This doctoral thesis investigates treating and modifcation of silicon and gallium nitride surfaces and their interaction with metals in metal-semiconductor contacts. The key factor behind the experiments and results in this thesis is the utilization of ultra-high vacuum conditions in preparation and characterization of the materials. The interface between the metal and the semiconductor determines the type and quality of the formed contact, which is why understanding and developing of it is crucial to reduce electrical losses and enhance the effciency and life span of semiconductor devices.

In this work, the surfaces of n-type silicon and p-type gallium nitride were investigated. The surfaces were treated with wet chemistry and by heating and exposing the surfaces to gasses in ultra-high vacuum. The surfaces were measured with scanning tunneling microscopy/spectroscopy, x-ray photoelectron spectroscopy, and low-energy electron diffraction. The metal-semiconductor contacts were characterized by measuring the contact resistivity, and with gallium nitride, the interface was also investigated with synchrotron radiation x-ray photoelectron spectroscopy.

The results show that i) nickel forms a silicide with the silicon already at the room temperature but the electrical properties of the said silicide depend on the silicon surface treatments, and the doping concentration has a clear impact on the path of the current fow between the contacts; ii) by heating the Ni-GaN interface in ultrahigh vacuum, the gallium diffuses to the nickel layer and the vacancies created in the interface act as a route for the charge carries to tunnel through the potential barrier, leading to an ohmic contact; iii) by treating silicon devices, which already contained metal contacts, in ultra-high vacuum, the quality of the surface can be improved, and hence it is possible to improve the performance of devices made from silicon.