Modification of electric transport properties of carbon based systems by MeV-ion beam irradiation

Dipl.-Phys. Alexander Arndt

In a cooperation between the group of Nuclear Solid State Physics and Superconductivity and Magnetism a newly developed experimental setup was realized to determine the electric properties of multigraphene layers during the ion irradiation. Furthermore, also the electric transport properties should be investigated in situ. In the frame of this work a special sample holder and a suitable electromagnet was build to fit into the LIPSION chamber for the external beam with exit nozzle. The sample holder allows the in situ resistance and magneto transport measurements. The used ions were 2.25 MeV protons with a fluence between 9E12 cm^-2 and 6E15 cm^-2. After the proton irradiation the samples exhibits an increase of resistance and a reduction of the magneto resistance effect. This effect is correlated with the ion induced ferromagnetic order within the graphene system. During the actual investigations this behavior was not established in the samples.

Figure 1: In situ measurement of electric resistance during irradiation with quoted fluence values.

Surprisingly, during the in situ irradiation measurements of the resistance a clearly visible reduction in the electric resistance could be observed within the used fluence range. Furthermore, after the minimum of the resistance a relaxation behaviour was found. Figure 1 shows this effect in the normalized resistance values. The figure depicts six resistance curves of a multi graphene system of  60 nm thickness. Between the starting point (0 s) and the end point (40 s) of the irradiation process the resistance drops by up to 5% of the initial value. After this irradiation the resistance exhibits a relaxation curve to a resistance value higher than the initial one of each irradiation process (0.2% up to 1%). This process is still not fully understood. One part of this process could be identified as a thermal relaxation process due to the temperature increase during the irradiation. Another possible effect could be the recombination of the induced vacancies and interstitial carbon atoms.