Substrate temperature and spraying are  the possible reasons for this column formation. You may optimize the substrate temperature and concentration of the solution you are spraying. 

@Jalal Uddin: Thanks for your advice. I have already tried various temperatures ranging from 250°C - 400°C. The columnar growth ontop of the layer can be observed at all spraying temperatures. For a standard process: 100 nm of CuO would take 5 min of spraying of an 0.05 molar copper nitrate ethanol solution. A higher molarity will probably change the spraying duration. I have learnt a higher molarity also results the deposition of Cu2O aside from CuO. The principal goal of my work is the deposition of single CuO phase films. Maybe a small change of concentration would be enough to get rid of the columar growth. I will attempt an experiment and keep you posted. Have you ever tried to deposit CuO by spray pyrolysis method?

Dear Robert,

You have to increase the the number of seeds formed on the top of the 100 nm film. I think if you let the substrate vibrating with some ultrasound vibrations this will disperse the formation of the seeds for the columnar growth. You may also spin the wafers during the deposition. This is a proposition that i hope to. work. One oher silution you may decrease the substrata temperature further if this not decrease the reaction rate. This will also helps increasing the the number of seeds.

Best wishes 

@ Abdelhalim Zekry: Thank you very much. I will work with your advice. Best regards

Dear Robert, I am unable to get your question clearly.  Does columnar growth commence once you exceed approximately 100 nm?  pH is one important parameter and what is the pH of the present  solution? If you are going to get vertically aligned CuO columns, is not advantageous?

@Sundara Raja Vanjari: I think that the columnar growth start from the beginning of the spraying process. Mostly you will get a thin film, but in between there are these columar growths (dimensions d= 1µm, h=1µm). I have already tried different spraying durations and I always get this structures. I have to admit that I don't know the pH of the solution. But I will check it. Have you had any expericence with change the pH value for a spray pyrolysis process?

We have not worked on CuO but we have worked on the spray pyrolysis deposition of compound semiconductor thin films like CuInS2, CuInSe2, CuIn(S,Se)2, Cu2ZnSnS4, SnO2, FTO, ITO etc.

With regard to  dimensions you have given, is it the height or  length of the column since the film thickness you have mentioned is about 0.2 micrometer?

I have forgot to answer your question.  We have worked   on the  effect of pH in case of spray deposition of CuInS2 thin films. 

@Sundara Raja Vanjari: The layer is around 100 nm. The growth of these pillars (height 1µm, diameter 1µm) starts with the growth of the layer itself. So the pillars are a part of the layer. I have heard from another researcher here on researchgate that he has seen the same growth. I was wondering if this has happend for other materials as well. The layers you have deposited are smooth or have you seen a similar growth behaviour at any other material. Thanks for your input. Best regards, Robert