dear Raja,

I am not sure what you mean by silicon "converted into oxide" when deposited on metals, but I think that you mean - "why is silicon oxidized' when deposited on metals.

Since you do not specify the deposition method, I can try to explain  4 key parameters you check for your deposition process.

1) First, let's emphasize that metals and metallic impurities ALWAYS enhance silicon reactivity and thus oxidation, because metals in contact with silicon tend to delocalize the bonding which is initially very covalent (See the model shown in Fig 3, p. 279 of the last paper attach to this answer).

This delocalization lowers the activation barrier to break the bonds  for oxidation - so metals in contact with silicon will always act as catalysts (I attached several papers below - the first 7 are all references -  recent papers, a review and 3 classic papers on silicon oxidation, the 8th discusses Si deposition & contamination from metals (Section IV, Fig. 6), the 9th paper shows in Figure 3 a model explaining  the bond delocalization/'metallization' occurs with covalent Si thin films when in contact with metals and how this increase in the metallic character of the bond ( which is less directional, has lower energy) increases silicon reactivity.

2) Second, silicon can oxidize while being deposited on a substrate only if a significant oxygen partial pressure pO2 is present in the deposition vessel ambient.

This partial pressure can enable - depending on the sticking coefficient, which can be less

Which deposition process are you using ? 

What is the substrate material ? 

dear Raja,

I am not sure what you mean by silicon "converted into oxide" when deposited on metals, but I think that you mean - "why is silicon oxidized' when deposited on metals.

Since you do not specify the deposition method, I can try to explain  4 key parameters you check for your deposition process.

1) First, let's emphasize that metals and metallic impurities ALWAYS enhance silicon reactivity and thus oxidation, because metals in contact with silicon tend to delocalize the bonding which is initially very covalent (See the model shown in Fig 3, p. 279 of the last paper attach to this answer).

This delocalization lowers the activation barrier to break the bonds  for oxidation - so metals in contact with silicon will always act as catalysts (I attached several papers below - the first 7 are all references -  recent papers, a review and 3 classic papers on silicon oxidation, the 8th discusses Si deposition & contamination from metals (Section IV, Fig. 6), the 9th paper shows in Figure 3 a model explaining  the bond delocalization/'metallization' occurs with covalent Si thin films when in contact with metals and how this increase in the metallic character of the bond ( which is less directional, has lower energy) increases silicon reactivity.

2) Second, silicon can oxidize while being deposited on a substrate only if a significant oxygen partial pressure pO2 is present in the deposition vessel ambient.

This partial pressure can enable - depending on the sticking coefficient, which can be less

Dear Nicole Herbts,

Thank you very much Madam. Your answer explain all of my confusions. I will definitely read all these articles to make my ideas clear about my work.

By the way I am depositing Silicon on metal by CVD method at 550 degree Celsius by means of Silane(SiH4) gas in Helium and flushing with Argon. But as you mentioned, may be I have to increase silicon deposition rate to avoid oxide.

Thanks for your very kind detailed answer :)

I think the key point is oxygen or other oxidizing environment (water) - check your system for vacuum background.

You must create very high vacuum before filling your chamber with silane gas to avoid any possibility of oxygen left in the chamber. Also degass the metallic substrate in vacuum before filling silane in the chamber. See whether you get your problem of oxidation reduced.

If your silicon layer is very thin, or if the silicon layer is very porous, the silicon may simply be oxidising from atmospheric exposure, nothing to do with deposition conditions. The surface of silicon oxidises immediately on exposure to air, so if the layer is thin, or if there are many porous grain boundaries, a substantial portion of the silicon can be oxidised just from this.

Yes, K.S.A. Butcher, since Raja did not quite specify the thickness of his films - any thickness under 10-20 nm could easily oxidize since he is depositing by CVD at 550°C.

Metal diffusion into Si is significant at that temperature and would enhance surface oxidation well above the typical thickness of native oxides found  high purity (ppb), low doping, high density Si, which is about 1 nm.

As Raja figured out, increasing his CVD deposition rate is the first step to try. I am not sure how low the oxygen partial pressure can be in his CVD reactor since CVD is not a vacuum method.

Any residual moisture/H2O adsorbates would increase oxidation, so outgassing the reactor and then the metal substrates for at least 10-15 minutes before initiating deposition would be best. One needs also to make sure to flush the reactor and gas lines with dry gasses . What is the purity of the Helium and Argon gasses used?

Silicon is oxidized in an Deposition process, since typical vacuum Systems all contain many water layers on the internal surface, which also are part of the residual gas pressure, and which normally are not cleaned by a full chamber Plasma cleaning and outgassing procedure. Therefore evaporated hot Silicon Atoms find abundant Oxygen in the water layer to become oxidized.

 Have a happy new Year , Hans