It sounds like your chamber walls might have some carbon contamination that is redepositing on your substrate during the puff off flashing step. Your recipe sounds about right, the only reason I can think of for SiC particle formation is contamination. When was the last chamber clean/bakeout performed? Do you utilize this chamber for SiC growth?

The base pressure in the UHV chamber should be in the low -11 scale, preannealing should be done at 700C, followed by as short as possible flashes at 1300C  while trying to keep the pressure during the flashes in the -10 range. Lifetime of the clean surface after the treatment again depends on the base pressure in the chamber.

Dear Mr. Tanuj Trivedi,

We don't employ our MBE set up for SiC deposition.

We have performed the 'bake out' process ~ 3 months back. So to rule out the possibility of formation SiC nanoparticles due carbon contamination of the chamber wall, we will do the 'bake out' of the UHV chamber again. 

Except such contamination coming from the chamber wall, is there any other possibility of  formation of SiC particles on the reconstructed Si(hkl) specimens ? 

With regards,

Anjan

Dear Mr. A. A. Zakharov,

In our MBE set up we achieve UHV condition with the systematic operation of four types of pump i.e. Diapharm, Turbomolecular, Ion and TSP.  

So, for minimum achievable base pressure is  ~ 1.5 × 10 -10 mbar and it rises to ~ 8.0 × 10 -10 mbar during 'flashing' of the specimen. 

However, we will try to keep base pressure as low as possible during next specimen preparation.

With regards,

Anjan

To have the pressure on the -11 scale it might be an idea to cut off the turbo and leave the chamber only on the ion pump. If turbo (and especially forpump) is in a good shape it is OK to have the turbo on all the time. But if the maintenance of the turbo + forpump is not done regulary they may compromise the pressure and cause a problem to reach the ultimate pressure which chamber allows. 

I would question the accuracy of your temperature reading of the silicon.  it is likely you are not at 1200 and closer to 1100 where carbon islands (pyramids) will form and not evaporate.  Get to 1250 and no carbon.  Short flashes are all that is needed after your 600 anneal.  You can verify your temperature by looking for melting damage at that usually starts around 1350.

Anjan, I actually like Jason's idea better, I didn't think of that before. Always shave with Occam's razor first. Once you confirm that you have good temperature calibration, only then you should worry about contamination.

If your chamber isn't heavily utilized, then a 3 month old bakeout should be fine. Do you have a loadlock that is used heavily? It is also possible that the loadlock has a lot of contamination. Do you have a way to bakeout the load-lock separately from the chamber? If the gate-valve separating the chamber and the loadlock is not that strong, then it might not be a good idea.

Dear Mr. Jason L Pitters,

Instead of contamination of reconstrucred ultra-clean Si(hkl) substrates from the chamber wall, your described reason is more probable. 

I would like to know your suggestion about another aspect of specimen preparation. 

I generally perform the flash heating at high temperature (~ 1200 0C) for typically ~ 30 seconds. 

Is the time duration (~ 30 sec) is suitable for preparation of atomically smooth reconstructed Si(hkl) substrate ?

Thanking you,

Anjan

Dear Tanuj Trivedi,

Yeah, in our MBE set up we have a small fast entry load lock (FEL) chamber for transferring the specimen from ambience to UHV. Your described reason i.e. contamination coming from the FEL chamber during specimen transfer is also probable. In that case, I can do partially 'bake-out' the FEL chamber. 

However, I think the reason described by  Mr. Jason L Pitters is more probable for formation of SiC nanoparticles on the resconstructed Si(hkl) substrate.

With regards

Anjan

I believe it is your temperature that is the problem.  Fix that and you will be fine with you anneal.  But to expand slightly... I will assume you are using a direct current heating method of heating the silicon.  Two things to consider, pressure in the chamber and the heat you pass to your sample holder (hotter holder can mean more dirt passed to the sample). Keep the pressure in e-10Torr range. Most of the pressure rise during heating comes from passing heat to the holder and manipulator, causing degassing.  Also, it is suggested that your first heating to remove the oxide should be quick, so don't be slow passing through the 700 to 800 range.  Long anneals at 1200 only really degas the sample holder.  Your final flash is best done after you have let the sample holder cool.  Then flash to 1250 and straight back to RT.  multiple flashes are  fine.  Usually, heating manually, this can be accomplished in 5 seconds or so.  Cool down can be rapid (just turn off power supply).  This procedure is good for 100 and 111 with low miscuts (typical crystals).  Sometimes you may need to cool slowly if you want to achieve 111 7x7 as you pass through the 1x1 phase.  Other orientations and miscut angles or desire for other reconstructions may need modification of the above.  

Dear Mr. Jason L Pitters,

We use direct current (DC) heating method for preparing the reconstructed Si(hkl) substrates. In comparison with low index Si substrates like Si(001), Si(111), for high index Si(5 5 12), Si(553) and Si(557) substrates, we use some tricky steps to avoid step-bunching. 

With a base pressure ~ 1.5 × 10 -10 mbar and it rising  to ~ 8.0 × 10 -10 mbar during 'flashing' , we have prepared ultra-clean reconstructed Si(hkl) substrates without formation of SiC.

The substrate temperature in DC heating method depends of the dimensions (length, width and thickness) and resistivity. So it might be true that for some specimens our substrate temperature was inappropriate with the calibrated value. I will thoroughly check these aspects.

Thank you very much for your valuable suggestion.

With regards,

Anjan