I think SiO2

I think SiO2

SiO2 will be better, U should using grazing angle XRD for this, preferably grazing angle below 1 degree will give you better peaks of the film only. You should also do a XRD of bare sample at the same grazing angle. Only then you can compare the peaks. Since, it is always a possiblity of getting a substrate peak.

In standard XRD you'd still get huge peaks from Si regardless of the thick SiO2

For (001) Si wafers the most prominent peak is the (004) at around 69 degrees, and if your source is bright enough and your alignment is good enough you might observe the forbidden (002) reflection at around 32~33

If you don't monochromate the source you'll observe many satellites such as K-beta and others.

If the material that you are looking to grow has relevant/interesting peaks far enough away from these peaks and their satellites - you shouldn't care about the substrate's contributions. If the peaks of interest overlap the Si - the SiO2 layer will not help you anyway and you should choose another substrate.

While I don't believe the latter case is likely, there are a few solutions:

(a) You can buy a highly-miscut Si wafer and use that as a substrate. These have no reflections below >>100 degrees. For example:

https://www.mtixtl.com/Zero-Si-24D05C1.aspx

Alternatively you can just use some cheap glass etc., just measure it bare before to make sure it has no peaks of its own.

(b) If you tilt the sample (omega) by about ~1 degree from the (001) direction of Si, you will not observe the substrate peaks. This is only relevant if the measurement you want to perform is suitable. For example, if you want to measure thick polycrystalline films with no preferred orientation it should be fine, but if you're measuring textured film or very thin films you will lose much, or all, of the signal.

If you share details of the actual experiment (materials, parameters) it might be possible to provide further guidance.

Thank you for your answer. I really appreciate your help and support.

One more point that I forgot to mention: (almost) nothing that you can grow on Si will have peaks as fine and narrow as those of Si. Therefore it is quite easy to tell if a peak originates from the substrate (including its many satellites and artefacts) or from the film, just by the width. Substrate-related peaks would be as thin as a needle (typically limited by your average instrumental resolution).

si and sio2