I am trying to get ohmic contact between Aluminum and p-type silicon (resistivity 1 to 10 ohm.cm). I did check IV with two separated Al contacts on top and it is showing ohmic nature. Also, showing ohmic if taking two separate contacts on bottom of silicon.
But, if i am taking one bottom and one top contacts, it is showing schottky. Here, bottom is instrument chuck ground. According to band diagram of Al/Si/Al, one junction will always give schottky behaviour in forward or reverse bias as i understood.
My goal to have bottom Al/silicon contact ohmic.
I am confused how do people check contact nature. Is it ok to check ohmic nature by taking two contacts on one side of wafer !.
First make sure that Al will make a rectifying contact with p-type Si. For making a ohmic contact with p-type Si, you have to choose a metal with a work function higher than the work function of p-type Si, e.g. Au.
Yes, I dipped in HF to remove native oxide. Only issue why both contacts on top / bottom is giving ohmic nature, but not one top and one bottom contacts. point to be noted here is that the distance between two metal contacts is higher than silicon wafer thickness, if we consider voltage drop in bulk silicon.
To check contact types, metal contacts only on one side of the Substrate (in this case p-Si 1-10 ohm-cm) is sufficient. Al makes ohmic contact with p-Si, which is also observed by you when probed on two contact pads on the same side.
However, for measuring Al/Si/Al structure you are using chuck as bottom contact. I have few questions in this regard. 1. What is the Al contact pad size on the bottom side ? 2. Whether you have switched on the chuck vacuum to make a proper contact between bottom contact pads and the chuck (generally chuck surface is quite rough) ? If this contact is not proper, then you may get distorted I-V characteristics. Those who want to use chuck as bottom contact, generally go for large area bottom contact to avoid any such issue.
Further, if you want to check the how good is the contact is, you can make a transmission line method (TLM) structure (contact pads with increasing different spacing between them on the same side of substrate) and measure the contact resistance too.
Thanks Gourab
i am getting ohmic by taking contacts on same surface. But, still I have doubt how it is coming ohmic because according to band diagram any one interface in structure Al/p-Si/Al will always show non-ohmic behaviour means an increase in barrier height. may be i am not able to get here !
Yes, i am using chuck with proper vaccum hold on bottom side. Yes, Bottom side we have complete blanket deposition. Yes, TLM, Thanks
Al is p-type dopent for Si. Moderate temperature annealing generates a p+ region near the metal/Si interface which reduces the depletion region width and the conduction is dominated by tunneling.
You can see the slide-7 of the following PPT.
https://inst.eecs.berkeley.edu/~ee143/sp08/lectures/Sec8-Metallization.pdf
A highly doped p-type silicon would give good ohmic contact; annealing would improve too.
The most used method to measure the type the type of the metal semicondcutor MS contact is the three electrode method where one deposit three regions of metal on the same side of the wafer and then pass a constant current between two adjacent of them and measure the voltage across the remaining one and its adjacent current carrying one. In this way measure only the I-V behavior of the meddle electrode. The three electrode method is well recommended to check the i-v type of contact whether ohmic or rectifying. Aluminum can form rectifying nonlinear contact with p-silicon when it is mot properly annealed after deposition. When it is properly annealed it forms microalloying which is resulting in p+ contacting aluminum. This forms tunneling ohmic contact For more information please refer to the link:Article A. Zekry and G. El-Dllal, “ effect of MS contact on the elec...
Thank you for adding your answer.
Dear Shaimaa,
welcome,
In integrated circuit the most used metallization metal is Aluminum where it is used for metal contact of both n and p type silicon. To form an ohmic contact irrespective of the type of semicondcutor, it must be interfaced to the metal by heavily doped layer that is / Al/n+/n 0r Al/p+/p.
In this way you can use aluminum for both n and p-type silicon.
Best wishes
Dear Prof. Dr./ Abdelhalim Zekry
Thank you for your interesting answer. Already, I increased the doping of n-type Si nanowires and I will try to deposit different thicknesses of Al to get an ohmic contact with lower resistance by measuring it from top to bottom. Also. I will try the annealing to enhance the measured resistance.
Thanks Again
Dear Shaimaa,
Generally Au-Sb alloy is used to make ohmic contact with n-Si. You need to anneal additionally. Generally heavily doped region will increase tunnelling probability for electrons and there by good conduction. All the best!
>> any one interface in structure Al/p-Si/Al will always show non-ohmic behaviour >>
That's not the case! Your experiments on either top or bottom surface of the wafer demonstrate this!
Your findings that bottom to top contacts show rectifying behavior might be explained by the layered structure of the wafer. Are you sure that it's not some epitaxial one, with two or more layers on top?
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n-Si is indeed typically contacted by the annealed Au-Sb (1%).
Funnily, Al also works in this case but for a very limited time around ~1 day. Then the Al diffusion into the Si material makes the surface p-doped and the contact becomes rectifying.
Making contacts of Al with n type semiconductor is a problem and one needs to implant a n+ well. But Al itself can be thought of a p++ doped silicon hence, no need of implanting p+ well in Al/p junction.
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