample (check the principal paper from Michael Toney and Sean Brennan, http://dx.doi.org/10.1103/PhysRevB.39.7963). The shift is btw largest exactly at the critical angle, so working above or below is a good idea in order not to misinterprete shifts of the Bragg peaks in terms of lattice stress and strain, stacking faults, etc.
From the experimental point of view, working with a line focus in a laboratory diffractometer is more helpful compared to a point focus (the point focus is usually larger in height compared to the line focus, so you'll get more flux on the sample if the incidence angle is small) You may calculate the actual heigt of the sample in the beam by means of simple trigonometry and check thereby if point and line focus make a difference or not.
Hope this helped, kind regards, Dirk
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