I agree with Michel but also take about the decomposition temperature of your precursors. I had same issues by the time that I was spraying at low temperatures. If high temperatures are required for your experiments then you can add a highly boiling point solvent as AcAc or TEG.

Also for the case of very "thick" thin films there is also the possibility of a kind of "delamination" because of an intrinsic stress, leading to a powder-like deposition. 

I think you should go for DTA TGA analysis of the initial ingredients. That will give you correct deposition temperature. Again one has to see that initial should not react with each other at room temperature. If you check all these your problem may solve.

I think that you have touched the fundamental problem of spray pyrolysis. It's good only for very thin layer of nanoparticles production. There is  strong instablility of film growth at droplet deposition process.

May be low speed motion of nozzle along substrate could help.

Good luck

Thank you very much for valuable suggestions.

Dear Michael,

As I read, for the spray deposition, there should not be physical contact between substrate and droplet. If I increase the flow-rate how will it improves the quality of film deposition?

I agree with Michel ,  try to make your substrate closer to he nozzle or reduce the substarte temperature  this should  resolve the issue 

what is your present spray rate? Which metal oxides you want to deposit? and What are the ingredient you are using. Give this information some more suggestion may come.

dear

what is your deposition temperature

do you use a suitable temperature controller

reduce substrate temperature

good luck

Dear  Karanth,

First of all, heterogeneity and restriction to the thickening may be sourced by many  parameters as you have mentioned. But, i would suggest to focus on further parameters like; surface characteristics of the metal oxide particles, particle size and shape should be taken into account as well.

Best Regards. 

PS: Defect formation and heterogeneous cleaning may occur due to the US assisted cleaning may occur with wrong choice of instrument and procedure. 

Dear all,

I am maintaining substrate temperature of 450+/-10 degree C during deposition, spray rate of 4 ml/ min, solvent is mixture of alcohol and DI water.

I don't much about it. But ultrasonic cleaning after the deposition may result in peeling off your films from the substrate if there not enough adhesion. I don't think ultrasonication can create defects.

If all growth parameters are good than most probably dopant is creating a large number of lattice disorders. The crystalline solid solution is not forming. I have observed such kind of powder growth (in small amount) on the surface of Al doped ZnO thin films when Al doping is increased beyond 2 at. %. XRD studies showed that higher doping concentration of Al decreases crystallinity by a large amount suggesting the presence of large number of lattice disorders thus hindering the formation of proper crystalline solid solution in the form of thin film. 

Solvent used for deposition also matters a lot. If it is any of the alcohol or volatile, then it may get decomposed prior to reaching substrate. You can reduce the nozzle to substrate distance and try again or  also you may try to decrease the temperature but it may loose your phase formation and crystallinity of the films. Also check that solution is clear after adding dopant into it and observed the stability of the precursor solution if it gets precipitated after sometimes. Generally for spray pyrolysis clear solutions yield uniform films.

Reduce the temperature by controller or ultrasonic cleaning will help u. If u decrease temperature may possible change in phase formation.

I feel using a good atomizer during spray deposition is very important.

Do not continously spray the solution. Wait for the some time till the sprayed solution on the heated substrate to react, burn the organics away, and turn into a good transparent layer. The spray again another thin layer and repeat in case you wish to build your film thickness. the solution viscosity and the chemical structure should play a vital role. If the process is too fast, you will end up with clusters after the deposited product converts to an inorganic product, and then it will become powdery.

K. Sreenivas

Temperature increase can be a solution

I suggest, If you try lower concentration of the solution.

Try adding to the solution acetylacetone, from my experience in many cases the film deposits in a much more uniform way. mixing ratio of 1:1 with your organometallic precursor. this solvent has a high boiling point and therefore instead of lowering the temperature and risking your film quality, you can use it. good luck.

control your substrate temperature 

I think if you deposit onto glass don't exceed 400 °C

what are the solutes, solvent etc. should be mentioned, so that it can be answered more accurately

I guess that you have to gradially decreases the substrate temparature on order to avoid nanoparticles formation. Please look over my paper "Isothermal Deposition of Droplets and Nanoparticles from the Incident Gas Jet onto a Substrate"/You can find it at RG. 

Good luck

Try to add acetic acid to the solution. It will overcome the low boiling temperature of alcohol and will prevent the evaporation of the droplet before reaching the substrate. In addition, the doped atom used plays an important role in changing the properties of your films. So you have to choose it carefully (ionic radius, valence electrons ....)

I suggest that control both  substrate temperature and solution viscosity

Dear PV Raghavendra Karanth,

1-To avoid the origin of the mentioned problems: 

A-For "films are not uniform" you can try rotating substrate holder

B- For "powders are forming"  to set the suitable substrate temperature, firstly, why this temperature exactly? from the DTA or DSC analysis of the mentioned metal oxide. you can compare the substrate temperature with glass transition temperature and crystallization temperature.

2-To treat the produced film you can try ultrasonic bath.

K Sreenivas

Prof., in case of semiconductor oxide film deposited at 350 degree Celsius i found small bubbles/black holes/ black particles on the film. I am struggling with that bubbles to make a very uniform film. i am using a stabiliser in 2ME solvent to make ZnO film , in this case no bubbles. But if i want to make AlOx or ZrOx i find that problem. Can you suggest?

I tried by reducing the flow rate but i need a thick layer. I repeat the process 6 or times. As no. of layer increases the density of bubbles/black particle increasing.