Dear colleague Jan Mischke,

Have a nice time and good day.

To  reduce the amount of defects in these films and improve the surface flatness of the films you can try one of the next items according to your conditions.

1- Annealing of the prepared films after spin coating to remove pores.

2- Try variable thickness (from low to high).

3- Try different spin coating speed.

I wish that is is useful and help you.

Yours

A. EL-Denglawey

Dear colleague A. Eldenglawey,

Annealing doesn´t work, but I will try the different spin coating speed.

Thank you for your answer!

The holes in your films indicate both a wetting issue and a cleanliness one. Clean the substrate impeccably with 5% phosphate-free surfactant in DI water at 40 - 50 C with 15 minutes sonication.  Rinse thoroughly in 18 Meg DI water.  A film of water should result - no breaking away.  Dilute your material with the appropriate solvent (this will also produce a thinner film as will increased spinner speed) in order to improve the wettability.

Now, I'm not sure how you're depositing down your ZnO - you can't spin coat ZnO directly.  You have start with a soluble zinc salt (isopropoxide? acetate ester?) in an appropriate organic reagent (isopropanol? ethyl acetate?) in order to prepare a film that is sintered at sufficiently high temperature to produce ZnO (and not Zn(OH)2 or other intermediates).

Dear Jan Mischke 

Have a nice times and good day

There are many parameters control the physical properties of the nano-crystalline films like:

1-The deposition rate or the spin coating speed.   

2- The substrate temperature or the deposition temperature.

3- The film thickness.

The decreasing the deposition rate and increasing of both the substrate temperature and the film thickness lead to increase the crystallization process. Increasing the crystallinity leads to the improving of the structure and electrical properties of films. As well as, the annealing of the substrate to higher temperature and the film samples also lead to the same results. 

good luck

@Alan: I don't think that there is a problem in the cleaning process with my samples as this process is very well understood. However, you concept sounds reasonable and I will try your approache to see the difference in surface properties. For the ZnO spin coating: I have the ZnO particles disolved in Ethanol. Thus, I can trickle the solvent with the particles directly on my substrate.

@Ahmed: I will start to have a look into the temperature of the substrate during spin coating. So far, we have not seen any progress by changing the temperature after spin coating the particles.

@Ankur: Thank you for the literatur. I will have a look into it. Dip coating is sadly not an option for my purposes due to technical and structural difficulties. Nethertheless, good idea and maybe we will try it in the future.

Thank you all very much for your contribution to my topic.

# Jan  I'm still confused as to how you're applying your layers.  ZnO is not soluble in ethanol.  One creates ZnO by high temperature decomposition of appropriate Zn precursors that are soluble in appropriate liquids for spin or dip coating.  If you have a suspension of ZnO in ethanol then it wouldn't surprise me that poor layers result - the firing temperature would have to be very high to produce a film (probably close to the melting point).  In terms of cleanliness I spent 7 - 8 years in the LCD business learning to apply photoresists and sol gel materials to ITO substrates.  Cleanliness for applying films to ITO layers is vital, I can assure you.  The cleaning recipe I gave you was the outcome of a lot of research on cleaning glass and ITO coated substrates.

Dear colleague

 to improve the structural properties of ZnO you must play on the temp and annealing temperatur and the number of layer and drying between each layer against electrical properties for what I am about to try his realisation  you must introduce a doping . I remain at your disposal for further questions on the subject

@ Alan: I get the already dispersed ZnO-nanoparticles delivered. "Disolved" was the wrong term. So they are dispersed in ethanol as nanoparticles. It is not the point that they solute in ethanol. That's not possible or desirable. Via the dispersion I can spin on the nanoparticles onto my substrate, heat it up so the ethanol can evaporate and thus get a ZnO nanoparticle layer. Still, this layer should show uniformity in surface flatness, because the nanoparticles are very small (few nm). Thank you very much for youzr recipe to clean the ITO substrate!

@ Kheyar: Very good tips. Thank you very much.

Dear Mishke,

In order to improve the structural properties of your ZnO films, you have to sweep the annealing temperature and see what  temperature gives the best structure. Also, you can try the effect of the thickness. I think that structural qualities could be destroyed upon a certain film thickness. For improving the electrical properties, you have to choose an appropriate dopant. In fact, ZnO is n-type semiconductor, so try doping with donor elements in order to increase conductivity. 

best wishes,

Abdelmounaim.