Dear Vladimir Burtman ,

you are right:

that is an crystallographic issue.

In your last sentence above you talk about an 111 Si substrate.

This implies that you deal with a single crystalline Si Wafer, which is cut parallel to the 111 planes of its crystal lattice.

The arrangement of the lattice points in the 111 plane is a hexagonal one.

See for example

https://www.globalsino.com/EM/page4489.html

and/or

https://nanowiz.tripod.com/sisteps/si111.htm

The 111 plane has got a 6 fold symmetry. With respect to the existance of 111 peaks in the diffraction pattern this means, that when the direction of the primary x-ray beam is aligned at (or close to) these six directions one will have pronounced 111 peaks.

So, in your case of your blue pattern, you have accidentally got a sample orientation in phi, which matches one of the directions of the hexagonal lines. But when rotating the sample by 90° (in phi, as you did) you do not meet one of the hexagonal directions, but instead you will will end up just in the middle between them. Thus the 111 peak is failing in your red pattern.

I would like to encourage you to do a phi scan at your bare 111 Si wafer.

Good luck and

best regards

G.M.

Not much to add to Gerhard's answer - as usual for him.

You can buy single crystal silicon wafers designed to be "zero background holders" by being cut a few degrees off-axis, so theta/2theta scans will give no peaks for any phi.

Dear Boyer, thank you for affirming Gerhard's response, which is very helpful indeed! I went back to XRD studies after a 20-year gap, and I was glad to find the new powerful instrumental possibilities, but at the same time, I was very puzzled by the results. The problem was that I was running the phi scan for film composed of organic nanofibers, which was deposited in such a way that it should not have any anisotropy (spray deposition). But phi scan shows a significant anisotropy (peaks appear and disappear at certain phi angles), and I was not sure if this is structural anisotropy or an artifact of measurement. So, I'm very glad that this issue was sorted out! Many thanks!

Dear Boyer, can you please share where you used to purchase the silicon wafers designed to be "zero background holders"? They should be undopped, correct?

From Google ZBH diffraction

https://www.universitywafer.com/x-ray-diffraction-zero-background-specimen-holder.html?srsltid=AfmBOoqNE14JE3xi1GtxIokYWPKsd6xO7pDXkBCQxZB9sOQ74L--qt4h#

I have bought some here

https://mtixtl.com/collections/zerodiffractionplates?page=2

Bruker: C79298A3244B249 in catalogue

https://nanoqam.ca/wiki/lib/exe/fetch.php?media=ps_xrd_18_sample_holders_doc-s88-exs018_v7_high.pdf

Dear Dr. Boyer, thank you very much for your advice! I end up buying these zero-background wafers from MTI, the University Wafers quote was 8 times more expensive.

Dear Dalibor, thank you for your advice as well! I would like to measure thin films on a Silicon substrate, not a powder on Silicon. So, the C79298A3244B249 Si low background sample holder for small sample amounts will not work for my case.

I really appreciate your feedback, folks! thanks a lot

Vlad Burtman

Folks, thank you all for your help again! Just for the common records, below I append here an official response from Bruker that I got recently:

"For single crystals the rotation and tilt of the sample could certainly cause the lattice planes to be misaligned relative to the incident and diffracted beams. Hard to say more without knowing more about your sample and how you measured it. If it's a symmetric scan on a 111-oriented Si wafer, the peak disappearing might be due to sample wobble or a significant miscut."

The Gerhard Martens's response (with valid comments of Dr. Boyer) is still the best

Folks, eventually, the last update. I bought a ZBSi from MTI Corporation and ran 3 XRD tests using Bruker D8 XRD. The 1st test is a regular Tetha/2Tetha scan 6-120 deg. No peaks observed. The Decay from 6 deg to about 40 deg is due to the low angle and may be due to the fact that the beam was touching sample-holder,