This peak could appear if during the ZnO synthesis takes place a chemical reaction which produces Zn(OH)2 phase with (211) reflections. See Fig.4.2(b) from the attached PhD thesis.

Though there may be another explanation because, theoretically, this peak exists for ZnO powder which contains crystals in random directions producing multiple visible peaks. While sputtered ZnO usually contains only grains which are oriented normally to the substrate surface (producing the major peak around 34.5 degrees). Check Fig.16 from another PhD thesis. Unfortunately, it does not say to which Miller index the peak at 32.8 degrees is actually related.

I agree with Sergei, probably this is a minor intensity feature of zinc oxide or hydroxide - however, there is only little information on the experiments given. What about the intensity of the peak under discussion - what is the intensity relative to the leading peak? ZnO in its bulk form should have the 100 at around 32° for Cu K_alpha, so strain, etc may affect the peak position and result in a shifted peak, have a look into the literature.There are many ZnO-studies with XRD which obey a peak at about 32-33 2theta range, see enclosure (from Nanoscale Research Letters 2008, doi: 200810.1007/s11671-008-9136-2)

So I think this is not a true extra peak ...

I have deposited the samples at 350 C substrate temperature. Is it possible to observe OH related phases?

Idris, if my first answer regarding the origin of this peak is correct, then this is not about the deposition. It's about the quality of your original ZnO film. Did you prepare it by yourself or got it from somebody else? Zn(OH)2 phase usually appears during a low temperature synthesis of ZnO films (read the text in the first PhD thesis around Fig.4.2).

Dirk, I also thought that this peak is related to (100) reflection. But according to Fig.16 (from the second PhD thesis, take a look), (100) peak is actually situated around 31 degrees, that is below that misterious peak at 32.8. In terms of intensities, (100) has 90 a.u., (002) has 70 a.u. while the peak at 32.8 has 40 a.u. So, it's not that small. Perhaps we could solve this mistery if one can get an access to PDF (powder diffraction file) library. The PDF file which is mentioned in Fig.16 for ZnO powder is called "03-0891".

sergei, if you read my question carefully you can see "I" have deposited the films. I only wonder if this type of phase show s itself after 90 degrees according the document you sent.

I think OH related phase can probably survive at higher temperatures. The only question is how big it is (in comparison with a more stable ZnO phase). In terms of relative intensity, how this peak at 32.8 degrees is related to (002) and (100)? By the way, do you see the (100) peak around 31 degrees?

Yes. I see it but its intensity is so low to compare with this phase.

By the way, I have found a peak in an ASTM card matching this angle at 32.72 °. (Al2O3.ZnO.monoclinic). What is your opinion about this?

It looks like you solved the mystery of this peak. Perhaps this is not very common phase for depositions but makes sense to me. Can you link it with any particular Miller index? Just curious.

The presence of monoclinic Al2O3.ZnO could be an explanation, however, in terms of a full structural analysis - is a single peak enough to state the presence of such a phase? What about the other diffraction peaks of monoclinic Al2O3.ZnO - those usually have quite a lot of diffractions and should be detectable ... Have a nice day, Dirk

Idris, I don't know for what purpose you prepared your film but I would suggest to test its electric and magnetic responses. The results may surprise you because you have a rather unconventionally oriented film with a suppressed (100) peak and developed monoclinic phase (which is probably a secondary phase). It's quite possible that you've inadvertently cooked a very peculiar sample.

Sergei, I agree with you. I will try to deposit these films with different parameters to find out the real reason of this unexpected phase. Thank you very much for your comments.