Normally the best annealing temperature is found empirically. This can be accomplished by annealing the contacts under N2 as both Si and Ge form oxides by reacting with oxygen. You must Know that GeO2 is hygroscopic and disolves in water. So, you have not to allow any residual oxygen in the system. The mechanism of building strong contact is the microalloying where aluminum dissolves silicon and builds aluminum silicon compound. So, i would assume that you must anneal at temperatures below the eutectic temperature which may be lower than Melting Point of Al-Ge at 427 °C.

Best wishes

Thank you very much for your reply. It is very helpful. One more question though. If I anneal at a temperature lower than 475 °C I will not dissolve the SiO2 to form Al2O3, right? And the Al-Ge alloy will not be harmful for electrical conductance, so for better electrical contacts a higher temperature is better, I imagine. What is the temperature required to break GeO2 and form Al2O3?

Thank you very much.

Dear Gabriel,

welcome again!

Aluminum, silicon and Ge have high affinity to oxygen. So when you deposit aluminum it will share in the native very thin oxide layer on the surface. This leads to strong adhesion. If this native oxide is very thin it will be tunnelable and has negligible impedance for mobile charge carriers.

What happens by annealing is that this interface oxide layer will be broken and aluminum will be microalloyed with both Ge and Si. But aluminum builds eutectic solution with Si and Ge at different temperatures with that of Ge is lower than that of Silicon. Since we do not want to reach the eutectic temperature of any one you must anneal at temperatures below that of Ge as hinted in the my first post, That is below 427 degree centigrade.

Best wishes