Try to use another substrate - for example, Lithium Niobate. If it will not help, firstly, try to make ZnO grains using a spin-coating method and a precursor for the hydrothermal grow process.

Thank you Oleksandr Ovsianytskyi, The second part is a good idea, but is there any idea for only using spin coating method?

Of course, you can use only the second part.

You may try one of the physical deposition techniques in which coating form from evaporation phase.

Thanks Dr. Mohamed A Hafez. Well, I actually try to obtain films with good adhesion and emission, which means larger thickness films, focusing on the spin coating method.

I got some good results, but I need more enhancements, so any ideas will be helpful.

Thank you