I need to create ohmic (non-rectifiyng) contacts to a germanium wafers, using only manual tools, on an area of about 1 to 2 mm2.
The wafers are raw, P or N doped.
The procedure I use is the following:
- Wetting the area with a few drops of very strong flux (for inox soldering)
- Scraping the area with a surgical knife
- Tinning the area with Sn/Pb 60/40 solder
However I cannot obtain consistent result and the joints are often very delicate.
Do you have suggestion for a better connection?
The sample has to be subject to thermal stress (cycled between -200 and +200°c in 1 hours a few dozen times).
Thank you in advance for your help
Actually, you have a bit of a problem from a fundamental point of view. Due to fermi level pinning it can be very hard to get a very good ohmic contact to n-type Ge. For p-type you should have better luck.
For your application it is probably a question of what is good enough. There is a difference between trying to make a contact to a pristine semiconductor layer that doesn't have any defects and a layer that has a lot of surface states. One way you can get something that resemble an Ohmic contact is to create a lot of surface states at the surface between the metal and the semiconductor. This might not be practical depending on your application. Roughening up the surface with a 0.2 or less micron diamond grit might help.
One thing that may help, is to etch the surface of the germanium before soldering. This should remove the thin layer of Germanium oxide. I would look around some to find the right etchant.
Also the oldest paper I attched recommends InGa Eutectic as the solder. This will be liquid a little above room temperature, but you can probably move away from the eutectic point some.
In general almost all solders are going to have a problem at +200 C.
For a student perspective on soldering to Ge, this went around the internet a few years ago as something that was supposed to be kind of funny.
http://pages.cs.wisc.edu/~kovar/hall.html
What is your application?
Thanks for your answer.
The application is an experiment to measure the hall effect on different materials across a wide range of temperature.
Your suggestion about the diamond grit it's very interesting and i will try it as soon as possible.
I'm afraid the InGa solder has a melting point too low for my needs... i can easily stop the experiment at 120-150°C .
Do you think the content of the solder heavily influence it's adhesion to the germanium or most solders that work on copper/gold/silver are going to work as well?
Best regards.
Davide Bortolami
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