Since the plasma source has been installed in our MBE, honestly I did not really know what the significant of it. What actually was the reason of it sitting there?
The isolation chamber connects the plasma source and the growth chamber. It means there is one shutter controller for nitrogen plasma source and one gate valve that comes from the isolation chamber. Below is the simple illustration:
Now, how about the MBE where the plasma source is connected directly to growth chamber of MBE (without isolation chamber)? Simply, only shutter of the nitrogen plasma source that governs the nitrogen plasma flow to the sample surface. Here is the simplified sketch:
The impact of the isolation chamber can be noticed when one has to work with more than one sample in short transition time interval with the next sample. As the plasma is ignited and introduced into the growth chamber, the pressure in in increase remarkably, from 10E-9 to 10E-4 Torr. It is a common procedure that, to exchange the sample with another sample (in buffer chamber), pressure of the growth chamber must be recovered to its normal working vacuum level. This can be achieved either by closing the gate valve (for the system having isolation chamber) or turning off the nitrogen plasma completely (for the system without isolation chamber).
This is the main difference. Terminating nitrogen plasma and evacuating nitrogen, venting it out from the growth chamber takes much more time for the vacuum level back to ultra high level. According to my experience, normally 60 min or more. Meanwhile, for the MBE with isolation chamber, nitrogen plasma source (and hence the nitrogen gas) is still there but isolated from the growth chamber. This is where the isolation chamber plays its role. Compared to the former design, recovery time of the pressure to ultra high vacuum takes at most 6 times faster, barely 10 minutes on the MBE system with isolation chamber.
If the growth plan is finished that day, the evacuation of the MBE with isolation chamber takes the similar manner as the MBE without the isolation chamber. In other words, the isolation chamber increase the time efficiency in growing between samples.
In addition, surface nitridation process is unavoidable reaction taking place for the MBE without isolation chamber. Sometime, one prefers to suppress surface nitridation by depositing, for example Al on the surface of Si and nitridation can be done afterward. This fashion can only be done on the MBE with isolation chamber, as the Nitrogen plasma can be generated before introduced inside the growth chamber. For the MBE system connected directly with nitrogen plasma source, the ignition of the plasma happens inside the growth chamber, where the sample is sitting, resulting nitridation treatment on the surface of the substrate, for some time even without realizing it.
The only consequence I have not discovered is the physical impact of the nitrogen plasma source to the substrate as the distance gets longer. Update will come soon, I hope!