Crystalline Si(111) substrates are available commercially. These substrate contains oxide layer on top of it. If you carry out standard cleaning procedure to remove the oxide layer from the top in an ultra high vacuum chamber, you will get Si (111)7x7 surface.
What is done in the standard substrate cleaning steps is as follows.
The Silicon substrate first cleaned in ultar high vacuum pressure (~10^ -10 mbar). Resistive heating at 400-500°C is done for two and half hour to decrease the resistivity of the Si substrate.
After completion of resistive heating, Direct Heating is done for degassing the substrate. This time current is passed through the Silicon substrate. By this direct heating the absorbed gases is degassed out of the substrate. This process continues for 14-16 hours during this time sample temperature is around 600 °C.
After keeping the sample at a temperature of around 600 °C for 14-16 hours, the sample temperature is swept to quickly 1100 -1200 °C for 60 second. This is called Sample Flashing. By sample flashing the top oxide layer on the surface of silicon substrate is removed. As the oxide layer is removed from the surface, it becomes clean silicon substrate with (111)1x1 surface.
Then it is again lowered at a temperature of around 850 °C for 30 minutes, cool it down to RT. During this time the silicon atom on the cleaved surface gets reconstructed to saturate the dangling bonds to lower the surface free energy.
Due to this rearrangement of atoms the unit cell of this reconstructed Si (111) surface becomes seven times larger than that of the bulk silicon (111)1x1 unit cell. So, we call this surface as Si(111)7×7 surface.
Hi!
Has anyone seen change in colour to whitish gray after reconstruction?
How you are doing all these, may I know?? Then possibly I can give you better idea. To be specific , you cann't see anything in naked eye.
Hi! Jagadish,
First of all thank you very much for your reply.
I am new to this reconstruction business. I have tried your recipe but didn't work very well for me. However, I improvised it a little bit and the image of what I received is attached herewith. After degassing I flashed the sample for 1 min and then kept it at around 900°c for a minute, followed by cooling down. I repeated the process several times (7-8). This is the best reconstruction I have got so far. But as you may see that there's a lot of defect sites. The chamber pressure during flashing is 4.5*10E-9 mbar. But all the samples that got reconstruction has some whitish tint if viewed from some angle. I am worried about the defect sites now. Can you please tell me where am I going wrong?
Hi, Soumyadip,
1. Nice to see you captured the STM images of clean Si(111)7x7. I understand the surface contains defects. If you wish to improve this surface to well order Si(111)7x7, you may following. The essence is that you may anneal this sample at 700-870 C. You shall see, a lot of defects gets converts into 7x7, due to the fact that 7x7 is the energetically most favorable reconstruction.
2. Other thing you can try, is that you may keep the sample ~ 850-900C immediately after flashing the sample for a longer time (~15-30 minute) instead of just i minute as you did. Then slowly decrease the temperature to RT. I hope you will get better reconstructed surface.
By the way, in which research group your are working, all these STM research work. What else facility is there ?
best wishes
Jagadish
Hi Jagadish,
Thanks for your suggestion. I am working in 'Spectroscopic Imaging and Nanoelectronics' group led by Dr. Goutam Sheet. We have different cryogenic facilities like He3 refrigerator. Apart from that we have different thin film deposition facilities like PLD and scanning probe microscopes etc. We also have projection lithography sysytem.
Regards,
Soumyadip
It's great! You have excellent facility in your group.
Best wishes
Jagadish
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