Dear colleague Zhiqiang Yu ,

Have a nice and good day.

You mentioned that "I’m performing an experiment on fabricating compound thin films by Thermal Evaporation method." many thing is don,t mentioned as compound name, adjusted substrate temperature and substrate type!!! this is important.

Your aforementioned temperature may by high or low.

Any how to adjust the substrate temperature at a certain degree, first you should have the DSC or DTA of your compound. From DSC or DTA you can determine the glass transition temperature, Tg , the crystallization temperature, Tc and the melting point, Tm. So you should have such this analysis to adjust your substrate temperature.

Yours

A. EL-Denglawey

Dear colleague Zhiqiang Yu ,

Many factors may affect the obtained film by thermal evaporation technique as:

1-Density of the evaporated material

2- Distance between boat and substrate

3- Deposition rate

4- Substrate temperature

5- Substrate type

6-Pressure of the deposition

7- Ambient gas

8- Thermal literature of the evaporated material.

9- Sample holder position; Fixed or rotated.

10- Surface tension,..............etc

You should have to know the background about the foregoing items.

Yours

A. El-Denglawey

Dear colleague A. Eldenglawey,

It's really kind of you for so earnest and detailed advices.

I forgot to point out that my material is SnS, maybe later I'll try other Sn-based chalcogenides such as SnSe. The substrate is quartz plate without temperature controlling or rotations, because I think it's hard to realized in my equipment. And that is truly, one important factor in the deposition process, as you mean.That's why I used two spacers (thermal insulating bricks) to reduce the temperature of substrate slightly, and I know it only worked a little bit, further improvments are neened (My idea is to use quartz shelf to hold the substrate to further lower down Ts, I think it's likely work on account of the heating sources are some IR lamps, so that the heat change between sources and substrate by thermal radiation would be restricted, where radiation is the main way of thermal transfer).

You are right, it is necessary to perform thermal analysis (I think TGA maybe more appropriate) to define the properties of the material, TGA will be done soon.

Thank you again for your wonderful answer on affecting factors! It's really useful for me and I conclude them as 4 aspects: [1] substrate parameters (4,5); [2] source material properties (1,8); [3] fabrication methods (TE) and [4] experimental conditions (2,3,6,7,9).

Kind regards

Zhiqiang

Hello Aswin kumar Anbalagan,

Thanks for your answer! As for your aforementioned aspects, I have some thoughts,

1) the effect of distance can be roughly attributed to the improper temperature of the substrate (Ts), I thought if the Ts was low enough, the deposition would certainly happen although the thin film grew results may varied with the diastance or other factors.

2) the solid angle related issues are indeed need to be considered , and here, what I adopted was more like free surface source and as you said, the solid angle was large really. But the key point is that there was nothing deposited on the substrate, so I think the solid angle is a side issue here.

3) yes, the temperature of sample was also probably inappropriate, and I'll change that to see whether it will work or not.

Kind regards

Zhiqiang

Hi Zhiqiang Yu ,

It looks like your substrate has the temperature close to the one of the evaporation boat.

Therefore the material deposits and then immediately reevaporates from its surface.

Try to mount the substrate further away from the boat (is it possible to attach it to the chamber walls ? - they are definetely cold enough ), use massive metal mounting with good attachment to provide thermal contact, and also switch off the top heating lamp, use just the bottom one.

In fact, may be the last point may be enough to get the desired result.

Kind regards,

Victor.

Dear Victor Solovyev,

Thanks for your answer! I agree with your points and I'll have a try.

But I didn't understand the idea of "...use massive metal mounting with good attachment to provide thermal contact ..." you aforementioned, do you mean I should contact the substrate with massive metal to conduct the heat away and stable the temperature of substrate?

Yours,

Zhiqiang Yu

Dear Zhiqiang,

You're perfectly right: the metal attachment will help to reduce the substrate temperature (but remember this will not last long - its heat capacitance is not infinite).

Please be advised to choose the most appropriate material: it must have high melting point, and must be chemically compartible with your process. E.g. brass is not good (zinc evaporates at low temperatures already), stainless steel is typically fine though its thermal conductivity/capacity is not the best. Copper is the most suitable for heat removal (gold and silver are even better but expensive), but can interfere with the process.

You can also try sapphire - it has excellent thermal conductivity, though heat capacitance is not so big. Also it hardly can be machined, but you can find something suitable in the lab.

Actually, I would start with simply switching off the top heating lamp, this might be sufficient.

Kind regards,

Victor.

Hi Zhiqiang Yu. You may simply find that the material you are trying to deposit has such poor adhesion to your substrate that it "blows off", with the convection currents rather than sticking. You could try cleaning your substrate in a plasma, or try coating with a thin layer of another material first.

Dear Zhiqiang Yu,

you are absolutely right. The substrate temperature in your case was above critical condensation temperature. Therefore nothing is deposited on.

This effect is described in numerous handbooks. See "Handbook of Thin Film Technology" by L.Maissel and R.Glang (1970) for example.

You have to reduce substrate temperature cause thermal insulationg bricks are not efficient in this case. You have to put substrate as far as possible from evaporator to avoid direct contact with a substrate. It's better to put W or Mo screen with a hole or slit between boat and substrate to reflect back part of the thermal irradiation.

There is no substrate cooling in your setup, because lack of convection in a vacuum. Furthermore, substrate is praced almost directly on evaporator. What's why your sample surface is too hot for condensation.

On the other hand, outside wall of quartz tube is cooled by air (convective mechanism). That's why inner wall temperature is below critical point and layer is deposited on it.

So, you have to modify your setup to reduce substrate temperature below critical point for sucessful deposition.

Best regards

Yevhen

Dear Victor Solovyev, Phil Denby and Yevhen Zabila,

Thank you for your useful answers! It's really kind of you ! I know that my reply is late because I tried to find the best solution for the deposition and then to show the results for you.

Briefly, I almost succeeded in preparing the SnS thin films on the glass substrate (see the figure below) mainly by:

1) enlarging the distance between the evaporator and the substrate,

2) adopting a quartz shelf to support the substrate,

3) reducing the amounts of the source material,

4) lowing down the evaporation temperature slightly,

5) lasting the process in a proper time scale.

In addition, the fabrication should be improved and the film uniformity and component uniformity need further investigations for photovoltaic applications.

Thnak you again!

Zhiqiang

Dear Phil Denby,

Your thought about "the film was blowed off by the evaporated species" is wonderful and very possible, my results told me that you are right, and another reason except for the poor adhesion is the improper temperature of the substrate.

Thank you !

Zhiqiang

Dear Yevhen,

You are right in "thermal insulationg bricks are not efficient in this case" and then I changed the bricks to customized quartz shelf to reduce the temerature of the substrate.

The handbook you recommended is also useful to me.

Thank you!

Zhiqiang